Simulations

Precipitation forms within clouds through two main processes: the collision-coalescence process and the Bergeron process. In the former, raindrops grow by colliding and joining with other drops and droplets. The Bergeron process occurs as water droplets evaporate in supercooled clouds and their moisture is transferred to growing ice crystals. A third mechanism, condensation, is important for the formation and growth of cloud droplets but a very minor contributor in the production of precipitation-sized particles. Precipitation types such as ice pellets, snow pellets, and freezing rain result mainly from temperature variations in the air through which rain or snow fall.

Worldwide, annual precipitation averages approximately 1 meter per year. This figure masks enormous variations in precipitation from place to place and in time.

Most efforts at precipitation modification are founded on the theory that some clouds are deficient in freezing nuclei. Seeding clouds with dry ice and silver iodide increases the number of freezing nuclei, sometimes induces evident changes in cloud structure, and may increase precipitation under the right circumstances by 10% to 15%. However, many scientific, legal, and ethical questions about this new technology remain unanswered.

In Chapter 6, you became familiar with the Shew T diagram, learning how to find and interpret some of the information on that chart. Since the diagram shows temperature conditions through the depth of the part of the atmosphere that contains most of our weather, you can use such a diagram to determine what form precipitation should take when it reaches the ground.

The main forms of precipitation are rain, freezing rain, sleet, hail and snow. Most precipitation starts its descent as snow. If the falling snow encounters temperatures at or colder than freezing all the way down, it will reach the surface as snow.

Occasionally, snow will survive a shallow layer of warmer than-freezing air near the surface. One way for this to happen is through evaporational cooling. If snowflakes fall into air that is not saturated (such as the air below the clouds), but is warmer than freezing, the flakes will start to melt. However, the outer "skin" of water will quickly cool because of evaporational cooling. This keeps the remaining part of the flake chilled through its descent.

If snow melts on the way down, we notice rain when it reaches the surface. Suppose the air at ground level is colder than freezing. In this case, the liquid drops will freeze on contact, resulting in glaze. This is called freezing rain. When the ice builds up enough, it can snap power lines, break tree limbs, damage shrubbery, etc. A storm consisting of freezing rain is often called an ice storm, and the worst ones have caused many millions of dollars in damage. When thousands of individual power lines snap, the job of restoring power is enormous. Each break has to be repaired, and crews often have to battle a maze of broken branches just to reach the broken lines. Residents in the affected areas often become quite frustrated because their power is out for days. They may have experienced power outages caused by lightning, and wonder why the ice storm outages take so long to repair. In lightning cases, there is often just one point of damage. In an ice storm, entire systems can be affected.

If snow melts on the way down, but then encounters a relatively thick layer of colder than freezing air, the rain drops can freeze into pellets called sleet. Note: the drops do not freeze back into snow. Snow forms directly from water vapor, not from the freezing of raindrops. Sometimes, people hear the sleet ticking against the windowsill and think they are hearing hail. However, hailstones develop higher up in the atmosphere in thunderstorms. Strong updrafts keep precipitation aloft long enough to allow drops to pass through one or more layers of colder than freezing air. As the resulting balls of ice move through the heavy precipitation, they grow larger, until finally even the updrafts cannot keep them aloft. Hailstones the size of grapefruit have been seen on occasion in the Great Plains, where some of the most fierce thunderstorms develop. Hail can cause considerable damage, breaking glass and denting cars and trucks. Fortunately there have been few injuries or deaths caused by hail. This is probably because the approaching thunderstorm looks so dark and menacing that people take shelter before the hail falls.

In the following diagrams, we trace the changes in precipitation type using actual skewT diagrams. After looking at several examples, you will have the opportunity to diagnose several cases.

Types of Precipitation Simulation (2194.0K)