Supercooled liquid drops (SLD): SLD are water particles, that due to their purity and lack of particulate matter in a given airmass fail to freeze at 0 c. In stead they remain liquid . They will remain liquid until they come in contact with particulate matter, dust, sand, smoke, smog, etc. At which point they will instantly adhere to these "freezing nuclei" and form an ice crystal. Typically, a cloud is said to be supercooled and containing a large percentage of liquid betwwen 0 and - 10c. This is known as the Burgeron (sp) process. The hazard to airman lies in the fact that the SLD can not distinguish between particulate matter in the atmosphere and a "foreign object", such as an airfoil, passing through it. Flight through supercooled clouds cause the impact of the SLD on the airfoil and other aircraft parts causing them to freeze on contact and cause accretion of airframe icing.
Virga, as you probably know, is rain that falls from a cloud but evaporates before it hits the ground. Virga is usually associated vith nimbostatus or cumulonimbus clouds.
A check valve is, simply put, a one-way valve. It prevents the back flow of hydro-dynamic forces, (fluid, or gaseous) in the opposite of the intended direction of flow. You will typically see check valves used in fuel, hydraulic, or bleed-air sytems. A good illustration, in laymans terms, would be the the valve stem on your car's tire. It allows the air to enter the tire but prevents the reverse flow of air out of the tire. Of course you can manually open the valve stem to let air out, which you couldn't or would't want to do on an a/c hydraulic line. But you get the point.
Note that supercooled water may remain in the liquid state as low as -40 degrees. It's not the presence of a foriegn object or impurities (condensation nuclei; something entirely different) that causes it to freeze, but the breaking of surface tension on the droplet that allows freezing to occur. It is the surface tension that prevents the freezing from occuring. Impact with a wing or flying surface breaks the surface tension on the droplet, and freezing is instantaneous.
I've wondered about this stuff for awhile and have postulated a theory:Since the boiling point of water increases 3F for every pound you pressurize it,is the inverse true ? Will water freeze at a lower temp if it is at a lower pressure ? Since pressure generally decreases with altitude,it would seem possible.Then,along comes an airplane with it's associated pressure field (We're talking sub sonic here) and viola ! Instant ice.Somebody better versed in science help me out here.
At subfreezing temperatures, surface tension on the individual water droplet resists expansion and formation of the ice crystal. Relieving or breaking that surface tension allows the water to freeze.
Raising boiling point is a function of pressure is due to an increase in the vapor pressure; the water must be heated to a higher temperature to produce adequate pressure to boil. However, the inverse is not true, for the same reason.
Yeah if you think about a really cold winter day, with below freezing temps and with clouds that are not cirrus, well you have super cooled liquid water. The water is still in liquid form, even though air temps and the temp of the water droplets is well below freezing. Yes, water can in the right circumstances be liquid down to -40 (C or F, take your pick)
Thats how you get icing.
32F,0C is when water melts, but not neccessarily freezes. There are other factors that come into play with it.
Virga is not really dependent on a type of cloud, its when the rain evaporates before hitting the ground. Its extremely common in the southwest. It can be hazardous, because evaporative cooling can make the air sink, causing a downdraft under the virga.
A check valve closes to maintain integrity of a system following failure of part of it. For example, twin engines have 2 vacuum pumps. Well if one failed, that could screw up the entire system. however a checkvalve closes that side to keep it the system working.
Freezing temp of water isnt dependent on vapor pressure.