When temperatures drop unexpectedly, meteorological services issue alerts to protect people, property, and agricultural resources from cold-weather damage. These alerts communicate the severity and timing of cold air events, allowing communities to prepare. Understanding the exact temperature thresholds and terminology used in these forecasts is necessary for taking appropriate action.
Understanding Freeze Warning Temperatures
A Freeze Warning is typically issued when temperatures are expected to drop to or below 32°F (0°C) over a widespread area for a sustained period of time. This threshold is significant because it is the point at which water freezes, causing damage to sensitive vegetation and potentially threatening infrastructure.
The impact of the cold is categorized further by temperature, distinguishing between a “light freeze” and a “hard freeze.” A light freeze occurs when temperatures drop to 32°F or slightly below, which can kill tender plants and flowers. A hard freeze is triggered when the air temperature is forecast to fall to 28°F (-2.2°C) or lower for several hours. This lower temperature is sufficient to kill most types of commercial crops and residential plants. These alerts are based on the air temperature measured four to five feet above the ground, the standard height for meteorological readings.
Distinguishing Warnings From Advisories
The terminology used in cold-weather alerts indicates both the severity and the expected extent of the event. A Freeze Warning signifies that sub-freezing temperatures are imminent or highly likely across a broad region, requiring immediate protective action. This warning addresses the danger of widespread freezing of water within plant cells and exposed plumbing.
A Frost Advisory, by contrast, is issued for less severe conditions, usually when temperatures are expected to range from 33°F to 36°F (0.5°C to 2°C). Frost can still form at these temperatures, especially on clear, calm nights, because the surface temperature of objects can fall below the air temperature. The advisory suggests that only sensitive vegetation is at risk from the patchy formation of ice crystals.
The Science of Radiative Cooling
Surface temperatures can reach the freezing point even when the air temperature measured by official instruments remains slightly above 32°F. This phenomenon is explained by radiative cooling, the process by which objects lose heat to the atmosphere and outer space through thermal radiation. On nights with clear skies and little wind, objects on the ground, such as grass, car windshields, and pavement, radiate heat upward faster than the surrounding air can replenish it.
This imbalance causes the temperature of the object’s surface to drop below the air temperature measured a few feet above the ground, creating a temperature inversion. The surface temperature may reach the dew point, the temperature at which the air becomes saturated with water vapor. If the dew point is below freezing and the surface temperature reaches it, moisture deposits as frost or ice directly onto the cold surface. Clouds prevent this rapid cooling by absorbing upward radiation and re-radiating it back down, meaning radiative cooling is most effective under clear, dry conditions.
Immediate Preparation Steps
When a Freeze Warning is issued, immediate action is necessary to safeguard property and prevent costly damage. Protecting plants should be a priority, which involves moving any potted or container plants indoors to a sheltered location. For sensitive plants rooted in the ground, covering them with blankets, towels, or burlap provides a layer of insulation against heat loss.
Attention must also be given to preventing damage to exposed plumbing, as the expansion of freezing water can burst pipes. When preparing for a freeze, focus on the following actions:
- Insulate outdoor faucets and exposed pipes with foam pipe insulation or rags.
- Disconnect and drain all garden hoses completely.
- Allow indoor faucets to drip slowly to relieve pressure in the pipes, which helps prevent ruptures.
- Ensure the welfare of animals by providing pets and livestock with warm, dry shelter and a source of non-frozen drinking water.