At What Time Is Solar Intensity Greatest?

Solar intensity refers to the amount of solar radiation or sunlight energy reaching a given area on the Earth’s surface, usually measured in kilowatt-hours per square meter (kW/m2). The solar intensity varies over the course of a day as the position of the Sun in the sky changes.

During midday when the Sun is highest in the sky, the rays have the shortest path through the atmosphere resulting in the maximum intensity of sunlight. In the morning and evening when the Sun is lower in horizon, the sunlight has to travel through more atmosphere resulting in greater scattering and absorption leading to lower intensity.

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Position of the Sun

The sun’s position in the sky changes throughout the day as the Earth rotates on its axis. At sunrise, the sun appears along the eastern horizon. As the morning progresses, the sun rises higher in the sky, reaching its maximum altitude around solar noon. After noon, the sun begins descending toward the western horizon, where it sets in the evening.

The sun’s changing position affects its angle relative to the ground. When the sun is low in the sky, its rays strike the Earth at an oblique angle. At solar noon, the sun reaches its highest point in the sky and its rays hit the Earth more directly. This varying solar altitude impacts the intensity of the sun’s radiation at different times of day.

Solar Noon

Solar noon is the moment when the sun reaches its highest position, or altitude, in the sky each day. This occurs when the sun is directly overhead along a meridian, an imaginary line from the North Pole to the South Pole passing through a given location. At solar noon, the sun will appear due south in the sky for locations in the Northern Hemisphere and due north for locations in the Southern Hemisphere.

The time of solar noon changes throughout the year and depends on the observer’s longitude and the equation of time. It occurs between 11am and 1pm local clock time. Solar noon is approximately 12pm (noon) for locations along the median longitude within a time zone. Moving east or west within the time zone, solar noon occurs earlier or later. The exact time also shifts several minutes over the course of a year due to the ellipticity of Earth’s orbit and the tilt of its axis.

For a given location, solar noon marks the middle of the sun’s daily journey across the sky when it reaches its highest altitude for that day. This is the point when shadows are shortest as the sun is most directly overhead.

Solar Altitude

The altitude (height) of the sun in the sky has a significant impact on solar intensity. The higher the sun is in the sky, the shorter the path sunlight has to travel through the atmosphere before reaching the ground. This results in less scattering and absorption of sunlight by the atmosphere, increasing the intensity at the surface.

At solar noon, when the sun is at its peak altitude for the day, intensity is maximized. As sunrise and sunset approach, when the sun is low on the horizon, intensity drops off sharply due to the increased air mass. The solar altitude angle (measured from the horizon) can be used along with air mass to quantitatively determine the intensity at a given time and location.

The maximum solar altitude varies throughout the year and with latitude. Areas closer to the equator experience higher peak sun angles, resulting in greater maximum intensities. The altitude effect explains why solar intensity is generally greatest within several hours of solar noon each day.

Air Mass

The air mass refers to the path length that light from the sun travels through the atmosphere before reaching the Earth’s surface. As the sun’s position in the sky changes throughout the day, the air mass changes as well. When the sun is directly overhead, the air mass is at its minimum (around 1). But when the sun is close to the horizon, its light must travel through much more atmosphere, increasing the air mass (up to around 38 at sunset).

A higher air mass leads to greater scattering and absorption of light by atmospheric gases and particles. This filtering effect reduces the intensity of solar radiation at the surface. Therefore, the solar intensity is greatest when the air mass is lowest – that is, when the sun is closest to directly overhead around solar noon.

Time of Maximum Intensity

The time of day when solar intensity reaches its maximum is when the sun is at its highest point in the sky, which is known as solar noon. This occurs when the sun is directly overhead, passing through its highest elevation angle relative to a location on Earth. At solar noon, the sun’s rays have the shortest path through the atmosphere and undergo the least amount of scattering and absorption.

The exact time of solar noon depends on the time zone and season. It typically occurs around 12pm local standard time. However, due to daylight saving time changes and the eccentricity of Earth’s orbit around the sun, it ranges between 11am and 1pm over the course of a year for a given location. Solar noon occurs earliest in early January and latest in late December in the Northern Hemisphere.

At solar noon, the sun’s rays are perpendicular to the Earth’s surface at a given latitude. With the sun directly overhead, the rays have the shortest atmospheric path length. This minimizes scattering and absorption of light by atmospheric gases and particles that would otherwise reduce intensity. As a result, more of the sun’s rays reach Earth’s surface leading to maximum solar irradiance.

Other Factors Impacting Solar Intensity

In addition to the position of the sun in the sky, several other factors impact the intensity of solar radiation reaching the Earth’s surface.

Weather conditions play a major role. Cloud cover can dramatically reduce intensity by blocking incoming sunlight. Atmospheric moisture and particulates like dust also absorb and scatter sunlight. Areas with very humid or hazy conditions will have lower peak intensity compared to dry, cloudless regions.

Geography is also important. Solar intensity decreases at higher latitudes as sunlight strikes the Earth at an oblique angle. Elevation is another consideration—solar radiation is more intense at higher altitudes due to less atmospheric interference. The intensity can vary by over 25% between sea level and 6000 feet.

Seasonal changes impact the angle of incoming sunlight and length of day. The most direct sunlight occurs during the summer solstice in June in the Northern hemisphere and December in the Southern hemisphere. The peak solar intensity will be higher during these seasons.

Local landscape and artificial structures provide shading which can significantly reduce the intensity in affected areas. Reflective surfaces like snow, water and white paint can conversely increase intensity through additional reflected light.

Measuring Intensity

Solar intensity is measured using instruments called pyranometers. A pyranometer is a sensor that measures solar irradiance – the power per unit area produced by the sun in the form of electromagnetic radiation. There are a few different types of pyranometers:

Thermopile pyranometers – These contain a thermoelectric junction that generates a voltage proportional to the solar irradiance. They can measure both direct beam radiation and diffuse sky radiation.
Photovoltaic pyranometers – These contain a photovoltaic sensor that generates a current proportional to the solar irradiance. They are only used for measuring the global solar irradiance.

Photodiode pyranometers – These contain a photodiode sensor and measure visible light and portions of ultraviolet and infrared radiation from the sun.

Professional-grade pyranometers are calibrated against a world radiometric reference to ensure accuracy. The voltage or current output from the pyranometer sensor is logged to record the solar irradiance data over time. This allows the measurement of peak solar intensity during the day.

Applications

Understanding the daily and seasonal patterns of solar intensity has many practical applications, including:

Solar power generation – The output of solar photovoltaic panels peaks when the sun is highest in the sky around solar noon. Tracking solar intensity patterns allows optimal orientation of panels.

Architecture and construction – Designing buildings and shade structures to account for sunlight patterns improves lighting, heating, and cooling. Understanding peak solar intensity informs spacing and shading elements.

Agriculture – Knowing when plants will receive the most intense sunlight allows scheduling planting, irrigation, and harvesting for optimal results.

Skin protection – Solar intensity data guides recommendations on sunscreen usage and exposure during peak daily and seasonal intensity.

Spacecraft and satellite design – Orbital periods and orientations are planned with solar intensity in mind to ensure adequate power and thermal control.

Solar energy research – Monitoring and analyzing solar intensity provides insights into solar cycles and variability for solar physics and climatology.

Conclusion

In summary, solar intensity is dependent on the position of the sun in the sky, which changes throughout the day. The key points are:

Solar noon is when the sun reaches its highest point in the sky and is due south (in the northern hemisphere). This corresponds to roughly 12pm, but varies.

The sun’s altitude, or angle above the horizon, impacts intensity due to the air mass. The higher the altitude, the less atmosphere sunlight must travel through.
Intensity is greatest when the sun is at its peak altitude around solar noon. The precise timing depends on location and time of year.

Understanding when maximum solar intensity occurs allows effective use of sunlight for applications like solar power generation. Careful tracking of the sun’s position can optimize solar energy capture.