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Difference between Winter and Summer Storms (Credits: NWS, AMS, Weather.gov)

2/27/2019

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Major storms can occur in every season, but there are significant differences between winter and summer storm systems.

During the summer, hurricanes are one of the primary storm systems that meteorologists pay close attention to Hurricanes, or tropical cyclones are localized areas of intense low pressure wind systems, which form over tropical oceans. The center of a hurricanes is associated with warm cores, which mean the center of the storm (where the low pressure is) is warmer than the outside of the storm. This well-defined center makes it so there is a closed wind circulation around the storm. The tropical storms that hit the eastern part of the United States are usually formed off convective systems that move off the coast of Africa. These systems move across the Atlantic Ocean and through latent and sensible heating, they increase in intensity. This process is part of WISHE (Wind Induced Surface Heat Exchange) which helps enhance the storm. If and when a tropical cyclone makes landfall, these systems can bring along heavy rainfall, flooding, and damaging winds.

On the other hand, during the winter, snowstorms are one of the major storm systems that meteorologist will pay attention to. Winter storms have a completely different make up then a hurricane does. As previously mentioned, hurricanes and tropical cyclones are symmetric, have a well-defined center, and move over tropical places. Winter storms are known as mid latitude cyclones or extra-tropical cyclones. An extra-tropical cyclone is a low pressure system associated with cold or warm fronts. A weather front is the boundary which separates two different air masses of different density and temperature. Snowstorms are a form of an extra-tropical cyclone associated with a cold front, which brings along colder temperatures, and stronger winds. This will lower the pressure within the storm, making it more intense. Storms like these develop best where the jet stream is prevalent because of the difference in temperatures in these areas. The northern half of the United States is a prime location for these storms because of how prevalent the jet stream is in these areas. Lastly, one of the biggest difference between a snowstorm and hurricane is that they bring snow, ice, and frigid temperatures to the areas they affect.

Even though both of these storms can create lots of damage and can be life threatening, it is important to know the difference between the two.

To learn more about other severe weather topics from around the world, click here!

©2019 Weather Forecaster Allison Finch

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What is the Dryline and How Does it Contribute to Severe Weather? (Credit: University of Wisconsin-Madison)

2/26/2019

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DISCUSSION: As we start to head into the spring months, a common meteorological boundary will be frequently mentioned. A boundary can be one of the most important ingredients when forecasting severe weather. One of these boundaries is known as the dryline. The dryline is a boundary that separates moisture. It’s important to note that it is not a front since a front is a boundary which has a strong density gradient such as the difference in temperatures across the boundary. It is essentially a boundary which has moist air to the east and dry air to the west and is a very common feature. The dryline can occur on a nightly basis during the summertime in the High Plains, although it is mostly referenced during springtime. The dryline is more notable when low pressure systems move into the plains. These systems can cause a contrast between moist and dry air. Dry air comes from the Rocky Mountains, while humid air is brought up from the Gulf of Mexico, thus a sharp gradient. The southerly winds help to supply moisture from the Gulf of Mexico ahead of the dryline. In the diagram below, the dryline is depicted as the scalloped line running down from eastern Oklahoma through eastern Texas. Out ahead of this line in Texas the dewpoints are in the mid 50s to around 60 degrees Fahrenheit. Behind the line, dewpoint temperatures are in 30s and 40s. It is not uncommon to have dewpoints in the 70s ahead of the dryline and dewpoints in the 20s behind it. Along this line is where the strongest storms would be expected to form. This is because of the convergence of the differing moisture boundaries and wind shift. Usually, there is also a southeast flow along and ahead of the boundary while there is a westward flow behind it. This further contributes to the convergence.

Image Courtesy: University of Wisconsin-Madison

As the strong winds come into contact with each other, it has nowhere to go so it is forced upward. The stronger the convergence, the greater the updraft, and hence the stronger the storms which develop. The strongest convergence occurs when the winds are more direct to one another, such as a westerly wind and an easterly wind.

Image Courtesy: University of Illinois

The dryline is forced to the east during the daytime hours as the shallow layer of moist air near the boundary gets mixed out by the daytime heating.The turbulent mixing of the dry air above and the moist air below reduces the dewpoint temperature which helps to mix the dryline eastward. During the evening hours the dryline retreats back to the west, but the storms which formed are able to continue eastward as momentum carries them.This propagation of the dryline back to the west occurs because the diurnal heating has been lost, thus turbulent mixing also ends. Radiative cooling is more effective in dry air which enable the dryline to retreat back to the west as the temperature is cooled back to its dewpoint temperature. The animation below clearly depicts where the dryline is. The yellow and orange colors indicate dry air while the blues and greens indicate moist air.

Image Courtesy: University of Wisconsin-Madison

A large majority of violent tornadoes in the Midwest are due to the dryline. An example is the El Reno tornado of May 31st, 2013 which resulted in eight deaths. With spring right around the corner, we can expect the dryline to become more active once again.

To learn more about other severe weather topics from around the world, click here!

©2019 Meteorologist Corey Clay

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The Process of Hail Formation (Credit: World Atlas/ Weather Underground/ The Weather Channel)

2/9/2019

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Hailstones measured after a thunderstorm in Zurich, Switzerland.
Credit: Arnd Weigmann/ Reuters (Newsweek)

Hail is a type of precipitation that forms from water droplets within cumulonimbus clouds. During the updraft of a thunderstorm, water droplets become supercooled as they rise up into temperatures below freezing. At such cold temperatures, these water droplets condense onto condensation nuclei, such as dust particles within the cloud, and then freeze onto this condensation nucleus. The small ice particle that is formed during this condensing process within the cloud then begins the growth process before precipitating from the cloud as hail.

Credit: Daphne Johnson (WeatherOps)

One way that hailstones can grow is throughout a process known as wet growth. During wet growth, supercooled water droplets continue to collide with the ice particle formed during the initial condensing onto the condensation nucleus. During the collision, the water from the water droplets spreads out surrounding the ice particle, causing the ice particle to freeze slowly. This type of growth creates ice particles that are relatively clear, as the supercooled droplets slowly freeze onto the particle. Hailstones can also grow throughout a process known as dry growth, in which the supercooled water droplets collide with the ice particle and freeze immediately on contact. Hailstones formed during dry growth formation tend to have air bubbles from the supercooled droplets trapped and frozen into the ice particle. These air bubbles can occur because the freezing process occurs so quickly. This process can create hailstones that appear cloudy as well as have irregular shapes.

Large hailstones measured after a severe thunderstorm in South Dakota.
Credit: NOAA

Hailstones remain within the below freezing area of the cloud until they collide with more and more supercooled water droplets, and thus grow greatly in size. When a hailstone becomes too large or too heavy to be supported by the updraft of the thunderstorm, the hailstone falls from the cloud due to gravity during the downdraft. Hailstones vary from an average size of approximately 5 mm to 15 mm, but records show that they can grow much larger. The hailstone that holds the record for being the largest hailstone in the United States is a hailstone from South Dakota that is over 8 inches in diameter. Hailstones that grow to these sizes can cause severe damage to cars, trains, livestock, crops, and airplanes. Cloud seeding is a process that is currently aiming in part to prevent hail from occurring in certain areas by shooting chemicals, such as silver iodide, into the sky. These chemicals act as the condensation nuclei for precipitation formation within the cloud, and thus can alter the type of precipitation that falls from the cloud. Hailstones propose high hazards to people located wherever they may fall, especially due to the fact that they are able to grow to such destructive sizes.

To learn more about hail and other severe weather phenomena, be sure to click here!

©2019 Weather Forecaster Christina Talamo

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Severe Weather Topics

Difference between Winter and Summer Storms (Credits: NWS, AMS, Weather.gov)

What is the Dryline and How Does it Contribute to Severe Weather? (Credit: University of Wisconsin-Madison)

The Process of Hail Formation (Credit: World Atlas/ Weather Underground/ The Weather Channel)

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