All things in nature have a diurnal, or daily pattern simply because they change throughout the course of a day. In meteorology, the term "diurnal" most often refers to the change of temperature from the daytime high to the nighttime low.
Daily High Temperatures
The process of reaching a daily high (or low) temperature is a gradual one. It begins each morning when the Sun rises and its rays extend toward and strike the Earth's surface. Solar radiation directly heats the ground, but because of land's high heat capacity (ability to store heat), the ground doesn't immediately warm. Just as a pot of cold water must first warm before coming to a boil, so must the land absorb a certain amount of heat before its temperature rises. As the ground's temperature warms, it heats a shallow layer of air directly above it by conduction. This thin layer of air, in turn, heats the column of cool air above it.
Meanwhile, the Sun continues its trek across the sky. At high noon, when it reaches its peak height and is directly overhead, sunlight is at its most concentrated strength. However, because the ground and air must first store heat before radiating it to surrounding areas, a maximum air temperature isn't yet achieved. It actually lags this period of maximum solar heating by several hours! Only when the amount of incoming solar radiation equals the amount of outgoing radiation does the daily high temperature occur. The time of day this generally happens depends on a number of things (including geographic location and time of year) but is usually between the hours of 3-5 p.m. (Prior to this time, there is a buildup of incoming heat energy within the atmosphere. This is why the hottest, and most dangerous part of a day is said to be between the hours of 10 a.m. and 3 p.m.)
Daily Low Temperatures
After noon, the Sun begins its retreat. From now until sunset, the intensity of incoming solar radiation continually declines. When more heat energy is being lost to space than is incoming at the surface, a minimum temperature is reached.
Determining Diurnal Variation
On average, locations experience a daily temperature swing of 20-30°F. A number of conditions can widen or lessen this range, including:
- Day length. The greater (or shorter) the number of daylight hours, the more (or less) time the Earth is subject to heating. Length of daylight hours is determined by geographic location as well as season.
- Cloudiness. Clouds are good at both absorbing and giving off longwave radiation, and at reflecting shortwave radiation (sunlight). On cloudy days, the ground is shielded from incoming solar radiation because this energy is reflected back out into space. Less incoming heat means less --and a decrease in diurnal temperature variation. On cloudy nights, diurnal range is also decreased, but for opposite reasons--heat is trapped near the ground, which allows the day's temperatures to remain constant rather than to cool.
- Elevation. Because mountain areas are located farther from the radiating heat source (the sun-heated surface), they receive less warming, and also cool more rapidly after sunset than do valleys.
Humidity. Water vapor is good at absorbing and giving off longwave radiation (energy that's released from the Earth) as well as absorbing in the near-infrared part of solar radiation, which reduces the amount of daytime energy reaching the surface. Because of this, daily highs are typically lower in humid environments than they are in dry environments.
This is a primary reason why desert regions experience some of the most extreme day-to-night temperature fluctuations.
- Wind speed. Winds cause air at different levels of the atmosphere to mix. This mixing lessens the difference in temperature between warmer and cooler air, thus decreasing the diurnal temperature range.
How to "See" the Diurnal Pulse
In addition to feeling the diurnal cycle (which is done easily enough by enjoying a day outside), it's also possible to visibly detect it. Watch a global infrared satellite loop closely. Do you notice the "curtain" of dark to light that rhythmically sweeps across the screen? That's Earth's diurnal pulse!