Geographical & Temporal Distributions of Thunderstorms

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Instructor: Linda Fye
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. 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.

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