The Difference between Sleet and Rain

It may be surprising to some, but there really isn’t a lot of difference between sleet and rain, except important one. That one difference is well worth looking at in more detail, especially with the confusion about precipitation terms.

In clouds, precipitation gradually builds up around particles, such as of dust. Often, the droplets also merge. When they become heavy enough, they fall. This is a basic condensed version of a much more complex happening (no pun intended), but it should serve our needs here.

As the droplets are jostling around and merging, they are also losing heat. Especially at higher altitudes where the air is quite cold, this means that the temperature of the droplets can be below the freezing point of water though they remain in a liquid state.

If the layers of air they fall through are relatively warm, the falling drops of super cooled water gradually heat up on the way down, finally hitting the surface as rain. However, if the intervening layers of air are cold, the liquid drops can freeze, landing as small frozen ice pellets or sleet.

The basic difference then is that rain makes it to the surface in a fluid state, while sleet is frozen droplets. This is where it becomes more complex and interesting, however.

Under special circumstances, the super cooled droplets fall through cold layers of air without freezing, if they can pick up just enough heat to prevent it. It is still rain, but is below the freezing temperature of water. If the temperature near the surface is also below the freezing point, something unusual happens when the super cooled raindrops land. They instantly freeze on anything they strike.

Since they strike the ground or objects in a fluid state, this is rain rather than sleet, however the fact that they immediately turn to ice gives this form of precipitation the special term, “Freezing rain”. Freezing rain is a dangerous phenomenon, in part because it coats everything it hits, and partly because the ice builds up so fast. Ice has a great deal of weight, so as it coats power lines for instance, they can snap under the weight of the ice. A roof can collapse for the same reason.

It is unfortunate that many weathermen persist in sometimes referring to sleet as freezing rain. In a way, this can be understandable since the rain is technically freezing on the way down. However, freezing rain and sleet are two different forms of precipitation. Again, one strikes in a fluid form while the other strikes in a solid form. This means that the complex part is what happens immediately after it strikes the surface, and in what form it is.

Sleet will usually bounce off anything it hits, rather like hail except in much smaller form. Rain hits and coats what it hits in water. If the conditions are right, it will freeze solid. It doesn’t bounce off objects.

Note that anyone who has been in a major rain storm will probably have seen what appears to be the raindrops bouncing off the water in puddles and so forth. This is an illusion. The raindrops are actually hitting with such force they cause the water they strike to splash. A raindrop striking a lake as an example is well below the surface of the water before it slows down. To the observer, though, it may appear that it bounces back into the air because of the splash that occurs as water is displaced and rebounds.

All forms of precipitation are similar in some ways, and dissimilar in others. This is true of all forms of rain, and sleet. Few people would have trouble telling the difference between regular rain and sleet. Freezing rain, however, is not the same thing as sleet and the terms should not be used interchangeably as they often are.