Climate in Illinois
(United States)

The number of hours of sunshine refers to the time when the sun is actually visible. That is, without any obstruction of visibility by clouds, fog or mountains. With 10 hours per day, July is the sunniest month in the state of Illinois. In December, the sun shines the least.

A rain day is a day on which at least 0.1 mm precipitation (=0.1 liter) per square meter falls. This can be rain, snow, hail or even dew. So it does not have to rain the whole day. With 11 rain days, May offers the most rain days, while September has the fewest.

The amount of precipitation is measured in millimeters per square meter. Thus, at two mm/day, two liters of water fall on one square meter within 24 hours. With only 1.7 mm, the least amount of rain falls in January. June, on the other hand, has the most rain.

Warm air can absorb more moisture than cold air. The relative humidity indicates how much moisture can be physically contained in the air. At high humidity, a person feels uncomfortable and perceives this as oppressive. In general, a relative humidity of 40-60% feels pleasant. With humidity averaging 76%, December is the most uncomfortable. In April, on the other hand, it is easier to endure.

In warm months, there is also a higher absolute humidity almost everywhere. At a temperature of 25°C, air can absorb up to 23 grams of water per cubic meter. At 20°C, it is only 17.3 grams. A relative humidity of 40% at 25°C thus corresponds to an absolute humidity of 9.2 grams of water. From a proportion of approx. 13.5 grams, humans perceive the air as muggy. This limit is only reached in a few months in Illinois, although these are always average values.

The "Humidex" (humidity index) is an index of well-being in warmer areas, calculated from the air temperature, relative humidity and dew point. The index corresponds approximately to the temperature felt. At high temperatures, the body transports heat away by sweating. At high humidity, the ambient air can only absorb a small amount of sweat and thus one feels uncomfortable. An index of 20-29 is considered comfortable. Up to 39, people perceive the air as slightly uncomfortable, and above 40, as very uncomfortable. Values above 45 are dangerous in the long run and often lead to heat stroke.