Weather for Kids

Subject: Thunderstorms

On a warm sunny day, moisture in the air rises and cools and clouds become visible. If you could look inside a cloud you would see tiny raindrops and snowflakes bumping into each other in a maze of rising and sinking air. Eventually a cloud grows taller and taller and the number of raindrops and snowflakes increase. As they bounce off each other a small electrical charge is created eventually leading to a buildup of negative and positive charges within the cloud. When the amount of electricity becomes very strong in the cloud, a lightning bolt is discharged striking from cloud-to-cloud, or cloud-to-ground, and even cloud-to-air. At this stage, the cloud is called a thunderstorm.

Raindrops and snowflakes caught in the updrafts and downdrafts of a thunderstorm can remain suspended in a cloud for several minutes. When some of the supercooled raindrops are lifted into the colder, freezing air of a thunderstorm cloud, a layer of ice builds around the drop producing a hail stone. Hail is most often smaller than a dime, but can be golf ball-sized and in rare cases as large as grapefruits. A hail stone grows larger when strong updrafts keep it suspended in a cloud for a long duration, collecting many layers of supercooled water as the stone rises and falls. Eventually the weight of the hail stone is too great for the thunderstorm updraft to keep aloft and it falls to the ground.

Precipitation from a thunderstorm falls toward the ground, creating a downburst. The wind associated with a downburst spreads out rapidly once it reaches the ground and can encompass an area from only a few hundred yards wide or up to several miles wide. Very strong downburst winds can reach speeds in excess of 100 mph and are capable of causing significant damage to trees and property.

Heavy rain from a slow moving thunderstorm can produce an abundance of water in a short amount of time that turns normally dry washes, ditches, and even streets into fast-moving water ways. This is called flash flooding, and occurs more often in urban areas where water runoff from streets and parking lots quickly overwhelms storm drains. Flash flooding is extremely dangerous and is the number one thunderstorm-related, and weather-related killer each year.

Although rare events in Montana, thunderstorms can produce damaging tornadoes. Strong thunderstorms sometimes form a lower cloud base that rotates when updrafts are very strong. This can result in development of a funnel cloud near this part of the thunderstorm. If the funnel lowers to the ground it is then called a tornado. Tornadoes are extremely dangerous and can have wind speeds over 200 mph capable of causing major destruction.

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