Notes on Precipitation 10-6-04

General information on Precipitation

Types:

- Rain: precip in the form of liquid water drops that have diameters greater than drizzle drops

o Size range à .5 mm to 5 mm (the largest drop on record was discovered by Dr. Ken Beard, a professor here in my department. It was 8 mm in diameter)

- Drizzle: small water drops between 0.2 and 0.5 mm in diameter (smaller than rain drops)

o These tend to fall slowly compared to raindrops and often obscure vision

- Snow: solid (frozen) form of precip that is composed of ice crystals in a complex (usually hexagonal) form

- Sleet: frozen form of precip that consists of semi-transparent ice-pellets of 5 mm diameter or less.

- Freezing rain/drizzle: rain or drizzle that falls in the liquid form and freezes upon striking a cold object (i.e., a tree or your car) or the ground. Can produce a coating of ice called glaze.

- Graupel: ice pellets with size raging from 2 – 5 mm in diameter that form in a cloud. They are created by a process called accretion, which is the growth of a precip particle (usually ice pellet or snow flake) by collection of super-cooled drops (liquid drops with temps below 0°C) that freeze once they contact the precip particle. These particles also contribute to the static electricity build up that helps form lightning.

- Hail: transparent or partially opaque particles of ice that have a general size range of pea size to golf ball size with some reaching 3-5 inches in diameter (baseball to grapefruit size). The largest I have ever heard of happened last summer in a storm over Nebraska where 7-inch diameter (~17-inch circumference) hail fell! That’s roughly the size of a 16-inch softball.

- Virga: any type of precip that falls from a cloud but evaporates before it hits the surface

o Even though the radar sees it, the station observation doesn’t report it. This is because the radar beam is above the surface where the precip is while the station is at the surface, which is below where the precip evaporated.

In terms of size range:

Cloud drop < drizzle drop (freezing drizzle too) < rain drop (freezing rain too), graupel, sleet < hail

Precipitation type from soundings

SLEET

Freezing Rain

Snow

Rain

Radar information

What is radar? Radar is a large antenna that sends out microwave frequency energy waves. Microwave (with wavelength 2-15 cm) are “sensitive” to precipitation sized particles in that when a microwave hits a rain drop it will bounce off back to the antenna that sent it. The strength of the “echo” is proportional to the size of the drop that reflects it. So, larger drops create larger echoes. These echoes, or more commonly called “reflectivity” have units of decibels (dBZ). The range of reflectivity values indicates how heavy precipitation is. This range of reflectivity is usually color coded so that heavy precip (i.e., thunderstorms) looks bright yellow to reds and eventually to purples, while lighter precipitation starts in light blues and moves toward dark greens as the precip becomes more moderate. One major thing scientists can derive from radar is a precipitation rate (or rainfall rate inches/hour). The larger the reflectivity value the greater the rainfall rate.

A typical radar can “see” about 130 miles. In other words a radar has a radius of about 130 miles.

NOTE: The radar can only “see” precipitation sized particles. Any particles that are smaller (i.e., cloud drops) do not produce a strong enough reflectivity for the radar to “see” them. Therefore, radar only lets you know where it is precipitating, it does not tell you where clouds are! (However, you can infer that where it is raining, there are clouds but if there is no rain on the radar screen, that DOESN’T mean that there aren’t clouds.)