September is usually noted for having heavy dews that bejewel cobwebs and soak your feet when walking through grass. The reason is that nights are getting longer, which allows the grass and other objects more time to drop below the dew point temperature. This is when moisture in the air will condense on a cool surface.
Clear nights with still air are ideal conditions for dew, making it an excellent predictor of fair weather: “When dew is on the grass, rain will never come to pass.” Dew will not form during heavy cloud cover or much wind, both of which occur during foul weather. The heaviest dews occur when moist warm air moves in over surfaces already very cold, such as when the weather suddenly changes after a cold spell. Dew becomes frost when the surface temperature of objects drops below 32 degrees.
It is interesting to note that condensation cannot occur without a surface to form on. Dew only forms when grass or some other cool surface is present. The tiny droplets that form clouds must also form on something, usually dust. For rain to occur the drops must grow large enough to reach the ground before evaporating. Most rain drops start their journey as snowflakes originating high in the frozen cloud tops of nimbostratus or cumulonimbus clouds. Way up there cloud droplets freeze into tiny ice crystals, which collide with each other and grow into snowflakes, which when heavy enough will fall and melt as they pass through the warmer layers of cloud. If they are large enough to keep falling they will collide with and gather up more cloud droplets, and when they reach 1/25th of an inch in diameter they will be able to make it to the ground before evaporating and fall as rain.
Because of their resistance to the air, falling raindrops are not tear-shaped like cartoonists like to draw, but resemble small hamburger buns. The largest raindrops are around a quarter-inch in diameter and fall at a speed of 26 feet per second. The occasional really large raindrops were likely hailstones or large snowflakes that have melted just before reaching the ground.
Steve Roark is with the Tennessee Department of Agriculture, Forestry Division.