⏳ Estimated Reading/Watching Time: 8 - 10 minutes

Explain how the sun's energy affects the Earth's surface.

Solar Radiation

The Sun is the closest star to Earth. Even so, light from the Sun takes approximately 8 minutes to get to Earth. This is very quick, given that the next star is several light years away from Earth. The Sun is so close, that we can feel the Sun’s energy when we are outside on a sunny day.

The Sun emits ultraviolet radiation, which is a type of electromagnetic energy that travels in a wave-like form. When it reaches the Earth, some is absorbed by the atmosphere and some can penetrate the atmosphere, hitting the surface of the Earth.

A graphic with information about the electromagnetic spectrum, showing that as wavelength increases, frequency decreases. Radios have the lowest frequency and highest wavelength, visible light is in the middle, then ultraviolet, and gamma rays have the shortest wavelengths and highest frequency. — **The electromagnetic spectrum includes visible light, ultraviolet light, x-rays, microwave, and gamma rays.**

Ultraviolet (UV) light from the Sun can also be harmful. Therefore, we wear sunblock when we are going to be outside for a long time – we need to protect our skin from UV-A and UV-B rays. “Ultra” means “beyond” so ultraviolet light is just past, or just beyond violet light on the electromagnetic spectrum.

Radiation is part of the electromagnetic spectrum, along with visible light. UV radiation can damage protein and DNA, but luckily, more than 99% is absorbed by the atmosphere. There are 3 types of ultraviolet light that penetrate the atmosphere.

An image showing the sun shining on Earth and the Ozone layer. Visible light and UV-A fully penetrates the Ozone layer, Most of UV-B is absorbed by the Ozone layer, and all of the UV-C is absorbed by the Ozone layer. — **The ozone layer protects Earth from the most harmful rays of the sun's radiation.**

UV-C is the most harmful type of ultraviolet light. Thankfully, the ozone layer absorbs all of this radiation and none of it makes it to the Earth’s surface. UV-B is the next most harmful form of ultraviolet radiation. This form of UV cannot penetrate past the upper layers of the skin. It is responsible for tans, skin cancer, and enhances skin aging. Most of UV-B radiation is absorbed by the atmosphere and ozone layer.

UV-A can penetrate to the deepest layers of skin. It is responsible for that immediate tan look you get when you go outside (if you are a person that tans). It is also responsible for causing wrinkles and aging and may enhance skin cancer. Some of this type of UV radiation is trapped by the ozone layer, but most penetrates to the surface of the Earth.

Sunscreens will also protect you from UV-A and UV-B radiation if they are applied frequently when you are outside in the sun, but only the ozone layer can protect you from UV-C.

Essential Knowledge

Incoming solar radiation (insolation) is the Earth's main source of energy and is dependent on season and latitude.

Because the Earth is round, the energy from the sun is not distributed equally across the globe. The sun strikes the Earth most directly at the equator where the angle of incidence (angle of strike) is 90 degrees. At the north and south poles, the angle of incidence is much shallower, so the sun’s rays bounce off more easily leading to much colder temperatures than those found at the equator.

An image of the Earth and the sun. Solar radiation is hitting the earth at different latitudes. As the latitude increases in either direction, the amount of surface area lit by an equal amount of light increases, causing the intensity to decrease. — Sunlight that hits the Earth at the equator, hits at almost 90 degrees, whereas at other places on the globe, sunlight hits much more indirectly, with sunlight being the least direct at the North and South poles.

The tropics are the warmest area on Earth because sunlight hits the Earth most directly at the equator. As you move away from the equator in either direction, north or south, the climate tends to get cooler and cooler until you reach the poles, where it is coldest of all. These varying climates determine which plants and animals can live at each latitude. For example, polar bears tend to live in the Arctic and not near the equator because they are adapted to living in very cold climates.

Essential Knowledge

The angle of the sun's rays determines the intensity of the solar radiation. Due to the shape of the Earth, the latitude that is directly horizontal to the solar radiation receives the most intensity.

Difference in heating of the Earth’s surface is the basic cause of wind and other atmospheric currents like jet streams. A jet stream is a high altitude ‘river’ of air that is constantly flowing.

Jet streams influence weather and climate by steering air masses around the Earth. The video below describes the jet streams and how they warm and cool the planet. See if you can find the jet stream that affects the weather in the United States.

Essential Knowledge

The highest solar radiation per unit area is received at the equator and decreases towards the poles.

Seasons

A graphic showing the Earth tilting on its axis. The North and South Celestial Poles are labeled. The Equator is also labeled. — **Earth's tilt is one of the main factors causing Earth's seasons.**

Earth rotates on its axis and also revolves around the sun. This rotation and revolution are the basic causes of the Earth’s seasons, as we know them.

The Earth is also tilted on its axis at an angle of 23.5 degrees away from vertical. The axis always tilts the same direction in relation to the sun. In addition to the tilt of the Earth, the revolution of Earth around the sun changes the orientation of the northern and southern hemispheres in relation to the sunlight coming from the sun.

Essential Knowledge

The tilt of Earth's axis of rotation causes the Earth's seasons and the number of hours of daylight in a particular location on the Earth's surface.

When the northern hemisphere is tilted towards the sun, that hemisphere is heated more directly and experiences what we call summer. When the northern hemisphere is tilted away from the Sun, that hemisphere receives less direct sunlight and experiences colder temperatures. We call that season winter in the northern hemisphere.

When Earth is neither tilted toward nor away from the sun, equal amounts of heating occur in both hemispheres and we call those two seasons either spring or autumn, depending on which hemisphere you are in at the time.

Four significant dates occur during Earth’s revolution around the Sun.

The diagram illustrates Earth's position at each solstice and equinox. — **The diagram above refers to the seasons that occur in the northern hemisphere. At the same time, the seasons in the summer hemisphere are the reverse of those in the northern hemisphere.**

The longest day of the year (the day with the most sunlight) in the Northern Hemisphere is on or around June 20 or 21st and is known as the summer solstice. The shortest day of the year (day with the least amount of sunlight) in the Northern Hemisphere occurs on or around December 21 or 22nd and is called the winter solstice. These are reversed in the Southern Hemisphere.

During an equinox, there are approximately equal amounts of daylight and darkness during that 24-hour period. The vernal, or spring, equinox occurs in March in the Northern Hemisphere. The autumnal, or fall, equinox occurs in September in the Northern Hemisphere.

The names of the equinoxes are reversed in the Southern Hemisphere with the vernal equinox occurring in September and the autumnal equinox occurring in March.

It can get tricky when you consider the seasons in both hemispheres at once.

The interactive below can help you understand the differences between the seasons in the two hemispheres. Just tap on the white dots at the bottom to uncover all the fascinating details!

Essential Knowledge

The solar radiation received at a location on the Earth's surface varies seasonally, with the most radiation received during the location's longest summer day and the least on the shortest winter day.

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