Earth Science

Why are summer rainstorms so violent?

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Rainstorm clouds forming; rain will follow shortly
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Ferocious storms can strike during the summer months

It is the middle of July, a perfectly glorious summer day, ideal for a backyard get together, when ominous, dark clouds start rolling in. Batten down the hatches! It appears that it is going to be one of those knuckle-biting severe summer rainstorms that can scare the daylights out of you.

Violent rainstorms can produce lightning that occurs as far as 10 miles away from the actual rain. Lightning kills between 75 and 100 people annually. Although thunderstorms are small (approximately 15 miles in diameter) compared to winter storms and hurricanes, do not under-estimate their wallop. A thunderstorm typically lasts for half an hour. A severe thunderstorm can result in flash floods, hail the size of grapefruit and 140 mile per hour winds. Flash floods kill approximately 146 people each year. Hail results in millions of dollars of property and crop damage.


Tornadoes are part and parcel of summer. Prior to the formation of a thunderstorm, wind direction changes and wind speed increases. The height of the wind also increases and this creates a horizontal, invisible spinning effect in the lower atmosphere. The air within the thunderstorm rises and this updraft tilts the rotating air from horizontal to vertical. The area of rotation can extend from two- to six miles in width. It is within this strong area of rotation that most tornadoes form.

Thunderstorms occur when there is warm, moist air that advances in a cold front that is moving to the east. When these thunderstorms develop, large hail, strong winds and tornadoes are likely to result. Tornadoes are often the result of strong frontal systems that have formed in the central states and are moving eastward. Sometimes tornadoes accompany hurricanes and tropical storms that reach land. The tornado will be to the right and head of the path of the storm center as it comes onshore.



Summer rainstorms tend to get violent particularly when they are paving the way for an oncoming mass of cold air. When you combine high pressure (cold areas) with low pressure (warm areas) and the jet stream this results in nasty summer storms. The higher the temperature and the greater the volume of water present, the more likely a kick-butt storm is going to develop. Put on the kids' rain gear and brace yourselves!

Jet streams also contribute. A jet stream is a swift narrow air current that forms at the boundaries that flank air masses and which possess considerably different temperatures. These currents of air travel north and south in snakelike patterns, changing their location as the planetary axis tilts each year.

A thunderstorm will develop when there are significant temperature differences in side-by-side zones, combined with large amounts of water vapor. When warm air goes up it cools and condensation occurs. When condensation occurs, heat is released, which causes the air to lift even more. If the atmosphere is unstable, this process will last long enough for cumulonimbus clouds to form. These clouds support thunder and lightning.

Water can freeze as it rises into the colder air. When the wind speed is great enough, the frozen water is lifted and then repeatedly drops, adding layers of ice. When the wind can no longer support the weight of these pellets they fall to the ground in the form of hail.

And that lightning! Yikes! Lightning is a discharge of electricity. Each strike is made up of three or four strokes. The air around and in the lightning bolt expands rapidly, which produces an increase in temperature and pressure in the air. The air expansion causes a sonic shock wave and that is what produces thunder. Lightning is a fascinating albeit dangerous phenomenon. It occurs in all thunderstorms and strikes the earth 20 million times each year. The energy from just one lightning flash could keep a 100 watt light bulb charged for a quarter of a year.


Lightning can manifest from cloud to air or cloud to ground, cloud to cloud or within a cloud. Lightning forms when positive and negatively charge areas build up and then discharge. The positive and negatives charges are separated by the rising and descending air that occurs when there is a thunderstorm. Ice particles as well as water can have an impact on where the charge is distributed. When a channel of electrically charged air nears an object on the ground, a surge of electricity from the ground moves from upward to the cloud and this is when you see the lightning strike.


Tornado Guide

NOAA: tornado index

NOAA: Brochures

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