Chapter 2. Weather and Climate The Structure of the Atmosphere While the Earth’s atmosphere extends upward for hundreds of kilometers until it merges with interplane- Surrounding the Earth is a gaseous envelope or tary space, more than half of the atmosphere’s total atmosphere, held in place by the planet’s gravitational mass is below an altitude of only about 6 kilometers attraction. The Earth’s (3.75 miles) above the surface (Figure 2-1). The atmosphere is a complex lowest region of the atmosphere, the troposphere, dynamical, physical, and extends from the surface to an altitude that varies from chemical system. Dyna- 10 to 15 kilometers (km) (6.2 to 9.3 miles (mi.)), mic processes cover a depending on latitude and season. The top of the large range of scales from troposphere is called the tropopause. The regions of the microscopic-scale the atmosphere above the troposphere are the dynamics of evaporation, stratosphere (from between 10 and 15 to 40 km condensation, cloud (between 6.2-9.3 and 25 mi.)), the mesosphere (40 to formation and precipi- 80 km (25 to 50 mi.)), the thermosphere (80 to 500 km tation, to small-scale, (50 to 310 mi.)) and the exosphere (begins at about localized vertical and 500 km (310 mi.)). The exosphere merges with horizontal wind motions, interplanetary space. The ionosphere is the region of to medium-scale atmosphere between 40 and 300 km (25 and 185 mi.). cyclones, anticyclones, It is the region of positively-charged atoms and hurricanes, typhoons, molecules and negatively-charged electrons. tornadoes, thunderstorms, fronts, etc., to the large- scale general circulation of the atmosphere. Physical processes in the atmosphere include the transfer of incoming solar radiation through the atmosphere to the surface, the heating of the surface, the emission of outgoing infrared radiation, the absorption of infrared radiation by atmospheric gases, the evaporation of water, the condensation of atmos- pheric water vapor into clouds, and precipitation. Chemical processes include the transformation and production of atmospheric gases, such as atmospheric ozone, via chemical reactions involving many dozens of gases in the atmosphere. Figure 2-1. Regions of the atmosphere. Meteorology Activities for Grades 5-9 5 NP-2006-08-97-LaRC The Chemical Composition Instruments to Measure Weather of the Atmosphere Weather is the instantaneous or current state of the The Earth’s atmosphere is a complex mixture of atmosphere and is measurable in terms of tempera- gases: nitrogen (N2) (about 78% by volume), oxygen ture, atmospheric pressure, humidity, wind speed and (O2) (about 21% by volume) and argon (Ar) (about direction, cloudiness and precipitation. Climate is the 0.9% by volume) with small and varying amounts of state of the atmosphere over long time periods, such water vapor (H2O) (0 to 4% by volume) and still as over years, decades, centuries or greater. In gen- smaller amounts of carbon dioxide (CO2), methane eral, the weather that impacts the surface of the Earth (CH4), nitrous oxide (N2O), ozone (O3) and dozens of and those that live on the surface takes place in the other gases at still smaller concentrations. The chemi- troposphere. Weather parameters are measured with cal composition of the atmosphere is given in Table different instruments. Atmospheric temperature is 2-1. The protection afforded by the atmosphere is measured with a thermometer. very important for life on Earth. The atmosphere shields the Earth’s surface and its myriad forms of Atmospheric pressure is a measure of the force life from biologically damaging high-energy cosmic exerted by the mass of atmosphere on the surface at radiation. In addition, ozone, found mostly in the a given location. The average pressure of the atmo- stratosphere, absorbs ultraviolet radiation from the sphere at mean sea level is about 1 kg per square cm, Sun, shielding the Earth’s surface from this bio- which is equivalent to about 14.7 pounds per square logically damaging radiation. inch or a pressure of 1013.25 millibars (mb), and which is also referred to as 1 atmosphere. Atmo- Table 2-1. Chemical Composition spheric pressure is measured with a barometer. of the Earth’s Atmosphere Gas Concentration* Humidity is a general term that refers to the water Nitrogen (N2) 78.084% vapor content of the air. Absolute humidity is the Oxygen (O2) 20.946% actual amount of water vapor per volume of air. Argon (Ar) 0.934% Relative humidity is the percentage of water vapor in the atmosphere compared with the maximum Carbon Dioxide (CO2) 0.037% † amount of water vapor that the atmosphere could Water Vapor (H2O) 0.01 to 4% contain at that temperature. The dew point of a given Neon (Ne) 18.2 ppmv parcel of air is the temperature to which the parcel Helium (He) 5.0 ppmv must be cooled, at constant pressure, for the water Methane (CH4) 1.8 ppmv vapor component to condense. Humidity is measured Krypton (Kr) 1.1 ppmv with a psychrometer. Hydrogen (H2) 0.5 ppmv Nitrous Oxide (N2O) 0.5 ppmv Wind speed is measured with a 4-cup anemometer Xenon (Xe) 0.09 ppmv and wind direction is measured with a weather vane. † Ozone (O3) 0.0 to 0.07 ppmv Winds are named after the direction from which they flow. For example, the northeast trade winds flow in Nitrogen Dioxide (NO2) 0.02 ppmv a southward direction from the northeast. The amount *Concentration units: % or parts per million by volume (ppmv) (1 ppmv = 0.0001%). of cloud cover is estimated either visually or photo- †Highly variable. graphically. The amount of precipitation is measured The mean molecular mass of dry air is 28.97 atomic mass with a rain gauge. units or Daltons. Meteorology Activities for Grades 5-9 6 NP-2006-08-97-LaRC Solar Radiation, the Greenhouse Effect and the Temperature of the Earth To a large extent, the temperature of the Earth’s surface is determined by the amount of radiation received from the Sun. Most of the incoming radiation from the Sun is in the form of visible radiation. The atmosphere is mostly transparent to incoming solar radiation, i.e., this radiation is not absorbed by gases in the atmosphere, with the notable exception of solar ultraviolet radiation, which is absorbed by ozone mostly located in the stratosphere. However, some of Figure 2-2. Transfer of incoming solar radiation the incoming solar radiation is reflected back to space through the atmosphere. by clouds (Figure 2-2), by ice and snow at the poles, and by desert areas. The surface of the Earth is heated of greenhouse gases in the atmosphere has led to by the absorption of incoming solar radiation and national and international concern about global reaches a mean global temperature of about –18 °C warming and its accompanying environmental (0 °F). Once heated to the mean temperature, the Earth consequences. emits radiation in the form of “long-wavelength,” or infrared, radiation back to space. Unlike incoming The Intergovernmental Panel on Climate Change solar radiation, which is not strongly absorbed by (IPCC) released its Fourth Assessment Report in atmospheric gases and passes through the atmosphere February 2007 with the following conclusions: to the surface, outgoing infrared radiation is strongly absorbed by several different atmospheric gases, • Warming of the climate system is unequivocal. including carbon dioxide, water vapor, methane, nitrous oxide and ozone. • Most of the observed increase in globally averaged temperatures since the mid-20th Immediately after being absorbed by these atmo- century is very likely due to the observed spheric gases, the infrared radiation is quickly re- increase in anthropogenic greenhouse gas emitted or released back to the atmosphere in both concentrations. the upward and downward directions. The downward component of the re-emitted infrared radiation strikes • Hotter temperatures and rises in sea level the surface and causes additional heating, increasing “would continue for centuries” no matter the mean temperature of the Earth to about 15 °C how much humans control their pollution. (59 °F). This additional heating is called the “green- house effect” and the gases that absorb and then re- • The probability that this is caused by natural emit infrared gases are called “greenhouse gases.” climatic processes alone is less than 5%. Measurements show that atmospheric concentrations of greenhouse gases—carbon dioxide, methane and • World temperatures could rise from anywhere nitrous oxide —are increasing with time most between 1.1 and 6.4 °C (1.98 to 11.52 °F) with probably due to human activities. Atmospheric a corresponding sea level rise of 18 to 59 concentrations of water vapor will increase as the centimeters (cm) (7 to 23 inches (in.)) during temperature of the atmosphere increases. The buildup the 21st century. Meteorology Activities for Grades 5-9 7 NP-2006-08-97-LaRC Solar Heating and Atmospheric Motion Weather is a very complex phenomenon and is con- trolled by many factors and processes, such as the heating of the Earth’s surface and atmosphere by in- coming solar radiation. Incoming solar radiation is absorbed by the Earth’s surface, which in turn warms Figure 2-3. Rising air masses and low-pressure areas the lower atmosphere. Because warmer air is less are usually associated with clouds and stormy condi- dense than cooler air, the heated air will begin to rise tions, while descending air and high-pressure areas at through the atmosphere. The rising air creates a low- the surface usually mean fair weather conditions. pressure area at the surface. The background, or ambient, temperature of the atmosphere decreases areas. Regions of descending or falling air cause high- with altitude (Table 2-2) as the distance from the pressure areas at the surface, and, in general, bring Sun-heated surface increases.