Page 2 >{ Articles 11-19} 19: Foehn Effect, 18: Cyclone formations, 17.Jetstream &Weather, 16.High & Low Pressures, 15:Coriolis Effect, 14:Coldest Place on Earth, 13: Rainv/s showers, 12: Alto clouds with Virga,11: Crepuscular Rays

Weather Knowledge - 19

Foehn effect

The foehn effect causes warming and drying of air on the lee side of cross mountain wind.

What is the foehn effect?In simple terms, this is a change from wet and cold conditions one side of a mountain, to warmer and drier conditions on the other (leeward) side. 

Foehn winds (sometimes written "Föhn") are common in mountainous regions, regularly impacting the lives of their residents and influencing weather conditions for hundreds of kilometres downwind. Their notoriety has led to recognition by a multitude of names including: the Chinook or "snow eater" of the North American Rocky Mountains; the Zonda of the South American Andes; and the Helm wind of the English Pennines.

On 14 - 15 January 1972 in Montana, USA, a foehn chinook event was responsible for the greatest temperature change over a 24 hour period ever recorded in the United States: according to the US National Weather Service the temperature rose a staggering 57 °C; from -48 to 9 °C.

Foehn events are often accompanied by dramatic cloud formations above the mountains, such as towering lenticular clouds and lower-level rotor clouds. This is seen in the photo below revealing overturning and turbulence during a foehn event over the Antarctic Peninsula.

How does the foehn effect work?

Explanations of the foehn effect in popular literature or on the web often single out just one causal mechanism (#1 in the below), but there are in fact four known causes. These mechanisms often act together, with their contributions varying depending on the size and shape of the mountain barrier and on the meteorological conditions, for example, the upstream wind speed, temperature and humidity.

There are four mechanisms which combine to create the foehn effect:

1) Condensation and precipitation

When air is forced upwards over elevated terrain, it expands and cools due to the decrease in pressure with height. Since colder air can hold less water vapour, moisture condenses to form clouds and precipitates as rain or snow above the mountain's upwind slopes. The change of state from vapour to liquid water is accompanied by heating, and the subsequent removal of moisture as precipitation renders this heat gain irreversible, leading to the warm, dry foehn conditions in the mountain's lee. This mechanism has become a popular textbook example of atmospheric thermodynamics and it lends itself to attractive diagrams. However, the common occurrence of 'dry' foehn events, where there is no precipitation, implies there must be other mechanisms.

2) The draw-down of air from aloft

When the approaching winds are insufficiently strong to propel the low-level air up and over the mountain barrier, the air is said to be 'blocked' by the mountain and only air higher up near mountain-top level is able to pass over and down the lee slopes as foehn winds. These higher source regions provide foehn air that becomes warmer and drier on the leeside after it is compressed with descent due to the increase in pressure towards the surface.

3) Turbulent mixing

When river water passes over rocks, turbulence is generated in the form of rapids, and white water reveals the turbulent mixing of the water with the air above. Similarly, as air passes over mountains, turbulence occurs and the atmosphere is mixed in the vertical. This mixing generally leads to a downward warming and upward moistening of the cross-mountain airflow, and consequently to warmer, drier foehn winds in the valleys downwind.

4) Radiative warming

Dry foehn conditions are responsible for the occurrence of rain shadows in the lee of mountains, where clear, sunny conditions prevail. This often leads to greater daytime radiative (solar) warming under foehn conditions. This type of warming is particularly important in cold regions where snow or ice melt is a concern and/or avalanches are a risk.

Courtesy: Met Office U.K.

Weather knowledge - 18

Know about Cyclones Formations:

Whilst intense storms are very common throughout the Tropics, tropical cyclones need a very specific set of ingredients in order to form. These are:

• Warm ocean temperatures, greater than 26°C to a depth of at least 60m. Tropical cyclones only form over oceans, never over land, as they need sources of both heat and moisture. Even if the surface waters are very warm, the strong winds in a developing cyclone cause mixing of the surface layer of the ocean, bringing water up from below the surface. If this water is very much cooler than the surface, the supply of heat to the system will be reduced.

• Latitudes greater than 5° north or south of the equator. Cyclones are rotating systems and so only form in regions with a sufficient component of the Earth’s rotation about the local vertical (the Coriolis Effect) . This is zero on the equator itself and only becomes large enough to generate rotation within a weather system poleward of 5° of latitude.

• No large changes in wind speed and direction with height (known in meteorological terms as low wind shear). Cyclones rely on the development of tall columns of convective cloud which extend of the order of 10 km in the vertical. If the wind is changing too much with height, these columns of cloud cannot form through a great enough depth of the atmosphere as the tops will be constantly blown away.

• Lots of moisture through the depth of the atmosphere. As well as large wind shear, a dry atmosphere can also act to prevent deep columns of cloud forming.

• A pre-existing disturbance in the atmosphere. Tropical cyclones do not form spontaneously from nothing. There needs to be an area of enhanced thunderstorm activity which can act as a focus for the development of the cyclone in the presence of all the other conditions listed above. Forecasting the formation of tropical cyclones relies on early identification of these pre-existing disturbances and then recognising which ones will amplify into full-blown cyclones.

The development of tropical cyclones is a complex process and is still the subject of much research. In simple terms, a cyclone acts like an engine. It converts the energy available from a warm ocean surface into strong vertical air currents and horizontal wind speeds via the evaporation of warm water from the ocean surface and the subsequent condensation of this water vapour in deep columns of cloud around the centre of the storm. This deep cloud often forms an almost circular ring called the eyewall around the very centre of the storm which is itself free from clouds. Thankfully cyclones are rather inefficient engines, converting less than 10% of the available heat energy from condensation in the clouds into the mechanical energy of the motion of the winds.

There is an important positive feedback mechanism which allows cyclones to develop into intense systems. As the system starts to form, evaporation from the ocean surface acts as the source of heat and moisture for the formation of deep clouds. As these clouds intensify, strong rising currents of air in the eyewall around the storm centre. Near the surface, air is drawn into the centre of the storm to replace the rising air. This inrushing air near the surface results in strong winds which increase the evaporation from the ocean surface. This evaporation provides more heat and moisture to the clouds making the rising air currents within them stronger and thus intensifying the surface winds even further. As the system develops the Earth’s rotation acts on the inrushing winds, deflecting them into a pattern that rotates about the centre of the storm, spiralling in towards the centre.

A result of the mechanism for development described here is that the most severe weather, the heaviest rain and strongest winds, is strongly focused in the centre of the cyclone. In Figure 2 the diameter of the almost circular region of cloud associated with a Hurricane is about 800km. However, the most damaging winds and heaviest rain are all concentrated within the innermost 200km of the storm. The physics behind why there is sinking, warming and therefore clear air in the eye of the storm is still an area of active research.

 Diagram of a tropical cyclone.© NASA

The decay of a tropical cyclone usually occurs when the source of energy to the storm, the warm ocean surface, is removed. This may be due to the cyclone moving over land or into an ocean region with lower surface temperatures. An increase in vertical wind shear can also bring about the decay of a tropical cyclone.

Courtesy: University of Reading and Royal Meteorological Society

Weather Knowledge - 17

The Jetstream and The Weather 

In order to understand the jet stream you need to have some idea of what is occurring in the atmosphere nearest the ground; known as the troposphere. It is also helpful to understand the formation of Hadley, Ferrel and Polar cells and how The Coriolis Effect influences these tropospheric cells. The jet stream flows in both hemispheres around the earth, but for ease this article will consider the Northern Hemisphere only.

The jet stream is a strong flowing ribbon of air that flows around our planet high up in the atmosphere, at around the level of the tropopause. Situated between the troposphere and the stratosphere, the Jet Stream is approximately 11 kilometres above the surface of the Earth at the poles and around 17 kilometres above the surface of the Earth at the equator. The jet stream flows at around 160kmph (100mph). We often hear that the jet stream is responsible for influencing the weather in the UK, so it is natural to wonder what causes the jet stream and why it has such an influence on the weather we experience on the ground.

Jet streams form and are strongest where variable air temperature gradients are steepest. This is normally seen in two zones:

• The boundary between the polar and mid latitude air . The Polar Front Jet or Polar jet
• The boundary between the mid latitude air and tropical air . The Subtropical Jet.

Both these jets are separate entities but they can join up from time to time across an area of the earth. The polar jet is the strongest as the temperature gradient across Polar and mid latitude regions is greatest. This is increased again in the Northern Hemisphere during winter. Both the subtropical jet and the polar jet travel from west to east and both would travel uniformly and evenly around the earth if it wasn.t for other influencing factors.

See Fig 1. below for an example of a typical jet stream chart across the earth.

Typical JetStream

Click to enlarge

Fig 1: The jet stream is shown in red at its strongest point fading to yellow at its weakest. The STJ is the area where the subtropical jet is flowing, the PJ is an area where the polar jet flows and the P&STJ shows where the jet streams have combined.

Influencing factors on the Jet Stream flow
The factors that influence the flow of the jet stream are the landmasses and the Coriolis effect. Landmasses interrupt the flow of the jet stream through friction and temperature differences, whilst the spinning nature of the earth accentuates these changes. So the jet stream meanders across the earth, like a river meanders before it reaches the sea. The meandering sections of the jet stream continue to change as they interact with landmasses once again, creating an ever-changing state of flux and subsequent temperature differences.

In winter the temperature of the stratosphere can also have an effect on the strength and position of the jet stream. The cooler the polar stratosphere, the stronger the polar/ tropical differential becomes; encouraging the jet stream to gain in strength. The warmth of the landmasses and oceans (such as the El Nino Southern Oscillation) can also have a bearing on the strength and amplitude of the jet stream.

Jet Stream Variables
The strongest areas of the jet stream are known as jet streaks. These are areas where the jet stream has increased in speed by as much as 100kmph. A typical jet streak is 160km wide, 2-3 km thick and lasting several hundred km in length. The strongest jet streams are seen where the upper air temperature differentials are greatest, the weakest jet streams appear when the opposite is true. Strong jet streams tend to have very little meandering associated with them whereas weaker jet streams have a considerable amount of meandering associated with them.

A Summary of the Jet Stream

• Jet streams are strong upper air currents circumnavigating the globe.
• There are two main jets: the polar jet and the subtropical jet.
• Jet streams can fluctuate in strength between 100-200mph.
• The jet stream meanders in waves.

How does the jet stream affect the weather in the UK?
To try explain how the jet stream affects the weather in the UK, it is worth going back to the jet stream chart seen in Fig 1, now zoom in to the jet stream analysis over the Atlantic as seen in Fig 2.

Jet Analysis

Click to enlarge

The jet stream is shown in pink at its strongest point fading to yellow at its weakest. CA=cold polar air, WA=warmer air, PJ=polar jet, STJ=subtropical jet, JS=jet streak.

From Fig 2. you can see that the polar front jet meanders across the country in a wave like pattern. These waves introduce pockets of colder air southwards and warmer northwards. The temperature change is demonstrated by looking at the air temperature at a high enough altitude in the atmosphere, where ground and sea temperatures do not affect it .around 1500 metres in altitude where the air pressure is around 850 hPa.

Fig 3. shows how these temperatures match the fluctuations in the jet stream.

Jet + 850hpa Temps

Click to enlarge

From looking at Fig 3 and the 850-hPA temperatures you can see the clear boundaries between the cold and warm air masses, which is being divided by the polar jet. You can also see the demarcation between warm and hot air that the sub tropical jet is demarcating. This demonstrates the pattern of the jet stream, showing how it is linked to and by the difference in warmer and cooler air masses.

Furthermore, looking back at FIG 2, the wave pattern associated with the polar jet has distinct peaks (ridges) and troughs. It is no surprise to find that when we overlay the jet stream with a chart showing sea level pressure, that the ridges occur where sea level pressure is highest and the troughs occur where sea level pressure is lowest (as FIG 4 demonstrates). This is because air rises where troughs are situated and sinks where ridges are occur. Rising and sinking parcels of air will therefore determine the type of weather that a region experiences.

Ridges and Troughs

Click to enlarge

R=ridge, T=trough, X=deep sea level depression or low.

Look at the area X on Fig 4. which is situated towards the west of Iceland. This is a common cyclonic or baroclinic depression that is caused by the difference in pressure and temperature over a region. This is feeding strength to the jet stream, which is then invigorating the depression.

In contrast, Fig 5 shows the same chart from FIG 2. However, note the following points:

• The circled area is where the jet stream is strongest . the jet streak.
• The purple box marks the winds entering the jet streak.
• The winds leaving the jet streak are marked by the black box.

The winds leaving the jet streak are rapidly diverging, creating a lower pressure at the upper level (tropopause) in the atmosphere. The air below rapidly replaces the upper outflowing winds. This in turn creates the low pressure at the surface (marked X on fig 4). This surface low pressure creates conditions where the surrounding surface winds rush inwards. The Coriolis effect creates the cyclonic rotation that is associated with depressions. The strongest surface winds in any developing depression are normally seen at the left exit point of the jet streak, where the jet streak is strongest.

Jetstreak

Click to enlarge

Fig 5: The black oval indicates a jet streak, the purple square indicates entry winds into the jet streak and black box indicates rapidly diverging exit winds.

The positioning and strength of the jet stream determines where ridges and troughs are associated and this in turn influences the surface weather.

Zonality and Meridional flow.
During periods when the jet stream is flat and strong with little amplification or meandering, the UK is likely to experience weather that is driven straight in from the Atlantic. This is characterized by wet and windy weather with temperatures near to average. This condition is often termed .zonality. as the warm and cold air masses are clearly defined by a straight fast flowing jet.

During periods when the jet stream is amplified (such as Fig2) the pattern will be different. This is often termed meridional with polar air travelling further south than usual and warmer sub tropical air travelling further north. The exact positioning of the amplification of the jet stream will determine whether or not the UK is in cold polar air or warmer air from lower latitudes. If a meridional pattern becomes stagnant then the UK may experience either of these conditions for a period of time and the pattern may be known as .blocked..

Summary of the Jet Stream and the weather it creates:

• The position of the jet stream over the UK determines the type of weather we experience.
• If the polar front jet is situated significantly to the south of the UK we will experience colder than average weather.
• If the polar front jet is situated to the north of the UK we will experience warmer than average weather.
• If the polar front jet is situated over the UK we will experience wetter and windier than average weather.
• If the polar front jet has a large amplification then cold air will travel further south than average and warm air will travel further north than average.
• The direction and angle of the jet stream arriving at the UK will determine what source of air (i.e. cold, dry, warm, wet, from maritime or continental sources) the UK experiences.

Article written by Ed O'Toole

Weather Knowledge - 16

High and Low Pressure areas.

We live on a planet that rotates, so this simple wind pattern is distorted to such a degree that the air is twisted to the right of its direction of motion in the Northern Hemisphere and to the left in the Southern Hemisphere.  Today we know this effect as the Coriolis Force and as a direct consequence, great wind spirals are produced which we know as high and low pressure systems.

In the Northern Hemisphere, the air in low pressure areas spirals counterclockwise and inward — hurricanes, for instance, are Coriolis mechanisms, circulating air counterclockwise.  In contrast, high pressure systems the air spirals clockwise and outward from the center.  In the Southern Hemisphere the direction of the spiraling of the air is reversed.

So why do we generally associate high pressure with fair weather and low pressure with unsettled weather? 

High pressure systems are “domes of density” that press down, while low pressure systems are akin to “atmospheric valleys” where the density of the air is less.  Since cool air has less of a capacity to hold water vapor as opposed to warm air, clouds and precipitation are caused by cooling the air. 

So by increasing the air pressure, the temperature rises; underneath those high pressure domes, the air tends to sink (called “subsidence”) into the lower levels of the atmosphere where temperatures are warmer and can hold more water vapor.  Any droplets that might lead to the formation of clouds would tend to evaporate.  The end result tends to be a clearer and drier environment.

Conversely, if we decrease the air pressure, the air tends to rise into the higher levels of atmosphere where temperatures are colder.  As the capacity to hold water vapor diminishes, the vapor rapidly condenses and clouds (which are composed of countless billions of tiny water droplets or, at very high altitudes, ice crystals) will develop and ultimately precipitation will fall.   Of course, we could not forecast zones of high and low pressure without employing some sort of device to measure atmospheric pressure.

Weather Knowledge - 15

The Coriolis Force (also called the Coriolis Effect)

The Coriolis Effect is named after 19th-century French engineer-mathematician Gustave-Gaspard Coriolis. In 1835 he expanded on Sir Isaac Newton’s 3 laws of motion; by describing how an inertial force acts upon things with in a rotating frame of reference. This force is to the right of the direction of body travel for counterclockwise rotation of the reference frame or to the left for clockwise rotation. 

It affects weather patterns, it affects ocean currents, and it even affects air travel. The Coriolis Effect makes things (like planes or currents of air) traveling long distances around the Earth appear to move at a curve as opposed to a straight line. This is the reason; air tends to rotate counterclockwise around large-scale low-pressure systems and clockwise around large-scale high-pressure systems in the Northern Hemisphere. In the Southern Hemisphere, the flow direction is reversed. .

Most of us can agree that the Earth is a very large round object that is rotating on its axis. We can also agree that a day is the length of time it takes the Earth to make one rotation on that axis, which is 24 hours. If this is true, then depending on latitude parts of the Earth are moving at different speeds. 

That might sound crazy, but think of it like this. If one of y’all were standing a foot to the right of the North Pole, this would make the circumference of that circle about 6 feet. It would take 24 hours for that spot to rotate back around that circle on a round rotating Earth. That’s about 0.00005 miles per hour. OK now we move and stand on the equator. The day is still 24 hours long; but the circle circumference is much bigger. At the equator the Earth's circumference is about 25,000 miles. Which would mean you’re moving about 1040 miles per hour just by standing there. So even though we are all on Earth, how far we are from the equator determines our forward speed. The farther we are from the equator, the slower we move. 

Okay how does this stop things like hurricanes from moving in a straight line? Going back to our imaginary surroundings. This time we’re moving at 70 mph down the interstate. As we move, we come up on a slower moving bus. As we pass the bus, we see an open window. Since you have a baseball in your hand, you decide to try and throw that ball through the window. You take aim, and make an extraordinary dead on throw. But in spite of that, the baseball travels to the side and misses the window. That’s because the ball is traveling not only in the direction of the window, but it is also going in the direction (and speed) of your car. And that’s the deflection we are talking about! 

Anything traveling long distances, like air currents, ocean currents, even hurricanes and airplanes, will all be deflected because of the Coriolis Effect! Uncanny but weirdly true. 

Credit WX4cast Rebecca Ladd

Weather Knowledge - 14

Disclaimer: This page is kept for historical purposes, but the content is no longer actively updated

What is the coldest place on Earth?

It is a high ridge in Antarctica on the East Antarctic Plateau where temperatures in several hollows can dip below minus 133.6 degrees Fahrenheit (minus 92 degrees Celsius) on a clear winter night.

Scientists made the discovery while analyzing the most detailed global surface temperature maps to date, developed with data from remote sensing satellites including the new Landsat 8, a joint project of NASA and the U.S. Geological Survey (USGS). Ted Scambos, lead scientist at the National Snow and Ice Data Center in Boulder, Colo., joined a team of researchers reporting the findings Monday at the American Geophysical Union meeting in San Francisco.

 

Researchers analyzed 32 years' worth of data from several satellite instruments. They found temperatures plummeted to record lows dozens of times in clusters of pockets near a high ridge between Dome Argus and Dome Fuji, two summits on the ice sheet known as the East Antarctic Plateau. The new record of minus 136 F (minus 93.2 C) was set Aug. 10, 2010.

That is several degrees colder than the previous low of minus 128.6 F (minus 89.2 C), set in 1983 at the Russian Vostok Research Station in East Antarctica. The coldest permanently inhabited place on Earth is northeastern Siberia, where temperatures in the towns of Verkhoyansk and Oimekon dropped to a bone-chilling 90 degrees below zero Fahrenheit (minus 67.8 C) in 1892 and 1933, respectively.

"We had a suspicion this Antarctic ridge was likely to be extremely cold, and colder than Vostok because it's higher up the hill," Scambos said. "With the launch of Landsat 8, we finally had a sensor capable of really investigating this area in more detail."

The quest to find out just how cold it can get on Earth -- and why -- started when the researchers were studying large snow dunes, sculpted and polished by the wind, on the East Antarctic Plateau. When the scientists looked closer, they noticed cracks in the snow surface between the dunes, possibly created when wintertime temperatures got so low the top snow layer shrunk. This led scientists to wonder what the temperature range was, and prompted them to hunt for the coldest places using data from two types of satellite sensors.

The coldest place on earth is in the East Antarctic Plateau, but not at the highest peak. Rather, the coldest spots develop just downhill from a ridge that runs from Dome A to Dome Fuji.  Movie

They turned to the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA's Terra and Aqua satellites and the Advanced Very High Resolution Radiometer (AVHRR) on several National Oceanic and Atmospheric Administration satellites. These sensitive instruments can pick up thermal radiation emitted from Earth's surface, even in areas lacking much heat.

Using these sensors to scan the East Antarctic Plateau, Scambos detected extremely cold temperatures on a 620-mile stretch of the ridge at high elevations between Argus and Fuji, and even colder temperatures lower elevations in pockets off the ridge. Then, with the higher resolution of the Thermal Infrared Sensor (TIRS) aboard Landsat 8, the research team pinpointed the record-setting pockets.

The team compared the sites to topographic maps to explore how it gets so cold. Already cold temperatures fall rapidly when the sky clears. If clear skies persist for a few days, the ground chills as it radiates its remaining heat into space. This creates a layer of super-chilled air above the surface of the snow and ice. This layer of air is denser than the relatively warmer air above it, which causes it to slide down the shallow slope of domes on the Antarctic plateau. As it flows into the pockets, it can be trapped, and the cooling continues.

"By causing the air to be stationary for extended periods, while continuing to radiate more heat away into space, you get the absolute lowest temperatures we're able to find," Scambos said. "We suspected that we would be looking for one magical site that got extremely cold, but what we found was a large strip of Antarctica at high altitude that regularly reached these record low temperatures."

This narrated animation shows the process by which the coldest place on Earth develops its extreme low temperatures.  Play it

The study is an example of some of the intriguing science possible with Landsat 8 and the TIRS instrument, which was built at NASA’s Goddard Space Flight Center in Greenbelt, Md. Since its launch Feb. 11, Landsat 8 has captured approximately 550 scenes per day of Earth's land surface. USGS processes, archives and distributes the images free of charge over the Internet.

"With Landsat 8, we expect to see more accurate and more detailed maps of the landscape than we've ever been able to see," said James Irons, the mission's project scientist at Goddard. "If change is occurring, I think we'll be able to detect it earlier and track it."

Researchers also are eager to see what new results come out of Landsat 8, both from icy plateaus and Earth's warmer regions.

"What we've got orbiting Earth right now is a very accurate and consistent sensor that can tell us all kinds of things about how the land surface of Earth is changing, how climate change is impacting the surface of Earth, the oceans of Earth, and the icy areas of Earth," Scambos said. "Finding the coldest areas on Earth is just the beginning of the discoveries we're going to be able to make with Landsat 8."

Credits:

Production editor: Dr. Tony Phillips | Credit: Science@NASA

Weather Knowledge -13

Rain VS Showers: What is the difference between the terms Rain and Showers.

When you hear or see showers on the forecast do you think, there won’t be very much and when you the term rain is used do you think, it will be soaking?

The difference between the two is kind of tricky and subtle. There is no doubt that showers are indeed rain. Taking at face value the term rain or showers, has nothing to do with how much precipitation is going to fall. Instead it tells you how it is going to fall. It has to do with the type of cloud they come from.

What are showers?

A shower is a short duration event, that can last a couple of minutes to perhaps 15 minutes or so. But they can sometime last over half an hour. They typically start quickly and end quickly. There can be heavy downpours when dealing with showers.

Showers come from Cumuliform clouds - Cumulus or Cumulonimbus (thunderstorm) the puffy ones that look like they are bubbling up. often separated by blue sky. Showers are pushed around by the wind, so you only experience a particular shower if you are in its path Since they are hit and miss, your house could be getting wet, while your next-door neighbor could be dry. Cumulus normally result in lower totals while amounts can get quite high from Cumulonimbus.

The customary way of talking about them is isolated showers or scattered/widespread showers. Because they are hit and miss you will never see 100% chance of showers.

Cumulus 

Cumulonimbus

Cumulonimbus

What is rain?

Rain is a moderate to long duration event. That can last for a couple of hours to all day. It typically starts gradually then ramps up and ends gradually.

Rain comes from Stratiform clouds- Altostratus and Nimbostratus. These types of clouds are more or less featureless and cover the sky in a grey, widespread sheet, with little to no blue sky to be seen, and of the two, Nimbostratus is thicker and produces heavier rains.

Rain covers a wide area, so most or everyone over a large area are getting wet. Rain can come down lightly or heavily. Because of its long-lasting duration, it can lead to flooding issues.

Altostratus 

Nimbostratus

Wx4cast

Weather Knowledge -12

 Altocumulus with Virga

July 14th, 2020 by Roy W. Spencer, Ph. D.

These are altocumulus clouds with strong upward motion in them producing precipitation, in this case snow, falling in streaks and not reaching the ground (“virga”). They form when the upper troposphere is unstable, and warm advection (a warm air mass moving into a cool air mass) produces the uplift.

Weather Knowledge - 11

Watch "What are Crepuscular Rays? | 

Weather Wise Lessons" on YouTube

https://youtu.be/3mQibPr5bmI

Page 4 >{Articles 35-44} 44:Peculiiar Cloud over Caspian, 43: Research on 2021 Heatwave in NWIndia,42:Red Sprites, 41: Lenticular Clouds, 40Lightning risks in S.E.Asia, 39: Worlds Longest Lightning Bolt, 38: Mumbai Dust Storm 2024, 37:Tonga Volcano Effect on Weather, 36: FAQ: coldest time of the day, 35: FAQ: What is UVIndex ?.

 Weather Knowledge - 44

Peculiar Cloud Over the Caspian

May 28, 2022JPEG

On most days, clouds hover over at least part of the Caspian Sea, the planet’s largest inland body of water. But a cloud that drifted across the Caspian on May 28, 2022, looked more peculiar than most.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired this view on the morning of May 28. The lone cloud had well-defined edges resembling something from a cartoon, making it stand out from the more typical diffuse and dispersed cloud cover.

According to Bastiaan van Diedenhoven, an atmospheric scientist at SRON Netherlands Institute for Space Research, the cloud is a small stratocumulus. The cloud type is puffy like a cumulus cloud; cumulus is the Latin word for “heap” or “pile.” But unlike a cumulus cloud, the “heaps” in a stratocumulus cloud are clumped together, forming a widespread horizontal cloud layer. (Stratus is from a Latin verb that means “to spread out,” or “cover with a layer.”) The stratocumulus pictured here formed a layer spanning about 100 kilometers (60 miles) across.

Stratocumulus clouds form at low altitudes, generally between 600 and 2,000 meters (2,000 and 7,000 feet). This cloud was probably hovering at an altitude of about 1,500 meters (5,000 feet).

In the late morning (shown above), the cloud was poised over the central Caspian. By the afternoon it had drifted toward the northwest and hugged the coast of Makhachkala, Russia, along a low-lying plain near the foothills of the Caucasus Mountains. According to van Diedenhoven, the cloud could have formed when warm, dry air—possibly from the Balkans—encountered colder, moist air over the Caspian. It then drifted across the sea and dissipated when it reached land.

The scenario also explains the cloud’s well-defined edges. “Sharp edges are often formed when dry, warm air coming from land collides with colder moist air over the ocean, and the cloud forms at that boundary,” van Diedenhoven said. “You often see this off the west coast of Africa, but at much larger scales.”

NASA Earth Observatory image by Joshua Stevens, using MODIS data from NASA EOSDIS LANCE and GIBS/Worldview. Story by Kathryn Hansen.

Weather Knowledge - 43

Research: Pacific storm clouds strengthened 2021 Northwest heat wave

Weather Knowledge - 42

Red Sprites Generated By Hurricane Mathew
Taken by Frankie Lucena on October 1, 2016 @ Cabo Rojo,Puerto Rico

These red sprites were captured last night when Hurricane Mathew was near Aruba and the northern tip of Colombia. I used a Canon EOS T3 camera with no IR filter and a 50mm lens at F/1.8 for 1 second at iso 6400.

Sprites are a strange and beautiful form of lightning that shoot up from the tops of electrical storms. They reach all the way up to the edge of space alongside meteors, auroras, and noctilucent clouds. Some researchers believe cosmic rays help trigger sprites, making them a  true space weather phenomenon.

Seeing sprites above a hurricane is rare. Many hurricanes don't even have regular lightning because the storms lack a key ingredient for electrical activity: vertical winds. (For more information read the Science@NASA article "Electric Hurricanes.") But Matthew is not a typical hurricane.  It's one of the most powerful in recent years, briefly reaching Category 5 at about the time Lucena photographed the sprites.  Perhaps extra-strong winds in the vicinity of the storm set the stage for upward-reaching bolts.

Weather Knowledge - 41

 Here is a picture taken by Darlisa on Friday (looking north, west is to the left).  

Look closely and you will see one lenticular right over Mount Adams, nearly symmetric over the peak.  That is often called a cap cloud, and it results from air being pushed up by the mountain until it reaches saturation (100% relative humidity).  

Picture by Darlisa Black

But even more impressive are the stacked lenticular clouds downstream (east) of Mount Adams).  These clouds extend to great heights in the atmosphere and are associated with vertical propagating mountain waves.   Translation:  vertically propagating means waves that can extend through great vertical distances.

From Cliff Mass Weather Blog

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Assessing Lightning Risk in South Asia

2001 - 2017JPEG

In August 2021, a wedding in northwestern Bangladesh took a tragic turn when lightning struck and killed 17 guests along the Padma River during a downpour. Though such mass casualty incidents are rare, fatal lightning strikes in Bangladesh and nearby Nepal are not.

Hundreds of people lose their lives to lightning in these two countries each year, and hundreds more are injured. Due to sharp increases in lightning deaths and injuries in Bangladesh and Nepal in recent years compared to the past few decades, the problem is getting more attention from both scientists and government officials. As awareness of lightning dangers grow, satellite observations and ground-based lightning networks are playing key roles in sizing up the extent of the problem and helping people formulate strategies to minimize the risks.

The number of reported lightning deaths and injuries in Bangladesh has increased from dozens of deaths per year in the 1990s to more than 300 per year now. Trends are more difficult to determine in Nepal due to a lack of long-term data, but one recent study estimated about 100 people are killed by lightning there each year. For comparison, lightning kills about 17 people per year in the United States, a country with more than 10 times as many people as Nepal. The photograph below, taken by an astronaut on the International Space Station, shows lightning flashing over Nepal in 2021.

Lightning experts cite a variety of reasons for the apparent increases in deadly strikes, including population growth, better reporting, and increasing storminess due to climate change. Though the cause is not clear, the timing is quite clear.

September 14, 2021JPEG

“The frequency of lightning is highest in Bangladesh during the pre-monsoon period from mid-April to June,” said Ashraf Dewan, a remote sensing scientist at Curtin University. Using observations from NASA’s Lightning Imaging Sensor (LIS), he found that 69 percent of the lightning strikes in Bangladesh occur during the period before the heaviest seasonal rains set in. And lightning flashes were particularly common in the morning. The LIS instrument flew on NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite from 1997 to 2015, and a duplicate sensor has operated on the International Space Station since 2017.

“Bangladesh has more lightning from sunrise to midday than anywhere else in the world,” added Ron Holle, a meteorologist with the National Lightning Safety Council. “Unfortunately, that is when people are farming in huge numbers across the country.” Holle and Dewan worked together on a study of ground-based lightning data from the Global Lightning Detection Network that documented a large number of lightning deaths among Bangladeshi farmworkers.

Dewan and other colleagues have been analyzing decades of satellite observations from LIS as well as data from ground-based lightning networks to map lightning patterns in Bangladesh. Other researchers have done similar mapping for Nepal.

One team of researchers in the DEVELOP program at NASA’s Marshall Space Flight Center worked to turn LIS data into a product tailored to save lives. The team integrated maps of lightning flash density together with data on land elevation, population density, and socioeconomic vulnerability to lightning (including housing conditions and employment type). One of their lightning risk maps is shown at the top of the page; areas with the highest risk are depicted in yellow.

“Our goal was to map where lightning posed the highest risk to people—not just where the most lightning flashes occur,” explained Essence Raphael, a member of the DEVELOP team and a research associate at the University of Alabama in Huntsville. “That’s the kind of information people who develop safety interventions and educational campaigns can use to target their efforts most effectively.”

Flash density relative to population density was critical for defining the high-risk areas, explained Patrick Galtin, a lightning researcher at NASA Marshall. Though the number of lightning strikes in both countries is similar, the risk is higher in Bangladesh due to its higher population density.

Terrain also plays an important role in defining high-risk areas. Dry air in the higher elevations of northern Nepal prevents storms from forming. However, in lower-elevation areas along the country's southern border, warm, moist winds from the Bay of Bengal collide with cool air from the north to produce towering cumulonimbus clouds and extreme lightning.

Likewise in Bangladesh, the northeastern part of the country has a higher risk because the area is buffeted by moisture-laden winds that run into hilly terrain, which promotes convection and storm development. It is also an area with large numbers of farm and other outdoor workers in harm’s way.

After creating the maps, the DEVELOP team shared them with meteorologists at the Bangladesh Meteorological Agency and Nepal’s Department of Hydrology and Meteorology. Targeted information about lightning risk could be used to help advance ongoing lightning safety initiatives, noted Raphael. Such efforts include building early warning systems, constructing lightning shelters, and conducting public education campaigns.

Given the increasing risks, Dewan has some practical advice for people in the two countries. “If you can, get inside a large building with grounded wiring and plumbing or a fully-enclosed, metal-topped vehicle during thunderstorms. Don’t shelter under isolated trees or on high ground if you are stuck outside,” he urged. “Stay away from water and open spaces.”

NASA Earth Observatory image by Lauren Dauphin using data from NASA DEVELOP (Raphael et al.) Astronaut photograph ISS065-E-386846 was acquired on September 14, 2021, with a Nikon D5 digital camera using a focal length of 24 millimeters. Story by Adam Voiland.

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The Tonga Volcano Affects the Weather and Water of the Pacific Northwest

Yesterday, around 0400 UTC 15 January (8 PM PST 14 January), there was a massive, explosive eruption near Tonga, in the southern tropical Pacific, about 5642 miles from Seattle (see map).

The volcano was clearly evident in satellite imagery from the massive ash cloud (see below, about 1-h after the eruption)

The explosive eruption created shock waves in the atmosphere (pressure waves) that rapidly propagated away.  These waves are evident in some infrared (water vapor channel) imagery as concentric rings (shown below).

The oceanic eruption also pushed away a massive amount of water, which created a tsunami on nearby islands (such as Tonga) and deep water waves that moved away at the speed of a jet plane, reaching the West Coast this morning.  This is why some local tsunami warnings went out this AM.

The Pressure Wave Reaches the Northwest

Local barometers indicated a well-defined pressure wave passing over our region around 4:30 AM this morning.  Here in Seattle, the University of Washington barometer showed the feature, with an amplitude of roughly 2 hPa (2 mb).  The arrow indicates the feature. Very impressive.

So it took about eight hours and 30 minutes to go about 5643 miles--thus a speed around 664 miles per hour.  

The water wave moves slower, around 400 mph (and occasionally approaching 500 mph)....so a later arrival was expected.   Thus, at Neah Bay, at the entrance to the Strait of Juan de Fuca,  the water wave arrived around 9 AM (17:00 UTC as shown on the chart), as indicated by the waviness in the water level after that time.  The amplitude of the variation is around 2 feet.

From Cliff Mass Weather Blog

Pressure spikes from around the globe due to the eruption

Weather Knowledge-36

FAQ...What is Coldest time of the day

Lowest temperature is just after sunrise

The coldest time of the day is normally about plus/minus 15 minutes of sunrise.

A lot of people ask this, because it seems logical that the coldest time should be in the middle of the night. However, all through the night the ground is losing heat and the air is steadily getting colder.

When the sun rises over the horizon, its first rays are very weak. and there is still more heat being lost then is being received. 

After around 15 minutes, the sun is strong enough to add a little warmth and the temperatures can start to rise.

Of course there are plenty of things which can cause the temperature to change during the day. Warm air can drift in from another area or clouds/rain can drag the temperature up or down. If sky is clear and there is no cloud or wind draft affecting the weather, the coldest time of the day will be just after dawn.

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What is UV index? An expert explains what it means and how it’s calculated

Written by Sarah Loughran

You’ve probably seen the UV index in the day’s weather forecast, and you know it tells you when you need to cover up and wear sunscreen. But where does that number come from? We produce it at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). Australia has one of the highest rates of skin cancer in the world, accounting for about 80 per cent of cancers diagnosed in Australia each year. Most skin cancers are caused by exposure to UV radiation from the sun.

What is the UV index?

The UV index tells you how much ultraviolet radiation is around at ground level on a given day, and its potential to harm your skin. UV radiation is a component of sunlight that can cause tanning and sunburn in the short term. In the longer term, too much exposure to UV can cause cataracts and skin cancer.

In 2002, the World Health Organisation devised the UV index in an effort to make people around the world more aware of the risks.

The index boils down several factors into a single number that gives you an idea of how careful you need to be in the sun. A score of 1 or 2 is low, 3 to 5 is moderate, 6 or 7 is high, 8 to 10 is very high, and 11 and above is extreme.

What is UV radiation?

The Sun showers Earth with light at a huge spectrum of different wavelengths, and each wavelength can have a slightly different effect on human skin. An important part of the spectrum is ultraviolet or UV radiation: light with wavelengths too short for our eyes to see, from around 400 nanometres to 10 nanometres.

There are two important kinds of UV radiation: UV-A, with wavelengths from 400 to 315 nanometres, and UV-B with wavelengths from 315 to 280 nanometres. (Shorter wavelengths are called UV-C, but are mainly blocked by the atmosphere so we don’t need to worry about it.)

How is the UV index calculated?

The UV index takes into account how much UV radiation of different wavelengths is around and how each of those wavelengths affects our skin.

ARPANSA has a network of sensors around Australia measuring sunlight at different wavelengths to determine the UV index, with the information available online in real time. This data is combined with other information about location, cloud cover and atmospheric conditions to produce maps and forecasts of the UV index for the whole country.

How are UV levels different around the world?

The UV index you see reported is usually the daily maximum — that’s the highest it will be all day. How high it gets depends on lots of factors, including your location, the time of year, the amount of cloud cover, and ozone and pollution in the atmosphere.

The index tends to be higher closer to the Equator and at high altitudes, as the sunlight has to pass through less air before it reaches the ground. People often experience the sun in Australia as particularly harsh, compared with places in North America or Europe.

In a British summer, for example, the maximum UV index might be between 6 and 8. In an Australian summer it can range from 10 to 14.

There are a few reasons for this. One is that Australia’s cities are closer to the Equator than many big cities in Europe and North America. Another is that Earth is very slightly closer to the Sun in the southern hemisphere’s summer than the northern summer, meaning the sunlight is a few percent brighter. A third reason is the ‘hole’ in the ozone layer. The layer of ozone in the upper atmosphere, which absorbs some UV-B, is thinner towards the South Pole. This was caused by the use of chemicals called chlorofluorocarbons or CFCs, and it has been improving since they were banned by an international agreement in 1987. And finally, the air in Australia generally has less smoke, dust and other small particle pollution than many places in the northern hemisphere. While this makes the air nicer to breathe, pollution does absorb or block some UV radiation.

Is UV changing over time?

We know UV levels have increased in recent decades. In Australia, a study in 2011 found the average UV index had increased by 2 to 6 per cent between the 1970s and the period 1990 to 2009, due to depletion of the ozone layer. A NASA study found similar results for 1979 to 2008. It’s harder to say what will happen in the future, as there are several uncertain factors.

We expect the ozone layer to slowly recover from the impact of CFCs, which is likely to reduce UV levels. However, we also expect less fossil fuel will be burned, which would mean less air pollution and higher UV levels.

On the flip side, we may also have more bushfires due to climate change, which would mean more air pollution and lower UV. Clouds are also likely to behave differently due to climate change, but we’re not sure exactly how. Researchers in Japan found reductions in clouds and tiny particles in the air are expected to have a bigger impact than the recovery of the ozone layer, which would mean UV levels are likely to go up overall.

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