Global Environment.com

Weather and Climate

How Does Weather Differ from Climate?

At every moment at any spot on the earth, the troposphere (the inner layer of the atmosphere containing most of the earth’s air) has a particular set of physical properties. E.g. temperature, pressure, humidity, precipitation, sunshine, cloud cover, and wind. These short-term properties of the troposphere at a given place and time are what we call weather.

Climate is the physical properties of the troposphere of an area based on analysis of its weather records over a long period (at least 30 years). The two main factors determining an area’s climate are temperature, with its seasonal variations, and the amount and distribution of precipitation.

How Does the Global Circulation of Air Affect Regional Climates?

The temperature and precipitation patterns that lead to different climates are caused primarily by the way air circulates over the earth’s surface. Several factors determine global air circulation patterns:

Long-term variations in the amount of solar energy striking the earth.

The uneven heating of the earth’s surface. Air is heated much more at the equator (where the sun’s rays strike directly throughout the year) than at the poles (where the sunlight strikes at an angle and is thus spread out over a much greater area). Theses differences help explain why tropical regions near the equator are hot, polar regions are cold, and temperate regions in between generally have intermediate average temperatures.

Seasonal changes occur because the earth’s axis is tilted; as a result, various regions are tipped towards or away from the sun as the earth makes its annual revolution. This creates opposite seasons in the northern and southern hemispheres.

The earth rotates on its axis, which prevents air currents from moving due north and south from the equator. Forces in the atmosphere created by this rotation deflect winds to the right in the northern hemisphere and to the left in the southern hemisphere in what is called the Coriolis effect.

Climate and global air circulation are affected by the properties of air and water.

How Do Ocean Currents Affect Regional Climates?

The factors just listed, plus differences in water density, cause warm and cold ocean currents. Ocean currents, like air currents, redistribute heat and thus influence climate and vegetation, especially near coastal areas. E.g. without the warm Gulf Stream, which transports 25 times more water than the world’s rivers, the climate of north-western Europe would be subarctic. Currents also help mix ocean waters and distribute nutrients and dissolve oxygen needed by aquatic organisms.

Along some steep, western coasts of continents, almost constant trade winds blow offshore, pushing surface water away from the land. This outgoing surface water is replaced by an upwelling of cold nutrient-rich bottom water. Upwellings, whether far from shore or near shore, bring plant nutrients from the deeper parts of the ocean to the surface and support large populations of phytoplankton, zooplankton, fish, and fish-eating seabirds. Changes in prevailing winds can change the temperature of surface waters, weaken or alter ocean currents, suppress upwellings, and trigger weather changes over at least two-thirds of the globe.

How Does the Chemical Makeup of the Atmosphere Affect Climate? The Greenhouse Effect and the Ozone Layer

Small amounts of heat-trapping gases such as water vapor (H2O), carbon dioxide (CO2), ozone (O3), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFC's) play a key role in the earth’s average temperature and thus its climates.

Together, these gases, known as greenhouse gases, act somewhat like the glass panes of a greenhouse: They allow light, infrared radiation, and some ultraviolet radiation from the sun to pass through the troposphere. The earth’s surface then absorbs much of this solar energy and degrades it to longer-wave, infrared radiation (heat), which then rises into the troposphere. Some of this heat escapes into space; some is absorbed by molecules of greenhouse gases, which warms the air; and some radiates back towards the earth’s surface. This natural trapping of heat in the troposphere is called the greenhouse effect.

The amount of heat trapped in the troposphere depends primarily on the concentrations of greenhouse gases and the length of time they stay in the atmosphere. The primary heat-trapping gas is water vapor; because its concentration in the atmosphere is high (1-5%), inputs of water vapor from human activities have little effect on this chemical’s greenhouse effects. By contrast, the concentration of carbon dioxide in the atmosphere is so small (0.036%) that fairly large input of CO2 from human activities can significantly affect the amount of heat trapped in the atmosphere.

Without its current greenhouse gases (especially water vapor), the earth would be a cold and lifeless planet with an average surface temperature of –18oC instead of its current 15oC.

Measured atmospheric levels of certain greenhouse gases – CO2, CFC's, methane, and nitrous oxide – have risen substantially in recent decades. Although molecules, of CFC's, methane, and nitrous oxide trap much more heat per molecule than CO2 makes it the most important greenhouse gas produced by human activities. Most of the increased levels of these greenhouse gases since 1958 have been caused by human activities: burning fossil fuel, agriculture, deforestation, and use of CFC's.

Ice core analysis reveals that at the beginning of the industrial revolution the atmospheric concentration of CO2 was about 280 parts per million (pre industrial level). Between 1860 and 1995 the concentration of CO2 in the atmosphere grew exponentially to 360 parts per million, higher than at any time in the past 150,000 years. The atmospheric concentrations of CO2 and other greenhouse gases are projected to double from the pre industrial (1860) levels sometime during the nest century – probably by 2050 – and then continue to rise.

We and other species currently benefit from a comfortable level of greenhouse gases that typically undergo only minor, slow fluctuations over hundreds to thousands of years. However, mathematical models of the earth’s climate indicate that natural or human-induced global warming taking place over a few decades could be disastrous for human societies and many forms of life.

In a band of the stratosphere 11-16 miles above the earth’s surface, oxygen (O2) is continuously converted in to ozone (O3) and back to oxygen by a sequence of reactions initiated by ultraviolet radiation from the sun. The result is a thin veil of protective ozone at very low concentrations that absorbs at least 95% of the harmful incoming ultraviolet radiation from the sun and prevents it from reaching the earth’s surface.

Ultraviolet (UV) radiation reaching the stratosphere is composed of three bands: A, B, and C. The ozone layer blocks out nearly all the highest-energy, shortest-wave-length radiation (UV-C), approximately half of the next highest band (UV-B), a small part of the lowest-energy radiation (UV-A). Besides preventing most of the sun’s harmful ultraviolet radiation from reaching the earth’s surface, stratospheric ozone creates warm layers of air that prevent churning gases in the troposphere from entering the stratosphere. This thermal cap is important in determining the average temperature of the troposphere and thus the earth’s current climate.