Rain, Snow, Sleet and Freezing Rain: What Determines What Falls From The Sky
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Rain, snow, sleet and freezing rain can all begin in surprisingly similar ways. What determines what eventually reaches the ground has less to do with the temperature where you are standing than with the temperatures that precipitation encounters on its entire trip through the atmosphere.
A winter day can be below freezing at ground level and still bring rain. Snow can fall when the thermometer reads above 32°F. And sometimes a snowflake melts on the way down, freezes again, and arrives as a tiny pellet of ice.
The key is what meteorologists call the temperature profile of the atmosphere — essentially, a vertical map of temperatures between the clouds and the ground.
Most Precipitation Starts High In The Clouds
Clouds are made of tiny water droplets, ice crystals or a mixture of both. In many clouds, especially those high enough to extend into very cold air, precipitation begins as ice crystals.
Those crystals grow as water vapor freezes onto them and as they collide and combine with other crystals. Eventually they become heavy enough to fall. What happens next depends on the air below.
A snowflake may remain frozen all the way to the ground. It may pass through warmer air and melt into a raindrop. Or it may melt and then encounter another layer of cold air before it reaches us.
That journey produces many of the different forms of precipitation we see at the surface. And there's more happening along the way than you might expect: here's what can happen to a raindrop as it falls from a cloud toward the ground.
Why It Rains
Rain reaches the ground when liquid drops remain liquid during the final part of their fall. Sometimes precipitation begins as liquid water within a warm cloud. But much of the rain we experience actually starts higher in the atmosphere as snow or ice.
If a falling snowflake encounters a sufficiently deep layer of air above freezing, it melts. From that point on, if temperatures remain warm enough, it reaches the ground as rain.
This is why looking only at the temperature outside doesn't necessarily tell you what will fall from the sky. The air several thousand feet above you may be considerably warmer or colder than the air at ground level.
Why It Snows
Snow reaches the ground when ice crystals and snowflakes remain frozen for most or all of their journey downward. That doesn't mean the temperature at the surface must always be below 32°F.
Snow can fall when surface temperatures are a few degrees above freezing, particularly when the layer of warmer air near the ground is shallow. A snowflake takes time to melt, and if it doesn't spend long enough in warm air, at least some of it can survive the trip.
Whether snow accumulates is another question. Warm pavement, soil and other surfaces may melt snow even while flakes continue to fall.
That is why you can sometimes watch large snowflakes falling on a 35°F afternoon while the roads remain wet.
Sleet: Snow That Melts And Freezes Again
Sleet — called ice pellets by meteorologists — requires a more complicated trip. It typically begins as snow high in the atmosphere. The snow then falls through a layer of warmer air and partially or completely melts. Below that warm layer is a deeper layer of subfreezing air.
The melted drops have enough time in this cold layer to freeze again before reaching the ground. Instead of snowflakes or raindrops, they arrive as small, hard pellets of ice.
Sleet often makes a distinctive tapping sound against windows, roofs and cars. Unlike freezing rain, it is already frozen before it lands.
That difference is important.
Freezing Rain: Liquid Water That Freezes When It Lands
Freezing rain begins much like sleet. Snow falls into a layer of warmer air and melts into rain. But this time, the layer of freezing air near the ground is relatively shallow. The drops don't have enough time to freeze before they reach the surface.
Instead, they become supercooled — liquid water that remains liquid even though its temperature has fallen below the normal freezing point. When those drops strike a cold road, sidewalk, tree branch, power line or other surface, they freeze on contact. The result can be a smooth coating of ice, sometimes called glaze.
A relatively small amount of freezing rain can cause serious problems because the ice accumulates directly on surfaces. Roads become slippery, and layers of ice can add substantial weight to trees and utility lines.
The Difference Between Snow, Sleet And Freezing Rain Is Often Just A Layer Of Air
Imagine looking at a vertical slice through the atmosphere during a winter storm. Snow reaches the ground when temperatures remain below freezing through most or all of its journey. A deep layer of warmer air melts falling snow into rain. When those melted drops then travel through a substantial layer of freezing air near the ground, they can refreeze into sleet. But when that cold layer is very shallow, the drops remain liquid until they strike a frozen surface, where they freeze on contact as freezing rain.
Small changes in the depth or temperature of those layers can change the type of precipitation falling at a particular location.
That's one reason winter storms can be so difficult to forecast precisely. A shift of only a few degrees at a particular altitude can mean the difference between several inches of snow, a period of sleet or an ice storm. There are fundamental limits to how far ahead we can accurately predict the weather, and small temperature differences can be particularly important in winter forecasts.
It also explains why neighboring towns sometimes experience very different weather from the same storm. Rain and other precipitation can vary considerably over surprisingly short distances.
Can It Rain When It's Below Freezing?
Yes. If the temperature where you're standing is 30°F, you might reasonably expect anything falling from the sky to be frozen. But a raindrop doesn't instantly freeze just because it enters air below 32°F.
If the cold layer near the ground is shallow enough, liquid drops can reach the surface before they have time to freeze. That's how freezing rain is possible.
Liquid cloud droplets can also remain unfrozen at temperatures well below 32°F. These supercooled water droplets are common in clouds and play an important role in both precipitation and aircraft icing.
Water's freezing point tells us the temperature at which freezing can occur; it doesn't mean every drop of liquid water automatically turns into ice the moment it reaches that temperature.
What About Hail?
Hail is often grouped with sleet because both involve pieces of ice falling from the sky. But hail forms in a completely different way.
Sleet is primarily a temperature-layer problem. Hail is a thunderstorm problem.
Hail develops inside powerful thunderstorms with strong updrafts — currents of rapidly rising air.
A small piece of ice is carried upward into very cold parts of the storm, where supercooled water droplets freeze onto it. As the hailstone moves through different parts of the cloud, it can collect additional layers of ice.
Strong updrafts can keep hailstones aloft while they grow. Eventually they become too heavy for the rising air to support them, or they move out of the strongest part of the updraft, and they fall.
This is why a cut-open hailstone can sometimes reveal layers rather like the rings inside an onion. And unlike sleet or freezing rain, hail doesn't require freezing weather at the ground.
In fact, hail is most strongly associated with warm-season thunderstorms. It can fall on a hot summer afternoon because it formed thousands of feet overhead, where temperatures were far below freezing. The same powerful updrafts and downdrafts that create hail are responsible for many of the other signs you may notice as a thunderstorm approaches.
Why Hail Doesn't Always Melt Before Reaching The Ground
A hailstone falling into warm air does begin to melt. But a large hailstone may fall quickly enough — and contain enough ice — that it doesn't have time to melt completely before reaching the surface.
Smaller hailstones may melt significantly on the way down, particularly when the layer of warm air below the storm is deep.
Once again, what reaches the ground depends on the entire journey through the atmosphere, not simply the temperature at ground level.
Why Precipitation Can Change During A Storm
You've probably seen a storm begin as snow, turn to sleet, become freezing rain and eventually change to ordinary rain — or do the reverse. The storm hasn't necessarily changed completely. The temperature profile above you has.
As warmer or colder air moves into different levels of the atmosphere, the layers that falling precipitation travels through change in depth and temperature.
A warm layer may gradually deepen until falling snow begins to melt. Later, temperatures near the ground may rise above freezing and freezing rain becomes ordinary rain. Cold air moving back in can reverse the process.
This transition can happen gradually across an entire region or occur along a relatively narrow boundary. That's why weather maps sometimes show bands of snow, sleet, freezing rain and rain lined up next to one another within the same storm system.
The Atmosphere Is Layered
We tend to experience weather from the bottom up. We step outside, look at the thermometer and see what is happening around us. But precipitation works from the top down.
A raindrop, snowflake, ice pellet or hailstone may have traveled thousands of feet before it reaches your yard. Along the way it encounters layers of air with different temperatures, humidity levels and movements.
What finally lands in the rain gauge — or bounces off it — is the result of that entire journey. And that is one of the reasons precipitation is more interesting than simply asking whether the temperature outside is above or below freezing.
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