Howdy everyone and thank you for continuing on with module 8. In this video, we're going to continue talking about air pressure, but begin to define it a little bit more clearly and then start talking about what is controlling air to move in our atmosphere in the x, y, and z direction. So, finally, what is air pressure? Air pressure is a measure of the weight of the air above a point of observation and it's measured as a force over an area. Now the baseline for air pressure is mean sea level which is about 1,3.25 mibars which is equivalent to 14.7 lb per square in or 760 mm of mercury or 29.92 in of mercury. Now, pressure also changes more quickly with vertical distance than it changes over a horizontal distance. And pressure also decreases at a constant rate as you move up in elevation. So, that makes sense. The higher up you go in the atmosphere, the less air there's going to be above you. An Italian physicist named Evangelista Torichelli in 1643 invented the first instrument to measure air pressure. He used a calibrated glass tube inserted open and down into a shallow dish of mercury and noticed that the mercury would rise up into the tube. So that's why some of those units we just went over included millimeters or inches of mercury. This is exactly why. And he concluded correctly that the pressure of the air on the mercury is what it was actually forcing it up into the glass too. And on average he noticed the mercury rose to 760 mm or 29.92 in. Inches of mercury are actually still used today for measuring air pressure. And this device that he had that measures air pressure and the devices we use today are called barometers. And some of the common units of air pressure again include height of mercury, usually in inches, 29.92, at least here in America. But the bar is what's most commonly used in meteorology. And the bar is based on the force of a thousand dimes per square centimeter. And a dime is the force of acceleration of 1 meter per second per second. You don't need to memorize how a bar is defined and what a dine is, but just for context, one bar equals approximately 14 12 pounds per square inch. But you'll see in a lot of weather reports that atmosphere is reported in millibars. This is an air pressure map. All of these lines that you see on the map are called isobars and are connecting points of equal air pressure and shown here in millibars which again is the most common unit of measurement for atmospheric pressure. So you can see where we have high pressure where that's changing to lower pressure and keep in mind the mean sea level pressure MS LP is 1013 approximately. So when we see pressure above that, we consider it higher, below that, lower. You get the point. But this is how air pressure maps generally look. Just like our topographic maps that are showing elevation of the land surface, this is showing changes of air pressure over distance. Okay. So what is driving air to move horizontally, vertically, and in all directions? The x, y, and z directions. Well, there are four primary driving forces. The primary force exerting an influence on air molecules is the gravitational force. The atmosphere is literally being held against the Earth by this force. And our planet wouldn't even have an atmosphere if it wasn't for gravity. Now, the difference between surface pressure over a given distance between two locations is called a pressure gradient. And when there's a pressure gradient, it produces a force that causes air to move from high to low pressure. And that's going to become very important. The corololis force, which we saw with our atmospheric circulation, also drives how air moves. And this is due to the rotation of the earth that deflects moving objects in air. And then finally, we also have frictional forces which work in the opposite direction to surface winds. And generally these forces reduce its speed. So slow it down. Now pressure gradient is the rate of pressure change and that's occurring over a given distance. The pressure gradient force is the net force produced when there's a difference in horizontal air pressure. The pressure gradient force always points from high to low. Always. always points from high pressure to low pressure. The force is also always directed at right angles to the isobars. And remember, isobars are lines that connect equal air pressure. So when you see isobars on an air pressure map, when they're close together, that indicates a rapid change in air pressure. That produces a steep pressure gradient, which results in strong, high-speed winds. Now when our isobars are farther apart that represents a slower change in air pressure producing a gentler or gradual pressure gradient which results in weak low speed wents. Important to remember that the pressure gradient force is what's controlling wind direction. So it's always going to go from high pressure to low pressure always. But this force is also controlling the wind velocity. Where the air pressure is changing the slowest is going to produce slower speed winds. When that pressure is changing quicker, it's going to produce faster winds. So pressure gradient force controls wind direction and wind velocity. Coralis force varies with speed, altitude and latitude of a moving object. Now the coralolis force is almost zero at the equator but it's greatest near the poles. The higher the velocity of the moving object, the stronger the corololis affects it. Now corololis effects only controls wind direction. It does not do anything about wind velocity. And this coralis force is an apparent force and it's the result of that rotation of the earth on its axis. We saw how it deflects our trade winds in the atmosphere and our atmospheric circulation cells. This is also going to deflect weather patterns. And this force appears to deflect any free moving object. Doesn't just have to be air. It also deflects planes, ships, rockets, bullets. So the military and snipers really have to account for that. and even ocean currents and it deflects these freemoving objects from their original straight line path. Now the deflection is to the right in the northern hemisphere and it's to the left in the southern hemisphere. Now frictional force works in the opposite direction to surface wind and this usually reduces its speed. The closer individual air molecules are to the surface, the more they're going to be influenced by that surface, which slows it down by surface drag. But how much it does this, the magnitude of that friction force depends primarily on the roughness of the surface. And there is less friction over smooth snow or water than there is over mountains or forests or city skylines. Now this friction layer of the earth varies in height but for the most part it lies within about a kilometer of the surface. So any higher than that we usually just ignore that frictional force. But frictional force is going to work in the opposite direction and it's going to control velocity. It's going to reduce its speed. Now here showing the forces in wind directions acting together. We have our pressure gradient force that's controlling that air is going to move from high to low which is exactly why you have air moving out of high pressure systems and moving into low pressure systems. But that's not the only force that's controlling direction. We also have the corololis force and the frictional force. But the corolises force is going to deflect that moving air. And this is what's going to cause it to rotate. So air is always going to move into the center of a low pressure flow uh air pressure system, a low air pressure system. We call that cyclonic air flow. And air is always going to move out of a high pressure system. We call that anti-cyclonic air flow. So in the northern hemisphere, we get a counterclockwise rotation into the center of a low and we get a clockwise rotation out of the center of the high. So air is moving into the low because of the pressure gradient force, but it's rotating counterclockwise because of the corolises force. Now, in the southern hemisphere, we're moving clockwise into the center of a low and counterclockwise out of the center of a high. Okay, so hopefully that makes a little bit more sense as we start building on to some of the more um processes that are going to control a lot of the inclement weather that we'll be talking about later on this semester. I'll see you in our next video.