Insolation

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.

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.

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 associated with increased release of solar energy.

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:

• Polar night occurs when nighttime lasts for over 24 hours

• Polar day (also called midnight sun) occurs when the Sun remains above the horizon for over 24 hours

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.

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.

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.

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.

What's the equation for standard deviation?

\begin sqrt{\dfrac{\sum\left(x-\overline{x}\right)^{2}}{12-1}}=SD 

• x̄: mean of the data set

• x: individual data measurement

• Σ: sum of

• n: sample size

• √: square root

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.

^{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}