Precipitation type defined: Rain, freezing rain, sleet, and snow

Weather Glossary

ROCHESTER, NY (WROC) – From innocent rain to crippling ice, Western New York and the Finger Lakes can see a plethora of different types of precipitation. Let us go through them all here.

To start off, nearly all precipitation begins as snow. Temperatures are well below freezing where a cloud forms, so when precipitation begins, it often starts as snow.

TYPES OF PRECIPITATION:

RAIN: defined as water droplets. Temperatures are above freezing (32°). The most common form of precipitation worldwide. This can be confined within a mile-wide thunderstorm or span large areas, thousands of miles along a cold front. The biggest threat posed from rain is flooding.

FREEZING RAIN: defined as water droplets that land on a frozen surface and immediately freeze. Note the example above. Snow falls out of the cloud (~15,000′) and there is a part of the atmosphere in which temperatures are above freezing (8,000′ – 2,000′). That snow completely melts into a water droplet. The air temperature directly above the surface (2,000′-Sfc) is below freezing. This water droplet did not have enough time to freeze in the air and freezes on contact. This accumulates on surfaces and can cause slick conditions and property damage. The biggest threat from freezing rain is difficult travel, outages, and property damage from downed power lines and trees.

SLEET/ICE PELLETS: defined as a frozen water droplet. Snow falls out of the cloud (~15,000′) and there is a part of the atmosphere in which temperatures are above freezing (10,000′-5,000′). That snow melts down into a water droplet. The droplet eventually falls into a thick layer of below freezing temperatures above the surface (5,000′-Sfc) and that turns the droplet completely into either sleet or an ice pellet. These terms are mostly interchangeable, although sleet may have a more watery texture. NOTE: THIS IS NOT HAIL.

HAIL: Ice formed from a strong thunderstorm. Read more about hail here.

SNOW: defined as ice crystals. Snow falls when the entire layer of the atmosphere is below freezing, but it can take on many different shapes and sizes depending on the type of cloud, amount of moisture, temperature, height of cloud, warmer layers, and much more. The biggest threat from snow is difficult travel and potentially power outages.

Note the “Warm nose” above where you find ice, freezing rain, and sleet.

ANALYSIS:

When forecasting for precipitation type, it is important to see where the atmosphere is above freezing. Basically, how deep is the above freezing layer when talking about precipitation? This can be found with “Skew-T log P” graphs that show the atmosphere as a slice. The name stands for “skewed temperature – log pressure”. Imagine a generic x/y graph. Except the vertical lines are skewed (for temperature) and the horizontal lines are in log form (pressure) since pressure decreases with height in this way.

Below are two graphs that show examples of some of the hardest precipitation types to forecast for, freezing rain and sleet on Skew-T log P graphs. Red represents temperature and blue is dewpoint. This is as we go up in the atmosphere. The data shows what we may see from launching a weather balloon.

Note the deep above-freezing later and the later right at the surface that is below freezing.

The graph above points out how we could potentially see freezing rain if precipitation was falling at this point. Focus on the red line (temperature) and start from the top. As you go down toward the surface (like a snowflake would) you enter a layer of warmth. That melts the snowflake into a water droplet. That droplet then falls into a below freezing later and does not have time to re-freeze. That means it will freeze on whatever surface it hits.

Note the shallow above-freezing layer and deeper below freezing layer. This sounding is very similar to the sleet sounding, but with very different results.

The sleet sounding will allow that melted snowflake to completely re-freeze and fall as sleet.

BONUS ROUND: Here is a list of some other types of precipitation that are important in their own right, and may get highlighted on another page

DRIZZLE: Similar to fog or mist. Very light rain. We could put smoke in this section too. Smoke would likely be from wildfires.

ICE CRYSTALS: Only in very cold regions (sometimes Western New York) it is just as it sounds. Crystals often in the form of needles, columns, or plates.

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