METEOROLOGIST JEFF HABY
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
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