[MUSIC] The last engine, that of the fronts. Perhaps the one on the fronts is best known by people who live in, for people who live in middle latitudes, in most cases we think it is the only engine of weather, and that is not true. More often than not, weather is powered by other engines, and not that of frontal systems. What are the fronts? You have seen them many times. If we have a mass of cold air acting on a mass of warm air, this is cold moving air. The cold air is more dense, so it behaves as if it were a kind of liquid, which is not a liquid, it is a gas, but that moves more densely, therefore, has a wall that is very steep and moves on top of the hot air. And this is hot air moving on cold air. The hot air is less dense, therefore, when it is in cold air, it is placed on top. It is placed on top of the cold air, So the separation between hot air, cold air, is a kind of line on which warm air is put on top. How do we draw this? You have seen it many times, the cold front, which is a blue line with blue triangles. The warm front, it is a red line with red semicircles. How does the wind behave around the fronts? As this is a moving steep line, what the wind is doing here is to generate a parallel ascent to the line where the air mass change. This is the front line, the front by definition is where the mass of air cuts the ground, here is the front. But high up it does not have to correspond one thing with the other. You notice that this is going backwards, therefore, the surface front does not have to be the same position high up, and even less on the warm front. Notice that this warm air is put on top of the cold, therefore, here what is happening is the wind. It is rising parallel to this separation of air masses, the frontal line is here, but notice that the front is very large, is very extensive. Therefore warm fronts can be enormously large, the associated cloudiness or the disturbance of weather associated with the warm front, much larger in extent, because this slope can be quite pronounced. What will happen with the warm fronts? You have seen it many times, because there is a whole series of cloud types, these are the Cirrus. They are high clouds, later they form another type of high clouds, that are Cirrocumulus, Cirrostratus, and finally there is a cloudiness associated with warm fronts, that can even give precipitation. They are of Nimbostratus types. Cold fronts, on the other hand, remember that the wind ascends in a way vertical in its front, therefore the cloudiness associated with a cold front, here... Cumulonimbus, of the shower, or the storm type. Cold fronts typically associated to showers and storms, while those associated with warm fronts typically have areas with lots of cloudiness, extensive cloud cover. And some areas of precipitation, which may be generally stratiform, It means that it falls in a continuous but not very intense way. And another very defining thing of the fronts is the variation in the direction of the wind. The wind before and after the front will change, You know that because it was visible in the isobars. Associated to the front, typically before a front we will find Southwest, after the front we will find the north-west, typically associated with the change of a cold front. While on a warm front the difference is not so marked, here it is more difficult to know when the front has passed, before we will usually have more southern winds, and here they will be more from west. But that will depend a lot on how this warm front is configured. In addition, the warm fronts are very well defined in high latitudes, above the fifty degrees, sixty degrees, there they will behave well because the difference between the masses of cold and hot air are going to be very distinct. But at lower latitudes, toward the 40 degree latitude, the warm fronts typically pass very blurred, and are a bit more difficult to monitor.
