Sea Level Pressure in hPa (solid lines) and equivalent potential temperature at 850 hPa (dashed and coloured) The equivalent potential temperature is calculated from the specific humidity (ratio of mass of the water vapor to the total mass of the system) and the temperature, both at 850 hPa. Therefore, from this map one can read the fronts even better than from the 'RH 700 hPa' or other maps.