Chapter 4

Atmospheric Moisture and Precipitation

Precipitation

• Precipitation-the falling of liquid or solid water from the atmosphere to reach the Earth’s land or ocean surfaces

• How does is form?– What is the name of the nucleus required for water

droplets to form?– KEY=Upward Movement (what happens?)

• 2 types of upward movement:– 1. Orographic Lifting– 2. Buoyed Air (surface)

Phases or States of Substances

• Three phases: solid, liquid, and gas• In terms of water: ice, water, water vapor• Importance of latent heat

– KEY: A change in state requires an exchange of latent heat

– Changes in latent heat lead to changes of state– Absorption of latent heat warming– Release, loss of latent heat cooling

Changing States of Water(See Figure 4.1 in textbook)

Water vapor

Ice Water

Subl

imat

ion

Dep

ositi

on

Freezing

Melting

CondensationEvaporation

The Hydrosphere

• Portion of the Earth system that contains all three phases of water

• Mostly ocean saltwater: ~ 97.2%• Fresh water: ~ 2.8%

– 2.15% of the 2.8% is in glaciers/ice– .63% is groundwater

• 3 categories of fresh water:– 1. Subsurface– 2. Groundwater– 3. Surface

The Hydrosphere(See Fig. 4.2 in textbook)

Small, but significant

The Hydrologic Cycle• Hydrologic Cycle-the complete process whereby

Earth’s free water moves through the gaseous, liquid, and solid states

• 2 main processes:– 1. Evaporation from oceans/land surfaces– 2. Water vapor condensing and falling back as precip.

• On land it can evaporate and return as water vapor, be absorbed by the soil, or runoff into rivers and streams

• In atmosphere, water can condense or deposit and then fall as precipitation (snow, rain, sleet, etc.)

• Frozen precipitation takes longer to re-enter the system (snow, ice, glaciers, permafrost)

The Hydrologic Cycle

Global Water Balance• Like with what we saw with the balance of

incoming (solar/shortwave) and outgoing (terrestrial/longwave) radiation, there must be a balance in the hydrologic cycle

• The total evaporation from land and ocean must equal the total precipitation to land and ocean

• Whole cycle and balance very complicated, and as an example...

Humidity• Humidity-a measure indicating how much water

vapor is present in the air– Given as a Dew Point or RH measure

• RULE=Warmer air can hold more water vapor than colder air

• Saturation occurs when air is “holding” the maximum amount of water vapor– What happens when the air is saturated?

Measures of Water Vapor Content

• 3 main measures of water vapor:– 1. Specific humidity– 2. Dewpoint temperature– 3. Relative humidity

Specific Humidity• Actual amount of water

vapor held by a parcel of air

• Mass of water vapor in a mass of air (g of water vapor per kg of air)

• Important because describes amount of water vapor is available for precipitation

• Max SH varies greatly with temp (Fig. 4.4)

Dewpoint Temperature• The temperature of an air

mass at which the air holds its full capacity of water vapor

• If continued cooling occurs condensation (dew, fog) or deposition (frost)

• Given in °C or °F

• Scale of comfortability

• Important for forecasting clouds (fog) and precipitation

Relative Humidity

• RH-a measure, expressed as a %age, that indicates the amount of water vapor currently present compared to the maximum amount that the air can hold at that specific temperature

• When RH = 100%, the air is saturated…meaning?• Important for weather forecasting and fire

applications• What’s the problem with RH?• Measured with a sling psychrometer

Relative Humidity

• It can change slow or fairly rapidly.

• Method of change #1: increase or decrease in water vapor (slow)

• Method of change #2: increase or decrease in air temperature

• In general, highest RH in morning, lowest RH in afternoon – why?

The Adiabatic Process• A change in temperature that results from a

change in pressure (no heat is added or removed by outside influences) (compression or expansion)

• If air is forced upward, it will expand and cool; if air is descending, it will compress and warm.

• Air will cool differently if it above or below the dew point mark

The Adiabatic Process

• Three lapse rates of importance– Dry adiabatic (“dry air”) (DAR) lapse rate

• Rate of temperature change when a parcel of air is not saturated

• Approximately 10°C/1000m

– Wet/adiabatic (“wet air”) (PAR) lapse rate• Rate of temperature change when a parcel of air is

saturated (above lifting condensation level)• Ranges from 4 – 9°C/1000m• We will use 6°C/1000m

– ELR rate?– LCL?

Lifting Condensation Level (LCL)

• LCL (m) = Air Temp (°C) – Dew Point (°C)

DAR

X 1000 meters– Example: Air temperature = 30°C, Dew Point

temperature = 10°C…LCL?• 2,000 meters

The Adiabatic Process

ELR, DAR, and PAR

• ELR=ΔTemperature/ ΔAltitude• Example: Find the ELR given a surface

temperature of 32°C and a temperature at 5,000m of -8°C– Answer: 8°C/1,000m

• Air Stability– ELR<6°C=Stable– ELR>10°C=Unstable– ELR between 6 and 10°C=Conditionally stable

• Examples

Clouds

• Dense concentrations of liquid water droplets, ice particles, or a combination of liquid and ice that is suspended in the air

• Required Components—water vapor and cloud condensation nuclei (dust, pollen, sea salt)

• Temperature determines whether liquid droplet or ice particle will form

• 4 families (High, Middle, Low, and Vertical)• 2 Classifications (Stratiform and Cumuliform)

Cloud Nomenclature

• Named using Latin bases– By height: alto- (mid-level), cirro- (high)– By form: strato- (layered, blanket-like, greater

horizontal extent), cumulo- (globular, puffy, great vertical extent)

– By process: nimbo- (rain cloud, rain-bearing, shower)

• Obtain names by combining the above terms

Basic Cloud Families and Types

Fog

• A cloud (generally stratiform) that is close to or in contact with the ground

• Some types and their formation:– Radiation fog: forms at night when temp of air at the

ground cools to dewpoint and condensation occurs (CA Central Valley)

– Advection fog: forms when warm moist air moves over a cooler surface (CA coast)

– Upslope: moist air moves up a slope, cools to dewpoint, and condensation forms (northeastern US in fall)

– Evaporation (mixing) fog: cold, dry air moves over a warm body of water (river valleys in northeastern US)

Precipitation

• Liquid or solid water that falls from the atmosphere (more specifically from clouds) and reaches the surface of the Earth

• Contrast with virga, which is liquid or solid water falling from clouds that does not reach the surface

• Together precipitation and virga are called hydrometeors (hydro- “water;” meteor- “something falling from/through the atmosphere”)

Importance of Precipitation

• Global energy flow – movement of heat

• Life – supply of water

• Landforms/landscapes – shapes Earth

• Aesthetics – falling snow

• Allergies – removes pollen, dust, etc. from air

Forms of Precipitation• Rain: liquid water falling from a cloud• Snow: ice particles falling from a cloud• Sleet: falls as liquid water, then passes through a thick

layer of freezing/sub-freezing air and becomes an ice grain/pellet

• Freezing rain: falls as liquid water and freezes upon contact with ground b/c ground and very thin layer of air is < 0°C

• Hail: masses of ice that fall from a thunderstorm; form when water freezes in a storm cloud, and motions cause ice to circulate and gain mass

Intensity of Precipitation

• Some different names for precipitation, depending on the intensity at which it falls:– Drizzle, flurries, freezing drizzle: very light,

continuous or intermittent– Rain, snow, freezing rain: light to moderate

intensity, usually continuous– Shower/storm: heavy intensity, usually short-

lived– Hurricane, blizzard: extreme intensity

precipitation combined with strong winds, long-lived/continuous

Measurement of Precipitation• Rain

– Cylinder rain gauge, tipping bucket– Measure to nearest 0.01”– Below 0.01” is considered a trace

• Snow– Use ruler at different locations on flat surface– Measure to nearest 0.1”– Melt to obtain water equivalent– “Standard” equivalent is 10” of snow to 1” of

rain, but varies from 30:1 in light, powdery snow to 2:1 in “old” snow

Four Precipitation Processes• Cyclonic/frontal

– Air is forced upward due to air temperature/density differences along fronts

– Results in light, steady precip or intense precip, depending on temp differences

– Not too common in Arizona or the southwestern US, occasionally see during winter and with remnants of hurricane or tropical storm

– Cold fronts act as an anvil pushing warm air up from the surface (Midwest)

Four Precipitation Processes

• Orographic– Air is forced over a

mountain, hill, or other topographic barrier

– Results in light, steady precip or intense precip, depending on strength of uplift

– Common in Arizona during the monsoon

Rainshadow

• Dry area that exists on leeward side of topographic barrier

• So which type of precipitation process?

• Why does this happen?• Examples:

– Coastal Ranges

– Sierra Nevada

Four Precipitation Processes

• Convectional– Air is forced upward

due to heating of the surface/lower levels of the atmosphere

– Generally results in intense precipitation

– Common in Arizona during the monsoon

Figure 4.20

Four Precipitation Processes

• Convergence– Air is forced upward as

opposing wind “collides”

– Results in light, steady precip or intense precip, depending on strength of “collision”

– Fairly common in Arizona during monsoon

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