The diurnal cycle

The fact that the insolation varies diurnally (with the day and night) is a "trivial" example of how the atmosphere responds to changes in the available solar energy. Every 24 hours, the Earth makes a complete rotation about its own axis and the sunlight varies as the Sun is nearly above the equator. The atmosphere tends to cool during night, with greatest differences between day and night-time in the low-latitude, mid-altitude regions with continental climate (spring and autumn) and the smallest diurnal variations near the sea. The diurnal cycle also has more subtle effects on the atmosphere, such as the nocturnal jet which is a result of strong radiative cooling near the ground. During the day, the ground heats due to the effective absorption of sunlight, and since warm air tends to be lighter than cold air, surface air rises and mixes with the air above, resulting in a more uniform vertical temperature profile. The mixing gives rise to a frictional stress acting on the free atmosphere above the boundary layer. During night, the ground cools off more quickly than the atmosphere, and the coldest air is accumulated near the ground, hence producing a much stronger vertical temperature gradient. The vertical temperature profile inhibits mixing, and the free atmosphere no longer feels the friction as it does during daytime. Thus, a different flow structure is set up (Gill, 1982).

The diurnal cycle varies with latitude and seasons. The days and nights are of approximately equal duration near the equator, but in the polar regions the sun does not set in the summer and the polar nights (the Sun is below the horizon) during winter last for a couple of months. The reason for these seasonal variations in the diurnal cycle is that the Earth's axis is tilted with respect to its orbital plane and this axis points in the same direction throughout the year. June 21st is the longest day in the northern hemisphere and the daylight hours are shortest on December 21st.

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