Types of Winter Precipitation

Winter storms often produce a wide variety of precipitation types throughout New York and New England. It is a relief to a meteorologist when a storm is all rain or all snow as the forecast process is somewhat simplified. But those types of storms occur less frequently than you might think. Ultimately it is the temperature profile from the ground up to the layer where the precipitation is forming that is the key to what type eventually falls to the ground. Large winter storms are in part driven by dramatically contrasting air masses coming together. The circulation around classic winter cyclones forces air of differing temperature around the storm center at all levels of the atmosphere which is why a storm, depending on track and intensity, may produce any combination of rain, freezing rain, sleet, and snow over a particular region. Of course, it's determining how much of each type of precipitation will fall and over what regions that makes forecasting the weather in upstate New York and western New England very challenging during the winter. This page will outline the most common types of winter precipitation and briefly describe some of the mechanisms responsible for producing them.

Snow:

Snow is precipitation in the form of solid white or translucent ice crystals mainly branched in a hexagonal form. Most precipitation forms as snow. If the vertical temperature structure of the atmosphere is such that the temperature is below thirty two degrees Fahrenheit (the freezing point) from the top of the troposphere down to the ground, then snow will fall to the surface. Snow can also fall when the temperature adjacent to the ground is above freezing. In this case, the layer of above freezing air close to the ground is to shallow to melt the snow into rain drops before the snow reaches the ground. Occasionally during the spring and autumn snow will reach the ground when the air temperatures at the surface are in the mid to upper 30s.

Giant Snow Flakes:

Giant snowflakes can form in what are called isothermal environments where the temperature profile from the ground up through a deep layer is right about at the freezing point. In this case individual snowflakes become very wet and sticky and as a result coalesce to form very large flakes sometimes reaching the size of silver dollars or slightly larger.

Snowfall Rate Classification (Manual):

Snowfall intensity is determined manually by an observer using the visibility:

Very Light (Flurries): Scattered flakes do not completely wet or cover the ground.

Light Snow: When the visibility is 5/8 of a statute mile (1.0 km) or more.

Moderate Snow: When the visibility is less than or equal to 5/8 of a statute mile but not more than 1/4 of a statute mile (0.5 km).Heavy:

When the visibility is less than 1/4 of a statute mile (0.5 km).

Snow Showers:

Snow showers are bursts of snow affecting localized rather than widespread areas. The snow can fall at various intensities accumulating as little as a coating to as much as several inches in short time periods.

Snow Squalls:

Snow squalls are simply a variation on the more generic snow shower. Snow squalls, however, are generally rather intense, but still only affect localized rather than widespread areas. Snow squalls are typically accompanied by a period of strong wind gusts blowing and drifting the snow. Accumulations in a snow squall may be significant but generally quite localized.

Sleet or Ice Pellets:

Sleet is a type of solid precipitation consisting of transparent or translucent pellets or balls of ice generally on the order of 5 mm in diameter or less. The shape may vary, but usually sleet is spherical. Sleet is formed when snow falls into a fairly deep layer of above freezing air causing it to melt into rain drops. The rain drops then fall into a moderately deep layer of subfreezing air which may extend to the ground. The rain drops then freeze into ice pellets which bounce off of surfaces on contact. Sleet generally does not stick to surfaces but will accumulate on the ground. Sleet is often mistaken for hail. Hail, however, must be 5mm in diameter or larger and is formed by warm season convective processes in thunderstorms which is very different from the process that generates sleet.

Freezing Rain:

Freezing rain is often confused with or used incorrectly as a synonym for sleet when in reality it is quite different. Simply, freezing rain is rain that falls onto a surface that is below thirty two degrees Fahrenheit, thus immediately freezing into a glaze on contact. Freezing rain occurs when only a very shallow layer of sub-freezing air exists adjacent to the ground. The vertical temperature profile in the atmosphere above the ground through a very deep layer must be above the freezing point to allow liquid drops to reach the subfreezing surface. Freezing

rain is nasty stuff. If air temperatures at the ground do not warm to above the freezing mark and a storm is able to generate hours of rain, then significant ice accumulations can occur bringing down trees and power lines. In general, when ice accumulations are expected to reach a half an inch or greater, the storm is considered a significant ice storm.

Snow Pellets or Graupel:

Graupel is a form of solid precipitation consisting of white, opaque, nearly round ice particles having a structure similar to that of snow. Graupel ranges in size from 2 to 5 mm in diameter. In most cases graupel falls in a shower, often before and sometimes together with snow and mainly when the surface temperatures are at or slightly below freezing. Graupel is formed as a result of snow falling though a layer of supercooled water droplets (very small

liquid cloud droplets that remain in a liquid form in a subfreezing environment) Supercooled cloud droplets that contact snow will immediately freeze or accrete on the snow crystal's surface, a process called riming. Eventually a rimed snow crystal will morph into graupel providing enough liquid accretes on the surface of the flake.

Snow Grains:

Snow grains are a form of solid precipitation consisting of very small, white, opaque particles of ice. Snow grains are essentially the solid equivalent of drizzle and resemble snow pellets in appearance but generally have diameters less than 1 mm. Snow grains never fall in the form of a shower like snow pellets do, but instead fall in small quantities from stratus clouds or from fog.