Precipitation type is based on an intricate balance between moisture and temperature through the atmosphere. A slight change in temperature over a few thousand feet can mean the difference between a blizzard, a crippling ice storm, or a dismal chilly rain.
Throwing aside the small talk, let's jump right over the fold for a basic explanation on how various types of precipitation form.
State of the Skies is a new diary series bringing the DailyKos community a small snapshot of the previous day's significant weather events, as well as a look at how different weather phenomena develop, occur and impact regions of the country. Use this diary to discuss the weather, effects the weather has on your life or the lives of others, or ask any weather/climate questions you have. Enjoy!
*Disclaimer: For the purpose of keeping this diary uncomplicated and relatively simple to understand, I won't delve too deeply into SKEW-T soundings and all possible ways a certain precipitation type will/could develop.
As I mentioned in my Weather Maps 101 edition of State of the Skies, 69 sites across the country launch a weather balloon every 12 hours (at 00z and 12z) to get a vertical profile of the atmosphere. The data collected from these releases include pressure, altitude, temperature, dew point (the temperature at which air will reach saturation, or a humidity of 100%), wind speed and direction, and a few other variables.
Meteorologists graph this information on a specialized chart called a Skew-T/Log-P chart. The Skew-T (short name) is named such because the pressure markings on the chart go up logarithmically, and this causes the temperature markings on the chart to skew, or increase on a 45 degree slant.
This is a Skew-T chart. I won't go into all the details, but the 5 basic things you need to know are the following:
The numbering of this list corresponds with the numbering of the above image.
1 - Isobars Each horizontal line on the chart marks a new isobar, or a line of constant pressure. The height of these pressure levels vary depending on what kind of weather system is present at the time, but the pressures themselves stay the same.
2 - Isotherm Each positively-slanted line on the chart is an isotherm, or a line of constant temperature. The temperatures are denoted in Celsius, and appear on a 10°C interval.
5 - Dew Point The bold squiggly line that appears on the left is the dew point line. To read the dew point measurement at a specific pressure level, interpolate (estimate) the point's position between the two nearest isotherms.
6 - Temperature The bold squiggly line that appears on the right is the temperature line. To read the temperature measurement at a specific pressure level, follow the same process as you would with the dew point -- interpolate the point's position between the two nearest isotherms.
7 - Wind Plots The barbs along the right side of the Skew-T chart are the wind plots, which denote the wind speed and direction at the corresponding pressure level. The barbs point in the direction from which the wind blows, and the speeds are denoted by the short spikes, longs pikes, and flags on the barb. A short spike is equal to 5 knots; a long spike is equal to 10 knots, and a flag is equal to 50 knots.
These Skew-T charts are extremely crucial in helping to determine what type of precipitation will fall.
Rain is a small water droplet that falls from a cloud. Simple enough. These droplets are usually greater than or equal to 0.5mm in diameter. If any of these droplets that fall are smaller than 0.5mm, they are called drizzle. For reference, if viewed on a normal computer in standard font, 0.5mm is about half the diameter of the letter "o" in this sentence.
For rain/drizzle to fall, the atmosphere usually has to be above freezing for at least the last few thousand feet above the surface. Actually, most rain that falls begins as snow in the upper atmosphere, but melts to rain well before it reaches the ground.
You can see in this Skew-T sounding that the atmosphere is moist and above freezing from top to surface, signaling that this particular event would be all rain.
Sometimes, the rain won't reach the ground because it falls through a dry layer and evaporates. In this case, the precipitation is called virga. Virga frequently appears on radar (especially during the onset of a snowstorm), and you can even see it if the terrain and lighting is right:
Image Source The virga is the wispy cloud-like stuff extending towards, but not reaching, the ground.
Snow forms when water vapor undergoes deposition onto the surface of a suspended particle (dust, pollution, insect, etc.) in the atmosphere. The best region for snow growth is the area of the atmosphere that has temperatures of - 12°C to - 18°C, because this is the region wherein the most successful snow type, the dendrite, is favored to form.
The atmosphere needs to be below freezing for almost the entire journey of the snowflake for the snow to remain as snow by the time it reaches the surface. The following is a typical snow sounding on a Skew-T chart. The dew point and temperature lines are close together from the snow growth region down to the surface, showing that the snow has ample opportunity grow and make it to the ground.
Sleet is a small ice pellet, roughly the same size of a raindrop. Sleet forms when a raindrop falls through a subfreezing layer of the atmosphere, and has enough time to thoroughly freeze before reaching the surface. Sleet can also form when a snowflake briefly melts, and has enough time to refreeze into sleet before reaching the ground.
In this Skew-T sounding, you can see that the atmosphere is below freezing until a layer of the atmosphere between 800mb and 950mb. In this layer, the temperature shoots up to about 5°C above freezing, then rapidly falls down to around -5°C right before the surface. This 150mb layer of above freezing temperatures allowed any snow that fell into that layer time to partially melt, then solidly refreeze before reaching the surface.
Freezing rain is supercooled rain (water that is below freezing, but doesn't have a nuclei, or particle, on which to freeze) that strikes the surface and freezes on contact. Anyone who's lived through an ice storm involving freezing rain knows that this is one type of winter weather that isn't fun for anybody.
Freezing rain forms similarly to sleet, but this time, the snowflake (or sleet) melts completely and doesn't give the raindrop a catalyst on which to begin to freeze.
Hail forms when a thunderstorm's updraft lifts raindrops high into the clouds of a thunderstorm, where the raindrop freezes into a frozen pellet. As the frozen pellet becomes too heavy to stay in the upper levels of the thunderstorm, it falls back down through the cloud and accumulates rain on its surface. As the pellet descends back into the heart of the updraft, it gets lifted back up into the upper levels of the thunderstorm, and the water on its surface refreezes and creates a larger pellet. This cycle continues until the newly-formed hailstone moves into a part of the thunderstorm where the strength of the updraft can no longer support the hailstone, and it falls to the surface.
So, that's how a few of the major precipitation types form. I'm sure I forgot something, so if you have anything to add, or anything to add about your weather today, tell me about it! :)