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Earth's atmosphere extends from the surface to an altitude of about 500 km, when it effectively fades into the vacuum of space (Figure 3.7). The lowest part of the atmosphere is called the troposphere. This region is where most weather phenomena take place, e.g. cloud formation. The troposphere contains about 80% of the atmosphere's mass, including almost all of its water.

Air temperature generally decreases at a steady rate (the “lapse rate”) with altitude, until the tropopause, the upper boundary of the troposphere, is reached.⁴ The height of this boundary extends up to 17 km near the equator, reducing to about 8 km above the polar regions.

Since the troposphere is characterized by a continuous decrease of temperature with increasing height, any parcel of air that is warmer than its surroundings is likely to become buoyant and unstable, sometimes rising to great heights. The result of this convection is towering clouds and heavy precipitation (rain, snow, hail, etc.).

Figure 3.7 The vertical structure of Earth's atmosphere. Most of the atmosphere and weather is found in the lower region – the troposphere. Temperature decreases at a steady rate (the “lapse rate”) until it reaches the top of the troposphere. It then increases in the stratosphere, where ozone gas absorbs solar radiation. In the ionosphere, about 100 km above the surface, positively charged particles (ions) reflect radio waves.

(UCAR)

In the upper troposphere, the temperature drops to about –50°C. At this height, jet streams occur at the boundaries between air masses that have significantly different temperatures. The jet streams are relatively narrow bands of very high wind speeds (up to 360 km/h) which meander around the globe at about 60° latitude – above the polar – and at 30° latitude. During the winter, Arctic and tropical air masses create a greater temperature contrast, resulting in a stronger jet stream. However, during the summer, when surface temperature variations are less dramatic, the jet streams are weaker.