16 Atmosphere

Lecture 16&17 Atmosphere

Goals

• Earth's Envelop, Weather and Climate

• Energy from the Sun - Heat and Temperature

• Structure in the Atmosphere

• Moisture in the Atmosphere

• Clouds

We now turn to the second large fluid body associated with the Blue Planet, the atmosphere. We shall find that this envelop surrounding the solid earth and supporting life is responsible to a large extent for the daily weather we experience and the long term climate. The principal energy source for these effects comes from the sun and are dependent upon how this energy is dispersed in the atmosphere. Water will be special constituent of the atmosphere that leads to precipitation and ultimately erosion at the earth's surface. The concepts introduced during this discussion will lay the foundation for an understanding of daily weather and long term climatic fluctuations, the subject of subsequent lectures.

A. Earth's Envelop - Weather and Climate

The unique aspect of the earth as a planet is the composition of its atmosphere and the presence of a biosphere. Photosynthesis is a critical part of this system as plants take in carbon dioxide and water, producing oxygen, organic matter and calcium carbonate.

Weather - Daily variations in temperature, wind and precipitation.

Climate - Long term (many years) trends in weather.

The key variables are:

Temperature
Air Pressure
Humidity
Cloudiness
Wind speed and direction

Composition of atmosphere includes aerosols, humidity and gases. The key gases are N₂ (78%), O₂ (21%) and Ar (<1%)

B. Energy from the Sun - Heat and Temperature

The primary energy driving processes in the atmosphere comes from the sun in the form of solar radiation. This energy goes into heating, evaporation and photosynthesis while a portion is reflected back into space. Although the portion used in photosynthesis is very small compared to the others, it is critically important as it removes carbon dioxide from the atmosphere and supplies oxygen. The amount of solar radiation reaching the earth varies with latitude, with higher latitudes experiencing less solar radiation because of the longer travel paths through the atmosphere for the solar energy. Because of the tilt of the earth's axis of rotation relative to the plane of the earth's orbit around the sun, the amount of solar energy reaching the northern and southern hemisphere varies with the time of the year. These effects are important but as we say in the last lecture weather can also be affected by the earth's other large fluid body, the oceans.

We use two terms to characterize thermal energy:

Heat - Total energy of a material.

Temperature - average kinetic energy.

C. Structure of the Atmosphere

Just as we divided the interior of the earth into four distinct regions (crust, mantle, outer core and inner core), we divide the atmosphere into four layers:

Troposphere (0 to 10-16 km): Temperature is greatest at earth's surface and then decreases with altitude since it is warmed by earth's surface. Most all weather occurs in this layer. Height changes with latitude.
Stratosphere (10-16 to 50 km): Temperature increases with altitude. Increase is caused by absorption of sunlight by O₃ (Ozone).
Mesosphere (50 to 80): Decreasing temperature with altitude.
Thermosphere (80 to 400 km): Increasing temperature with altitude due absorption by O.

Absorption of solar energy by the atmosphere is very important. In the Stratosphere ozone is able to absorb ultraviolet energy (UV) that can be carcinogenic (skin cancer). Within the troposphere the build up of carbon dioxide can lead to the absorption infrared energy that again raises temperatures. This effect is known as the greenhouse effect and can lead to run away temperature increases if increases in carbon dioxide levels go unchecked.

Finally as altitude increases the density of gas molecules decreases as well as pressure. In fact at the highest altitudes the number of molecules and the average distance between them is large. Thus the atmosphere in the Thermosphere can have very high temperatures (1500 degrees C) but little total energy or heat.

D. Moisture in the Atmosphere

Water in the atmosphere plays a critical role in weather and erosion as we have already seen in our studies of rivers and oceans. The most important property of water is that within the range off temperatures experienced on the surface of the earth water exists as a liguid(l), solid(s) and gas(g). Heat is either released or absorbed (latent heat) when water changes phase. Heat is released when water goes from gas to liquid or from a liquid to a solid. Heat is absorbed when it goes from solid to liquid or liquid to gas.

Liquid water that evaporates goes into the atmosphere as a gas. This gas can condense forming precipitation. When the amount of water vapor that precipitates equals the amount of liquid water that is evaporating, one has reached the saturation water vapor pressure. The saturation vapor pressure is the maximum water that the air can hold in vapor pressure and is dependent upon temperature (increases with temperature). The relative humiduty is defined as the vapor pressure of an air sample divided by the saturation vapor pressure (at same T) multiplied by 100 (per cent).

E. Clouds

We begin our discussion of clouds at this point as we have begun to explore how moisture is retained in the atmosphere. Our discussion will extend into the next lecture as well.

We first consider adiabatic processes or ones in which no heat is gained or loss from or to an external source.

Rising Air- Volume increases, temperature decreases and pressure decreases.

Sinking Air- Volume decreases, temperature increases and pressure increases.

The cooling associated with rising air leads to the condensation of water vapor and the formation of clouds. This process is slowed somewhat by the release of latent heat as water vapor condenses to liquid form.

Movement of air upward is key to cloud formation. There are four ways in which air will move upward:

Density lifting
Frontal lifting
Orographic lifting
Convergence lifting