Download Chapter 24 notes - Kasson-Mantorville Middle School

Water in the Atmosphere
Chapter 24
24.1 Atmospheric Moisture
A. Heat Energy and Water
- Water exists in the atmosphere in all 3 states of matter
- Solid (ice, snow), liquid (cloud droplets, rain), and gas (water vapor)
- It also changes from one state to another in the atmosphere.
- All changes in state are energy changes.
- Some terminology for changes in state:
Physical change
solid to liquid
liquid to solid
liquid to gas
gas to liquid
solid to gas
gas to solid
Name
melting
freezing
evaporation
condensation
sublimation
deposition
Energy change
Absorbs heat from atmosphere
Releases heat to atmosphere
Absorbs heat from atmosphere
Releases heat to atmosphere
Absorbs heat from atmosphere
Releases heat to atmosphere
- All of these changes, and their respective changes in energy, can be shown in a diagram.
- Latent heat is the energy stored in the motion of the water molecules
- When the water goes from gas, to liquid, to solid it will give this latent heat back to the
atmosphere warming it up
B. Humidity
1. Capacity and specific humidity
- The amount of water vapor actually in the air is called the specific humidity.
- The specific humidity (S.H.) is the number of grams of water per kilogram of air.
g(H2O)/kg(air)
- This room has a specific humidity of about 6 or 7 g/kg.
- Remember, this is the actual amount of water vapor in the air.
- The capacity (cap.) of the air is how much water the air can possibly hold.
- Capacity of the air is always greater than or equal to the specific humidity.
- Capacity depends on the temperature.
- The higher the temperature the more water vapor it can hold.
- This means that higher temperatures have higher capacities.
- Look at the graph on page 480. Air at 10°C has a capacity of 9 grams but air at 20° has a
capacity of 18 grams.
- Air that is holding all the water vapor it can is called saturated.
- When the air is saturated the specific humidity is equal to the capacity.
2. Relative Humidity
- Relative humidity (R.H.) is a percent comparison between the specific humidity and the
capacity.
%RH = SH/CAP X 100
Examples:
- Saturated air has a relative humidity of 100%
3. Measuring relative humidity
- Instruments used to find relative humidity are hygrometers.
- The simplest hygrometer is a hair or paper hygrometers.
- Some hair or paper changes shape (curls) when the humidity changes.
- Cheap but not very accurate.
- Electronic hygrometers pass electric current through a moisture attracting chemical
- The psychrometer (sling psychrometer) is made of 2 thermometers.
- The drier the air the more evaporation cools the thermometer and the greater the
difference in the temperature between the wet and dry bulb.
- Using a table like the one on page 481 relative humidity can be determined.
Example: What is the relative humidity if the wet bulb is 12°C and the dry bulb is 18°C?
C. Dew Point
- As the temperature goes down the capacity of the air also goes down.
- If the specific humidity stays the same, the relative humidity goes up.
- The temperature at which the RH = 100%, the air is saturated, and the SH = Cap,
condensation will begin.
- This temperature is the dew point temperature.
- At the dew point dew, frost, fog, or clouds will start to form.
1. Dew
- If the ground cools rapidly in the evening due to reradiation it will chill the air above it
by conduction.
- If the chilled air drops below its dew point, dew will condense on the surfaces.
- Dew only forms if the dew point temperature is above 0°C or 32°F.
2. Frost
- Frost will form if the dew point temperature is less than 0°C or 32°F.
- Frost forms from deposition of water vapor directly into ice crystals.
24.2 Clouds and Fog
- Clouds are masses of cloud droplets (1 million droplets per drop) or ice crystals in the air
- Fog is clouds on the ground.
A. Cloud formation
- 3 things needed to form clouds (water vapor, condensation nuclei, and cooling air)
1. Water vapor
- There is always water vapor in the air (1 - 4%)
- Warm air will have higher capacities so it will usually have more water vapor.
2. Condensation nuclei
- Cloud droplets must have something to condense on to.
- Particles of dust, ash, sea salt and pollution form the condensation nuclei for droplets
- Ice particles need freezing nuclei which are much less common in the air.
3. Methods of cooling air
a. Convective cooling
- Convective cooling or adiabatic cooling happens when air rises vertically.
- As warm air rises it moves into lower pressure and it will expand.
- It takes energy to expand the air so it will cool.
- It cools at a rate of 1°C for each 100m
- As the air rises and cools it eventually reaches its dew point and the water vapor starts to
condense on condensation (or freezing) nuclei to form clouds.
- This is how thunderstorms and puffy cumulus clouds usually form
b. Forceful lifting
- When air is forced to rise over mountains the air will expand and cool to form clouds.
- This also happens when warm air is forced over colder air at fronts (next chapter)
c. Mixing
- When warm and cold air mix the warm air can chill to form clouds.
- This is actually not very common because cold air sinks and warm air rises instead of
mixing.
d. Advective cooling (wind driven cooling)
- When warm moist air blows over cold ground the air is chilled by conduction.
- This happens along the seacoast and when warm humid southerly winds blow over snow
covered ground in Minnesota.
B. Classification of Clouds
- Clouds are classified by their form and altitude.
1. Cloud forms
a. Stratus clouds – horizontal or layered clouds
- Typically form in stable air that does not want to rise naturally
- Air is only rising because it has to so creates low clouds
b. Cumulus clouds – vertical or “puffy” clouds
- Form in unstable air that wants to naturally rise on its own.
2. Cloud height or altitude
- High clouds (>6,000m) are called cirrus clouds and are made of ice crystals
- Can be just cirrus, cirrostratus, or cirrocumulus
- Middle clouds (2,000 – 6,000m) are called alto clouds
- Can be altostratus or altocumulus
- Low clouds (<2,000m) are just called stratus and cumulus
- If the clouds contain precipitation they are called nimbus clouds
- There could be stratonimbus or even altostratonimbus.
- Cumulonimbus clouds are called thunderheads.
- All the common cloud types are shown in the diagram on page 487.
C. Fog
- Fog is clouds at the ground
- 4 types of fog
1. Radiation fog (reradiation fog or ground fog)
- As the ground reradiates its heat to space on clear calm nights the cold ground chills the
air above below its dew point by conduction to create fog
- Commonly forms during a temperature inversion.
2. Advection fog (wind blown fog)
- Basically the same as advection cooling to form clouds
3. Upslope fog
- Basically the same as forceful lifting to form clouds in mountains.
4. Steam fog
- Whisps of vertical fog coming off warm lakes or rivers on cool Fall days.
24.3 Precipitation
A. Why does it rain?
- 2 theories of how rain forms from cloud droplets
1. Coalescence (Warm cloud process)
- Generally it is large droplets falling and hitting smaller droplets until it grows into a
drop. Given that 1 drop = 1 million droplets, difficult to imagine it happens often.
- Probably does not happen in middle latitudes but maybe in tropics.
2. Super cooling (cold cloud process)
- A complex process where droplets are cooled below 0°C (32°F) but do not freeze because
there is no freezing nuclei.
- When a freezing nuclei is available all the super cooled droplets rush to it to from a
snowflake.
- As the snowflake gets larger it falls apart creating many more freezing nuclei for more.
- Cloud quickly becomes a mass of millions of snowflakes.
- This means that all our precipitation starts as snow in the cloud.
B. Forms of Precipitation
- In cold air (<0°C) snow will fall all the way to the ground.
- Average snow to water ratio is 10 to 1, dry snow 20 to 1, and wet snow 6 to 1
- If the temperature is above freezing the snow will melt to rain (drops 0.5 to 5mm)
- Small rain drops (<0.5mm) is called drizzle
- Cold rain that hits cold ground will form freezing rain (also called glaze or sheet ice)
- If the cold rain falls through very cold air it will form ice pellets called sleet.
- Hail forms in cumulonimbus clouds as the ice pellets are carried up and down several
times in convection currents.
- Rain gauges measure rainfall.