Topic 1: Sun Angle

It is important to have an understanding of astronomy since it has a significant influence on the weather. One of the most significant influences is the position of the sun in the sky and how long each day the sun is in the sky. The season (and day of year) determines how long the sun will be in the sky each day. The sun brings warmth. The warming power is not only how long the sun is in the sky but also how close it is to being overhead. A sun that is closer to directly overhead will bring much more warmth than a sun near the horizon.

Since the earth is a ball shaped, is tilted 23.5 degrees on axis and has an elliptical orbit around the sun, the amount of warming available from the sun varies throughout the year. The curving nature of a ball means the sun will be more directly overhead in the tropical areas and will be closer to the horizon in the mid-latitude and polar areas. This is the primary reason the tropical regions are warmer than the polar regions. The tilt of the earth causes the sun angle to vary throughout the year. When the earth is tilted more toward the sun (summer) the sun will be at a higher position in the sky and there will be more warming. Without the tilt, seasons would not have near the influence they currently have.

One skill that comes up time and time again in a basic meteorology and climatology course is sun angle. They can be some of the most challenging problems for students. An important bit of information to know is what is the highest the sun can get in the sky at a particular location. This is known as the noon solar sun angle (where noon does not mean exactly noon but rather the time when exactly half the daylight is complete). At this point the sun will be the highest in the sky before it starts dipping down toward the horizon.

To find the noon solar sun angle first determine the latitude of your location. For this example I will use Starkville, Mississippi. The latitude of this location is at 34 degrees North latitude. Latitude varies from 0 at the equator to 90 degrees at the pole. The North means Starkville is in the Northern Hemisphere. Second, you need to know the day of the year. Since the earth is titled, the day of the year will determine where the sun is directly overhead at solar noon. The sun, during the course of a year, can only be overhead in the tropics (somewhere between 23.5 N and 23.5 S). A chart known as an analemma is used to determine the latitude the sun is directly overhead given a certain day. There are 4 values that we know intuitively. They are the first day of winter, summer, fall and spring. On the first day of winter the sun is directly overhead at 23.5 S, on the first day of both fall and spring the sun is directly overhead at the equator 0 degrees, and on the first day of summer the sun is directly overhead at 23.5 N. Once we know the latitude of our location and the day of the year we can determine the noon solar sun angle.

What is the noon solar sun angle in Starkville, MS on the first day of summer?

Starkville is at 34 N while the sun is directly overhead at 23.5 N. The difference between these two latitudes is 10.5 degrees. For every degree latitude you move north from the 23.5 N latitude the sun will be 1 degree less in the sky. Since 90 degrees is a sun directly overhead, the noon solar sun angle on the first day of summer in Starkville, MS is 90 - 10.5 = 79.5 degrees.

What is the noon solar sun angle in Starkville, MS on the equinox?

The equinox occurs on the first day of fall and spring. At both times the sun passes directly overhead at solar noon at the equator. The difference between the Starkville latitude and the equator is 34 - 0 = 34 degrees. Thus, the noon solar sun angle on the equinox in Starkville, MS is 90 - 34 = 56 degrees.

What is the noon solar sun angle in Starkville, MS on the first day of winter?

On the first day of winter the Northern Hemisphere is tilted away from the sun at the maximum extent it can have. When it is winter in the Northern Hemisphere then it is summer in the Southern Hemisphere. The noon solar sun is directly overhead at 23.5 S on this day. The difference between the Starkville, MS latitude and the sun directly overhead is 34 + 23.5. They are added since the values are in different hemispheres. They add to 57.5. Thus, the noon solar sun angle on the first day of winter in Starkville, MS is 90 - 57.5 = 32.5 degrees. With such a low sun angle even in the middle of the day it is no surprise that the coldest weather occurs in winter.

Topic 2: Solar Intensity

The curvature of the earth and the sun angle produce different solar intensities depending on latitude. The sun's rays spread out over a larger area as the sun gets closer to the horizon. Thus, the solar energy from the sun is more intense when the sun is high in the sky and less intense when the sun is closer to the horizon. The solar intensity has the following formula of:

Solar Intensity (I) = Solar Constant * Sin(Sun Angle)

The solar constant = 1,370 units (W/m^2). The solar constant is the amount of energy from the sun at the top of the atmosphere and perpendicular to the sun's rays. At the earth's surface this value will be less due to scattering, clouds, atmospheric absorption and other factors. Let's say for argument sake that on a particular day the solar "constant" at the earth's surface for a specific location happens to be 700 units and is steady throughout the day. Here is what the solar Intensity would be for 6 different sun angles (5, 15, 30, 45, 70 and 90 degrees).

I = 700*sin(5) = 61 units
I = 700*sin(15) = 181 units
I = 700*sin(30) = 350 units
I = 700*sin(45) = 495 units
I = 700*sin(70) = 658 units
I = 700*sin(90) = 700 units

From this data you can see that the solar intensity is much lower when the sun is closer to the horizon. When the sun is at a 30 degree angle it only has half the intensity of the sun being directly overhead. This is one reason why polar climates are much colder than tropical climates and why in the polar and mid-latitudes that it is much colder in winter as compared to summer.

Topic 3: Weather and Sun Energy

Sun angle and solar intensity are important controls of the weather. Another important control is what gets in the way between the sun and the earth's surface... clouds.

It is no surprise that overcasting clouds during the daytime tends to make the weather cooler than it would be if the clouds were not present. Clouds are great reflectors of solar energy. This reflected energy is energy that can not be used to warm the earth's surface.

When it comes to astronomy, clouds are a nuisance. They get in the way of seeing that rare eclipse and being able to see the heavens. Many observatories are located at higher elevations in dry climates where clouds, lights and atmospheric influences are reduced.

When the sun is low on the horizon there is a tendency for there to be more clouds in moist climates. This is because a weaker solar intensity produces a weaker evaporative potential. When the solar intensity is strong (sun high in the sky) and the atmosphere is stable, the warming tends to produce less clouds since moisture is evaporated away into the dry air.

Topic 4: Moon and Weather

The moon is the most marvelous heavenly body in the sky. I would say the sun but you can not look at the sun since it damages the eyes. The moon though is a sculpture that can be marveled at. As moons go in the solar system, the earth moon is huge. Most moons on other planets are much smaller. Our moon can give us enough light to see even when dark outside. The light is not enough to warm the surface by more than a fraction of a degree but it is a nice night light. One of the most significant astronomical influence of the moon is tides. Because the moon is big it has a significant enough gravitational field to influence the earth's surface. Since air and water have a lower density they are most influenced by the moon. The gravity from the moon tugs on the earth under the location of the moon. Since the earth is rotating, the tide field moves producing high and low tides at locations. Since the sun also produces tides, the sun and moon work together to produce complex tidal variations across the earth. The greatest tides occur when the sun, earth and moon are in alignment (make a line) with each other.

One of the greatest wonders is to look at the moon through a powerful telescope on a clear dry night. It is amazing to see all the features that can be seen.

The moon can also influence the weather during a solar eclipse. During this event the moon is directly between the earth and sun. Certain eclipses can last for an hour or more. During this reduced solar radiation the temperature can stop warming up or even cool at the surface. The solar eclipses (especially totality) does not happen very often. When it happens it makes big news.

Topic 5: Stars and Planets

The stars and planets have very little influence on the weather since they are so far away. They do though give beauty to the night sky and many of the planets have their own atmosphere and weather. What a bore the clear night sky would be without all the stars and planets. They can also be used as navigation aids. Since the North Pole points toward the star Polaris, that particular star will stay at a fixed point in the sky. It can be used as a reference for the direction of north. The stars and planets also allow us to contemplate the vastness and diversity of the universe. Each star is a sun and many have planets revolving around them. Just as the human body has trillions of cells, the universe has many trillions of heavenly bodies (stars and planets). We can only see a tiny fraction of these in the night sky.