How much sunlight?

Question

Is there a number or rating that tells you how much sunlight or sun power there currently is? (I'm thinking of something where like at night the sunlight is 0, on a cloudy day it is 30-40, on a sunny day the sun rating may be 90+ even if it is winter, etc..) And if so, how is it measured? Do weather services monitor this?

Answer 1

Light is measured in footcandles (standard) or lux (SI).

Wikipedia has a nice chart:
http://en.wikipedia.org/wiki/Lux

It also varies seasonally. During summer, sunlight is more direct and during winter less direct. The amount of sunlight received will be 4-6 times higher in the summer than in the winter. It also varies with latitude.

Answer 2

Sunlight is not the same as sun power;

Sunlight is the brightness -
32000 lux (32 klx) on an average day (min.)
100000 lux (100 klx) on an average day (max.)


Power from the sun, there is the solar constant;
It is measured by satellite to be roughly 1366 watts per square meter, (it fluctuates by about 6.9% during a year - from 1412 W/m2 in early January to 1321 W/m2 in early July and by a few parts per thousand from day to day). Thus, for the whole Earth, with a cross section of 127,400,000 km², the power is 1.740×1017 W plus-minus 3.5%.

Different parts of the earth have different average cloud covers. San Diego has very few cloudy days, Chicago is usually cloudy about 40% of the time. These numbers can be found in ASHRAE.