You opened your weather app this morning, saw 33 degrees beside a small icon of something falling, and reasonably assumed the number would tell you what to expect. That number describes the bottom few feet of the atmosphere, while the decision about what lands on your windshield is being made about a mile above it.
Early November is when this starts to matter. The first strong cold fronts of the season slide beneath air that still carries October's warmth, and that arrangement — cold below, warm above — produces the first mixed-precipitation forecasts of the year.
We think surface temperature is the wrong place to begin reading these forecasts. The better question is what happens to a snowflake on the way down, and that question has a readable answer.
What Decides the Precipitation Type
Snow survives when the whole column stays at or below freezing. Sleet forms when a warm layer melts snow and deep cold air refreezes it. Freezing rain forms when that cold layer is too shallow to refreeze the drops.
In a mid-latitude winter storm, most precipitation begins as ice inside the cloud, regardless of what eventually reaches you. A flake then falls through thousands of feet of air, and each layer it crosses either preserves it, melts it, or refreezes it.
Meteorologists call this vertical record of temperature the profile, and they plot it on a chart known as a sounding. Accordingly, all four winter outcomes are the same falling object with a different journey.
The Warm Nose: The Layer That Makes the Call
A warm nose is a layer of above-freezing air sitting over colder air near the ground. If it peaks near 37°F or higher and runs a few thousand feet deep, falling snow generally melts completely.
On a sounding, this layer bulges to the right of the freezing line, which is where the name comes from. It usually sits somewhere between about 2,500 and 8,000 feet, and the 850-millibar level — roughly 5,000 feet — is the height forecasters check first.
The warm nose arrives through warm advection, as moist southerly air rides up and over a dense wedge of cold air left behind by a front. That said, the air at the ground can sit several degrees below freezing the entire time, entirely unaware of what is passing overhead.
Two properties of the layer matter, and both are worth looking for in a forecast discussion. Keep in mind that a thin layer barely above freezing and a deep layer at 40°F are very different storms, even when both are described as a warm layer aloft:
- Peak temperature. A nose peaking near 33°F or 34°F may only wet the flake, while a peak around 37°F or above tends to melt it through when the layer has some depth.
- Depth. Melting takes time, so a flake crossing 3,000 feet of warm air absorbs far more heat than one crossing 500 feet at the same temperature.
Forecasters combine those two properties into a single estimate of melting energy, and that is why two storms with the same warm-layer peak can deliver different results. Overall, the warmth of the nose determines whether a flake survives, and everything below it determines what the melted drop becomes.
Four Profiles, Four Outcomes
The table below compares the four standard temperature profiles from the cloud to the ground. Read each row from left to right, the way a flake would experience it.
| What lands | Warm layer aloft | Cold layer near ground | What you notice |
|---|---|---|---|
| Snow | None, or very thin | Column stays near or below freezing | Quiet, soft accumulation on every surface |
| Sleet | Shallow to moderate | Deep, often a few thousand feet | Pellets ticking off glass and bouncing on pavement |
| Freezing rain | Deep and well above freezing | Shallow, often under about 1,500 feet | Ordinary-looking rain that glazes whatever it touches |
| Cold rain | Deep, merged with the surface | None | A raw, wet day, with no ice on the ground |
Notice that the warm layer gets more dominant as you move down the table, while the cold layer retreats. As a result, the order on the map during a storm tends to run from snow on the cold side through sleet and freezing rain to plain rain on the warm side.
Why Cold-Layer Depth Separates Sleet From Freezing Rain
Sleet freezes before it lands and bounces as ice pellets. Freezing rain lands as liquid and freezes on contact, glazing every cold surface. The difference is the depth of the cold air beneath the warm layer.
A drop leaving the warm nose needs time in cold air to freeze. A partly melted flake keeps an ice core and refreezes readily, while a fully melted drop is harder to freeze and needs a deeper, colder layer to finish the job.
If the cold layer is too shallow, the drop reaches the ground still liquid but chilled below 32°F, a condition called supercooled. It then freezes the moment it touches a road, a branch, or a power line.
This is why freezing rain is such a narrow and treacherous outcome. A few hundred feet of cold-layer depth can separate pellets that skitter harmlessly across a sidewalk from a glaze you cannot stand on.
A worked profile. Consider an illustrative sounding: 30°F at the surface, 29°F at 1,500 feet, 39°F at 4,500 feet, and 20°F at 9,000 feet.
The flake starts frozen, melts through a warm nose peaking well above 37°F, and then has only about 1,500 feet of cold air left to refreeze. Expect freezing rain, even though the same 30°F reading in a fully cold column would bring snow.
Why the Number on Your Phone Misleads
Snow can reach the ground at 36°F to 38°F when the air is dry, because evaporation cools it toward the wet-bulb temperature. Freezing rain can fall at 31°F. Surface temperature alone cannot settle the question.
The thermometer reading at street level is taken about six feet off the ground, and it tells you nothing about the layers above. Moreover, it can change during the storm itself, as falling precipitation reshapes the air it passes through.
Two cooling processes do most of that work, and both explain forecasts that seem to flip within an hour. Here's how each one fits into the profile:
- Evaporative cooling. When precipitation falls into dry air, some of it evaporates and pulls heat from its surroundings, dropping the temperature toward the wet-bulb value.
- Melting cooling. Snow melting in a warm layer absorbs heat from that layer, which can erode a marginal warm nose and turn a rain forecast back to snow during heavy precipitation.
This is why the dew point deserves a look alongside the temperature, much as it does when you are reading a frost forecast. A 37°F afternoon with a dew point of 22°F has far more cooling available than a 37°F afternoon with a dew point of 35°F.
How to Read the Profile Yourself
Open the local Area Forecast Discussion, find the words warm nose or warm layer, and note the timing of the changeover. A forecast sounding then shows how deep the above-freezing layer runs.
You do not need a meteorology degree to do this, though you do need to look past the icon. The National Weather Service publishes everything required, and a few minutes of reading will tell you more than any summary symbol:
- Start with the discussion. Your local forecast office's Area Forecast Discussion names the warm nose, its expected height, and how confident the forecaster feels about the changeover.
- Check the freezing level. A freezing level that sits at several thousand feet while the surface is below 32°F signals an inversion, which is the shape that creates mixed precipitation.
- Look at a forecast sounding. Public model viewers plot predicted soundings, and any stretch of the temperature line to the right of the 0°C line is your warm layer.
- Compare two or three models. Different forecast models handle shallow cold air differently, so their disagreement is useful information.
- Watch the radar's melting layer. A bright ring or band of high reflectivity often marks where flakes are melting, which is one reason reading weather radar takes extra care in a mixed storm.
Remember that the radar sees what is falling aloft, and that weather balloons are launched only twice a day at around 90 U.S. sites. All of the above means the profile between those launches is a model estimate, so treat it with the same discipline you would bring to forecast confidence in general.
Where These Forecasts Go Wrong
Mixed-precipitation forecasts fail in recognizable ways, which is useful to know before you commit to a plan. Shallow cold air is the most frequent culprit, because a dense wedge only 1,000 feet deep is difficult for a coarse model grid to capture.
East of the Appalachians, this appears as cold-air damming, in which cold air banks against the mountains and resists the warm flow trying to push it out. Similarly, the Columbia River Gorge can funnel cold air westward beneath a warm storm, and the ice can linger after nearby areas have changed to rain.
For a storm like this, we would favor the high-resolution regional models over the global ones for the final 24 hours. That said, a 1°F error in a marginal profile can move the freezing rain band by many miles, so the location of the line carries more doubt than the existence of the storm.
This is also where a percentage does less work than readers expect. A high chance of precipitation tells you something will fall, yet it says nothing at all about which of the four outcomes you will receive.
What Each Type Asks You to Decide
The National Weather Service generally issues an Ice Storm Warning at a quarter inch or more of ice. A half inch can add as much as 500 pounds to a span of power line.
Translation is the point of reading any of this, so here is what each outcome changes about your day. Before you assume the worst or the best, match the forecast to the decision in front of you:
- Snow. Plan around depth and timing, and take a look at snow photography if the light cooperates afterward.
- Sleet. Wear boots with real tread and expect a dense, sand-like layer that is heavy to shovel but offers some grip for tires.
- Freezing rain. Delay the drive if you can, charge your devices, and keep clear of tree limbs and lines once the accumulation passes a quarter inch.
- Cold rain. This calls for a technical shell, wool socks, and little else — a miserable day, though a safe one.
Of course, November adds one local wrinkle, because the ground has not yet lost its summer heat. Pavement may stay merely wet while railings, windshields, and overpasses glaze, which makes the road ahead look safer than it is.
When we write a daily brief for a day like this, the warm layer is the story. Our rule is simple: if the discussion mentions a warm nose and your surface reading is at or below 33°F, leave early, or do not leave at all.
Common Questions
Why do bridges ice over before the road does?
Air flows above and below a bridge deck, so it loses heat from both sides, while a road sits on ground that still holds warmth. In early November that stored heat can keep pavement wet while overpasses, railings, and parked cars glaze over during the same hour.
Does sleet mean the same thing in the United Kingdom?
No. American forecasters use sleet for ice pellets, the small frozen grains that bounce and report as PL in airport observations. In British usage, sleet means a mix of rain and melting snow, which is why a London forecast and a Chicago forecast describe different things with one word.
How much does sleet accumulate compared with snow?
Far less. An inch of liquid might produce ten inches of snow but only about three inches of sleet, because dense pellets pack tightly. A forecast of heavy precipitation that turns to sleet therefore lands well under its snow total, though the pack is hard to shovel.
Why do models lose track of shallow cold air east of the mountains?
Shallow cold air is dense and hard for a coarse model grid to resolve, so global guidance can erode it too quickly. East of the Appalachians, cold air wedged against the mountains often holds hours longer than forecast, which extends the icing well past the predicted changeover.
What is the 540 line, and can I trust it?
It marks where the 1,000–500 millibar layer is 5,400 meters thick, a rough rain-snow boundary from the era of paper charts. It averages temperature through the whole column, so it can hide the shallow warm layer that creates sleet and freezing rain. Treat it as a first glance.