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The atmosphere is a mixture of gases, solids and liquids, held to the earth by gravitational attraction. 98% of the mass of the atmosphere lies within 16 miles of the surface. Very low-weight hydrogen and helium atoms extend as far as 20,000 miles from the earth’s surface, but the atmosphere at these distances is extremely rarified and for practical purposes the top of the atmosphere is considered to be about 300 miles (480 km) in altitude. Harry Williams, Earth Science
The atmosphere can be divided into layers according to composition, function and temperature Harry Williams, Earth Science
Composition Profile In terms of composition, the atmosphere is divided into the heterosphere and homosphere. In the heterosphere (above 80 km), gases are sorted into layers according to their atomic weight – the lightest gases, such as hydrogen and helium, form the higher layers; heavier gases, such as oxygen and nitrogen, form lower layers. Less than one thousandth of 1 percent of the atmosphere’s mass is in the heterosphere. In the homosphere (below 80 km) gases are almost* completely mixed together into what is commonly called “air” (* exceptions are ozone and some variable gases – these are explained later). Much of the homosphere is made up of uniform gases – meaning the proportion of these gases remains constant from place to place and over time. Harry Williams, Earth Science
Major Components Nitrogen (78.1%), oxygen (20.9%) and argon (0.9%) together make up about 99.9% of gases in the atmosphere. These gases do not greatly affect the weather. Water vapor is the gaseous form of water and can vary from about 0-4% of the atmosphere's volume. The variation in the percentage of water vapor can be from place to place (e.g. on a given day there may be a lot more water vapor in the air over Louisiana than over Arizona) or it can be at one location over time (e.g. there could lots of water vapor in the air over Dallas after a spring thunderstorm, but little water vapor after a long summer drought). Unlike nitrogen, oxygen and argon, water vapor is very important for its effect on the weather - it is the source of all clouds and precipitation, and is also involved in the storage, transportation and release of heat energy. Harry Williams, Earth Science
Carbon dioxide also has an important effect on weather, because it has the ability to absorb infrared radiation (radiant heat) given off by the surface of the earth, which helps to maintain warmth in the lower levels of the atmosphere. Carbon dioxide is fairly uniformly distributed, but the amount has been slowly increasing over the last 100 years or so, due to the burning of fossil fuels (a by-product of burning is CO2). Many scientists believe that this will cause GLOBAL WARMING, with unpredictable and serious consequences for the earth's climates. Ozone is concentrated in a layer 12 - 31 miles above the surface. It is particularly important in that it absorbs ultraviolet radiation (UV) from the sun. Absorption of UV heats this part of the atmosphere and prevents most of this harmful radiation from reaching the surface. Harry Williams, Earth Science
Pollutant Gases make up most other minor gases from factories, cars, etc. These include carbon monoxide, nitrogen oxides and hydrocarbons. For the atmosphere as a whole these are present in very small quantities, but these can be concentrated in certain locations and can have significant impacts on life and climate. Particles include solid and liquid particles, mainly of natural origin - i.e. ice particles, volcanic ash, wind-blown dust (Figure 4) and wildfire smoke particles. There are also man-made particles, again mainly pollutants such as soot and smoke. Although the volume of particles is very small compared to gases, they do have significant effects on weather by: 1. many collect water and help to form clouds. 2. some absorb or reflect insolation - decreasing the amount that reaches the surface. Harry Williams, Earth Science
Function Profile In terms of function, the atmosphere is divided into two layers – the ionosphere and the ozonosphere. Both layers function to remove harmful radiation and charged particles from insolation. The ionosphere, extending from 50 miles to 300 miles high, absorbs cosmic rays, gamma rays, X-rays and shorter wavelengths of UV radiation. Harry Williams, Earth Science
Absorption of insolation by the ionosphere and the ozonosphere. Harry Williams, Earth Science
Atoms in the ionosphere are changed into positively charged atoms or ions by this absorption of energy – giving the ionosphere its name. The ozonosphere is named after the ozone layer (12-31 miles high). The molecule ozone (O3) is concentrated in this layer and absorbs UV radiation. The result of filtering of insolation by the ionosphere and the ozonosphere is that it is mainly visible light and infrared radiation that reaches the surface of the earth. Harry Williams, Earth Science
Temperature Profile The temperature profile of the atmosphere results from differences in how different parts of the atmosphere absorb energy from the sun and earth. The temperature profile of the whole atmosphere is shown by the yellow line. Abrupt changes in temperature define four layers: Harry Williams, Earth Science
Troposphere - this is the lowest layer where temperature decreases with height. On average the troposphere is about 11 miles deep over the equator and 5 miles deep over the poles. This layer is warmed by the earth's surface, which in turn is warmed by absorption of visible light and infrared radiation from the sun. The farther from the surface, the colder the air becomes. The top of the troposphere is the tropopause, where temperature no longer decreases with height. Virtually all WEATHER occurs within the troposphere.Weather is defined as atmospheric conditions prevalent in a given place at a given time. Weather is essentially a temporary phenomenon, changing from day to day or even from hour to hour. Harry Williams, Earth Science
Stratosphere – this layer lies between approximately 11 and 31 miles high and is generally a zone of increasing temperature: the maximum temperature of about 0o C is reached at 31 miles, after which temperature stops increasing with height - this marks the upper boundary to the stratosphere - the stratopause. The stratosphere is heated by the absorption of UV radiation by the ozone layer it contains. Harry Williams, Earth Science
Mesosphere – this layer extends up to about 50 miles and is simply the layer of the atmosphere that becomes colder with increasing distance from the heated stratosphere. The top of the mesosphere, where temperature again begins to increase, is the mesopause. Harry Williams, Earth Science
Thermosphere - temperatures again rise in the thermosphere due to the absorption of insolation by atmospheric gases. Temperatures can reach 2200oF however, the air is extremely rarified (low density), so the actual amount of heat present is low, despite the high temperatures. Harry Williams, Earth Science
Air Pressure Air pressure can be thought of as the weight of the overlying air (the atmosphere is composed of atoms with mass, and is acted upon by gravity, so it does have weight). At the surface, the whole atmosphere is above, and so pressure is greatest; higher up, there is less overlying atmosphere and so pressure decreases. Harry Williams, Earth Science
However, the rate of pressure decrease with height is not constant, because air is compressible. Air near the surface is the most compressed because it bears the greatest weight; because of its greater density, pressure decreases more rapidly with height near the surface. Higher up, air is less dense and therefore pressure decreases less rapidly. Air pressure is measured in millibars (mb) – average sea-level pressure is about 1,013 mb. Harry Williams, Earth Science
