PART II: Water in the Atmosphere
CHAPTER 5. Atmospheric Moisture
Chapter Overview:
The chapter explores the concepts of evaporation and condensation of water and related humidity
parameters in the Earth’s atmosphere system. Particulars regarding saturation of air, curvature of
cloud drops, and condensation nuclei are also expressed. Various types of condensation are also
considered with particular attention paid to related diabatic and adiabatic processes.
Chapter at a Glance:
Over 70% of the planet is covered by water, a unique substance that can simultaneously exist in all
three states at the same temperature. Also, water is readily able to shift between these states easily.
The hydrologic cycle refers to the regular cycle of water through the earth-atmosphere system.
• Water Vapor and Liquid Water - Liquefaction of water vapor occurs frequently at
normal Earth temperatures. This occurs when air is saturated with respect to water vapor
so that the addition of more water vapor causes a change of state to a liquid or frozen form.
<ME5.1>
A. Evaporation and Condensation - Evaporation will occur if energy is available
to a water surface. With increased evaporation, water vapor will increase into air
until it approaches saturation. Upon saturation condensation will begin and water
will return to the surface. Saturation, therefore, marks an equilibrium between
evaporation and condensation. This situation may occur in the presence or not in
the presence of dry air so that the statement that air holds water is erroneous.
These changes of state may also occur with regard to water vapor changing
directly to ice, called deposition, or the inverse situation, known as sublimation.
<CD3.2>
• Indices of Water Vapor Content - Humidity is a term that indicates the amount of water
vapor in air. It may be expressed in a variety of ways, each of which has advantages and
disadvantages. All indices refer solely to water vapor and exclude liquid and frozen states.
<CD3.3>
A.
Vapor Pressure - Vapor pressure is simply the amount of pressure exerted
on the atmosphere by water vapor. Although vapor pressure is dependent upon both
temperature and the density of the vapor, density plays the larger role. The
maximum water vapor pressure that can occur is saturation vapor pressure, which is
solely temperature dependent. Saturation vapor pressure exponentially increases
with temperature such that high temperatures may have extremely high saturation
vapor pressures as compared to lower temperatures. <CD3.3>
B. Absolute Humidity - Absolute humidity indicates the density of water vapor in
air expressed as g/m3. Because the absolute humidity term changes as air volume
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changes, it is not widely used.
C. Specific Humidity - Specific humidity is commonly used in scientific
applications as it represents a given mass of water vapor per mass of air (in g/kg).
Because it indicates mass, the term does not vary with air volume fluxes. As most
volume changes are invoked by temperature variations, specific humidity again
remains constant when temperature changes. It is frequently used to gain insight
into the water vapor content of air at different locations and temperatures. Saturated
air has the highest specific humidity possible for a given temperature and pressure
and is expressed as saturation specific humidity. <CD3.3>
D. Mixing Ratio - Mixing ratio is very similar to specific humidity in that it
expresses the mass of water vapor relative to air mass. However, the mixing ratio
expresses the amount of water vapor relative only to a mass of dry air. The
maximum mixing ratio in air is expressed as the saturation mixing ratio.
E. Relative Humidity - The most commonly used expression of water vapor
content is relative humidity, which indicates the amount of water vapor in the air
relative to the possible maximum. As such, it is given as a percentage that does not
indicate the amount of air that is water vapor but simply describes the amount
present relative to a saturation point. The saturation point, and thus the relative
humidity term, is relative to air temperature and total water vapor present. Because
more water vapor can exist in warm air than cold air, the term is sometimes rather
misleading. An example is the diurnal distribution of RH, which typically sees the
greatest RH during the early morning hours, when temperatures are at a minimum.
Lowest RH amounts will typically occur during the late afternoon, the time of
greatest air temperatures, making high temperature, high humidity’s (90% or so)
impossible. Because of this temperature dependency, the term cannot be used to
compare moisture content at different locations having different temperatures.
<CD3.3>
F. Dew Point - The dew point temperature, the temperature at which saturation
occurs in air, is reached, holding moisture content constant, through air cooling to
the saturation point. The dew point is a good indicator of moisture content in that
relatively high dew points indicate abundant atmospheric moisture. Dew points
can be only equal to 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 in the place of dew point. <CD3.3>
• Distribution of Water Vapor - Water vapor enters the atmosphere from local
evaporation or from advection from other locations. With advection, the amount of water
vapor in the atmosphere generally decreases with distance from the source. Further, the
amount of atmospheric water vapor increases dramatically during the warm season as low
air temperatures preclude the existence of high water vapor content.
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• Methods of Achieving Saturation - Air may become saturated through the addition of
water vapor to air at a constant temperature, mixing cold air with warm, moist air, or by
cooling air to the dew point. The first process is exemplified by light fogs that develop
beneath clouds as vapor is added to the air through falling raindrops. The second process is
characterized by contrails and steam fogs that develop as cold air passes over a relatively
warm water body. However, most condensation processes occur by the last process, air
chilled to the dew point. <CD3.4>
• Effects of Curvature and Solution - Condensed water suspended in the atmosphere is
typically curved. In addition impurities exist. Both play a role in phase shifts.
A. Effect of Curvature - Suspended water droplets are essentially spherical with
smaller drops exhibiting greater curvature than larger ones. Curvature influences
saturation vapor pressure such that highly curved drops of pure water require RHs
in excess of 100% to remain liquid. For very small cloud drops this supersaturation
may approach 300%. Hygroscopic aerosols acting as condensation nuclei <Web>
help keep RHs below these extremes. Condensation onto such particles, called
heterogeneous nucleation, causes dissolution of the aerosol.
B. Effect of Solution - Evaporation from solutions is less than from pure water.
This directly opposes curvature influences such that condensation typically occurs
at relative humidity near 100%. Hygroscopic nuclei abound in the atmosphere,
deriving from many natural (salt, dust, ash, etc.) and anthropogenic (combustion
derivatives) sources. Very small condensation nuclei may lead to very tiny water
droplet formation resulting in haze.
C. Ice Nuclei - Atmospheric water does not freeze at 0oC, leading to the presence
of supercooled water. Ice crystal formation in the atmosphere requires ice nuclei, a
relatively rare temperature dependent substance which requires a similar shape to
ice (clay, ice fragments, bacteria, volcanics, etc.). Ice nuclei become active at
temperatures below -4oC. At temperatures between -10 and -30oC (14–22oF),
saturation may lead to ice crystals, supercooled drops, or both. Below -30oC,
clouds are composed solely of ice crystals, while at temperatures at or below -40oC
(-40oF) spontaneous nucleation, the direct deposition of ice with no nuclei present,
occur.
• Measuring Humidity - Water vapor is an invisible gas mixed together with all other
atmospheric gases. The simplest way of measuring water vapor content is with a sling
psychrometer, which consists of a pair of thermometers. A cotton wick, wetted with
water, exists around the bulb of one. The two are spun through the air as evaporation is
encouraged to produce a wet bulb temperature which will be compared to the dry bulb
temperature. If air is unsaturated, water will evaporate from the wet bulb. This will be
indicated by a falling temperature which will eventually reach a steady state relative to
the amount of water contained in the atmosphere. The difference between the dry and
wet bulb temperatures is called the wet bulb depression. This is used to ascertain the
amount of water vapor content in the air. Some psychrometers are equipped with fans
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which circulated air across the bulbs. These are known as aspirated psychrometers. A
hair hygrometer contains human hair which expands and contracts in response to relative
humidity. Readings are recorded on a rotating drum in a hygrothermograph.
• High Humidities and Human Discomfort - Temperature is one of the most important
weather variables with regard to human comfort. High temperatures combined with high
humidity causes more fatalities in North America than hurricanes, lightning, floods, and
tornadoes combined. Humidity combined with high temperatures may be expressed in
terms of a heat index. Perspiration guards the body from overheating. Sweat is allowed to
evaporate which consumes latent heat and cools the body. In a moist atmosphere, the
amount of evaporation is reduced. This may cause overheating to a fatal point. The
apparent air temperature combines the effect of temperature and humidity. Between 41oC
and 54oC (105-129oF) muscle cramps or heat exhaustion is likely as is the threat of heat
stroke. Above 54oC, heat stroke is likely for at-risk people.
• Cooling the Air to the Dew or Frost Point - Most condensation processes occur as air is
chilled to the dew point. Air temperature changes occur either from direct energy
exchanges, termed diabatic processes, or from those involving no net energy exchange,
termed adiabatic processes.
A. Diabatic Processes - Diabatic processes involve the direct addition or removal
of heat energy. Air passing over a cool surface loses energy through conduction, a
diabatic process. Energy is always transferred from areas of high temperature
toward those of lower temperature in accordance with the Second Law of
Thermodynamics. Diabatic processes are typically associated with fog
development.
B. Adiabatic Processes - Cloud formation typically involves temperature changes
with no net exchange of energy, an adiabatic process. <CD4.1> Such processes
occur 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. <CD4.2> Parcels expand and cool at the
steady rate of 1oC/100 m (5.5oF/1000 ft), the dry adiabatic lapse rate (DALR).
<CD4.3> Sinking parcels experience exactly proportional compression warming.
A rising parcel reaching saturation at the lifting condensation level (LCL), <Web>
the height at which saturation occurs cools at the lesser saturated adiabatic lapse
rate (SALR) of 0.5oC/100 m (3.3oF/1000 ft) due to released latent heat of
condensation. The saturated adiabatic lapse rate is an approximation dependent on
actual moisture content and temperature. <CD4.5>
C. The Environmental Lapse Rate - The environmental (ambient) lapse rate
(ELR) refers to an overall decrease in air temperature with height. This rate, which
changes diurnally from pace to place, is initiated as air located farther from surface
heating is typically cooler than that nearer the surface.
• Forms of Condensation - Many forms of either liquid or solid condensation can occur
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depending upon particular process characteristics.
A. Dew - Dew refers to liquid condensation on surface objects. Diabatic cooling of
surface air typically takes place through terrestrial radiation loss on calm, cool,
clear nights. By morning, surface air is typically saturated and condensation forms
on objects, which act as condensation nuclei.
B. Frost - Frost formation is similar to dew except that it forms when surface
temperatures are below freezing. Deposition occurs instead of condensation. Such
frost may also be referred to as white or hoar frost which is different from frozen
dew.
C. Frozen Dew - When normal dew formation processes occur at temperatures
above freezing, then a temperature drop below freezing is initiated, dew will freeze
in place. This frozen dew process ensures a tight bond between ice and the surface
and is the cause of dangerous black ice roadway conditions.
D. Fog - Fog is simply a surface cloud formed when air near the surface either cools
to the dew point, has moisture added, or when cooler air is mixed with warmer
moister air. Some minor types of fog include precipitation fog, formed from
evaporating raindrops in cool air, and steam fog which forms when cool air passes
over a warm water body. More prominent forms include radiation, advection, and
upslope fogs.
1. Radiation Fog - When surface air cools diabatically to saturation
through terrestrial radiation loss on cool, clear nights, radiation fogs
develop. Unlike dew, radiation fogs require a slight breeze to vertically mix
air through a shallow near surface air column. However, if winds exceed
about 5 km/hr (3 mph) then vertical mixing with warmer air aloft prohibits
condensation near the surface. After sunrise, radiation fogs will likely lift,
or evaporate from below, as insolation warms the surface. In some cases,
fog may persist through the day as its thickness may effectively scatter
insolation.
A Tule fog refers to Central Valley, CA radiation fogs which are common
in winter as clear skies, light breezes, and abundant moisture from crops
provide ideal conditions for formation. Another common type is the valley
fog which occurs as dense cold air drains into lower elevations.
2. Advection Fog - Advection fogs take place as warm, moist air moves
horizontally across a cooler surface. The air, chilled diabatically to
saturation from below, is transported downstream. Such conditions are
common along the US west coast as warm, moist air from the central
Pacific advects over the cold California ocean current near the continent.
These fogs may also occur near boundaries of opposing ocean
temperatures, as is the case off the northeast coast of the US.
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3. Upslope Fog - Upslope fogs are the only fogs which form through
adiabatic processes. The fogs are formed as air is advected over land
surfaces which increase in elevation. This is a common occurrence in the
Great Plains of the US where warm, moist air advects from the lowland
Mississippi River Valley toward the Rocky Mountains.
• Formation and Dissipation of Cloud Droplets - Clouds result from adiabatic cooling
associated with rising air parcels. Dew points also decrease as air rises but at the shallow
dew point lapse rate of 0.2oC/100 m (1.1oF/1000 ft). <CD4.4> Approximately 50 m above
the LCL virtually all condensation nuclei have condensed water attached, leading to
additional growth of those droplets over the creation of new cloud drops. This process
soon halts leaving the existing drops to slowly evaporate or sublimate. Sinking parcels
above the LCL may warm at the saturated lapse rate as warming is offset by evaporation.
Thus, the concept of parcel cooling and condensation being an entirely reversible process
is valid, however, not common.
Chapter Boxes:
5-1 Forecasting: Dew Point and Nighttime Minimum Temperatures - Knowledge of
the current dew point temperature is a useful tool to the forecaster fro the prediction of the
following morning’s low temperature. Given no significant weather changes, the
minimum temperature will often approximate the dew point. In a condition where
nighttime temperatures reach the dew point, and a radiation fog is induced, the fob would
inhibit further cooling as water drops absorb longwave radiation. Without additional
radiation loss, the temperature will remain constant. This relationship will break down
with the advection of a mass of warm or cold air into the region. Heavy cloud cover and
strong winds will also alter the relationship as both will lead to higher surface
temperatures. Also, if the dew point depression is very large, as in a desert, the chance of
cooling the surface to that temperature is minimal.
5-2 Forecasting: Vertical Profiles of Moisture - Plotting temperatures and dew points on
a vertical profile chart gives considerable information on cloudiness. When both air
temperature and the dew point temperature lines intersect, air is saturated. This typically
occurs at some distance above the surface. Plotting also allows one to calculate the actual
specific humidity and saturation specific humidity at any level.
5-3 Physical Principles: The Varying Value of the Saturated Adiabatic Lapse Rate The SALR is dependent upon the temperature of the saturated air parcel with higher
temperatures causing lower lapse rates. At low temperatures, relatively little latent heat is
released so that the SALR and the DALR are similar. Warmer air lifted to saturation
condenses larger amounts of water leading to greater differences.
CD Rom Unit 3 - Atmospheric Moisture:
1. Introduction - The CD Unit begins by describing water as a key ingredient to a
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number of atmospheric processes. Clouds, precipitation, and the energy balance
are all greatly affected by water. It is also an important thermal sink as heat is
consumed and released through changes of state associated with water.
2. Saturation - The concepts of saturation are explained through an animation of
an empty pan surrounded by a vacuum. As water is added, pressure increases.
Some water evaporates, some condenses. Pressure remains constant as the number
of molecules equals that condensing. Thus, the volume is saturated with water
vapor and the measured pressure equals saturation vapor pressure. The animation
continues as temperature increases leading to a higher pressure (expressed in mb).
An interactive temperature slide creates accompanying pressure changes. This is
also depicted graphically through a plot of temperature (x axis) and pressure (y
axis). The animations best apply to the text section describing evaporation and
condensation.
3. Measures of Moisture - A new situation is considered in this section; one in
which a volume of atmosphere exists with water vapor. The section details the
different ways of expressing water vapor such as vapor pressure, specific humidity,
dew point, and relative humidity. Each is detailed through text explanations,
interactive graphics, and interactive mathematical expressions. Because multiple
text sections illustrate these concepts the best use of the CD Unit might be as a
summary of topics covered.
4. Temperature - Moisture Relations - An interactive graph and animation is
provided to illustrate relationships between temperature and specific humidity
through changes in temperature. The text section, Methods of Achieving
Saturation captures this notion.
CD Rom Unit 4 - Adiabatic Processes:
1. Introduction - The CD introduction details the fact that vertical atmospheric
motions are weak or nearly absent compared to horizontal motions which may be
100 times stronger. Vertical motions, however, are important to many weather and
climate processes, especially considering that any change in altitude leads to large
changes in temperature. Additionally, rising and sinking air changes temperature
with no gain or loss of heat. These adiabatic changes are important to nearly all
precipitation. These topics are explained in the text sections addressing adiabatic
processes.
2. Fundamentals - An animation of a rising parcel expanding into a lower
pressurized atmosphere is used to demonstrate the fundamentals of parcel theory.
The animation also shows that the reverse situation causes compression.
Appropriate temperature changes are presented as a function of the addition or
subtraction of mechanical energy. A further animation depicts adiabatic
temperature changes with changes in height. Again, the text section on adiabatic
processes examines these concepts.
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3. Dry Adiabatic Lapse Rate - The dry adiabatic lapse rate of 1oC/100m is
described as being constant as energy extraction is the same for every parcel due to
a nearly constant mix of gases and gravitational acceleration. Interactive graphics
portray the cooling rate as well as warming which accompanies a sinking parcel.
The reversible dry adiabatic process is emphasized. The text explores this topic in
the section entitled Adiabatic Processes.
4. Transition to Saturation - An animation portrays the concept of the dew point
lapse rate, which is eventually overtaken by the larger DALR to create a saturated
parcel. The animation leads to the notion that above the saturation point the parcel
follows a saturated process. Two sections of the text, Formation, and Dissipation of
Cloud Droplets, examine topics related to this unit section.
5. The Saturated Process - Upon saturation, water condenses in the parcel and
thus, falls out of the parcel to create a cloud. The parcel continues to rise and cool,
but at the lesser Saturated Adiabatic Lapse Rate. This lesser rate is due to the
release of latent heat of condensation. The SALR is variable (between 4oC/km and
9oC/km) as it is determined by the total amount of condensation taking place. This
condensation rate is dependent upon the total amount of water vapor available,
which in turn is related to air temperature. Therefore, lower rates of cooling are
associated with higher air temperatures containing more water vapor and higher
condensation rates. An interactive graph depicts the relationships between air
temperature, water vapor and condensation rates. An animation of a parcel rising
over a mountain, considering condensation, reiterates the concepts above. The
animation also details the additional warming that takes place on the lee side of the
mountain due to the differential cooling rates (DALR & SALR) and the constant
warming rate (DALR). Lastly, the graphics show that the saturated adiabatic
process is not reversible, and therefore, is not truly adiabatic. These concepts are
also illustrated in the Adiabatic Processes area of the text.
Related Web Sites:
Condensation Nuclei: http://earth.agu.org/revgeophys/rasmus00/node26.html
Lifting Condensation Level: http://personal.accuweather.com/wx/accudata/ampsskewt.htm
Humidity: http://www.usatoday.com/weather/whumcalc.htm
Heat Index: http://www.wunderground.com/US/Region/US/Heatindex.html
Dew Points: http://www.ems.psu.edu/wx/usstats/dewpstats.html
Media Enrichment:
ME5.1 - Movie of global January and July water vapor.
Key Terms:
evaporation
condensation
saturation
relative humidity
dew point
frost point
vapor pressure
wet bulb depression
sublimation
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wet/dry bulb temperature
lifting condensation level
precipitation fog
deposition
steam fog
hair hygrometer
dew
humidity
hygrothermograph
frost
vapor pressure
hygroscopic
diabatic process
frozen dew
absolute humidity
condensation nucleus radiation fog
advection
specific humidity
haze
supercooled water
adiabatic process
upslope fog
advection fog
mixing ratio
saturation mixing ratio
sling psychrometer saturated (wet) adiabatic lapse rate
second law of thermodynamics
aspirated psychrometer
homogeneous nucleation
heat index
saturation vapor pressure
ice nucleus
saturation specific humidity
dew point lapse rate
dry adiabatic lapse rate
environmental lapse rate
Review Questions:
1. Why is it incorrect to refer to the air “holding” water vapor?
Because water vapor is not held by atmospheric gases, it simply coexists with the other
gases that make up the atmosphere.
2. What are deposition and sublimation?
Both terms refer to a change of state which skips a step. Specifically, deposition refers to
the changed of state from a gas to a solid while sublimation indicates the reverse.
3. What is vapor pressure? What units of measurement is it expressed in?
The pressure exerted on the atmosphere by the presence of water vapor. Expressed another
way it is the part of the total atmospheric pressure due to water vapor. It is expressed in
millibars in the US and kilopascals in Canada.
4. Explain the concept of equilibrium and saturation.
Saturation is an equilibrium state denoting proportional loses of water from the surface to
the atmosphere through evaporation and gains from the atmosphere to the surface through
condensation.
When energy is available and evaporation occurs, the low water vapor content of the
atmosphere keeps condensation from occurring. With increasing water vapor content, the
rate of condensation increases. Eventually, the amount of water vapor in the air is enough
for the rates of condensation and evaporation to be equal. The resulting equilibrium state is
termed saturation.
5. What units of measurement are used to describe mixing ration and specific humidity? Why are
the two values nearly equal?
Specific humidity expresses the mass of water vapor existing in a given mass of air in
grams of water vapor per kilogram of air. The mixing ratio is a measure of the mass of
water vapor relative to the mass of the other gases of the atmosphere. The difference is that
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mixing ratio separates the water vapor from the other gases while specific humidity
measures water vapor against all the atmospheric gases, including water vapor. They are
nearly the same value because the amount of water vapor in the air is always small, so that
whether or not it is counted in the denominator of the equations hardly changes the ratio.
6. Why is absolute humidity seldom used?
Absolute humidity is the density of water vapor expressed as the number of grams of water
vapor contained in a cubic meter of air. This value changes whenever air expands or
contracts (i.e. with temperature changes). Therefore, it is not widely used.
7. Define relative humidity.
Relative humidity is the amount of water vapor in the air compared to the maximum
amount possible at the current temperature.
8. Why is relative humidity a poor indicator of the amount of water vapor in the air?
Because it is temperature dependent and it is inversely proportional to air temperature.
Therefore, high relative humidity values in cold air actually relate to minuscule absolute
humidity values. Conversely, low relative humidity values in warm air may relate to rather
large absolute humidity values.
9. Define dew point. What characteristics make this measure superior to relative humidity?
Dew point is the temperature at which saturation is reached. It is useful in that the dew
point can be compared directly to the current temperature to determine the amount of water
vapor present. When the dew point is close to the actual temperature then the air is near
saturation, when it is far from the air temperature, it is far from saturation.
10. Why can’t the dew point temperature exceed the air temperature? What happens if the air
temperature is lowered to a value less than the initial dew point?
If air temperature is lowered to the dew point, saturation will occur. If the temperature dips
below the dew point, then the dew point will decrease as well. This is offset by the
condensation of water vapor in the air. Because of this, the dew point can never exceed the
air temperature.
11. Describe the distribution of average dew point across the United States in summer and winter.
The total amount of water vapor generally decreases with distance from the Gulf of
Mexico, the primary source of water vapor. This is true in both seasons as well as in
north-south and east-west directions. During winter the amount of moisture extending into
the Great Plains is low and only a minimal amount of east-west variation exists. In
summer, a decline is seen in a north-south direction and also moving westward from about
the Mississippi River to the Rocky Mountains. Obviously, more water vapor is present
57
during the summer months.
12. What are the three general methods by which the air can become saturated?
By adding water vapor to air, mixing cold air with warm, moist air, and/or lowering the
temperature of air to the dew point.
13. What are the effects of droplet curvature and solution on the amount of water vapor needed for
saturation?
Highly curved droplets of pure water have higher saturation vapor pressures than lesser
curved drops. Thus, extremely high states of supersaturation would be required to keep the
drop from evaporating. Solution refers to impurities in the water, gained mainly from
atmospheric aerosols acting as condensation nuclei. Solutes oppose the effect of curvature
which keeps the droplets in equilibrium at more realistic relative humidity values (near
100%).
14. Why doesn’t homogeneous nucleation form water droplets in the atmosphere?
Because droplets would form by chance collision and bonding of water vapor molecules
under supersaturated conditions. As such they would have a small number of molecules
and a high degree of curvature so that they would exist only at high levels of
supersaturation.
15. What are condensation nuclei and ice nuclei? Are they typically made of the same materials?
Which is more abundant in the atmosphere?
Condensation nuclei are small, airborne particles that enhance condensation and ice nuclei
are particles onto which ice crystals can form when the air becomes saturated.
No, they are not typically made of the same materials.
Condensation nuclei are more abundant in the atmosphere.
16. What is supercooled water?
Water having a temperature below the freezing point of water but nonetheless existing in a
liquid state.
17. What are psychrometers? How do they work?
A pair of thermometers, one of which has a cotton wick around the bulb that is saturated
with water, the other with no covering. Swinging the device will cause evaporation of
water on the wet bulb, giving the wet bulb temperature. This is compared to the dry bulb
temperature to determine the amount of water vapor present in the air.
18. Define dry bulb temperature, wet bulb temperature, and wet bulb depression.
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The dry bulb temperature is the temperature of the dry thermometer, the wet bulb
temperature is the temperature of the wet thermometer. The wet bulb depression is the
difference between the two temperatures, which is used to measure water vapor content of
the air.
19. What is the heat index?
The heat index is a measure of the high air temperature with the effect of humidity factored
in. This gives an indication of how high heat and high humidity will affect the human
body.
20. What is the first law of thermodynamics and why is it important?
The first law of thermodynamics states what happens when heat is added to or removed
from gases. Specifically, if heat is added, there will be some combination of an expansion
of the gas and an increase in its temperature. This is important towards a proper
understanding of the adiabatic process.
21. Explain the difference between diabatic and adiabatic processes.
Diabatic processes refer to temperature changes that involve a direct exchange of heat
energy. Adiabatic processes refer to temperature changes without an exchange of heat.
22. What are the numerical values of the dry and saturated adiabatic lapse rates? Under what
circumstances are they applicable?
As long as air is unsaturated it will heat or cool when ascending or descending,
respectively, at a rate of 1.0oC/100 m, the dry adiabatic lapse rate. Once saturated,
ascending air will cool at the saturated adiabatic lapse rate which on average is
0.5oC/100m.
23. What does envirnametl lapse rate refer to?
(ELR) the rate of vertical temperature decrease in the air column
24. Describe the various processes that can lead to the formation of dew.
Lowering of temperature to the dew point near the surface. Favored under clear skies and
no wind.
25. What is the differece between frozen dew and frost?
Structure and manner of formation
26. Describe the various processes that can lead to the formation of fog.
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cooling of layer of air with light winds, evaporating water from falling precipitation or by
mixing warm moist air with cold air
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