While the Valley deals with rain during the warm season, Mother Nature brings her entire deck of cards during the winter- from rain to snow, with sleet, freezing rain, and even graupel- in between.

While most precipitation starts as snow well above our heads, summertime warmth melts these flakes before they reach the ground. However, intricate changes to the temperature above our heads can spell major differences in what we see.

 

 

Remember, the weather can be very different above our heads. Usually, temperatures decrease as one increases in elevation. For example, the temperature at the top of a mountain will almost always be lower than the temperature at the base of the mountain, because places aloft are subject to less heating from the ground. The graphic below shows how temperatures tend to decrease in the atmosphere, decreasing as one goes up in elevation. If the surface temperature is close to or below 32°F, then snow will fall all the way from clouds to the ground. If the temperature at the surface is above this number with typical atmospheric conditions, then rain falls.

 

 

Note: While snow melts at 32°F, snow can reach the ground with temperatures higher than this number, as long as flakes do not have enough time to melt before reaching the ground. With conditions like the one displayed above, appreciable snow can fall with temperatures as high as the upper-30s.

 

While temperatures typically decrease with elevation in this case, it is possible for there to be increasing temperatures, called an inversion. In cold-weather terms, we call this pocket of warmer air a warm nose. Warm noses tend to be most prominent between 1,000 and 3,000 feet, sometimes with temperatures “bulging” above freezing, even as both the air at the surface and aloft are below freezing. If the warm nose is strong enough, this thin layer can partially melt falling snowflakes into a ball of rain and slush. However, as the drop re-encounters cold air, it freezes back. The larger drop cannot freeze back into a snowflake, so it hardens into a ball of ice known as sleet. Sleet is most common to fall when surface temperatures are between 20°F and 30°F.

 

  

Freezing rain forms under this same process. However, warm noses in these cases need to be just a bit stronger, both in depth and temperature. Just before reaching the surface, though, the liquid re-encounters cold air, but it does not have enough time to re-freeze into ice. As a result, plain rain falls and immediately freezes upon contacting cold surfaces.

 

 

When warm noses get all the way down to the surface, this paves the way for plain rain to fall without the threat of refreezing.

Out of these four precipitation types, freezing rain is by far the most dangerous. In addition to sleet, it is incredibly difficult to pinpoint where and when it will fall, thanks to a number of factors coming together. Additionally, freezing rain blankets any sub-32°F surface it contacts, allowing for roads, driveways, and sidewalks to become a giant sheet of slippery ice.

 

 

Generally, any ice accretion over a tenth of an inch brings challenges to travel. A quarter of an inch of freezing rain could have an impact on small branches, which could break from the added weight of the frozen ice. Roads usually become impassable with a half-inch of ice, while widespread electric issues could come as wires break after an inch or so of ice.

While folks may confuse graupel with small hail pellets, these balls of ice form in their own distinct way. Starting as snow, the flakes fall within an area of supercooled water droplets, with sub-freezing temperatures. These drops freeze upon impact with these snowflakes, creating a small, white ball that looks like a mix of small hail and sleet.

 

 

For graupel to reach the ground, surface temperatures need to be between 30°F and 45°F. Its ability to fall with temperatures above freezing can differentiate graupel from sleet.