It is a pleasantly warm day, which is heating up rapidly. As you watch, a small cloud forms in the sky above you. The cloud grows bigger, seeming to expand out of nothing. Before long, the cloud is large, dark, anvil shaped, and you head for cover, recognizing it as a thundercloud.
Many of us have had this or similar experiences. But where did the thundercloud come from and what caused its formation?
It starts when the ground begins to warm. As the ground heats up, a thermal updraft is created, since warm air rises. Warm air is also able to hold more water vapor than is cold air, and often does, so the warm rising air carries with it a considerable amount of moisture.
The higher up the air goes, the colder the temperatures are that it encounters. In the first 10,000 feet, for instance, temperatures drop about 3 degrees Fahrenheit for every 1,000 feet of altitude. The flow of warm air into colder air produces an enormous amount of energy as it begins to cool.
The cooling updraft also reaches a point when it is super-saturated with moisture. The moisture, unable to be contained in the air anymore, condenses. This forms the clouds that we see. Since more moisture-laden air is rising under the cooling air mass, it can't release the condensed water as rain, so the cloud grows progressively larger.
As the cloud builds, sometimes tens of thousands of feet from top to bottom, it continues to produce energy. Friction from the water droplets rubbing past each other increases the amount of energy further. When the top of the cloud reaches the jet stream, it is contained and chopped off', which gives the top of the developing thundercloud the characteristic anvil shape.
Warm moist air is still rising at this point, so the cloud expands and the water droplets grow larger. At this time, too, the rising air is producing a low-pressure area below the cloud. Air rushes in to fill the low-pressure area, creating a huge pressure difference between the top and the bottom of the cloud. This causes the formation of down drafts from the top and leading edge of the cloud. The down drafts can cause wind sheering and are a particular hazard around airports.
The condensed water now becomes too heavy for the clouds to hold it, and it begins to fall as it continues to cool. The raindrops are still buffered by rising warm air however, which halts the fall and lifts the drops back up where they cool even more. In a short amount of time, they reach a state of super-cooling. This is when the water temperature is well below freezing.
When the super-cooled moisture contacts a dust speck, it freezes instantly. Then, as other cold moisture contacts the pellet of ice, it also freezes. This is the formation of hailstones. At some point, the weight of the hailstones exceeds the ability of the updraft to keep them in the air, and they fall. The size of the stones is controlled by the strength of the updrafts. Very small hailstone may melt before they reach the ground, which results in heavy but usually cold rain. If they are too large, they strike the ground as hailstones and can sometimes be of considerable size.
By this time, the cloud contains a fantastic amount of energy, and when the hailstones finally begin to fall, since more energy is produced from this, a point is reached where the cloud must discharge the energy, in the form of static electricity. The discharge, lightning, can be to anything that has an opposite or lesser charge. Sometimes this results in cloud-to-cloud lightning, but very commonly it results in cloud to ground lightening. (In the latter case, it actually follows a very small tendril from the ground up, so properly speaking, it is a ground to cloud lightning strike.)
Since it takes awhile for the hailstones to reach the ground, either frozen or as rain, this is why it is so common to see lightning and to hear thunder before the rain or hail actually hits the ground.
Once the moisture and energy are dissipated, the thunderstorm tears itself apart.
Start to finish, the entire process may take no more than about twenty minutes, or it can last for hours, depending on the amount of moisture and energy. Thunderstorms can also happen at anytime of the year, over water as well as over land. All that is required is that the air above is colder than the air below so the air can rise. Most thunderstorms happen in the summer and fall since that is when the conditions are most ideal.
The anatomy of thunderstorms is very interesting, even though the storms can be highly destructive, with extremely fierce winds, wind sheer, and water or hail damage. If you did as in the opening; watching the formation of the storm before seeking shelter when you identified it as a thunderstorm, you were wise to head for cover.