[MUSIC] Let's come back to the global picture, let's think about what we expect at mid latitudes when we combine these two balances, hydrostatic and geostrophic balance. Because of the differential heating, we said that the tropics will be warmer than the poles. For simplicity, let's assume the pressure is uniform at the surface. Given the temperature difference at a given height, the air is less dense in the tropics. So, pressure decreases more slowly in the vertical in the tropics. As a result, there should be an increasing horizontal gradient of pressure as you go up in height implying westerly winds that increase with height, leaving high pressures on the right in the northern hemisphere and on the left in the southern hemisphere. And if you consider seasonal variations, the pole is much colder in winter so the temperature difference is greater in winter and you expect stronger winds in winter. These are just expectations from very simple considerations. Well, here's a climatology of the zonal wind in northern winter from a meteorological center. First, I need to explain what we're looking at. This describes the wind in the west east direction, that's zonal wind. It's in meters per second with the latitude on the horizontal axis, and height indicated using pressure in the vertical. This cross section results from an average over all longitudes and over many years. So now we can describe the winds themselves. At mid latitudes, for both hemispheres, as expected from the simple considerations stated previously, there's a westerly jet which increases with height in the troposphere. It is stronger in the winter hemisphere. Things are drastically different in this stratosphere where the temperature of distribution is radically different. It's coldest at the winter pole, warmest at the summer pole which is constantly lit, but that's another story. So what about the surface? Well, the winds are weaker and note some easterly winds near the surface in the tropics, these are the trade winds. One then can understand them if we say a couple of words about the circulation in the tropics. Here's a schematic of the general circulation of the atmosphere. In the tropics, a lot of energy is received from the sun. The air is warm, it is very moist, leading to very strong, deep moist convection. The air that has thus been transported up then spreads out to higher latitudes and slowly subsides in cloud free regions. This circulation closes with air moving equatorwards towards the surface. In this branch of what we call the Hadley cell, the Coriolis force deviates the wind towards the west in both hemispheres. This results in regular easterly winds which have been relied upon by navigators in the 16th-17th century, hence the name of trade winds. Keep in mind now that these pictures are simplified, and just describe averages in time and longitude. At any given time, the flow is much more complicated than this. And other way to get the feeling for what the atmosphere looks like, is to look at images from satellites, and this allows to introduce flow features, that is structures which organize the flow at a given time. Here's a color image, built from observations in the infra red, from geostationary satellites. In the tropics, recognize patches of convection along the inter-tropical convergence zone. See the wide regions of cloud-free air where the air goes back down in the descending branches of the Hadley cells. At mid latitude, see how the flow organizes into long forms over thousands of kilometers and also in low pressure systems. There's a nice one to the southeast of Madagascar turning clockwise and another east of New England and Canada. Let's zoom in on this storm east of New England and Canada. Here, you can recognize a low pressure system, also called an extratropical cyclone on the East Coast of the United States. Cold air is brought from the north in the rear of the storm. There's a front ahead of it and ahead of the front, there's warmer, moisture air with plenty of thicker clouds and rain. Stronger winds are found near the front. In the case of this storm, which was significant storm, wind gusts reached up to 45 meters per second at the surface. The picture to the top left shows the latitude bands in which extratropical storms form. The storms typically move eastward by about 1000 km per day or 10 meters per second. Okay, to be fair, here's a beautiful picture of an extratropical cyclone in the southern hemisphere with winds turning clockwise around the low pressure system. Now, as a concluding remark, I wish to stress that I have focused on the troposphere and on the extratropics, how surface winds are related to the winds above, what other phenomena affect the surface winds and the distribution. Those will be topics for the next sessions.
