A frost advisory is an alert issued by the National Weather Service (NWS) notifying the public of approaching cold weather that poses a threat, primarily to outdoor vegetation. This notification is specific to the growing season when sensitive plants and crops are vulnerable to cold temperatures. The advisory signals that conditions are favorable for frost formation, meaning temperatures are expected to drop near the freezing mark. It prompts people to take precautionary measures to protect items that could be damaged by the cold.
Defining the Conditions
The NWS issues a frost advisory when the minimum air temperature is forecast to be between 33 degrees Fahrenheit (0.5 degrees Celsius) and 36 degrees Fahrenheit (2 degrees Celsius). This temperature range is high enough to prevent a hard freeze but low enough to allow frost to form on surfaces. The meteorological conditions that facilitate frost formation include clear skies and light winds, typically less than five miles per hour. Clear nights allow heat to escape rapidly from the ground into the atmosphere, a process known as radiational cooling.
The frost itself is not frozen rain or dew, but rather water vapor that turns directly into ice crystals upon contact with surfaces that have cooled below the freezing point of water. This process, called deposition, requires calm air so that the coldest air pools close to the ground and objects. Even when the air temperature measured a few feet above the ground is slightly above 32°F, surfaces like grass blades or car windshields can cool more rapidly and drop below freezing, allowing the delicate ice crystals to develop.
Distinguishing Advisories, Watches, and Warnings
The National Weather Service uses a specific hierarchy to communicate the severity and timing of cold weather events. An advisory represents the lowest level of concern, meaning areas of frost are expected, posing a threat to sensitive vegetation. This alert is non-life-threatening and generally indicates localized damage is possible during the growing season. A more serious alert is the Freeze Watch, issued when there is a potential for significant, widespread freezing temperatures (typically 32°F or below) within the next 24 to 36 hours.
A watch indicates conditions are favorable for a freeze, giving the public time to prepare and protect susceptible items. The most urgent alert is the Freeze Warning, which means significant, widespread freezing temperatures of 32°F or lower are imminent or occurring. This condition poses a threat of major damage to plants and could signal the end of the growing season.
Essential Preparation and Safety Measures
When a frost advisory is issued, protecting sensitive vegetation is the primary concern for homeowners and gardeners. Tender plants, such as annuals, herbs, and vegetables like tomatoes, should be brought indoors into a garage or shed if they are in containers. For plants remaining in the ground, covering them loosely with a sheet, blanket, or horticultural cloth can provide an insulating barrier, offering a few degrees of protection. Watering the ground around plants earlier in the evening can also be beneficial, as moist soil holds and releases heat more effectively overnight than dry soil.
Protecting outdoor plumbing and irrigation systems is another important step during an advisory. Hoses should be disconnected from outdoor spigots, and if possible, the spigots should be drained and covered to prevent water inside the pipes from freezing and bursting. Vehicles also require attention, as frost can quickly form on glass surfaces overnight. Covering the windshield with a towel, cardboard, or a commercial frost shield prevents the need to scrape ice in the morning.
People with pets or livestock should ensure their animals have adequate shelter, as even temperatures slightly above freezing can be dangerous, especially for smaller animals. Pet owners should also be mindful of de-icing salts used on walkways, as these chemicals can irritate or burn an animal’s paws.