Download condensation

Chapter 5 Lecture
Understanding
Weather and
Climate
Seventh Edition
Water in the
Atmosphere
Frode Stordal, University of Oslo
Redina L. Herman
Western Illinois University
© 2015 Pearson Education, Inc.
The Hydrologic Cycle
• Earth has been called the “water planet” as over ¾ of
the Earth’s surface is covered by water.
• Water is unique in that it can simultaneously exist in
all three states (solid, liquid, gas) at the same
temperature and shift between states easily.
• The hydrologic cycle refers to the cycle of water
through Earth and atmosphere.
© 2015 Pearson Education, Inc.
The Hydrologic Cycle
© 2015 Pearson Education, Inc.
Water Vapor and Liquid Water
• Evaporation and Condensation
– Molecules escape into the overlying volume as water vapor
during evaporation. Energy must be available at the water
surface.
– Water vapor increases in air as surface water evaporates.
– Water vapor molecules randomly collide with the water
surface and bond with adjacent molecules during
condensation.
– There is an equilibrium between evaporation and
condensation during saturation. Upon saturation,
condensation begins and water returns to the surface.
– The air/atmosphere does not “hold” water vapor. Water vapor
exists in gas form just like any other atmospheric gas: oxygen,
nitrogen, etc.
© 2015 Pearson Education, Inc.
Water Vapor and Liquid Water
• Evaporation and Condensation
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Introduction
– Humidity is the amount of water vapor in air.
– Humidity is expressed in several ways.
– Each has advantages and disadvantages, depending upon
use.
– All humidity refers to water vapor and NOT solid or liquid
states of water, as in ice, snow, or rain.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Vapor Pressure
– Vapor pressure is the amount of pressure exerted on the
atmosphere by water vapor.
– Vapor pressure is dependent upon both density and
temperature of the vapor.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Vapor Pressure
– Saturation vapor pressure is the vapor pressure of the
atmosphere when it is saturated.
The movement of water vapor molecules
exerts vapor pressure on surfaces.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Vapor Pressure
– Saturation vapor pressure is temperature dependent.
– At low temperatures the saturation vapor pressure increases
slowly, but it increases rapidly at higher temperatures. It is not
a linear increase.
Nonlinear increase in saturation vapor
pressure with increase in temperature.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Absolute Humidity
– Absolute humidity is the density of water vapor, expressed
as the number of grams of water vapor per cubic meter of
air (g/m3).
– The absolute humidity value changes as air volume expands
and contracts.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Specific Humidity
– Specific humidity represents a given mass of water vapor in
a given mass of air.
– Specific humidity does not change as air expands and
contracts.
– Saturation specific humidity is the specific humidity of the
atmosphere when it is saturated.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Mixing Ratio
– Mixing ratio is a measure of atmospheric moisture; it is the
mass of water vapor per unit mass of dry air, usually
expressed in grams per kilograms (g/kg).
– Mixing ratio is very similar to specific humidity in that it
expresses the mass of water vapor relative to air mass.
– Maximum mixing ratio is the saturation mixing ratio.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Relative Humidity
– Relative humidity (RH) is the amount of water vapor relative
to the maximum that can exist at a particular temperature.
• RH = (specific humidity/saturation specific humidity) x 100%
– Relative humidity describes the amount of water vapor relative
to a saturation point.
– The saturation point, or the relative humidity term, is relative to
air temperature and total water vapor.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Relative Humidity
– The highest RH occurs in the morning, during the coolest time
of the day.
– The lowest RH occurs in late afternoon, during the warmest
time of the day.
– Because of temperature dependency, the term cannot be used
to compare moisture content at different locations having
different temperatures.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Relative Humidity
The relationship between RH and temperature.
© 2015 Pearson Education, Inc.
Indices of Water Vapor Content
• Dew Point
– The dew point temperature is the temperature at which
saturation occurs in the air and is dependent upon the amount
of water vapor present.
– High dew points indicate abundant atmospheric moisture.
– Dew points can be only equal or less than air temperatures.
– If saturation is reached and air temperatures cool further, water
vapor is removed from the air through condensation.
– When air reaches saturation at temperatures below freezing,
the term frost point is used.
© 2015 Pearson Education, Inc.
Distribution of Water Vapor
• Water vapor is supplied to the atmosphere by local
evaporation or horizontal transport (advection).
© 2015 Pearson Education, Inc.
Processes That Cause Saturation
• Air can become saturated in three ways:
– The addition of water vapor
– Mixing cold air with warm air
– Moist air—by cooling the air to dew point
• If enough vapor is added to the air to saturate it, a
precipitation fog forms beneath the cloud (#1 listed
above)
© 2015 Pearson Education, Inc.
Factors Affecting Saturation and
Condensation
• Effect of Curvature
– Small drops exhibit greater curvature than larger ones.
– Curvature influences saturation vapor pressure with highly
curved drops.
– For very small drops, requires supersaturation of 200
percent.
– Hygroscopic (water attracting) aerosols act as condensation
nuclei (particles onto which water droplets form).
– Condensation onto condensation nuclei, called
heterogeneous nucleation, causes dissolution of the
aerosol.
© 2015 Pearson Education, Inc.
Factors Affecting Saturation and
Condensation
• Effect of Curvature
Larger drops have less curvature than smaller ones.
© 2015 Pearson Education, Inc.
Factors Affecting Saturation and
Condensation
• The Role of Condensation Nuclei
– Evaporation from solutions is lower than for pure water.
– This opposes curvature influences in a way that
condensation typically occurs at RHs near 100 percent.
– Hygroscopic nuclei is abundant in the atmosphere from
many natural sources (salt, dust, ash, etc.) and
anthropogenic sources (combustion derivative).
– Small condensation nuclei lead to very tiny water drops,
or haze.
© 2015 Pearson Education, Inc.
Factors Affecting Saturation and
Condensation
• Effect of Solution
Small droplets require higher RHs to remain liquid.
© 2015 Pearson Education, Inc.
Factors Affecting Saturation
• Ice Nuclei
– Atmospheric water does not normally freeze at 0°C.
– Supercooled water refers to water having a temperature
below the melting point of ice but nonetheless existing in a
liquid state.
– Ice crystal formation requires ice nuclei, a rare temperaturedependent substance similar in shape to ice (six-sided).
• Examples: clay, ice fragments, bacteria, etc.
• Ice nuclei become active at temperatures below -4°C
– Between -10° and -30°C, saturation may lead to ice crystals,
supercooled drops, or both.
– Below -30°C, clouds are composed solely of ice crystals.
– At or below -40°C spontaneous nucleation, the direct
deposition of ice with no nuclei present, occurs.
© 2015 Pearson Education, Inc.
Cooling the Air to the Dew or Frost Point
• Diabatic Processes
– Diabatic process involves the addition or removal of energy.
• Example: Air passing over a cool surface loses energy through
conduction.
© 2015 Pearson Education, Inc.
Cooling the Air to the Dew or Frost Point
• Adiabatic Processes
– Cloud formation typically involves temperature changes with no
exchange of energy (adiabatic process), according to the
first law of thermodynamics.
– Rising air expands through an increasingly less dense
atmosphere, causing a decrease in internal energy and a
corresponding temperature decrease.
– Parcels expand and cool at the dry adiabatic lapse rate
(DALR), 1°C/100 m.
– Parcels may eventually reach the lifting condensation level,
the height at which saturation occurs.
– Parcels then cool at the saturated adiabatic lapse rate
(SALR), ~0.6°C/100.
© 2015 Pearson Education, Inc.
Cooling the Air to the Dew or Frost Point
• Adiabatic Processes
Dry adiabatic cooling.
© 2015 Pearson Education, Inc.
Cooling the Air to the Dew or Frost Point
• The environmental (ambient) lapse rate (ELR) refers to
an overall decrease in air temperature with height.
• This rate, which changes from place to place, stems
from the fact that air located farther from surface
heating is typically cooler than that nearer the surface.
A comparison of adiabatic and
environmental cooling rates.
© 2015 Pearson Education, Inc.
Forms of Condensation
• Dew
– Dew is liquid condensation on surface often occurring during
the early morning hours.
– Loss of longwave radiation at night can cause the surface to
cool diabatically.
– Surface air becomes saturated and condensation forms on
objects acting as condensation nuclei.
© 2015 Pearson Education, Inc.
Forms of Condensation
• Frost
– Frost is similar to dew except that it forms when surface
temperatures are below freezing.
– Deposition occurs instead of condensation.
– May be referred to as white frost or hoar frost.
© 2015 Pearson Education, Inc.
Forms of Condensation
• Frozen Dew
– Frozen dew occurs when normal dew formation processes
occur, followed by a drop in temperature to below freezing.
– Ensures a tight bond between ice and the surface.
– Causes “black ice” on roadways.
© 2015 Pearson Education, Inc.
Forms of Condensation
• Dew (a), frost (b), and frozen dew (c).
© 2015 Pearson Education, Inc.
Atmospheric Moisture and Climate Change
• The IPCC studies around the world have found that
increases in specific humidity near the surface have
been associated with increasing temperatures since
1976.
• Over most oceanic areas, relative humidities have
remained fairly constant, as increasing water vapor
contents have been offset by increases in the
saturation specific humidity.
• Over some land areas, increases in specific humidity
have been more than offset by increases in the
saturation levels, leading to locally unchanged or
slightly reduced relative humidities.
© 2015 Pearson Education, Inc.