Snow, Sleet & Freezing Rain

In short: most winter precipitation starts high in the cloud as snow. What lands depends on the air it falls through. Cold all the way down, it stays snow. A layer above freezing aloft, the warm nose, melts it. Then the cold layer underneath decides. A deep, cold one refreezes the drops into sleet. A shallow one leaves them liquid but below freezing, so they ice over on contact as freezing rain. No cold layer at all means rain. A degree or two decides between a snow day and an ice storm.

How the type is decided

The usual setup is warm, moist air riding up over a wedge of cold air at the ground, ahead of a warm front or a storm moving up the coast. That is why one storm can drop snow in one county, sleet in the next and freezing rain in the one after. On a Skew-T, the temperature line bends right of 0 °C aloft and back left near the ground; see Reading a Skew-T.

The warm nose: how warm is warm enough

How much a warm nose melts depends on its temperature and its depth. WPC’s winter training gives rough numbers using the layer’s peak wet-bulb temperature (see below):

• Under about 1 °C (34 °F): only partial melting; the flakes usually land as snow or refreeze.
• About 1 to 3 °C (34 to 37 °F): partial melting, usually refreezing into sleet, or a sleet and freezing rain mix.
• Above about 3 to 4 °C (37 to 39 °F): the snow melts completely, giving rain or freezing rain.

These are guides, not rules: a thin layer at 3 °C can melt less than a deep one at 2 °C. The Bourgouin method (Bourgouin 2000) handles that by measuring the area between the temperature line and 0 °C on a sounding: a warm area much larger than the cold area under it favors freezing rain, the reverse favors sleet. One more catch: if the whole cloud is warmer than about -10 °C (14 °F), it may hold no ice to start snow, and the result can be freezing drizzle with no warm nose at all.

Snow

Snow needs the column at or near freezing all the way down. Flakes survive a shallow layer just above freezing, and steady snow often falls with the surface at 1 to 2 °C (34 to 36 °F). That marginal air makes the wet, heavy snow that breaks branches.

Sleet

Sleet, or ice pellets, is melted snow that refroze in the cold layer below. The pellets bounce and pile up like coarse sand. How easily the drops refreeze depends on how much they melted. Partly melted flakes still hold some ice and refreeze readily, even in a mild cold layer. Drops that melted completely need much colder air: WPC’s training, after Stewart and King (1987), says they stay supercooled, and land as freezing rain, when the cold layer is warmer than about -8 °C (18 °F), and refreeze reliably only near -10 °C (14 °F) or colder. WPC adds that the cold layer’s temperature matters more than its depth. Sleet makes roads slick but rarely brings down trees, because it does not stick to branches.

Freezing rain

Freezing rain is liquid drops colder than 0 °C. The cold layer is too thin or too mild to freeze them in the air, so they freeze on contact into a clear glaze. Bridges, branches, power lines and cars ice first, often with the thermometer near 0 °C (32 °F) while the ground still looks only wet.

Freezing level, wet-bulb zero and the changeover

The freezing level is the height where the air temperature crosses 0 °C; the wet-bulb zero is where the wet-bulb temperature does. The wet-bulb is the temperature air reaches when water evaporates into it, so in dry air it sits well below the thermometer reading.

That matters once precipitation starts. Evaporating drops and melting flakes both take heat from the air, pulling it down toward the wet-bulb. Rain at 3 °C (37 °F) with a dew point of -5 °C (23 °F) can turn to snow within an hour of the first drops, with no change in the pattern. Heavier bursts cool faster, which is why heavy precipitation can flip rain to snow and a lull can flip it back. Once the air is saturated the evaporative cooling stops. Melting can keep cooling the layer in heavy snow, but after that only colder air moving in pushes the line further.

In MesoRadar: the Skew-T tool lists Freezing level in Moisture and levels, and, under More, Wet-bulb zero in Lapse rates and levels. The National tab's Freezing Level (0°C Height) maps the 0 °C height (Pro).

The rain-snow line on radar

Radar cannot see temperature, but it can see snow melting. Flakes coated in water look like huge raindrops, so reflectivity jumps in the melting layer, then falls as they collapse into smaller drops. On a tilted scan that shows as a ring around the radar, the bright band: rain inside it, snow outside it. Dual-pol makes it clearer:

• Correlation coefficient (CC) dips, often below 0.9, in a noisy ring, because the mix of snow, wet snow and drops is less uniform. NWS training says CC is often the best product for finding the melting layer.
• ZDR rises in the same ring. Reflectivity up, ZDR up and CC down together mean melting, not heavier rain.
• A fainter ring of higher ZDR closer to the radar can mark drops refreezing into sleet below (Kumjian et al. 2013).

The trap: the ring sits where the beam crosses the melting layer, often thousands of feet up, not on the line at the ground. Far from the radar the beam can pass over a low melting layer entirely, and when the 0 °C level slopes or changes fast the ring is too ragged to see (see What Radar Can’t See).

In MesoRadar: Correlation Coefficient and Differential Reflectivity (ZDR) in the Local tab's Radar Product picker. On the National tab, Precipitation Type (rain or snow, from MRMS) and Rain / Snow / Ice (which adds a model-assisted Ice class) are free to view; Pro adds the full Precipitation Type loop. How MRMS draws rain or snow.

Ice storms

An ice storm is freezing rain that lasts long enough to build a thick coat. Radial ice, the thickness around a branch or wire, is what breaks things: the NWS notes a quarter inch can snap branches and bring down power lines, and less is needed in wind. Heavier ice brings widespread outages.

An Ice Storm Warning is issued when ice on a flat surface is forecast to reach the local office’s criterion. The national directive leaves the number to regions and offices; a quarter inch is common, and some offices set a higher one. Some NWS offices have also used the Sperry-Piltz Ice Accumulation Index (SPIA), which combines ice and wind into a 0 to 5 scale of damage to power systems; a 5 means outages that can last weeks.

In MesoRadar: Ice Storm Warning is one of the warning types Pro lets you pick for the places you save. Alerts in the app.

How much snow from how much water

The snow-to-liquid ratio is the inches of snow one inch of melted water makes. 10 to 1 is the old rule of thumb; a 30-year study of NWS cooperative observations found an average nearer 13 to 1 over much of the country, with wide swings (Baxter et al. 2005). Wet snow near freezing can run well under 10 to 1. Cold, dry snow made of branching crystals, which grow best between about -12 and -18 °C (10 and 0 °F) in the cloud, can run 20 to 1 or more, as lake-effect snow often does (see Lake-Effect Snow). Sleet is far denser than snow, so counting it as snow overstates the depth badly.

In MesoRadar: the Snow tab. Each forecast map names how it turns water into inches. The HRRR and ECMWF rows multiply the model's snow water by ten ("10:1"), so they run low in fluffy snow and high in wet snow. The HRRR's snow water leaves out sleet. In sleet or wet snow the ECMWF total can read high. The RRFS rows use the model's own snowfall ("model ratio"). The model sets the ratio from the temperature near the ground, from about 8 to 1 at freezing up to 17 to 1 in colder air. It does not look at the air higher up, where flakes form, so it can miss too. The NBM 10-day row (Pro) is NOAA's National Blend of Models ("blend ratio"): many models, several ratio methods, sleet counted at 2 to 1. All are snowfall, not depth on the ground, and none shows ice from freezing rain. No one method is right in every storm. Snow in the app.

Snow squalls

A snow squall is a short, intense burst of snow and gusty wind, often on a sharp cold front. It usually passes in 30 to 60 minutes and may leave an inch or less, but visibility can drop to near zero in seconds while wet roads flash-freeze. Squalls have caused deadly highway pileups.

The NWS issues a Snow Squall Warning for visibility of a quarter mile or less in snow with roads below freezing, or a sudden temperature drop that can flash-freeze roads with poor visibility. Like a thunderstorm warning, it covers a small area along the squall’s path for 30 to 60 minutes. Warnings tagged SNOW SQUALL IMPACT…SIGNIFICANT set off Wireless Emergency Alerts on phones in the path; untagged ones do not. Delay travel until it passes; if already driving, slow down, turn on headlights and hazard lights, and avoid hard braking.

In MesoRadar: Snow Squall Warning is one of the warnings pushed by default for the places you save, as an urgent alert.

The WPC winter products

The Weather Prediction Center publishes two national winter products worth a look before any storm:

• Winter Storm Severity Index (WSSI): a forecast of impact, not inches, built from snow amount, snow load, ice and blowing snow, for Days 1 to 3. Levels run from a winter weather area through minor, moderate and major to extreme.
• Probabilistic Winter Precipitation Guidance: the chance of reaching set snow and ice amounts over the next three days. A 40% chance of 4 inches says more about risk than one number.

Your local NWS office’s watches, warnings and advisories remain the official word (see Watches, Warnings & Outlooks).

In MesoRadar: the Outlooks sheet's Winter impact row shows the WSSI, Day 1 free and Days 2 and 3 in Pro. Its Chance chips swap in WPC's chance of 4 inches of snow or a quarter inch of ice, on the same Day chips (Day 1 free, Days 2 and 3 in Pro).