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Water in the Atmosphere Chapter 23 Table of Contents Section 1 Atmospheric Moisture Section 2 Clouds and Fog Section 3 Precipitation
Section 1 Atmospheric Moisture Chapter 23 Objectives • Explainhow heat energy affects the changing phases of water. • Explainwhat absolute humidity and relative humidity are, and describe how they are measured. • Describewhat happens when the temperature of air decreases to the dew point or below the dew point.
Section 1 Atmospheric Moisture Chapter 23 Changing Forms of Water • Water in the atmosphere exists in three states, or phases. • One phase is known as a gas called water vapor. • The other two phases of water are the solid phase known as ice and the liquid phase known as water. • Water changes from one phase to another when heat energy is absorbed or released.
Section 1 Atmospheric Moisture Chapter 23 Changing Forms of Water, continued Latent Heat latent heatthe heat energy that is absorbed or released by a substance during a phase change • When liquid water evaporates, the water absorbs energy from the environment. • When water vapor changes back into a liquid through the process of condensation, energy is released into the surrounding air and the molecules move closer together.
Section 1 Atmospheric Moisture Chapter 23 Changing Forms of Water, continued Evaporation • Most water enters the atmosphere through evaporation of ocean water near the equator. • However, water vapor also enters the atmosphere by evaporation from lakes, ponds, streams, and soil. • Plants release water into the atmosphere in a process called transpiration. • Volcanoes and burning fuels also release small amounts of water vapor into the atmosphere.
Section 1 Atmospheric Moisture Chapter 23 Changing Forms of Water, continued Sublimation sublimationthe process in which a solid changes directly into a gas (the terms is sometimes used for the reverse process) • Ice commonly changes into a liquid before changing into a gas. • When the air is dry and the temperature is below freezing, ice and snow may sublimate into water vapor.
Section 1 Atmospheric Moisture Chapter 23 Changing Forms of Water, continued The diagram below shows the different phases of water.
Section 1 Atmospheric Moisture Chapter 23 Reading Check Summarize the conditions under which sublimation commonly occurs.
Section 1 Atmospheric Moisture Chapter 23 Reading Check Summarize the conditions under which sublimation commonly occurs. When air is very dry and the temperature is below freezing, ice and snow change directly into water vapor by sublimation.
Section 1 Atmospheric Moisture Chapter 23 Humidity dew point at constant pressure and water vapor content, the temperature at which the rate of condensation equals the rate of evaporation • Water vapor in the atmosphere is known as humidity. • Humidity is controlled by rates of condensation and evaporation. • When the rate of evaporation and the rate of condensation are in equilibrium, the air is said to be “saturated.”
Section 1 Atmospheric Moisture Chapter 23 Humidity, continued • The rate of evaporation is determined by the temperature of the air. • The higher the temperature is, the higher the rate of evaporation is. • The rate of condensation is determined by vapor pressure. • When vapor pressure is high, the condensation rate is high.
Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Absolute Humidity absolute humiditythe mass of water vapor per unit volume of air that contains the water vapor, usually expressed as grams of water vapor per cubic meter of air
Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Absolute Humidity, continued • However, as air moves, its volume changes as a result of temperature and pressure changes. • Therefore, meteorologists prefer to describe humidity by using the mixing ratio of air. • The mixing ratio of air is the mass of water vapor in a unit of air relative to the mass of the dry air.
Section 1 Atmospheric Moisture Chapter 23 Humidity, continued The diagram below shows the effects of vapor pressure.
Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Relative Humidity relativehumiditythe ratio of the amount of water vapor in the air to the amount of water vapor needed to reach saturation at a given temperature • If the temperature does not change, the relative humidity will increase if moisture enters the air. • Relative humidity can also increase if the moisture in the air remains constant but the temperature decreases.
Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Reaching the Dew Point • When the air is nearly saturated with a relative humidity of almost 100%, only a small temperature drop is needed for air to reach its dew point. • Air may cool to its dew point by conduction when the air is in contact with a cold surface. • The resulting form of condensation is called dew.
Section 1 Atmospheric Moisture Chapter 23 Humidity, continued Reaching the Dew Point, continued • If dew point falls below the freezing temperature of water, water vapor may change directly into solid ice crystals, or frost. • Because frost forms when water vapor turns directly into ice, frost is not frozen dew. • Frozen dew is relatively uncommon. Unlike frost, frozen dew forms as clear beads of ice.
Section 1 Atmospheric Moisture Chapter 23 Reading Check How does dew differ from frost?
Section 1 Atmospheric Moisture Chapter 23 Reading Check How does dew differ from frost? Dew is liquid moisture that condenses from air on cool objects when the air is nearly saturated and the temperature drops. Frost is water vapor that condenses as ice crystals onto a cool surface directly from the air when the dew point is below freezing.
Section 1 Atmospheric Moisture Chapter 23 Measuring Humidity Using Thin Polymer Film to Measure Humidity • Humidity is commonly measured by a humidity sensor that uses a thin polymer film. • The polymer film’s ability to conduct electricity is affected by the relative humidity of the surrounding air. • Thus, by measuring the polymer film’s ability to store electricity, relative humidity can be determined.
Section 1 Atmospheric Moisture Chapter 23 Measuring Humidity, continued Using Psychrometers to Measure Humidity • It consists of two identical thermometers. The bulb of one thermometer is covered with a damp wick, while the bulb of the other thermometer remains dry. • When the psychrometer is held by a handle and whirled through the air, the air circulates around both thermometers. • The difference between the dry-bulb temperature and the wet-bulb temperature is used to calculate relative humidity.
Section 1 Atmospheric Moisture Chapter 23 Measuring Humidity, continued Other Methods for Measure Humidity • Dew cells consist of a ceramic cylinder with electrodes attached to it and treated with lithium chloride, LiCl. • By detecting the electrical resistance of LiCl as it is heated and cooled, the dew cell can determine dew point. • The hair hygrometer determines relative humidity based on the principle that hair becomes longer as relative humidity increases.
Section 1 Atmospheric Moisture Chapter 23 Measuring Humidity, continued Measuring Humidity at High Altitudes • The hygrometer may be carried up into the atmosphere in an instrument package known as a radiosonde. • The electric hygrometer is triggered by passing electric current through a moisture-attracting chemical substance. • The amount of moisture changes the electrical conductivity of the chemical substance. The change can then be expressed as the relative humidity of the surrounding air.
Section 1 Atmospheric Moisture Chapter 23 Measuring Relative Humidity
Section 2 Clouds and Fog Chapter 23 Objectives • Describethe conditions that are necessary for clouds to form. • Explainthe four processes of cooling that can lead to the formation of clouds. • Identifythe three types of clouds. • Describefour ways in which fog can form.
Section 2 Clouds and Fog Chapter 23 Cloud Formation clouda collection of small water droplets or ice crystals suspended in the air, which forms when the air is cooled and condensation occurs • For water vapor to condense and form a cloud, a solid surface on which condensation can take place must be available. • In addition, for clouds to form, the rate of evaporation must initially be in equilibrium with the rate of condensation.
Section 2 Clouds and Fog Chapter 23 Cloud Formation, continued condensation nucleusa solid particle in the atmosphere that provides the surface on which water vapor condenses • Although the lowest layer of the atmosphere, the troposphere, does not contain any large solid surfaces, it contains millions of suspended particles of ice, salt, dust, and other materials. • Because the particles are so small—less than 0.001 mm in diameter—they remain suspended in the atmosphere for a long time.
Section 2 Clouds and Fog Chapter 23 Cloud Formation, continued The diagram below shows the molecular formation of a water droplet.
Section 2 Clouds and Fog Chapter 23 Adiabatic Cooling adiabatic coolingthe process by which the temperature of an air mass decreases as the air mass rises and expands • As a mass of air rises, the surrounding atmospheric pressure decreases. • Thus, fewer collisions between the molecules happen. • The resulting decrease in the amount of energy that transfers between molecules decreases the temperature of the air.
Section 2 Clouds and Fog Chapter 23 Adiabatic Cooling, continued Adiabatic Lapse Rate • The rate at which the temperature of a parcel of air changes as the air rises or sinksis called the adiabatic lapse rate. • The adiabatic lapse rate of clear air is about –1°C per 100 m that the air rises. • The slower rate of cooling of moist air results from the release of latent heat as the water condenses.
Section 2 Clouds and Fog Chapter 23 Adiabatic Cooling, continued Condensation Level • When air cools to a temperature that is below the dew point, net condensation causes clouds to form. • The altitude at which this net condensation begins is called the condensation level. • The condensation level is marked by the base of the clouds. • Further condensation allows clouds to rise and expand above the condensation level.
Section 2 Clouds and Fog Chapter 23 Reading Check What is the source of heat that warms the air and leads to cloud formation?
Section 2 Clouds and Fog Chapter 23 Reading Check What is the source of heat that warms the air and leads to cloud formation? The source of heat that warms the air and leads to cloud formation is solar energy that is reradiated as heat by Earth’s surface. As the process continues, latent heat released by the condensation may allow the clouds to expand beyond the condensation level.
Section 2 Clouds and Fog Chapter 23 Mixing • Some clouds form when one body of moist air mixes with another body of moist air that has a different temperature. • The combination of the two bodies of air causes the temperature of the air to change. • This temperature change may cool the combined air to below its dew point, which results in cloud formation.
Section 2 Clouds and Fog Chapter 23 Lifting • Air can be forced upward when a moving mass of air meets sloped terrains, such as a mountains range. • As the rising air expands and cools, clouds form. • The large cloud formations associated with storm systems also form by lifting. • These clouds form when a mass of cold, dense air enters an area and pushes a less dense mass of warmer air upward.
Section 2 Clouds and Fog Chapter 23 Advective Cooling advective coolingthe process by which the temperature of an air mass decreases as the air mass moves over a cold surface • As air moves over a surface that is colder than air is, the cold surface absorbs heat from the air and the air cools. • If the air cools below its dew point, clouds form.
Section 2 Clouds and Fog Chapter 23 Classification of Clouds • Clouds are classified by their shape and their altitude. • The three basic cloud forms are stratus clouds, cumulus clouds, and cirrus clouds. • There are also three altitude groups: low clouds (0-2,000 m), middle clouds (2,000 to 6,000 m), and high clouds (above 6,000 m).
Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Stratus Clouds stratus clouda gray cloud that has a flat uniform base and that commonly forms at very low altitudes • Stratus means “sheet-like” or “layered.” • Stratus clouds form where a layer of warm, moist air lies above a layer of cool air. • Stratus clouds cover large areas of sky and often block out the sun.
Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Stratus Clouds, continued • The two variations of stratus clouds are known as nimbostratus and altostratus. • Unlike other stratus clouds, the dark nimbostratus clouds can cause heavy precipitation. • Altostratus clouds form at middle altitudes. They are generally thinner than the low stratus clouds and produce very little precipitation.
Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Cumulus Clouds cumulus clouda low-level, billowy cloud that commonly has a top that resembles cotton balls and has a dark bottom • Cumulus means “piled” or “heaped.” • These clouds form when warm, moist air rises and cools. As the cooling reaches its dew point, the clouds form. • The flat base that is characteristic of most cumulus clouds represents the condensation level.
Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Cumulus Clouds, continued • High, dark storm clouds known as cumulonimbus clouds, or thunderheads, are often accompanied by rain, lightning, and thunder. • If the base of cumulus clouds begins at middle altitudes, the clouds are called altocumulus clouds. • Low clouds that are acombination of stratus and cumulus clouds are called stratocumulus clouds.
Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Cirrus Clouds cirrus clouda feathery cloud that is composed of ice crystals and that has the highest altitude of any cloud in the sky • Cirro– and cirrus mean “curly.” • Cirrus clouds form at altitudes above 6,000 m. These clouds are made of ice crystals because the temperatures are low at such high altitudes. • Because these clouds are thin, light can easily pass through them
Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued Cirrus Clouds, continued • Cirrocumulus clouds are rare, high-altitude, billowy clouds composed entirely of ice crystals. Cirrocumulus clouds commonly appear just before a snowfall or a rain fall. • Long, thin clouds called cirrostratus clouds form a high, transparent veil across the sky. • A halo may appear around the sun or moon when either is viewed through a cirrostratus cloud.
Section 2 Clouds and Fog Chapter 23 Classification of Clouds, continued The diagram below shows the different types of clouds in the atmosphere.
Section 2 Clouds and Fog Chapter 23 Reading Check Why are cirrus clouds commonly composed of ice crystals?
Section 2 Clouds and Fog Chapter 23 Reading Check Why are cirrus clouds commonly composed of ice crystals? because cirrus clouds form at very high altitudes where air temperature is low
Section 2 Clouds and Fog Chapter 23 Fog fogwater vapor that has condensed very near the surface of Earth because air close to the ground has cooled • The obvious difference between fog and clouds is that fog is very near the surface of Earth. • However, fog also differs from clouds because of how fog forms.
Section 2 Clouds and Fog Chapter 23 Fog, continued Radiation Fog • One type of fog forms from the nightly cooling of Earth. • The layer of air in contact with the ground becomes chilled to below the dew point, and the water vapor in that layer condenses into droplets. • This type of fog is called radiation fog because it results from the loss of heat by radiation.