Titan U glossary

Precipitation Type

Precipitation type is the form precipitation takes when it reaches the ground, such as rain, snow, sleet (ice pellets), freezing rain, or a mix of these. In winter it depends mostly on the temperature of every layer the precipitation falls through on the way down, not only on the temperature at the surface.

Storm Forecasting Storms in the Field Winter Precipitation

Following a snowflake down

Most winter precipitation starts as ice crystals in cold air aloft. If the air stays at or below 32°F (0°C) from cloud base to the ground, the crystals arrive as snow.

If the flakes fall into a layer warmer than freezing, often called a warm layer, they start to melt. How deep and how warm that layer is decides whether they melt partly or completely.

What happens near the ground

Flakes that only partly melt can refreeze in a deep layer of below-freezing air near the surface and land as sleet, pellets of ice that bounce. Flakes that melt completely and then meet only a thin below-freezing layer stay liquid and freeze on contact as freezing rain.

If the air near the ground is above freezing, the result is usually rain, though snow can still reach the ground when it is not too warm. Ice can also be missing from the start: when a cloud is between about 32°F and 16°F (0°C and -9°C), it will likely hold supercooled liquid rather than ice, so whatever falls begins as liquid.

How forecasters judge it

Forecasters read the depth and temperature of the warm and cold layers on a sounding, and which one sits on top. When the air is not saturated they use the wet-bulb temperature, because precipitation falling into dry air cools it by evaporation.

Layer thickness is a fast first guess at type, but Weather Prediction Center guidance notes that thickness methods can miss a very thin warm layer.

What the type alone does not tell you

The ground has a say too. If the ground is warmer than freezing while the air is colder, heat from the ground may keep freezing rain from icing streets.

Type can also change during a storm. Melting pulls heat from the air, and that cooling is strongest where precipitation is heaviest, so a heavy burst can shift rain toward snow.

The freezing level and wet-bulb zero mark where melting begins aloft, and on radar the bright band shows the melting layer itself.

Keep going

How Precipitation Type connects

Builds on 2

Melting Layer
Precipitation type builds on the melting layer, since its height and depth help decide whether snow arrives as snow, rain, sleet or freezing rain.
Wet-Bulb Temperature
Forecasters judge precipitation type with the wet-bulb temperature when air is unsaturated, because falling precipitation cools dry air by evaporation.

Leads to 2

Wintry Mix
Wintry mix is everyday shorthand for several precipitation types at once, so reading it builds on knowing snow, sleet, freezing rain and freezing drizzle.
Snow-to-Liquid Ratio
A snow-to-liquid ratio only applies once precipitation type and the rain-snow line say the precipitation will actually fall as snow.

Includes 2

Freezing Rain
Freezing rain is one of the precipitation types forecasters sort winter precipitation into, along with rain, snow and sleet.
Sleet
Sleet, meaning ice pellets in U.S. usage, is one of the precipitation types winter precipitation is sorted into.

Contrasts with 1

Liquid Equivalent
Liquid equivalent gives the amount of water that fell, while precipitation type gives its form, and the same liquid can arrive as rain, snow or ice.

Measured by 3

Hydrometeor Classification Algorithm
The Hydrometeor Classification Algorithm offers a first guess at precipitation type, though it works at beam height and cannot tell freezing rain from rain.
Radar Precipitation Type
Radar precipitation type estimates whether rain, snow or hail is falling, often with help from model temperatures, and ground reports remain the check on it.
Atmospheric Thickness
Layer thickness gives a fast first guess at precipitation type, though thickness methods can miss a very thin warm layer.

Seen in 1

Atmospheric Sounding
Precipitation type is read on a sounding, where forecasters check the depth and temperature of the warm and cold layers and which sits on top.