CyberAirVA

__________________________________________________

Thunderstorms

One of the most destructive weather phenomena for aviation is the thunderstorm. Who haven't heard of wind shear, or microbursts. Below I will explain how and why the thunderstorm is just such a destructive force of nature.

How does it form?
As the name implies a thunderstorm is a storm which thunder :) so where does the thunder come from? The key ingredients to a thunderstorm and thunder are warm air and vapors. As the warm air rises it passes through colder areas which cools some of the air of and the vapor forms clouds. In the case of the thunderstorm these clouds will be cumulonimbus (CB) clouds.

In the cloud the warm air keep rising until the vapor in it chills down and becomes precipitation, i.e. rain. this then falls to the ground. Note that the thunderstorm hasn't formed yet. Anyway when the air and vapor rises into cooler areas it sometimes becomes hail, little ice crystals. These hail particles fall quite rapidly and when they do this they strike the new vapor particles on their way up.

As they do this they knock electrons out of the rising particles which then becomes positively charged (+). The falling hail becomes negatively charged (-). What this means is that the top part of the cloud becomes positive and the lower part becomes negative. As you might know nature likes things nice and even, so it tries to equalize the charge, usually by lightning.

Lightning
So now we have lightning which can be in cloud (IC), cloud-to-cloud (CC), cloud-to-water (CW) or cloud-to-ground (CG). Lightning is always a warning sign. When there is lightning there is a thunderstorm. Since some forms of lightning can travel ahead of the storm by as much as 10 miles so the storm isn't always visible.

Precipitation
Precipitation is a very good indicator of thunderstorms, sometimes the CB cloud can be obscured, hidden in other clouds and can be somewhat hard to spot. But usually the precipitation can tell you about it. Rain can be a warning of a upcoming thunderstorm, especially with CB clouds. Usually though the precipitation which is most associated with thunderstorms is hail, even light hail.

Turbulence
One of the big problems with thunderstorms is the turbulence, Turbulence is simply air moving around a bit. Turbulence associated with thunderstorms can be anywhere from light to severe, usually though it's at the top of the scale. In the more severe thunderstorms wind gusts can be expected.

Wind Shear and Microbursts
A couple of very "popular" terms are wind-shear and microbursts, but what are they? The definition of wind shear is a rapid change in the winds direction or speed. Severe wind shear can be expected around thunderstorms, meaning there can be a wind speed change of more than 15kt and/or a change in vertical speed of 500ft or more.
There are a couple of subgroups to the wind shear phenomena which is low-level wind shear (LLWS) and microbursts.
Low-level wind shear (LLWS) is when the wind shear is encountered at 2000ft AGL (above ground level) or below. The problem with LLWS is that at those altitudes the aircraft can't afford those kinds of losses of altitude and airspeed. This is because it is most often encountered in the approach or landing sequence, when altitudes and airspeed are close to minimums.
A microburst is severe wind shear (can be up to wind speed differences of 200kt). When rain cooled air leads to a severe downdraft (less than one-half mile in diameter). The downdraft (downburst) has a very sharp edge and forms a ring vortex upon contact with the ground. This vortex then forms an area of fast outward moving air.

There are some guidelines on how to recognize wind shear:

Precipitation or dust curls carried up towards the clouds base
Horizontal bulging in a rain shaft forming a foot-shaped prominence near the surface
An increase in wind speed as the microburst expands over the ground
Abrupt wind gusts

If you encounter any of signs above you should really consider heading for your alternate airport.
If you don't see any signs of wind shear, but you suspect that there are some out here, i.e. thunderstorms in the vicinity, here's what you can do:

Select the longest runway for take-off
Select the longest runway for landing and use a higher landing speed
Go to your alternate airport if landing
Don't take-off, if you are still on ground

Here are a couple of guidelines to follow if you encounter wind shear when in-flight:

Aggressively apply maximum rated power
Never try to land
Use your maximum rate of climb

Avoid
The key to not having problems with thunderstorms and especially any form of wind shear is avoidance. Most of the weather report will mention thunderstorms (TS), cumulonimbus clouds (CB), precipitation: hail (GR or GS), snow (SN), rain (RA), ice pellets (PE), ice crystals (IC), snow grains (SG) or just precipitation (UP). Also reports of convective areas, activity are available. It's important not to ignore the signs that are available to you.

_______________________________________