Radiational Frost: Why a Surface Freezes Above 32

Why this matters

The forecast low is 37 F, so nobody expects ice. At 6:30 am the truck windshield is white, the ladder rungs are slick and the low-slope roof the crew is about to walk is glazed with frost. The night before, a coating crew finished a roof at 3 pm with the low forecast at 46 F, comfortably above the data sheet's minimum, and by morning the coating has dew sitting in it. In both cases the forecast was right about the air. It said nothing about the surfaces, and on a clear, calm night the surfaces are the coldest thing outside.

The mechanism

Every surface gives off heat as infrared radiation, and receives some back from whatever it faces. A roof deck facing a clear night sky faces something very cold: the clear sky returns far less infrared than the surface sends up, so the surface loses heat steadily all night. Its effective temperature, the temperature a surface would need to be to radiate what the clear sky radiates back, is well below the air temperature, often tens of degrees below.

Cloud cover shuts that off. Clouds are much warmer than the clear sky and radiate heat back down to the surface. Under overcast, a roof deck stays within a degree or two of the air.

Wind limits it. On a calm night, the air touching the cold surface cools and stays put, so the surface and the thin layer of air on it keep chilling together. A breeze keeps mixing warmer air down onto the surface and holds its temperature closer to the air reading.

The dew point is a brake. When the surface falls to the dew point, water condenses on it, and condensation releases heat into the surface. That slows the fall, which is why a humid night produces heavy dew and a dry night produces a surface that falls much further. If the surface reaches 32 F and keeps losing heat, the condensed water freezes or frost grows directly as ice crystals.

The air reading is not where the surface is. Forecast and station temperatures are air measured at about head height, in shade. On a calm clear night the air near the ground cools faster than the air at that height, so even the air at your boots is colder than the reading, and the surface below it is colder still. A difference of several degrees between the official air temperature and a roof or car surface at dawn is ordinary; an insulated roof can run further.

Which surfaces chill first

The surfaces that drop furthest below the air share three traits: they see a lot of open sky, they hold little heat, and they are cut off from any heat below them.

• Low-slope roofs over insulation. The insulation is doing its job, keeping the building's heat from reaching the membrane, so the membrane has nothing to replace what it radiates away. These are among the coldest surfaces on a clear night, and the first to frost.
• Car and truck roofs, windshields and ladders on a rack. Thin, light, facing straight up. If the truck roof is frosted, assume the roof deck is too.
• Metal roofing and sheet metal. Very little heat stored; it follows the sky closely.
• Deck boards, stair treads and wood walkways on open framing. Air below them is as cold as air above, and wood stores little heat. A deck can frost while the driveway beside it stays wet.
• Fresh thin coatings, stain and sealant. A wet film on any of the surfaces above takes the substrate's temperature and is the first place dew lands.

The surfaces that stay warmer: masonry walls and slabs on the ground, which store the day's heat and release it overnight; vertical walls, which see only half the sky; and anything under eaves, porch roofs, trees or a carport, which see a ceiling instead of the sky.

 CLEAR, CALM NIGHT             OVERCAST NIGHT
   open sky, very cold           cloud base
     ^     ^     ^                 v     v     v
     |     |     |  heat out       |     |     |  heat back

 air 37 F at head height        air 37 F at head height
 ====================           ====================
  roof membrane 29 F             roof membrane 36 F
 --------------------           --------------------
  insulation (blocks heat        insulation
  from below)
 warm building                  warm building
 (readings illustrative, same air temperature both nights)

The air reading is the same on both nights. The surface difference, 29 F against 36 F, comes entirely from what the roof faces.

Why the forecast low misleads two kinds of job

Early starts. A forecast low of 37 F suggests a wet morning at worst. On a clear, calm night with a dew point a few degrees below that, the exposed surfaces may have crossed 32 F hours earlier. The crew arriving at 7 am reads the weather from the cab thermometer and walks onto frost. The check is the surface itself: an infrared thermometer pointed at the roof, the deck or the ladder rungs before anyone climbs, plus the cheapest gauge there is, the frost on the truck.

Overnight cure. A coating, stain or sealant applied in the afternoon spends its first night on a surface that cools below the air. A data sheet minimum temperature that the forecast low clears can still be violated at the surface, and the dew point that the air never reached can be reached on the film. Dew on an uncured water-based coating can leach surfactants and leave a white or sticky surface; dew on a moisture-sensitive coating can blush or stay soft. The same coating on the same night under cloud would have been fine. The coating rules themselves belong to The Surface Five Degrees Above Dew Point Rule for Coatings and Humidity and Cure: Moisture-Cure, Evaporative and Reactive.

Worked example: two jobs, one clear night

The night forecast for a Thursday in late October reads: clear skies, calm wind, low 37 F at 6 am, dew point 33 F through the night.

The early start. A crew is scheduled onto a low-slope commercial roof over rigid insulation at 7 am. The crew lead arrives at 6:30 am and takes readings before access:

The membrane is 8 F under the air (37 minus 29) and 3 F under freezing, with frost visible on it. The forecast low was 5 F above freezing (37 minus 32), and the surface had crossed 32 F anyway. The crew holds access until the sun reaches the roof and the membrane reads above freezing and visibly dry, which on this east-facing roof takes until about 9 am, and starts with the interior work on the ticket in the meantime.

The afternoon coating, the day before. On Wednesday, a different crew had been offered the same roof for an elastomeric coating. Say the coating's data sheet reads: apply when air and surface are 50 F or above and rising, the surface at least 5 F above the dew point, and no dew or rain before the film is dry to the touch, which the sheet puts at several hours in cool weather. The Wednesday forecast had an afternoon high of 58 F and the same clear, calm night, low 37 F and dew point 33 F.

At 2 pm, the start conditions pass: air 56 F, surface in the sun 64 F, dew point 33 F, a spread of 31 F at the surface (64 minus 33). The problem is the night. After sunset the membrane falls below the air as fast as it did on Thursday morning. If it reaches 29 F at dawn, it passed the 33 F dew point some hours earlier, well before the low, and dew then frost formed on a film that had been applied only a few hours before sunset. The crew lead turns the job down for Wednesday and books it for a day with an overcast forecast night, or for a day when the coating can finish by late morning and have the whole afternoon to dry.

What would have changed both calls. An overcast forecast with a light breeze. The same 37 F air with clouds keeps the membrane within a degree or two of the air, about 36 F on the diagram above, which is 3 F above the 33 F dew point: no frost at 6:30, no dew on the coating, and both crews work.

Planning around it before you have readings

The board for tomorrow gets set before anyone can point a thermometer at tomorrow's roof, so the shop needs a default. A reasonable starting assumption for a forecast night that is clear and calm: exposed, sky-facing surfaces (roof membranes, decks, metal, vehicles) run 5 to 10 F below the forecast air temperature at dawn, toward the colder end on insulated roofs. Under overcast or with a steady breeze, assume a degree or two. Then replace the assumption with your own readings: a crew lead who logs the dawn surface readings on a few clear mornings at the shop's usual job types will know the local number within a season.

Two consequences follow from the default. A forecast low within 10 F above freezing on a clear calm night is a possible frost morning for exposed surfaces, whatever the icon shows. And the surface reaches the dew point well before dawn, often in the evening, so a coating's critical hours are the first ones after sunset, not the forecast low at 6 am.

Reading a surface correctly

An infrared thermometer reads what a surface radiates, so it depends on the surface's emissivity. Membranes, shingles, painted surfaces and wood read well at the common default setting. Bare, shiny metal does not: it reflects the cold sky into the reading and can show a number that is wrong by a wide margin. On shiny metal, read a painted or taped patch, or use a contact thermometer. Read the surfaces that face the sky, not the wall beside the hatch, and read before sunrise if the decision is whether to climb, because the first sun on one slope tells you nothing about the slope in shade.

References

• National Weather Service guidance on frost and freeze conditions and on air temperature measurement height
• Product data sheets for application, dew point and dry-to-touch limits
• See related: What Dew Point Predicts
• See related: Snow and Frost on a Roof Before Anyone Climbs It
• See related: The Deck Stain That Turned White Overnight