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The Impact of Atmospheric Moisture on the Landscape

The Nature of Water: Commonplace but Unique

  • Water is both the most distinctive and the most abundant substance on Earth.
  • Surface water makes up more than 70 percent of Earth’s surface.

The Hydrologic Cycle

    • The hydrologic cycle  is the ceaseless interchange of moisture in terms of its geographic location and its physical state.
      1. Water evaporates, becomes water vapor.
      2. Goes into the atmosphere.
      3. Vapor condenses and becomes liquid or solid state.
      4. Returns to Earth.
    • The hydrologic cycle is intricately related to many atmospheric phenomena.
    • Important determinant of climate:
      • Rainfall distribution
      • Temperature modification

The Water Molecule

      1. Inside a water molecule there are two atoms of hydrogen and one atom of oxygen.
      2. They are linked together by a covalent bond.
      3. Water molecules possess a polarity, with the oxygen side being negative and the hydrogen side being positive.
      4. Water molecules link together through their hydrogen bonds.

Important Properties of Water

      1. Liquidity—water is abundant in liquid form at numerous locations on Earth’s surface.
        • This property increases its versatility.
      2. Ice Expansion—as water freezes, it contracts until it reaches about 4°C and then expands (as much as 9%) as it cools from 4°C to 0°C.
        • As it cools and freezes, it begins to form hexagonal structures held together by hydrogen bonding.
        • This expansion also allows ice to become less dense than liquid water and float.
      3. Surface Tension—because of its electrical polarity, water also possesses “sticky” properties, and this is what gives it the ability to bead.
      4. Capillarity—surface tension and adhesion combined allow water to climb upward in narrow openings in an action called capillarity.
        • Water can also stick to many surfaces.
        • This is called adhesion.
      5. Solvent Ability—water is also a “universal solvent” in that it can dissolve almost anything.
      6. Specific Heat—water also has a great heat capacity.
        • When it is warmed, it can absorb an enormous amount of energy with only a small increase in temperature.
        • This high heat capacity is attributed to the large amount of energy required to overcome the hydrogen bonds between water molecules.

Phase Changes of Water

  1. Water comes in three states (liquid, solid, and gas) and can change from one state to another.
    • Evaporation—liquid water converted to the gaseous form.
    • Condensation—water vapor converted to the liquid form.
    • Sublimation—the process by which water vapor is converted directly to ice or vice versa.
  2. In each of the change processes, there is a gain or loss of heat, or latent heat.
    • Converting 1 gram of ice to 1 gram of liquid water at 0°C requires 80 calories of heat absorbed.
    • To raise the temperature of 1 gram of liquid water at 0°C to the boiling point, 540 calories of heat must be absorbed.
    • For ice to sublimate to water vapor or for water vapor to sublimate to ice, 680 calories must be absorbed or released, respectively.
    • The energy that is absorbed when water undergoes a phase change from a solid to a liquid or a liquid to a gas is known as the latent heat of evaporation.
    • The energy that is released when water undergoes a phase change from a gas to a liquid or a liquid to a solid is known as the latent heat of condensation.
    • In nature, water requires 540 to 600 calories of energy for every 1 gram of liquid water at 20°C.
  3. Importance of Latent Heat in the Atmosphere—the absorption and release of energy during evaporation and condensation have several effects.
    • Water can store energy when it evaporates.
    • Water can release heat back to the atmosphere when it condenses.

Water Vapor and Evaporation

Water vapor—the gaseous state of water; atmospheric moisture.

Changes easily from one state to another with temperature and pressure changes.

This ease of changing results in erratic distribution around the world.

Can be virtually absent in some parts of world but can constitute as much as 4 percent of atmospheric volume in other parts.

Essentially restricted to the lower troposphere.

Evaporation and Rates of Evaporation

Rate of evaporation from a water surface depends on three factors: the temperature of the water and the air, the amount of water vapor already in the air, and whether the air is still moving.

Evaporation—process by which liquid water is converted to gaseous water vapor.

Molecules of water escape the liquid surface into the surrounding air.

Water vapor is added to the air when the rate of evaporation exceeds the rate of condensation—net evaporation, in this instance.

Rates of Evaporation

  • Temperature—a key factor in evaporation, both in water and in the air around it.
  • Molecules become more agitated the higher the temperature, and this agitation leads to evaporation.
  • Temperature works in conjunction with pressure.

Vapor pressure—the pressure exerted by water vapor in the air.

At any given temperature, there is a maximum vapor pressure that water vapor molecules can exert.

Saturated air—the point at which some water vapor molecules must become liquid because the maximum vapor pressure is exceeded.

The warmer the air, the more water vapor it can hold before becoming saturated.

Evapotranspiration  –  the process of water vapor entering the air from land sources.

Evapotranspiration occurs through two ways:

Transpiration—the process by which plant leaves give up their moisture to the atmosphere.

Evaporation from soil and plants.

Most evapotranspiration occurs through plants.

Potential Evapotranspiration—the maximum amount of moisture that could be lost from soil and vegetation if the ground were sopping wet all the time.

Potential evapotranspiration rate and actual rate of precipitation play a key role in determining a region’s groundwater supply (or lack of it).

Measures of Humidity

Humidity—the amount of water vapor in the air

Absolute Humidity  –  a direct measure of the water vapor content of air.

  • Expressed as the weight of water vapor in a given volume of air, usually as grams of water per cubic meter of air.
  • Amount is a function of how much volume is being considered.
  • If the volume of air doubles, the absolute humidity halves.
  • Absolute humidity is limited according to temperature.
  • The colder the air, the less vapor it can hold.

 Specific humidity—a direct measure of water vapor content expressed as the mass of water vapor in a given mass of air (grams of vapor/kilograms of air).

Vapor pressure—the contribution of water vapor to the total pressure of the atmosphere.

Saturation vapor pressure—the maximum possible vapor pressure at a given temperature.

Relative humidity—an expression of the amount of water vapor in the air in comparison with the total amount that could be there (capacity) if the air were saturated. This is a ratio that is expressed as a percentage.

  • Relative humidity = Actual water vapor in air/Capacity × 100
  • Relative humidity changes if either the water vapor content or the water vapor capacity of the air changes.
  • Temperature–Relative Humidity Relationship
  • Relative humidity also changes if temperature changes.
  • Relationship between temperature and relative humidity is one of the most important in all meteorology.
  • Inverse relationship—as one increases, the other decreases.
  • Relative humidity can be determined through the use of a psychrometer (see Appendix IV for a description of humidity measurement via this instrument).

Related Humidity Concepts

  • Dew Point Temperature—the critical air temperature at which saturation is reached.
  • Cooling is the most common way that air is brought to the point of saturation and condensation.
  • Sensible Temperature—a concept of the relative temperature that is sensed by a person’s body

Condensation —process by which water vapor is converted to liquid water; opposite of evaporation.

  • For condensation to take place, air must be saturated.
  • Condensation cannot occur, however, even if the air is saturated, if there is not a surface on which it can take place.
  • Air becomes supersaturated  if a surface is not available.
  • In the upper atmosphere, surfaces are available through hygroscopic particles or condensation nuclei—tiny atmospheric particles of dust, smoke, and salt that serve as collection centers for water molecules.
  • Most common are bacteria blown off plants or thrown into the air by ocean waves.
  • Clouds often are composed of liquid water droplets even when the temperature is below freezing.
  • If water is dispersed as fine droplets, it can remain liquid at temperatures as cold as –40°C (–40°F).
  • Water that persists as a liquid below freezing is known as supercooled.
  • Supercooled water droplets promote the growth of ice particles in cold clouds.

Adiabatic  Processes – Key physical geographic fact:

Large masses of air can be cooled to the dew point only  by expanding as they rise.

Because of this limitation, adiabatic cooling is the only prominent mechanism for the development of clouds and the production of rain.

Dry adiabatic lapse rate—the rate at which a parcel of unsaturated air cools as it rises; this rate is relatively steady (5.5°F per 1000 feet) (10°C/km).

Air is not necessarily dry; it is just not saturated.

Descending air warms, and it does so at the dry adiabatic lapse rate.

Lifting condensation level  (LCL)—the altitude at which rising air cools sufficiently to reach 100 percent relative humidity at the dew point temperature and condensation begins.

Saturated adiabatic lapse rate—the diminished rate of cooling, which occurs when air rises above the lifting condensation level. It depends on temperature and pressure, but averages about 3.3°F per 1000 feet (6°C/kilometer).


Not all clouds precipitate, but all precipitation comes from clouds.

At any given time, about 50 percent of Earth is covered by clouds.

Clouds play an important role in the global energy budget.

Receive insolation from above and terrestrial radiation from below.

They absorb, reflect, scatter, or reradiate this energy, and so influence radiant energy.

Classifying Clouds

Clouds are classified on the basis of two factors:

  1. Form
  2. Altitude

Cloud Form

Three forms of clouds:

  1. Cirriform clouds—a cloud that is thin, wispy, and composed of ice crystals rather than water particles; it is found at high elevations.
  2. Stratiform clouds—a cloud form characterized by clouds that appear as grayish sheets or layers that cover most or all of the sky, rarely being broken into individual cloud units.
  3. Cumuliform clouds—a cloud that is massive and rounded, usually with a flat base and limited horizontal extent, but often billowing upward to great heights.

These three cloud forms are subclassified into 10 types based on shape.

One type may evolve into another.

Three of these 10 are purely one form, whereas the other 7 are combinations of these 3.

Three pure forms:

Cirrus cloud—high cirriform clouds with a feathery appearance.

Cumulus cloud—puffy white cloud that forms from rising columns of air.

Stratus cloud—low clouds, usually below 6500 feet (2 kilometers), that sometimes occur as individual clouds but more often appear as a general overcast.

Precipitation comes only from clouds that have “nimb” in their name, specifically, nimbostratus or cumulonimbus.

Cumulonimbus cloud—cumuliform cloud of great vertical development often associated with a thunderstorm.

Nimbostratus cloud—a low, dark cloud, often occurring as widespread overcast and normally producing precipitation.

Cloud Families – Four categories based on altitude:

  1. High clouds—altocumulus clouds—found above 6 kilometers (i.e., cirrus clouds)
  2. Middle clouds—between about 2 and 6 kilometers (i.e., altocumulus and alto stratus).
  3. Low clouds—below 2 kilometers (i.e., stratocumulus and nimbostratus).
  4. Clouds with vertical development (i.e., cumulus clouds).

Fog—a cloud whose base is at or very near ground level.

No physical differences between a cloud and fog.

Important differences in how fog and clouds form.

Most clouds develop as a result of adiabatic cooling in rising air.

Most fogs are formed either when Earth’s surface cools to below its dew point or when enough water vapor is added to the air to saturate it.

  1. Radiation fog—forms through loss of ground heat.
  2. Advection fog—forms when warm, moist air moves over a cold surface.
  3. Upslope fog (orographic fog)—caused by adiabatic cooling when humid air climbs a topographic slope.
  4. Evaporation fog—when water vapor is added to cold air that is already near saturation.


Dew—the condensation of beads of water on relatively cold surfaces; if temperature is below freezing, ice crystals (white frost) form.

Clouds and Climate Change

Clouds are important in their influence on radiant energy.

They receive both solar and terrestrial radiation and then can absorb, scatter, reflect, or reradiate the energy.

Their influence must be taken into account when attempting to anticipate the causes and consequences of climate change.

Atmospheric Stability

Buoyancy—the tendency of an object to rise in a fluid.

A parcel of air moves vertically until it reaches a level at which the surrounding air is of equal density (equilibrium level).

The Stability of Air

  • Stable Air—resists vertical movement; nonbuoyant, so will not move unless force is applied.
  • Unstable Air—buoyant, so will rise without an external force or will continue to rise after a force is removed.
  • Conditional instability—intermediate condition between absolute stability and absolute instability. Occurs when an air parcel’s adiabatic lapse rate is somewhere between the dry and wet adiabatic rates. Acts like stable air until an external force is applied; when forced to rise, it may become unstable if condensation occurs (release of latent heat provides buoyancy).

Determining Atmospheric Stability

Temperature, Lapse Rate, and Stability—air stability is related to adiabatic temperature changes, as discussed in Review Questions 24–28.

Visual Determination of Stability—accurate determination of the stability of any mass of air depends on temperature measurements, but one can get a rough indication from looking at cloud patterns.

Unstable air is associated with distinct updrafts, which are likely to produce vertical clouds.

Cumulous clouds suggest instability.

Towering cumulonimbus clouds suggest pronounced instability.

Horizontally developed clouds, most notably stratiform, characterize stable air that was forced to rise.

Cloudless sky indicative of stable, immobile air.


Most clouds do not yield precipitation.

Condensation alone is insufficient to produce raindrops.

The Processes of Precipitation Formation

It is still not well understood why most clouds do not produce precipitation.

Two mechanisms are believed to be principally responsible for producing precipitation:

  1. Collision and coalescence of water droplets
  2. Ice-crystal formation

Collision/Coalescence—most responsible for precipitation in the tropics and produces much precipitation in the middle latitudes.

Rain is produced by the collision and coalescing (merging) of water droplets.

No ice crystals form because cloud temperatures are too high.

Must coalesce enough that the droplets become large enough to fall.

Coalescence is ensured only if atmospheric electricity is favorable, so that positively charged droplets collide with negatively charged ones.

Ice-Crystal Formation—known as the Bergeron process,  the process by which ice crystals form; it is believed to account for the majority of precipitation outside of tropical regions.

Ice crystals and supercooled water droplets in a cloud are in direct competition for water vapor not yet condensed.

Ice crystals will attract most of the vapor if liquid droplets are in a state of equilibrium.

If ice crystals grow at the expense of water droplets, the crystals will become large enough to fall.

As they descend, they get warmer and pick up more moisture, growing still larger.

They then either precipitate as snowflakes or melt and precipitate as raindrops.

Forms of Precipitation

    • Rain—the most common and widespread form of precipitation, consisting of drops of liquid water. Most rain is the result of condensation and precipitation in ascending air that has a temperature above freezing, but some results from thawing of ice crystals.
    • Snow—solid precipitation in the form of ice crystals, small pellets, or flakes, which is formed by the direct conversion of water vapor to ice.
    • Sleet—small raindrops that freeze during decent, reaching the ground as small pellets of ice.
    • Glaze—rain that turns to ice the instant it collides with a solid object.
    • Hail—rounded or irregular pellets or lumps of ice produced in cumulonimbus clouds as a result of active turbulence and vertical air currents. Small ice particles grow by collecting moisture from supercooled cloud droplets.
    • Virga—if the relative humidity of the air below a precipitating cloud is low, falling precipitation may evaporate before reaching the surface.


Atmospheric Lifting and Precipitation

Significant amounts of precipitation can originate only by rising air and adiabatic cooling.

There are four principal types of atmospheric lifting:

  1. Convective lifting
  2. Orographic lifting
  3. Frontal lifting
  4. Convergent lifting

More often than not, the various types operate in conjunction.

Convective Lifting

Showery precipitation with large raindrops falling fast and hard; caused by convective lifting, which occurs when unequal heating of different air surface areas warms one parcel of air and not the air around it.

This is the only spontaneous of the four lifting types; the other three require an external force.

Orographic Lifting

  • Caused when topographic barriers force air to ascend upslope; only occurs if the ascending air is cooled to the dew point.
  • Rain shadow—area of low rainfall on the leeward side of a topographic barrier; can also apply to the area beyond the leeward side, for as long as the drying influence continues.

Frontal Lifting

Occurs when air is cooled to the dew point after unlike air masses meet, creating a zone of discontinuity (front) that forces the warmer air to rise over the cooler air (frontal lifting).

Convergent Lifting

Showery precipitation caused by convergent lifting, the least common form of lifting, which occurs when air parcels converge and the crowding forces uplift, which enhances instability. This precipitation is particularly characteristic of low latitudes.

Global Distribution of Precipitation

The spatial distribution of precipitation is the most important geographic aspect of atmospheric moisture.

Broad-scale pattern is based on latitude, but many other factors are involved in this complex pattern.

Isohyet—a line joining points of equal quantities of precipitation.

Nature of the air mass and the degree to which that air is uplifted determine the amount of precipitation in an area.

Humidity, temperature, and stability are mostly dependent on where air originated and on the trajectory it has followed.

Uplifting (and its amount) is determined largely by zonal pressure patterns, topographic barriers, storms, and other atmospheric disturbances.

Regions of High Annual Precipitation

Because they have warm trade winds that are forced to rise, tropical latitudes contain most of the wettest areas in the world.

Coastal regions usually receive more precipitation than interior regions because they are closer to moisture sources.

The remaining wettest areas are narrow zones along the western coasts of North and South America.

Caused by a combination of onshore westerly airflow, frequent storms, and mountain barriers that run perpendicular to the westerly winds.

Regions of Low Annual Precipitation

Dry lands are most prominent on the western side of continents in subtropical latitudes.

Influenced by sinking air from subtropical highs and reinforced by cold ocean currents offshore.

Regions that have little access to moist air masses (such as in continental interiors).

Regions on the leeward side of orographic belts experience the rain shadow effect and therefore receive limited amounts of precipitation.

Very high latitudes because of limited water surfaces possessing low temperatures.

These regions are referred to as “cold deserts.”

Seasonal Precipitation Patterns

Summer/winter variation in precipitation occurs over most of Earth.

Strongest over continental interior because strong summer heating of the surface causes instability.

Coastal areas often are more balanced in their seasonal precipitation regime (always close to moisture sources).

The displacement of wet and dry zones mirrors the seasonal shifting of major pressure and wind systems, which follows the Sun—northward in July and southward in January.

Summer (in each hemisphere) is the time of maximum precipitation over most of the world.

Monsoon regions present the most conspicuous variation in seasonal precipitation, with very wet summers and generally dry winters.

Precipitation Variability

In any given year or any given season, the amount of precipitation may or may not be similar to the long-term average.

Precipitation variability—expected departure from average precipitation in any given year (expressed as a percentage; can go above or below the average).

Regions of normally heavy precipitation experience the least variability.

Normally dry regions experience the most variability.

 Acid Rain

Acid rain—involves either the wet or dry deposition of acidic materials from the atmosphere onto Earth’s surface; precipitation with a pH less than 5.6.

World regions have been experiencing a rapidly increasing intensity, magnitude, and extent of acid rain.

Evidence indicates the principal human-induced sources are sulfur dioxide emissions from smoke stacks.

Acid precipitation harms aquatic ecosystems; several hundred lakes in the United States and Canada have become biological deserts in the past quarter century.

Most fish perish at a pH less than 4.5, and precipitation is increasingly being recorded at this level.

Increasing evidence shows that it may be the major culprit in forest diebacks taking place on every continent except Antarctica, which has no forests.

In some parts of eastern and central Europe, 30 to 50 percent of the forests have been affected or killed by acid rain.

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Geog 001

1.  Introduction to Earth