While instability and moisture help determine the strength of a thunderstorm's precipitation (rate of rain release, hail generation), the wind shear determines the storm's movement and life span. Wind shear has two forms: speed shear and directional shear. Speed shear is a change in wind speed with height while directional shear is a change in wind direction with height. Each of these shears have varying magnitudes. To simplify, we will have two categories: weak and strong. Thus, we have four combinations. Each of these combinations will be diagrammed using a hodograph sketch. The type of weather to expect from the storm will also be explained. Assume the updraft is of moderate strength for each case (moderate instability).
CASE 1: WEAK SPEED SHEAR, WEAK DIRECTIONAL SHEAR
A storm in this environment will move slowly and will be short lived. Since the storm moves slowly, the downdraft will cut-off the updraft and will thus diminish the storm. Storms in this environment are often termed "air mass thunderstorms" or "garden variety thunderstorms". If storms form in a moisture rich environment, rain can be heavy for brief periods of time. Severe weather is not likely.
EXAMPLE HODOGRAPH SKETCH #1
**note for sketches: 1= 950 mb, 2= 850 mb, 3= 700 mb, 4= 500 mb, 5=300 mb
SKETCH 1: SSE 10 knots @ 950 mb, S 22 knots @ 850 mb, SSW 23 knots @ 700mb, SW 10 knots @ 500 mb, S 13 knots @ 300 mb
CASE 2: STRONG SPEED SHEAR, WEAK DIRECTIONAL SHEAR
This situation is often termed "unidirectional shear". The speed shear will allow the storm to move. The movement insures the storm will last longer than an airmass thunderstorm. Unidirectional shear often produces storms that form into lines (Mesoscale Convective Systems (MCS's)). Since the storm moves, outflow produces lift that enables new storms to grow on the storm's periphery. Over time, a line a storms result. These storms primarily produce small hail, weak tornadoes and heavy rain when they are associated with severe weather.
EXAMPLE HODOGRAPH SKETCH #2
SKETCH 2: SE 19 knots @ 950 mb, SE 35 knots @ 850 mb, SSE 45 knots @ 700mb, S 70 knots @ 500 mb, SSW 85 knots @ 300 mb
CASE 3: WEAK SPEED SHEAR, STRONG DIRECTIONAL SHEAR
When speed shear is weak the directional shear is not of significance. Storms in this environment will take on the characteristics of those in CASE 1. Hodograph wind speed will have similar pattern to CASE 1 and wind direction change with height will be high but often unorganized.
CASE 4: STRONG SPEED SHEAR, STRONG DIRECTIONAL SHEAR
This situation can produce single-cell super-cells. This is the best situation in order to produce a rotating updraft. The speed shear enables the storm to move quickly and helps keep the updraft and downdraft separated while the directional shear helps rotate the updraft into the storm. These storms can produce large hail, strong tornadoes and heavy rain.
EXAMPLE HODOGRAPH SKETCH #3
SKETCH 3: ESE 24 knots @ 950 mb, S 43 knots @ 850 mb, SW 52 knots @ 700mb, WSW 77 knots @ 500 mb, W 95 knots @ 300 mb