We love to see it on trees, we hate to see it on our cars, and we fear it when our tomatoes are ripening: frost.
Frost forms when water vapor in the air freezes onto cold objects. The Oxford English Dictionary defines three kinds: “hoarfrost,” the thick, crystalline stuff that makes your backyard look like a Christmas card (“hoar” means “white”); “rime,” the more generic stuff you scrape off your car, and “black” or “killing” frost, which is what you get when the air is too dry to form white frost but cold enough to freeze your begonias.
The amount of water vapor in the air, the specific humidity, is measured in grams of water per kilogram of air, and generally increases with temperature. Hot, moist air may be as much as five percent water, whereas cold, dry air may hold almost none. But at each temperature, there is a specific point at which the air is completely saturated, like a wet paper towel.
At -40 at sea level, the saturation point is about one tenth of a gram of water per kilogram of air; at freezing point it’s about 3.8 grams; at room temperature it’s about 15 grams, and on a record-breaking summer day, at 40 degrees, it’s about 50 grams.
Relative humidity — what we hear about in weather reports — is how close to saturation the air is. Thus, a relative humidity of 90 percent at -40 would still mean the air only holds about .09 of a gram of water per kilogram, whereas relative humidity of 90 percent at room temperature would mean it holds a whopping 13.5 grams per kilogram.
Since the saturation point decreases as the temperature falls, if warm air is cooled it eventually reaches a point at which it is unable to hold all its water vapor. That’s the dew point, the temperature at which dew — or, if it’s below freezing, frost — forms.
Because the temperature drops at night, that’s when dew and frost usually form. Cars, tree branches, grass blades and other things not in direct contact with the ground collect it first, because whereas the ground stores some of the day’s heat, most objects quickly radiate it into the sky. Heat radiates more quickly into a clear sky than a cloudy one, which is why clear nights are the coldest — and the most likely to have frost.
How thickly frost forms depends on the humidity, how cold it gets, and how quickly it gets that cold. When we get really, really thick hoarfrost — the picture-postcard stuff — it’s usually because something else has been added to the mix: fog.
Fog, consisting as it does of drops of water suspended in the air, provides large quantities of freezable moisture. It usually forms when warm, moist air flows over cold ground or snow cover. As the warm air is cooled from below, it falls below its dew point and the moisture in it condenses. Ice fog is frozen fog, made of tiny ice crystals instead of water droplets. Cities get a lot of it because of all the warm, moist air pumped out of chimneys and car exhausts.
The “fog” that rises off open water in very cold weather is properly called “Arctic sea smoke.” When cold dry air passes over water, some of the water evaporates, but almost immediately condenses again, which makes it look like the water is steaming.
These basic principles explain a lot of familiar winter phenomena. Your breath turns to fog because you’re very warm inside and you’re mostly made of water, so you breathe out lots of water vapor that condenses in the cold air. Your glasses frost up when you come inside because they’re still at the outside temperature, way below the dew point of the room’s air.
Cars frost up on the inside for the same reason. The car windows are cold, so the moisture you breathe out condenses and freezes on them: or snow gets into the engine compartment, melts as the engine heats up, turns into vapour, gets blown into the car and condenses on the windows. (This usually happens as you’re passing a truck on an icy two-lane.)
But condensation is more than a nuisance: water vapor condensing into clouds gives us rain, and in some dry climates, dew itself is a vital source of water.
Tell yourself this the next time you’re scraping frost from your window on a dark, snowy morning when it’s -40 and the wind is slicing your skin off one strip at a time.
Maybe it’ll make you feel better.
