How much stronger is sunlight in June?

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Dear Tom,
I know that the hours of daylight now are much greater than the minimum of hours in late December, but what is the difference? And what units are used to measure the strength of sunlight?
— Milton Shores, Chicago
Dear Milton,
Ultraviolet radiation, measured at peak times (around midday), grows eight times stronger on June 21 than on Dec. 21. That’s the word from dermatologist Dr. Bryan Schultz.
Over a full day, incoming sunlight is six times stronger in June than in December. The sun delivers about 76 units of energy per day in December, but 458 units per day in June — and that is an increase of 603 percent.
A “langley” is the unit used to describe the energy in incoming sunlight. One langley is the amount of energy required over a square centimeter to raise the temperature of one gram of water by one degree Celsius.

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