 View PDF
The science of meteorology gets our attention when a storm is on the horizon! Have you ever wondered how snow depth is predicted or why the forecasts can sometimes be so far off?
Many variables come into play-not just moisture and temperature. Other important factors are vertical transport or vertical motion (how fast the snowflakes fall) and cloud physics, including evaporation and crystal growth rates. According to Wes Junker of the National Weather Service, snow forms around small particles in the atmosphere. The small particles act as nuclei and can have different shapes. The rate of snowflake growth and the temperature at which the snow will form are affected by the nuclei shape. The size of the clouds as well as how fast new snow-producing clouds are created also factor into snowfall predictions.
As to the question of whether the precipitation will be snow, sleet or freezing rain, the type of precipitation depends greatly on the temperature of the atmospheric layers. To begin with potential snowflakes are groups of ice crystals that stick together as they fall toward the ground. If the temperature throughout their fall stays below freezing, then the ice crystals arrive as snowflakes, because they did not pass through a warmer layer, and reach the ground as snow.
The story of sleet is more complicated. Sleet is defined as frozen raindrops that bounce on impact with the ground. According to the Weather World 2010 Project, sleet is less prevalent than freezing rain and there is a distinction. Sleet occurs when ice crystals pass through a warm atmospheric layer, partially melting. They then pass through another cold layer just before hitting the ground. Forecasting sleet is tricky business, as the layer thicknesses and positions have to be just right. This is also why sleet is less often seen in comparison to freezing rain.
Freezing rain occurs when falling snow travels through a layer of warm air in which the crystal melts completely and becomes rain. As the drops continue to fall, they pass through a cold layer in which the drops do not freeze but become “supercooled”. “Supercooled” describes a state in which the drop is still actually unfrozen below the freezing point, which can happen when a drop is smooth, pure and clean. In fact it happens quite often in the atmosphere, until the supercooled drops come in contact with a nuclei or the wing of an airplane. (There are also some neat YouTube videos showing supercooled water freezing when disturbed.)
So, we have these “supercooled” drops that will freeze on contact with the cold ground, or your car’s windshield, tree branches, or electrical wires, forming a thin layer of ice. This is the definition of ice storm. These can be among the most devastating of weather phenomena causing accidents, loss of power, and other damage, such as was experienced this winter.
Ordinary rain develops when growing cloud droplets become too heavy to be held in the cloud and fall toward the ground. Rain can begin as ice crystals that pass through warm layers, melting completely. Another subtlety about precipitation is hail. Hail requires updrafts. An updraft, such as that found in thunderclouds, is a vertical movement of air. If a crystal exists in a cloud with updraft, it will travel up growing as more drops or crystals collect; then fall through the cloud, growing as it travels. If it gets caught in another updraft, the process is repeated. The longer it bounces around the cloud and updrafts, the bigger the hail pellet becomes. At some point, it becomes too heavy for the cloud system, and falls to the ground as hail, not melting because it has not spent enough time at the warmer lower layer to melt.
These many and varied atmospheric conditions determine the resulting form of a water drop on the ground and continue to present meteorological forecasting challenges.
Date published on web site:
03-26-2009
|
