It would seem that having lots of moisture in the entire troposphere would be the most beneficial to severe thunderstorms. However, while lots of moisture near the surface is important, if the air higher aloft is dry it contributes to storms being stronger. Dry means that the dewpoint depression is large. For example if the temperature is 8 C with a dewpoint of -15 C then that air is dry. If the dewpoint depression is 5 or less than that air is moist. The dewpoint depression is found by taking the temperature minus the dewpoint. For example if the temperature is 20 C and the dewpoint is 17 C, then the dewpoint depression is 20 - 17 = 3. Note that dewpoint depression is not a temperature but rather a temperature difference. First I will explain how dry air higher aloft contributes to stronger convective wind gusts and larger hail at the surface. As a cumulus updraft rises it will become exposed to dry air higher aloft in a situation in which there is a high dewpoint depression aloft. If there is strong wind shear aloft then the dry air will effectively penetrate into the thunderstorm. The dry air will penetrate into air that is saturated and full of precipitation. A downdraft of precipitation will fall toward the surface. When dry air mixes into the saturated air, cooling will occur due to evaporative cooling. This makes the downdraft colder and more dense than it otherwise would be. Cold dense air will accelerate toward the surface. Since the downdraft falls at a faster rate the convective wind gusts experienced at the surface will be stronger. The evaporative cooling aloft keeps the downdraft colder than it otherwise would be. This makes it more likely hail will reach the surface and more likely the hail will be bigger once it reaches the surface. Hail has less time to melt when the freezing level is closer to the surface and the downdraft is colder. Next I will explain how dry air aloft leads to higher instability. When instability is higher the storm updraft is stronger and this contributes to a stronger downdraft and the generation of larger hail. The term instability is a situation in which the temperature cools with height at a rate that can support a storm. An unstable situation is to have very warm moist air under very cold air. Since very warm air is less dense than cold air, the warm air can rise freely. Think of steam rising from a pot of boiling water. The steam rises to the ceiling since it is warmer than the surrounding air. For the troposphere to be unstable there must also be adequate moisture near the surface. If the air is both very dry at the surface and higher aloft then there will be no instability generated. When the air is warm and moist at the surface and the air is very dry higher aloft this is a situation that is termed convective instability. A convectively unstable environment becomes more unstable when all the air within the troposphere is lifted (such as a low pressure system causing the air within the troposphere to rise). Air that is dry will cool more rapidly than air that is saturated. Thus, when lifting occurs the dry air higher aloft will become much colder as it is lifted. Instability increases because the air at the surface is still warm and moist while the dry air higher aloft becomes much colder. Once a storm occurs the updraft will be stronger when the surrounding air rapidly decreases in temperature with height. |