The strength of the sun refers to the intensity of its invisible ultraviolet (UV) radiation, not visible light or heat. UV radiation is the component of sunlight responsible for sunburn, skin aging, and damage to cellular DNA. The sun’s strength is directly related to the risk of harm to unprotected skin and eyes. In the Northern Hemisphere, the sun reaches its maximum intensity during the late spring and summer months, a predictable consequence of the Earth’s orbit and tilt.
Defining Peak Solar Strength and the UV Index
The highest levels of solar intensity in the Northern Hemisphere typically occur in June and July. This peak is quantified using the Ultraviolet Index (UVI), an international, standardized measurement. The UVI provides a daily forecast of the expected risk of overexposure to UV radiation.
The UVI scale ranges from 1, representing a minimal risk of harm, to 11 or more, signifying an extreme risk. A UVI of 3 to 5 is considered moderate, where sun protection is advised for extended exposure. When the index reaches 6 or 7, the risk is classified as high, and unprotected skin can burn quickly.
Values of 8 to 10 are considered very high, and anything above 11 is categorized as extreme exposure. During these periods, even short periods of unprotected exposure can cause significant skin damage. The UVI is a more reliable indicator of risk than temperature, as UV radiation levels can remain high even on cool or overcast days.
The Role of Solar Angle and Atmospheric Path Length
The sun is strongest in summer due to the change in the angle at which solar radiation strikes the Earth, caused by the planet’s axial tilt. During summer in the Northern Hemisphere, the Earth is tilted toward the sun, causing the sun to appear higher in the sky. This effect is measured by the solar zenith angle, which is the angle between the sun’s rays and a point directly overhead.
When the sun is positioned nearly overhead, the solar zenith angle is at its minimum. The ultraviolet radiation travels through the shortest possible distance of the Earth’s atmosphere. This shorter atmospheric path length means there is less opportunity for the atmosphere to absorb, scatter, or filter the incoming UV rays. Consequently, a greater percentage of the sun’s intense radiation reaches the surface, leading to a higher UV Index.
Conversely, in winter, the sun is lower in the sky, resulting in a larger solar zenith angle and a much longer path through the atmosphere. The extended journey allows for more scattering and absorption of the UV radiation by gases and particles. This natural filtration process significantly reduces the intensity of the UV rays that ultimately reach the ground, making the sun less potent compared to the summer months.
Geographic and Daily Modifiers to Sun Strength
Latitude and Altitude
While the seasonal cycle determines the general period of peak intensity, several other factors modify the local strength of the sun. Geographic latitude is a major influence, as areas closer to the equator receive more direct, overhead sunlight year-round. The nearly perpendicular angle of the sun at lower latitudes means the atmosphere filters less UV radiation, resulting in consistently higher UVI values compared to regions farther north or south.
Altitude also plays a substantial role, as UV intensity increases with elevation. With every 1,000 meters (about 3,280 feet) increase in altitude, UV levels can rise by approximately 10 to 12 percent because there is less atmosphere overhead to absorb the radiation. This means a clear, sunny day in the mountains can present a much higher UV risk than a day at sea level.
Time of Day and Reflection
The time of day is another predictable modifier, with UV radiation levels peaking around solar noon, typically between 10 a.m. and 4 p.m., regardless of the season. During this midday window, the sun is highest in the sky, and its rays are most direct, minimizing the atmospheric path length. Additionally, certain surfaces can dramatically amplify UV exposure through reflection. Fresh snow can reflect between 50 and 88 percent of UV radiation, while sand reflects up to 25 percent and water reflects between 10 and 30 percent.
Protecting Yourself During Peak Intensity
Understanding the factors that contribute to peak sun strength allows for effective sun protection strategies. The most straightforward measure is to limit outdoor time during the midday hours, typically from 10 a.m. to 4 p.m., when UV rays are most intense.
Essential Protection Methods
- Limit outdoor time during peak hours (10 a.m. to 4 p.m.).
- Seek shade under trees, umbrellas, or canopies to reduce direct exposure.
- Apply broad-spectrum sunscreen with an SPF of 30 or higher liberally to exposed skin.
- Reapply sunscreen at least every two hours, or immediately after swimming or excessive sweating.
- Wear a wide-brimmed hat and UV-blocking sunglasses to protect sensitive areas.
- Use clothing labeled with an Ultraviolet Protection Factor (UPF) rating, or tightly woven, dark-colored, long-sleeved shirts and pants.