Lecture 34Outline of Last Lecture I. Review about Difference between Extratropical Cyclones and Tropical CyclonesOutline of Current Lecture II. RemindersIII. Weather of the dayIV. Air Masses and Frontsa. Illustration of different frontsV. Extratropical CyclonesCurrent LectureRemindersHomework due this FridayCloud Project due December 14Final is 2 weeks from todayReview Friday next week Weather of the dayIt is pretty nice outside. It looks like it is going to be nice for a while. It is related to movement into a zonal pattern. We have some cold air coming in though. We are in the middle of the gradient as you spread out. We will be in the middle of a high-pressure system by midnight tonight. We would expect for the weather to be clear skies with calm wind. What is causing rain on the map? We can see the thickness line or the mean temperature of the lower troposphere. There is a wedge of cold air stationary along the gulf coast that is slanting upward and to the north. The low pressure is drawing air up. The wedge of cold air on the surface and the low pressure has circulation associated with it, the air is raising semi parallel to the isobars. As air progresses upward it is lifted and cools until it reaches cloud base and then it condenses water into precipitation forming ice aloft that then falls to thesurface through the cold air and produces the rainfall. Atm Ocn 100 1nd EditionRainfall that is shown on the map is rainfall over the past 12 hours. Precipitation with these systems is from lifting of air over fronts. The fronts are the boundarys between the warm and the cold air. Warm air is like a ramp, air is driven up the ramp and the ramp is indicated by the radiant or decreasing thickness as you go north. This means that air getting colder as you go north. Does the low we are looking at have a warm front?There is not a warm front drawn on the map. The front is indicated on the map, it is the boundary where the cold air reaches the surface. There is a strong gradient of thickness getting lower as you go north. This is also where the ramp of cold air is, it starts at the surface and goes deeper as you go north. The warm front is right below the ramp. You can see it by the trough of low pressure. There are kinks in low pressure, a sharp curve; it is a trough of low pressure going left. The trough is caused by the change in temperature resulting from the front. The trough that goes from over Minnesota to above Montana is also where the warm front is. Air is being lifted over the warm front, so there is a band of precipitation. Why? The low is lifting air over that front so air is being pushed up the ramp which is the front sloped down from the surface from the north to the south. When air is pushed up the front it gets colder because it’s lifting the lowerpressure, it reaches dew point and saturation and condenses water forming ice and liquid droplets. This turns into snow and then it reaches the surface and sometimes melts. This is why there is precipitation is by the warm front. California is getting a ton of rain. This is what happens in El Nino. Back to lecture Air Masses and FrontsIdealized picture of what a front looks like. As we form a wave over the cold air that is moving from the left to the right. We expect the front to drop south. The warm front is by the right. Then the cold air wraps the warm air underneath the top. And then warm air is over the top of the cold air. They close off…At this point where cold air has reached warm air it is an occlusion. The three fronts are cold front, warm front, and occlusion fronts of air. What happens is that one front movesover the other front. As occlusion occurs, the cold air will move across the country and remove the gap. The cold air will advance over the warm air until the open sector is completely gone. A cyclone forms following the occlusion process by advancing the cold air to the south. The way the cold air advances toward the equator is through the extratropical cyclones, the spinning motion, occlusion, it then pushes the cold air south. If you look at slope of the cold air it is less, it is still on slope but it is reduced.The colder the air is the higher the mass is. What you find happening is that cold air advances to the south, warm air to the north, the average center mass is lower. The sun by heating the mass puffed up the atmosphere, difference in temp north to the south, elevated center of gravity. The center of gravity is dropped then. The energy that supplies the tropical cyclone is from the drop of center of gravity in the mass. It is a lower energy state after the cyclone advances the cold air. We have converted potential energy keeping the center of gravity high, by dropping the cold air height and raising the warm air height, which decreases center of gravity. That energy is released and drives the cyclone. All the winds, that energy is associated with the released energy from dropping the center of gravity. We need extratropical cyclones in the northern hemisphere to get cold air to get to the south. Otherwise the poles would get extremely cold and middle latitudes would stay warm. All of the energy is mixed by having extratropical cyclones. Illustration of the different frontsThe cold front is in blue. The warm front is in red. The stationary front is alternating red and blue. The half circles and triangles alternate for the warm and cold front. The stationary front is no different but its stationary. The cold front tends to be steeper and the warm front is less steep. The occluded front, or warm and cold front put together, is purple. And you have circles and triangles. There are also dry lines occasionally which are half circles and in brown. The dry lines occur in the mountains usually. Not all of these illustrations are used anymore. Now we have computer analysis with thickness line and can show where the fronts are. In television they still use signals sometimes. But now on most weather maps you have to find the fronts.Lecture 17 Extratropical Cyclones Chapters 10 and 11From satellite we can see the extratropical cyclone systems. The cold front is by Texas because that is where airis lifted. The warm front goes through Kentucky. Classic Norwegian Model of Frontal CycloneHave a cold front and a warm front. It is basically the low we saw in the animation. They don’t all look classic because cyclones come across mountains or they interact with the Gulf of Mexico or so on like