Course Icon
Natural Science - Year I

Unit 3: Scientific study of the Weather

Course Materials are always under revision! Weblecture content may change anytime prior to two weeks before scheduled chat session for content.

SO Icon

Science Web Assignment for Unit 2


This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 2: Weather - Movements of Air

For Class
Outline/Summary

Weather's Source of Energy: the Sun

Science Web Reading
Please look at the pages at the National Center for Atmospheric Research site on Weather Basics. NOTE: This website did not display properly in Safari. Use FireFox to see all the links in left navigation bar.] Follow the links inside the pages as you need to in order to understand the material and answer the questions in your homework. Depending on how much you already know about weather, you may need to read more or less on these pages. In particular, look at

  • Under the Big Picture, be sure to look at:
    • Sun
    • The Layered Atmosphere (Air Composition)
    • The Water Cycle
  • Under Building Blocks of Weather, be sure to look at (we'll read more next time from here):
    • Wind
    • Pressure changes
    • Under Climate Change, be sure to look at:
      • How Does Climate Change
      • Ancient Climate Changes
    Beaufort Scale: Check out the Beaufort scale here to get information about windspeeds.

This science web assignment covers a somewhat confusing array of different aspects of the weather: the atmosphere, climate, winds, fronts. A single principle about air movement can help us to understand how these phenomena are related. As we look at how masses of air move around the earth, let's pose the following hypothesis: Energy from the sun heats air, causing the air to expand and rise in a column (called a thermal), which in turn creates a partial vacuum that is filled by cooler air. As we apply this testable statement to a number of phenomena, we can start to put together a theory that all weather phenomena in the atmosphere depends on the energy from the sun warming the air in different locations by different amounts.

Let's see how this uneven or differential heating applies to the different phenomena you've read about.

Energy and the Up-Down Flow of Air on Earth

Many of the elements in Figure 1 are shown in more detail in your reading assignment, but we need to put them together differently in order to see how our hypothesis explains most of them.

We can compare two "beams" of sunlight striking near the equator and near the north pole. Each brings the same amount of energy, but the one near the north pole has to spread its light out over a larger area, because of the slant of the earth's surface to the direction of the incoming sunlight. This means a square foot of land near the poles gets less energy from the sun than a square foot of land near the equator. The air temperature at the poles will be cooler than the air temperature at the equator.

Let's apply our principle that warm air expands and rises. If the air at the equator is warmer than the air to the north and south of it, it will start to move upwards and expand, pushing outwards horizontally north and south. Cooler air from the north and south will rush toward the equator to take the place of the rising air.

Atmosphere Layers and Cells

Click on the diagram at the right to work through a 7-slide movie which animates each of the diagram's main areas of information. When you are prompted, click to advance to the next slide. If that doesn't work, check out this animated slide presentation, which is not quite as polished, but has all the same information. When you see the prompt to "Press Any Key for Next Slide", use the spacebar, or hover over the bottom of the display to use the forward and backward arrows to move through the slides. If you cannot see either the movie or the slides, click on the slide numbers below to view the completed slides in separate windows, one at a time..

1 2 3 4 5 6 7

Study the large "cell" to the north of the equator, called the tropical cell, in which air moves in a vertical (up and down) circle. Hot air rises up at the equator, and moves away from the equator to the north, and is cooling as it goes. The cooler denser air starts to fall back toward the surface of the earth, usually around 30° north latitude. It brings down air from another cell to the north. In this second cell, the temperate cell, air flows high above the southward from the north, drops down to ground level, and flows back north along the ground. It rises back up around 60° north of the equator. There is a third and final polar cell in which the air rises, moves north at high altitudes, and falls back to earth at the equator, flowing south along the ground. These three huge cells account for the north-south movement of the major wind systems.

The atmosphere is not the same near the earth's surface as it is near the top. As the rising warm air moves up, it has more volume to cover, and so it expands, becomes less dense, and cools. This thinner high atmosphere can't hold as much water vapor, so most of the water in our atmosphere stays near the ground, in the troposphere. All the weather that we experience occurs in this layer.

That doesn't mean that the other layers are unimportant. In the middle of the stratosphere is a trapped band of a special type of oxygen molecule called ozone. The ozone layer is transparent to most light but reflects light in the ultraviolet range, protecting life on the earth's surface from high-energy radiation that can trigger the development of cancers. The ionosphere lets most radiation through, but reflects radio waves, causing radio transmissions to "bounce" around the curvature of the earth. Before satellites became available to retransmit their programs, radio stations used this phenomenon to extend their range.

The composition of the atmosphere also affects the transmission of energy. All light reaching the earth's surface warms the land and sea, which then radiate some of this energy back out toward space in the infrared region. Even though it is a very small amount, the carbon dioxide in the earth's atmosphere acts like an opaque window, and reflects this energy back to the ground, much like the glass windows in a greenhouse trap heat energy and keep it inside the greenhouse. This greenhouse effect is responsible for the earth's current temperature being much warmer than it would be without CO2 in the atmosphere, warm enough to support life.

The Surface Directions of Wind

So far, we've seen how differential heating causes air in the tropical, temperate, and arctic cells to move up or down, and the composition and layers of the atmosphere can affect how fast this happens. Now we have to add in the fact that the earth is rotating eastward, making one complete revolution a day. This means that a person on the ground is moving westward underneath the north or south rotating air cells. For someone in the tropical zone, as air flows southward and the earth rotates eastward, the resulting wind will appear to move from northeast to southwest. An observer on the ground in the temperate zone will see the air flowing from the southwest to the northeast. It almost looks like something is pushing the air to one side. This kind of apparent push (which is really the result of the movement of the observer) is called the Coriolis effect.

The combination of north-south cells and the Coriolis force gives us our system of dominant atmospheric movements. If we look at a map of air currents, we see that in the northern hemisphere, the predominant winds in the tropics near the equator are the "trade winds" that blow from the northeast. The predominant winds in the temperate zone between 30° and 60° latitude are "westerlies".

The global wind pattern has had important historical consequences. Explorers setting out from Europe were unable to travel due west, and instead were blown south and west, landing in the West Indies rather than Massachusetts. To get back to Europe, they had to go north along the East Coast until they could pick up the "easterlies" that would blow them back to Europe.

While the theory of large cells and the Coriolis force can explain the major wind systems, we need to also understand that local geography can itself cause a difference in the amount of energy available to heat the air, resulting in other directions for air flow that may run counter to the global wind systems. Because land heats up faster than water, and cools faster than water, seashores have their own "microsystems" for wind. During the day, the land heats faster than the ocean (or large lake). By midday, air rises over the warmer land, and the cooler sea air rushes in to take its place, creating a refreshing "sea breeze". In the evening, as the land cools more quickly than the water, air begins to rise over the ocean instead, and the cooler land air rushes out to sea, creating a "land breeze".

Air Pressure

All that atmosphere, even though it is a gas, weighs an incredible amount and puts pressure on everything on the earth's surface. At sea level, with no air moving in a wind, the air pressure is about 14.7 pounds per square inch, which works out to nearly a ton per square foot. This is enough to raise a column of mercury in a glass tube about 30 inches.

When warm air rises and expands, it becomes less dense, and the air pressure in the area it left drops. A low pressure area corresponds to a thermal or rising warm air column. Thermals suck in air at the ground level, push it up, and blow it out the top of the column in the high troposphere. When they really get going, thermals can generate winds moving at hundreds of miles per hour. They are responsible for the formation of hurricanes and tornadoes.

When cooling air falls, it creates a high pressure area that pushes air out from the center along the ground. Often this air is trapped by other air masses, and the weather "stalls", unable to move forward or back. Since the cooler air carries less humidity, the weather is often dry and pleasant at the center of a high pressure zone.

Results: Seasons and Climates

We've seen that a city at a high latitude (such as Seattle, near 47 degrees north) receives less sunlight per foot than a city such as Baton Rouge (near 31 degrees north), but that isn't the whole story. As we'll learn in a few weeks in more detail, the earth is tilted on its axis, causing the sun's position in the sky to change over the course of a year, and the length of a day to grow and shrink. This creates even more variation in the amount of sunlight that reaches a particular location, and that will necessarily the general weather patterns. These variations over the course of the year result in seasons and the general average weather over the course of many years for a given location is its climate.

In another unit, we'll cover how the earth's motion in space creates seasons. For now, you need to realize that while the length of days and amount of sunlight per day in the equatorial regions is nearly constant, both change as latitude (which is really just distance from the equator) increases. In the areas beyond the arctic circles, days go from 24 hours long (the sun never sets) in high summer to 0 hours long (the sun never rises) in winter. All the latitudes in between have longer days and more direct1 sunlight in summer, and shorter days with less direct sunlight in winter. This means that the boundary locations for the tropical, temperate, and polar cells change with the seasons, and that changes the local wind patterns.

Even though wind patterns can change daily depending on the land and sea locations, or elevations, and will change depending on seasons, the weather for a given location like Seattle usually stays within a particular range of temperatures and rainfall. The combination determines the climate for the location. In Seattle's temperate climate, the temperatures generally stay between 25°F and 90°F with an average temperature of around 53°F and easterly winds blowing humid air up from Hawaii give the area an annual rainfall near 40 inches. In Los Angeles, which gets drier easterlies from further south in the Pacific, the average temperature is higher (63°F) and the range much higher, with summer days over 90°. Because of the drier winds, Los Angeles gets only 12 inches of rain a year, a near-desert climate. In our units on ecology, we'll see how the climate of a region determines what vegetation and wildlife it can support.

By "direct", I mean that the sunlight hits the ground nearly straight on. More precisely, it hits the ground at a right angle, so the amount of sunlight coming in covers a minimal area. The sunlight is con centrated, and able to heat a small area more efficiently. In winter, when the sunlight is less direct, it hits the ground at an angle and the amount of ground the incoming sunlight must cover is larger, so the sunlight can't cause the ground's temperature to rise the same amount as it does in summer.

Study/Discussion Questions

Further Study On your Own (Optional)