Learn must be forced high into the atmosphere

Learn about the factors and mechanisms necessary for thunderstorm development. Understand where storms are likely to be found on Earth and when they are most likely to occur.

Thunderstorm Development

Did you know that at any given moment in time, there are about 2,000 thunderstorms that exist on Earth? Considering storms can be very dangerous due to strong wind, lightning, tornadoes, and hail, that many thunderstorms at one time sounds pretty scary! But where are all these storms at? There are actually certain areas of the planet where you’ll find lots of storms and other areas where there are hardly any. In this lesson, you’ll learn about where and when all these thunderstorms occur.A thunderstorm is a localized storm that has lightning and thunder and is short-lived. Some factors that are necessary for thunderstorm development are vertical air movement, humidity, and instability. Air that has a lot of moisture in it must be forced high into the atmosphere for formation to begin and this only happens when the weather is unstable.

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Several things can cause the necessary lifting of air that forms a storm. This lesson will look at the reasons for thunderstorms.

Thunderstorms Due to Convection

One way that thunderstorms are created is through convection, or the vertical movement of air due to density differences.

Warm air is less dense than cold air because, as it absorbs heat, the molecules expand, making it less dense or lighter than cold air. Think of a hot air balloon. As soon as the air inside the balloon is heated up more than the air around it, it starts floating upward. Convection can happen simply from the air near the ground being warmed up by the sun on a hot day.Convection plays a very important role in several places around the world that have warm climates. The latitudes near the equator are heated by the sun all year long and this heating warms the surface of the planet.

The warm ground or ocean heats the air above it, causing the air to rise and creating instability. The rising air leads to cloud formation, rainfall, and thunderstorm development. These latitudes are called the intertropical convergence zone (ITCZ).

This is just an area near the equator where winds blowing from the north and south collide and rise upward to create instability and thunderstorms.Convection also causes thunderstorms and rain in some coastal areas, where it leads to a shift in wind direction. These types of storms are known as monsoons, or a seasonal reversal of winds. Certain times of the year, when heating at Earth’s surfaces is the greatest, the terrain and specific conditions of landmasses and oceans causes a persistent wind to blow from the ocean onto land. When coastal land is heated by the sun, wind blows from the ocean on to land, where thunderstorms and heavy rain occur.There are two major monsoon systems.

One is the South Asian monsoon in the Indian Ocean and the other is the East Asia monsoon in the Pacific Ocean, off the coast of China. There are also two minor monsoons. One is of off the Northern coast of Australia and the other off the West coast of Africa. Monsoons are very important because over half the world’s population live where they occur. Agriculture, meaning both food production and cash crops, depends on the rains. Without the monsoons, there could be widespread hunger problems or economic disasters.

Thunderstorms Due to Cold Fronts

But there are other things that can cause air to rise and thunderstorms to develop.

Thunderstorms are often produced when a cold front moves into an area. A cold front is the leading edge of a cold air mass. Because cold air is denser than warm air, when the two mix the cold air stays close to the ground and pushes the warm air upward, causing thunderstorms to develop.

These kinds of storms are often more severe than those caused by simple convection. We see these types of storms in places where warm and cold air mix, like in the midlatitudes at about 30 to 60 degrees north or south latitude. Cold air blows down from the Polar Regions and warm air moves up from the tropics. It is only when the two meet that a cold front forms and storms develop.

And this only happens in the midlatitudes.

Thunderstorms Due to Orographic Lifting

Lastly, orographic lifting is another reason thunderstorms develop. If air moving across a land mass runs into a mountain, it has nowhere to go but up, forcing it to rise and causing thunderstorms. Imagine a cold front in the Pacific Ocean as it starts to move eastward across the United States. It moves along, bringing cold air with it until it reaches the Rocky Mountains. The tall mountains force the air high into the atmosphere, creating strong thunderstorms that move east across the Great Plains.

That same phenomenon happens across the world, but it is a very localized occurrence. Thunderstorms form in this way in various places on Earth, given these conditions.

Polar Regions & Deserts

So far, you’ve learned where thunderstorms form frequently, but just as important is where they do not form. In the cold Polar Regions, thunderstorms rarely develop. There’s just not enough moisture available and the air is too cold to rise up and create the instability needed for storm development. Thunderstorms also occur more often over land than ocean. This is because the ground heats up much more quickly than water, so unstable rising air develops more over land.

Finally, thunderstorms rarely occur over the deserts of the world. There are areas in each continent with constant high-pressure systems that prevent air from rising, so there is no instability. Plus, there is simply not enough moisture available for storm development in an arid desert.

Temporal Distribution of Thunderstorms

It is not only important to understand where thunderstorms form, but also when they do. Most importantly, they develop during the warm season of the year, which includes spring and summer. This is because the heating of the ground that leads to convection is greatest during warm months. So, thunderstorm frequency varies depending on the hemisphere of Earth. When it is summer in the Northern hemisphere and there are a lot of storms, it is winter in the Southern hemisphere, where there are fewer storms. The opposite is true when it’s winter in the Northern hemisphere.

This basic idea also affects precisely where the storms associated with the ITCZ near the equator are. The ITCZ shifts north or south by about 4 to 9 degrees of latitude, depending on whether it is summer in the Northern or Southern hemisphere. The exception is the area around Africa, Asia and Australia.

There, the ITCZ shifts north as far as 25 degrees latitude during the Northern hemisphere summer and creates the monsoons in Asia. Likewise, the ITCZ shifts as far south as 20 degrees latitude during the Southern hemisphere summer, creating the African and Australian monsoon.

Lesson Summary

A thunderstorm is a localized storm that has lightning and thunder and is short-lived.

Several things can cause the necessary lifting of air that forms a storm. One is convection, or the vertical movement of air due to density differences that happens when the sun warms the surface of the Earth. Also, a cold front forms when a cold air mass meets a warm air mass and pushes warm air upward. Lastly, orographic lifting, or air being forced upward because of a mountain, is another reason thunderstorms develop.

Convection is important where the latitudes near the equator are heated by the sun all year long and this area is called the intertropical convergence zone (ITCZ). It also causes thunderstorms and rain in some coastal areas, where it leads to monsoons, or a seasonal reversal of winds. These happen in Southern and Eastern Asia, Northern Australia, and Western Africa and are vital to the growth of crops.

A cold front forms when cold polar air meets warm tropical air in the midlatitudes, causing the lifting of warm air. Orographic lifting is a localized occurrence of storms that happens in places like the Rocky Mountains. Thunderstorms do not happen often in the Polar Regions and deserts, because there is no moisture and rising of air, and they happen more over land than ocean. Thunderstorms happen most in the warm season of the year, which is opposite in the Northern and Southern hemispheres.

The ITCZ shifts north of the equator during the Northern hemisphere’s summer, creating monsoons in Asia, and it shifts south for the Southern hemisphere’s summer, to cause monsoons in Africa and Australia.

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