3.2- Introduction to Weather and Sun Angles
What is weather?
Watch: Instructor's Video Links to an external site.
The term "weather" refers to temporary conditions of the atmosphere. It is the day-to-day temperature and precipitation activity. It is what is going on outside your door. Climate is the average of atmospheric conditions over long periods of time, generally 30 plus years. While weather may determine what you grab out of the closet to wear in the morning, climate will determine what you have in your closet to begin with.
It is important to make this distinction because we so often see these terms confused in the press. A record hot or cold day may or may not have anything to do with a change in climate. To draw that conclusion, you have to see these changes hold up over time.
The Components of Weather
Weather includes: temperature, atmospheric pressure, wind and atmospheric moisture. Temperature is the first building block of weather. It is the first domino you flick over to set the whole thing in motion. Temperature around the world starts with the Sun and the atmosphere and is influenced by latitude, the angle of the sun, duration of daylight, and the depth of the atmosphere. It is further influenced by the differential heating of land and water, ocean currents, altitude and geographic position.
We will begin by exploring the energy from sun and its interaction with our atmosphere.
Sun Angles
We already know that the seasons on Earth are caused by the combined effects of the:
- Rotation of the Earth (24 hours in a day)
- Revolution of the Earth around the Sun (364 1/4 days in a year)
- Inclination (or tilt) of the Earth's axis (23.5°)
- Polarity of the Earth's axis (the North Pole always points towards Polaris)
We have talked about at what position the Earth's axis is at the date that is the start of each of the four seasons:
- Equinoxes (March - Spring/Vernal and September - Fall/Autumnal) the declination of the Sun is 0° and the length of day is 12 hours everywhere
- The Solstices (December and June) the declination of the Sun on the Winter Solstice is 23.5°S and on the Summer Solstice the declination of the Sun is 23.5°N. The Summer Solstice is the longest day of the year in the Northern Hemisphere and the Winter Solstice is the shortest.
We know that the angle of the noon Sun above the horizon is 90° at the Equator on the Equinoxes and at the Tropics of Cancer and Capricorn on the Solstices. The angle of the noon sun is important because it determines the intensity of solar radiation received at a location on a specific date. Where the Sun
Sun's rays are striking vertically or at a 90° angle indicates where the greatest intensity of solar radiation is received on any given date.That location is, as you will recall, the Sun's declination. The sun is only directly over head (at a 90° angle) at locations between 23.5° N and 23.5°S. This happens at those locations two times per year. Here in Sonoma County, therefore, the Sun is never directly overhead.
The Analemma
To use the Analemma you find the date on the chart (e.g. October 11). You follow the grid line to the left and you will find the declination of the Sun for that date (7° S). So, on October 11 at solar noon the Sun will be directly overhead at 7°S. Pick a few dates that are important to you - anniversaries, birthdays etc - and practice finding the declination of the Sun using the Analemma. Its important that you stop and think about what you are finding and whether it makes sense. For October 11th, 7°S makes sense as the Sun's declination. Why? On September 21st the declination of the Sun is 0° (the Equinox). We know that in December the declination of the Sun will be 23.5°S (the Winter Solstice) so in October we would expect the declination of the Sun to be in the southern hemisphere but not too far below the Equator (because October is not too long after the Equinox).
Another change that comes with the seasons is the length of day at different latitudes. At the Equator, the length of day is always about 12 hours, with the Sun rising at 6 am and setting at 6 pm. But as you move north and south of the Equator, the length of day becomes more variable until you reach the Arctic and Antarctic circles and beyond where we experience 24 hours of daylight and 24 hours of darkness seasonally. So, the angle of the Sun changes for a latitude throughout the Seasons as does its length of day. The more consistent these two things are - consistently high sun angles, consistent length of day - the more solar energy that latitude will receive. Clearly the Equator and the Tropics are the big winners here.
Open this pdf and see how the length of day varies by latitude and season. TIME OF SUNSET.pdf Download TIME OF SUNSET.pdf
Finding the Angle of the Sun
In addition to finding the declination of the Sun on any given day, we can find the angle of the noon sun for any location on any given day if we have our Analemma, a little information, and some math. To find the the angle of the noon Sun for a location on a given day, we need three pieces of information:
- the date we are interested in
- the latitude of the location
- and the declination of the Sun on that date (we find that on the Analemma)
This information and some simple math will give us the Sun angle at solar noon for any given date at any location.
The steps for finding the Sun angles are:
- Know the latitude of the location you are working on and the specific date you are interested in.
- Using the Analemma, find the declination of the Sun for that date
- If the location’s latitude and the declination are in the same hemisphere, subtract those two numbers
- If the location’s latitude and the declination are in opposite hemispheres, add those two numbers
- Take the answer to 3 or 4 above (whichever is appropriate) and always subtract from 90. (90-x = angle of the noon sun)
Let's do a couple of examples:
Example 1: Location = Santa Rosa, Ca. Latitude = 38°N. Date = March 6th
Step 1: Use the Analemma to find the declination of the Sun on March 6th
The declination is 6°S
Step 2: Since the latitude of Santa Rosa (38°N) and the declination (6°S) are in opposite hemispheres add
38 + 6 = 44
Step 3: To calculate the final answer, always subtract from 90
90 - 44 = 46° On March 6th the angle of the noon Sun above the horizon in Santa Rosa will be 46°
Example 2: Location = St. Andrews, Scotland. Latitude= 56°N. Date= April 30th
Step 1: Use the Analemma to find the declination of the Sun on May 30th
The declination is 14°N
Step 2: Since the latitude of St. Andrew's (56°N) and the declination (14°N) are in the same hemispheres subtract
56 - 14 = 42
Step 3: To calculate the final answer, always subtract from 90
90 -42 = 48° On May 30th the angle of the noon Sun above the horizon in St. Andrew's Scotland will be 48°
You will be doing this in an assignment so make sure that you understand how and why you are finding the angle of the noon Sun.
Learn More
See how the Analemma is created Links to an external site..
