Almost all aspects of weather are (at least partially) caused by the fact that the Earth is hotter near the equator and colder near the poles. This uneven distribution of heat has an obvious impact on temperature, but also governs the large scale distribution of precipitation, and creates the pressure differences that drive our regional wind patterns. Since the temperature difference between the poles and the tropics is such an important factor in controlling weather, it is critical that we understand the relations between the Earth and Sun that create these variations in heat (energy).
The Earth rotates (spins) on its axis (the imaginary line between the North pole and the South Pole) while it revolves around the sun. The rotation of the Earth controls the day/night cycle but has only a mild effect on the weather. Most of our seasonal and weather variations are caused by the revolution of the Earth around the sun.
The Earth moves around the sun in a nearly circular orbit ( average distance of 150,000,000 km). The Earth’s path is not quite perfectly circular; it is actually an ellipse (slightly flattened circle) so that the Earth is not always at exactly the same distance from the sun. At its closest point to the sun, called perihelion, it is approximately 147,000,000 km from the sun. At its most distant point, called aphelion, it is about 152,000,000 km from the sun. CLICK FOR DIAGRAM Many people mistakenly believe that these slight variations in distance cause the hot and cold seasons. When you consider the fact that perihelion occurs on Jan. 3 and aphelion occurs on July 4, this claim is clearly incorrect. The slight changes in distance (about 2% above and below the average) does have a very minor effect on our temperature, but this effect is overwhelmed by the much larger effects caused by the tilt of the Earth’s rotation axis.
If the rotation axis of the Earth was perpendicular to the path on which the Earth revolves around the sun (standing upright), the northern and southern hemispheres would always receive the same amount of solar energy, although the equator would still get more than the poles. There would be no seasons, and temperature would mainly depend on latitude (distance from the equator).
The rotation axis of the Earth is not perpendicular, however. It is tilted 23 ½°. This means that for several months of the year (April – August) the northern hemisphere is tilted toward the sun and receives lots of solar rays, while the southern hemisphere is pointed away from the sun and is experiencing cold winter temperatures. During other months (Oct. – Feb.) the southern hemisphere is tilted toward the sun and receiving lots of solar rays (summer) while the northern hemisphere is pointed away from the sun and receiving less solar energy (winter).
In order to understand the variations in seasons throughout the planet, it is necessary to understand two questions:
1) Why does the northern hemisphere point toward the sun part of the year while the southern hemisphere is pointed toward the sun for the other part of the year?
2) Why is the hemisphere that is pointed toward the sun warmer than the hemisphere that is pointed away from the sun?
1) Why does the northern hemisphere point toward the sun part of the year while the southern hemisphere is pointed toward the sun for the other part of the year?
The rotation
axis of the Earth is tilted 23½° with respect to the plane of the ecliptic (flat
plane of the solar system on which the Earth moves around the sun). The axis always points to the North Star
(Polaris) and maintains its orientation with respect to the distant stars, but
changes with respect to the nearby Sun.
Sometimes the Sun is between the Earth and the North Star, so the N.
pole (which always points to the N. Star)
is tilted toward the sun. At
other times, as the Earth moves around the sun, the Earth is between the Sun
and the North Star, so the N. pole is still tilted toward the N. Star, but is
now tilted away from the sun. At this
time the S. pole is facing toward the sun.
example: (use your imagination a bit here and don’t
worry about tiny details like roads and stuff)
Consider what would happen if you were to drive due north from
Los Angeles. You could always have your
car pointed directly toward Seattle (like the N. pole stays pointed to the N.
star). During the early part of your
trip, San Francisco would be in front of you, on the hood side of your
car. During the later part of your
trip, San Francisco would be behind you, on the trunk side of your car.
Since your car is staying oriented toward a distant object
(Seattle), movement of your car along the road changes its orientation with
respect to a closer object, San Francisco.
If you think of, Seattle as the N.Star, San Francisco as the
Sun, and your car as the Earth (hood=N. pole; trunk=S. pole), you can see how the drive north from LA is
similar to half of a revolution of the Earth around the sun. During the early part of the trip San
Francisco is shining on the hood of your car (sun shining mostly on N.
hemisphere) and during the second part of the trip, San Francisco is shining on
the trunk of your car (sun shining mostly on S. hemisphere). The hood of your car (N. pole) stays pointed
toward Seattle (N. Star) the entire time.
Therefore it is these three critical facts that control which hemisphere is facing more directly toward the sun: CLICK HERE FOR DIAGRAM
the Earth’s rotation axis is tilted 23 ½ ° with respect to the plane of the ecliptic
the N. pole always points to the N. Star
the Earth revolves around the sun, changing the relative positions of the Earth, Sun, and N. Star.
2) Why is the hemisphere that is pointed toward
the sun warmer than the hemisphere that is pointed away from the sun?
The hemisphere that faces the sun more directly receives more solar energy (also called solar radiation) than the hemisphere that faces away from the sun. There are three main factors that control the amount of incoming solar radiation (insolation) received by any location on the planet. (Local variations can be caused by cloud cover, elevation, etc., but we are going to ignore these effects for now, and concentrate on the larger picture.)
1) concentration of the sun’s rays – The sun’s rays travel a long distance to reach the Earth, and can be considered as a series of rays that are roughly parallel to the plane of the ecliptic. These incoming rays hit directly on the part of the Earth that lies on the plane of the ecliptic (somewhere between 23½° N and 23½° S depending on the season), but hit other parts of the Earth at an angle. The part of the Earth receiving the direct rays gets a more concentrated dose of sunshine than the parts that get hit at an angle. The lower the angle at which the sun’s rays hit the Earth, the more spread out and diffuse is the solar radiation once it reaches the surface.
example-
When I spend the day hiking, I tend to get the most intense sunburn on the top of my shoulders, the tops of my ears, the tops of my feet, and on the part in my hair. These are the parts of my body that are perpendicular to the sun’s rays and receive the most direct and concentrated solar radiation. Other parts of my body that are not oriented perpendicular to the sun’s rays get hit at an angle instead of directly, so my arms, legs, and chest don’t tend to burn quite as badly.
2) amount of atmospheric filtering – As solar rays travel through the atmosphere, some of their energy gets reflected back into space or absorbed by the atmosphere. Solar rays that travel through a lot of atmosphere are less likely to reach the surface than those which travel a shorter distance through the atmosphere. The parts of the Earth closest to the plane of the ecliptic have the least atmosphere between them and the sun, so most of the sun’s rays reach the surface. Farther from the plane of the ecliptic (and closer to the poles), fewer incoming solar rays actually make it through the atmosphere to reach the surface, so these areas receive less solar radiation.
example-
Tahoe has less atmosphere between it and the sun than San Jose (about 6500 feet less). This makes it easier to get sunburned in Tahoe than San Jose, even if all other weather attributes are the same.
3) hours of daylight – There are more hours of sunlight in a day during the summer than in the winter. This gives the Earth more hours in which to absorb solar radiation during the summer than the winter. Although this effect is minor compared to the first two, it can have an impact on temperature.
Solstices
and Equinoxes-
As the Earth moves through its orbit around the sun, there are four important points that mark the important changes of the rotation axis with respect to the Sun, and hence, mark the changes in the seasons.
There are two solstices (June and Dec) when there is a straight line between the Earth-Sun-N. Star. At these times, one of the poles is pointing directly toward the sun (S. pole in Dec.; N. pole in June), so that hemisphere is receiving the most direct possible radiation from the sun. The sun is shining directly on the 23½° latitude in that hemisphere (Tropic of Cancer and Tropic of Capricorn). The solstices are the longest day of the year in one hemisphere while they are the shortest day of the year in the opposite hemisphere.
The other two important days of the year (March and September) are the equinoxes. These occur when the Earth is off to the side of the sun (90°) with respect to the N. star. On these days the equator is on the plane of the ecliptic and is receiving the most direct and concentrated solar rays. Everywhere on the planet has 12 hours of daylight and 12 hours of darkness. There are equal amounts of daylight and dark, and it is equal everywhere on Earth – hence the term equinox (equal night).
Please answer these questions thoroughly, including diagrams as needed. These answers (TYPED!) are due on Thursday January 19.
1. . Both rotation and revolution involve the movement of the Earth, but in very different ways. Explain the differences between them. How long does it take for the Earth to complete one rotation? How long does it take to complete one revolution?
2. Describe the three factors that cause the northern hemisphere to point toward the sun part of the year while the southern hemisphere is pointed toward the sun for another part of the year?
3. Explain three reasons why the hemisphere that is pointed toward the sun is warmer than the hemisphere that is pointed away from the sun?
4. Draw a sketch of the Earth’s orbit around the sun, showing the relative positions of the Earth and Sun at each solstice and equinox. How are the equinoxes and solstices related to the amount of daylight? When does each occur?