Chapter 4: Humidity

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Hydrologic cycle (water cycle)

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

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Evaporation

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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

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70 Terms

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Hydrologic cycle (water cycle)

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

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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

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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

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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

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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

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Groundwater

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

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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)

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What is the largest source of moisture in the atmosphere?

Oceans

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Humidity

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

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Actual humidity

amount of water vapor that is actually in the air

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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)

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Saturation

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

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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

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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

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Absolute Humidity

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

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Dew Point

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

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Radiational cooling

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

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Temperature inversion

increase in temperature with height

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When are temperature inversions common?

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

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Apparent temperature or heat index

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

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Heat stroke

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

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Body temperature during heat stroke

Possible if body temperature greater than 106 F

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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

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Wet bulb

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

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Dry bulb

kept dry, gives the air temperature

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Wet bulb depression

temperature difference between the dry bulb and wet bulb

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Large depression

low relative humidity

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Small depression

near saturation, high relative humidity

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What happens when the bulbs are equal?

Relative humidity is 100%

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Hygrometer

instrument for measuring relative humidity

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Hair hygrometer

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

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Electrical hygrometer

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

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Dew point hygrometer

used in automated weather stations

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Dew (condensation)

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

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Frost (deposition process)

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

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Deposition

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

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Frost point

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

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Fog

cloud with its base at the Earth's surface

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Radiation fog

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

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Advection fog

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

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Upslope fog

moist air flows upward over a topographic barrier

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Evaporation fog

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

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Clouds

visible aggregate

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High clouds

thin, icy, little moisture, mostly white

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Middle clouds

water droplets and possibly ice crystals, possible precipitation

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Low clouds

largely water droplets

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Cirrostratus

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

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Cirrus

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

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Cirroculmulus

White cloud patches, ripple pattern, mostly ice

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Altostratus

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

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Altocumuls

white or gray, layers or patches, wavy rounded masses

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Stratus

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

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Stratocumulus

mostly uniform, lumpy rounded masses with blue sky in between

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Nimbostratus

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

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Cumulus

flat bases, puffy tops, cottonball, well-defined

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Cumulonimbus

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

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Geostationary and Polar-orbiting satellites

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

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Geostationary

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

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Polar-orbiting

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

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Visible Satellite Imagery

Visible satellite images display the sunlight reflected from the cloud tops

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Cloud condensation nuclei

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

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Diurnal cycle of relative humidity

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

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

lower, greater

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States with abundant fog

Washington, Oregon, Maine, New Hampshire, West Virginia

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Low clouds

stratus, stratocumulus, nimbostratus (nimbo = raining)

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Mid-level clouds

altostratus, altocumulus

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High clouds

cirrus, cirrostratus, cirrocumulus

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Vertical clouds

Cumulus, cumulonimbus (thunderstorms)

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Types of weather satellites

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

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Main controls of regional temperature variations

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