Why does the temperature drop a few degrees just before sunrise?

Answer 1

Right before sunrise (where you are) it IS sunrise just over the horizon. There, the sun is coming up and beginning to warm. This causes an outward expansion of air which in turn disturbs the air in areas around it (where you are) This disturbance stirs the air up, causing air close to the surface, which has been able to stay slightly warmer (due to earth temperature, industry, nearby bodies of water, condensation of dew etc, all of which release small amounts of energy to their environment) to mix with air further from the surface, which didn't have these warming influences. Now some of that colder "high" air is on the surface, so the surface air temperature drops slightly. This phenomenon is known as the Solar Semi-diurnal Tide.

There is another cause of this phenomenon, and that is something known as the latent heat of vaporization. When the Sun's rays first hit dew or frost on the ground, but before the radiant heating of the Earth has begun, the Sun's rays actually begin the process of evaporation and it is this process of evaporation that removes heat from the environment thus registering a noticeable drop in temperature. Obviously, the conditions would have to be just right for this phenomenon to occur, but it does happen.

Answer 2

The temperature drops just before sunrise because without the sun's heat energy, the Earth's surface cools down overnight through radiation. This cooling effect is most pronounced just before dawn when the sun is about to rise.