Interesting Weather Information

Friday, March 15, 2013

Thunderstorm Primer - Part 1: Storms Formerly Known As Air Mass Thunderstorms

As I mentioned in my previous post, the Introduction to this Thunderstorm Primer, we probably should not call the simplest of thunderstorms "Air Mass" thunderstorms because they really are not. This is an old, overused concept and it should be retired to the "Of Historical Interest" section of your meteorology lexicon.

Meteorologists originally thought that thunderstorms were with the air mass type or frontal type. Air mass thunderstorms were thought to pop up at random on a hot, humid summer afternoon and the sole source of energy is differential surface heating. In fact thunderstorms are never random. We may not know precisely why one develops where it does but it is not random.

The lifetime of a simple or ordinary or single-cell or garden-variety thunderstorm - all are better than the old term air mass thunderstorm) is illustrated below and ranges generally from 15 to 45 minutes. Because the updraft is vertical and the heavy thunderstorm rains fall against the updraft  the lifetime of s single-cell thunderstorm is limited to a short duration. One meteorologist said it is like pouring water down a chimney.

The life cycle of a typical single-cell thunderstorm.

Because the storm does not last very long it has little time to interact with its environment. Remember this point because we come back to storm-environment interaction in the next post.

In Summary:
Simple Single-Cell Thunderstorms
· are primarily convective but other sources of lift can contribute
· have an updraft that is typically vertical so rising air has to push against the rainfall
· mostly have a lifetime of 15 – 45 minutes
· can be severe (with 1” hail and winds >=58 mph) but mostly storms are below severe limits
· are individual cells but several can grow together to form a MCS (mesoscale convective system).

Frontal type thunderstorms were thought to develop in lines along fronts. Differential surface heating along  lift along the front supplied energy to the storm. Lines of thunderstorms are now classified as a type of mesoscale convective systems. This will be covered in the next post.

So, what's in a name! The term air mass thunderstorm still hangs around even though it is obsolete. A better name is isolated or single-cell or simple or garden-variety thunderstorm. Naming issues will be visited again when we distinguish between mesocyclone tornadoes and leading edge vortices.

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