The Atmosphere

• Meteorology is the study of the atmosphere and the processes that cause weather.

I. Weather vs. Climate

• In order to understand weather, you have to understand how the earth’s surfaces impact weather and climate.

•Weather is the conditions of the atmosphere that change day to day• Climate is the long term average of weather conditions in an area

•Weather encompasses a wide variety of atmospheric variables (the conditions of the atmosphere that change and influence conditions on earth)

• Climate generally references air temperature and moisture levels only. So climate is described as warm or cool and arid or humid.

II. Characteristics of the Atmosphere

• The atmosphere is the layer of gases that surrounds the earth.• As altitude above the surface increases, the concentration of air molecules decreases.

• Consequently, the density of the atmosphere decreases as altitude increases.

• The atmosphere is 10% as dense at ~16 km as the air above sea level,• and 1% as dense at ~50 km above sea level

• The thickness of the atmosphere is less than 2% of the circumference of the earth. It can be likened to the thickness of the peel of an apple.

• Because of its relative thinness, most of the movement is horizontal (wind) rather than vertical (air currents). However it is the vertical movement that determines most of the behavior of the atmosphere.

A. Composition

• The atmosphere is composed of:–Invisible gases–Suspended microscopic particles (aerosols)–Water droplets

•When analyzing the composition of the atmosphere, the first 80 km is called the homosphere due to the homogenizing efffect of the vertical air currents

• The region of the atmosphere more than 80 km above sea level is called the heterosphere because its gases are segregated by molecular weight – with molecular weight decreasing with an increase in altitude…H and He are found highest up.

1. The Homosphere

• The homosphere consists of permanent gases, which are found everywhere in the same proportion

• and variable gases, which are distributed unevenly in time and space throughout the homosphere. These gases are rare and do not significantly affect global concentration averages.

a. Permanent gases of the homosphere:

Gas Formula % by volume

Nitrogen N2 78.08

Oxygen O2 20.95

Argon Ar 0.93

Neon Ne 0.002

Helium He 0.0005

Krypton Kr 0.0001

Xenon Xe 0.00009

Hydrogen H2 0.00005

• Nitrogen is relatively unreactive and has a long residence time (measured in millions of years).

• Residence time refers to how long it takes for a particular gas to be added or removed from the atmosphere

• Oxygen has a residence time of ~6,000 years.• And you know how important it is!• Together with N2 it makes up 99% of the gases in the atmosphere.

• Argon makes up the bulk of the remaining 1% of the atmosphere and also has a long residence time. However, it plays no significant role in weather or climate.

b. Variable gases of the homosphere:

Gas Formula % by volume

Water Vapor H2O 0.25

Carbon Dioxide CO2 0.039

Ozone O3 0.01

•Water vapor is the most abundant of the variable gases. Its concentration decreases with an increase in altitude. The majority of the water vapor is found in the lower atmosphere.

• Evapotranspiration from the earth is the primary source of water vapor in the atmosphere.• It is rapidly exchanged between the earth and the atmosphere through the hydrologic cycle.

• Because of its rapid exchange, the residence time of water vapor is approximately 10 days.

•Water vapor content varies according to location.• It is ~1% over the deserts, 4% over the tropics and ~2% everywhere else.

•Water vapor is important as it is the source of moisture for cloud formation as well as being an effective absorber of infrared radiation (making it a greenhouse gas)

•Water vapor is a gas…like every other gas. But, unlike other gases, water vapor changes phase with ease. This process generates large amounts of energy which fuels meteorological events.

• Even though water vapor is an invisible gas, satellites can indirectly measure the amount of water vapor in the air and show this in images.

• These images can indicate wind patterns in the atmosphere as well as the boundaries between adjacent air masses.

• Carbon dioxide is unobtrusive, but very important in the discussion of weather.• It has a residence time of 150 years.

• Because it is present in such small amounts, its concentration is often stated in parts per million (ppm) rather than as a percentage (parts per hundred)

• In doing this; the 0.039% that carbon dioxide represents in the atmosphere becomes 390 ppm.

• CO2 is put into the atmosphere through both natural and anthropogenic means.• It is removed from the atmosphere by photosynthesis

• For many decades, the amount of CO2 put into the atmosphere has exceeded the amount removed leading to a global increase in the level of atmospheric CO2

• This information is obtained at the Mauna Loa Observatory at an elevation of 3400m. (these measurements are considered to be representative of the levels throughout the Northern Hemisphere.

• Since the 1950s, the concentration has increased at ~2.15 ppm/year. It is believed to be largely due to human activities – combustion and deforestation

• This increase in the levels of CO2 has fostered great discussion in the scientific, political and environmental arena due to its atmospheric warming ability.

• Ozone: O3

•When found in the upper atmosphere (the stratosphere) it is essential to life on earth.• In the lower atmosphere (the troposphere) it is one of the main atmospheric pollutants.

• In the lower atmosphere, it can cause irritation to lungs and eyes and damage to plants.

• Luckily, it is in relatively low concentrations in the troposphere• Even in highly polluted urban areas, the ozone concentration may only be 0.15 ppm.

• In the stratosphere, it may reach concentrations up to 100 times higher.• Even at those concentrations, ozone is still a small part of the stratospheric gases

• Stratospheric ozone is essential to life on earth as it absorbs much of the harmful ultraviolet (UV) radiation from the sun.

• Ozone forms in greater amounts in the upper atmosphere because that is where atomic oxygen is found in greater numbers; allowing for a greater number of collisions with diatomic oxygen to form ozone

•When ozone absorbs uv radiation, it splits back into atomic oxygen and diatomic oxygen which can then recombine to form another ozone molecule.

• Through this process, ozone is constantly being broken down and reformed to provide a relatively constant concentration of ozone in the ozone layer.

• Methane: CH4

• Methane has a residence time of ~10 years.• It is a greenhouse gas as it is an effective absorber of IR radiation.

• The concentration of methane has begun to increase – its levels have doubled over the past 200 years to 1.8 ppm

• Most of the increase is believed to be a result of increased rice cultivation, biomass burning and fossil fuel extraction

c. Aerosols

• Aerosols are small solid particles and liquid droplets in the air (other than clouds and precipitation)

• Aerosols are added to the atmosphere through both natural and anthropogenic means. • Pollen, dust, volcanic ash, & sea spray are all examples of natural aerosols.

• The residence time of most aerosols ranges from a few days to several weeks. Their small size allows vertical air currents to keep them aloft in the atmosphere.

• Since aerosols are the condensation nuclei for cloud formation, they are most effectively removed from the atmosphere as precipitation falls to the surface.

• “On average, each breath brings into the lungs ~1cm3 of air. With the average size and concentration of aerosols, we bring 1 trillion aerosols into our lungs several times each minute…which amounts to ~ 2 tablespoons of solids per day!

B. Vertical Structure

• The atmosphere can be stratified by a number of characteristics…pressure, temperature, density, composition and/or electrical properties.

1. Density

• As altitude within the atmosphere increases, density decreases.• This is due to the compressibility of atmospheric gases and the mass of the overlying atmosphere

2. Pressure

• At sea level, the atmosphere exerts a pressure of–14.7 psi (pounds/in2)–1013 mb

• As altitude within the atmosphere increases, air pressure decreases…but not at a constant rate.• Pressure falls rapidly near Earth’s surface, but more slowly higher up in the atmosphere.

• Even though pressure and density are closely related, meteorologists reference pressure rather than density due to its ease in measuring.

3. Temperature

• Because air movement is greatly influenced by temperature (and its rate of change), scientists frequently describe the layers of the atmosphere based on changes in temperature

a. The Troposphere

• The lowest of the four temperature based layers of the atmosphere.• Tropos: turn…named based on the turbulent nature of this layer in which overturning frequently occurs.

• In the troposphere, temperature decreases with an increase in altitude.• It is heated by the energy reradiated by the surface of the Earth

• The troposphere is where most of the weather events occur.• It is the shallowest layer, but contains 90% of the mass

• The depth ranges from 8 to 20 km with an average of ~15 km.• It is thicker over the tropics than the polar regions and thicker in the summer than in the winter

b. The Stratosphere

• Strato: layer• Very little weather occurs here other than some of the stronger thunderstorms• Here, temperature increases as altitude increases

• Heating occurs in the upper stratosphere due to the absorption of UV radiation by the ozone layer and in the lower stratosphere by the energy radiated by Earth’s surface

• The stratosphere contains ~19.9% of the total mass of the atmosphere• The ozone layer is found in the stratosphere (20 – 30 km above Earth)

• The stratosphere has very little water vapor; the decreasing temperature prohibits the vertical movement that creates clouds, so there are few clouds in this layer

• Because of the lack of precipitation in this layer, particulates can remain here for many months. In addition, the strong stratospheric winds distribute these particulates across the globe – greatly impacting global temperatures.

c. The Mesosphere

• The mesosphere extends up to ~80 km above sea level.• Here, temperature decreases as altitude increases

d. The Thermosphere• Here, temperatures increase to values in excess of 1500ºC. Even though these values are high, they do not represent a corresponding heat value. The molecules of air are too spread out to have many effective collisions.

• The thermosphere is the source of atomic oxygen that is needed to create ozone…the high intensity UV radiation photodissociates O2 into two atoms of oxygen

e. The Exosphere

• The exosphere is the outermost layer of the earth’s atmosphere extending from the thermosphere into outer space.

• It extends from approximately 250 - 500 km to approximately 190,000 km (halfway to the moon!!!)

4. By electrical properties

• The Ionosphere.• It extends from the upper mesosphere into the thermosphere

• It contains large numbers of ions (electrically charged particles)

• These ions are formed when electrons are lost as atoms and molecules are blasted by solar energy creating positively charged ions and free electrons

• The ionosphere affects radio communication from satellites; and is responsible for the formation of auroral displays.

• Auroral displays are created when the subatomic particles from the sun are captured by our magnetic field; collisions excite some of the atoms which then emit radiation as the electrons fall back to their lower levels or when ions regain free electrons.