Sounding data for Albany, NY at 0Z 4 mar, 12Z 4 Mar shows that the daily temperature variation is in fact restricted to the very lowest layer of the atmosphere. At 0Z, the surface temperature at Albany is just about -8 C, whereas at 12 Z it has decreased to about -11 C. Note however, that above 950 mb, the temperature has barely budged. For example, at 950 mb the temperature remains constant at about -16 C.

This is typical of diurnal (daily) temperature fluctuations. These are restricted to a region of the troposphere called the boundary layer. Although the height of this layer can vary depending on weather conditions, it typically is restricted to below 850 mb. It's common to use temperature data from above this layer (usually 850 mb maps) to determine temperature trends based on temperature advection to remove the effect of the sun.

Data is taken from the University of Wyoming website.