T. James Noyes, El Camino College Winds Unit (Topic 8A-1) page 1 Name: Section: Winds Unit (3 pts) The Ocean and the Atmosphere We need to learn about the atmosphere, because the ocean and atmosphere are tightly interconnected with one another: you cannot understand what is happening in one without understanding what is happening in the other. For example, the atmosphere s winds push water around, causing ocean currents and waves. Ocean currents shift around the warm and cold water that produces winds, and ocean water evaporates, giving the atmosphere the moisture needed to produce weather like storms and rain. Convection Cells The major winds that blow over the surface of the ocean are one part of a phenomenon called a convection cell. A convection cell is air (or another fluid like water) circulating due differences in density. Sunlight travels through the atmosphere, warming the surface of the Earth (both the land and the ocean). Some places, though, become warmer than others. The air above warmer places is warmed by the Earth, while the air above cooler places is cooled, and this imbalance in temperature is what sets the air in motion. The warmer, lower-density air rises over the warm spot, and the cooler, higher-density air sinks over the cold spot. (Remember: warming air makes its molecules move faster. This helps the High Altitude Winds warm air molecules push outwards (push aside the neighboring air molecules), allowing them to spread and thus lowers their density. The opposite happens to cold air molecules.) Cool, Sinking Air Surface Winds Warm, Rising Air At the surface, cold air slides away from the cold place, replacing and lifting up the Cold Warm warm air due to the cold air s higher Ocean density. The air moving right in the picture is the surface winds. This air is moving to the warm spot to fill in the empty space left behind by the warm, rising air, and is pushed to away from the cold spot by the cool, sinking air. The warm, rising air pushes aside the air that was above it, moving it towards the cold place to replace and push down the air that is sinking there. The air moving left in the picture above is the high altitude winds. This air is moving to the cold spot to fill in the empty space left behind by the cold, sinking air, and is pushed away from the warm spot by the warm, rising air.
T. James Noyes, El Camino College Winds Unit (Topic 8A-1) page 2 Overall, then, the air ends up moving in one big circle or loop. This, though, is not the end of the story: The cold air will be warmed by the warm place on the surface of the Earth, and the process will repeat again. And again. And again. The air moving horizontally is what we call wind. This is how the major winds of the world are created! 1. We get our fresh water from rain and snow. Where does the water vapor in the air come from? How or why does it enter the atmosphere? 2. Where does air rise, at the warm spot or the cold spot? Why? 3. Where does air sink, at the warm spot or the cold spot? Why? 4. Does the air at the surface of the Earth (the surface winds) move towards the warm spot or the cold spot? Why? 5. Does the air higher in the atmosphere (the high altitude winds) move towards the warm spot or the cold spot? Why?
T. James Noyes, El Camino College Winds Unit (Topic 8A-1) page 3 Sea Breezes and Land Breezes You might be wondering why some places on the Earth become warmer than others. A classic example, a sea breeze, results from our old friend heat capacity (see your 4A notes). Both the land and ocean are warmed by the Sun, but the land becomes warmer than the water owing to its lower heat capacity. (When water is heated, its temperature only goes up a little bit.) As a result, the air over the land rises, and cool air from ocean comes in to replace it, a sea breeze. The opposite happens at night. Both the land and ocean radiate heat into space, but when the land gives up heat, its temperature drops a lot more than water, so the land becomes colder than the ocean. The air over the ocean is not warm, but it does have a lower density than the cold air over the land, so the air over the ocean rises, and cold air from the land moves out to the ocean to replace it, a land breeze. Sea Breeze Land Breeze Wind Wind Warm HOT Cool COLD Notice that the ocean warms up too during the day in the picture above. The land is warmer, so the air over the land has a lower density than the air over the ocean. Similarly, both the land and ocean cool down at night, but the land gets colder. Technically speaking, it is not warm air that rises, but the lowest density air in the environment. 6. Which warms faster during the daytime, the land or the water? 7. During the daytime, where will air rise, over the land where the air is warmer or over the ocean where the air is less warm? 8. Which cools down faster during the nighttime, the land or the water? 9. During the nighttime, where will air rise, over the land where the air is colder or over the ocean where the air is less cold? 10. Are the names of these winds (sea breezes and land breezes) based upon where the air is coming from or are going to?
T. James Noyes, El Camino College Winds Unit (Topic 8A-1) page 4 What the global wind pattern would look like if the Earth did NOT rotate The goal of this entire section is for you to understand the overall motion of the atmosphere: the pattern and its causes. Here s your first step: Where is air warmer, at the Equator or the Poles? Clearly, air is warmer at the Equator. Therefore, air rises at the Equator and sinks at the Poles, and the cool air from the Poles will slide to the Equator to replace the air rising at the Equator. In other words, surface winds tend to blow away from the Poles and towards the Equator. The picture on the right is two pictures in one (in a way). The view is from outer space above the Equator. The green arrows within the globe are giving you a bird seye-view of the winds that are blowing over the surface of the Earth. They show the air over the ocean moving south in the Northern Hemisphere, and air over the ocean moving north in the Southern Hemisphere. The red, green, and blue arrows on the side of the globe are showing you what the air is doing vertically as well as horizontally (i.e., the convection cells). They show the air rising up at the Equator, moving towards the Poles, sinking down at the Poles, and moving towards the Equator near the surface of the Earth. Note that the red dotted arrow at the Equator represents air moving up, and the blue dotted arrows at the Poles represent air moving down. Up means away from the surface of the Earth, and Down means towards the surface of the Earth. (Do not describe them from your own perspective. The world does not revolve around you!) Similarly, North means towards the North Pole (the high altitude green arrows), and South means towards the South Pole (the low altitude green arrows). This is how the winds would blow if it were not for one teeny tiny little fact: the Earth rotates! 11. At what latitudes is the air rising? 12. At what latitudes is the air sinking? 13. What is the direction of the wind at 45 o N: north, east, south, or west? 14. What is the direction of the wind at 45 o S: north, east, south, or west?
T. James Noyes, El Camino College Winds Unit (Topic 8A-1) page 5 The Coriolis Effect As you probably know, the Earth is not stationary: it is rotating/spinning/turning around its axis once per day. This leads to a phenomena known as the Coriolis effect: objects traveling over the Earth bend off course. This happens because moving objects go straight forward while the Earth is turning beneath them. As the Earth turns, the directions north, east, south, and west change (see the figure below), but moving objects continue going in their original direction; in other words, an object moving north continues to move towards the old north, not the new north that results from the Earth s rotation. To you and me and other stationary objects on the Earth, it looks like nothing has changed except the direction of moving objects, because everything travels with us. Moving objects do not bend off course because they are changing direction, but because we and our point of view (the north-east-south-west arrows) are moving with the rotating Earth. North Pole N North Pole N E S Earth turns towards the East W E W S I will not ask you to explain how the Coriolis effect works in detail, but I do expect you to be able understand a few things about how it works: Objects bend off course to their right in the northern hemisphere Objects bend off course to their left in the southern hemisphere (As you can see from the picture above, the wind really bends in the same direction. They only appear to be different in each hemisphere because one person s right is the other person s left.) The Coriolis effect is stronger near the Poles (weaker near the Equator). The Coriolis effect is only significant for objects that travel a large distance (or a long time) over the surface of the Earth. Therefore the Coriolis effect is not an important factor in everyday situations; it does not cause curve balls in baseball or the direction that water goes down your sink or toilet. To achieve pin-point accuracy, our military needs to take the Coriolis effect into account when they fire shells or missiles more than a mile. Airplane pilots who fail to adjust for the Coriolis effect end up in the wrong city! Important: The Coriolis effect does not cause winds, just like it does not cause a missile or airplane to move. All the Coriolis effect only changes the direction of whatever is moving. What does causes winds? If you are not sure, review the section on Convection Cells.
T. James Noyes, El Camino College Winds Unit (Topic 8A-1) page 6 The bird s-eye-view maps below show the directions that the winds bend under the influence of the Coriolis effect. (Hint: If this confuses you, turn the paper so that the wind the arrow is pointing away from you, then your right and the wind s right will be the same!) The green arrows show the directions that the winds "want" to go, and the black dotted arrows show how they are bent off course by the Coriolis effect. 15. What causes the Coriolis effect? 16. Under what conditions do winds and currents turn towards the right of their direction of motion? 17. Under what conditions do winds and currents turn towards the left of their direction of motion? 18. Examine the maps below. Sketch arrows showing the direction each wind will turn towards into the maps below, and write the direction it turns towards (north, east, south, west, etc.) next to each arrow in the maps below. (Examples of what to do are shown at the top of the page.) NH indicates that the wind is in the northern hemisphere. SH indicates that the wind is in the southern hemisphere. 19. Where do winds and currents turn sharply to the side, the Equator or Poles, and where do winds and currents bend the least and thus go mostly straight? 20. True or false? The Coriolis effect causes water to spiral down toilets. 21. True or false? The Coriolis effect creates winds.
T. James Noyes, El Camino College Winds Unit (Topic 8A-1) page 7 The Global Wind Pattern As the air tries to move from the Poles to the Equator, it is bent off course towards the west. The problem is that the farther the air travels, the more it gets turns off course. At some point, it is no longer heading north or south, and therefore it cannot make any more progress towards the Equator. Instead it rises up at this new latitude. The air at 60 o N/S is not warm (it is in Canada), but it is warmer than that air at the Poles, so its density is low enough to rise at this latitude. Similarly, after air rises at the Equator, it moves north towards the Poles, cooling down as it travels. It is bent towards the east by the Coriolis effect, and at 30 o N/S cannot move any farther towards the Poles. The air at 30 o N/S is not cold (it is close to southern California), but it is colder than the air at the Equator, so its density is high enough for it sink at this latitude. (Remember that the dotted arrows show air rising and sinking air going towards or away from the surface of the Earth.) Air is always pushed away from the place where it sinks and towards the place where it rises. Between 30 o N/S and 60 o N/S, the air is forced to move in another convection cell by the sinking air at 30 o N/S and the rising air at 60 o N/S, respectively. Notice that these middle convection cells are the reverse of the convection cells by the Equator and the Poles; they move in the opposite direction. You need to memorize the global wind pattern shown above. I suggest that you memorize the directions of the trade winds: they both blow towards the Equator and towards the west. This should not be too hard to remember, because the Equator is the warmest place in the world: the air on the Equator rises, so the nearby air moves toward the Equator to replace the rising air. As the air moves towards the Equator, it is bent off course by the Coriolis effect. In the northern hemisphere the air moving south goes to its right, and the in the southern hemisphere the air moving north turns to its left: in both cases, the air turns towards the west. Once you have the trade winds memorized, the other winds are a piece of cake: the westerlies are the opposites of the trade winds, and the polar easterlies are the opposites of the westerlies (same as the trade winds). (Remember: Winds and currents are often named for the direction that they come from, not the direction that they are going to, so the westerlies come from the west and blow towards the east.)
T. James Noyes, El Camino College Winds Unit (Topic 8A-1) page 8 Even this picture of the global wind pattern is a gross oversimplification. The winds shift with the seasons as the warmest spot on the Earth shifts north and south of the Equator and the land becomes warmer or cooler than the ocean. All this complexity contributes to the weather patterns that we experience every day, our next subjects. 22. What is the direction of the winds at 75 o N? 23. What is the direction of the winds at 45 o N? 24. What is the direction of the winds at 15 o N? 25. What is the direction of the winds at 15 o S? 26. What is the direction of the winds at 45 o S? 27. What is the direction of the winds at 75 o S?