Have you ever spent too long in the sun, and then felt dizzy and sick? The combination of high temperatures and physical activity can lead to heat exhaustion. Make sure that you drink lots of water when it's hot, especially when you're exercising.
Severe heat exhaustion can cause heatstroke – a condition also called insolation.
Insolation has another meaning. What do you think it could be? (Hint: focus on the first two syllables).
That's right, it refers to incoming sun – a.k.a. solar radiation.
Solar Insolation: Definition
Let's begin with the definition of insolation.
The units of measurement of solar insolation are kWh/m2/day (kilowatt-hours per square metre per day).
Insolation is regulated by a planet's distance to the Sun.
Why is Insolation Important?
Solar insolation enables life on Earth. Without incoming solar radiation, it would be too cold for organisms to survive.
It's important for scientists to know insolation data. Knowledge of insolation helps meteorologists to understand weather and climate patterns. In turn, this helps botanists understand patterns of plant growth around the world. This information is used by farmers to help maximise their crop yields and provide enough food for the population.
The Earth has a whole maintains its temperature – it doesn't accumulate or lose heat. But Earth can only maintain its temperature if the amount of heat received by insolation = the amount of heat lost by terrestrial radiation. This balance between insolation and terrestrial radiation is called the heat budget.
Insolation vs Irradiance
The terms insolation and irradiance are often confused. Let's clear up the differences between the two.
Irradiance is a measure of solar power. Power refers to the rate of energy transfer over time – i.e. the amount of solar energy arriving in an area in a given moment. It's measured in Watts/m2.
In contrast, insolation is a measure of solar energy. The irradiance value is converted to express the total amount of energy received over an interval of time, so is communicated using Watt-hours. As we learned earlier, its unit of measurement is kWh/m2/day.
Insolation is calculated by using measurements of irradiance.
Fig. 1 – Insolation is represented by the blue area underneath the curve.
Irradiance is measured using a piece of equipment called a pyranometer. There are two kinds of pyranometer: thermophiles and reference cells.
Thermophiles measure the temperature difference between their exposed surfaces and their shaded surfaces. Reference cells are silicon solar cells that measure the photocurrent of sunlight.
Insolation and Temperature
Earth's surface temperature is directly related to solar insolation.
Factors Affecting Insolation
Solar insolation is not uniform around the globe. What factors affect insolation, and therefore surface temperature?
Solar Constant
Insolation received at the top of the atmosphere is known as the solar constant. At the thermopause (between the thermosphere and exosphere), the average solar constant is 1370 Watts/m2.
The solar constant varies slightly, depending on sun spots.
Sunspots are darker and cooler visible regions on the Sun's surface.
Sunspots are associated with increased release of solar energy.
The number of sunspots varies according to an 11-year cycle.
Angle of Incidence
The Sun’s rays strike the surface at different angles, depending on the latitude. The higher the latitude, the smaller the angle of incidence, thus the less solar insolation reaches the surface.
This is one of the reasons why the equator is warmer than the poles.
Duration of the Day
The length of the day determines how much solar radiation can reach Earth’s surface. The longer the day, the more insolation there is. At the equator, the day length remains constant at 12 hours throughout the year. But as latitude increases, the difference between day and night becomes more extreme.
The northernmost and southernmost regions of Earth experience two phenomenons:
Fig. 2 - Tromsø, a city in northern Norway, experiences polar night. The sun does not rise between November 27th and January 15th. Source: unsplash.com
Distance from the Sun
The Earth revolves around the Sun in an elliptical orbit.
Eccentricity is the measurement of how much Earth's orbit deviates from a perfect circle.
The eccentricity of the Earth varies over a 100,000 year cycle. When Earth's orbit is at its most circular, it receives 23% more solar radiation than when it is at its most circular.
The Earth is furthest from the Sun on the 4th of July. This position is called the aphelion. In contrast, the Earth is nearest the Sun on the 3rd of January. This position is called the perihelion.
Transparency of the Atmosphere
Earth's atmosphere is not transparent. It's composed of gases, water vapour, and particulate matter.
The less transparent the atmosphere is, the less solar insolation is received.
Volcanic eruptions release ash, dust, and sulfur gases into the atmosphere. High concentrations of atmospheric particulate matter reflect incoming solar radiation, leading to a reduction in insolation.
Major eruptions can lead to volcanic winters; a reduction in global temperatures caused by reduced insolation.
An unidentified eruption in the year 536 led to an eighteen month volcanic winter, with temperatures falling by 2.5ºC. Harvests failed, leading to famine and starvation.
Average Solar Insolation by Country
Generally, countries near the equator have higher rates of solar insolation due to limited seasonal variation. However, solar insolation can also depend on elevation, climate, and cloud cover.
Fig. 3 – Solar irradiance, and therefore insolation, is greatest in equatorial and other hot countries. Source: SolarGIS
The area with the greatest solar irradiance is the Atacama Desert in Chile, reaching 310 Watts/m2. Thus, the Atacama Desert will have the greatest solar insolation.
Solar Insolation Map of the UK
Although solar insolation is low in the UK (averaging 2-3 kWh/m2), it varies geographically. Areas with the most solar insolation are found in the south of the country.
Fig. 4 – The southern coastline of the UK has the greatest solar insolation. Source: SolarGIS
Standard Deviation of Insolation: Worked Example
The main disadvantage of solar power is its unreliability. So, when constructing a new solar farm, the managers should pay attention to the variability of insolation levels.
The managers want to build a solar farm where the insolation is less variable. Using the data, we can perform a standard deviation test to assess variability.
| Month | Average Daily Insolation (kWh/m2) |
| Site A | Site B |
January | 1.4 | 1.8 |
February | 1.6 | 1.9 |
March | 1.7 | 2.0 |
April | 2.4 | 2.1 |
May | 2.9 | 1.9 |
June | 3.4 | 2.7 |
July | 3.5 | 2.6 |
August | 2.6 | 2.6 |
September | 2.6 | 2.5 |
October | 2.3 | 2.3 |
November | 1.9 | 2.0 |
December | 1.5 | 1.9 |
Mean | 2.32 | 2.19 |
Standard deviation measures the variability of a dataset from its mean.
What's the equation for standard deviation?
\begin sqrt{\dfrac{\sum\left(x-\overline{x}\right)^{2}}{12-1}}=SD Now, let's insert the data from Site A into this equation. The mean insolation is 2.32, and the sample size is 12.
\sqrt{\dfrac{\sum\left(x-2.32\right)^{2}}{12-1}}=0.72 So, the standard deviation of Site A is 0.72.
Now, let's do the same with Site B. The mean insolation is 2.19, and the sample size is 12.
\sqrt{\dfrac{\sum\left(x-2.19\right)^{2}}{12-1}}=0.33 So, the standard deviation of Site B is 0.33.
Which site is less variable, therefore will be the future location of the solar farm?
I hope that this article has explained insolation for you. Remember that insolation is the amount of solar radiation received (measured in kWh/m2/day). Surface temperature is dependent on insolation. The equator has a greater insolation than the poles, so its surface temperature is warmer.
Insolation - Key takeaways
- Insolation is the amount of solar radiation received by a planet. It's measured in kWh/m2/day. Insolation is regulated by a planet's distance from the Sun.
- Irradiance is a measure of solar power, while insolation is a measure of solar energy.
- Earth's surface temperature is directly related to solar insolation.
- Insolation is affected by the solar constant, the angle of incidence, the duration of the day, the distance from the Sun, and the transparency of the atmosphere.
- Countries near the equator have higher rates of insolation due to limited seasonal variation.
- Insolation is relatively low in the UK. Regions with less insolation variability are most suitable for solar power farms.
1. Alan Buis, Milankovitch (Orbital) Cycles and Their Role in Earth's Climate, NASA’s Jet Propulsion Laboratory, 2020
2. Fjord Tours, Polar night season in Tromsø, 2020
3. John Kennewell, The Solar Constant, Australian Government Bureau of Meteorology, 2022
4. Kristine De Abreu, Apocalypse Then: The Volcanic Winter of 536AD, Explorers Web, 2022
5. Roberto Rondanelli, The Atacama Surface Solar Maximum, Bulletin of the American Meteorological Society, 2015
6. UCAR Center for Science Education, The Sunspot Cycle, 2012
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