## Properties of the Atmosphere

N4BEn Atmospheric properties, such as temperature, air pressure, and humidity describe weather conditions.

Real-World Reading Link Have you noticed the weather today? Maybe it is hot or cold, humid or dry, or even windy. These properties are always interacting and changing, and you can observe those changes every time you step outside.

### Temperature

When you turn on the burner beneath a pot of water, thermal energy is transferred to the water and the temperature increases. Recall that particles in any material are in random motion. Temperature is a measure of the average kinetic energy of the particles in a material. Particles have more kinetic energy when they are moving faster, so the higher the temperature of a material, the faster the particles are moving.

Measuring temperature Temperature is usually measured using one of two common temperature scales. These scales are the Fahrenheit (°F) scale, used mainly in the United States, and the Celsius (°C) scale. The SI temperature scale used in science is the Kelvin (K) scale. Figure 11.9 shows the differences among these temperature scales. The Fahrenheit and Celsius scales are based on the freezing point and boiling point of water. The zero point of the Kelvin scale is absolute zero—the lowest temperature that any substance can have.

Fahrenheit Celsius Kelvin

373 K

273 K

Fahrenheit Celsius Kelvin

Section 2 • Properties of the Atmosphere 289

David Hays Jones/Photo Researchers

Vocabulary

Science usage v. Common usage

Force

Science usage: an influence that might cause a body to accelerate

Common usage: violence, compulsion, or strength exerted upon or against a person or thing

Figure 11.10 The density and pressure of the layers of the atmosphere decrease as altitude increases.

### Air Pressure

If you hold the your hand out in front of you, Earth's atmosphere exerts a downward force on your hand due to the weight of the atmosphere above it. The force exerted on your hand divided by its area is the pressure exerted on your hand. Air pressure is the pressure exerted on a surface by the weight of the atmosphere above the surface.

Because pressure is equal to force divided by area, the units for pressure are N/m2. Air pressure is often measured in units of millibars (mb), where 1 mb equals 100 N/m2. At sea level, the atmosphere exerts a pressure of about 100,000 N/m2, or 1000 mb. As you go higher in the atmosphere, air pressure decreases as the mass of the air above you decreases. Figure 11.10 shows how pressure in the atmosphere changes with altitude.

^p Reading Check Deduce why air pressure does not crush a human.

Density of air The density of a material is the mass of material in a unit volume, such as 1 m3. Atoms and molecules become farther apart in the atmosphere as altitude increases. This means that the density of air decreases with increasing altitude, as shown in Figure 11.10. Near sea level, the density of air is about 1.2 kg/m3. At the average altitude of the tropopause, or about 12 km above Earth's surface, the density of air is about 25 percent of its sea-level value. At the stratopause, or about 48 km above Earth's surface, air density has decreased to only about 0.2 percent of the air density at sea level.