The atmosphere and the Weather

Precipitation

Rain and Hail

Rain develops when growing cloud droplets become too heavy to remain in the cloud and as a result, fall toward the surface as rain.

Rain can also begin as ice crystals that collect each other to form large snowflakes. As the falling snow passes through the freezing level into warmer air, the flakes melt and collapse into rain drops.

Hail stone measuring 21 centimeters. (Source: NOAA Photo Collection Website).

Hail is a large frozen raindrop produced by intense thunderstorms, where snow and rain can coexist in the central updraft.

As the snowflakes fall, liquid water freezes onto them forming ice pellets that will continue to grow as more and more droplets are accumulated.

Upon reaching the bottom of the cloud, some of the ice pellets are carried by the updraft back up to the top of the storm.

As the ice pellets once again fall through the cloud, another layer of ice is added and the hail stone grows even larger.

Once the hail stone becomes too heavy to be supported by the updraft, it falls out of the cloud toward the surface.

Freezing Rain

Ice storms can be the most devastating of winter weather phenomena and are often the cause of automobile accidents, power outages and personal injury.

Ice storms result from the accumulation of freezing rain, which is rain that becomes supercooled and freezes upon impact with cold surfaces.

The diagram on the left shows a typical temperature profile for freezing rain with the red line indicating the atmosphere's temperature at any given altitude. The vertical line in the center of the diagram is the freezing line. Temperatures to the left of this line are below freezing, while temperatures to the right are above freezing.

Freezing rain on tree branches

Freezing rain on clothes line - Imagine what it will do to your utility cables.

Freezing rain develops as falling snow encounters a layer of warm air deep enough for the snow to completely melt and become rain. As the rain continues to fall, it passes through a thin layer of cold air just above the surface and cools to a temperature below freezing.

However, the drops themselves do not freeze. When the supercooled drops strike the frozen ground (power lines, or tree branches), they instantly freeze, forming a thin film of ice, hence freezing rain.

An intense ice storm can paralyze a region in a matter of hours, greatly affecting the people who live there.

Freezing rain is dangerous because it is almost invisible on smooth surfaces and consequently, people are often unaware of its presence. Sidewalks become extremely slick when covered with freezing rain, increasing the likelihood of someone slipping and injuring themselves. Automobile accidents are more likely to occur during an ice storm because of the icy roads.

Sleet

Sleet is less prevalent than freezing rain and is defined as frozen raindrops that bounce on impact with the ground or other objects.

The diagram on the right shows a typical temperature profile for sleet with the light blue line indicating the atmosphere's temperature at any given altitude.

Sleet is more difficult to forecast than freezing rain because it develops under more specialized atmospheric conditions. It is very similar to freezing rain in that it causes surfaces to become very slick, but is different because its easily visible.

Snow

Close-up photograph of a snowflake. (Source: NOAA Photo Collection Website).

Progressing even further away from the warm front, surface temperatures continue to decrease and the sleet changes over to snow.

Snowflakes are simply aggregates of ice crystals that collect to each other as they fall toward the surface.

The diagram shows a typical temperature profile for snow with the red line indicating the atmosphere's temperature at any given altitude. The vertical line in the center of the diagram is the freezing line. Temperatures to the left of this line are below freezing, while temperatures to the right are above freezing.