As you'll learn in later in the course, tight packings of isobars (large pressure gradients) correspond to strong surface winds. and eastern Canada, indicating a large pressure gradient over this region (relatively large pressure changes over a short distance). Note the tight packing of isobars over the northeastern U.S. Check out the 06Z analysis of sea-level pressure on Ma(below), which shows a powerful low-pressure system along the New England Coast. Now let's take what we've learned from topographical maps and generalize it to weather maps. Thus, a small gradient, which marks little change in elevation over a relatively short horizontal distance (fairly flat terrain), corresponds to a loose packing of contours. Meanwhile, the "packing" of contours of constant elevation is rather loose where the terrain is flatter. ![]() Thus, a large gradient on a topographical map, which marks a large change in elevation over a relatively short horizontal distance (steep terrain), corresponds to a tight packing of contours. Remember that the contours of constant elevation are packed close together in areas where the elevation changes rapidly over short distances (like near the mountain summits). How do we distinguish between a large and small gradient on a topographical map? Let's return to the video showing the virtual topography of Hawaii ( video transcript) you saw earlier. Similarly, a small gradient means that the elevation of the road changes very little with distance (the road is relatively flat). In other words, you're driving on a steep hill. What does that mean in practical terms? For the mountain road to have a large gradient, the elevation of the road must change relatively rapidly over a short traveling distance. Driving to the summit of the beautiful mountains in central Pennsylvania, motorists are greeted by signs that warn of "steep grades" ahead, which refers to large gradients. Zones where weather variables have large changes are often zones of active weather, so meteorologists like to keep tabs on areas with so-called "large gradients." Tuck this idea away, as the importance of gradients will come back again and again in our studies of meteorology.īut, to start us off in our discussions of gradients, let's return to the example of elevation. The change in a variable over a certain distance is called the gradient, and gradients are very important in meteorology. Similarly, maps of isobars allow meteorologists to find areas of high and low pressure, and see how pressure changes over certain areas. Maps of isotherms allow meteorologists to identify regions of warm and cold air, as well as how temperatures change over certain areas. In other words, contour maps make it easy for meteorologists to see how a weather variable (like temperature or pressure) is changing over a large area. Sometimes tornadoes form when very high pressure air collides with very warm, moist low pressure air.Besides allowing meteorologists to estimate values of weather variables at specific points, contour maps are also useful tools that help meteorologists to see patterns in the data. Clouds begin to form and move across the sky -thunderhead clouds forming when moist air is thrust very high. With very low pressure systems, storms are on the way (if they aren't there already).(Clouds are simply water droplets that are small enough to be kept aloft). ![]() thus, rising low pressure air will only produce rain if it gets up where the air is cool enough to condense the water vapor into droplets too heavy to be kept aloft by the rising air. But droplets won't form if the glass is only slightly cool. You see this effect when air's invisible water vapor is forced to condense into droplets when it contacts the outside of a cold glass). ![]() Surrounding air draws inward toward the low system's center as the lighter air balloons upward, often causing clouds or precipitation because that moist air cools as it rises. A low pressure system is an air mass that has less dense air because its air is moister and/or warmer. Low pressure is usually associated with humid air and in some cases, precipitation. Understand what a low pressure system is.
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