Chapter 4: Humidity

Hydrologic cycle (water cycle)

continuous movement and exchange of water among the Earth's surface, oceans, and atmosphere including phase changes of water

Evaporation

process by which a liquid changes into a gas. Occurs when air is warmed. The sun's energy transforms enormous quantities of liquid water into water vapor. Water can evaporate from the oceans, lakes, soil, etc. 85% of water evaporated into the atmosphere

Condensation

Process by which water vapor becomes a liquid and can form clouds or dew. Occurs when air is cooled. Prescient of cloud condensation nuclei (microscopic bits of dust, smoke, ans sea salt in the air) is important for condensation and cloud formation

Precipitation

Any form of water particles, either solid or liquid, that falls from the atmosphere and reaches the ground. Under certain conditions, the liquid cloud particles or solid ice crystals collide, coalesce and form, grow in size, and fall to the surface as rain, snow, sleet, or hail. Some falling precipitation evaporates back into the atmosphere

Transpiration

process by which water is pants is transferred as water vapor to the atmosphere through stomata, or small openings on the underside of leaves

Groundwater

once precipitation reach the ground,s some water soaks into the ground by percolating downward through small openings in soil and rock

Runoff

water from rivers, lakes, and the ground returning to the oceans. Occurs when there is excessive precipitation and the ground is saturated (cannot hold anymore water)

What is the largest source of moisture in the atmosphere?

Oceans

Humidity

amount of water vapor (doesn't count liquid water) in the air

Actual humidity

amount of water vapor that is actually in the air

Saturation humidity

amount of water vapor that could be in the air, depending on air temperature (warmer air can hold more water vapor than colder air)

Saturation

Atmospheric condition where the level of water vapor is the maximum possible at the existing temperature and pressure

Air parcel

imaginary volume of air. Large enough to contain a great number of molecules. Small enough that is has relatively uniform properties. Total air pressure inside the parcel is due to the collision of all molecules against the walls of the parcel

Relative humidity

Ratio of the amount of water vapor in the air compared to the amount required for saturation (maximum the air can hold) at a particular temperature and pressure

Absolute Humidity

Mass of water vapor in a given volume of air... depends on volume

Dew Point

Temperature is which air must be cooled, at constant pressure and constant water vapor content, for saturation to occur (units

Radiational cooling

At night, the ground and overlying air cool by radiating infrared, longwave radiation

Temperature inversion

increase in temperature with height

When are temperature inversions common?

clear (little cloud cover and moisture), calm nights, especially with snow on the ground

Apparent temperature or heat index

what the air temperature feels like to the average person for various combinations of air temperature and relative humidity

Heat stroke

Physical condition induced by a person's over-exposure to high air temperatures, especially when accompanied by high humidity

Body temperature during heat stroke

Possible if body temperature greater than 106 F

Psychrometer

measures relative humidity and dewpoint. Includes 2 liquid in glass thermometers (wet bulb and dry bulb) mounted together on a piece of metal with a handle or chain at the end

Wet bulb

cloth covering the bulb dipped in water. Lowest temperature achievable by evaporating water in the air

Dry bulb

kept dry, gives the air temperature

Wet bulb depression

temperature difference between the dry bulb and wet bulb

Large depression

low relative humidity

Small depression

near saturation, high relative humidity

What happens when the bulbs are equal?

Relative humidity is 100%

Hygrometer

instrument for measuring relative humidity

Hair hygrometer

traditional instrument, rarely used today. Inaccurate. Increases relative humidity causes hair/fiber length to increase

Electrical hygrometer

As water vapor is absorbed, the electrical resistance of the carbon coating changes, and these changes are translated into relative humidity

Dew point hygrometer

used in automated weather stations

Dew (condensation)

water that has condensed onto objects near the ground when their temperatures have fallen below dew point of the surface air

Frost (deposition process)

covering of ice produced by deposition on exposed surfaces when air temperature falls below the frost point

Deposition

water vapor directly changing into ice without becoming a liquid. When temperature is below 0C

Frost point

temperature (<0C) at which air becomes saturated with respect to ice when cooled at constant pressure and constant water vapor content.

Fog

cloud with its base at the Earth's surface

Radiation fog

ground fog/valley fog. produced over land due to radiational cooling (light winds)

Advection fog

warm, moist air moves over a cold surface and is cooled. Wind moves warm air over cool surfaces

Upslope fog

moist air flows upward over a topographic barrier

Evaporation fog

water vapor is added to air by evaporation, and the moist air mixes with the relatively drier air can occur during rain

Clouds

visible aggregate

High clouds

thin, icy, little moisture, mostly white

Middle clouds

water droplets and possibly ice crystals, possible precipitation

Low clouds

largely water droplets

Cirrostratus

Layered, sheetlike, uniform, mostly ice, can produce halo

Cirrus

Wispy, fibrous, feathery, white, mostly ice, fair weather

Cirroculmulus

White cloud patches, ripple pattern, mostly ice

Altostratus

Grey/bluish sheets, layered, mostly water, often cover whole sky

Altocumuls

white or gray, layers or patches, wavy rounded masses

Stratus

gray sheetlike layer near ground, often blocks out sunlight, often drizzle

Stratocumulus

mostly uniform, lumpy rounded masses with blue sky in between

Nimbostratus

dark gray, layered, light/moderate precipitation, rarely severe

Cumulus

flat bases, puffy tops, cottonball, well-defined

Cumulonimbus

deep dark rain cloud, top anvil, thunderstorms, can be severe

Geostationary and Polar-orbiting satellites

Weather satellites provide valuable images of clouds over areas without ground-based observations (hurricanes over oceans)

Geostationary

located above equator; orbit at the same rate as Earth spins; provide continuous monitoring of a specific region

Polar-orbiting

pass over north and south polar regions, eventually covering the entire planet

Visible Satellite Imagery

Visible satellite images display the sunlight reflected from the cloud tops

Cloud condensation nuclei

dust, salt, smoke, etc in air that helps condensation and cloud formation

Diurnal cycle of relative humidity

peaks at sunrise and lowest in mid-afternoon = inverse relationship with air temperature

Desert has ___ relative humidity but ___ water vapor than polar region

lower, greater

States with abundant fog

Washington, Oregon, Maine, New Hampshire, West Virginia

Low clouds

stratus, stratocumulus, nimbostratus (nimbo = raining)

Mid-level clouds

altostratus, altocumulus

High clouds

cirrus, cirrostratus, cirrocumulus

Vertical clouds

Cumulus, cumulonimbus (thunderstorms)

Types of weather satellites

geostationary (always sitting over equator) and polar-orbiting (orbits around North and South Poles)

Main controls of regional temperature variations

latitude, land-sea distribution, ocean currents, and elevation