Wind Shear
Wind shear is the change in wind direction or speed with height. Wind shear plays important roles in meteorology: it can help to dissipate tropical cyclones and has interesting effects on thunderstorms. Along with instability, wind shear is the second huge factor when it comes to understanding thunderstorms and forecasting the severe weather they produce. Naturally, we would expect there to be wind shear in the atmosphere as wind speed increases with height, from zero at the surface. In the troposphere, the fastest winds are generally from 300 - 150 hPa, or 8000 - 12000 m, in a region called the jet stream and they are the fastest in the winter. However, wind shear can be generated through a number of different processes, such as low-level jets, the passage of a front, or even mountain ranges. 
 
Wind shear can influence the internal dynamics of a thunderstorm, as well as the organisation of a group of thunderstorms. There are two main types of wind shear: directional shear and speed shear. 
 
Directional shear - this is a change of wind direction with height. The wind direction throughout the troposphere is rarely uniform. In fact, although the wind at the surface may come from every direction, aloft (above 500 hPa) it almost always comes from the west, in the middle latitudes (such as the UK). An approaching extratropical cyclone can lead to significant directional shear: winds at the surface may come from the south, but may be from the west a few thousand metres above the surface. The best way to see this is in the summer, when cumulus clouds close to the surface often move in a different direction to the high-up cirrus clouds. 
 
Speed shear - this is a change of wind speed with height. As already stated, wind speed is rarely constant with height too: unless you are under a stagnant high pressure system, wind speed will usually increase with height. The jet stream often has winds above 150 mph in the winter, yet winds are rarely half this speed at the surface (thankfully). Speed shear can also be generated by low level jets, which are ribbons of fast flowing air lower in the atmosphere, at 850 hPa, or 1500 m, for example. To see speed shear in the atmosphere is a little harder, since high clouds move faster, but this effect is masked by the fact that they are further away. However, speed shear can easily be measured by a radiosonde.
 
 
 
 
The wind shear diagram above illustrates how speed and directional shear look in the vertical. The directional shear can be easily seen by the arrows pointing in dirrerent directions at different heights. The speed shear is represented by the top three arrows, which all point in the same direction, but at different speeds.
From http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/modl_shear.rxml