Knowing Which Way the Clouds are Moving Helps Forecast the Weather
POSTED: Monday, February 10, 2014 - 12:40pm
UPDATED: Monday, February 10, 2014 - 12:46pm
Watching the Direction the Clouds are Moving at Both the Lower and Upper Levels Helps Forecast Temperatures and Storms
Monday, February 10th, 2014 — The Borderland has a cold front backing in from the east/northeast Monday into Tuesday. As you know from reading previous, “Weather Talk” articles I love watching the clouds! Watching the clouds will tell you what direction the winds are blowing from. When you know the surface wind direction and the upper level wind direction you know temperature advection. Temperature advection is the process by which the wind transfers heat energy horizontally through the atmosphere. Cold air advection involves wind carrying cooler air into an area that already contains warmer air. By contrast, warm air advection involves wind carrying warmer air into an area that already contains cooler air. Temperature advection is measured in terms of the amount of temperature change that occurs in a given interval of time. The units used to express temperature advection are “degrees Fahrenheit per hour. I will not get too scientific, but I will explain how knowing the changing direction of wind with height can help you forecast that day’s weather.
Having knowledge of temperature advection at the upper levels of the atmosphere is a valuable tool in forecasting the weather, especially in the summer. During the warm months the air near the surface is warm, when upper level cold air moves in or cold advection, this makes the atmosphere more unstable and increases the chances of towering cumulus, clouds and showers. The opposite occurs when there is warm advection aloft. The warm air pushing in usually increases the temperature of the air, making the atmosphere more stable. When warm advection happens during the winter months this usually leads to smoke, pollution and haze getting trapped near the surface or an inversion. This is the cause of El Paso’s light brown cloud during the early half of the day in the cold months. Read my “Weather Talk”; Our Smoky, Brown Haze Is Enhanced By Winter Weather http://www.ktsm.com/weather/weather-talk/our-smoky-brown-haze-enhanced-w...
By watching the movement of the clouds, we get a good indication as to the wind direction at cloud level, which tells the type of advection. For example, a cloud moving from the west indicates a west wind, a cloud moving from the north a north wind, and so on. Clouds at different levels very often move in different directions. This means that the wind direction is changing with height. The surface wind direction can be you starting point if there are no lower clouds. Wind that changes direction in a clockwise sense (north to northeast) is a veering wind. Wind that changes direction in a counter-clockwise sense (north to northwest to west) is a backing wind. There are two rules that will help you determine where cold or warm advection is occurring at the upper layer of air above us. First, winds that back with height (change counter clockwise) a backing wind, indicate cold advection. The second winds that veer with height (change clockwise) a veering wind, show warm advection.
For example, say we see lower clouds moving from a southerly direction (south wind) and higher clouds moving from an easterly direction ( east wind), the wind is backing with height and the atmosphere between the cloud layers is probably becoming colder. On a different day we observe lower level clouds moving from a southerly level and high clouds drifting from a westerly direction. The wind direction veering with height; warm advection is happening between the cloud layers and the air should be getting warmer.
We can use this information to improve upon a weather forecast. For instance, if you happen to be located ahead of an advancing warm front and the winds are veering with height, the chances are that even if precipitation begins as snow, it may change to sleet or rain as the warm air mixes in overhead. Behind a cold front where the winds are backing with height, the push of cold air may drop the temperature enough so that the rain first becomes mixed with snow and then changes to snow before the storm moves out.
I like to use the website www.blastvalve.com to get the current and forecast upper level winds in one spot. I use this site when I am forecasting the winds for the hot air balloon pilots during the “KLAQ Balloon Fest” every Memorial Day Weekend. That way you can quickly see the winds at upper levels, if you do not have the time to watch clouds or the clouds are not moving much, if at all. Go to the El Paso National Weather Service site to find out about the current surface wind; http://www.srh.noaa.gov/elp/
When you are forecasting every little tool or advantage you can add to your forecasting arsenal surely helps. When you have some time take a look at which direction the surface wind is blowing from and which way the clouds are moving, this may help you adjust your plans for your outdoor activities later on that day.
Check out my other “Weather Talk” articles on clouds; Hey! That Cloud Looks like Grandpa! http://www.ktsm.com/weather/weather-talk/hey-cloud-looks-grandpa and "Foehn" Clouds Over The Franklin's http://www.ktsm.com/weather/weather-talk/foehn-clouds-over-franklins