This morning, we are in-depth on "diurnal range," that's the difference in temperature between the morning low temperature and the afternoon high. Typically, the Quad Cities has a 20 degree diurnal range, meaning if it's 50 degrees in the morning there's a high chance it will be 70 degrees in the afternoon.
That's the standard. But there are five things that determine whether we will see that 20 degree rise, see steady temperature through the day, or even observe falling temperature.
- Cloud cover. Clouds are good reflectors of sunlight which keeps the lower part of the atmosphere shadowed. Shade and the subsequent cooler temperature produces a decrease in the diurnal temperature. In addition, cloudy nights tend to be warmer than clear nights because the clouds trap shortwave and longwave radiation...especially in the winter.
- Day length. The longer the day (June and July), the more available sunlight. At the end of June, this is maximized with more than 15 hours of daylight. This can increase the diurnal range significantly, especially when humidity is low.
- Humidity. The water in the atmosphere (water vapor) is really efficient at holding onto heat. That's why humid regions tend to see fairly steady temperatures from night to day. Think about Florida where nights are usually warm and humid with hot and humid afternoons. Where humidity is low (desert regions), it can be quite chilly at night but really hot in the afternoon. So we would say that desert areas have very high diurnal ranges.
- Elevation. The tops of mountains are farther away from the warmed surface of the sun. In addition, shaded areas stay out of the sunshine and remain cool day to night. Conversely, valleys can be shaded more than flatter areas.
- Wind speed. Winds are the greatest variables to change the temperature. In addition, winds can cause cooler or warmer air to move down to the surface from higher elevations. As winds change direction, they advect cooler and warmer air into a specific region.