In general, the wind speed increases with height from the surface to the upper troposphere. There are several reasons that explain this tendency. First, especially in the middle latitudes, the pressure gradient increases with height. This is shown in the diagram below in (1). The height of the troposphere is taller in warmer air (right side of diagram) since warmer air is less dense and thus occupies a greater volume. Going up in altitude, the pressure gradient between the warm air and the cold air increases with height. This can be noticed by the dashed line for pressure being slanted more as height is increased. A higher slant results in a greater pressure gradient between the warm and cold air and thus stronger wind.

A second reason for the wind speed increasing with height, especially near the ground, is due to surface friction. Surface objects such as trees, rocks, houses, etc. slow the air as it collides into them. The influence of this friction is less with height above the ground, thus the wind speed increases with height.

A third reason is due to air density. The density of the air is highest at the surface and decreases with height. A force imparted on air will cause the air to move more easily when the mass of the air is less. Dense air requires a greater force to move it the same speed as less dense air. With air density decreasing with height, it is easier to move the less dense air at a higher wind speed.