Download Chapter 5 Fog, Clouds, and Precipitation

Chapter 5 Fog, Clouds, and Precipitation
5.1 Production of Dew, Fog, and Clouds
We have seen that air becomes saturated whenever the relative humidity is
equal to 100%. Any further cooling of the air causes water vapor to condense out of
the air. As we saw in chapter 4, condensation is the change of state from water
vapor to liquid water. Condensation can occur in two ways: (1) by increasing the
amount of water vapor into the air, and (2) by cooling the air down to the dew point
temperature. When the air is cooled to the dew point, the relative humidity becomes
100% and the air becomes saturated. Any further cooling produces condensation. If
the condensation occurs directly on the surface of the earth the result is called dew
or frost. If the tiny water droplets formed become suspended in the air then either a
fog or a cloud is produced depending on whether the condensation occurs close to
the surface of the earth or aloft. In most of what follows we assume that the
temperature of the air is high enough for the condensation to occur as water
droplets. If the temperature of the air is below freezing, the water vapor will
condense as ice crystals. Thus fog and/or clouds can consist of water droplets, ice
crystals, or both.
5.2 Fog
Fog results when atmospheric water vapor condenses (or sublimes) to the
extent that the new forms, water droplets or ice crystals, become visible and have
their base in contact with the ground. Saturation of the air and sufficient
condensation nuclei are ordinarily prerequisites for the formation of fog. One of the
great hazards associated with fog is a lack of visibility. Visibility in fog is defined as
the greatest distance in a given direction at which common objects like buildings or
trees are visible to the unaided eye. Prevailing visibility is defined as the greatest
visibility that prevails over at least one-half of the horizon. The principle processes
that cause saturation are cooling of the air and evaporation of water into it.
Classification of Fog.
Fog is classified according to the way it is produced. Fog can be caused by
adding water vapor to the air to cause saturation or cooling the air to produce
saturation.
(1) Fogs resulting from evaporation.
(a) Steam Fog. Steam fog is fog that is produced by intense evaporation of
water into relatively cold air. Saturation occurs, then condensation, then fog.
Steam fog is observed over bodies of water in mid- and high latitudes.
Sometimes it occurs over warm, wet land immediately after a rain.
(b) Frontal Fog. Frontal fog, as the name implies, is fog found along the
boundary of two air masses. Evaporation from warm rain falling through the
Chapter 5 Fog, Clouds, and Precipitation
drier air below may be followed by saturation and condensation in cooler
layers to form frontal fog, figure 5.1.
warm air
cool air
Fog
Figure 5.1 Frontal Fog
(2) Fogs resulting from cooling.
(a) Radiation Fog or Ground Fog. Radiation fog is fog produced when
fairly calm moist air, which is in contact with the ground, is cooled to
saturation and then condensation by nighttime radiation. If air is completely
calm only dew or frost will form. Slight turbulence increases the depth of the
fog, but if it is violent enough it will prevent or dissipate the fog. Valleys are
particularly susceptible to radiation fog or frost. In polar regions fogs are
composed of ice crystals and are called ice fog. The process of formation of fog
by radiation is called a diabatic process, that is, it is a non-adiabatic process.
The cooling occurs by taking heat energy away from the air.
Another form of radiation fog occurs when direct radiation from the
moist air itself causes a cooling of the air down to saturation; further cooling
causes condensation and the formation of the fog.
(b) Advection Fog. Advection fog is fog that is formed when moist air is
transported over a cold surface. The cold surface causes the air in contact
with it to cool. If the air cools to saturation, condensation occurs and dew is
formed on the ground. Further cooling causes the air close to the surface to
also become saturated, and the motion of the air causes a mixing of the air
and causes the condensation to occur at higher levels above the surface until
the fog covers the entire area. Advection fog is especially common at sea.
Winds blowing onshore tend to carry the fog inland. The fog also occurs over
land when warm moist air is transported over snowy surfaces.
(c) Upslope Fog. Upslope fog is fog that forms when there is a gradual
orographic ascension of moist air up a sloping plain or hilly region. The moist
air will cool adiabatically to form upslope fog providing the air is already
close to saturation.
(d) Mixing Fog. Mixing fog is fog that occurs when warm moist air comes in
contact with cool moist air. The mixture may have a temperature low enough
to produce saturation and condensation, producing the mixing fog. Mixing fog
occurs at fronts between air masses of maritime origin. A special case of the
physical process associated with mixing fog occurs when you “see your
breath” on a cold day. The warm moist air expelled from your lungs mixes
with the colder air. For a moment, the mixture becomes saturated and you
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can see the condensation in the air coming from your mouth. Of course, this
process doesn’t last long because the mixed air mixes with even more drier
air until the air is no longer saturated and the water droplets from your
breath quickly evaporate.
5.3 The Formation of Clouds
A cloud is physically an aerosol, that is, a visible aggregate of minute water
droplets, ice crystals or a mixture of both suspended in the air. In order to form the
cloud, the water vapor in the air must condense into these minute water droplets.
As we saw previously, condensation of water vapor into water droplets can occur in
two ways: (1) by increasing the amount of water vapor into the air, and (2) by
cooling the air down to the dew point temperature. Since a cloud forms at a
significant height above the surface of the earth, there is no source of water
available to evaporate into the air at that level. Whatever moisture that is presently
in the air was obtained when water evaporated into the air when it was at the lower
level. Thus increasing the amount of water vapor into the air is not the main
technique for causing condensation for the formation of clouds. Therefore the only
practical way that the relative humidity can be increased to 100% is to cool the air
down to saturation.
Hence, the process that is responsible for the formation of clouds is the cooling
of the air down to saturation. There are two necessary conditions for the formation of
clouds. They are:
(a) A Cooling Mechanism.
(b) A Lifting Mechanism.
(a) A Cooling Mechanism.
The mechanism for the production of clouds is the adiabatic cooling of
rising air. An adiabatic process is a thermal process that occurs in which there is
no heat exchanged. Most heating or cooling processes that you are familiar with are
actually non-adiabatic processes. For example, if you wish to warm water, you put it
into a pot, place it on the stove and apply heat. The warming occurs because you
have applied heat. Similarly, if you wish to cool water, you put it into a glass and
place it in the refrigerator. The refrigerator uses electrical energy to remove the
heat from the water thereby cooling it. Both these processes are non-adiabatic,
because you either added heat or removed it during the process.
When a parcel of air rises into the atmosphere it finds it self at a new level.
However as we mentioned in chapter 1 the pressure of the air decreases with
height. Thus, the pressure of the air aloft is less than the pressure at the surface of
the earth. Consider the parcel of air as though it were a balloon. At the surface, the
air inside the balloon has a certain pressure and this pressure is exerted outward
against the wall of the balloon. The pressure of the atmospheric air outside the
balloon is pushing inward on the balloon, and an equilibrium condition exists
between the force pushing in on the wall of the balloon and the force pushing out.
When the balloon rises into the atmosphere the outside pressure becomes less but
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the inside pressure is the same. Therefore the balloon will expand until the
pressure inside the balloon is the same as the pressure outside the balloon and
equilibrium is again established. The air inside the balloon must do work in order
for the balloon to expand. It takes energy for the air to do that work, and that energy
comes from the internal energy of the air. But the internal energy of the air comes
from the motion of the air molecules and is directly proportional to the temperature
of the air. So if the internal energy of the air decreases, so does the temperature of
the air. Therefore, when the air expands it is cooled. Notice, however, that the
cooling has taken place without some external agent removing the energy, as in the
case of the glass of water in the refrigerator. Thus this cooling is an adiabatic
cooling.
Therefore the rising air expands and cools adiabatically. When the air is
cooled to the dew point temperature Td the air becomes saturated, the relative
humidity becomes 100%, and any further cooling will cause condensation. The tiny
water droplets formed in the air becomes the cloud.
(b) A Lifting Mechanism. As we have just seen, rising air can cool to the point
where condensation can began and cloud droplets can form. But how do we get
rising air? The air must be lifted by some mechanism so that it rises into the
atmosphere. The four lifting mechanisms for cloud formation are:
(1) Convection Normal heating of the ground by short-wave radiation from
the Sun causes the ground to warm up. The ground radiates long-wave
radiation that is absorbed by water vapor and carbon dioxide in the air. The
air at the surface of the earth is thus warmed. The warm air expands and
becomes lighter. The lighter air now rises by convection, and expands and
cools adiabatically. If the rising air cools to saturation, condensation occurs
and cloud droplets form. The usual type of cloud that is formed by convection
is the cumulus cloud.
(2) Convergence The convergence of wind currents or air masses causes a
lifting of the air. As pointed out in the quick survey of meteorology in chapter
1, air spirals into a low-pressure surface at the surface of the earth.
Where can all this air go? The only place for it to go is upward. Hence there
is vertical motion upward in a low-pressure area. If the rising air cools to
saturation, condensation occurs and cloud droplets form.
Convergence also occurs over a region like Florida. Air blows inward
from the Gulf of Mexico and the Atlantic Ocean. The air converges over
Florida. The only place for it to go is upward. If the rising air cools to
saturation, condensation occurs and cloud droplets form
(3) Frontal Lifting A front is a boundary between two different air masses.
When these two air masses collide the warmer air mass, being lighter, will
move up over the colder air mass. Hence, the front will cause lifting of the
air.
(4) Orographic Lifting Orographic lifting occurs when air pushes up
against a mountain barrier. There is no place for the air to go but upward.
Hence the air is forced to rise.
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5.4 Classification of Clouds
Of all the many varied clouds that are observable in the sky, they fall into
only two basic types of clouds. Those clouds associated with strong rising air
currents have vertical development and a puffy appearance and are called cumulus
clouds. Those resulting from gentler lifting tend to spread out into layers and are
called stratus clouds.
Clouds are primarily classified on the basis of their height into the following
scheme:
(1) High Clouds. High clouds are found at levels above 6000 m above the ground
and can extend up to the tropopause. High clouds belong to the family of Cirrus
clouds. These clouds are made up of ice crystals. The different types of high clouds
are:
(a) Cirrus. Cirrus clouds are nearly transparent, white, fibrous or silky.
Figure 5.2 Cirrus clouds
(b) Cirrocumulus. A cirriform layer, or patch of small white flakes arranged
in groups or lines. Sometimes they have the appearance of ripples, similar to
sand on a beach. (This is not a very common cloud.)
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Figure 5.3 Cirrocumulus clouds.
(c) Cirrostratus. A thin white veil of cirrus, nearly transparent (the sun,
moon and stars can be seen through them).
Cirrus clouds create “halos”. A halo is refraction of light by ice crystals.
Figure 5.4 Cirrostratus clouds.
(2) Middle Clouds. Middle clouds are found at levels between 2000 m and 6000 m
above the ground. Middle clouds belong to the family of Alto clouds. These clouds
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are made up of water droplets, ice crystals, or both. The different types of middle
clouds are:
(a) Altocumulus. An altocumulus cloud is in the form of layers or patches of
globular clouds. The clouds may build upward. From the ground, the clouds
often look very much like cirrocumulus clouds, but they are lower.
Figure 5.5 Altocumulus clouds.
(b) Altostratus. An altostratus cloud is defined as a fibrous veil of clouds
that is gray or blue gray. When the cloud becomes thick and rain starts to fall
it is called a nimbostratus cloud.
Figure 5.6 Altostratus clouds.
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(3) Low Clouds. Low clouds are found at levels just above the ground to 2000 m
above the ground. Low clouds all have the prefix stratus. These clouds are made up
of water droplets. The different types of low clouds are:
(a) Stratus. A stratus cloud is a low uniform layer of cloud resembling fog
but not resting on the ground. Because the thickness of the cloud is small,
precipitation, if it occurs, is light.
Figure 5.7 Stratus clouds.
(b) Stratocumulus. Stratocumulus clouds are a low, gray layer of clouds
composed of globular masses or rolls. They have the same appearance as
altocumulus cloud only they are lower.
Figure 5.8 Stratocumulus clouds
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(4) Clouds of Vertical Development. Clouds of vertical development are found at
levels from just above the ground and can extend all the way up to the tropopause.
These clouds are made up of water droplets and ice crystals. The different types of
clouds are:
(a) Cumulus. Cumulus clouds are dense, dome-shaped clouds that have flat
bases. Cumulus with little vertical development and a slightly flattened
appearance are usually associated with fair weather.
Figure 5.9 Cumulus clouds.
(b) Cumulonimbus. A cloud of great vertical development, towering to 18
km or more where they spread out to leeward and form an anvil of cirrus.
The cumulonimbus is the thunderstorm cloud that has heavy showers of
rain, snow, or hail, lightning and thunder.
(a) Side view. Cloud still building up. (Probably better called a Cumulus congestus
cloud at this stage of its development.)
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(b) When viewed from beneath the cloud, you cannot make out the detail of the
towering cloud.)
Figure 5.10 Cumulonimbus clouds.
5.5 Cloud Observations
Because of the obvious relation between clouds and weather, cloud observations are
an integral part of every weather observation. The items listed in every cloud
observation include:
(1) Cloud Types. A statement as to what types of clouds are present in the
atmosphere. That is, cirrus, cumulonimbus etc.
(2) Sky Cover. This is a statement of the fraction of the sky that is covered by
clouds. The categories are
(a) Clear. The sky is considered clear if the sky is completely clear or
contains less than 1/10 of clouds.
(b) Scattered or sometimes called partly cloudy. The sky is considered
scattered if the sky is covered by more than 1/10 of clouds but less than 6/10
of clouds.
(c) Broken or sometimes called cloudy. The sky is considered broken if
the sky is covered by more than 6/10 of clouds but less than 9/10 of clouds.
(d) Overcast. The sky is considered overcast if the sky is covered by more
than 9/10 of clouds.
(3) Cloud Height. The cloud height is the distance from the ground to the base of
the cloud. The ceiling is the distance from the ground to the lowest broken or
overcast cloud cover. To determine the distance from the ground to the base of the
cloud, the following techniques are used
(a) Ceiling Balloon. A weather balloon is released from the surface of the
earth and is observed as it rises into the atmosphere. The balloon rises at a
constant rate v in the atmosphere. Hence, by measuring the time t for the
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balloon to rise to the point where it enters a cloud, figure 5.11, the base of the
cloud is determined. That is, if the vertical velocity is v, then the height h
that the balloon will rise to in the time t is
h=vt
h=vt
Figure 5.11 Ceiling balloon.
The ceiling balloon is mostly used in the day time, but it can also be used at
night if it carries a light source. This is not a very good technique for very
high clouds. The balloon will move too far in the horizontal while it is rising
and can move out of the field of observation.
(b) Ceiling Light. A vertical light is projected onto the base of a cloud, figure
5.12. The angle θ is measured and the distance d is known. The height of the
base of the cloud is determined by trigonometry. That is,
tan  = h
d
Solving for the height h we get
h = d tanθ
h
θ
d
Figure 5.12 The ceiling light.
(c) Ceilometer. The ceilometer is an automatic ceiling light. The same
principle of the ceiling light is used but a photoelectric element which reacts
selectively to the light spot gives an automatic indication of the height.
(d) Pilot Reports. Pilots regularly radio information on the types and bases
of clouds back to the airport.
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(4) Cloud Direction. The motion of the clouds is usually determined by radar and
satellite observations.
5.6 Stability and Clouds
In the section 5.4 we saw the many different types of clouds that can form in
the atmosphere. Why does one type of cloud form rather than another? One of the
things that determines the type of cloud that will form is the stability of the
atmosphere. Let us, therefore, first try to understand the concept of stability. The
simplest way to understand the concept of stability is to visualize a ball placed in
the bottom of a bowl as seen in figure 5.13a. The bottom of the bowl is called the
(a) stable
(b) unstable
(c) neutral
Figure 5.13 The concept of stability.
equilibrium position. If the ball is given a slight push it will momentarily move
away from the bottom of the bowl but then the ball will roll back toward the bottom
of the bowl, the equilibrium position. The ball may oscillate a few times about this
equilibrium position but eventually it will come to rest at the bottom of the bowl.
The ball in the bottom of the bowl is said to be stable because when displaced from
the equilibrium position the ball always returns to its equilibrium position.
Now consider the ball placed at rest at the top of the inverted bowl, figure
5.13b. If the ball is given a slight push, the ball will now move away from the
equilibrium position and will keep on moving, never returning to the equilibrium
position. The ball at the top of the inverted bowl is said to be unstable because when
displaced from the equilibrium position the ball continues to move away from the
equilibrium position.
Now consider the ball placed on the level surface in figure 5.13c. If the ball is
given a slight push, the ball will move away from the equilibrium position. It will
not, however, return to the equilibrium position, as in stable motion, nor will it
continue in motion as in the case of unstable motion. The ball on the level surface is
said to be in neutral stability because when displaced from the equilibrium position
the ball neither continues to move away from the equilibrium position, nor returns to
the equilibrium, but stays at the place where it was displaced.
Let us now apply this concept of stability to the atmosphere. If atmospheric
air is stable, when displaced from its equilibrium position, the air will return to its
original position. If atmospheric air is unstable, when displaced from its
equilibrium position, the air will continue to move away from its original position. If
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atmospheric air is neutral, when displaced from its equilibrium position, the air will
remain at the new position.
How do we determine if the atmospheric air is stable? The stability of the
atmosphere is determined by the atmospheric lapse rate. As you recall, the
lapse rate is defined as the change in temperature ∆T with height ∆h, that is,
L = − T
h
Knowing that the atmosphere is heated by the absorption of long wave infrared
radiation from the surface of the earth, and the fact that we see permanent snow
caps on high mountains, even in the tropics indicates that the temperature of the
air decreases with altitude. The normal or average atmospheric lapse rate is
6.5 0C/km. This means that for each kilometer that we move upward into the
atmosphere the temperature will decrease by 6.5 0C. If the temperature at the
surface of the earth is 20 0C then the temperature at 1 km will have cooled to 20 0C
− 6.5 0C = 13.5 0C. At a height of 2 km the air temperature will now be 13.5 0C − 6.5
0C = 7.00 0C. Using the temperature lapse rate in this way we can find the
temperature at any height h. We should note that the normal lapse rate is an
average over time and space and the actual lapse rate at a particular time and place
will probably be different. A typical graph of the variation of temperature with
altitude is shown in figure 5.14.
h
4 km
3 km
2 km
1 km
0
10
20
T 0C
Figure 5.14 The variation of temperature T with altitude h for the normal lapse
rate.
Remember that when air rises, it cools adiabatically. The rate at which
unsaturated air cools as it rises is called the dry adiabatic lapse rate and is LDA
= 10 0C/km. The dry adiabatic lapse rate is shown in figure 5.15. Air will cool at
this rate as it rises and will warm at this rate if it subsides. Notice that the dry
adiabatic lapse rate (10 0C/km) is greater numerically than the normal lapse rate
(6.5 0C/km). But because the rate is negative, the slope of the line for the dry
adiabatic rate is not as steep as the line for the normal lapse rate.
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h
4 km
3 km
2 km
1 km
Normal rate
LDA
Dry Adiabatic Rate
0
10
20
T 0C
Figure 5.15 The dry adiabatic lapse rate.
If the air cools to saturation and condensation begins, then the heat of
condensation (Lv = 600 kcal/kg) is released into the air reducing the rate of cooling.
The new rate of cooling is called the wet or moist adiabatic lapse rate. It is the dry
adiabatic lapse rate modified by the heat of condensation. Hence, the saturated or
wet adiabatic rate of cooling is less than the dry adiabatic rate of cooling. The wet
adiabatic lapse rate varies between 5 0C/km and 9 0C/km depending upon the
amount of water vapor in the air. We will take the wet adiabatic lapse rate to be
LWA = 6 0C/km in all our examples. The wet adiabatic lapse rate is sometime called
the pseudoadiabatic lapse rate because some latent heat is added to the air as the
water vapor condenses. Figure 5.16 shows the wet and dry adiabatic lapse late on
the same graph.
h
4 km
Wet Adiabatic rate
3 km
2 km
1 km
LWA
LDA
Dry Adiabatic Rate
0
10
20
T 0C
Figure 5.16 The wet and dry adiabatic lapse rates.
An example of adiabatic cooling as air rises into the atmosphere is shown in
figure 5.17. Surface air is at a value of 30 0C. If it is lifted into the atmosphere it
cools at the dry adiabatic rate LDA = 10 0C/km. Hence, at a height of 1 km the
temperature of the air will be 30 0C − 10 0C = 20 0C. That is the air will have cooled
by 10 0C to a value of 20 0C at the height of 1 km. At 2 km it will have cooled, 20 0C
− 10 0C = 10 0C, and at 3 km we will have 10 0C − 10 0C = 0 0C. Let us assume that
the actual dew point temperature of the air is 0 0C at 3 km. When the air cools to
this temperature at 3 km, the air is completely saturated and condensation begins.
Now, if there is any further cooling, the air will cool at the wet adiabatic lapse rate
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of LWA = 6 0C/km. As the air rises another kilometer it now cools as 0 0C − 6 0C = − 6
0C, and then for the next kilometer, − 6 0C − 6 0C = − 12 0C. Figure 5.17 shows the
temperature of the rising air at each of these heights.
5 km
- 12oC
4 km
- 6oC
3 km
Condensation Level
0o C
2 km
10oC
1 km
20 C
Surface
30oC
Wet adiabatic
rate L = 6oC/km
Dry adiabatic
rate L = 10oC/km
o
Figure 5.17 Adiabatic cooling in the atmosphere.
An example of an absolutely stable atmosphere is shown in figure 5.18. Let
us assume that the actual lapse rate found in the atmosphere at this time and place
is 5 0C/km. This means that the actual temperature of the air at each kilometer is
Actual Lapse
rate
L = 5 oC / km
5 km 5 oC
- 12oC
4 km 10 oC
- 6oC
3 km 15oC
0o C
20 oC
10oC
1 km 25o C
20 C
2 km
Surface 30 oC
o
Wet adiabatic
rate L = 6oC/km
Condensation Level
Dry adiabatic
rate L = 10oC/km
30oC
Figure 5.18 An Absolutely stable atmosphere.
at 1 km 30 0C − 5 0C = 25 0C
at 2 km 25 0C − 5 0C = 20 0C
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at 3 km 20 0C − 5 0C = 15 0C
at 4 km 15 0C − 5 0C = 10 0C
at 5 km 10 0C − 5 0C = 5 0C
As the air starts to rise it cools at the dry adiabatic rate of LDA = 10 0C/km, until the
air becomes saturated at 3 km and thereafter it cools at the wet adiabatic rate of
LWA = 6 0C/km, as shown in figure 5.17 and now again in figure 5.18. Notice from
figure 5.18, that as the air rises and cools in the atmosphere, it is always cooler
than the air of the original atmosphere. Being cooler it is more dense than the
original air, and hence heavier. Therefore, if it can, it will fall back to the original
height it had in the atmosphere. This air is thus absolutely stable. The type of
clouds that are formed in stable air are the layered or stratus type of cloud, because
there is no tendency for the air to rise on its own. Instead it spreads out into layers.
When flying in an airplane in stable air, the ride will usually be very smooth due to
a lack of turbulence in the stable air.
The criteria for air to be absolutely stable is that the actual lapse rate be less
than the wet adiabatic lapse rate. That is,
L < LWA
Condition for absolute stability
That is, if the actual lapse rate is 5 or 4 or 3 0C/km etc., this lapse rate is less than
the wet adiabatic lapse rate of 6 0C/km, and hence the air is absolutely stable. Thus
if the air is lifted to any level in the atmosphere it will be cooler than the
surrounding air at that level and will fall back to its original level if possible.
An example of an absolutely unstable atmosphere is shown in figure 5.19. Let
us assume that the actual lapse rate found in the atmosphere at this time and
Actual Lapse
rate
L = 12 oC/ km
5 km -30oC
- 12oC
4 km -18oC
- 6oC
3 km -6 oC
0o C
6 oC
10oC
1 km 18 o C
20 C
2 km
Surface 30 oC
o
Wet adiabatic
rate L = 6oC/km
Condensation Level
Dry adiabatic
rate L = 10oC/km
30oC
Figure 5.19 An absolutely unstable atmosphere.
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place is 12 0C/km. This means that the actual temperature of the air at each
kilometer is
at 1 km 30 0C − 12 0C = 18 0C
at 2 km 18 0C − 12 0C = 6 0C
at 3 km 6 0C − 12 0C = − 6 0C
at 4 km −6 0C − 12 0C = −18 0C
at 5 km
−18 0C − 12 0C = −30 0C
As the air starts to rise it cools at the dry adiabatic rate of LDA = 10 0C/km,
until the air becomes saturated at 3 km and thereafter it cools at the wet adiabatic
rate of LWA = 6 0C/km, as shown in figure 5.17 and again in figure 5.19. Notice from
figure 5.19, that as the air rises and cools in the atmosphere, it is always warmer
than the air of the original atmosphere. Being warmer it is less dense than the
original air, and hence lighter. Therefore, it will continue to move upward in the
atmosphere. This air is thus absolutely unstable. The type of clouds that are formed
in unstable air are the cumulus or puffy building up type of cloud, because once
displaced from its original position, the air will continue to rise on its own. When
flying in an airplane in unstable air, the ride will be very bumpy due to a great deal
of turbulence in the unstable air.
The criteria for air to be absolutely unstable is that the actual lapse rate be
greater than the dry adiabatic lapse rate. That is,
L > LDA
Condition for absolute instability
As an example, if the actual lapse rate is 11 or 12 or 13 0C/km etc., this is greater
than the dry adiabatic lapse rate of 10 0C/km, and the air is absolutely unstable.
An example of a conditionally unstable atmosphere is shown in figure 5.20.
Actual Lapse
rate
L = 9 oC / km
5 km -15oC
- 12oC
4 km -6 oC
- 6oC
3 km
3 oC
0o C
2 km 12 oC
10oC
1 km 21 o C
20 C
Surface 30 oC
o
Unstable
Air
Wet adiabatic
rate L = 6oC/km
Condensation Level
Stable Air
Dry adiabatic
rate L = 10oC/km
30oC
Figure 5.20 A conditionally unstable atmosphere.
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As can be seen, as the air rises and cools in the atmosphere at the dry adiabatic rate
it is always cooler than the air of the original atmosphere. Being cooler it is more
dense than the original air, and hence heavier. Therefore, if it can it will fall back to
the original height it had in the atmosphere. This air is thus stable. If the air is
forced to continue to rise, condensation begins and the air will now cool at the wet
adiabatic rate, LWA = 6. Now the rate at which the rising air cools is less, and when
the air reaches the 4 km level it will have cooled to − 6 0C which is the same
temperature of the original air at the 4 km level. Hence at this level the air is said
to be neutral because if the cause of the lifting is removed the air will remain at this
4 km level. As the air continues to rise it becomes warmer than the original air and
now becomes unstable. Thus this air is conditionally unstable. The condition for the
air to be conditionally unstable is that the actual lapse rate fall between the wet
adiabatic lapse rate and the dry adiabatic lapse rate. That is ,
LWA < L < LDA
Requirement for conditional instability
Hence, if the actual lapse rate is 7 or 8 or 9 0C/km, this is greater than the wet
adiabatic lapse rate of 6 0C/km, and yet is less than the dry adiabatic lapse rate of
10 0C/km, and the air is conditionally unstable. For this example, if the air is only
lifted to levels below the 4 km level the air will be stable. But if it is lifted above the
4 km level the air will be unstable and will continue to rise up into the atmosphere.
5.7 Precipitation
Precipitation is defined as water in liquid or solid forms falling to the earth.
It is always preceded by condensation or sublimation or a combination of the two,
and is primarily associated with rising air. Fog, dew, or frost are not considered to
be precipitation because they do not fall from the sky. A cloud is an aerosol - a
suspension of minute water droplets or ice crystals. These water droplets or ice
crystals are held up by the rising air. In order for the water droplet or ice crystal to
fall from the cloud, the droplets or crystals must grow to sizes which can no longer
be held up by the rising air. The simplest process for the conversion of a cloud
droplet to a rain droplet is by coalescing with other cloud droplets until the size of
the droplet is so large that it can not be suspended by the rising air.
Precipitation is classified in two ways: (1) according to the form taken by the
falling water, and (2) on the basis of the process which leads to its formation.
I. Precipitation based upon the form taken by the falling water.
Rain is the most common form of precipitation. It falls from clouds that are
formed in rising air when the temperature, at least at the lower levels, is above the
freezing level of 0 0C. Raindrops may begin as snow but melt as they descend into
the warmer air below.
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Chapter 5 Fog, Clouds, and Precipitation
Snow is formed when the temperature is below 0 0C when the saturation
and condensation process occurs. Ice crystals are then formed. If these ice crystals
reach the ground we have snow.
Sleet (Ice Pellets) Sleet, or as it is now called, ice pellets, is rain which
freezes as it falls from a warmer layer of air aloft through a cold layer of air near
the surface, figure 5.21.
cold air
warm air
rain
sleet
Figure 5.21 The formation of sleet.
Freezing Rain Freezing rain is similar to sleet in that it is rain that falls
from warm air aloft through cold air below and freezes upon striking the cold
surfaces at the ground. The air itself is not cold enough to freeze the rain as in the
formation of sleet, but the surfaces are all below freezing and when the rain hits the
surface it freezes on contact. Freezing rain is very dangerous. Automobiles slide all
over the road when it occurs and many accidents occur. Many tree limbs and shrubs
are broken from the weight of the ice, figure 5.22. Freezing rain usually presages a
warmer mass of air that moves in behind a warm front.
Figure 5.22 The effects of freezing rain.
Drizzle Drizzle are minute droplets of water that fall so slowly that they
seen to float in the air following the slightest movement of the air. Drizzle falls
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Chapter 5 Fog, Clouds, and Precipitation
continuously from low stratus clouds. It is often accompanied by fog and poor
visibility.
Freezing Drizzle Freezing drizzle is drizzle that freezes when it strikes the
cold surfaces at the ground. The formation of freezing drizzle is similar to the
formation of freezing rain except that the size of the drizzle particle is much smaller
than the rain particle.
Hail Hail is a form of precipitation that falls from violent summer
thunderstorms. It starts as a rain drop in the convective cloud but is carried to
higher levels by the convective currents. As the rain drop passes the freezing level,
it freezes. As the size of the ice crystal increases, it falls to lower altitudes below the
freezing level where water drops now form all over the ice crystal. It is again caught
in a convective current and is again carried to higher levels above the freezing level
where the water on the ice crystal freezes again. Repeated rising and falling of the
hailstone occurs until the hailstone becomes so large that it falls to the earth. The
largest hailstone ever recorded had a mass of 1 kilogram.
Snow Pellets Snow pellets are small opaque balls of compacted snow
crystals that are white in appearance but they do not have the ice coat found on a
hailstone. Snow pellets are usually found in convective storms of winter and spring.
II. Precipitation based upon the processes which lead to its
formation.
(a) Convectional Precipitation Convectional precipitation is precipitation
that results from convective overturning of moist air and is found mostly in
thunderstorm type clouds. The precipitation is usually heavy and showery and
consists usually of either rain, snow showers, hail, or snow pellets.
(b) Oragraphic Precipitation Oragraphic precipitation is precipitation
that forms when air rises and cools as it tries to rise over a topographic barrier such
as a mountain. An example of a location that receives large amounts of oragraphic
precipitation is Cherrapunji, India. During the monsoon season moist air from the
Indian Ocean moves over India heading north. As the air tries to rise up and over
the Himalayas mountains it cools to saturation, then condensation, and then
eventually rain occurs. The mean annual amount of precipitation throughout the
world is about 86 cm (34 inches). The average rainfall in Cherrapunji is 1144 cm,
and the record is 2299 cm in 1861.
(c) Frontal precipitation Frontal precipitation is precipitation that forms when
air currents converge and rise over a frontal surface. In warm fronts, figure 5.23,
the frontal convergence is characterized by the more gradual sloping ascent of
warm air over cooler air. Usually the air is more stable and hence layered type
clouds usually form and the type of precipitation is usually steady rather than
showery.
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Chapter 5 Fog, Clouds, and Precipitation
Warm front
warm air
cool air
rain
Figure 5.23 Warm front precipitation.
In cold fronts, figure 5.24, the front is steep and the warm moist air is
pushed up with greater vertical velocity and the clouds tend to be more of the
cumulus type. Usually the air is a little more unstable and hence the form of the
precipitation is usually of the showery convectional type. If the cold front is fast
moving a squall line may develop out in advance of the front.
cold front
cool air
warm air
rain
Figure 5.24 Cold front precipitation.
The Language of Meteorology
1. Fog - results when atmospheric water vapor condenses (or sublimes) to
the extent that the new forms, water droplets or ice crystals, become visible and
have their base in contact with the ground.
Fogs resulting from evaporation.
(a) Steam Fog - fog that is produced by intense evaporation of water into
relatively cold air. Saturation occurs, then condensation, then fog.
(b) Frontal Fog - fog found along the boundary of two air masses.
Evaporation from warm rain falling through the drier air below may be
followed by saturation and condensation in cooler layers to form frontal fog,
Fogs resulting from cooling.
(a) Radiation Fog or Ground Fog - fog produced when fairly calm moist
air, which is in contact with the ground, is cooled to saturation and then
condensation by nighttime radiation.
(b) Advection Fog - fog that is formed when moist air is transported over a
cold surface. The cold surface causes the air in contact with it to cool and the
air close to the surface becomes saturated, and the fog forms.
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Chapter 5 Fog, Clouds, and Precipitation
(c) Upslope Fog - fog that forms when there is a gradual orographic
ascension of moist air up a sloping plain or hilly region. The moist air will
cool adiabatically to form upslope fog providing the air is already close to
saturation.
(d) Mixing Fog. Mixing fog is fog that occurs when warm moist air comes in
contact with cool moist air.
A cloud is physically an aerosol, that is, a visible aggregate of minute water
droplets, ice crystals or a mixture of both suspended in the air. The process that is
responsible for the formation of clouds is the cooling of the air down to saturation.
The two necessary conditions for the formation of clouds are:
(a) A Cooling Mechanism.
The mechanism for the production of clouds is the adiabatic cooling
of rising air. An adiabatic process is a thermal process that occurs in which
there is no heat exchanged. Therefore the rising air expands and cools
adiabatically. When the air is cooled to the dew point temperature the air
becomes saturated, the relative humidity becomes 100%, and any further
cooling will cause condensation. The tiny water droplets formed in the air
becomes the cloud.
(b) A Lifting Mechanism. Rising air can cool to the point where
condensation can began and cloud droplets can form. The four lifting
mechanisms for cloud formation are:
(1) Convection Air at the surface of the earth is warmed. The warm air
expands and rises by convection, and cools adiabatically. If the rising air
cools to saturation, condensation occurs and cloud droplets form. The usual
type of cloud that is formed by convection is the cumulus cloud.
(2) Convergence The convergence of wind currents or air masses causes a
lifting of the air. Air spirals into a low-pressure surface at the surface of the
earth. The only place for this air to go is upward. Hence there is vertical
motion upward in a low-pressure area. If the rising air cools to saturation,
condensation occurs and cloud droplets form.
Convergence also occurs over a region like Florida. Air blows inward
from the Gulf of Mexico and the Atlantic Ocean. The air converges over
Florida. The only place for it to go is upward. If the rising air cools to
saturation, condensation occurs and cloud droplets form
(3) Frontal Lifting A front is a boundary between two different air masses.
When these two air masses collide the warmer air mass, being lighter, will
move up over the colder air mass. Hence, the front will cause lifting of the
air.
(4) Orographic Lifting Orographic lifting occurs when air pushes up
against a mountain barrier. There is no place for the air to go but upward.
Hence the air is forced to rise.
Types of Clouds - clouds associated with strong rising air currents have
vertical development and a puffy appearance and are called cumulus clouds.
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Chapter 5 Fog, Clouds, and Precipitation
Those resulting from gentler lifting tend to spread out into layers and are called
stratus clouds.
Clouds are primarily classified on the basis of their height into the following
catagories:
(1) High Clouds. High clouds are found at levels above 6000 m above the ground
and can extend up to the tropopause. High clouds belong to the family of Cirrus
clouds. These clouds are made up of ice crystals. The different types of high clouds
are:
(a) Cirrus. Cirrus clouds are nearly transparent, white, fibrous or silky
(b) Cirrocumulus. A cirriform layer, or patch of small white flakes
arranged in groups or lines. Sometimes they have the appearance of ripples, similar
to sand on a beach.
(c) Cirrostratus. A thin white veil of cirrus, nearly transparent (the sun,
moon and stars can be seen through them).
(2) Middle Clouds. Middle clouds are found at levels between 2000 m and 6000 m
above the ground. Middle clouds belong to the family of Alto clouds. These clouds
are made up of water droplets, ice crystals, or both. The different types of middle
clouds are:
(a) Altocumulus. An altocumulus cloud is in the form of layers or patches of
globular clouds. The clouds may build upward. From the ground, the clouds often
look very much like cirrocumulus clouds, but they are lower
(b) Altostratus. An altostratus cloud is defined as a fibrous veil of clouds
that is gray or blue gray. When the cloud becomes thick and rain starts to fall it is
called a nimbostratus cloud.
(3) Low Clouds. Low clouds are found at levels just above the ground to 2000 m
above the ground. Low clouds all have the prefix stratus. These clouds are made up
of water droplets. The different types of low clouds are:
(a) Stratus. A stratus cloud is a low uniform layer of cloud resembling fog
but not resting on the ground. Because the thickness of the cloud is small,
precipitation, if it occurs, is light
(b) Stratocumulus. Stratocumulus clouds are a low, gray layer of clouds
composed of globular masses or rolls. They have the same appearance as
altocumulus cloud only they are lower.
(4) Clouds of Vertical Development. Clouds of vertical development are found at
levels from just above the ground and can extend all the way up to the tropopause.
These clouds are made up of water droplets and ice crystals. The different types of
clouds are:
(a) Cumulus. Cumulus clouds are dense, dome-shaped clouds that have flat
bases. Cumulus with little vertical development and a slightly flattened appearance
are usually associated with fair weather.
(b) Cumulonimbus. A cloud of great vertical development, towering to 18
km or more where they spread out to leeward and form an anvil of cirrus. The
cumulonimbus is the thunderstorm cloud that has heavy showers of rain, snow, or
hail, lightning and thunder.
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Chapter 5 Clouds and Fog
Sky Cover. This is a statement of the fraction of the sky that is covered by clouds.
The categories are
(a) Clear. The sky is considered clear if the sky is completely clear or
contains less than 1/10 of clouds.
(b) Scattered or sometimes called partly cloudy. The sky is considered
scattered if the sky is covered by more than 1/10 of clouds but less than 6/10
of clouds.
(c) Broken or sometimes called cloudy. The sky is considered broken if
the sky is covered by more than 6/10 of clouds but less than 9/10 of clouds.
(d) Overcast. The sky is considered overcast if the sky is covered by more
than 9/10 of clouds.
Cloud Height. The cloud height is the distance from the ground to the base of the
cloud. The ceiling is the distance from the ground to the lowest broken or overcast
cloud cover.
Atmospheric stability. If atmospheric air is stable, then when it is displaced
from its equilibrium position, the air will return to its original position. If
atmospheric air is unstable, when displaced from its equilibrium position, the air
will continue to move away from its original position. If atmospheric air is neutral,
when displaced from its equilibrium position, the air will remain at the new
position.
Precipitation is defined as water in liquid or solid forms falling to the earth.
Rain is the most common form of precipitation. It falls from clouds that are
formed in rising air when the temperature, at least at the lower levels, is above the
freezing level of 0 0C. Raindrops may begin as snow but melt as they descend into
the warmer air below.
Snow is formed when the temperature is below 0 0C when the saturation
and condensation process occurs. Ice crystals are then formed. If these ice crystals
reach the ground we have snow.
Sleet (Ice Pellets) Sleet, or as it is now called, ice pellets, is rain which
freezes as it falls from a warmer layer of air aloft through a cold layer of air near
the surface.
Freezing Rain - Rain that falls from warm air aloft through cold air below
and freezes upon striking the cold surfaces at the ground.
Drizzle Drizzle are minute droplets of water that fall so slowly that they
seen to float in the air following the slightest movement of the air. Drizzle falls
continuously from low stratus clouds. It is often accompanied by fog and poor
visibility.
Freezing Drizzle Freezing drizzle is drizzle that freezes when it strikes the
cold surfaces at the ground.
Hail Hail is a form of precipitation that falls from violent summer
thunderstorms. It starts as a rain drop in the convective cloud but is carried to
higher levels by the convective currents where it freezes. Repeated rising and
falling of the hailstone occurs until the hailstone becomes so large that it falls to the
earth.
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Chapter 5 Clouds and Fog
Questions for Chapter 5
1. On a very clear night, radiation fog can develop if there is sufficient
moisture in the air. Explain.
2. Discuss the concept of stability and how it applies to the weather in the
atmosphere.
3. If you are on the ground without any weather instruments and you see
some clouds that might be stratus clouds or alto stratus clouds. Is there any way
that you can really tell the difference.
4. If you are on the ground and you see some clouds that are building up very
rapidly. Can you guess what type of clouds they are and what type of weather you
are probably going to get.
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