Boundary Layer » Maximum Temperature Seasonal Variations
There are also seasonal variations that must be taken into account when using this method. At elevations that are within a few hundred feet (100 m) of sea level, 850 hPa is an appropriate reference level in the spring or fall.
In the summer, with higher sun angles and greater solar heating, the reference level may be as high as 700-750 hPa.
In winter, with relatively low sun angles and less solar heating, you may choose a reference level at or below 900 hPa.