Thursday, January 20, 2011

Winter Weather 101: The Factors(s) That Determine Precipitation Types

WARNING: Nerdy weather-ness ahead with some low quality Skew-T diagrams and plenty of technical terms (actually included this time)

If you are the type who upon occasions find yourself staring blankly ahead thinking of the possible atmospheric conditions required for snow, sleet, or freezing rain, and the differences/requirements for one versus type versus another, read on my friend....

Quick Notes:
A sounding is a measure of several variables in a column of air - Temperature, Dewpoint, Pressure, and Wind - at different heights (ft/millibars). This is what "weather balloons" measure. Technically they are called RAOBs and are launched twice daily from select locations. Greensboro is the closest/best for our area.

A Skew-T diagram (see below) is simply a visual representation of the observed data from the balloon. The name refers to the temperature lines - they are "skewed" to the right. So, the right solid line is the observed temperature and the left dashed line is the dewpoint.

This following is edited and copied from NWS Louisville's Forecast Office. 
Click the title to jump to their page.

"PRECIPITATION CONCEPTS; FACTORS AFFECTING PRECIPITATION"

Freezing Rain:
Precipitation starts in cold air aloft as ice crystals/snowflakes that have formed via heterogeneous nucleation, deposition, and ice multiplication and grown through aggregation and riming. Crystals then fall through a melting (warm) layer that is sufficiently deep or warm enough to completely melt the crystals to water drops. The drops then become supercooled as they fall through a subfreezing (refreezing) layer near the surface and freeze on contact if ground objects are colder than 0 C (32 F). A typical freezing rain sounding is shown in Figure 2.

Melting layers greater than 1200 ft deep usually cause complete melting, although for large flakes or for maximum melting layer temperatures less than or equal to 1 C, a deeper melting layer may be required for complete melting.

Figure 2

If the ground temperature is warmer than 0 C but the air temperature is colder than 0 C, then heat conduction may prevent freezing on the ground/streets, but freezing will occur on elevated cold surfaces, i.e., trees, power lines, and cars. If the ground is frozen, then freezing rain can occur despite air temperatures above 0 C (at least for awhile).

Sleet:
A sleet sounding is similar to that for freezing rain, i.e., a melting layer aloft and subfreezing layer below are present. The main differences are the depth and temperature of these two layers and the crystal/snowflake size as to whether complete (freezing rain) or partial (sleet) melting occurs. A typical sleet sounding is shown in Figure 5. Partially melted snowflakes can refreeze much more readily resulting in sleet (or even snow) than completely melted crystals, since ice nuclei still exist in partially melted particles.

As the melting layer decreases in depth and temperature and the subfreezing layer increases in depth, the probability of sleet or a mix of sleet and snow graupel increases.

Figure 5

Unmixed (no freezing rain) sleet events usually are most probable when the average minimum temperature of the low-level subfreezing layer is colder than -5 C. Mixed (sleet and freezing rain) events can occur when the minimum temperature of the subfreezing layer is as warm as -2.5 C.

Snow Versus Rain:
A borderline rain/snow sounding (Figure 6) is different (opposite) from freezing rain and sleet soundings in that the melting/warm layer usually exists near the surface while subfreezing temperatures are located aloft above the warmer boundary layer air.

The depth of low-level warm air (below the freezing level) needed to melt snow falling from above to rain varies from about 750-1500 ft and depends on the mass of the flakes and the lapse rate of the melting layer. When the lapse rate is small (i.e., temperature decreases slowly with height in the layer; left diagram in Figure 7), the melting layer is weak so it must be deeper to melt snow completely. When the lapse rate is large (right diagram in Figure 7), the layer can be smaller and still melt snow.
Figure 6 Figure 7
Precipitation likely will be snow at the surface if the height/depth of the melting layer is less than 900 ft (i.e., a 50 percent or greater chance of snow reaching the ground). If the depth is only about 200 ft (i.e., surface temperatures are above freezing but the freezing level is at 200 ft above the ground), then the probability of snow is about 90 percent. If the depth is more than 1000 ft, then the probability of snow decreases rapidly below 50 percent.

It is crucial to evaluate the depth and temperature of warm and cold layers (as well as which layer lies on top versus underneath) in soundings to help determine precipitation type in the winter.

That's it for now. If you have any questions or would like me to (try to) explain certain terms/ideas in greater detail, let me know.