What is frost and how does it differ from a freeze?
ROCHESTER, N.Y. (WROC) — As we slowly begin to get to the colder months, our temperatures will also begin to get colder. Some days and most nights, we stay in the mid 30s. In the event this happens, we sometimes see frost develop. What exactly is frost and what causes it?
The fundamentals first: What is frost?
According to the National Weather Service Glossary , "frost describes the formation of thing crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans." The National Weather Service says that a frost can form when the temperature fall below 36°F, with it being more than likely in rural areas. However, frost will become more widespread when the temperature falls to 32°F.
When can Rochester expect its first frost?
What causes frost?
According to the National Weather Service , the following list are some meteorological conditions that can lead to frost conditions:
Clear skies lead to radiational cooling , allowing the greatest amount of heat to exit into the atmosphere.
Calm to light winds prevent stirring of the atmosphere, which allows a thin layer of super-cooled temperatures to develop at the surface. These super-cooled temperatures can be up to 10 degrees cooler than 4-5 feet above the surface, where observations are typically taken. For example, if conditions are favorable, air temperatures could be 36 F, but the air in contact with the surface could be 30 degrees or colder.
Cool temperatures, with some moisture, that promote ice crystal development. If the super-cooled, freezing temperatures can cool to the dew point (the temperature at which, when cooled to at constant pressure, condensation occurs; moisture will have to come out of the atmosphere as fog, frost, etc) frost could develop on exposed surface.
Local topography has a large role in determining if and where frost develops. Cold air will settle in the valleys since it is heavier than warm air, therefore frost conditions are more prone in these regions. Valleys also shelter the area from stronger winds, enhancing the potential for frost.
So, what's the difference between a frost and freeze?
The main difference between frost and freeze is the temperature at which they occur. Remember, a frost occurs when the air temperature is between 36°F and 32°F and water droplets freezes on surfaces like cars, leaves and the ground. A freeze occurs when the air temperatures falls below 32°F.
Does the National Weather Service alert us when either a frost or freeze will occur?
Yes, the National Weather Service will issue alerts for frost and freeze. They include:
Frost Advisory: Issued when the minimum temperature is expected to be between 33° and 36° Fahrenheit on clear and calm nights during the growing season.
Issued when significant and widespread freezing temperatures are expected in the next 24 to 36 hours.
Freeze Warning: Issued when significant, widespread freezing temperatures are expected
