Heating and Cooling of Earth's Atmosphere
The Earth's atmosphere is primarily heated by the Sun's radiation, but various processes affect how heat is distributed and transferred within it. The main process of heating is solar radiation, where the Sun's energy reaches the Earth. A small portion of this energy is reflected by clouds, aerosols, and the Earth's surface, but most is absorbed by the land, oceans, and atmosphere. As the Earth’s surface absorbs solar energy, it warms up and subsequently radiates heat in the form of longwave infrared radiation. This process warms the lower layers of the atmosphere by direct contact and by convection, where warm air rises and cool air sinks, leading to a constant vertical movement of air.
At night, the absence of solar radiation causes the Earth’s surface to cool by radiating heat into space. The cooling of the surface is then transferred to the air by conduction (direct heat exchange between the surface and the air) and radiation, leading to cooler temperatures in the atmosphere.
Horizontal and Vertical Distribution of Temperature
The distribution of temperature in the atmosphere is not uniform and varies both horizontally and vertically.
- Horizontal Distribution: The horizontal distribution of temperature is influenced by the Earth’s rotation, the tilt of its axis, and the Sun's position relative to different latitudes. The equator receives more direct sunlight year-round, making it the warmest region of the Earth. In contrast, the poles receive sunlight at a much more oblique angle, resulting in lower temperatures. This temperature difference causes the formation of large-scale wind patterns and ocean currents as warm air moves towards the poles and cold air moves toward the equator.
- Vertical Distribution: Vertically, temperature decreases with height in the troposphere (the lowest layer of the atmosphere) because the atmosphere is primarily heated from the Earth's surface. The temperature in this layer typically decreases by about 6.5°C for every kilometer of altitude (known as the lapse rate). However, in the stratosphere (above the troposphere), temperature increases with height due to the absorption of ultraviolet radiation by the ozone layer.
Concept of Adiabatic Lapse Rate
The adiabatic lapse rate refers to the rate at which the temperature of an air parcel changes as it moves upward or downward without exchanging heat with its surroundings. There are two types of adiabatic lapse rates:
- Dry Adiabatic Lapse Rate (DALR): For unsaturated air, the temperature decreases at approximately 10°C per kilometer as the air rises. As the air rises, it expands and cools because the pressure decreases with altitude.
- Moist Adiabatic Lapse Rate (MALR): For saturated air, the cooling rate is slower, typically around 5-6°C per kilometer. This is because the latent heat released during condensation (as water vapor turns into liquid) offsets some of the cooling as the air rises.
The concept of the adiabatic lapse rate is important for understanding weather patterns, as it determines the stability of the atmosphere and influences cloud formation and precipitation.
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