When there is still sunlight at 5pm, and the temperature is just as hot as it is at 1pm, why is the UV index significantly lower in the evening?
In: Planetary Science
Temperature is more a quality of the air, not sunlight exposure at that moment. And while there is still sunlight in the late evening the angle at which the light arrives means it travels through more atmosphere to reach the ground. It is still plenty bright to see by but the UV light has more opportunity to be absorbed and so your exposure is much reduced.
The UV index has nothing to do with temperature. It has to do with how much UV light is kaking it from the sun to your skin, which can happen at any temperature and even through clouds.
In the evening, the Sun is lower in the sky, so the sun light needs to pass through more air to reach your skin, so more UV light gets blocked.
The UV index is mostly affected by how much and what type of atmosphere the sunlight goes through. It’s why higher altitudes get more, for example; the higher altitude means there’s less atmosphere, which means more sunlight makes it through.
Now, what happens when you take a straw that is straight up in a glass, but then lean it diagonally across the cup? Suddenly, the straw is going through more water.
Similarly, if you take the sunlight that is mostly vertical at noon but then have it more diagonal in the evening, it is suddenly going through more atmosphere than before. Therefore, the UV index will be lower in the evening than when the sun is the highest.
In addition to what has been mentioned: The atmosphere absorbs UV pretty well, but doesn’t absorb visible light much. At noon you get almost all the sunlight and maybe 10% of the UV compared to space. At 5 pm the light crosses twice as much air, you still get almost all the visible light but only 10% of 10% = 1% of the UV. The numbers depend on the location and the day, but the general concept is the same everywhere. Temperature and visible light are not an indication for the UV index.
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