Friday, April 24, 2015

Thunderstorms Part 1

One viewer suggested talking about supercell thunderstorms. As a severe weather nerd myself, how could I say no to talking about this?!
Since there is so much to talk about in regards to severe weather, this week I will be focusing on two types of thunderstorms in this post: single-cell and multi-cell thunderstorms. Supercell thunderstorms are more complex so I will dedicate next week’s blog post just to talk about supercells. So let’s dig in and get learning about thunderstorms!  
The first question we should ask ourselves is “What is a thunderstorm?” Well a thunderstorm is a storm that produces thunder and lightning and can also have gusty winds, heavy rain, and hail. Thunderstorms need three major ingredients in order to form and grow: moisture, instability, and lift. You want warm and moist air in a conditionally unstable environment/atmosphere. The warmer the air parcel is to the surrounding air, the stronger the upward force (lift) which can lead to larger and stronger thunderstorms.  
A thunderstorm is classified as ‘severe’ when it contain at least one of the following criteria:
  1. Hail that is one inch in diameter or bigger
  2. Wind gusts that reach 58 mph or greater 
  3. Produces a tornado 
Thunderstorms are broken down into three main types: single-cell thunderstorms, multi-cell thunderstorms, and supercell thunderstorms (I will talk about this type next week).
Single cell or airmass thunderstorms are relatively weak and form in low wind shear environments. They are short-lived (between 30 to 60 minutes) and typically are not severe. These types of thunderstorms are typical during the warm summer months. These thunderstorms follow the typical life cycle of thunderstorms with its development, maturity, and decay stages. To read more about this life cycle click here.

Multi-cell thunderstorms contain multiple number of cells with each of them being in a different stage of development. These storms typically form in moderate to strong wind shear environments. Wind speed increases with height in multi-cell thunderstorms, causing the storm to tilt and separate the updraft from the downdraft (the region where the precipitation falls out from a thunderstorm).

Image from: Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere
Image from: Brett Rathbun
Mammatus clouds in Miles City, Montana
From this image you can see that precipitation does not fall though the updraft, causing the storms fuel supply to not be cut of, meaning that these storms can last a longer period of time. Other parts of a multi-cell thunderstorm include the gust front (the leading edge of cold air from falling precipitation and where you will see a sharp drop in temperature and the strongest winds), the shelf cloud (region of warm, moist air rising along the forward edge of the gust front), the overshooting top (layer of clouds above the anvil region into the stable atmosphere above), and the sinking air along the anvil forms an area of mammatus clouds (my favorite cloud type). 
Image from: Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere

Unlike single-cell thunderstorms, there are various types of multi-cell thunderstorms. The first type is microbursts. These are strong downbursts of air extending 4 km or less from a thunderstorm. These storms can produce damaging straight-line winds of 150 mph or greater. Microbursts are very dangerous to aircraft. 
Image from: Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere
A second type is squall line thunderstorms. Squall lines are composed of multi-cell thunderstorms that form in a line along a cold front and can extend for hundreds of miles. New cells form ahead of the older cells, causing squall lines to be able to last for several hours. 

Image from: Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere

A third type are bow echoes in whichStrong winds push out the center of the squall line into a 'bow appearance'. Another type are derechos in which straight-line winds cause damage over a long distance along the squall line’s path. Derechos cannot be identified until after the storm ends (can tell by surveyed damage).

The last type of multi-cell thunderstorms is the Mesoscale Convective Complex (MCC). A MCC is a large, organized cluster of individual thunderstorms that typically move in a circular pattern. These storms can be rather large and can extend over an area of 100,000 sq. km! MCC’s also bring hail, high winds, flooding, and tornadoes. 
Image from: Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere
This ends the first posting about thunderstorms and severe weather. Next week I will continue with this topic and focus on supercell thunderstorms. If you have any more questions related to severe weather or any weather topic, feel free to ask them in the comment section down below! You can also ask your questions on Twitter using the hashtag #WeatherwithHeather. See you all next week! 

Works Cited
Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. 6th ed. Andover: Cengage Learning, 2012. Print.
"What Constitutes a Severe Thunderstorm?" National Weather Service Birmingham, AL Weather Forecast Office. N.p., n.d. Web. 24 Apr. 2015.

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What's the Weather Heather? by Heather Janssen is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.

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