Radiosonde observations provide the condition of the atmosphere above the launch site (typically within 25 miles/40 km) at the time of launch. While they do not provide any direct forecast information, they do help explain why we experience different types of weather. The following sample soundings are typical for different weather conditions.
Snow
The atmosphere is very moist, as indicated by the small amount of separation between the air temperature (red line) and the dew point (blue line). Even though the air temperature increases a few hundred feet above the ground (a temperature inversion), the air temperature throughout the entire atmosphere remains below freezing.
Therefore, precipitation will be in the form of snow and will remain frozen as snowflakes reaching the ground.
Ice pellets (Sleet)
As with the previous sounding, the atmosphere is very moist, so much so that the air temperature and dew point are the same from near 900 millibars (3,000 ft. / 1,000 m) to a little above 700 millibars (10,000 ft. / 3,000 m).
At the surface, an arctic cold front had moved south of the observation station with an air temperature well below freezing. The air temperature begins to decrease with height (which is normal), dropping from 23°F to 12°F (-5°C to -11°C).
However, the density of the arctic air is such that it lays close to the ground with its vertical extent fairly small, in this case only about 3,000 feet (1,000 meters) deep. Above 900 millibars (3,000 ft. / 1,000 m), the air becomes considerably warmer. This area is called an inversion, where temperature change with height is inverted as it increases with height instead of the typical decrease. This type of inversion is often also referred as a "warm nose".
Eventually, the temperature of the atmosphere will return to the typical decrease with height (near 800 mb) and will continue to cool until it falls to below freezing again (about 720 mb).
While there may be some precipitation forming as rain in the warm nose region where the air temperature is above freezing, the vast majority of precipitation will form as snow in the colder below-freezing air above the inversion.
As snow falls into the warm nose, it melts into a liquid drop/rain. The liquid drops then pass into the arctic air mass (near the ground) that is cold enough and deep enough for the liquid to freeze into ice pellets before reaching the ground.
Freezing Rain
The basic pattern for freezing rain is similar to ice pellets. The main difference is that the sub-freezing air near the surface is very shallow and/or the warm nose is large, and thus the melted snow does not have sufficient time to freeze into ice pellets before it reaches the ground. Rain drops may become supercooled, where the temperature of the water falls below freezing, but the drop has not yet frozen solid.
As a result, precipitation falls as rain but freezes upon contact with an elevated surface such as a tree, power line, automobile, or bridge.
These elevated surfaces may be capable of accumulating ice as soon as the air temperature falls below 32°F (0°C). For road surfaces in contact with the ground, they usually begin to ice when the air temperature falls to 28°F (-2°C).
Of all winter weather situations, freezing rain causes the most havoc. There are more car accidents, injuries, and deaths from freezing rain than in any other type of winter weather.
Hurricane
Inside hurricanes, the velocity of the air helps keep the air mixed. Therefore, other than the normal decrease with height, variations in temperature (and dew point) are fairly minimal.
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