Different Types of Frost: Air, Ground, Black, White, Silver, Hoar & Rime

The crisp winter weather includes snow, sleet, hail, and frost. Frost is the frozen coat of ice that dresses the lawn in the early winter and the spring flowers and vegetables planted too early in the spring. Frost comes in several types.

When the outside air temperature (6 feet above ground level) is 0ºC (32ºF) or less, there is air frost. When the air very close to the ground (2 inches above ground level) is at 0ºC (32ºF) or less, there is ground frost. Depending on the season of the year, the temperature of the ground and the temperature of the air can vary by several degrees, as the sun warms the air first at sun rise, and the ground hold the heat longer at sunset. This difference explains why there are two temperature-related measurements of frost.

When the temperature of outdoor objects is below freezing, and there is an incoming air flow containing precipitation that is close to freezing, then moisture will collect and freeze on the exposed objects. This is called glaze. When the exposed object is a road surface, it is called black ice. When the incoming air mass is quite warm and the ground surface is quite frozen, there can be a partial thaw and refreezing of the newly deposited surface moisture creating a condition known as silver thaw. When the glaze becomes very thick, it is an ice storm.

When the frost forms inside plants because there is more moisture inside the plants than in the air, it is a dry freeze called black frost. There may be no visible ice crystals, but the plants turn black and die.

When frost from supersaturated air (the relative humidity is greater than 100%) forms on plants or solid objects that are already frozen, there is hoarfrost. Hoarfrost is recognized by the ice feathers or ice needles that protrude from the ice covered surfaces. If there is a heavy coating of hoarfrost, it is called white frost.

Hoarfrost is less dense and softer than glaze. There is a frost type in between the two called rime. Whether the frost is hoarfrost, rime, or glaze depends on the size of the moisture drop, the rate that ice forms, the degree of supercooling (temperature below freezing while still a liquid), and how quickly the surface becomes frozen.

Rime is usually formed when there is very rapid freezing of supercooled water, like from a dense fog. Both hard and soft rime can look like hoarfrost and are associated with fog. Rime and glaze can happen at the same time on the same surfaces. When ice forms on airplane wings, it is usually rime.

Understanding the different types of frost helps people to better understand weather conditions, to drive more safely, to know when the vegetable garden needs an extra layer protection, and to dress appropriately before going outdoors.

Glossary of Meteorology, http://amsglossary.allenpress.com/glossary/browse
Weather Online, http://www.weatheronline.co.uk/reports/wxfacts/