Mixing Height & Wind Speed / Mixing Height Profiles


Mixing Height or Mixing Depth is used by meteorologists to quantify the vertical height of mixing in the atmosphere. It is the height at which vertical mixing takes place. Forecasting of mixing height is done with the aid of the vertical temperature profile. A radiosonde is sent aloft and temperatures at various altitudes are radioed back. The altitude at which the dry adiabatic line intersects the radiosonde measurements is taken as the maximum mixing depth (MMD). The dry adiabatic line is defined as a decrease of 5.4 degrees Fahrenheit over height of 1000 feet. The MMD is a function of Stability . In Unstable air the MMD is higher and in Stable air the MMD is lower. Looking at the figure at the top if the same amount of pollutants are emitted you would expect a larger amount of ambient air pollution in the case on the right. There is a smaller volume which the pollutant can be dispersed. There is a seasonal variation of mixing height. For Summer daylight hours MMD can be a few thousand feet where as for Winter it can be a few hundred feet. It varies also in the course of a day. It is lowest at night and increases during the day. With a measure of both MMD and wind speed with respect to height and we get a very good idea of the amount of pollutant dispersion.

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