Looking up in the sky and imagining shapes or animals with cloud formations has been with us ever since we could walk. Some of these objects actually hold fluid dynamic science behind their shape. Ever look up and notice a strange cloud that looks very similar to a wave that could be seen on a lake or the ocean? This cloud formation is actually a rare type of cloud that holds some of the more scientifically advanced atmospheric dynamics within it.
Named after William Thomson Kelvin and Hermann von Helmholtz, Kelvin-Helmholtz instability can occur when there is velocity shear in a continuous fluid. This instability can occur when there is a velocity difference across the interface between two fluids. To think about this in a visible sense, think about wind blowing over water. The instability that the wind creates manifests in waves that are produced on the surface of the water. Helmholtz studied the dynamics of two fluids of different densities when a small disturbance, like a wave, was introduced at the boundary of the connecting fluids. Helmholtz noticed how the formation of these wave-like structures were almost always because of different velocities interacting with one another. This can happen with any fluid, most notably, with clouds. When these Kelvin-Helmholtz waves are spotted they tend to look like ocean waves traversing across the sky.
To explain how Kelvin-Helmholtz clouds form one will have to think about shear. When two different layers of air in the atmosphere are moving at different speeds, this is called shear. When the upper layer of air is moving at a higher speed than the low-level air, the air tends to scoop the top of an existing cloud layer into wave-like rolling clouds. Due to the amount of shear and difference between air levels, windy days are often the best days to spot these clouds, and can also indicate aircraft turbulence due to the shear associated with the formation of these clouds.
If one of these clouds ever traverses across the sky above, just know that atmospheric and fluid dynamics is at work with ample amount of shear to create such a cool looking cloud.
For more weather education, click here!
©2019 Weather Forecaster Alec Kownacki
As the seasons change, you may often hear of the terms “astronomical Spring,” or “meteorological Spring” being used around this time of year. You may then be wondering what the difference between the two are, and which is the correct term to use? The answer is that they’re both correct, but when discussing the different seasons, it’s important to note which definition you’re using and what side of the equator you’re on. Simply, the astronomical seasons are defined by the position of the Earth compared to the sun, and the meteorological seasons are based on the annual cycle of temperatures.
The reason why we have seasons is because the Earth is tilted approximately 23.4 degrees on its axis. As the Earth orbits around the sun, different parts of the Earth receive varying amounts of solar energy at a time. This natural rotation of the Earth around the sun along with its tilt sets the foundation for the astronomical calendar in which we group the seasons into solstices and equinoxes. As you can see in the visual representation of Earth’s seasons below, the tilt of the Earth and alignment of the sun play a key role in defining our seasons.
The seasons are defined by most cultures as Spring, Summer, Fall (Autumn), and Winter. The astronomical definition uses the dates of solstices and equinoxes to define the start and end of the seasons. Spring and Fall (Autumn) begin on the Spring (vernal) and fall equinoxes, and Summer and Winter begin on the Summer and Winter solstices.
The Summer and Winter Solstices mark the longest and shortest days of the year where the Earth is tilted the most towards the Sun during the Summer solstice, and furthest away from the sun during the Winter Solstice. In the Northern hemisphere the summer solstice occurs around June 21st, the winter solstice around December 22nd, the Spring (Vernal) equinox around March 21st, and the Fall (Autumnal) equinox around September 22nd. In the Southern hemisphere the seasons are reversed but begin on the same dates as shown in the image above. However, the timings of the equinoxes and solstices vary each year, which changes the length of the astronomical seasons.
The Earth technically makes a complete orbit around the sun in 365.24 days, so every four years an extra day is needed to balance this out. The Leap Year was then created to accommodate the extra time it takes Earth to fully orbit around the sun. This in turn makes comparing climatological data for each season difficult from one year to the next. Therefore, the meteorological seasons were created.
Meteorological seasons are separated by Meteorologists and Climatologists into four groups of three months based on the annual temperature cycle. It’s based on the observations that summer is the warmest time of the year, winter is the coldest time of the year, and Spring and Fall are the transition seasons. With that being said, the seasons begin on the first day of each month. In the Northern Hemisphere:
Meteorological Spring starts March 1st and ends May 31st
Meteorological Summer starts June 1st and ends August 31st
Meteorological Fall (Autumn) starts September 1st and ends November 30th
Meteorological Winter starts December 1st and ends February 28th (29th during a leap year)
Below is a full chart showing how the seasons are defined by hemisphere, and which definition is being used:
This is why you often hear your local meteorologist inform you about the first day of the meteorological season at the start of each month that contains an astronomical season.
Defining seasons by the meteorological calendar helps long term forecasting and climatic data easier to calculate seasonal statistics in order to better understand and convey average temperatures to the public. This calendar is also useful for other important purposes such as agriculture and commerce.
Different countries around the world define seasons using multiple definitions and factors as well. For example, Australia and New Zealand rely on the meteorological definition to define their seasons, Ireland uses an ancient Celtic calendar, some cultures in South Asia have six seasons, and in Finland and Sweden seasons are based solely on temperatures above and below freezing, so their seasons begin on different days each year depending on their climate.
In summary, the reason for having two (or multiple) ways to express the start and end of the seasons are because of their different start times and durations and based on the climate and culture of an area.
To learn more about weather education topics, click here!
@ 2019 Weather Forecaster Christine Gregory
The doldrums and the horse latitudes are two interesting terms used by meteorologists and sailors to describe two distinct areas over the ocean. Although they share the calm wind characteristic, they are located on two different degrees of latitude and display different characteristics of climate and weather.
Calm winds, warm temperatures and very convective thunderstorms are used to describe the doldrums. Located at zero degrees latitude, also known as the equator, you will find this described area over the ocean. Here, there is little change in temperature and pressure in the horizontal direction but vertically is a different story. The warm air at the surface rises and condenses into large towering cumulonimbus clouds that then turn into highly convective thunderstorms. This is an area where the northeast trade winds in the northern hemisphere and the southeast trade winds in the southern hemisphere converge. This converging air at the surface collides and then lifts all the warm, moist air on the surface. That air then condenses and forms a line of strong convection and energy called the Inter Tropical convergence zone. There is so much energy and moisture here despite its relative calmness. Boats travelling in this area may experience bouts of calm weather and then quite the opposite when they are met with loud thunder and lightning.
Moving on up the ladder to an area of even calmer settings; located at 30 degrees north, you will find an area over the ocean that is famously known as the horse latitudes. This area, like that of the doldrums, is characterized by calm wind and small changes in horizontal temperature and pressure. The difference between this area and the doldrums, is that the westerlies and northeast trade winds diverge. This means the winds flow away from this area rather than converge and create thunderstorms. Storms are very scarce here as compared to the doldrums. It’s very calm. For example, if you were sailing to the new world back in the 1400s, you would find that your boat would come to a complete halt. With no winds to push the sails, sailors would be stranded for a long time in this area as they calmly drifted. What made this area take on its famous name was while sailors were stranded, they would run low on supplies and end up having to eat their horses or throw them overboard to make the ship lighter.
The doldrums and horse latitudes are a interesting couple of places to find yourself sailing overseas. Thankfully, today we have advanced in our shipping and travel. Many boats have engines that can tug them along in the horse latitudes and a lot of coverage from heavy rain and storms in the doldrums. Regardless of advances in shipping, these areas are significant when travelling overseas; an experience sailors will never forget.
For more fascinating meteorology terms, click here!
© 2019 Meteorologist Alex Maynard
Katabatic wind is a dense cold wind that sinks down from high mountain plateaus to valleys below. It is described by Meteorologists as heavy cold air that practically mimics the movement of mountain water as it flows downslope and waterfalls over steep canyons. Depending on the temperature of the air it displaces during its descent, katabatic wind can become violent, reaching speeds up to 100 mph.
Up on a high plateau of a mountain range, snow tends to be prominent during the winter months and into the spring. This snowpack keeps the air above it cold and dry. Air in the valley tends to be much warmer than the air on the plateau. To understand what makes this air start to move downslope, it is important to know the laws of temperature and pressure. Cold dense air is associated with higher pressure and warmer, less dense air is associated with lower pressure. Air tends to rise when it is warm and sink when it’s cool. Areas of higher pressure will move toward areas of lower pressure to try and replace the air that is lifted. The difference between areas of high pressure and low pressure is called a pressure gradient. The quicker the pressure changes, the stronger the pressure gradient. Therefore, the faster the air from the area of high pressure will move towards the area of low pressure.
The cold, dry air above the surface of the mountain plateau forms a small area of high pressure. As the air in the valley gets warmer, it starts to rise. This causes that high pressure over the snow to move toward that warmer, lifting air. This cold, dense air will eventually move off the plateau. With gravity taking hold, it will start falling downslope, picking up speed as it falls into warmer and warmer air. There is nothing to hold it back as it slips down slopes and plummets over cliffs. Reaching speeds up to 100 mph, this cold air becomes a violent wind that can topple trees and damage crops. Although it can reach such speeds, katabatic wind isn’t always as violent of a wind. It can rage anywhere between 10 mph to 100 mph depending the pressure gradient from the top of the plateau to the bottom of the valley. The stronger the pressure gradient the quicker the air will sink and pick up speed.
Katabatic wind is observed in many parts of the world with high mountain ranges and plateaus. You are most likely to experience katabatic wind in places northeast of the Adriatic sea that reside at the base of the Carpathian mountains, the Rhone Valley that sits at the base of the western mountain range in France, in Columbia at the base of the Cascade mountains, at Yosemite National Park in California and in places like Greenland and Antarctica. All these places where katabatic wind is observed, have other interesting names for the phenomenon. Areas south of the Carpathian mountains call it “Bora wind”, in France they call it “Mistral wind” and in Columbia they call it “Columbia Gorge wind”. Some areas see katabatic wind much stronger than others depending on the climate of the valley versus the elevational climates of the plateau it sits at the foot of.
Although it may become a violent wind, in most cases some natives of areas that see katabatic wind describe their experience as a cold wind sweeping through that forces them to put on a jacket. In another instance, trees toppled over at a campground in Yosemite National Park injuring a park employee that was sleeping in a tent. Wind can be both enjoyable and dangerous depending on the circumstances. It is certainly interesting to learn that there are many different types of wind that have fascinating terminology associated with them.
For more fascinating meteorology terms click here.
© 2019 Meteorologist Alexandria Maynard
Figure: Graphical explanation of land and lake breezes. (Courtesy of Vaughn Weather)
The Great Lakes in the Midwestern United States go through many phases based on the various season that are experienced. In this part of the country, the calendar tells what season it is, but precipitable phenomenon (i.e. snow) can definitely occur outside of winter months. We will steer our focus on the effects the lakes have on surrounding areas in the early spring. After the lakes have spent the duration of the winter experiencing cold air temperatures and partial freezing, and we progress into spring where air temperatures warm up, the lakes are bound to have an effect on the surrounding land.
The topic of discussion is somewhat of a slang term that came out of the Milwaukee National Weather Service office in the 1960s. The term Pneumonia Front refers to a sharp drop of air temperature on land of 16 degrees Fahrenheit in the period of an hour in the form of a cold front. Often times, the drop of temperature is generalized to 20-30 degree changes over the course of a few hours. This cold front moves inland in the form of a lake breeze and modifies the air around it, thus creating much cooler temperatures inland.
A recent example occurred on March 24, 2017 where a day of strong warm air advection with high winds from the southwest led to temperatures rising to a very warm high temperature for March of 81 degrees. The average high temperature for the O’Hare International Airport station on this date is 50 degrees, according to wunderground.com. Right around noon, local time in Chicago, the winds turned to northeast and temperatures began to drop. Due to the significantly cold lake paired with the northeast orientation of the winds flowing over the lake, the effects of significant cooling were felt inland. Below is the 7-hour period of data that shows a quick temperature drop which drastically changed conditions in a short period.
Figure: Historical Data from wunderground.com of observations on March 24, 2017 at O’Hare International Airport in Chicago, IL showing changes over a 7-hour period.
Lake breezes are not uncommon, but definitely strong during the springtime period. Lake breezes will form due to significant temperature differentials between the land and lake, where air can be easily modified and brought inland. Lake breezes can form and noticeably cool an environment inland in the early spring, but progressing closer towards summer, both the air temperature and the lake will warm up, and the effect of a lake breeze will not be as significant. As summer begins and warmer temperatures are more frequent, a lake breeze can bring decent relief in temperatures during afternoon and evening hours, but is likely not to change an environmental temperature by 30 degrees.
The effect of air modification due to a breeze from a body of water are also possible in other circumstances. For instance, Florida feels sea breeze that can modify weather conditions that usually lead to thunderstorms because of the great convergence over land due to two surrounding bodies of water being the Atlantic Ocean and Gulf of Mexico. Any sizable body of water near land can modify air around it. There have even been reports of breezes from rivers creating temperature fluctuations!
To learn more about other weather education topics, be sure to click here!
© 2019 Meteorologist Jason Maska
DISCUSSION: As many parts of North America and Europe begin to prepare and brace in earnest for the onset of the peak severe weather season during a typical calendar year, there a number of premiere issues which are worth understanding. That is, so everyone can have a better idea of how the atmosphere works and why various atmospheric phenomena evolve the way they do. One such example is understanding the different types of wind shear which can occur and their respective relevance.
To get started, wind shear is a naturally occurring aspect of the atmosphere which affects both atmospheric phenomena such as various types of convection as well as all aviation-related concerns. More specifically, wind shear comes in two primary forms which includes speed shear and directional shear. Speed shear is best characterized as any situation in which the wind shear changes in value from two given elevation points. Thus, when speed shear is present, this can often lead to situations in which there are convective storms which do not last for an overly long period as a result of core updrafts being tilted too much. Therefore, with too much speed shear, various thunderstorm forecasts can often end up being a bust in terms of forecast verification.
The other primary type of wind shear (as noted above) is known as directional shear. Directional shear is best characterized as any situation in which the direction of winds at two given points of elevation are different from one another. Often, when there happens to be rather large amounts of directional shear, there are frequent impacts to aviation safety in terms of crosswinds at any point during any flight (regardless of aircraft size) as well as to convective storm development. Impacts to convective storm development are quite often realized by way of there being a greater potential for the development of convective storms with rotating updrafts. Therefore, with greater amounts of directional shear, there can often be a greater threat for supercell thunderstorm assuming all other convective environmental conditions are conducive for supercell thunderstorm development.
Thus, this goes to show that whenever there is a substantial presence of speed and/or directional wind shear in the atmosphere, there a host of different considerations which always need to be made when it comes to both aviation and forecasting interests.
To learn more about other weather education topics, be sure to click here!
© 2019 Meteorologist Jordan Rabinowitz
Chances are, you’ve likely seen many rainbows in your life, whether it’s the classic bow shape of colors painted across the sky or just a quick glimpse of rainbow colors peaking through the spray of a sprinkler on a hot summer day. When white light from the sun hits a water droplet, some of the light is reflected while some of it is refracted, or bent, out of the droplet. The dispersion of the refracted light at varying angles can cause the appearance of the rainbow. While rainbows are something that we see during the day, a similar phenomenon occurs at night.
Moonbows, also known as lunar rainbows, are formed in a similar manner to your typical rainbows, but with a different light source. When light that is reflected off of the moon’s surface hits moisture in the air, the white light can be refracted into the spectrum of colors that we typically see in a rainbow. However, since the light reflected from the moon is much dimmer than the light from the sun, moonbows will appear much fainter than their daytime counterparts. If you happen upon a moonbow, you may not be able to see the full spectrum of colors and instead may just see a faint white light. This is because the dim light is usually not enough to excite the cone color receptors in our eyes. However, long-exposure photography can allow you to see the rainbow colors of a moonbow more clearly.
To learn more about various weather education topics, click here!
©2018 Meteorologist Stephanie Edwards
I’ve always wondered what the difference was between each of these things that fell from the sky. After learning about them, there are so many differences between them! Do you know the difference between Graupel, Hail, or Sleet?
Graupel is a snow particle (or snow pellet) that is irregularly-shaped. Graupel is formed when the snow pelts collect snow crystals, rian, or partially melted snow stick, like glue, to the snow pellet – which adds an icy layer on the outside. Graupel is usually no bigger than 5mm wide, and typically resembles a “chunkier” snowflake. Below is a picture of graupel.
Hail forms in convective processes, like severe thunderstorms or cumuliform clouds. It can either be white or more of an opaque color and is considered to be bigger than 5mm in diameter (which is already bigger than any types of graupel). In order to be considered hail, it must form within a convective process which allows the growth of the ice. A good example of where hail growth occurs is within thunderstorm clouds. Within the upper region of the cloud, water droplets freeze together. When they start to fall from the cloud, the hailstones get pushed back up by the updraft of the storm. When the hailstone is pushed back up into the cloud, it comes in contact with more water droplets, which adds another layer to the hailstone. Eventually, when the hailstone becomes too heavy for the cloud to hold, it falls to the ground. Below is a picture of hailstones.
Sleet is different from hail because its translucent and is usually a ball less than 5mm in diameter. Sleet is formed by the refreezing of liquid raindrops or the partial melting of a snowflake. The best example of this is in the winter. When the precipitation from a storm falls as a snowflake, it will enter a warm pocket and completely melt until it hits a colder pocket of air near the surface and freezes as a ball of ice. When it hits the ground it will be a clear ball of ice, which is considered sleet. The main difference between sleet and graupel is that sleet is the re-freezing of liquid raindrops or partial melting of a snowflake, while graupel doesn’t re-freeze, but rather collects melted snow. Below is a photo of sleet.
Now that you know the difference, I hope you’ll be able to recognize these when they fall from the sky.
To learn more about weather education and other related topics, click here!
Credits: National Geographic, National Weather Service
@2019 Weather Forecaster Allison Finch
Discussion: There are three stages of a thunderstorm: the cumulus stage, the mature cumulus stage, and the dissipating stage. Each stage are defined by certain characteristics and are outlined below.
Air that is warmer than its environment will start to rise by convection during this stage. As this warm, moist air rises, it will cool and condense, thus forming a cumulus cloud. The updraft is very strong at this point so as the small raindrops try to fall, they get suspended and pushed up even further in the cloud. The raindrops begin to form larger raindrops as they collide with one another. The cloud continues to grow vertically and eventually reaches a height above the freezing level. At this level, supercooled water molecules exist, meaning that there is liquid water that is below the freezing point of liquid water. This allow for the water droplets to grow very large and fast. When they become large enough, they fall from the cloud and start the initiation of a cool downdraft. This leads to the second stage.
2) Mature Stage
This stage is characterized by the presence of both updrafts and downdrafts. As the air descends to the ground, evaporative cooling takes place. This is because the air below the cloud is still relatively dry which allows for the water molecules to evaporate. This cools the atmosphere. Sometimes this process can cool the atmosphere by as much as 20 degrees in 30 minutes. Evaporative cooling acts to further strengthen the downdraft. When the downdraft hits the ground, it has nowhere to go so it spreads out in all directions. This can sometimes cause minor damage to trees and houses. The cloud will continue to grow vertically as there is still a very strong updraft. The strong updraft will continue to push some of the water molecules way up into the higher parts of the cloud. This is usually when the anvil shaped cloud becomes visible. Hail is also formed at this point and intense cloud-to-ground lightning can be seen. As long as the updraft remains strong, hail will continue to grow in size. Once the hail is too large to be supported by the updraft, it falls to the ground. The downdraft will eventually cut off the updraft which is supplying the storm with the warm and moist air. When this occurs, the final stage commences.
3) Dissipating Stage
At this stage, the downdraft is prominent. The downdraft continues to cut off the warm and moist air that is needed for the storm to maintain itself. The anvil that is seen during the mature stage starts to flatten out as the storm continues to weaken and the latent heat, which is the energy that is released when the water vapor transitions to liquid droplets, process has been cut off. The once towering cumulonimbus cloud then turns into wispy, non-threatening clouds.
This whole process takes about an hour but does vary with each storm. In some severe thunderstorms, the process can take several hours since the updraft is much stronger and can maintain the storm for a longer period of time. If the updraft is slanted, meaning that the thunderstorm is tilted as the height increases, as in the case for a severe thunderstorm, then the rain-cooled air cannot cut off the warm, moist air that is being brought up into the storm. This can cause the storm to maintain its strength for even longer.
Credit: North Carolina Climate Office
To learn more about weather education and other related topics, be sure to click www.globalweatherclimatecenter.com/weather-education
@2019 Meteorologist Corey Clay
Astronomical Spring is here for those in the Northern hemisphere, and more “spring-like” weather is approaching. With the present Spring season kicking in comes an increase in the amount of solar radiation. This is because the Earth’s Northern hemisphere starts tilting more towards the sun on its axis bringing with it longer and warmer days. Since the land is getting warmer than the surrounding bodies of water, the differences in temperature can create localized pressure differences between them. Once this happens a phenomenon called a sea breeze can occur during the day, and a land breeze can occur during the evening. But what exactly is a sea and land breeze?
Sea and land breezes are thermally forced circulations that occur from Spring to Autumn driven by differential surface heating. This means that the temperature differences between the land and water create areas of low and high pressure, and since high pressure always flows toward lower pressure a circulation forms to create the breezy conditions. Ideally the temperature difference between the land and water should be around 6-10 degrees Fahrenheit. The land is able to heat up much faster than water during the day while the water is able to hold in its heat better than the land during the night due to specific heat differences between them.
Specific heat is defined as the amount of energy it takes to raise the temperature of 1 gram of water 1 degree Celsius. More simply, it takes water much more time to heat up than it does for the Earth’s surface. The specific heat of water is 1 calorie/gram degrees Celsius, or 4.186 Joules/gram degrees Celsius. This amount is roughly four times higher than the specific heat of land, which gives water this important characteristic and sets the stage for this thermally direct phenomenon.
Below is a simplified graphic showing the difference in pressures that can occur during the day and night:
Notice the “cold front” indicated in the image above. This merely represents the cooler breeze flowing from higher to lower pressure. The sea breeze is shown on the left while the land breeze is shown on the right.
This phenomenon does not only occur to oceans, but lake breezes also occur over smaller bodies of water such as the Great Lakes in the Northern part of the U.S. for example. However, these breezes can occur along nearly any coastal boundary around the world; a common location being the Florida peninsula. One of the main triggers of daily thunderstorm activity in coastal areas like this result from sea breezes that converge directly over the land. Having large temperature gradients such as these are often the main trigger to any convective development that can result from this mesoscale phenomenon.
Forecasting for these events requires knowledge of the local synoptic and mesoscale weather forcings that can initiate this, the orography of the area, and the general shape of the coastlines. Are there calm, synoptic conditions such as an area of high pressure that won’t disrupt the flow? Are there concave or convex qualities to the shape of the land that could enhance or diminish the circulation? Is there a significant temperature difference between the water and the land? These questions can be helpful in deciding whether or not this type of phenomenon could occur.
Sea and land breezes can have a large impact on local temperatures, making coastal areas or cities difficult to forecast for as these events are challenging to predict. During the day as the land heats up and a sea breeze is initiated, the temperature can be kept cool or significantly cooled as a result. The same goes for a land breeze. Once the sun goes down, the land cools off and the water is relatively warmer than the land initiating a land breeze. This can keep the air at a steadier temperature during the night and prevent the lake from warming the air onshore.
To learn more about weather education and other related topics, click here!
@2019 Weather Forecaster Christine Gregory