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Name: Gail
Status: other
Age:  40s
Location: N/A
Country: N/A
Date: 2000-2001 - Revised May 2008


Question:
Why, just before sunrise, does the temperature seems to drop a few degrees? Gail,

The air near the Earth's surface cools at night in response to the loss of "heat" energy from the ground and from the air above it. This cooling begins even before sunset as the loss of energy from the ground and air becomes greater than the input of energy from the setting Sun, and then continues all night. Actually, the lowest air temperature tends to occur within the hour after sunrise, before the input of energy from the Sun equals the continuing loss of energy from the ground and the air. About an hour after sunrise, the input of energy from the Sun to the ground and the air equals the loss of energy from them; as the ground warms, it heats up and it releases energy to the air above more rapidly, and in combination with warming of the air directly by the Sun, the air temperature begins to rise.

To go into more detail, there are two factors that contribute to reduced temperatures at night. The first is the most important, by far.

First, virtually all nighttime cooling of the air (the mechanism spoken of above) is caused by what meteorologists call "radiational cooling" of the atmosphere. Long-wave energy from the Earth's surface and the air above it radiates into the upper part of the atmosphere and even to space, at all times of the day. However, at night, the input of energy from the Sun is missing, and therefore the radiational loss of long-wave energy results in a reduction of the ground surface and the air temperature of the lower atmosphere, including air near the surface. The longer the night is, the more cooling can occur. The closer it is to sunrise, the lower the air temperature tends to become. This cooling is especially strong if there are no clouds and the water vapor content of the air is small (in other words, there is little water vapor to absorb the energy). However, If condensation occurs on the surface (dew or frost forms, which results in the release of energy to the air), the rate of cooling of the ground and air is somewhat reduced. Radiational cooling is the overriding means by which the air is cooled and temperature decreases at night (aside from a cold front passage).

Second, the Solar Semi-diurnal Tide can cause a small decrease in atmospheric pressure during nighttime, with a minimum of pressure being reached a few hours before sunrise. As the Sun heats the atmosphere, (particularly the upper parts of the atmosphere, where ozone and water vapor are very effective at absorbing energy), and the Earth rotates, the mass of air is redistributed on the Earth. The heated, expanded atmosphere literally pushes air ahead of the rising Sun in a bi-modal wave, causing peaks and valleys of pressure, and resulting in a slightly decreased pressure a few hours before sunrise; the decrease in pressure can translate to a slightly decreased air temperature. These decreases are often too small to be detected amongst the usually much greater synoptic weather changes unless the observation point is a place on Earth (such as part of the year in Niger, Africa) where synoptic weather patterns do not mask it. I've seen the nighttime decrease in pressure from the Solar Semi-diurnal Tide in Niger and also under conditions of a very large and stable high pressure area in Oklahoma; the Solar Semi-diurnal Tide is particularly distinct in Niger and contributes in a very small way to decreased nighttime temperatures; the contribution to the temperature decrease is very hard to detect, even in Niger, and is not important in comparison to the radiational cooling mechanism.

As a point of information, the Solar Semi-diurnal pressure wave produces a minimum pressure and a maximum pressure twice a day; minimums occur about three hours before sunrise and a few hours before sunset; maximums occur a few hours after sunrise and a few hours after sunset.

Radiational cooling is the over-riding mechanism by which the air is cooled at night and the surface temperature thereby decreases.



David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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