Precipitation Weather Radar
We have talked about mechanisms that promote cloud and precipitation development. In today's lecture, we will take a more in-depth look at precipitation. We will also learn about radar, an instrument that helps meteorologists detect precipitation.
Precipitation
As air rises and cools to its dewpoint temperature, the moisture in the air begins to collect and accumulate onto cloud condensation nuclei. As this condition persists, the condensation nuclei grow larger through collisions. Once the droplets reach a certain size, they will fall to the Earth as precipitation. The merging of cloud droplets is a process called coalescence.
Coalescence is helped along by the fact that different sized cloud droplets move at different speeds. The fall speed of a particle is governed by its cross-sectional area, which is governed by its diameter. Therefore, the larger the droplet, the fast er it will fall. As the larger droplet falls, it sweeps out a path and collides with many smaller droplets which adhere to them and cause the main droplet to grow larger.
Warm rain refers to precipitation that is forming in an exclusively liquid water environment. Often times however, we don't need to go too high up into a cloud to encounter the freezing level, even in Florida. Once we get above the freezing le vel, we will likely encounter ice. Rain that forms in subfreezing environments is called cold rain.
Does all precipitation that forms below 32 F form as snow? It turns out this is not the case. How can liquid water exist below 32 F? To answer this question, we need to consider a chemical principle called Raoult's Law.
Raoult's Law basically describes that the freezing/melting point of water is affected by the amount of impurities in the water. The more impurities in the water, the lower the freezing point. And atmospheric water contains salts and many other substa nces aside from pure water. Salting the roads during snowfall is an illustration of Raoult's Law. It lowers the melting point so snow on the roads that would normally be frozen without the addition of salt at 15 F may melt at 15 F with the addition of s alt.
Water below 32 F that is in the liquid form is called supercooled. By the same principle that cloud condensation nuclei form, ice crystals can form below 32 F. As air rises and is cooled to the dewpoint temperature, the water vapor in the air is depositied onto ice nuclei. Cloud condensation nuclei are different than ice nuclei. A nuclei that is hydrophobic may not necessarily be physically suited to experience ice deposition and vice-versa. It has to do with the intrinsic chemical a nd physical nature of the nuclei.
In most clouds, below the freezing level (32 F), there is a mix of ice crystals and cloud droplets. As the temperature gets lower and lower, there is a higher concentration of ice crystals. At -40 C, all water in the cloud would be in the ice form.
There are different types of ice crystals. There are flat-faced plates and longer rods. Plates and rods form at different temperatures. With higher relative humidity values, ice crystals accumulate into snowflakes. The exact mechanism by which this happens is quite complicated. Snowflakes and ice crystals have a hexagonal type orientation due mostly to the chemical structure of ice.
Forms of precipitation: rain
Rain is simply liquid water falling out of the sky. When expressing the intensity of rainfall, the rainfall rate is often represented in terms of how much rain would fall in a given amount of time.
Raindrops, due to air resistance principles, do not fall out of clouds as tear shaped objects but rather as saucer shaped objects. Rain that reaches the ground often forms as snow or ice higher up in the cloud.
Sometimes, rain forms and falls through a dryer layer of air where the vapor pressure is less than the saturation vapor pressure and the rain actually dries up before hitting the ground. This is called virga.
Meteorologists, when making weather forecasts, will often characterize rain by its intensity and texture using terms such as showers, drizzle, etc. Showers usually refer to rain that is scattered and is not expected to last long. Drizzle is cl assified as very tiny rain drops.
Snow
Snow reaching the ground is primarily a wintertime phenomena. Snowflakes have different sizes and different water contents. The warmer the temperature is, the more liquid cloud droplets adhere to the surface of the snowflakes, thus increasing their w ater content. This is why some snow is very wet and heavy, and other snow is very dry and flaky.
Wet snow is usually much better for packing and making snowballs or snowmen.
During falling snow, it is usually very quiet. This is because snow actually serves as an insulator to sound.
If you melt down snow, you won't always get the same amount of liquid precipitation from equal depths of snow. Also, depending on the temperature and the type of snw crystals present, 12 inches of snow may melt down to 1 liquid inch or it may melt dow n to .2 inches. It depends on many factors including temperature and vapor pressure.
Sleet and freezing rain
These types of precipitation occur in temperature inversion situations. Remember that in the troposphere, the atmosphere typically cools with increasing elevation. In certain meteorological weather situations, including warm fronts, the temper ature sometimes rises with increasing elevation.
Sometimes in an inversion, the temperature at the ground will be below freezing while the temperature above the ground will be above freezing. In this case, rain may form above the freezing level, but then freeze as it falls through the freezing level .
If the rain falls through a very deep freezing layer, the drops will likely completely freeze into little ice pellets called sleet. However, if the layer of freezing temperatures is shallow, the rain will freeze with the first object it comes into con tact with. This type of precipitation is called freezing rain or glaze. It puts a slippery glaze of ice on all objects it comes into contact with.
Warm fronts are very notorious sleet and freezing rain producers. Warm air advances into cold air. Warm air is less dense than cold air, so is forced up and over the cold air. In this situation, there is warm air above cold air - perfect dynamics fo r sleet or freezing rain. Last month, Montreal and northern New England experienced one of the worst ice storms in history.
Hail
Hail forms exclusively in cumulonimbus clouds where there are large vertically propagating updrafts. Within the cumulonimbus cloud, there is a specific freezing level. Raindrops will form and get caught up in an updraft and be carried above th e freezing level where they will turn to ice. They'll fall back through the cloud via a downdraft accreting more water. Then they'll get caught back up in an updraft where the new water accreted will freeze, thus making the ice particle bigger. This pr ocess continues until the particle is too heavy to be lifted by the updraft and then falls out of the cloud as a hailstone.
Hailstones can sometimes turn into lethal objects. Some hailstones can grow to be as large as softballs or grapefruits. The largest hailstone ever recorded fell in Kansas and had a diameter of a record breaking 15 cm! That would give you quite a lum p on the head if it hit you.
There have been hailstones that have killed people. More commonly, extensive property damage to crops, buildings, cars, and houses is experienced in hailstorms. Hail can accumulate as snow does. In Dallas during the Summer of 1994, 2 feet of hail fe ll in the middle of the summer. It didn't stick around long but it provided Dallas with a rare event.
Because hail and tornadoes both occur frequently in the vicinity of strong updrafts, hail and tornado activity are closely correlated.
Measuring precipitation
Meteorologists use instruments called rain gauges to measure how much precipitation falls during a given time period of precipitation event.
Weather Radar
A radar is an instrument that sends radiation out in all directions. Some radiation encounters objects and is reflected back toward the radar. At the radar, there are instruments that can detect this reflected radiation. The larger an object that th e radiation comes into contact with, the more power will be returned. Thus, when sending out radar radiation beams, if they encounter precipitation, some of the radiation will be reflected back and measured at the radar location. The amount of radiation detected is the reflectivity.
Weather radars are an extremely important tool in weather forecasting. It is possible to correlate reflectivity (often given the symbol Z) to the rainfall rate, R. These relationships are called Z-R relationships.
A Doppler Radar is a special type of radar that can detect the movement of the precipitation.
A Radar Image of the Southeast United States
