How does this shear thing work? I have been trying to get my head around. What do you mean by a downshear and upshear?
Posted June 11, 2008
MIKE MOSS SAYS: Mike, Wind shear comes into play in a number of different contexts in meteorology, and you don't say what prompted your question. In a general sense, wind shear is simply the difference in wind speed and direction from one location to another, so for a specific value of wind shear to have meaning the two locations have to be specified. Wind shear can be horizontal (from one location to another at the same altitude) or can be vertical (from one altitude to another above the same point on the earth).
We can calculate wind shear as a scalar quantity (i.e. just the difference in wind speed from one point to another) or, more meaningfully in most cases, a vector difference. This results in a wind shear vector that has a direction and speed. A very simple example would be a wind at 1000 feet up that is from the west at 20 mph and a surface wind that is from the west at 5 mph - since they are both from the same direction, the shear vector would be from the west at 15 mph, so that adding by adding this to the surface wind you get the 1000 foot wind. In this case, "downshear" would be toward the east, while "upshear" would be toward the west.
It's a little more complicated when the two directions are different, involving some trigonometry to work out the the shear vector. A fairly simple example of this would be a 1000 foot wind from the west at 10 mph and a surface wind from the south at 10 mph. In this case, the shear vector would be from the northwest at 14.1 mph, so that upshear would be toward the northwest and vice versa.
Shear can have impacts in the organization of severe thunderstorms and the potential for tornadoes, in the formation of turbulence, and even in something as subtle as organizing cumulus "cloud streets," in which the lines of clouds tend to form in a direction that follows along the shear vector.