By: Steve Roark
Volunteer Interpreter, Cumberland Gap National Historical Park

September is 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 and moisture in the air condenses on the cooled surface. Dew forms on vegetation more readily than other surfaces such as pavement because leaves and grass typically are thin and suspended in the air, causing them to cool more readily to reach dew point temperature.

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 cold, such as when the weather suddenly changes after a cold spell. Dew becomes frost when the surface temperature of vegetation 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 cloud layers. 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 ¼ 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.