I have talked here before about why you should care about the dew point rather than relative humidity (as a rule of thumb, a dew point less than 65° is comfortable). Today in a similar vain I am going to talk about sunrise and sunset versus civil twilight.

The times that the weather forecasters give on TV for sunrise and sunset are useless for most people watching them. Why? Most people incorrectly think sunrise and sunset is respectively when it becomes light and when it becomes dark, while actually these terms have very technical definitions.

Sunrise and sunset refer to the times when the upper edge of the disk of the Sun is on the horizon, considered unobstructed relative to the location of interest. Atmospheric conditions are assumed to be average, and the location is in a level region on the Earth's surface.

This doesn't tell you when it will be "dark" so you will know how long you have to finish your yard work or how to plan a picnic. For this you need to know about civil twilight.

While civil twilight also has a very technical definition, it is of much more value to the average person:

Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities. Complete darkness, however, ends sometime prior to the beginning of morning civil twilight and begins sometime after the end of evening civil twilight.

Simply, it is when there is still enough sunlight outside to do stuff without turning on the lights.

I want to apply lawn fertilizer tonight after work and I wanted to see how much time I have. Sunset in my location is 6:02 PM, but civil twilight doesn't end until 6:30 PM.

If you want to find out this information for your location just go to this page that is run by the US Naval Observatory and under the first section select your state and type in your city and click the "Get Data" button.