air - DIGGING A LITTLE DEEPER
The Earth’s atmosphere is a blanket of gases approximately 350km (218 miles) thick. It is a large and complex system that interacts with the Sun, the land, and the oceans in order to produce both the Earth’s weather and climate. The Earth’s atmosphere has four distinct layers:
Thermosphere: The layer of atmosphere most distant from the Earth is the thermosphere, which begins approximately 80km in altitude. It is also the hottest layer “thermo” being Greek for heat. The temperatures in the thermosphere increase with altitude due to the absorption of intense solar radiation by the limited amount of remaining molecular oxygen. The source of this heat is through bombardment of solar particles carried on the solar wind that do not reach deeper into the atmosphere.
Mesosphere: The mesosphere extends from 50 to 80km in altitude with very sparse atmosphere, accounting for only about 0.1 percent of the mass of the atmosphere as a whole. Temperatures decline within the mesosphere as altitude rises, containing the coldest temperatures within the Earth’s atmosphere. At its upper boundary, the mesopause, average temperatures are near -110° C in the summer and -60° C in the winter.
Stratosphere: The stratosphere extends from approximately 10-12 km to around 50 km above the Earth’s surface. The air temperature remains relatively constant up to an altitude of 25 km, then increases gradually having a stabilizing effect on atmospheric conditions. The stratosphere contains nearly 90 percent of the atmospheric ozone, which plays a major role in regulating temperatures as solar energy is converted to kinetic energy when the ozone molecules absorb ultraviolet radiation, resulting in the heating of the stratosphere.
Troposphere: The troposphere is the layer closest to the Earth’s surface, containing more than 80 percent of total atmospheric mass – composed of 78 percent nitrogen, 21 percent oxygen, other trace gases, water droplets, dust, and other particles. The troposphere is where most weather occurs; the circulation of air in intensive vertical movements results in the formation of clouds, while horizontal movements results in wind. Both temperature and water vapor content decrease rapidly as altitude increases within the troposphere. Nearly 99 percent of atmospheric water vapor is contained within this layer, which plays a major role in regulating air temperature as it absorbs solar energy and thermal radiation from the planet’s surface.
In the troposphere, air rises as it is heated by the sun, falls earthward as it cools, and intermixes with evaporated water from the planet’s bodies of water to form clouds and precipitation. The uneven heating of the Earth’s surface by the sun, along with Earth’s rotation, creates rising (convection), falling (advection), and horizontal air movements (winds). The result of these processes occurring in the form of rain, snow, heat or freezing cold, at a particular place and time, is called weather. The longer term trends in patterns of temperature, rainfall, and other weather indicators over time, usually in blocks of 30 years, that can affect the entire Earth, is called climate.
Variations in the behavior of the weather over long periods, such as from one century to another, is referred to as climate change. Climate variation occurs as a response to climate forcing which can cause either a warming or a cooling of the atmosphere. Over most of the Earth’s history, the forcings have been entirely natural, caused by continental drift, variability in solar radiation, changes in the Earth’s orbit, and volcanic emissions. However, since the Industrial Revolution, human activity has had an effect on the global climate system, increasing the amount of greenhouse gases in the atmosphere, trapping heat and contributing to an overall global warming.