Water in the Atmosphere · • Specific Humidity – Specific humidity represents a given mass of water vapor in a given mass of air (dry + water vapour) (g/kg). – Specific humidity

© 2015 Pearson Education, Inc.

Water in the

Atmosphere

Chapter 5 Lecture

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal, University of Oslo


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.


The Hydrologic Cycle


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.


• Evaporation and Condensation

Water Vapor and Liquid Water


• Introduction

– Humidity is the amount of water vapor in air.

– Humidity is expressed in several ways.

– Each has advantages and disadvantages, depending upon

use.

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.



• 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.



• 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.



• 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.



• Specific Humidity

– Specific humidity represents a given mass of water vapor in

a given mass of air (dry + water vapour) (g/kg).

– Specific humidity does not change as air expands and

contracts.

– Saturation specific humidity is the specific humidity of the

atmosphere when it is saturated.



• 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.



• 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.




– 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.



The relationship between RH and temperature.




• 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.



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


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.




Condensation

Larger drops have less curvature than smaller ones.


• 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.


Condensation




Condensation

Small droplets require higher RHs to remain liquid.


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 temperature-

dependent 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.


• 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.

Cooling the Air to the Dew or Frost Point


• Adiabatic Processes

Dry adiabatic cooling.



• 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.



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.


• 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.

Atmospheric Moisture and Climate Change

Documents

Water in the Atmosphere · • Specific Humidity – Specific humidity represents a given mass of water vapor in a given mass of air (dry + water vapour) (g/kg). – Specific humidity