ATMO 171 Introduction to Meteorology and Climatology

Extra Cedit Assignment #2:

Find a location with a clear view of the western horizon. Make a sketch of the horizon that includes any local landmarks on the horizon (buildings, trees, mountains, etc.). Visit this location at least twice a week at sunset. On your sketch of the horizon, mark the position where the sun sets. Keep this diagram in a notebook and bring it to class. We will refer to it when we discuss the inclination of the earth's orbit and the seasons. I will announce when to hand this in at a later date. If you wish, you can do this for sunrise using the eastern horizon instead. Do either sunset or sunrise, but don't do both. You can get the exact times for sunrise and sunset from the local newspapers, local TV weather reports, and on the internet at www.wunderground.com.
9/8/99
Basic properties of matter (...and how they relate to pressure and temperature)

Mass refers to how much matter is contained within a given body (it is not the same as weight. Weight is a force, i.e. the product of mass and gravitational acceleration). For our purposes we will use the metric system, so mass will be measured in grams or kilograms.
Volume refers to how much physical space (in three dimensions) a body occupies. It is measured in terms of cubic centimeters or cubic meters.
Density is the ratio of an objects mass to its volume. It is expressed in units of grams (g) per cubic centimeter (cc) or kilograms per cubic meter. Water has a density of 1 g/cc. Air at standard temperature and pressure has a density of about 0.001 g/cc. From the definition of density, one can see that density is directly proportional to mass (an increase in mass causes an increase in density) and inversely proportional to volume (volume increases, density decreases).
The ideal gas law (or simply the gas law, also called the equation of state) relates the pressure, temperature, and density of a gas.
p = (constant) * density * temperature
Remember the definition of temperature from kinetic theory:
In a volume of gas, molecules are moving in all different directions at a variety of speeds - some faster, some slower; temperature is a measure of the average speed of air molecules' random motions
The demonstration of the balloon immersed in liquid nitrogen shows how the ideal gas law works:

as temperature of air inside ballon goes down, volume of balloon decreases, so the density decreases (this assumes the pressure remains constant. Is this right?).
In the case of the helium balloon, the density of the gas inside the ballon increases to the point that the helium is no longer buoyant; as the balloon warms up, it begins to rise - why?

9/10/99
Atmospheric Composition and Structure

The most basic constituent of the atmosphere that we will consider is the atom, which is made up of a nucleus of protons and neuterons, and electrons which orbit the nucleus. The number of protons in the nucleus (= number of electrons) is called the atomic number - this determines the chemical properties of each element. For a nice description of the periodic table and each element contained in it, check out www.shef.ac.uk/chemistry/web-elements.
Two or more atoms chemically bonded together make up a molecule. You should familiarize yourself with the chemical formulas for the most common atmospheric molecules (molecular nitrogen, molecular oxygen, water vapor, carbon dioxide, etc.
Compounds and mixtures: Know the difference between the two. Is our atmosphere a compound or a mixture?

Atmospheric Composition

Table 2.3 in Danielson shows the results of a survey of 1 million air molecules. Roughly 77 % are molecular nitrogen, 21% are molecular oxygen, and so on... (Can you find the typographical error in Table 2.3?).
A common method of expressing the concentrations of atmospheric constituents is the mixing ratio . The mixing ratio of molecule X is detemined by taking the number of molecules of X within a volume and dividing it by the total number of air molecules in that volume. The mixing ratio is commonly expressed in parts per million - i.e. 100 molecules of X within one million molecules total of air in a given volume is 100 parts per million or ppm.
Gases making up much less than 1% of the atmosphere's composition are called trace gases . Even though their concentrations are relatively small, they can be very important if they are chemically reactive, or especially good absorbers of sunlight. Some examples of important trace gases (e.g. ozone, methane, carbon monoxide, oxides of nitrogen), are discussed in Chapter 2. We will return to this topic when we discuss air pollution later in the course

Atmospheric structure

The atmosphere is divided into several distinct regions based on the vertical variation of temperature. These regions are :

The troposphere (0 - 10 km altitude) : here temperature decreases as altitude increases. The troposphere is generally an unstable region - Why?
The stratosphere (10 - 50 km) : here temperature increases with altitude; this is mostly due to the presence of ozone, which absorbs solar ultraviolet radiation and converts the energy to heat. The stratosphere is a very stable region - Why?
The mesosphere (50 - 90 km) : here temperature decreases again, mostly due to emission of infrared radiation to space by carbon dioxide.
The thermosphere (90 - ? km) : here temperature increases with height again. Pressures here are very low - less than 0.01 mbar.
The tropopause (approximately 10 km), stratopause (approx. 50 km), and the mesopause (approx. 90 km) mark the boundaries between the troposphere and stratosphere, stratosphere and mesosphere, mesosphere and thermosphere, respectively. At each point temperature tends to change little with height.

We will focus primarily on the regions of the troposphere and stratosphere.
Pressure falls of rapidly with altitude. At the tropopause (10 km), pressures are typially 250 mb (this tells you 75 % of the atmosphere's mass lies below you); At the stratopause (50 km), pressures are typically 1 mbar.

Atmospheric evolution

It is believed that our present atmospheric composition (77% nitrogen, 21% oxygen) has only been in place for the last billion years or so. When the earth first formed 4 billion years ago, its early atmosphere was most likely hydrogen and helium. These very light gases probably escaped earth's gravity rather quickly. Later, as the earth solidified and cooled, geologic activity established an atmosphere most likely consisting of carbon dioxide, water vapor, nitrogen, but very little oxygen. It is believed that the appearance of plant life roughly 1.8 billion years ago produced oxygen through photosynthesis, giving rise to our present atmosphere.
Of the other terrestrial planets in our solar system (i.e. bodies with a definite surface) Venus and Mars have atmospheres which are mainly composed of carbon dioxide. Table 2.6 in Danielson lists the differences between the atmospheres of Earth, Venus, and Mars.