Page 23 - Curriculum Visions Dynamic Book

 The Earth’s limb at sunset.
The lower atmosphere
The heated ground shares its warmth with the air by condUction, making the lowest layers of the air generally warmer than those above. That is a very unstable situation. Just as when water is heated in a saucepan, the heated gas
in the air becomes less dense than the air above it, and so it begins to rise through the more dense air. In turn, the denser air sinks. In this way a circulation is set in train that carries moist air from the surface to areas higher in the atmosphere. This is a process called convection.
Although the rising air is still warmer than its surroundings as it rises, it nonetheless cools as it moves away from its source of heating. The amount of water that air can carry as vapor gets smaller as the air gets cooler. So, as the air rises, some water vapor begins to condense, changing
to small droplets of liquid water. That is the origin of the Earth’s liquid water clouds, and they are quite unique in the Solar System.
Eventually the droplets gather and form larger drops (or snowflakes), which are then heavy enough to fall against the draft of rising air. They produce rain or snow. This once more begins the cycle of events that carries water and ice
to the land, where it can erode the rocks—again, a unique feature of the Solar System (see pages 28–29).
atmosphere The envelope of gases that surrounds the Earth and other bodies in the Universe.
condense To change state from a gas or vapor to a liquid.
condUction The transfer of heat between two objects when they touch.
convection The circulating flow in a fluid (liquid or gas) that occurs when it is heated from below.
limb The outer edge of a celestial body, including an atmosphere if it has one.
ozone A form of oxygen (O3) with three atoms in each molecule instead of the more usual two (O2).
radiation The transfer of energy in the form of waves (such as light and heat) or particles (such as from radioactive decay of a material).
radio waves A form of electromagnetic radiation, like light and heat. Radio waves have a longer wavelength than light waves.
Ultraviolet A form of radiation that
is just beyond the violet end of the visible spectrum and so is called “ultra” (more than) violet. At the other end of the visible spectrum is “infra” (less than) red.
water vapor The gaseous form of water. Also sometimes referred to as moisture.
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