Lecture 3

Lecture 3

SEASONAL AND DAILY TEMPERATURES

WHY THE EARTH HAS SEASONS

The earth revolves completely around the sun in an elliptical path (not quite a circle) in slightly longer than 365 days (one year).

As the earth revolves around the sun, it spins on its own axis, completing one spin in 24 hours (one day).

The average distance from the earth to the sun is 150 million km (93 million mi).

Because the earths orbit is an ellipse instead of a circle, the actual distance from the earth to the sun varies during the year.

The earth comes closer to the sun in January (147 million km) than it does in July (152 million km)

PERIHELION AND APHELION

The time around January 3rd, when the earth is closest to the sun, is called perihelion (from the Greek peri, meaning near and helios, meaning sun).

The time when the earth is farthest from the sun (around July 4th) is called aphelion (from the Greek ap, away from).

From this we might conclude that our warmest weather should occur in January and our coldest weather in July! But the primary cause of seasons is not just nearness to the sun.

Our seasons are regulated by the amount of solar energy received at the earths surface.

This amount is determined primarily by the angle at which sunlight strikes the surface and by how long the sun shines on any latitude (daylight hours).

Solar energy that strikes the earths surface perpendicularly (directly) is much more intense than solar energy that strikes the same surface at an angle [due to area covered see next slide].

In addition, the more the suns rays are slanted from the perpendicular, the more atmosphere they must penetrate.

And the more atmosphere they penetrate, the more they can be scattered and absorbed (attenuated).

As a consequence, when the sun is high in the sky, it can heat the ground to a much higher temperature than when it is low on the horizon.

Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to the same surface area than direct sun rays.

Length of time for which the sun shines

The second important factor determining how warm the earths surface becomes.

Longer daylight hours, of course, mean that more energy is available from sunlight.

From a casual observation, we know that summer days have more daylight hours than winter days.

Also, the noontime summer sun is higher in the sky than is the noontime winter sun.

Both of these events occur because our spinning planet is inclined on its axis (tilted) as it revolves around the sun.

As the earth revolves about the sun, it is tilted on its axis by an angle of 23 and 12.

The earths axis always points to the same area in space.

Thus, in June, when the Northern Hemisphere is tipped toward the sun, more direct sunlight and long hours of daylight cause warmer weather than in December, when the Northern Hemisphere is tipped away from the sun. (Diagram, of course, is not to scale.)

SEASONS IN THE NORTHERN HEMISPHERE

Warm Summer Season:

On June 21, the northern half of the world is directed toward the sun.

At noon on this day, solar rays beat down upon the Northern Hemisphere more directly than during any other time of year.

The sun is at its highest position in the noonday sky, directly above 23 12 north (N) latitude.

If you were standing at this latitude on June 21, the sun at noon would be directly overhead.

This day, called the summer solstice, is the astronomical first day of summer in the Northern Hemisphere and vice versa for Southern Hemisphere.

As the earth spins on its axis, the side facing the sun is in sunshine and the other side is in darkness.

Thus, half of the globe is always illuminated.

If the earths axis were not tilted, the noonday sun would always be directly overhead at the equator, and there would be 12 hours of daylight and 12 hours of darkness at each latitude every day of the year.

However, the earth is tilted.

Since the Northern Hemisphere faces toward the sun on June 21, each latitude in the Northern Hemisphere will have more than 12 hours of daylight.

The farther north we go, the longer are the daylight hours.

When we reach the Arctic Circle (66 12N), daylight lasts for 24 hours.

Notice the region above 66 12 N never gets into the shadow zone as the earth spins.

At the North Pole, the sun actually rises above the horizon on March 20 and has six months until it sets on September 22.

No wonder this region is called the Land of the Midnight Sun!

Land of the Midnight Sun. A series of exposures of the sun taken before, during, and after midnight in northern Alaska during July.

During the Northern Hemisphere summer, sunlight that reaches the earths surface in far northern latitudes has passed through a thicker layer of absorbing, scattering, and reflecting atmosphere than sunlight that reaches the earths surface farther south. Sunlight is lost through both the thickness of the pure atmosphere and by impurities in the atmosphere. As the suns rays become more oblique, these effects become more pronounced.

Each day past June 21, the noon sun is slightly lower in the sky.

Summer days in the Northern Hemisphere begin to shorten.

June eventually gives way to September, and fall begins.

September 22, the earth will have moved so that the sun is directly above the equator.

Except at the poles, the days and nights throughout the world are of equal length.

This day is called the autumnal (fall) equinox, and it marks the astronomical beginning of fall in the Northern Hemisphere.

At the North Pole, the sun appears on the horizon for 24 hours, due to the bending of light by the atmosphere.

The following day (or at least within several days), the sun disappears from view, not to rise again for a long, cold six months.

On December 21 (three months after the autumnal equinox), the Northern Hemisphere is tilted as far away from the sun as it will be all year.

Nights are long and days are short.

Daylight decreases from 12 hours at the equator to 0 (zero) at latitudes above 66 12 N.

This is the shortest day of the year, called the winter solstice, the astronomical beginning of winter in the northern world.

The date of March 20, which marks the astronomical arrival of spring, is called the vernal (spring) equinox.

At this equinox, the noonday sun is shining directly on the equator, while, at the North Pole, the sun (after hiding for six months) peeks above the horizon.

Longer days and more direct solar radiation spell warmer weather for the northern world.

LENGTH OF TIME FROM SUNRISE TO SUNSET FORVARIOUS LATITUDES ON DIFFERENT DATES IN THE NORTHERN HEMISPHERE

SEASONS IN THE SOUTHERN HEMISPHERE

At this time part of the world is now tilted away from the sun.

Nights are long, days are short, and solar rays come in at an angle.

All of these factors keep air temperatures fairly low.

The June solstice marks the astronomical beginning of winter in the Southern Hemisphere.

So, when it is winter and June in the Southern Hemisphere, it is summer and June in the Northern Hemisphere.

Conversely, when it is summer and December in the Southern Hemisphere, it is winter and December in the Northern Hemisphere.

Another difference between the seasons of the two hemispheres concerns their length.

Because the earth describes an ellipse as it journeys around the sun, the total number of days from the vernal (March 20) to the autumnal (September 22) equinox is about 7 days longer than from the autumnal to vernal equinox

Because the earth travels more slowly when it is farther from the sun, it takes the earth a little more than 7 days longer to travel from March 20 to September 22 than from September 22 to

March 20.

LOCAL SEASONAL VARIATIONS

Middle latitudes of the Northern Hemisphere, objects facing south will receive more sunlight during a year than those facing north.

This fact becomes strikingly apparent in hilly or mountainous country.

Hills that face south receive more sunshine and, hence, become warmer than the partially shielded north-facing hills.

Higher temperatures usually mean greater rates of evaporation and slightly drier soil conditions.

Thus, south-facing hillsides are usually warmer and drier as compared to north facing slopes at the same elevation.

In many areas of the far west, only sparse vegetation grows on south-facing slopes, while, on the same hill, dense vegetation grows on the cool, moist hills that face north.

DAILY TEMPERATURE VARIATIONS

DAYTIME WARMING

As the sun rises in the morning, sunlight warms the ground, and the ground warms the air in contact with it by conduction.

However, air is such a poor heat conductor that this process only takes place within a few centimeters of the ground.

As the sun rises higher in the sky, the air in contact with the ground becomes even warmer, and there exists a thermal boundary separating the hot surface air from the slightly cooler air above.

However, on a windless day, this form of heat exchange is slow, and a substantial temperature difference usually exists just above the ground.

Near the surface, convection begins, and rising air bubbles (thermals) help to redistribute heat.

In calm weather, these thermals are small and do not effectively mix the air near the surface.

On a sunny, calm day, the air near the surface can be substantially warmer than the air a meter or so above the surface.

The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earths surface.

Where incoming energy exceeds outgoing energy (orange shade), the air temperature rises.

Where outgoing energy exceeds incoming energy (blue shade), the air temperature falls.

At max point: end of heat-gain period due to gain>loss

Here, ground (and hence the air immediately above it) has taken in as much solar radiation as possible that day. Now, as soon as gain = loss, is the point of max heated ground. After this loss > gain, and ground temp will begin to drop.

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DAILY TEMPERATURE VARIATIONS

NIGHTTIME COOLING

As the sun lowers, its energy is spread over a larger area, which reduces the heat available to warm the ground.

Sometime in late afternoon or early evening, the earths surface and air above begin to lose more energy than they receive; hence, they start to cool.

Both the ground and air above cool by radiating infrared energy, a process called radiational cooling.

The ground, being a much better radiator than air, is able to cool more quickly.

Consequently, shortly after sunset, the earths surface is slightly cooler than the air directly above it.

The surface air transfers some energy to the ground by conduction, which the ground, in turn, quickly radiates away.

As the night progresses, the ground and the air in contact with it continue to cool more rapidly than the air a few meters higher.

Therefore, by late night or early morning, the coldest air is found next to the ground, with slightly warmer air above.

This measured increase in air temperature just above the ground is known as a radiation inversion because it forms mainly through radiational cooling of the surface.

On a clear, calm night, the air near the surface can be much colder than the air above.

The increase in air temperature with increasing height above the surface is called a radiation temperature inversion.