The circles and arrows represent
the movement of water molecules.
Temperature = 5C
Temperature = 15C
Figure 4.1 When the air above the water is saturated, the number of molecules
condensing equals the number of molecules evaporating from the water. The
number of molecules leaving the water increases with increasing temperature.
What will happen to the water in the beaker if it is uncovered? (Use other
techniques to demonstrate motion other than arrows. Replace circles with a
symobl that looks more like a water molecule as in Figure 4.28)
Height above the surface (km)
10
8
Tropical
Midlatitude summer
Artic w inter
6
4
2
0
0
2
4
6
8
10
12
14
16
18
20
Mixing ratio (g/kg)
Figure 4.2 The source of atmospheric water vapor comes from the surface of the Earth.
Explaining why values of mixing ratio typically decrease with altitude at different
regions of the world.
Figure 4.3 When the temperature of the web cooled to the dew point temperature, dew
formed making the web visible.
Figure 4.4 Frost, ice crystal formed by deposition of water vapor on subfreezing surfaces,
will form in open fields before before forming under a tree. Notice also the steam
fog in this picture. (Might want to label the whitish grass as FROST)
The circles represent
individual water molecules.
Figure 4.5 The smaller the drop the more curved the surface, reducing the number of
neighbors for each water molecule at the surface. This curvature effect makes it
easier for small drops to evaporate.
Figure 4.6 The mean annual number of foggy days with visibility less than 300 m over a
poriton of the United States (after Court and Gerston, 1966). Do you know
someone who lives in a foggy region of the United States?
Figure 4.7 Radiation fog often forms in low lying areas, particularly river valleys. This is
a satellite visible image of a radiation fog that occurred in many river valleys
through the Appalacian mountains.
Figure 4.8 Advection fog is common off the coast of California as warm moist air over
the Pacific is advected over the cold coastal waters. (Photograph courtesey of
NOAA).
Figure 4.9 Steam fog often occurs when cold air moves over warm water.
Orographic lifting
Convergence of air at the surface
solar heating
warm
air
cold air
Convection
cool air
Frontal lifting
Figure 4.10 Four mechanisms that cause air to ascend and form a cloud. (Add clouds, see
Ahrens Essential Figure 5.8)
Cirrus
Cirroc um ulus
Cum ulonim bus
Cirrostratus
Altoc um ulus
Altostratus
Nim bostratus
Stratus
Stratoc um ulus Cum ulus
Figure 4.11 The ten major cloud catagories. (Need a drawing of cloud types with
arrows point to pictures.
Figure 4.12 Stratus clouds (St) are low altitdue clouds in layers. (I have permission
forms and original prints of figures 4.12-4.21, or we can get new ones. Link
these figures to Figure 4.11)
Figure 4.13 Stratocumulus (Sc) are low-lying clouds. Stratocumulus are distinguished
from stratus by variations in color across the sky.
Figure 4.14 Nimbostratus (Ns) are deep clouds that bring precipitation and appear dark
gray to pale blue in color.
Figure 4.15 Cumulus clouds are often observed on a summer day.
Figure 4.16 Cumulonimbus clouds are thunderstorm clouds.
Figure 4.17 Altostratus clouds (As) are layered clouds that exist in the middle layers of
the troposphere.
Figure 4.18 Altocumulus (Ac) occur in the middle levels of the atmosphere when the air
is moist.
Figure 4.19 Cirrocumulus (Cc) are thin, white clouds that appear high in the troposphere.
Figure 4.20 Cirrostratus (Cs) are layered clouds that are sometimes observed with a halo.
Figure 4.21 Cirrus (Ci) are wispy, fibrous, white clouds that are composed of ice crystals.
Plate
Needle
Column
Flat plate
Dendrite
Figure 4.22 The five basic ice crystal habits are plate, column, dendrite, flat plate and
needle. Which shape an ice particle grows as depends on the temperature of the
environment it grows. (Need better drafted figures and temperature ranges
added.)
Figure 4.23 In the solar spectrum clouds tend to cool Earth while in the longwave
radiation they tend to warm the planet. (Need a figure that shows clouds
reflecting solar radiation back to space and reducing longwave radiation to
space, something like Lutgen's figure 3-11 page 57)
Figure 4.24 Large water drops fall faster than smaller ones. Because of the differenth fall
speeds, water drops collide and coalesce.
Figure 4.25 Ice crystals of different sizes or different shapes may collide and stick
together. (Need a similar figure to 4.24 but for ice crystals)
The circles and arrows represent
the movement of water molecules.
Ice
Super-cooled
water
Temperature = -5C
Figure 4.26 At a given temperature, the saturation vapor pressure over ice is less than the
saturation vapor pressure over water. Water molecules in the ice phase are more
strongly bonded to each other than when in the liquid phase. More energy is
required for the water molecules to escape from the ice phase. (Use other
techniques to demonstrate motion other than arrows. Replace circles with a
symobl that looks more like a water molecule as in Figure 4.28)
Saturation vapor pressure (mb)
1400
6
1200
5
Over water
Over ice
4
1000
3
2
1
800
0
-20
600
-15
-10
-5
Temperature (C)
0
400
200
0
-10
Relative humdity
less than 100%
Relative humdity > 100%
0
10
20
30
40
50
60
70
80
90 100 110
Temperature (C)
Figure 4.27 The relationship between temperature and the saturation vapor pressure over
water. Conditions along the line represent a relative humidity of 100%. The inset
depicts the differences between the saturation vapor pressure over ice and over
water. Since the line for water lies above the line representing ice, when the air is
saturated (relative humidity equals 100%) with respect to liquid water, it is
supersaturated with respect to ice.
Movement of water vapor molecules
Figure 4.28 The larger saturation vapor pressure over a liquid water surface than over an
ice surfaces causes the ice crystal to grow and the super-cooled drops to
evaporate. (Show sequence with water drop getting smaller and ice crystal
getting larger, see Arhens Essential figure 5.19. Should also use a dendrite
shape as Ahrens did.)
Figure 4.29 Virga is rain that falls out of the cloud but evaporates before reaching the
ground.
RAIN
SNOW
Cloud base
Cloud base
Temperature
below freezing
Temperature
below freezing
Temperature
above freezing
SNOW
Rain
Cloud base
Temperature
below freezing
Temperature
below freezing
Cloud base
Temperature
below freezing
Temperature
below freezing
Temperature Ice Pellets
below freezing (Sleet)
Rain freezes on contact
FREEZING RAIN
SLEET
Figure 4.30 The vertical variation of temperature will determine if precipitation falls as
rain, freezing rain, sleet or snow. In this example, all the precipitation particles are
initially ice crystals. Particle that fall into the melting layer become liquid drops.
To form freezing rain or sleet a surface temperature in version is required. (Add
more particles and indicate that they are falling.)
Figure 4.31 Freezing rain, or an ice storm, occurs when there is a shallow temperature
inversion near the ground. Freezing rain covers everything in ice. Making a
beautiful, though often treacherious traveling conditions. (Photograph from
NOAA).
Figure 4.32 Hailstones are the largest form of precipitation.
Figure 4.33 Rime forms when a super-cooled fog is present and surface temperatures are
below freezing. In this example, rime formed on a spider web.
Figure 4.34 As moist air flows over a mountain, clouds form and precipitation falls on the
upwind side. Sink motions on the leeward side of the mountain generate a
rainshadow. (Need to include parcel with temperatures at 1, 2 and 3 km on
each side of the mountain.)