Wind Gradient
Horizontally, on the Earth's surface wind always blows from areas of high pressure to areas of low pressure, usually at speeds determined by the rate of air pressure change between pressure centres. This situation is comparable to someone skiing down a hill. The skier will of course move from the top of the hill to the bottom of the hill, with the speed of their descent controlled by the gradient or steepness of the slope. Likewise, wind speed is a function of the steepness or gradient of atmospheric air pressure found between high and low pressure systems.
When expressed scientifically, pressure change over a unit distance is called pressure gradient force and the greater this force the faster the winds will blow.
Wind Shear
Wind shear is simply the change in wind direction and/or speed with height. We can differentiate between directional and speed shear, but generally speaking both occur simultaneously and hence the term wind shear. We have horizontal and vertical wind shear, but it is the vertical shear that creates the most trouble for aircraft. Be aware that the vertical shear may vary rather dramatically in the horizontal.
How does wind shear occur? It all comes down to friction, the closer you are to the surface of the sea or land, the slower the wind speed and the more the wind direction will veered (southern hemisphere) in relation to the wind gradient or the friction-free wind. It all comes down to the stability nature of the air-sea/land interface. Differences in speed range from very little in unstable air (around 5%) to enormous amounts in stable air (up to 300%). From a directional point of view, differences range from about 1 degree in unstable air to about 30 degrees in stable air.
It is wind shear and not changes to the air density that causes what is known as the "weight of the wind". Even though the air density will change with a change in the air temperature, the change in the actual value of the air density will be very small. It is the variation in wind speed and hence the wind shear that is largely responsible for this "weight" phenomenon.
When it occurs, it is this variation in wind speed that can lead to a huge difference in the drift moment of the aircraft. In stable air there will appear to be more "weight in the wind" since there will be generally a stronger wind speed aloft than at lower levels. In unstable air, there is generally good mixing or overturning taking place in the boundary layer so the wind speed is fairly constant between the upper and lower levels and hence the drifting moment is less.
Apart from temperature considerations, wind shear is generally associated with a developing sea breeze. It can be very marked whilst this breeze is building and will drop away to nothing once the breeze has reached maturity. Shear can also be marked by towering cumulus or cumulonimbus (thunderstorm) cloud and also if a cold front is close by.
Horizontal Wind Shear
Wind shear is encountered in an area where two winds moving in opposite directions "rub" or mix together. This shear zone creates small eddies and whirling masses of air that move in various directions. This generates tremendous turbulence. Some wind shears are predictable, but others may occur unexpectedly. Getting caught in wind shear can be devastating to an aircraft, especially if the wind shear occurs close to the ground. Currently airports are being outfitted with wind shear alarms that warn controllers and pilots of the potential windshear existence within runway takeoff and landing corridors.
Vertical Wind Shear
Vertical wind shear is the second critical factor in the determination of thunderstorm type and potential storm severity. Vertical shear, or the change of winds with height, interacts dynamically with thunderstorms to either enhance or diminish vertical draft strengths.
Low Level Jet Streams
A fast-moving river of air known as a "Low Level Jet Stream” sometimes forms at about 1,000 feet above the ground after sunset.
On a clear evening the atmosphere cools down. And if conditions are calm, a stable temperature "inversion" sets up where cool air aloft, which is heavier than warm air, sinks to the ground and any leftover warm air sits on top of it. The stable air in an inversion acts like a nearly solid object and allows the air above it to flow rapidly past the inversion like wind blowing over water.
Differences in air pressure on either side of the developing low-level jet help to concentrate the flow of air into a corridor or stream less than several hundred nautical miles wide. Winds in the stream can flow at speeds of 60 kt or more. Mountain ranges can further enhance low-level jet stream winds.
Night-time, low-level jet streams are marked by a rapid change in wind speed with height. The sudden shift in wind can catch late-night fliers by surprise and be hazardous to landing aircraft.
Once the sun begins to heat the land, the lower atmosphere begins to mix as the warm air rises, breaking the inversion. As this happens, the jet rises in some places and sinks in others like a giant roller coaster. Without a smooth surface to glide over, the jet encounters friction and slows down. But the same conditions the next night could allow the low-level jet to reform with equal strength and similar consequences.