1 Lab #1: Earth’s Atmosphere

Introduction and Objectives

Weather is considered the state of the atmosphere at a given place and time. Thus study of weather, Meteorology, evaluates the atmosphere’s characteristics, processes, and variations in terms of key elements of temperature, pressure, humidity, stability, precipitation, and winds. In order to understand more complex meteorological phenomena, a firm grasp of the atmosphere is necessary. This laboratory exercise is designed to acquaint you with the basic composition and structure of the atmosphere, while building skill and confidence in mathematical calculation.

Specific learning objectives of this lab are to:

• Explain how atmospheric pressure changes with height
• Convert between common units of pressure measurement
• Identify the primary fixed and variable gases of the atmosphere
• Calculate the partial pressures of the atmosphere’s gases
• Describe the primary layers of the atmosphere
• Calculate a lapse rate and differentiate normal from inverted lapse rates

Atmospheric Pressure

Earth’s atmosphere is a mixture of different gases that collectively exert a force on the underlying land surface proportionate to the gases’ mass. So to say, the more massive (or heavier) the gases above a specific location are, the greater the force they exert. This force is referred to as atmospheric pressure, the force the atmosphere exerts per unit area. At the surface, atmospheric pressure accounts for the entire atmospheric column above it, meaning a higher mass and thus a higher atmospheric pressure. Atmospheric pressure at the surface averages 14.7 lb/in2. At higher elevations, atmospheric pressure is less as less of the atmosphere (and its mass) are overhead.

The rate of change between atmospheric pressure and height above the surface is not linear. Because Earth’s atmosphere is compressible, a larger proportion of the atmospheric mass is contained close to the surface and atmospheric mass decreases rapidly with height. Due to the proportionate relationship between atmospheric mass and atmospheric pressure, atmospheric pressure also decreases rapidly with height in close alignment with atmospheric density.

Concept Check #1:

Within the Earth’s atmosphere, where would you expect to observer higher atmospheric pressure on average?

a. Death Valley, CA, elevation: -282 ft

b. Mount Marcy, NY, elevation: 5343 ft

 Atmospheric pressure can be reported with a number of different units. Millibars (mb) have historically been the most common unit used in meteorology for atmospheric pressure. However, the norm in the discipline today is the hectopascal (hPa). Converting between these two units is simple as 1 mb is equal to 1 hPa, making transition to the preferred hPa straight forward. Within aviation and some sectors of the U.S., atmospheric pressure is reported with the units of inches of mercury (Hg or inHg). This unit dates to when Mecury was commonly used as a means of measuring atmospheric pressure. To convert between hPa (or mb) and inHg, the following conversions can be used: 1 inHg = 33.86 hPa                 

 Concept Check #2:

Standard atmospheric pressure at sea-level is approximately 29.92 inHg. What is this pressure value in units of hPa? Round your answer to 1 decimal place and include the units “hPa” in your answer.

a. 0.9 hPa
b. 1013.1 hPa
c. 1.1 hPa
d. 981.6 hPa

Atmospheric Composition

As noted above, the atmospheric is made up of a mixture of gases and compounds. There is not a specific gas called “air”. Within much of the dry atmosphere (i.e., excluding water vapor), the gases are well-mixed. This means the relative proportions of each individual gas by volume at any point is roughly equal to the proportion by volume at any other point. The following table provides approximate values of key fixed and variable gases in units of percent by volume (Table 1-1).

Table 1-1. Percent by volume and concentration, in ppm, for fixed and variable gases.

With a value of percent by volume, it is possible to determine the pressure exerted by each individual gas of the atmosphere. The larger the percent by volume, the larger the contribution of that gas to total atmospheric pressure. Dalton’s Law states that total atmospheric pressure of a parcel or column of air is equal to the sum of the partial pressures of each individual gas.

Using the following equation, we can calculate the partial pressure of individual gases:

( Partial pressure ) = ( Percent of volume of gas ) x ( total atmospheric pressure )

Where, the percent of volume of gas is reported as a fraction (e.g., 78% = 0.78)

Concept Check #3:

Atmospheric pressure today is observed as 1026.4 hPa. Assuming Oxygen accounts for 20.9% of the atmosphere by volume, what is the partial pressure exerted by Oxygen? Report your answer in units of hPa with 1 decimal place. Include “hPa” in your answer.
a. 21,451.8 hPa
b. 49.1 hPa
c. 214.5 hPa
d. 4911.0 hPa

Certain variable gases play an important role in the overall planetary system. We will explore water vapor in greater detail in future labs, but will draw special attention to two: Ozone and Carbon Dioxide. Ozone is the tri-atomic molecule of Oxygen (O3) that is observed in two different layers within Earth’s atmospheric. Highest concentrations of Ozone occur in the Stratospheric layer (see next section) of the atmosphere where it serves a critical role is absorbing ultraviolent (UV) radiation from the Sun. UV radiation results in cellular damage to many forms of life, and is what causes sunburns. Ozone in the Stratosphere is naturally occurring, however can and has been impacted by human activity. The Ozone Hole refers to the thinning of the Stratospheric Ozone layer as Ozone molecules interact with chloroflurocarbons (CFCs) and other atmospheric pollutants. A thinner Ozone Layer (i.e., larger Ozone Hole) contributes to more UV radiation reaching the surface. Ozone also exists near the surface due to human activity and at this level is considered an atmospheric pollutant that can negatively impact air quality.

Concept Check #4

What would be a consequence of a larger Ozone Hole in the Stratosphere?

Carbon Dioxide (CO­2) is another naturally occurring gas in our atmosphere that can also be generated through human activity (primarily the combustion of fossil fuels). CO2 is considered a greenhouse gas due to its ability to efficiently absorb and re-radiate longwave radiation, or heat. In moderation, greenhouse gases are very important to keeping the planet livable. Without them, the average temperature of the planet would be approximately -18°C, well-below the freezing point of water. However, with too high a concentration of greenhouse gases, excessive amounts of thermal energy (heat) are retained within the climate system, increasing the temperature. The most robust observations of carbon dioxide have occurred at the Mauna Loa Observatory in Hawaii, which has tracked the concentration in the atmosphere since the 1950s. Below, you’ll see the “Keeling Curve” or a monthly timeseries of CO2 concentration (Figure 1-1), with more extensive details available at: https://scrippsco2.ucsd.edu/data/atmospheric_co2/primary_mlo_co2_record.html.

The above graphic (Figure 1-1) of CO2 exhibits two distinct features. One is a long-term increase in CO2 concentration over time from approximately 315 ppm in 1956-1960 to 415 ppm in 2018-2022. Current scientific consensus is that this increase in global CO2 concentration is primarily attributable to human actions. The second feature is an apparent seasonal fluctuation where CO2­ levels are highest during the winter and early spring, and then lowest in summer and early fall.

Concept Check:

What do you think is responsible for this apparent seasonal cycle in CO2 concentration?

Atmospheric Structure

Earth’s atmosphere has distinct layers in which air temperature changes differently with height. A lapse rate is the term for a change in temperature with height. We define a “normal” lapse rate as one in which the air temperature decreases with height. In contrast, an “inversion” refers to when air temperature increases with height. The graphic below highlights the four primary layers of the atmosphere. From the surface up, they are: Troposphere, Stratosphere, Mesosphere, and Thermosphere. Between each layer is a transition zone, or “pause” named for the layer below (Figure 1-2). For instance, the “Tropopause” marks the transition from the Troposphere below, to the Stratosphere above. Overall, the Troposphere and Mesosphere exhibit normal lapse rates, while the Stratosphere and Thermosphere are overall indicative of inversions. Additional details about these layers are noted in class, your textbook, or at online resources.

Often profiles of atmospheric temperature and other characteristics are displayed graphically. Being able to quickly read and extract information from these graphics is important to understanding atmospheric conditions. Standard graphics for analyzing atmospheric profiles are call Skew-T Log-P diagrams. At this stage in the semester however, we will leverage more simplistic representations by calculating lapse rates based on observations at different levels. Ultimately, a lapse rate is a change in temperature divided by a change in height. The following equation can be used to calculate a lapse rate from two observations (A and B):

( lapse rate ) = ( temperature B – temperature A ) / ( elevation B – elevation A). where location A is the location closer to the surface.

For example, if it is 78°F at an elevation of 0 m above the surface, and then 42°F at an elevation of 2000m. The lapse rate would be ( 42°F – 78°F ) / (2000m – 0m) = – 0.018 °F / m OR -18°F / 1000m. The value is negative meaning the temperature decreases as you go up: a normal lapse rate.

Concept Check:

What is the lapse rate of temperature between Location A and Location B, assuming the following observations. Location A: elevation 10m, temperature 30°C. Location B: elevation: 4150m, temperature: 0°C. Report your answer as °C per 1000m with 1 decimal place. E.g.: ##.# °C / 1000m

a. 6.5°C/1000m
b. 10.0°C/1000m
c. 7.2°C/1000m
d. 1.3°C/1000m