The+Atmosphere

T he atmosphere consists of the mixture of solids, liquids and gases that are held to the earth by gravitational force. The atmosphere consists of two elements- Weather & Climate. Weather refers to the state of the atmosphere at a local level, usually on short term timescale from minutes to months. It includes temperature, precipitation, atmospheric pressure, wind speed and wind direction. Climate is the average weather condition of a place taken over a given period of time, usually about 30 years. It is long term behavior of the atmosphere in a specific place.Climatic data temperature, precipitation, atmospheric pressure, wind speed and wind direction are used to predict elements of weather for daily, monthly or annually averages


 * The Structure of the Atmosphere **

Most of our atmosphere is located close to the earth's surface where it is most dense. The air of our planet is 79% nitrogen and just under 21% oxygen; the small amount remaining is composed of carbon dioxide and other gasses. There are many different types of gasses in the atmosphere. They include nitrogen, oxygen, argon, carbon dioxide and other noble gasses. The gas that is most abundant is nitrogen.

There are four distinct layers of the earth. These include:
 * Troposphere:
 * Stratosphere:
 * Mesosphere:
 * Thermosphere

The main source of earth’s energy is the sun. The earth receives incoming energy from the sun in the form of short-wave radiation or solar radiation. The amount of energy received is determined by various factors which would be discussed later. Once the Earth’s atmosphere receives shortwave solar radiation, the energy is referred to as insolation. When Insolation or the sun’s energy gets to the earth, it is either as it passes through the atmosphere
 * Energy in the atmosphere **
 * 1) Absorbed
 * 2) Reflected
 * 3) Scattered
 * 4) Transmitted
 * 5) Refracted.

Absorption is mainly by:
 * 1. Absorption **
 * ozone,
 * carbon dioxide,
 * particles of ice
 * dust.

Reflection takes place mainly by clouds and to a lesser extent, the surface of the earth. The ratio between in-coming radiation and the amount of energy reflected, expressed as a percentage is known as __//albedo.//__ The albedo varies with cloud type: Albedos also vary over different land surfaces:
 * 2. Reflection **
 * 30- 40% in thin clouds
 * 50 – 70% in thick stratos clouds
 * 90% in cumulo-nimbus clouds
 * >10% over ocean and dark soils
 * 15% over coniferous and urban areas
 * 25% over grassland and deciduous forest
 * 40% over light-coloured desert
 * 85% over reflecting fresh snow

Scattering occurs when solar radiation is diverted. It takes place in all directions and may reach the earth as diffuse radiation. Scattering or incoming solar radiation is diverted by:
 * 3. Scattering **
 * Particles of dust
 * Molecules of gas
 * Volcanoes
 * deserts

**4. Transmission** is another solar radiation pathway. It occurs when both shortwave and longwave energy, instead of scattering, pass through the atmosphere.

**5. Refraction** can also occur when solar radiation enters the atmosphere. This takes when energy moves from one type of space to another, such as from air into water. As the energy moves, it changes its speed and direction when reacting with the particles present there. The shift in direction often causes the energy to bend and release the various light colors within it, similar to what happens as light passes through a crystal or prism.

Only 24% of solar radiation reaches the earth due to the combined effect of absorption, scattering, reflection, transmission and refraction. When solar radiation reaches the earth, it is converted into //heat energy.// This comes about when earth’s surface of the earth radiates energy back into the atmosphere. This is called long-wave or infra-red radiation. About 94% of this energy is trapped by the ozone layer or absorbed by CO2 and water vapour - (the natural greenhouse effect) and 6% is lost to space. The natural greenhouse effect keeps the earth’s temperature at a level that makes it possible for us survive on this planet. This is summarized in the diagram below. What is the ozone layer and what does it do? Diagram of incoming solar radiation

source:Earth’s Energy Budget, NASA

Heat budget refers to the balance between incoming radiation and outgoing terrestrial radiation. The diagram below shows the areas of surplus energy and areas of energy deficit. Generally, there is a net gain in radiation everywhere on the earth’s surface except in the polar regions, whereas there is a net loss in radiation throughout the atmosphere. Additionally, t here is a net surplus between 35 degrees south and 40degrees north.
 * The Heat Budget **

You can see that there is a surplus of energy between 35N and 35S. In this region, incoming insolation exceeds outgoing radiation. There's an energy deficit between 35N and the north pole, and between 35S of the south pole. Here the outgoing radiation exceeds incoming insolation. Insolation rises sharply from 50 joules at the poles to 275 joules at the equator. Terrestrial radiation varies less, from 120 joules at the poles to 200 joules at the equator. Energy is transferred from the low-latitude energy surplus areas to the high-latitude energy deficit areas by atmospheric circulation. If there was no atmospheric circulation, the low latitudes would get hotter and hotter and the high latitudes colder and colder. Simply stated, the diagram tells us that tropical areas get more insolation than polar regions. There is a positive heat balance (surplus)within the tropics and a negative heat balance (deficit) at the polar regions and high latitudes.

For more on this subject see Climate Forcings and Global Warming

Two major transfers take place to prevent tropical areas from overheating. These are:

// Horizontal heat transfers: // Horizontal movement of air across the globe. This is done through:
 * (winds) jet stream,
 * hurricanes,
 * depressions(80%) and
 * ocean currents (20%)

// Vertical heat transfers: // Vertical movement of heat into the atmosphere. This is achieved through: //Latent heat// – it is the amount of energy needed to change the state of a substance without affecting its temperature. E.g ice – water- vapour
 * radiation,
 * conduction,
 * convection and
 * latent heat.

// Long Term factors: // Short Term factors
 * Factors affecting insolation **
 * Height above sea level: These areas experience a lower level of solar radiation because they are much colder, with others experiencing snow. This either reflects the sun's energy or absorb it leading to a low level of solar radiation.
 * Land and sea
 * Prevailing winds
 * Altitude of the sun
 * Ocean Currents
 * Seasonal changes
 * Length of day
 * Cloud cover
 * Aspect and urbanization