Intro to Weather

Editor's Notes

• #3: The vast majority of clouds that you see are formed via adiabatic cooling. In the first part of your experiment, you probably did not see a cloud form, because even though the air cooled, there was nothing for the water vapor to condense on. That's why I had you use the match in the second part of the experiment. The match put small particles of smoke into the bottle. The water then could condense onto those smoke particles, forming a cloud. Most clouds in Creation form around small particles in the atmosphere, like the smoke particles left by the match in your experiment. We call these particles cloud condensation nuclei (new' klee eye), because they form the “center” of water condensation.
• #11: When air stays in a particular region of the earth, it tends to pick up the region's temperature, humidity, and pressure. When this happens, we call the result an air mass. Air mass - A large body of air with relatively uniform pressure, temperature, and humidity Because an air mass is relatively uniform throughout, it tends to stay together, moving as a unit. When an air mass moves into a region, it brings with it the weather characteristic of the region in which it was formed. Wherever the air mass stays, the weather in that region of the earth is the same from day to day, until the air mass moves away and another takes its place. By tracing the movements of air masses across the earth's surface, meteorologists (scientists who study weather) can predict the kind of weather a region will have by predicting what air mass will be moving over the region. There are three basic types of air masses: arctic, polar, and tropical. As their names imply, arctic air masses are very cold, polar air masses are cold, and tropical air masses are warm. Polar and tropical air masses can further be divided into maritime (formed over the ocean) or continental (formed over a continent). Maritime tropical (abbreviated as mT ) air masses, for example, are warm air masses that form over the ocean. These air masses tend to have high humidity, so we typically say that they are warm and moist. Likewise, maritime polar ( mP ) air masses are cold and moist. Since continental air masses form over continents, they are considerably less humid than their maritime counterparts. Thus, continental tropical ( cT ) air masses are warm and dry while continental polar (cP) air masses are cold and dry. Arctic air masses are always low in humidity, so we say that they are very cold and dry.
• #14: In a cold front, a cold air mass is trying to move in on a region of warmer air. Since cold air tends to sink, the cold air mass wedges under the warm air mass and lifts it up off the ground, pushing it back. As a cold front lifts up the warm air in its way, moisture in the warm air condenses into clouds. If the warm air is really moist, then thick, severe weather clouds can form. Since the warm air gets lifted up, the clouds tend to pile up. Usually, cumulus clouds form first, and as the warm air continues to rise, altocumulus clouds can form. If the warm air is particularly moist, cumulonimbus clouds can form. In fact, if the cold front moves in fast enough, you can often see the curved shape of the cold front in the edge of the clouds that form. Cold fronts are usually the cause of the most severe weather systems. The violent upheaval of a warm air mass, especially a warm air mass that is high in humidity, can result in the rapid formation of thunderclouds with little warning. A storm can come just as quickly. As you might expect, the temperature drops as the cold front moves in. This is actually a good thing, however. Once the cold air mass moves in, the sky generally clears within a few hours.
• #15: A warm front, on the other hand, results when a warm air mass moves into a region that is occupied by colder air. Since warm air tends to rise, a warm front usually moves over a cold front. This flattens the cold air below and forms an upwardly-sloping front that can stretch for several hundreds of miles. This is a less violent collision than what I described for a cold front, so the resulting weather is usually less violent. The end result is still warm air rising, so clouds do form as the warm air cools. In a warm front, this usually starts out with cirrus clouds, followed by cirrostratus clouds. Then altostratus clouds are formed, and finally, nimbostratus clouds appear. The rain that results from this kind of front is typically less heavy than that formed by cold fronts, but it usually lasts longer.
• #16: When two air masses collide and neither moves, the result is a stationary front. As you can see from the figure, a stationary front looks much like a warm front. The main difference between a warm front and a stationary front is that winds typically blow along a stationary front in opposite directions up and down the front rather than against the front. Also, since neither mass makes headway in a stationary front, the weather tends to stick around longer than that produced by a warm front. Stationary fronts often turn into warm fronts or cold fronts when one of the air masses actually begins to make some progress.
• #17: An occluded (uh clue' did) front arises when two air masses traveling in the same direction collide. How can two air masses traveling in the same direction collide? Well, it turns out that cold air masses travel faster than warm air masses. So, it is possible for cold air masses to overtake and collide with warm air masses. When that happens, an occluded front forms. Usually, an occluded front forms when a warm front is traveling over a cold front. Another, fast-moving cold air mass comes in from behind the warm front and lifts the warm air mass up and away. Because an occluded front starts out as a warm front and ends up much like a cold front, the weather produced is a combination of that produced by each type of front. First, the clouds that herald a warm front form. They are then followed by the cumulus and cumulonimbus clouds of a cold front. As you might expect, the weather starts out as the slow, light rain of a warm front, and then the severe weather typical of a cold front moves in behind it.