Effect of Sun angle on climate

Last updated September 24, 2024

The amount of heat energy received at any location on the globe is a direct effect of Sun angle on climate, as the angle at which sunlight strikes Earth varies by location, time of day, and season due to Earth's orbit around the Sun and Earth's rotation around its tilted axis. Seasonal change in the angle of sunlight, caused by the tilt of Earth's axis, is the basic mechanism that results in warmer weather in summer than in winter.^[1]^[2]^[3] Change in day length is another factor (albeit lesser).^[2]^[3]

Geometry of Sun angle

Figure 2
One sunbeam one mile wide shines on the ground at a 90deg angle, and another at a 30deg angle. The one at a shallower angle covers twice as much area with the same amount of light energy. Seasons.too.png — **Figure 2**
One sunbeam one mile wide shines on the ground at a 90° angle, and another at a 30° angle. The one at a shallower angle covers twice as much area with the same amount of light energy.

Figure 1 presents a case when sunlight shines on Earth at a lower angle (Sun closer to the horizon), the energy of the sunlight is spread over a larger area, and is therefore weaker than if the Sun is higher overhead and the energy is concentrated on a smaller area.

Figure 2 depicts a sunbeam one mile (1.6 km) wide falling on the ground from directly overhead, and another hitting the ground at a 30° angle. Trigonometry tells us that the sine of a 30° angle is 1/2, whereas the sine of a 90° angle is 1. Therefore, the sunbeam hitting the ground at a 30° angle spreads the same amount of light over twice as much area (if we imagine the Sun shining from the south at noon, the north–south width doubles; the east–west width does not). Consequently, the amount of light falling on each square mile is only half as much.

Figure 3
This is a diagram of the seasons. Regardless of the time of day (i.e. Earth's rotation on its axis), the North Pole will be dark, and the South Pole will be illuminated; see also arctic winter. Seasons.svg — **Figure 3**
This is a diagram of the seasons. Regardless of the time of day (i.e. Earth's rotation on its axis), the North Pole will be dark, and the South Pole will be illuminated; see also arctic winter.

Figure 3 shows the angle of sunlight striking Earth in the Northern and Southern Hemispheres when Earth's northern axis is tilted away from the Sun, when it is winter in the north and summer in the south.

Obliquity, seasonality, and climate

Differing sun angle results in differing temperatures between lower and higher latitudes, and between winter and summer at the same latitude (although "winter" and "summer" are more complicated in the tropics.^{[lower-alpha 1]}^[4]

At fixed latitude, the size of the seasonal difference in sun angle (and thus the seasonal temperature variation) is equal to double the Earth's axial tilt. For example, with an axial tilt is 23°, and at a latitude of 45°, then the summer's peak sun angle is 68° (giving sin(68°) = 93% insolation at the surface), while winter's least sun angle is 22° (giving sin(22°) = 37% insolation at the surface). Thus, the greater the axial tilt, the stronger the seasons' variations at a given latitude.^[4]

In addition to seasonal variation at fixed latitude, the total annual surface) insolation as a function of latitude also depends on the axial tilt. At the equator (0° latitude), on the equinoxes, the sun angle is always 90° no matter the axial tilt, while on the solstices the minimum sun angle is equal to 90° minus the tilt. Therefore, greater tilt means a lower minimum for the same maximum: less total annual surface insolation at the equator. At the poles (90° latitude), on the equinoxes and during polar night, the sun angle is always 0° or less no matter the axial tilt, while on the summer solstice, the maximum angle is equal to the tilt. Therefore, greater tilt means a higher maximum for the same minimum: more total annual surface insolation at the poles. Therefore, lesser tilt means a wider annual temperature gap between equator and poles, while greater tilt means a smaller annual temperature gap between equator and poles.^[4] (At an extreme tilt, such as that of Uranus, the poles can receive similar annual surface insolation to the equator.) In particular, at Earth temperatures, and all else being equal, greater tilt warms the poles and thus reduces polar ice coverage, while lesser tilt cools the poles and thus increases polar ice coverage.^[4]

One of the first to publish on these effects was Milutin Milanković; the cyclic effects of axial tilt, eccentricity, and other orbital parameters upon global climate were named Milanković cycles. Although individual mechanisms (such as axial tilt and sun angle) are thought to be understood, the overall impact of orbital forcing on global climate remains poorly constrained.

Notes

↑ Consider: the equator and the poles (indeed all places on Earth) average around 12*365 = 24*182.5 hours of daylight per year. Yet, the equator's coolest is far warmer than the poles' warmest. The gap can only be due to the angle of the sun, not time under daylight.

Related Research Articles

A solstice is the time when the Sun reaches its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20-22 June and 20-22 December. In many countries, the seasons of the year are defined by reference to the solstices and the equinoxes.

<span class="mw-page-title-main">Orbital inclination</span> Angle between a reference plane and the plane of an orbit

Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object.

The Tropic of Cancer, also known as the Northern Tropic, is the Earth's northernmost circle of latitude where the Sun can be seen directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted toward the Sun to its maximum extent. It also reaches 90 degrees below the horizon at solar midnight on the December Solstice. Using a continuously updated formula, the circle is currently 23°26′09.9″ (or 23.43608°) north of the Equator.

In astronomy, an analemma is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram resembles a figure eight. Globes of the Earth often display an analemma as a two-dimensional figure of equation of time vs. declination of the Sun.

A circle of latitude or line of latitude on Earth is an abstract east–west small circle connecting all locations around Earth at a given latitude coordinate line.

<span class="mw-page-title-main">Milankovitch cycles</span> Global climate cycles

Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The term was coined and named after the Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he hypothesized that variations in eccentricity, axial tilt, and precession combined to result in cyclical variations in the intra-annual and latitudinal distribution of solar radiation at the Earth's surface, and that this orbital forcing strongly influenced the Earth's climatic patterns.

The polar regions, also called the frigid zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles, lying within the polar circles. These high latitudes are dominated by floating sea ice covering much of the Arctic Ocean in the north, and by the Antarctic ice sheet on the continent of Antarctica and the Southern Ocean in the south.

The thermal equator is a belt encircling Earth, defined by the set of locations having the highest mean annual temperature at each longitude around the globe. Because local temperatures are sensitive to the geography of a region, mountain ranges and ocean currents ensure that smooth temperature gradients are impossible, the location of the thermal equator is not identical to that of the geographic Equator.

Seasonal lag is the phenomenon whereby the date of maximum average air temperature at a geographical location on a planet is delayed until some time after the date of maximum daylight. This also applies to the minimum temperature being delayed until some time after the date of minimum insolation. Cultural seasons are often aligned with annual temperature cycles, especially in the agrarian context. Peak agricultural growth often depends on both insolation levels and soil/air temperature. Rainfall patterns are also tied to temperature cycles, with warmer air able to hold more water vapor than cold air.

Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m²) in SI units.

Many astronomical phenomena viewed from the planet Mars are the same as or similar to those seen from Earth; but some are quite different. For example, because the atmosphere of Mars does not contain an ozone layer, it is also possible to make UV observations from the surface of Mars.

A lunar standstill or lunistice is when the Moon reaches its furthest north or furthest south point during the course of a month. The declination at lunar standstill varies in a cycle 18.6 years long between 18.134° and 28.725°, due to lunar precession. These extremes are called the minor and major lunar standstills.

The five main latitude regions of Earth's surface comprise geographical zones, divided by the major circles of latitude. The differences between them relate to climate. They are as follows:

The North Frigid Zone, between the North Pole at 90° N and the Arctic Circle at 66°33′50.1″ N, covers 4.12% of Earth's surface.
The North Temperate Zone, between the Arctic Circle at 66°33′50.1″ N and the Tropic of Cancer at 23°26′09.9″ N, covers 25.99% of Earth's surface.
The Torrid Zone, between the Tropic of Cancer at 23°26′09.9″ N and the Tropic of Capricorn at 23°26′09.9″ S, covers 39.78% of Earth's surface.
The South Temperate Zone, between the Tropic of Capricorn at 23°26′09.9″ S and the Antarctic Circle at 66°33′50.1″ S, covers 25.99% of Earth's surface.
The South Frigid Zone, from the Antarctic Circle at 66°33′50.1″ S and the South Pole at 90° S, covers 4.12% of Earth's surface.

Daytime or day as observed on Earth is the period of the day during which a given location experiences natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the globe's hemisphere facing the Sun. In direct sunlight the movement of the sun can be recorded and observed using a sundial that casts a shadow that slowly moves during the day. Other planets and natural satellites that rotate relative to a luminous primary body, such as a local star, also experience daytime, but this article primarily discusses daytime on Earth.

Sun path, sometimes also called day arc, refers to the daily and seasonal arc-like path that the Sun appears to follow across the sky as the Earth rotates and orbits the Sun. The Sun's path affects the length of daytime experienced and amount of daylight received along a certain latitude during a given season.

The equator is a circle of latitude that divides a spheroid, such as Earth, into the Northern and Southern hemispheres. On Earth, the Equator is an imaginary line located at 0 degrees latitude, about 40,075 km (24,901 mi) in circumference, halfway between the North and South poles. The term can also be used for any other celestial body that is roughly spherical.

A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant. Various cultures define the number and nature of seasons based on regional variations, and as such there are a number of both modern and historical definitions of the seasons.

North African climate cycles have a unique history that can be traced back millions of years. The cyclic climate pattern of the Sahara is characterized by significant shifts in the strength of the North African Monsoon. When the North African Monsoon is at its strongest, annual precipitation and consequently vegetation in the Sahara region increase, resulting in conditions commonly referred to as the "green Sahara". For a relatively weak North African Monsoon, the opposite is true, with decreased annual precipitation and less vegetation resulting in a phase of the Sahara climate cycle known as the "desert Sahara".

The position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. As Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on the celestial sphere, along a circular path called the ecliptic.

The climate of Pluto concerns the atmospheric dynamics, weather, and long-term trends on the dwarf planet Pluto. Five climate zones are assigned on the dwarf planet: tropics, arctic, tropical arctic, diurnal, and polar. These climate zones are delineated based on astronomically defined boundaries or sub-solar latitudes, which are not associated with the atmospheric circulations on the dwarf planet. Charon, the largest moon of Pluto, is tidally locked with it, and thus has the same climate zone structure as Pluto itself.

References

↑ Windows to the Universe. Earth's Tilt Is the Reason for the Seasons! Archived 2007-08-08 at the Wayback Machine Retrieved on 2008-06-28.
1 2 Khavrus, V.; Shelevytsky, I. (2010). "Introduction to solar motion geometry on the basis of a simple model". Physics Education. 45 (6): 641. Bibcode:2010PhyEd..45..641K. doi:10.1088/0031-9120/45/6/010. S2CID 120966256. Archived from the original on 2016-09-16. Retrieved 2011-05-13.
1 2 Khavrus, V.; Shelevytsky, I. (2012). "Geometry and the physics of seasons". Physics Education. 47 (6): 680. doi:10.1088/0031-9120/47/6/680. S2CID 121230141.
1 2 3 4 Buis, Alan; Jet Propulsion Laboratory (27 February 2020). "Milankovitch (Orbital) Cycles and Their Role in Earth's Climate". climate.nasa.gov. NASA . Retrieved 10 May 2021. Over the last million years, it has varied between 22.1 and 24.5 degrees. ... The greater Earth's axial tilt angle, the more extreme our seasons are .... Larger tilt angles favor periods of deglaciation (the melting and retreat of glaciers and ice sheets). These effects aren't uniform globally – higher latitudes receive a larger change in total solar radiation than areas closer to the equator. ... Earth's axis is currently tilted 23.4 degrees, ... As ice cover increases, it reflects more of the Sun's energy back into space, promoting even further cooling. Note: See Axial tilt. Zero obliquity results in minimum (zero) continuous insolation at the poles and maximum continuous insolation at the equator. Any increase of obliquity (to 90 degrees) causes seasonal increase of insolation at the poles and causes decrease of insolation at the equator on any day of the year except an equinox.

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[4] Consider: the equator and the poles (indeed all places on Earth) average around 12*365 = 24*182.5 hours of daylight per year. Yet, the equator's coolest is far warmer than the poles' warmest. The gap can only be due to the angle of the sun, not time under daylight.

[1] Windows to the Universe. Earth's Tilt Is the Reason for the Seasons! Archived 2007-08-08 at the Wayback Machine Retrieved on 2008-06-28.

[SunModel-2] 1 2 Khavrus, V.; Shelevytsky, I. (2010). "Introduction to solar motion geometry on the basis of a simple model". Physics Education. 45 (6): 641. Bibcode:2010PhyEd..45..641K. doi:10.1088/0031-9120/45/6/010. S2CID 120966256. Archived from the original on 2016-09-16. Retrieved 2011-05-13.

[SunModel2-3] 1 2 Khavrus, V.; Shelevytsky, I. (2012). "Geometry and the physics of seasons". Physics Education. 47 (6): 680. doi:10.1088/0031-9120/47/6/680. S2CID 121230141.

[NASABBuis2021-5] 1 2 3 4 Buis, Alan; Jet Propulsion Laboratory (27 February 2020). "Milankovitch (Orbital) Cycles and Their Role in Earth's Climate". climate.nasa.gov. NASA . Retrieved 10 May 2021. Over the last million years, it has varied between 22.1 and 24.5 degrees. ... The greater Earth's axial tilt angle, the more extreme our seasons are .... Larger tilt angles favor periods of deglaciation (the melting and retreat of glaciers and ice sheets). These effects aren't uniform globally – higher latitudes receive a larger change in total solar radiation than areas closer to the equator. ... Earth's axis is currently tilted 23.4 degrees, ... As ice cover increases, it reflects more of the Sun's energy back into space, promoting even further cooling. Note: See Axial tilt. Zero obliquity results in minimum (zero) continuous insolation at the poles and maximum continuous insolation at the equator. Any increase of obliquity (to 90 degrees) causes seasonal increase of insolation at the poles and causes decrease of insolation at the equator on any day of the year except an equinox.

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