Chapter 4 Study Guide

evaporation:The process by which a liquid changes into a gas.

Condensation: The process by which water vapor becomes a liquid.

Precipitation: Any form of water particles—liquid or solid—that falls from the atmosphere and reaches the ground.

hydrologic cycle: A model that illustrates the movement and exchange of water among Earth, atmosphere, and oceans.

saturated air: An atmospheric condition whereby the level of water vapor is the maximum possible at the existing temperature and pressure.

condensation nuclei: Also called cloud condensation nuclei. Tiny particles upon whose surfaces condensation of water vapor begins in the atmosphere.

Humidity: A general term that refers to the air’s water vapor content. (See Relative humidity.)

actual vapor pressure: The pressure exerted by the water vapor molecules in a given volume of air.

saturation vapor pressure: The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water or ice. It represents the maximum amount of water vapor that the air can hold at any given temperature and pressure. (See Equilibrium vapor pressure.)

relative humidity: The ratio of the amount of water vapor in the air compared to the amount required for saturation (at a particular temperature and pressure). The ratio of the air’s actual vapor pressure to its saturation vapor pressure.

supersaturated air: A condition whereby the atmosphere contains more water vapor than is needed to produce saturation with respect to a flat surface of pure water or ice, and the relative humidity is greater than 100 percent.

dew-point temperature(dew point): The temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur.

wet-bulb temperature: The lowest temperature that can be obtained by evaporating water into the air.

heat stroke: A physical condition induced by a person’s over­exposure to high air temperatures, especially when accompanied by high humidity.

heat index (HI): An index that combines air temperature and relative humidity to determine an apparent temperature—how hot it actually feels.

apparent temperature: What the air temperature “feels like” for various combinations of air temperature and relative humidity.

Psychrometer: An instrument used to measure the water vapor content of the air. It consists of two thermometers (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.

Hygrometer: An instrument designed to measure the air’s water vapor content. The sensing part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).

Dew: Water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air.

Frost: A covering of ice produced by deposition on exposed surfaces when the air temperature falls below the frost point.

Haze: Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility. Dry haze particles are very small, on the order of 0.1 m. Wet haze particles are larger.

Fog: A cloud with its base at Earth’s surface.

radiation fog: Fog produced over land when radiational cooling reduces the air temperature to or below its dew point. It is also known as ground fog and valley fog.

advection fog: Occurs when warm, moist air moves over a cold surface and the air cools to below its dew point.

upslope fog: Fog formed as moist, stable air flows upward over a topographic barrier.

evaporation (mixing) fog: Fog produced when sufficient water vapor is added to the air by evaporation, and the moist air mixes with relatively drier air. The two common types are steam fog, which forms when cold air moves over warm water, and frontal fog, which forms as warm raindrops evaporate in a cool air mass.

cirrus clouds: A high cloud composed of ice crystals in the form of thin, white, featherlike clouds in patches, filaments, or narrow bands.

cirrocumulus clouds: A high cloud that appears as a white patch of clouds without shadows. It consists of very small elements in the form of grains or ripples.

cirrostratus clouds: High, thin, sheetlike clouds, composed of ice crystals. They frequently cover the entire sky and often produce a halo.

altocumulus clouds: A middle cloud, usually white or gray. Often occurs in layers or patches with wavy, rounded masses or rolls.

altostratus clouds: A middle cloud composed of gray or bluish sheets or layers of uniform appearance. In the thinner regions, the sun or moon usually appears dimly visible.

nimbostratus clouds: A dark, gray cloud characterized by more or less continuously falling precipitation. It is rarely accompanied by lightning, thunder, or hail.

stratocumulus clouds: A low cloud, predominantly stratiform, with low, lumpy, rounded masses, often with blue sky between them.

stratus clouds: A low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle.

cumulus clouds: A cloud in the form of individual, detached domes or towers that are usually dense and well defined. It has a flat base with a bulging upper part that often resembles cauliflower. Cumulus clouds of fair weather are called cumulus humilis. Those that exhibit much vertical growth are called cumulus congestus or towering cumulus.

cumulonimbus clouds: An exceptionally dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail. It is also known as a thunderstorm cloud

lenticular clouds: A cloud in the shape of a lens.

pileus clouds: A smooth cloud in the form of a cap. Occurs above, or is attached to, the top of a cumuliform cloud. Also called a cap cloud.

mammatus clouds: Clouds that look like pouches hanging from the underside of a cloud.

Contrail: A cloudlike streamer frequently seen forming behind aircraft flying in clear, cold, humid air.

nacreous clouds: Clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above Earth’s surface. They are also called mother-of-pearl clouds.

noctilucent clouds: Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the surface.

geostationary satellites: A satellite that orbits Earth at the same rate that Earth rotates and thus remains over a fixed place above the equator.

polar-orbiting satellites: A satellite whose orbit closely parallels Earth’s meridian lines and thus crosses the polar regions on each orbit.

Chapter 4 Study Guide Questions

1. Understand the basics of the hydrologic cycle. 

The hydrologic cycle describes the continuous movement of water on, above, and below the surface of the Earth, involving processes such as evaporation, condensation, precipitation, and infiltration.

2. What are evaporation, condensation and saturation?

Evaporation is the process of liquid water turning into vapor due to heat, while condensation refers to the transformation of water vapor back into liquid form as it cools. Saturation occurs when the air contains the maximum amount of water vapor it can hold at a given temperature, leading to the potential for precipitation.

3. What is Relative Humidity? Understand the basics as well as how it varies from

location to location.

Relative Humidity is the measure of the current amount of water vapor in the air compared to the maximum amount of water vapor that the air can hold at a specific temperature, expressed as a percentage. It varies from location to location due to factors such as temperature, altitude, and proximity to bodies of water, affecting the comfort level and weather patterns experienced in those areas. In warmer regions, for instance, higher temperatures can lead to increased evaporation rates, thus raising the relative humidity, while colder areas may have lower humidity levels despite having a similar amount of water vapor. Additionally, urban areas often experience different humidity levels compared to rural areas due to heat islands and vegetation differences.

4. What is Vapor pressure?

Vapor pressure is the pressure exerted by the water vapor present in the air, which is a crucial component in determining relative humidity. It represents the tendency of water molecules to escape from the liquid phase into the gas phase at a given temperature. As temperature increases, the vapor pressure also increases, leading to higher humidity levels if the amount of water vapor remains constant.

5. What is the dew point? What MUST occur when the dewpoint is reached?

The dew point is the temperature at which air becomes saturated with moisture, causing condensation to occur. When the dew point is reached, water vapor in the air begins to condense into liquid water, leading to the formation of dew, fog, or clouds.

6. Understand the formation of dew and frost and how they are different.

The process to form dew is initiated when the temperature of the air drops to the dew point during the night, allowing moisture in the air to condense on surfaces. In contrast, frost forms when temperatures fall below freezing, causing the water vapor to directly transition into ice crystals on surfaces. This distinction is crucial for understanding weather patterns and the impact of temperature on moisture in the environment.

7. Understand the basic types of fog: radiation, advection, Upslope (mountain).

Radiation fog is formed when the ground loses heat through radiation, cooling the air near the surface to its dew point, typically during clear nights. Advection fog occurs when warm, moist air moves over cooler surfaces, leading to condensation as the air cools to its dew point. Upslope fog forms when moist air is lifted over a mountain range, causing it to expand and cool, which results in condensation and fog formation.

8. How do we classify clouds? Three primary considerations.

Clouds are classified by their altitude, shape, and their ability to produce precipitation.