How Dew and Frost Form

Ever go outside early in the morning and breathe in that peculiar and quite particular scent that only arrives when fresh drew is on the lily? When little drops of dew appear on your front lawn, whether you’ve got lilies or not, that is the result of a specific weather situation that has to do air that is close to the earth’s surface being becoming saturated with water vapor.

The ground becomes heated during the waking hours by solar radiation beating down upon the planet like a flurry of jabs from Muhammad Ali atop George Foreman’s pate. When the sun goes down and the lights and lipstick go on, however, the ground tends to cool down quite quickly. This is especially true if the sky is clear. The air that comes into contact with ground comes into contact with the air above the ground, which is warmer. The result being that the air that is situation near the ground begins to cool to saturation near the dew point. The dew point, in case you were ever wondering, is the temperature at which water vapor begins to condense. Once the saturation point is above freezing, dew forms. Should the temperature be below the freezing point it is frost that forms and not dew.

Two types of frost exist. Hoar frost, and let’s all be adults here shall we, is the frost that forms when the air cools down and water condenses onto the grass in a way that is fragile and delicate in appearance. There is something truly beautiful about hoar frost; is the type of frost that Robert Frost may have written an ode to. Hoar frost will only form if there are very light winds and the result will be crystalline in appearance. In contrast is the second type of frost, which is called rime frost.

Rime frost happens when the formation of frost takes place quite rapidly. If you have awakened to a heavy fog in near-freezing temperatures, you will probably seen rime frost rather than hoar frost. Little droplets of water come into contact with the frozen surface and the result is a spiky, grain kind of frost that definitely does not have the beautiful crystalline appearance of hoar frost. Rime frost is the kind of frost that might inspire a poem by Allen Ginsburg. Rime frost is much more dense than hoar frost and as a result is harder. When you hear frost crunching beneath your feet, it is probably rime frost.