The Atmosphere and Winds

Transcription

1 Oceanography 10, T. James Noyes, El Camino College 8A-1 The Atmosphere and Winds 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 A convection oven is an oven in which the hot air circulates instead of just rising to the top, so it cooks food more evenly than a normal oven. Typically a fan is used to keep the air in motion. A convection cell is also air (or another fluid like water) circulating, but its motion is caused by changes in its density 1. 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 (tricky, huh? try to keep up ), 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. The cold air slides away from the cold place, replacing and lifting up the warm air. The warm air pushes aside the air that was above it, moving it towards the cold place to replace the air that is sinking there. Basically 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! In this section, you will learn about one cause of winds. The key concept in this section is the concept of a convection cell. Remember: warming air makes its molecules move faster. This helps the 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. 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. Cold High Altitude Winds Surface Winds Ocean Warm 1 It is a cell, because the air is trapped in one place, like a criminal walking around their cell, unable to actually go anywhere.

2 Oceanography 10, T. James Noyes, El Camino College 8A-2 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. 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 s? Clearly, air is warmer at the Equator. Therefore, air rises at the Equator and sinks at the s, and the cool air from the s will slide to the Equator to replace the air rising at the Equator. In other words, surface winds tend to blow away from the s 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. 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). Note that the red arrow at the Equator represents air moving up, and the blue arrows at the s 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, means towards the (the high altitude green arrows), and South means towards the South (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! (90 o N) 60 o N 30 o N Surface Equator (0 o ) Winds 30 o S 60 o S South (90 o S)

3 Oceanography 10, T. James Noyes, El Camino College 8A-3 The Coriolis Effect As you probably know, the Earth is not stationary: it is rotating/spinning/turning around its axis 2 once per day 3. 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 4 ; 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 5. 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. N N N E E Earth S turns towards N the East S Earth turns towards the East W WE E W W S 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 6 in the southern hemisphere The Coriolis effect is stronger near the s (weaker near the Equator) 7. The bird s-eye-view maps on the right 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!) South East west East West South (east) The green arrows show the directions that the winds "want" to go, and the black arrows show how they are bent off course by the Coriolis effect. 2 An imaginary line drawn from the to the South. 3 The Earth turns towards the east. This is why the Sun comes up in the east. 4 Think of your own experience: If you throw something like a ball, it keeps going in the direction you threw it. 5 We don t realize how fast we are moving, because the motion is so smooth; we don t feel speed does traveling in an aircraft at 400 mph feel any different from traveling in a car at 50 mph? we feel acceleration or bumpiness. 6 As you can see from the picture, 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. 7 Here is a way to think about it: the Equator is not in the northern hemisphere or the southern hemisphere, so objects don t bend left or right. In other words, there is no Coriolis effect on the Equator.

4 Oceanography 10, T. James Noyes, El Camino College 8A-4 There are a few more things that you should know about the Coriolis effect. 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 8. 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 starting on page 1. Coriolis Effect: A More Advanced Explanation (You do not need to read and understand this section. It is for students who want to understand the Coriolis effect in more detail.) The Coriolis effect arises because objects become too fast or too slow for their new latitude. The Earth and every object on it are constantly moving, because the Earth rotates (spins) all the way around each day. The closer you are to the Equator, the faster you are moving, because you are farther from the center of the circle (the Earth s axis, which hits the surface at the ). If an object begins traveling north, its momentum to the east (from its old latitude) will be too fast, carrying it a bit too far to the east. If an object begins traveling east, it has become too fast for its current latitude and so spirals outward, away from the, as the Earth turns but the object moves too straight. If you are "above" the looking down, an object going north towards the s is going from a latitude where the ground is moving faster to a latitude where the ground is moving slower (the itself rotates, but the ground doesn't move anywhere, so it has a speed of 0). The object keeps the momentum to the east that it has from the "faster" latitude where it began, so it is going faster to the east than its new latitude, causing it to bend off course to the east (to its right). If an object is in the Southern Hemisphere and going north, it is heading to the Equator, where the ground is moving the fastest. The object is moving too slowly to east, so the ground gets ahead of it, and the object bends off course to the west (to its left). Notice that I apply the same reasoning in both hemispheres. 8 unless you have a VERY large toilet in your house

5 Oceanography 10, T. James Noyes, El Camino College 8A-5 The Global Wind Pattern 9 (Figure 6.12 on page 174 in your textbook) Winds and currents are 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. (90 o N) Polar Easterlies 60 o N Westerlies 30 o N The Coriolis effect causes the extra convections cells and the winds that go Trade Winds backwards between 30 o N/S and 60 o N/S. Equator (0 o ) As the air tries to move from the s to the Equator, it is bent off course towards the Trade Winds west. The problem is that the farther the air travels, the more it gets turns off course. At 30 o S some point, it is no longer heading north or Westerlies south, and therefore it cannot make any more 60 o S progress towards the Equator. Instead it rises Polar Easterlies up at this new latitude. The air at 60 o N/S is South not warm (it is in Canada), but it is warmer (90 o S) than that air at the s, so its density is low enough to rise at this latitude. Similarly, after air rises at the Equator, it moves north towards the s, 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 s. 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. 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 s; 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). 9 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. (For example, see figure 6.13 on page 176 of your textbook.) All this complexity contributes to the weather patterns that we experience every day, our next subjects.

6 Oceanography 10, T. James Noyes, El Camino College 8A-6 Clouds, Rain, and Pressure Air mainly consists of oxygen molecules and nitrogen molecules. Remember that warm molecules move faster than cold molecules. This allows warm air molecules to push aside nearby molecules and spread out, which lowers their density and causes them to rise. As you ll recall, atmospheric pressure is caused by the weight of the air above. Thus, up in the mountains, air pressure is lower, because there is less atmosphere above you (less air pressing down on top of you). Therefore, as warm air rises higher into the atmosphere, it experiences lower pressure. Since the group of warm, rising air molecules are no longer being squeezed together as strongly by the air above, the group of warm, rising air molecules can now push outward (i.e., the warm air expands as it rises). However, in pushing outward against the neighboring cooler air molecules, they give their energy to the neighboring air, causing the warm, rising air to cool down. Experiment: Blow into your hand. First, keep your mouth opening small, then open wide as if yawning. In which case does the air feel warm? In which case does it feel cool? When the opening is small, the air is forced together, and quickly expands once outside your mouth. If the water molecules in the air cool down enough, they will begin to bond with one another. (The water molecules are no longer moving fast enough to fly apart when they get to close to one another and strong hydrogen bonds form between them 10.) Thus, rising air produces cloudy and rainy skies 11. As the rising air cools down more and more, it loses its water as rain. By the time the air reaches the location where air sinks, it is completely dry; dry air cannot produce clouds or rain. lower pressure low pressure high pressure water has fallen out as rain mountain If the locations of clouds & rain and clear skies do not make sense to you, then you might skip ahead to Weather, Climate, and Fronts where I try to explain why the explanation above may seen inconsistent with your own experience but is not. 10 In most situations water vapor needs additional help from aerosols tiny solid particles like dust or drops of liquid in the air to condense: it is easier for water molecule to bond with big, slower-moving objects. The kind and size of aerosols available can have a big impact on whether clouds form and rain occurs and how much rain occurs. 11 Moist, humid air has a lower density than dry air, and rises higher than dry air. When water evaporates from the ocean and enters the atmosphere, water molecules push aside other, heavier air molecules via collisions, thus reducing the density of the air it is mixed with (but the neighboring dry air becomes more dense). In addition, remember that water has to lose a lot of heat before it will cool down or condense into rain (put another way, water has more heat a higher heat capacity and latent heat than other substances), so the water molecules heat helps keep the air warm in spite of the cooling that occurs as air rises, allowing the air to rise higher than it otherwise would.

7 Oceanography 10, T. James Noyes, El Camino College 8A-7 Typically this cooling of rising air (called adiabatic cooling) does not cause the group of rising air molecules to become more dense and sink. Why not? Think about how the air became cooler: as a side-effect of expanding owing to lower atmospheric pressure. At this point in the class, most of my students know the following: if it is becomes colder, it becomes more dense, so it sinks. This is true most of the time in the ocean, but it is only part of the story in the atmosphere: pressure is also an important factor 12. If the group of rising air molecules cools by expanding (by spreading out, by becoming larger), then their density is not going up: they are spreading out! This is why the air up in the mountains is colder and stays colder (think of the snow that covers the peaks of mountains), even though you might expect it to sink down to sea level: the lower pressure on the air molecules allows them to spread out, keeping their density low. Incidentally, this is why the air at high altitudes is thin (harder to breathe, thin = low density ). Beneath regions of warm, low-density, rising air, the pressure at the surface of the Earth is lower (fewer air molecules above), and beneath regions of cold, high-density, sinking air, the pressure at the surface of the Earth is higher (more air molecules above). (Another way to think about this: if the air is rising going up it is not pressing down very hard, and if the air is sinking going down it is pressing down harder.) Thus, lower air pressure at the surface is associated with cloudy & rainy skies, and high pressure is associated with clear skies. (Just listen to weather forecasters on the news!) Clear Skies High Pressure Cold I expect you to memorize the global rainfall pattern, shown on the right, as well as the global wind pattern. If you know where surface winds come together and the air rises, then you know where it rains. Similarly, where surface winds move apart, air sinks, and the skies are clear. It is worth noting that often winds cannot reach all the way from the high pressure ( sinking, cold air ) place to the low pressure ( rising, warm air ) place, because they are bent off course by the Coriolis effect. As a result, winds of spiral out of regions of high pressure and into regions of low pressure. (See figure 6.14 on page 177 of your textbook.) 12 Pressure is also important in the ocean, but it is much less important than temperature and salinity. High Altitude Winds Surface Winds Ocean Low Pressure (90 o N) 60 o N 30 o N Equator (0 o ) 30 o S 60 o S South (90 o S) Warm Clear Rainy Clear Rainy Clear Rainy Clear

8 Oceanography 10, T. James Noyes, El Camino College 8A-8 Mountain Effect Air can rise for many reasons. For example, when winds hit mountains they are forced upwards to get up and over them. As the air rises, it cools and water vapor in it condenses into clouds are rain. If winds tend to come from one direction, the side of the mountain facing the winds gets lots of rain, so it tends to have lots of vegetation. This is why Palos Verdes (a hill by the coast) can often be quite foggy. The side facing away from the wind is gets very little rain (the moisture fell on the other side), so it tends to be dryer and more desert-like. About 70% of our water in California comes from snow that falls in our mountains. If we did not have mountains stretching from north to south, we would have a lot less water in California. Weather, Climate, & Fronts Up till now, we have been discussing climate, not weather. Climate is the long-term average of weather conditions (what the weather is usually like). For example, Southern California has a warm, dry climate. This does not mean that it is always warm (we have our cooler days) or that it does not rain in Southern California; it means that our weather is warm most of the time and that rain is less common here than elsewhere. Another way to think about it: weather is what conditions are like a particular day, climate is what conditions are like over a season or a year. Your own experience of actual storms and rain may contradict something that I said before: warm, rising air leads to clouds and rain. Many of you will say: wait a minute, the weather is cold when it rains! Storms often form along what meteor-ologists call fronts, a place where 2 air masses meet. An air mass is a collection of air with similar properties (e.g., temperature, moisture), often determined by where it comes from. For example, warm, moist air moves up into the United States from the Gulf of Mexico, while Cold Air Before: Warm Air Cold Air After: Warm Air cool, dry air comes down from Canada. We also use the word front to describe the location where two opposing armies meet and are shooting at one another. As in the military, the frontlines typically are where the action is (clouds, rain, hail, snow, etc.) in the atmosphere. At the locations where air masses meet (the front ), the cooler air pushes the warmer air up, sliding in underneath to replace it, or the warmer air can move up and over the cooler air. As the warmer air rises, it becomes cooler, and if the change in temperature is strong enough and the rising air contains enough moisture, the water vapor in the rising air will condense into rain. If the warmer, rising air does not contain water, there cannot be rain along the front. Thus, the weather is cooler when it rains, because cooler air is coming in and lifting up the warmer air. (Remember, the warmer air might not be very warm, it is just warmer than the cooler air on the other side of the front.)

9 Oceanography 10, T. James Noyes, El Camino College 8A-9 The Heat Balance of the Earth and the Seasons In this section, you will learn why temperature changes with the seasons, why some parts of the world are warmer than others, and how the motion of the ocean and atmosphere keep the warm places from getting too hot and the cool places from getting too cold. The Equator is warmer than the s, because it receives more heat from the Sun. Sunlight shines directly down upon the Equator, but approaches the s at an angle. As a result, sunlight is spread out over a wider area at the s (It is less concentrated, so these places are colder.) In addition, sunlight that comes in at an angle is more likely to get reflected back into space (the white snow and ice at the s help a lot too) rather than absorbed, and passes through more of the atmosphere (which absorbs a little bit more light than normal). Equator Spread Out Concentrated Experiment: Get a flashlight. Hold South your hand flat with your fingers pointing towards the ceiling. Hold the flashlight horizontal and shine it on your hand. Now, tilt you palm upwards towards the ceiling. What happens to the circle of light on your hand? You can see how sunlight is spread out at the s because it strikes the surface at an angle. Sun These factors also help explain why some parts of the year are warmer than other parts of the year (in other words, why we have seasons). Notice how the Earth is tilted relative to the Sun; the Earth s always points towards a star we call Polaris (creative, huh?), also known as the Star. So, as the Earth orbits (travels around) the Sun, its tilt never changes 13. Earth S. Equator N. ern Hemisphere Summer Location of the Warm Spot? Sun S. Equator N. ern Hemisphere Winter Earth Sun Earth 13 Actually, the Earth s tilt wobbles very slowly in a small circle over thousands of years.

10 Oceanography 10, T. James Noyes, El Camino College 8A-10 During our summer, the northern hemisphere is tilted towards the Sun, so we get more sunlight and become warmer. The southern hemisphere, on the other hand, is tilted away from the Sun, so it gets less sunlight and becomes cooler. It takes the Earth 1 year to travel all the way around the Sun, so in 6 months, the Earth will be on the other side of the Sun. The tilt does not change (it always points towards the north star, Polaris), so now the northern hemisphere is tilted away from the Sun. We get less sunlight during this part of the year, so it is our winter. The temperature of a place is not merely a matter of how much heat it receives, because if an object only gains heat, then it continues to get hotter and hotter. Objects also lose heat by conducting it to the neighboring environment (for example, your hand if you touch a cold surface) or radiating it away as infrared light (invisible to us because our eyes cannot capture it, but we can feel its heat when we get close to a hot object). Irrespective of how the heat is lost, the basic rule of heat loss is: The hotter an object is, the more heat it gives off. (Rocket science, huh?) As an object gives away heat, it cools down, and therefore it gives away less and less heat over time. This is kind of like someone who suddenly receives a lot of money: at first, they spend it freely (after all, they have plenty), but if they keep this up, sooner or later they are forced to lower their spending if they don t want to be left with nothing. Even frozen objects give off heat, and therefore get even colder! (They just get colder slower and slower.) Fortunately for us, the Earth does not run out of heat, because it gains more heat each day from the Sun. Every moment of the day and night, the Earth gives away heat to the atmosphere (via conduction) and radiates the rest towards outer space as infrared light (electromagnetic waves). The heat given to atmosphere is eventually radiated into space too, helped by the fact that warm air rises upward (transporting the heat through the greenhouse gases like carbon dioxide that absorb infrared light, trapping its heat in the atmosphere and keeping our planet from becoming a giant ball of ice!). Most of the atmosphere is made of nitrogen and oxygen. Carbon dioxide, water vapor, and all the other gases in the air combined only make up about 1% of the atmosphere! The s are colder than the Equator, so they give off less heat than the Equator, but they still radiate heat into space. Interestingly, observations 14 show that the s give off more heat each day than they receive from the Sun. Similarly, the Equator radiates less heat into space then it receives from the Sun. If the s are sending away more heat than they receive, they should get colder, and if the Equator sends away less heat than it receives, it should get warmer. But, of course, they are not getting warmer or colder; their temperatures are stable (global warming issues aside). An object s temperature is stable (does not increase or decrease) if the amount of heat it receives is exactly equal to the amount it gives away (just like how your bank account won t go up or down if the deposits are exactly equal to the withdrawals). 14 from satellites looking down, for example.

11 Oceanography 10, T. James Noyes, El Camino College 8A-11 The temperatures of the s and Equator are not increasing or decreasing, because the ocean and atmosphere are moving ( transporting ) heat from the Equator towards the s (so the Equator has less to spend and the s more to spend ). In convection cells, the cool air moves away from the cold spot and towards the warm spot. The air then warms up at the warm spot, and rises (absorbs heat from the warm spot, cooling it down). Similarly, the cold spot cools the air above it. In other words, heat goes from the air to cold spot, warming the cold spot. Thus, the air moving in the convection cell is cooling down the warm spot (the Equator) and warming up the cool spot (the s). As we will see in the next lecture, the ocean does the same thing by moving warm water from the Equator towards the s and cool water from the s towards the Equator. The movement of water between the ocean and atmosphere also plays an important role in transporting heat from low latitudes (e.g., the Equator) towards high latitudes (e.g., the s). Warm ocean water evaporates under the clear skies of 30 o N/S (e.g., southern California), moving heat from the ocean into the atmosphere. (Remember: the hot, fastest-moving water molecules tend to be the ones that evaporate.) Some of the air moves towards the s in the winds called the westerlies (the convection cell between 30 o N/S and 60 o N/S). The air gives up its heat to the cooler ground beneath (e.g., Seattle), causing the water to condense into clouds and rain. Thus, the motion of the atmosphere keeps the s from becoming too cold and the Equator from becoming too hot. As the air moves, it carries the heat away from the hot places and moves cold air away from the cold places. The motion of the ocean the ocean currents performs a similar job, making the Earth a much more pleasant place to live. Who would want to live in Canada if it were even colder and in Mexico if it were even warmer?

12 Oceanography 10, T. James Noyes, El Camino College 8A-12