colourless, odorless, tasteless invisible gas that mixes freely with other 

gases in the atmosphere

hydrogen bonds 

evaporative cooling 

vapor pressure 

Effect of water vapor in the air on evaporation and precipitation

- function of the amount of water vapor in the air- as water evaporates, vapor pressure increases and this slows the rate of 

evaporation and lowers the amount that can evaporate because 

most of spaces in the air that water molecules can occupy are 

already occupied

saturation value 

At any temperature and pressure there is a maximum amount of water vapor that the air can hold under normal 

circumstances

- air can still hold more water vapor

- low vapor pressure

- more evaporating than condensing is occurring

- air holding maximum amount of water vapor

- equal evaporation and condensation at water surface

Supersaturated

- unstable state where air holding water vapor in excess of maximum amount

- if disturbed, excess water condenses

combined effects of evaporation and transpiration

Water evaporated from the land is usually a result of both transpiration and evaporation from soil

- Whether area is wet or dry depends on balance between evapotranspiration and precipitation

- Maximum amount of evapotranspiration that would occur if the ground was soaking wet all of the time

- If precipitation >potential evapotranspiration will have excess water in ground and available for plants

- If precipitation < potential evapotranspiration will have water deficit and plants will not survive

absolute humidity 

• weight of water vapor/ volume of air (gm/m³)
• Varies with change in pressure, temperature and amount of moisture
• Pressure alters volume of air without changing the amount of water in the air

• Water vapor mass in gram/ Air mass in kilograms
• Not significantly altered by change in volume or temperature
• More useful for air masses moving vertically

• 100% X (Water vapor mass in air/ potential water vapor mass air can hold at that temperature when saturated)
•  % saturation
• does not tell amount of water vapor present only percent of potential that can be held

dew point 

• Temperature at which air reaches saturation value for the amount of moisture in the air
• relative humidity is 100%
• Function of temperature and amount of moisture
• As air cools, saturation value declines and undersaturated air becomes saturated for water vapor 
• Temperature of dew point is not an absolute but varies with water vapor content of air

sensible temperature 

• Temperature sensed by the human body as opposed to actual temperature-the temperature registered by a thermometer
• Function of actual temperature, relative humidity and wind

• water vapor is converted to the liquid state and releases 540cal/gm
• To occur, air must be saturated where condensation occurs
• Most commonly occurs because of decrease in temperature below dew point

surface tension 

• contraction of the surface of a liquid to form the smallest surface area
• of water forces it to form spheres, but shape makes it 
• difficult for additional water to enter droplet and easy for water
• molecules to leave (evaporate)

• particulates in air which act as condensation nuclei
• Include dust, sea salt, bacteria, pollen
• Particles are most abundant near sources: cities, volcanoes, sea, but they occur everywhere

dry adiabatic lapse rate 

• rate at which under-saturated air cools as rises
• Poor term because air not dry, just unsaturated
• If air rises high enough dew point will be reached and condensation begin to form clouds

Lifting Condensation Level

(LCL)

• elevation at which clouds begin to form for an air mass
• Generally coincides with the base of the clouds

• rate at which saturated air cools as rises 5^oC/Km  (3.3^oF/1000ft)

• change in temperature with altitude
• about 6.5^oC/km (3.6^oF/1000ft)
• does not involve the vertical motion of air masses

cirriform clouds 

• Cirrus = lock of hair
• Thin and wispy clouds composed of ice crystals

• Stratus = spread out, layer
• Grayish sheet that covers most or all of sky and rarely broken into individual units

• Cumulus = mass or pile
• Massive and rounded cloud with great vertical extent, limited horizontal extent and generally flat base

• Altitude > 6 Km  (20,000 ft)
• Low water vapor content and very cold temperatures
• Clouds tend to be thin, white and composed of ice crystals
• Tell of approaching storm or weather
• Includes :  cirrus, cirrocumulus, cirrostratus

• Altitude 2-6 Km  (6500-20000 ft)
• Composed of liquid water
• Include:

Altocumulus - indicate settled weather

Altostratus - indicate changing weather

• Altitude <2 Km  (6500 ft)
• Consists of individual clouds or general overcast
• Somber skies 
• Include:

Stratus

Stratocumulus

Nimbostratus - drizzly rain

• Grow upwards from base to 15 Km  (60000 ft)
• Horizontally restricted
• Indicate very active vertical movement of air
• Includes:

Cumulus - fair weather

Cumulonimbus - storm clouds

cirrocumulus clouds 

altostratus clouds 

stratocumulus clouds 

nimbostratus clouds 

drizzly rain

develop from stratus clouds

cumulus clouds 

cumulonimbus clouds 

storm clouds

develope from cumulus clouds

radiation fog 

• Ground looses heat through radiation
• When air at ground surface cools and dew point reached, fog develops

advection fog 

• Forms when warm moist air flows over cold surface such as snow or cold ocean current
• Common when have sea or land breezes (coastal fog)

orographic fog

(upslope fog)

• Produced by adiabatic cooling as humid air is forced up a topographic slope

• Water vapor is added to air already nearly saturated 
• where cold air flows across warm water
• more common with land breezes

• condensation that forms on objects
• Originates from night time radiation-cooling of objects at Earth’s surface
• Adjacent air cools by conduction and if reaches dew point, tiny beads of water collect on the cold surface
• If below freezing, ice crystals form (WHITE FROST or Hoar Frost)

white frost

(hoar frost)

• tendency of more dense fluids to sink and push less dense material upwards
• material ascends to the level of fluids of the same density

• is when material is at the level of its own density, such that there is no tendency to rise or sink

- Air mass is cooler and more dense than the surrounding air at a higher altitude 

- Air is stable and will only rise if forced, but when force is 

removed, air mass will subside

 that fluid below is more dense 

and that above is less dense

stable air 

• Air at its own density level and is neither ascending or descending
• Will only change its position vertically if force acts upon it and then will return to its original position when the force ceases

- Example: during inversions when cold air (more dense) lies below warm air (less dense)

• less dense air lies below more dense air
• Rises or falls without any external force applied or continues after force is removed
• As air rises or falls to its own density level will experience adiabatic cooling or warming:  If rising, cooling favors cloud formation

• Intermediate between absolute stability and instability
• Left alone, air mass is stable, but if forced to rise until it reaches its dew point, condensation releases heat of condensation and air cools more slowly gradually becoming less dense than the surrounding air and thus unstable
• When lapse rate is between dry adiabatic and saturated adiabatic lapse rates

• All precipitation begins in clouds, but not all clouds generate precipitation
• Condensation alone not sufficient to generate precipitation
• Tiny droplets are too small to fall and are kept suspended by updrafts or evaporate before reaching ground

Bergeron process 

• Main way precipitation forms outside of tropics
• Upper parts of clouds extend to where temperature is below the freezing point
• Ice forms here and can have mixture of ice and super-cooled water droplets
• Ice and water droplets compete for additional moisture, but vapor pressure around ice is lower and it attracts most of the moisture
•  

• As amount of moisture declines, water droplets evaporate faster than ice crystals sublimate 
• go directly from solid to gaseous state
• to evaporate requires only 540cal/gm

620 cal/gm (80 for latent heat of 

melting and 540 for latent heat of evaporation)

Drops of liquid water warmed above the freezing point that fall from 

clouds and reach the ground in a liquid state

• General name for solid precipitation in form of ice crystals, small pellets or flakes
• Formed from vapor condensed directly to ice without a water stage
• all flakes have basic hexagonal shape
• snowflake shapes are not unique; many can be identical

• in the U.S. is small rain drops that freeze during descent and reach ground as ice pellets
• Elsewhere is mixture of rain and snow

• Rain that becomes ice on impact
• Cause by rain falling through subfreezing air and becoming super-cooled
• Forms a coating of ice on surfaces

• Small to large pellets of ice consisting of roughly concentric layers of clear, larger ice crystals and cloudy smaller crystals (cloudiness is trapped air bubbles)
• air forming bubbles was dissolved in the water and is forced out as freezing occurs
• Produced in cumulonimbus clouds with strong updrafts
• Upper part of cloud below freezing and lower part warmer
• Updrafts carry droplets into freezing region where freeze and collect additional moisture
• When too large for updraft to support, they fall and collect  additional moisture

• Unequal heating of Earth’s surface means some areas are warmer than others
• Density of warmer air is less and it is pushed upward as adjacent cooler and denser air displaces it
• Air becomes unstable
• Pressure on air declines, air expands and adiabatically cools

• Topographic barrier that blocks path of air and forces it upslope
• Can occur at any latitude, any season, any time of day
• Likely to produce prolonged rain or drizzle because of steady upflow of air

• If rises high enough, adiabatic cooling may reach dew point and cloud formation may lead to precipitation

• Where air masses of different thermal characteristics meet, they do not mix, but warm air is forced up over cold

• Most common in the  midlatitudes because that is where cold and warm air masses meet

• Less common than other types of lifting
• Forms when air converges into a region and is forced to rise
• Commonly associated with cyclonic storms
• Most common in lower latitudes in the intertropical convergence zone and with tropical disturbances such as hurricanes andeasterly waves

• line connection points that receive equal amount of precipitation

• Form where unlike air masses meet
• Narrow zone of discontinuity within which properties of air change rapidly---steep pressure, temperature and humidity gradients possible
• Usually identified by temperature differences between the air masses (cold front or warm front) and movement, but also differ in humidity, density, and stability
• Typically front is inclined and less dense air mass is forced up over more dense

• Front that brings warm air into region (warm air displaces cold air)
• Warm air forced up over retreating cold air producing adiabatic cooling, cloud formation and possibility of precipitation
• - High cirrus clouds tell of approaching front several hours in advance
• - As front approaches, clouds descend and become altocumulus, altostratus, Stratocumulus, Stratus and Nimbostratus
• On maps, warm front identified by red or black line bearing red or black 
• semicircles pointing in the direction the front is moving

• Front that brings in cold air (cold air displaces warm air)
• Friction with the ground slows base of the front and base lags behind upper air, creating a bulge of cold air about 100 m above the ground
• Causes rapid uplift of warm air in front of the advancing front
• - Warm air unstable and winds can be blustery and violent
• - Vertical clouds (cumulus and cumulonimbus) with turbulence and showery precipitation

• Neither air mass is displacing the other, but must have warm winds opposing cold air mass or cold front would continue to flow across landscape under influence of gravity
• - warm winds rise upwards but produces limited precipitation
• - Weather is unpredictable
• On maps represented by line bearing alternating semicircles and triangles on opposite sides of the line (pointing in the direction the masses would be moving)

• When cold front overtakes warm front and merges with another (or the same) mass of cold air
• - creates a stable condition (inversion) with warm, less dense air over colder, denser air
• - On maps represented by alternating semicircles and triangles on the same side of the line and pointing in the direction both masses had been moving

• - disturbances in the atmosphere are caused by ascending air (low pressure) that can produce storms
• - in contrast, subsiding air generally produces extreme calm

Characteristics:

• - smaller than global circulation pattern and variable in size
• - Migratory and transient (relative brief duration of minutes to days)
• - Produce characteristic and relatively predictable weather conditions