Michael Pidwirny
LABORATORY 3: ATMOSPHERE COMPOSITION, PRESSURE, AND CIRCULATION
LEARNING GOALS
The objectives of this laboratory are to become familiar with the composition, structure, and circulation of the Earth’s atmosphere and to understand the various physical forces that control its behavior.
Upon completion of this laboratory you will be able to:
- Describe the composition of the atmosphere, including its constituents and vertical structure.
- Understand the association between horizontal pressure gradients and patterns in wind speed and direction.
- Examine the various forces that are responsible for the atmospheric motions near the Earth’s surface and upper troposphere.
- Explain why some surface patterns of pressure exist at the global scale.
COMPOSITION OF THE ATMOSPHERE
The Earth’s atmosphere (dry air) is composed of a number of different gases (Table 3.1). Most of these gases are found in the same relative proportions throughout the lower atmosphere and are referred to as the permanent gases. Mixed in with these permanent gases, are gases that vary in amount both spatially and temporally. These gases are called the variable gases.
During the last century, the proportions of the variable gases carbon dioxide and methane have been steadily increasing due to human activities. These gases play important roles in the greenhouse effect. Two other important variable gases are water vapor (H2O) and ozone (O3). Water vapor is found in the greatest concentrations near Earth’s surface and varies from 0% to 4% by volume. Ozone is found in very small amounts at 10 to 50 km above the Earth’s surface, with the maximum amount at about 25 km. Ozone helps to block some wavelengths of ultraviolet radiation from penetrating the lower atmosphere. Polar ozone holes have been detected, usually during the winter and early spring.
ATMOSPHERIC PRESSURE AND ALTITUDE
As a result of the Earth’s gravitational field, the atmosphere exerts a force on the Earth’s surface known as atmospheric pressure. It is commonly expressed in units of millibars (mb), or kilopascals (kPa), where 1 kPa = 10 mb.
The standard atmospheric pressure at the surface of the Earth is defined by international agreement as the average pressure at a latitude of 45°32′ and at a temperature of 0°C. At sea level, the standard atmospheric pressure is equal to 101.325 mb, or 1013.25 kPa. Atmospheric pressure drops rapidly with altitude above our planet’s surface (Figure 3.1).
Atmospheric pressure at the surface of the Earth varies with time, location, and changing weather conditions. It also varies with temperature, gravity, and elevation. Corrections are made to standardize the pressure data collected at all of the world’s weather stations with respect to these last three factors. These corrections make all the pressure readings on surface weather maps adjusted to sea level (regardless of weather station elevation).
FACTORS CONTROLLING WIND SPEED AND DIRECTION
Wind is simply air in motion. This m