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WHAT DOES THE WEATHER FORECAST MEAN WHEN THE GUY SAYS "COLD FRONT"? We all complain about the weather, and we all tease the people who predict the weather. Nobody has an inside track on what is REALLY going to happen in any arena, including the weather. However, over time, people have observed that there seem to be patterns in which things happen. We have observed that people, under certain types of circum- stances, display the same or similar kinds of behavior for those circumstances. We can avoid causing people to behave poorly in a social setting by seeking to allay hostility. By observing animal behaviors, we have devised methods of training animals to behave in specific ways. We can train dogs to lead blind people, to herd cattle or to find people who are hiding. We have learned many valuable lessons merely by careful observation. We cannot DO anything about the weather, but we can avoid being severely harmed by naturally-occurring events. As recently as last week, the news media were instructing people to take precautions against serious winds and flooding. They told us to pack emergency provisions in case we have to evacuate our homes. When one inch of rain falls instead of the predicted one foot of rain, we laugh at the weather guys and suggest that they are stupid. However, we must remember that they can only read the signs that are present and from those signs, make educated guesses as to what the signs mean. We can assume, when the news media issues storm warnings, that something serious is probably going to happen, and set about to ensure that harsh results are minimized. Those who earn their livings by growing and harvesting our food can make tentative plans for planting, spraying, etc., by paying attention to weather signs and predictions of meteorologists. Some information is better than none. Let's create some impending weather patterns and see how natural events develop: An aquarium can be structured to show what generally takes place when differently heated air masses come in contact. To do this, use plastic model cement or contact cement to glue plastic rib binders, such as the type used by students to hold report papers together, along the bottom and sides of the aquarium. Be sure the ribs are glued securely, to form a watertight guide for a glass partition. Now make a partition from a sheet of glass. Cut the partition so that it will just slide in the plastic guides and is higher that the top edge of the aquarium. Several different demonstrations pertaining to the contact of warm and cold air masses (weather fronts) can be observed: a. Warm and cold fronts. Place warm water in one side of the aquarium and cool water in the other. Use food coloring to color the warm water red and the cool water blue. Add half a spoonful of salt to the cool water. The red water represents a warm air mass and the blue represents a cool air mass. Such air masses are like huge invisible flat bubbles -- they are often more than 10 miles high and hundreds of miles across. Remove the partition, and describe what you see. The dividing line between the masses is called a front. When a cold mass (blue water) moves against a warm mass (red water), there is a cold front. You will see that the warm mass is forced upward over top of the cold mass. Predict what warm moist air would do under such circumstances. Generally, a sequence of cloud types progress along the front bringing a sequence of weather events: (1) The chilling of the warmer moist air near the ground causes a general haziness, light flow or fairly low clouds to appear. (2) The rapid rising and cooling of the warm moist air over the cold mass front causes the somewhat higher thunderclouds (cumulonimbus) to appear. Heavy rains fall, and, on the ground, strong winds blow. (3) Diminishing numbers of cumulus clouds can be seen high in the sky. On the ground the wind is calmer, and the air is cooler or even cold. There may be some variations in this sequence, depending upon the degree of difference in temperatures between the cold and warm masses. If the difference is slight, the stormy portion is less violent -- possibly just changing from mild to light rains and to slightly cooler temperatures. In the winter, the cold front may bring snow or sleet instead of rain. When a warm mass (red water) moves against a cold mass, there is a warm front. In the same demonstration, you can imagine how the warm mass can advance over the cold mass and bring a sequence of weather events: (1) Feathery bands or rows of cirrus clouds appear about 8 miles up in the sky where the air is so cold that water vapor freezes. The clouds are made up of ice crystals caused by the warm mass being chilled by the cold mass. (2) At a lower altitude, thin sheets of altostratus clouds appear. (3) Nimbostratus clouds appear, and steady rains fall. The clouds seem to come lower and lower as the front progresses and may touch the ground as fog. (4) The sky clears, and the air is warmer and damp. Many variables can affect this general sequence. Note that when a cold front advances, the progression of clouds increases in altitude. They decrease in altitude when a warm front progresses. By knowing what type of air mass is approaching and how fast, the meteorologist can generally predict the type of weather and when it will arrive. b. Occluded fronts. Insert the glass partition in the aquarium. Place warm water in one side of the aquarium and cool water in the other. Use food coloring to color the warm water red and the cool water blue. Add half a spoonful of salt to the cool water. Remove the partition, and allow the water to intermix. Reinsert the partition; stir the water on one side of it, then remove the partition again. The three colors of water now visible represent three different air masses -- warm, cold, and colder. You will see the air mass of intermediate temperature forcing its way between the warmer and colder layers to form three distinct layers. This relationship forms an occluded front. In a sequence of events, this triple-mass system passes high above the ground. The clouds move downward (e.g., cumulus, stratus, nimbostratus), then back up again (e.g., nimbo- stratus, altostratus, cirrus). The lowest clouds are almost always some distance above the ground. The weather changes from clear and cool to stormy, then to clear and cool again. Use the aquarium to see what would happen if no salt was used in the beginning, or if the two compartments contained water of the same temperature. You might find the temperature differences among the three layers in an occluded front. Other liquids, such as white syrup or glycerine, might be tried to see if they slow down the changes that take place.  Any problems with this page? Send URL to
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