Discussion: Thunderstorms are a powerful yet beautiful display of nature and can bring needed rainfall or devastating wind and hail. While they are most common in the afternoon during spring and summer, they can occur at any time of day, and in any season of the year. While thunderstorm formation is a complicated subject, meteorologists have become increasingly skillful in forecasting thunderstorm activity days in advance. When you think of a thunderstorm, you typically think of a warm and humid day. There is reasons behind this is chiefly due to three necessary ingredients for thunderstorm formation; instability, moisture, and a lifting mechanism.
The environment that thunderstorms thrive in consists of high moisture content especially closer to the surface of the Earth. It probably makes sense why this is the case. Without moisture, you cannot condense moisture into clouds and therefore no thunderstorms can be produced under such conditions. Typically, meteorologists use moisture variables such as the dew point temperature (the temperature at which air becomes saturated with respect to water vapor) to forecast if the atmosphere contains enough moisture.
The next ingredient is instability, and to think of instability, we must imagine a parcel of air. Meteorologists use the concept of a parcel because it is easier to visualize the intricate processes of the atmosphere. Now imagine you were standing on the ground and you released this parcel into the atmosphere. What happens to that parcel at this point can tell us if we have a stable or unstable atmosphere. A stable atmosphere is one in which a parcel will want to sink back down to the surface and resists vertical accelerations. This is typically what you see underneath the influence of high pressure systems. An unstable air mass on the other hand is one in which a parcel will want to keep rising because the temperature of that parcel is warmer than the air around it. Back to our parcel visualization, if we release this parcel and it keeps rising, then we know we have an unstable air mass. Meteorologists utilize a variety of tools to diagnose the stability of the atmosphere and even have equations to help us identify instability. One such that you may have heard of is Convective Available Potential Energy or CAPE. CAPE combines the moisture content of the atmosphere along with the stability to give us a proxy for environments conducive to thunderstorm formation. These values have a wide range that is dependent on season, and synoptic (or larger-scale) environment.
Now, we have a moist atmosphere, and we have a situation where air parcels will want to rise freely on their own. However, usually the atmosphere is not that simple. In fact, there can be a layer of stable air close to the surface underneath the more unstable air. We must find a way to push those air parcels above this layer into the unstable air. This is why thunderstorms usually form out ahead of a cold front. The cold front gives those air parcels the initial push they need to overcome the stable layer and rise freely on their own. There are other lifting mechanisms other than the cold front such as warm fronts, sea-breeze fronts, and even the cold-dense outflow produced from another thunderstorm. Thus the formation and sustenance of convection is simple from a standpoint of forecast operations.
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©2018 Meteorologist Allan Diegan