1. Moisture, Clouds, & Precipitation
Chapter 18
200

2. Water in the Atmosphere ch. 18.1
When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere.

3. Water’s Changes of State
Molecules are ALWAYS in motion!
Only substance that commonly exists in all 3 states (solid, liquid, gas) Solid (ice) 0° C & below
Liquid between 0° C & 100° C Gas (water vapor = invisible gas) above 100° C  Can’t see, but sometimes can feel (“humidity”)
Water often changes state in the atmosphere
By absorbing energy (heating) Ex. evaporation Liquid  gas (vapor) Ex. melting Solid  liquid Dew, fog, clouds
By giving off energy (cooling) Ex. condensation Gas (vapor)  liquid Dew, fog, clouds
Ex. freezing Liquid  solid snow, hail, frost

4. Humidity What is humidity?
general term for amount of water vapor in the air
There are two “types” of humidity:
absolute (specific) humidity
relative humidity
(1 kg)
(g)
1 L of water = 1 kg or 1 mL of water = 1 g

5. Humidity What is water vapor capacity?
amount of water air can “hold” at a given temperature
How does temperature affect the amount of water vapor needed to saturate the air?
Warm air can hold more water than cold air.
amount of water vapor needed to saturate the air = water vapor capacity… Think about it like the size of the container
Warmer air = bigger container; colder air = smaller container
How does temperature affect the amount of water air can hold?
Warmer air can hold more water (has a higher maximum capacity); colder air holds less water (has a lower maximum capacity)

6. What is the absolute humidity of this 1 kg parcel of air?
What is specific (absolute) humidity?
the actual amount of water vapor in the air (for a given time & place)
How do we express it?
specific humidity = mass of H2O vapor (g)
1 kg of air
(1 kg)
(g)
What is specific (absolute) humidity? the actual amount of water vapor in the air (for a given time & place)
How do we express it? Specific Humidity = Mass of water vapor (grams)
1 kg of air
Water Example = 15 g H20 / 1 kg of air
1 L of water = 1 kg
1 mL of water = 1 g
What is the absolute humidity of this 1 kg parcel of air?
15 g H20

7. Humidity What is relative humidity (RH)?
ratio of how close the air is to maximum capacity (How “full” of water vapor the air is.)
How do we express it?
relative humidity = specific humidity X 100
maximum capacity
Relative humidity (RH): how close the air is to maximum capacity (being “full” of water vapor) (for a given time & place)
Compares actual amount of water vapor in air with maximum amount (capacity) it can hold (at that temp. & pressure)
Expressed as a %:
To calculate: Relative Humidity = Specific Humidity X 100
Maximum capacity
Saturated (“full”) air RH = 100%
What is the special name for the which this occurs? Dew point temp
Air that contains NO water vapor  RH = 0%
Relative humidity can be changed in two ways. First, it can be changed by adding or removing water vapor. In nature, moisture is added to air mainly by evaporation from the oceans and smaller bodies of water.
RH = ?
100%

8. 50% 50% Humidity What is the relative humidity of each container? 50%
How can they be the same????
b/c both are ½ full to capacity
50%
50% for both b/c both are ½ full to capacity
50%

9. Humidity Relative humidity can be changed in two ways.
adding or removing water vapor
changing the air temperature

10. Relative Humidity & Temperature
How does temp. affect relative humidity?
As air temp goes down, RH goes up.
Why?
b/c cold air can hold less water than warm
As temp goes up, RH goes down.
b/c warm air can hold more water than cold
How does temperature affect the relative humidity? As temp goes down, RH goes up (b/c cold air can hold less water… lower maximum capacity… & therefore air is “more full” of water for that new temp…) As temp goes up, RH goes down (b/c warm air can hold more water… higher maximum capacity… & therefore air is “less full” of water for that new temp…)
Think about it as if: When water-vapor content stays the same… & air gets warmer, beaker gets bigger… so w/ same amount of water, it’s now less full… so, lower RH
air gets cooler, beaker gets smaller… so w/ same amount of water, it’s now more full… so, higher RH

11. Humidity When air has 100% relative humidity it is also called…
saturated
& air is “full” of water vapor (has reached capacity)
Saturated (“full”) air  RH = 100%
What is the special name for the which this occurs? Dew point temp

12. Humidity
What is the special name for the temp. at which saturation (100% RH) occurs?
dew point (temperature)
What happens at the dew point temp?
condensation (& precipitation)
gas  liquid
What happens if any more water evaporates (into saturated air)? an equal amount will condense… dynamic equilibrium (rate of condensation = rate of evaporation)… think of if like trying to add more water to a full bucket… any additional amount added, will also spill out…
Ex. Water drops on inside of lid of a container or outside of a glass
The dew-point temperature or simply the dew point is the temperature to which a parcel of air would need to be cooled to reach saturation. If the same air was cooled further, the air’s excess water vapor would condense, typically as dew, fog, or clouds.

13. Saturation & Temperature
How does temp. affect saturation?
It takes more water to saturate warm air & less to saturate cold air.
Why?
b/c warm air can hold more water & cold air can hold less.
How does temperature affect saturation? Amount of water vapor in saturated air depends on the temperature of the air
The warmer the air, the more water vapor it can hold.
~doubles for every 11°C rise in temp

14. What information can we gather from these figures?
Warmer air has greater capacity…
If amount of water stays the same, but temp changes, so does RH (temp goes up, increases capacity, RH decreases…. Temp goes down, decreases capacity, RH increases)

15. Measuring Relative Humidity
Using a sling psychrometer:
1. Wet the wick on the wet bulb.
2. Spin the psychrometer for the specified amount of time.
3. Read temperatures on both the wet & dry bulbs.
The wet bulb temp. should always be equal or cooler than the dry bulb temp. due to the evaporation of the water.
4. Record your data.
2 thermometers
“wet bulb” (bulb covered with water soaked cloth/wick) The water will evaporate off it.
Evaporation is a cooling process, so… this thermometer will have a lower temp.
2. “dry bulb” (regular thermometer) The dry bulb is measuring the ambient air temp. (temp in the room/outside).
The drier the air, the greater the cooling from evaporation, and the greater the difference in wet & dry bulb temperatures will be
If wet & dry bulb temps = air is saturated (100% RH)

16. Measuring Relative Humidity
Using the RH chart
1. Calculate difference between wet & dry bulb temperatures
2. Locate difference on the (top of) RH chart
3. Locate dry bulb temp. on the (left of) RH chart
4. Where they intersect gives the RH (in %)
Interactive Relative Humidity Chart
2 thermometers
“wet bulb” (bulb covered with water soaked cloth/wick) The water will evaporate off it.
Evaporation is a cooling process, so… this thermometer will have a lower temp.
2. “dry bulb” (regular thermometer) The dry bulb is measuring the ambient air temp. (temp in the room/outside).
The drier the air, the greater the cooling from evaporation, and the greater the difference in wet & dry bulb temperatures will be
If wet & dry bulb temps = air is saturated (100% RH)

17. Example: What is the RH?
If the wet bulb temp. = 16°C & dry bulb temp. = 26°C, what is the Relative Humidity?
34%
Find dry bulb
Find difference between dry & wet bulb (wet should be lower)
Read chart where lines for both numbers intersect
Interactive Relative Humidity Chart

18. Determining Dew Point Temperature
Same method as determining relative humidity, except use dew point chart
2 thermometers
“wet bulb” (bulb covered with water soaked cloth/wick) The water will evaporate off it.
Evaporation is a cooling process, so… this thermometer will have a lower temp.
2. “dry bulb” (regular thermometer) The dry bulb is measuring the ambient air temp. (temp in the room/outside).
The drier the air, the greater the cooling from evaporation, and the greater the difference in wet & dry bulb temperatures will be
If wet & dry bulb temps = air is saturated (100% RH)… this temp is also the dew point temp
Interactive Dew Point Chart

19. Example: What is the Dew Point?
If the wet bulb temp. = 16°C & dry bulb temp. = 26°C, what is the dew point?
9° Celsius
Find dry bulb
Find difference between dry & wet bulb (wet should be lower)
Read chart where lines for both numbers intersect
Interactive Dew Point Chart

20. Practice Determining Dew Point & Relative Humidity Using Psychrometer Readings
Dry Bulb Temp.
Wet Bulb Temp.
Diff. Between
Dew point Temp. in °C
% Relative Humidity
12 °C
EX.
24 C °C
20 C °C 7
49%
Find dry bulb
Find difference between dry & wet bulb (wet should be lower)
Read chart where lines for both numbers intersect 4
14 °C
66%

21. Cloud Formation ch. 18 sec. 2
Where, in the atmosphere, can clouds form?
anywhere in troposphere
Why?
b/c only layer w/ water vapor
What are clouds made of?
water (liquid or ice)
condensation nuclei
What are they made of? Condensation nuclei & water droplets (if temp above freezing)
Condensation nuclei & snow crystals & super-cooled water (below 0°C, but not frozen) (if temp below freezing)
If temp is below -20°C mostly snow & ice crystals

22. Cloud Formation How do clouds form? warm ground heats air above it
air rises & cools until reaches dew point & condenses (gas  liquid) or undergoes deposition (gas  solid)
cloud forms (LIQUID or solid water NOT gas)
What is the name for the atmospheric level where condensation occurs?
condensation level
Temperature changes that happen even though heat isn’t added or subtracted are called adiabatic temperature changes. They result when air is compressed or allowed to expand. When air is allowed to expand, it cools, and when it is compressed, it warms.
As you travel from Earth’s surface upward through the atmosphere, the atmospheric pressure decreases. This happens because there are fewer and fewer gas molecules. Any time a volume of air moves upward, it passes through regions of successively lower pressure. As a result, the ascending air expands and cools. Unsaturated air cools at the constant rate of 10°C for every 1000 meters of ascent. In contrast, descending air encounters higher pressures, compresses, and is heated 10°C for every 1000 meters it moves downward. This rate of cooling or heating applies only to unsaturated air and is called the dry adiabatic rate.
condensation
level

23. Processes that Lift Air
What is the orographic effect?
rainfall that results from the “lifting” of air over mountains
different effects on windward & leeward sides of mountain
windward  moist air forced over mountain & rises… the air expands and cools  precipitation
leeward  air is now dry & cool at top  air sinks & warms  less rain/cloud cover = “rain shadow desert”
Moist air blows toward mountain range on windward side of mtn (Think… wind is blowing toward mountain)
warm, moist air rises… as rises molecules spread out & air cools… once air reaches dew point temp, clouds form & rain occurs
Air that goes over the barrier (peak of the mtn) to the leeward side of mountain (Think… wind is leaving
dry & cool air sinks & warms up (due to molecules getting closer together b/c more pressure) (& can hold more moisture before reaching dew point)
“rain shadow” desert results b/c little humidity & harder to reach dew point
When elevated terrains, such as mountains, act as barriers to air flow, orographic lifting of air occurs. As air goes up a mountain slope, adiabatic cooling often generates clouds and precipitation. Many of the rainiest places on Earth are located on these windward mountain slopes. By the time air reaches the leeward side of a mountain, much of its moisture has been lost. If the air descends, it warms adiabatically. This makes condensation and precipitation even less likely. A rain shadow desert can occur on the leeward side of the mountain.
Internet Investigation ES1806 Which Way Does the Wind Blow?

24. Orographic Lifting & the Orographic Effect
Moist air blows toward mountain range on windward side of mtn (Think… wind is blowing toward mountain)
warm, moist air rises… as rises molecules spread out & air cools… once air reaches dew point temp, clouds form & rain occurs
Air that goes over the barrier (peak of the mtn) to the leeward side of mountain (Think… wind is leaving
dry & cool air sinks & warms up (due to molecules getting closer together b/c more pressure) (& can hold more moisture before reaching dew point)
“rain shadow” desert results b/c little humidity & harder to reach dew point
When elevated terrains, such as mountains, act as barriers to air flow, orographic lifting of air occurs. As air goes up a mountain slope, adiabatic cooling often generates clouds and precipitation. Many of the rainiest places on Earth are located on these windward mountain slopes. By the time air reaches the leeward side of a mountain, much of its moisture has been lost. If the air descends, it warms adiabatically. This makes condensation and precipitation even less likely. A rain shadow desert can occur on the leeward side of the mountain.

25. The Wind Blew Over the Mountain Tune: For He's a Jolly Good Fellow Written By: Unknown/Copyright Unknown
The wind blew over the mountain, The wind blew over the mountain, The wind blew over the mountain, And it was wet on the windward side
wet on the windward side wet on the windward side The leeward side of the mountain, The leeward side of the mountain, The leeward side of the mountain, Was as dry as it could be Was as dry as it could be, Was as dry as it could be, The leeward side of the mountain, Was as dry as it could be!
Moist air blows toward mountain range on windward side of mtn (Think… wind is blowing toward mountain)
warm, moist air rises… as rises molecules spread out & air cools… once air reaches dew point temp, clouds form & rain occurs
Air that goes over the barrier (peak of the mtn) to the leeward side of mountain (Think… wind is leaving
dry & cool air sinks & warms up (due to molecules getting closer together b/c more pressure) (& can hold more moisture before reaching dew point)
“rain shadow” desert results b/c little humidity & harder to reach dew point
When elevated terrains, such as mountains, act as barriers to air flow, orographic lifting of air occurs. As air goes up a mountain slope, adiabatic cooling often generates clouds and precipitation. Many of the rainiest places on Earth are located on these windward mountain slopes. By the time air reaches the leeward side of a mountain, much of its moisture has been lost. If the air descends, it warms adiabatically. This makes condensation and precipitation even less likely. A rain shadow desert can occur on the leeward side of the mountain.

26. Processes that Lift Air
frontal wedging
masses of warm & cold air collide producing a front
warmer, less dense air rises over cooler, more dense air
In central North America, masses of warm air and cold air collide, producing a front. Here the cooler, denser air acts as a barrier over which the warmer, less dense air rises.  called frontal wedging

27. Processes that Lift Air
convergence
air flows horizontally, collides, & gets pushed upward
whenever air in the lower atmosphere flows together horizontally, lifting results. This is called convergence. When air flows in from more than one direction, it must go somewhere. Because it cannot go down, it goes up. This leads to adiabatic cooling and possibly cloud formation

28. Processes that Lift Air
localized convective lifting
air is warmed more than surrounding air, becomes less dense, rises
On warm summer days, unequal heating of Earth’s surface may cause pockets of air to be warmed more than the surrounding air. For example, air above a paved parking lot will be warmed more than the air above an adjacent wooded park. Consequently, the parcel of air above the parking lot, which is warmer and less dense than the surrounding air, will move upward. These rising parcels of warmer air are called thermals. The process that produces rising thermals is localized convective lifting.

29. Condensation What causes condensation?
as air is cooled, water vapor capacity decreases (& RH ↑)
When air reaches dew point temperature, air becomes saturated (100% RH)
condensation occurs (gas  liquid)
forms dew, clouds, fog, precipitation
Dew point: The temp at which saturation occurs & condensation begins The more water vapor in air, the less the air has to cool in order for condensation to start, so the higher the dew point
How does air cool? Contact with a colder surface (dew & frost), Radiation (giving off) of heat, Mixing with colder air (fog), Expansion as it rises
clouds (at altitude in atmosphere)
Dew forms when the air temp above 0°C & the ground/leave/grass cool faster than the air (at night)
Frost forms (by deposition) when the air temperature is below 0°C
fog* (essentially a cloud at ground level) = LIQUID droplets NOT vapor!!!!
forms when warm air is cooled by a cold surface below it. Water vapor in the air condenses (around a condensation nuclei) into very tiny droplets that fall slowly Two types: 1. Radiation [Night sky is clear & ground loses heat rapidly (radiation), Layer of air cools to dew point, Fog forms at ground level & is colder than the air above it (temperature inversion-warm air above cold air)] 2. Advection [Warm, moist air (wind) blows over cool surface]

30. Condensation
Besides cooling air to the dew point, what else is needed for condensation to occur?
material for water vapor to condense onto
condensation nuclei
dust, sand, salt, aerosol particles
condensation nuclei = small particles in the air onto which water can condense
Examples: Dust/sand, Salt (from sea spray), Sulfate and nitrate particles (from natural sources like volcanoes, forest fires, burning of fuels and phytoplankton)
Also needed for ice crystals to form

31. Cloud Types & Precipitation ch. 18 sec. 3
Where, in the atmosphere, can clouds form?
anywhere in troposphere
Why?
b/c only layer w/ water vapor
What are clouds made of?
water (liquid or ice)
condensation nuclei
What are clouds made of? Condensation nuclei & water droplets (if temp above freezing)
Condensation nuclei & snow crystals & super-cooled water (below 0°C, but not frozen) (if temp below freezing)
If temp is below -20°C mostly snow & ice crystals

32. Cloud Types Names formed from one or more of 5 words/word parts
ch. 18 sec. 3
Names formed from one or more of 5 words/word parts
Classified according to:
shape
stratus or strato-
layers
cumulus or cumulo-
upward puffs/heaps
altitude (height in atm)
low
stratus, nimbostratus, cumulus, stratocumulus
alto (middle)
altostratus, altocumulus
cirrus or cirro- (high)
cirrus, cirrostratus, cirrocumulus
dark, rain clouds
nimbus or nimbo-
nimbostratus, cumulonimbus
vertical development
Names formed from one or more of 5 words or word parts
Classified according to:
Shape Stratus or strato- layers (horizontal air movement)
Cumulus or cumulo- upward puffs/heaps (vertical air movement)
Altitude (height in atmosphere)
Low (no prefix unless rain clouds) below 2000 meters  stratus, nimbostratus, stratocumulus
Alto (middle) meters  altostratus, altocumulus
Cirrus or cirro- (high) above 7000 meters  cirrus, cirrostratus, cirrocumulus
Dark, rain clouds
Nimbus or nimbo- Nimbostratus, cumulonimbus (shows vertical development)

33. Low Clouds

34. Middle Clouds

35. High Clouds

36. Dark, Rain Clouds shows vertical development
a.k.a. thunderheads
also considered a low-altitude cloud

37. cirrocumulus

39. Precipitation What is precipitation?
any form of water that falls to Earth from a cloud
H2O droplets/ice crystals have to be heavy enough to fall
examples:
drizzle
rain
glaze/freezing rain
snow
sleet
hail
Formation of Hail Animation
Growth of Water Droplets Tiny droplets bump into each other & combine, Big droplets fall faster than small droplets & can “capture” them Why do droplets differ in size?
1. Time in cloud In cloud longer  longer time to “grow”  bigger
2. Size of condensation nuclei Larger nuclei = larger droplet
3. Evaporation (if mix with air that is not saturated  shrink)
4. “Combining” of droplets
Growth of Ice Crystals Temps in upper part of clouds often below freezing
Ice crystals & supercooled droplets present, Droplets evaporate & vapor is deposited on ice crystals, Heavy enough  fall
~Grow by “capturing” smaller ice crystals & water droplets
Examples
Drizzle: very fine water droplets that fall slowly & close together
Rain: larger droplets that fall faster
Snow: ice crystals collide & clump
Sleet: supercooled raindrops  freeze in air
Glaze/Freezing rain: super-cooled raindrops  freeze instantly when hit a surface Causes sheet ice or glaze
Hail: balls or irregular clumps of ice frozen raindrop or small dense clump of ice crystals
collects ice, cloud droplets, super-cooled raindrops Kept aloft by strong updrafts (stronger  larger hailstones)
~when gets too heavy  falls Layers like onion (caused by partial melting & refreezing)

40. Measuring Precipitation
National Weather Service (NWS) measures in hundredths of an inch
What instruments are used to measure precipitation?
rain?
rain gauge
snow?
measuring stick
National Weather Service (NWS) measures in hundredths of an inch One of the only measurements in science that is NOT measured in metric units
What instruments are used to measure precipitation?
Rain  rain gauge
Snow  measuring stick Not accurate measure of how much water b/c dry snow is deeper than an equal weight of wet snow
Rain equivalent of snowfall figured out by melting the snow