Thunderstorms Part 2

Welcome back!

I will be continuing this week talking about severe weather and specifically supercell thunderstorms!

Supercell thunderstorms are individual cells with rotation. These storms form in strong vertical wind shear environments and the strong wind shear causes the thunderstorm to rotate (potentially leading to the formation of a tornado). 

This image below shows some of the features of a supercell thunderstorm:

Image from: Essentials of Meteorology: An Invitation to the Atmosphere.

Supercells are composed of many parts. The mesocyclone is a vertically rotating column of air that is typically 5-10 km across. The mesocyclone causes the updraft to rotate. 

The wall cloud is a rotating column of clouds under the base of a thunderstorm. This is where a funnel cloud and tornado may form. 

Image from: Heather Janssen 

Precipitation with supercells from south to north goes from hail, to heavy rain, and then light rain. 

Like multi-cell thunderstorms, supercells have the gust front, mammatus clouds, an overshooting top, and anvils. 

Supercell thunderstorms typically form in the late afternoon. What causes this? Well during the morning hours, storms are unable to grow vertically due to stable air. In the afternoon a conditionally unstable atmosphere becomes presents which can cause large growth in the vertical. This vertical growth will help with the formation of thunderstorms that eventually may become a supercell depending on the conditions in the atmosphere. 

If the conditions are right, a supercell may produce a tornado. A tornado is a rapidly rotating column of air extending down from the base of a supercell thunderstorm to the ground. Tornadoes can be of different shapes and sizes. The beginnings of a tornado is a funnel cloud in which the circulation has not reached the ground yet. Once the circulation had reached the ground, it can be classified as a tornado.

Image from: Essentials of Meteorology: An Invitation to the Atmosphere.

Tornadoes form most often in the presence of a supercell thunderstorm. Warm, humid air goes through the updraft of the storm while strong winds push heavy precipitation to the northeast portion of the storm. These winds help to produce a strong downdraft in the supercell.

The wind shear generates horizontal spin of air. The strong updraft in the thunderstorm can tilt the air vertically, creating the mesocyclone. Precipitation causes the creation of the forward flank downdraft and rear-flank downdraft. The rear-flank downdraft cuts off the warm, moist air causing the updraft to shrink horizontally and stretch vertically. This process forms the wall cloud where a funnel cloud develops and potentially a tornado if the circulation reaches the ground. 

Image from: Essentials of Meteorology: An Invitation to the Atmosphere.

This image shows these processes in a classic tornadic supercell thunderstorm: 

Image from: Essentials of Meteorology: An Invitation to the Atmosphere.

I hope you enjoyed this week’s posting! Make sure to comment down below with any weather-related you want answered by me. 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's the Weather, Heather? by Heather Janssen is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.

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.

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

Let's Talk Air and Temperature!

Hello and welcome back! This week we are going to be looking at answering this question:

“I thought heat rose, so why’s it often colder up on the hill where I live rather than down in the valley below?”

Yes that notion is true that heat does rise but it still does not help to answer the question. 

To get a better idea of how air and temperature work, let’s look at the vertical structure of the atmosphere!

The atmosphere contains billions of air molecules. How dense the air is depends on the number of air molecules in a given volume while the air pressure depends open the pressure exerted by the mass of air above a given point in the atmosphere. Both air density and air pressure decrease with increasing altitude. In other words, these components decrease with height. 

Here are two images to see this concept in a different way:

Image from: Essentials of Meteorology: An Invitation to the Atmosphere

This idea works the same when it comes to temperature in the layer of atmosphere which all weather occurs.

Image from: Essentials of Meteorology: An Invitation to the Atmosphere

There are four layer of the atmosphere: the troposphere, the stratosphere, the mesosphere, and the thermosphere. In each layer, the temperature change with height. In the troposphere and mesosphere, temperatures decrease with height while temperatures in the stratosphere and thermosphere increase with height. This figure to the right shows the layers of the atmosphere and the changes in temperature (the red line).

The rate at which temperature decreases is a called a lapse rate with the average lapse rate being 3.6°F (6.5°C) per 1 km. 

So how does this all relate to the question? Well there is a change in height between a hill and a valley. This goes along with the idea that the air pressure and density on the hill will be less than in the valley because the hill is at a higher altitude. This same idea goes with the differences in temperature between the hill and the valley. Temperatures decrease with altitude (or height) therefore, making the temperature of the hill colder than in the valley. 

Another way to look at this is through talking about air parcels. As an air parcel rises, the air pressure is lower. Therefore, the force exerted on the air parcel decreases, allowing it to expand and cool. And the opposite, when an air parcel sinks, air pressure increases. Therefore, the force exerted on the air parcel is greater causing it to compress and warm. The temperature of a parcel is based on the amount of molecular collisions. When the air parcels are in a smaller space, they collide more causing a warmer temperature and when the parcel is expanded, collisions are less causing a lower temperature. 

Image from: Essentials of Meteorology: An Invitation to the Atmosphere

This image shows this process through the use of a mountain. At the bottom of the mountain, the air parcel is warm and compressed. As it goes up in altitude, the air parcel expands and cools so by the top of the mountain the parcel is cold. It shows this process with a sinking air parcel in that it becomes smaller and increases in temperature as the air parcel reaches the surface.

These various elements show why the temperature at the top of the hill is colder than the valley below. 

I hope you enjoyed this week’s posting! Make sure to comment down below with any weather-related you want answered by me. You can also ask your questions on Twitter using the hashtag #WeatherwithHeather. See you all next week!

*Special thanks to Brett Rathbun who provided me with these images for this week’s posting.

Works Cited

Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. 6th ed. Andover: Cengage Learning, 2012. Print.

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

Welcome!

Hello and welcome to my blog! I am very excited for you to be joining me on this adventure to learn more about the weather!

Image from: Heather Janssen

Before I get into what this blog is all about, I will give you a small introduction about myself. My name is Heather (as you can tell by the name of the blog) and I am a soon-to-be college graduate who enjoys learning and talking about the weather. I have been interested in the weather since I was child. I would sit and watch The Weather Channel for hours, enthralled by the weather maps on my television screen. This love of learning about the weather continues to this day and has led me to create this blog.

So you are probably asking, “What is this blog about?” “Is it going to be like other weather blogs where the person gives a forecast for an area?” Well this blog will not be giving forecasts or me blabbing about what interests me about the weather. This blog is here for YOU to learn and discover about various meteorological phenomena.

Image from: Brett Rathbun

Each week there will be a new blog post talking about a weather topic from your questions! What I talk about each week is up to you so make sure to write your question(s) in the comment section below. You can also ask your weather questions on Twitter or Facebook, using the hashtag #WeatherwithHeather. For example, if you want to know the difference between freezing rain, sleet, and hail, write that down in the comment section then come back next week to see if your question was chosen. Do not be afraid to write multiple weather questions in the comments as well! The more the merrier!

Next week will be the first post to discuss a weather topic from your questions! Make sure after you read this blog post to comment down below with your questions. Also, if you have any suggestions for what you would like to see in my blog posts such as more pictures and videos, feel free to write them in the comment section as well. I look forward to reading and answering your questions in the coming weeks!

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