 The meteorology description of wind direction originates from the compass and facing into the wind where the wind comes from. The mathematical description of wind direction is based on the Cartesian XY grid and tracks the direction that the wind is going. We need to know both because the meteorology description, or medial angle, is used in the station weather plot, but we need the mathematical description, or math angle, for dynamics and in numerical weather prediction. Let's look at one example that relates the meteorology and math angles. First note that the grids are related with positive X corresponding to east and positive Y corresponding to north. Now let's add a wind. In this case, a wind from the northeast or northeasterly. From the station weather plot, the wind is from the northeast. Normally the wind barb would end in the center with the description of cloud cover. We extend it past the center toward the direction the wind is blowing, since that would be how we would draw the line and describe the wind direction in the mathematical XY coordinate system. Meteorology angles measure clockwise from the north axis, just as it is for a compass. 0, 90, 180, 270, 360, which is the same as 0. The math angle is measured counterclockwise from the X or east axis, 0, 90, 180, 270, 360, or 0. It turns out that the math angle equals 270 degrees minus the meteorology angle, and also therefore the meteorology angle equals 270 degrees minus the math angle. But for this case that we've drawn here, the meteorology angle equals 45 degrees, so the math angle equals 270 minus 45, which is 225 degrees. The meteorology angle is drawn clockwise, and the math angle is drawn counterclockwise. If a resulting angle is negative, simply add 360 degrees to make it positive.