When most people think of winter in the Tri-State often times thoughts of freezing temperatures and maybe snowfall comes to mind. However, in the cooler months the most dominant type of weather that occurs is overcast skies. In fact, the month of December is the cloudiest month of the year for Evansville. Out of the 31 days we average about 17.5 days of seeing gray skies. December 24th is considered the cloudiest day of the year; there is a 55% chance of cloudy skies every Christmas Eve.
The constant cloud coverage continues through most of winter months. Evansville averages 54% of overcast days in January and it only decreases to 48% by the end of March. However, most of our rainy days comes in the late-spring. So, what gives with the gray and gloom?
The long-lasting cloudy days in the winter can be properly explained by thinking about the thermal structure of the atmosphere. The thermal parts of the atmosphere are divided into 3 main layers. The lower layer is located from the ground to about 5,000 feet above the ground. The middle layer is about 5,000 ft -10,500 ft. The upper layer begins around 10,500 ft and higher. Clouds are generated when warm air moves upward towards cooler pockets of air. The vertical movement of warmer air mixing in with relatively cooler air allows water particles to condense and visible clouds appear. Think of water boiling in a pot and steam forms.
Although the boiling pot method of cloud formation is usually seen in the warmer months. Similar to the stove top being the source of heat for the pot; the summer-time heat and humidity provided by the sun allows heat from the surface to rise and form clouds. In the cooler months an interesting phenomenon occurs.
The roles are reversed; the surface or the lower level of the atmosphere is cooler. Meanwhile, warmer air is located in the middle levels. Unlike in the warmer months when clouds develop due to daytime heat moving upward to form bubbling cumulus clouds. In the winter months, stratus clouds usually form that are usually flat and blanket-like and dominants the sky. This temperature inversion of relative warmer air in the middle level can stick around for days at a time because of the lower angle of sun in the winter. Less sunlight decreases the chances of the surface heating up to balance out the vertical gradient of temperatures.