This lecture discusses seasonal radiation controls. The Sun emits radiation in all wavelengths, but by the time it reaches the top of the atmosphere, some of the radiation is lost. On average, around 1317 watts per meter square of radiation is received at the top of the atmosphere. This number is an average over all latitudes and all seasons and is called the solar constant.
It's a starting point for understanding how much energy is available to do the work of powering weather and climate. Insulation identifies the incoming or intercepted solar radiation. Not every location receives the same amount of radiation. Radiative receipt is controlled by two things, the incident angle of the Sun and the amount of time the Sun is above the horizon.
The incident angle refers to how high the Sun is in the sky relative to that location. The Sun is highest in the sky at solar noon. The angle between the Earth and the Sun is the noon solar altitude angle.
When the Sun is directly overhead at a 90 degree angle, that specific latitude is receiving concentrated radiation spread over a very small area. All other latitudes across the globe would have a lesser altitude angle at that time. Those locations would receive less radiation because the radiation is being spread over a larger distance. The amount of time the Sun is above the horizon is determined by day length.
The Earth spins around its axis of rotation. The axis of rotation is an imaginary line that connects the North and South poles. The North Pole always points to the North Star, which is currently Polaris.
It takes roughly 24 hours to complete one rotation. A rotation is moving about something that's internal. The Earth moves in orbit around the Sun. A revolution is to move about something that's external. Since physical and man-made processes don't always coincide, It takes a bit longer than one year to complete one revolution.
This is the reason for a leap year. The shape of the orbit is not perfectly circular. It's elliptical.
The Sun is not perfectly positioned in the center of the orbit. This causes the perihelion and the aphelion. The perihelion is the time of year when the Earth is closest to the Sun. That happens during early January.
The aphelion is the time of year when the earth is farthest from the sun, roughly around July 4th. Seasonality refers to a change in weather patterns throughout the year. Seasons are caused by the tilt of the earth's axis of rotation. The axis is tilted at 23.5 degrees relative to the Earth's orbital path. The tilt remains constant.
This constant tilt controls which latitudes get more radiation, where the solar angle is 90 degrees directly overhead. At times, the Sun's most direct radiation is in the northern hemisphere. This is the northern hemisphere summer.
Other times, The sun's most direct rays are in the southern hemisphere. This is the southern hemisphere summer. The terms summer, winter, fall, and spring are hemispheric-based terms. When it's summer in the northern hemisphere, it's winter in the southern hemisphere. The solar declination is the latitude receiving the most radiation.
In other words, the solar declination is the latitude that has a 90 degree solar angle. The solar declination changes every day, but stays within the tropics. An analemma shows the solar declination for any time of the year. An analemma is the figure 8 shape. often shown on a globe.
This table gives significant details about the solstices and equinoxes. The dates are all listed as the 21st just to provide a learning device. The actual dates can be on or about the 21st.
An equinox is when the sun's most direct rays are at the equator. In other words, an equinox is when the solar declination is at the equator. Again, the solar declination is the latitude with a 90 degree solar angle.
A solstice occurs when the sun's most direct rays are at their farthest position into a hemisphere. The June solstice is when the sun's most direct rays are on the Tropic of Cancer. The December solstice is when the sun's most direct rays are on the Tropic of Capricorn.
Day length is equal across the globe during an equinox. During a solstice, the circles to the poles experience the largest changes in day length. Half of the globe is always receiving some portion of sunlight.
During the day, locations across the earth rotate into and out of the lit portion. The dividing line between day and night is called the circle of illumination. The circle of illumination controls day length.
This is the other significant factor in controlling how much radiation is received at a location. This image shows the entire year in orbit. The lit parts are facing the sun.
When the circle of illumination cuts across the poles on the equinoxes, every latitude has 12 hours of day. When the circle of illumination just touches either the Arctic or Antarctic circle during a solstice, then the Arctic or Antarctic will have 24 hours of day length or 24 hours of darkness.