Mosquitoes usually do not fly in the rain, as the conditions make it too difficult to do so.
A collision with a raindrop can knock a mosquito to the ground, and the wind conditions of a rainstorm make flying extremely difficult.
But the period of time just after a rain storm subsides is perfect conditions for mosquitoes, and this is the time that the population of mosquitoes in the air is the thickest.
The standing water and humidity caused by the rain allows mosquitoes to travel far distances from the ponds and other stagnant water sources that they normally would stay close to.
