The Show Weather mode changes the display of APRS weather stations on the map to look like an old-style weather map, including wind direction tail, wind speed barbs, Fahrenheit "dry bulb" and "wet bulb" (dew point) temperatures to the left of the symbol, and the last three digits of the barometric pressure in tenths of millibars to the upper right of the symbol. Other stations' rendering is not changed.
The wind tail points in the direction the wind is coming from (as would the point of a weather vane). Triangular pennants indicate 50 knots (nautical miles per hour) of windspeed each, long barbs indicate 10 knots of windspeed, and a short barb indicates an additional 5 knots. Additional windspeed of 1 to 4 knots is lost. In the above image, WB5HSM-3's windspeed is 33 knots (hence three long barbs and nothing extra for the 3 knots).