1.2.1 Composition and temperature
Dry air is mainly composed of nitrogen (78.08% in volume), oxygen (20.95% in volume), argon (0.93% in volume) and to a lesser extent carbon dioxide1 (380 ppm or 0.038% in volume). The remaining fraction is made up of various trace constituents such as neon (18 ppm), helium (5 ppm), methane1 (1.75 ppm), and krypton (1 ppm). In addition, a highly variable amount of water vapor is present in the air. This ranges from approximately 0% in the coldest part of the atmosphere to as much as 5% in moist and hot regions. On average, water vapor accounts for 0.25% of the mass of the atmosphere.
On a large-scale, the atmosphere is very close to hydrostatic equilibrium, meaning that at a height z, the force due to the pressure p on a 1 m2 horizontal surface balances the force due to the weight of the air above z. The atmospheric pressure is thus at its maximum at the Earth's surface and the surface pressure ps is directly related the mass of the whole air column at a particular location. Pressure then decreases with height, closely following an exponential law:
where H is a scale height (which is between 7 and 8 km for the lowest 100 km of the atmosphere). Because of this clear and monotonic relationship between height and pressure, pressure is often used as a vertical coordinate for the atmosphere. Indeed, pressure is easier to measure than height and choosing a pressure coordinate simplifies the formulation of some equations.
The temperature in the troposphere, roughly the lowest 10 km of the atmosphere, generally decreases with height. The rate of this decrease is called the lapse rate :
where T is the temperature. The lapse rate depends mainly on the radiative balance of the atmosphere (see section 2.1.1) and on convection as well as on the horizontal heat transport. Its global mean value is about 6.5 K km-1, but varies with the location and season.
The lapse rate is an important characteristic of the atmosphere. For instance, it determines its vertical stability. For low values of the lapse rate, the atmosphere is very stable, inhibiting vertical movements. Negative lapse rates (i.e. temperature increasing with height), called temperature inversions, correspond to highly stable conditions. When the lapse rate rises, the stability decreases, leading in some cases to vertical instability and convection. The lapse rate is also involved in feedbacks playing an important role in the response of the climate system to a perturbation (see section 4.2.1).
At an altitude of about 10 km, a region of weak vertical temperature gradients, called the tropopause, separates the troposphere from the stratosphere where the temperature generally increases with height until the stratopause at around 50 km (Fig. 1.2). Above the stratopause, temperature decreases strongly with height in the mesosphere, until the mesopause is reached at an altitude of about 80 km, and then increases again in the thermosphere above this height. The vertical gradients above 10 km are strongly influenced by the absorption of solar radiation by different atmospheric constituents and by chemical reactions driven by the incoming light. In particular, the warming in the stratosphere at heights of about 30-50 km is mostly due to the absorption of ultraviolet radiation by stratospheric ozone, which protects life on Earth from this dangerous radiation.
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Atmospheric specific humidity also displays a characteristic vertical profile with maximum values in the lower levels and a marked decrease with height. As a consequence, the air above the tropopause is nearly dry. This vertical distribution is mainly due to two processes. First, the major source of atmospheric water vapour is evaporation at the surface. Secondly, the warmer air close to the surface can contain a much larger quantity of water before it becomes saturated than the colder air further away; saturation that leads to the formation of water or ice droplets, clouds and eventually precipitation.
At the Earth’s surface, the temperature reaches its maximum in equatorial regions (Fig. 1.3) because of the higher incoming radiations (see section 2.1.4). In those regions, the temperature is relatively constant throughout the year. Because of the much stronger seasonal cycle at mid and high latitudes, the north-south gradients are much larger in winter than in summer. The distribution of the surface temperature is also influenced by atmospheric and oceanic heat transport as well as by the thermal inertia of the ocean (see section 2.1.5). Furthermore, the role of topography is important, with a temperature decrease at higher altitudes associated with the positive lapse rate in the troposphere.
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