The feeling of warmth from the sun is distinct from the ambient air temperature, which is what a thermometer typically measures. This distinction helps explain why a shaded area can feel significantly cooler even when the air temperature remains the same.
Understanding Solar Radiation
The sun produces a broad range of electromagnetic energy, known as solar radiation, which travels through space to Earth. This spectrum includes visible light, alongside invisible forms such as infrared (IR) and ultraviolet (UV) radiation. Infrared radiation is primarily responsible for the heat we feel, while ultraviolet radiation can cause sunburn and skin damage.
Upon reaching Earth, solar radiation interacts with the atmosphere. The portion that reaches the surface heats objects it strikes, including our skin, leading to the sensation of warmth.
Why Sun Feels Hotter Than Shade
The primary reason direct sunlight feels hotter than shade is due to radiant heat transfer. When a person is in direct sunlight, their body absorbs solar radiation, specifically infrared and visible light, converting this energy into thermal energy. This absorption directly increases the temperature of the skin and underlying tissues, creating the sensation of warmth.
Ambient air temperature, measured by a thermometer, reflects the heat of the air itself, which is primarily transferred through convection and conduction. When in the shade, direct solar radiation is blocked, preventing this direct heating of the body. Even if the air temperature in the sun and shade is identical, the absence of direct radiant heating in the shade allows the body’s natural cooling mechanisms, like convection and sweat evaporation, to work more effectively, making it feel cooler. The air in the shade might also feel cooler because surrounding surfaces are not heated by direct sunlight and thus do not re-radiate heat.
Factors Influencing Perceived Heat
Beyond direct solar radiation, several other factors contribute to how hot a person feels in the sun. Humidity significantly affects perceived heat because it hinders the evaporation of sweat, which is a primary way the body cools itself. When humidity is high, sweat evaporates less efficiently, making the air feel warmer than the actual temperature. Conversely, wind aids in convective cooling by moving warm air away from the skin and promoting sweat evaporation, which can make a sunny environment feel less oppressive.
The type of clothing worn also plays a role; light-colored and loose-fitting garments reflect more solar radiation and allow for better airflow, enhancing cooling. Darker colors absorb more radiation, leading to increased heat absorption. Furthermore, the reflectivity and absorptivity of surrounding surfaces, such as dark pavement versus light grass, influence local temperatures by absorbing and re-radiating heat, impacting the overall perceived warmth. Individual factors like activity level and how acclimatized a person is to heat also affect their thermal sensation.
Measuring Heat in the Sun
Standard air temperature readings alone often do not fully capture the heat stress experienced in direct sunlight. To provide a more comprehensive indication of perceived heat, various metrics combine air temperature with other environmental factors. The Heat Index, for example, accounts for both air temperature and relative humidity, indicating how hot it feels to the human body when these two elements are combined. It is typically calculated for shaded areas.
Apparent Temperature is a broader measure that considers the combined effects of air temperature, relative humidity, and wind speed on human perception. It aims to provide a “feels like” temperature. A more comprehensive measure, the Wet-Bulb Globe Temperature (WBGT), specifically accounts for direct sunlight, along with air temperature, humidity, and wind speed. WBGT is particularly useful in settings like sports and outdoor work because it incorporates radiant heat, providing a more accurate assessment of heat stress in sunny conditions.
Staying Safe in Sunlight
Managing exposure to high temperatures and direct sunlight is important for health. Staying well-hydrated is a primary recommendation, as the body loses fluids through sweating when exposed to heat and sun. Drinking plenty of water or sports drinks helps replace lost fluids and electrolytes.
Seeking shade, especially during peak sun hours, typically between 10 a.m. and 4 p.m., significantly reduces direct solar radiation exposure. Wearing appropriate clothing also offers protection. Light-colored, loose-fitting clothes made from tightly woven fabrics can reflect sunlight and allow for better air circulation, keeping the body cooler. Some clothing is specifically designed with an Ultraviolet Protection Factor (UPF) to block harmful UV rays. Additionally, wearing wide-brimmed hats and sunglasses that block UV rays provides further protection for the face, neck, and eyes.