Chapter 4

Atmospheric Moisture, Condensation, and Clouds.

The sun’s electromagnetic spectrum and some of the descriptive names of each region. The numbers underneath the curve approximate the percent of energy the sun radiates in various regions.

0.4 μm = 400 nm 0.7 μm = 700 nm

Chapter 2-3 review

The hotter sun not only radiates more energy than that of the cooler earth (the area under the curve), but it also radiates the majority of its energy at much shorter wavelengths. (The area under the curves is equal to the total energy emitted, and the scales for the two curves differ by a factor of 100,000.)

Chapter 2-3 review

The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth’s surface. Where incoming energy exceeds outgoing energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming energy (gray shade), the air temperature falls.

Chapter 2-3 review

Chapter 2-3 review

The average annual incoming solar radiation (yellow line) absorbed by the earth and the atmosphere along with the average annual infrared radiation (red line) emitted by the earth and the atmosphere.

Chapter 2-3 review

http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Phase_Transitions/Phase_Diagrams_1

http://www.sci.uidaho.edu/scripter/geog100/lect/05-atmos-water-wx/ch5-part-2-water-phases.htm

Water can exist in 3 phases, depending upon pressure and

temperature.

Evaporation, Condensation, &

Saturation• Evaporation is the change of

liquid into a gas and requires heat.

• Condensation is the change of a gas into a liquid and releases heat.• Condensation nuclei

• Sublimation: solid to gaseous state without becoming a liquid.

• Saturation is an equilibrium condition in which for each molecule that evaporates, one condenses.

Latent Heat of Vaporization = 600 calories / 1g

Latent Heat of Condensation = 600 calories / 1g

Latent Heat of Fusion= 80 calories / 1g

http://clasfaculty.ucdenver.edu/callen/1202/Intro/EarthPlaceSpace/UniqueEarth.html

Latent Heat of Vaporization = 600 calories / 1gLatent Heat of Condensation = 600 calories / 1g

Latent Heat of Fusion= 80 calories / 1g

How much energy to sublimate?http://www2.chemistry.msu.edu/courses/cem152/snl_cem152_SS12/pracprob/practiceexam1.html

(a) Water molecules at the surface of the water are evaporating (changing from liquid into vapor) and condensing (changing from vapor into liquid). Since more molecules are evaporating than condensing, net evaporation is occurring.

(b) When the number of water molecules escaping from the liquid (evaporating) balances those returning (condensing), the air above the liquid is saturated with water vapor. (For clarity, only water molecules are illustrated.)

Condensation is more likely to occur as the air cools. (a) In the warm air, fast-moving H2O vapor molecules tend to bounce away after colliding with nuclei.

(b) In the cool air, slow-moving vapor molecules are more likely to join together on nuclei. The condensing of many billions of water molecules produces tiny liquid water droplets.

http://www.srh.noaa.gov/jetstream/atmos/hydrocycle_max.htm

Humidity

• Any of a number of ways of specifying the amount of water vapor in the air

• Absolute humidity: mass of water vapor/volume of air– Water vapor density– Not commonly used due to frequent change of

volume

AH = mass of water vapor (g) / Volume of Air (m^3)AH = mass of water vapor (g) / Volume of Air (m^3)

Humidity

• Vapor pressure: the pressure exerted by water vapor molecules in an air parcel– Fraction of total vapor pressure (1% or so)– More water molecules = high vapor pressure

• Saturation vapor pressure: the vapor pressure at which an air parcel will be saturated, changes with temperature

Fig. 4-4, p. 87

Saturation vapor pressure increases with increasing temperature. At a temperature of 10°C, the saturation vapor pressure is about 12 mb, whereas at 30°C it is about 42 mb. The insert illustrates that the saturation vapor pressure over water is greater than the saturation vapor pressure over ice.

Humidity

• Specific Humidity: mass of water vapor/mass of air

• Mixing ratio: mass of water vapor/mass of dry air

• Neither measurement changes with volume, must add or subtract water vapor.

Mixing Ratio = mass of water vapor (g) / Mass of Mixing Ratio = mass of water vapor (g) / Mass of dry Air (kg)dry Air (kg)

Humidity

• Relative Humidity: (actual water vapor/saturation water vapor)*100– RH can be changed two ways:• Change vapor content• Change saturation

– Decrease temperature causes an increase in relative humidity (inverse relation).

(a) At the same air temperature, an increase in the water vapor content of the air increases the relative humidity as the air approaches saturation.

(b) With the same water vapor content, an increase in air temperature causes a decrease in relative humidity as the air moves farther away from being saturated.

When the air is cool (morning), the relative humidity is high. When the air is warm (afternoon), the relative humidity is low. These conditions exist in clear weather when the air is calm or of constant wind speed.

Humidity

• Relative Humidity and Dew Point– Dew point is the temperature at which saturation

occurs– Cool air parcel to dew point and liquid water

condenses– A good measure of actual water vapor content– Relative humidity indicates how close to

saturation, dew point indicates the temperature to which air must be cooled for saturation to occur.

Average surface dew-point temperatures (°F) across the United States and Canada for January.

Average surface dew-point temperature across the United States and Canada (°F) for July.

Inside the cloud the air temperature (T) and dew point (Td) are the same, the air is saturated, and the relative humidity (RH) is 100 percent. However, at the surface where the air temperature and dew point are not the same, the air is not saturated (even though it is raining), and the relative humidity is considerably less than 100 percent.

The polar air has the higher relative humidity, whereas the desert air, with the higher dew point, contains more water vapor.

Humidity

• Relative humidity & human comfort– “It’s not the heat, it’s the humidity.”– High relative humidity equates to less evaporative

cooling.– Sweat cannot evaporate and cool the body.– Wet bulb temperature– Heat Index

Air temperature (°F) and relative humidity are combined to determine an apparent temperature or heat index (HI). An air temperature of 95°F with a relative humidity of 55 percent produces an apparent temperature (HI) of 110°F.

Humidity

• Measuring humidity– Sling psychrometer– Hygrometer

The hair hygrometer measures relative humidity by amplifying and measuring changes in the length of human (or horse) hair.

Dew and Frost

• Dew forms on objects near the ground surface when they cool below the dew point temperature.– More likely on clear nights due to increased

radiative cooling

• White frost forms when temperature cools below the dew point and the dew point is below 0°C.

Dew and Frost

• Particles suspended in the air around which water condenses or freezes– Hydrophobic/hygroscopic

• Dry condensation nuclei (above dew point) reflect and scatter sunlight creating blueish haze.

• Wet condensation nuclei (75% relative humidity) reflect and scatter sunlight creating greyish or white haze.

The high relative humidity of the cold air above the lake is causing a layer of haze to form on a still winter morning.

Fog

• Saturation reached condensation forms a cloud near the ground

• Radiation fog: forms when the ground cools through conduction and radiation; ground fog

• Advection fog: forms when the wind moves moist air over a cold surface and the moist air cools to its dew point.

• Upslope fog: forms as moist air slowly rises, cools, and condenses over elevated terrain.

Radiation fog tends to form on clear, relatively calm nights when cool, moist surface air is overlain by drier air and rapid radiational cooling occurs.

Radiation fog nestled in a valley in central Oregon.

• Advection Fog: warm, moist fog moves horizontally (advects) over a cool surface.– Summer fog on the Pacific coast

Advection fog rolling in past the Golden Gate Bridge in San Francisco. As fog moves inland, the air warms and the fog lifts above the surface. Eventually, the air becomes warm enough to totally evaporate the fog.

Fig. 4-20, p. 100

Upslope fog forms as moist air slowly rises, cools, and condenses over elevated terrain.

Even in summer, warm air rising above thermal pools in Yellowstone National Park condenses into a type of steam fog.

Average annual number of days with dense fog (visibility less than 0.25 miles) across North America. (Dense fog observed in small mountain valleys and on mountain tops is not shown.)

Clouds

• Classification of clouds: use Latin words to describe height and appearance

• Factors described– Height: low, mid, high, vertical– Appearance: shape, density, color

Cirrus clouds.

Cirrocumulus clouds.

Cirrostratus clouds with a faint halo encircling the sun. The sun is the bright white area in the center of the circle.

Altocumulus clouds.

Altostratus clouds. The appearance of a dimly visible “watery sun” through a deck of gray clouds is usually a good indication that the clouds are altostratus.

The nimbostratus is the sheetlike cloud from which light rain is falling. The ragged-appearing cloud beneath the nimbostratus is stratus fractus, or scud.

Stratocumulus clouds forming along the south coast of Florida. Notice that the rounded masses are larger than those of the altocumulus.

A layer of low-lying stratus clouds hides the mountains in Iceland.

Cumulus clouds. Small cumulus clouds such as these are sometimes called fair weather cumulus, or cumulus humilis.

Cumulus congestus. This line of cumulus congestus clouds is building along Maryland’s eastern shore.

A cumulonimbus cloud. Strong upper-level winds blowing from right to left produce a well-defined anvil. Sunlight scattered by falling ice crystals produces the white (bright) area beneath the anvil. Notice the heavy rain shower falling from the base of the cloud.

A generalized illustration of basic cloud types based on height above the surface and vertical development.

Some Unusual Clouds

• Not all clouds can be placed into the ten basic cloud forms.

• Unique atmospheric processes and environmental conditions create dramatic and exotic clouds.

• Unusual clouds and weather balloons often cause of UFO reports.

A lenticular cloud forming over Mt. Rainier in Washington State.

A pileus cloud forming above a developing cumulus cloud.

Mammatus clouds forming beneath a thunderstorm.

A contrail forming behind a jet aircraft.

The clouds in this photograph are nacreous clouds. They form in the stratosphere and are most easily seen at high latitudes.

The wavy clouds in this photograph are noctilucent clouds. They are usually observed at high latitudes, at altitudes between 75 and 90 km above the earth’s surface.