The Earth and Insolation

Insolation and Solar Energy §

Solar Activity §

• Insolation pertains to solar radiation transmitted to the Earth.
• The number of sunspots observed in the sun indicates interesting patterns. For example, the Maunder Minimum where there were few observed sun spots also occurred during the Little Ice Age and the number of sun spots now correlate with increasing heat.
• The amount of insolation is also influenced by solar activity which can change over time. The number of sunspots in turn positively correlate to the energy output of the sun.
• The amount of insolation can change depending on the location of the Earth on its solar orbit. The farther the Earth is from the sun, the less insolation it receives

Earth’s Geometry and Insolation §

• The amount of insolation that is transmitted to Earth is spread out over its surface area and its atmosphere: It is less than a quarter of the total energy received from the sun.
• The amount of insolation received at a given point on Earth’s is mostly related to latitude.
  • At latitudes closer to the Equator, the insolation is more focused and comes at a direct angle and thus, they receive more insolation.
  • By contrast, at latitudes closer to the Poles, the insolation is spread out over a wider area and they receive less.
  • The larger the zenith angle, the farther the energy has to travel through the atmosphere. Thus, the atmosphere at higher latitudes absorb more of the energy.
  • A more precise, empirical formulation of the above can be found in Reynolds Ch. 2.16

Insolation and Earth’s Surface §

• Considering various natural mechanisms, the Earth has a low average but highly variational surface albedo. This albedo is important since it determines heat balance.
  • Land gets warmer at a much faster rate than the ocean. This is because of the specific heat capacity of water compared to rocks. Also, water tends to transfer this heat to latent heat (i.e., evaporation)
  • Land cools faster than water for the same reason as the above.
  • Land only warms up to the surface; Water can get warm at lower depths. This is because of the opaqueness of land compared to water, as well as the fact water has convection currents to distribute the heat.
• Energy that is absorbed on Earth’s surface induces an increase in temperature and an increase in sensible heat. The heating of Earth’s surface also causes the air to heat up.
  • Because the transfer of heat between air and the surface takes a while, the warmest surface temperature occurs hours after noon and the coldest before sunrise
• Energy that is absorbed can also cause phase changes through latent heat. This causes energy to be released into the environment as sensible heat.

Counter-Radiation and Energy Balancing §

• Some materials that absorb shortwave radiation can also increase wavelength by being at the right temperature and emit radiation. (for why see Wien’s Law)
  • This counter-radiation is actually essential for keeping temperatures on Earth suitable for life.
  • Water vapor is the most abundant greenhouse gas that helps emit extra counter-radiation downwards. This moderates temperature changes between day and night.
  • Another thing to consider is the greenhouse effect wherein radiation, primarily thermal infrared that is permitted to pass through by the atmospheric window, becomes trapped in the Earth due to greenhouse gases in the atmosphere. This keeps the planet warmer than it would be otherwise.
• Ideally, there must be an energy balance to maintain hospitable temperatures. If too much of the incoming radiation is redirected back to space the earth will cool, otherwise if there is too little, it will be too warm.
  • The sun heats the Earth’s surface more than the atmosphere.
  • The main heat source for the atmosphere is actually from convection coming from the Earth’s surface.
  • This balance is maintained by having the surface return any absorbed temperature back to the atmosphere through radiation. This can come through radiation or evaporation.
  • The atmosphere then transmits any heat absorbed through longwave radiation
• Regions near the equator and the tropics receive more insolation that they transmit back. This excess energy is then transmitted towards the Poles. We refer to such regions as having an energy surplus.
  • More than half of the planet has an energy surplus when it comes to insolation. These contribute to wind patterns.
  • An increase in latitude means a decrease in energy surplus. As a result, in higher latitudes, the ocean tends to be warmer than the continent.

Seasons §

• The seasons are indicative of the amount of sunlight received by a region.
• The seasons are caused by the Earth’s axis of rotation being tilted relative to the plane in which we orbit the sun.
  • When the North Hemisphere has its summer, the South Hemisphere has its winter due to the North Hemisphere being tilted towards the sun.
  • The Solstice marks the date in which a pole of the earth is pointed most away (Winter) or towards (Summer) the sun
  • The Equinox pertains to the times when Earth’s axis is exactly sideways and the duration of daylight and darkness are equal.
  • The distance of the Earth to the sun is not a factor in explaining the seasons. During Northern winter, the Earth is closer to the sun.
• On the Tropics, latitudes which have an angle from the equator that matches the tilt off the Earth’s axis, the amount of insolation received is the maximal and minimal depending on which solstice.
  • In other times of the year, the sun delivers constant insolation between these regions which causes the tropics to be warm.
• On the Polar Circles, latitudes which have a complementary angle matching the tilt of the Earth so that they are perpendicular to the sun’s rays, during the day of the solstice, one pole will have no night while the other pole will have no day .

Daylight §

• Sunrise and sunset are brought about by the rotation of the Earth.

  • At any given point, half of the Earth is covered in darkness.
  • The boundary between night and day moves West as the surface rotates East. (counterclockwise from the perspective of the North Pole)

• Earth’s Geometry also affects the amount of daylight received.

  • At the poles, the most extreme cases happen where there is either 24 hours of sunlight or 24 hours of darkness.
  • During the Winter Solstice, the sun rises in the southeast and sets in the southwest. This is the shortest day and longest night.
  • During the Summer Solstice, the sun rises in the northeast and sets in the northwest. This is the longest day and shortest night.
  • During an equinox, every location on Earth has 12 hours of sunlight and darkness.

• Midnight Suns and White Nights are a thing. Midnight Suns happen when the sun remains visible at the local midnight. White nights happen when the sun has set but daytime activities remain possible.

Links §

• Reynolds Ch. 2.1 - 2.10, 2.13 - 2.17