4.4: Dew and Frost

Dew

Clear, calm nights allow the maximum release of longwave radiation from Earth's surface and surrounding objects into space. If the surface temperature falls to the dew point, water vapor in the air can begin to condense on the surface. We notice this early in the morning (when the temperatures are the lowest), as liquid droplets form on cars, and on blades of grass, as depicted in Figure \(\PageIndex{1}\). On the other hand, cloudy skies reflect and absorb sunlight, re-emitting longwave radiation to the surface, thereby reducing the amount of cooling that would otherwise occur. Windy conditions enable surface air to mix with drier air aloft. Thus, heavier dew tends to occur when the wind is light rather than when it is strong. Next time you walk across the grass and find your shoes completely wet, take note of the conditions that allowed dew to form.

Figure \(\PageIndex{1}\): As the ground cools by radiational cooling, water vapor condenses to form dew in the form of tiny droplets of water on blades of grass. (CC BY-SA 3.0; Kesoff via Wikimedia Commons)

During winter, as nights are longer than days, early-morning ground temperatures can fall below freezing. When temperatures drop below freezing and reach the dew point or frost point, the ice on the ground is referred to as frost or frozen dew. "Frost" can form in two ways: Either by deposition or freezing. Depositional frost or white frost occurs when the dew point (now called the frost point) is below freezing. When this frost forms, the water vapor goes directly to the solid state. Depositional frost covers vegetation, cars, and other objects with ice-crystal patterns (treelike branching). If the depositional frost is thick enough, it resembles a light snowfall as depicted in Figure \(\PageIndex{2}\).

Frost that forms when liquid water freezes is best referred to as frozen dew. Initially, both the dew point and temperature are above freezing when dew forms. Longwave radiational cooling gradually lowers the temperature to at or below freezing during the night. Cold-air advection at night can also cause temperatures to drop below freezing. Once the temperature falls to freezing, the condensed dew droplets freeze. Frozen dew looks different from white frost. Frozen dew does not have the crystal patterns of white frost. White frost appears whiter, while frozen dew tends to look slicker and less distinct.

Figure \(\PageIndex{2}\): When the ground cools to temperatures below freezing, ice appears on the ground instead of water droplets and is referred to as frost or frozen dew. (Public Domain; Rosendahl via Wikimedia Commons)

Compared to dew, frost and frozen dew can cause morning transport delays by sticking to car windshields. Some frost or frozen dew is much easier to scrape off a car than others. When the temperature is near freezing (29 to 32°F), ice is relatively easy to scrape from car windows. It is also quicker to warm up the car windows to above freezing with the defroster when temperatures are near freezing. Once temperatures drop below 28°F, the ice becomes more difficult to remove. It requires more "elbow grease" to remove the ice. It also takes longer to warm up the car windows to above freezing. Next time you witness ice in the morning, think about the processes that produced the frost or frozen dew.