Hydraulic Control Structures and Atmospheric Science and Precipitation

critical flow

going over structure

thin, fast

energy is at is min.

unstable

subcritical flow

steady uniform stable flow

supercritical flow

unstable

Humidity

• amount of water vapor in air

• maximum amount of moisture in air is temperature-dependent

Which holds more water vapor: warm or cold air

warm air

specific humidity

• the amount of water vapor available for precipitation

Relative Humidity

• the amount of water in the air compared to what the air could hold ( expressed as a percentage)

What humidity do we use in daily weather reports

relative humidity

relative humidity changes by

• gain of water vapor, i.e. evaporation from exposed surface

• change of temperature

Dew point

• temperature to which air must be cooled for condensation to occur( air is saturated at this point)

Moist air vs dry air dew point

• Moist air has a higher dew point than dry air

what does close air temps and dew points indicate

• high relative humidity

Heat Index

• combines air temp and relative humidity to describe how hot it feels

High relative humidity reduces

evaporation rate

condensation 

• happens when warm, moist air comes in contact cooler surface

• it is excess moisture in air condenses since cool air can’t hold as much water as warm air

Adiabatic Principle

• as gas expands, its temperature decreases

Adiabatic process

• as gas expands its tem[. decreases

• heating or cooling of air due solely to pressure change

• Precipitation forms when a large air mass steadily drops in temperature below the dew point

  • Occurs when air mass is lifted to higher level of the atmosphere

Dry Adiabatic Rate

• Atmospheric pressure decreases with increasing altitude

• As an air mass rises, pressure decreases, and air expands and cools

  • Descending air mass will compress and warm

• air mass is warming

Dry adiabatic lapse rate

• rate at which rising air is cooled by expansion when there is no condensation (air is not saturated)

• only applies to air masses in vertical motion

Dry adiabatic rates constant value

10°C/1000m

Wet Adiabatic Rate

• if air mass moves high enough, it will cool to its dew point

  • condensation will occur

  • lifting the condensation level

• —If cooling continues, water droplets will form, making a cloud

  • Condensation of water droplets releases latent heat

  • Slight warming of air mass

its cooling is due to uplift

the closer to the atmospher what happens to pressure and temperature

low pressure, and cooler

Atm. pressure decrease with

altitude

Percipitation is formed when

a large air mass steadily drops in tempurature below the dew pt.

wet adiabiatic rate cooling is due to

uplift

Cooling rate of saturated air vs dry air

• lower due to warming from condensation

wet adiabatic rate warms from

condensation

Wet adiabatic lapse rate

cooling rate for saturated air masses

—Not constant

—Ranges between 4 – 9 °C/1000m

—Usually use 5°C/1000m

• once condesation occurs the rate gets cut in half

Precipitation occurs as a result of

adiabatic cooling of moist air

Mechanisms for lifting air

—Frontal lifting

—Convectional lifting

—Orographic lifting

Frontal Lifting

• Forms when a cold air meets warm air (front)

  • forms when 2 air masses meet

—Warm air is less dense and rises, cools, and condenses

cold front 

isolated showers, cumulonimbus clouds

warm front

more widespread showers

• showers last longer

• cover more area

stratiform clouds( nimbostratus)

what are the clouds that give rain

cumulonimbus

nimbostratus

Lake effect snow

• forms when cold air moves over warm water

• cold air is being warmed as it goes over the warm water causing clouds to form and grow, resulting in heavy snow falls

Convectional Lifting

—Air parcel heated by warm ground and rises

—Can be very localized due to temperature differences

—Can produce intense, long-lasting storms over small areas

Convective precipitation

—Convection cells = vertical columns of rising air

—Even as cooling occurs with altitude, parcel remains warmer than surrounding air

eye of storm

lowest pressure, why hurricanes swirl(air rushing in to fill low pressure area

Orographic Lifting

• when an air mass is forced over mt. or topographic high

1. Moist air from ocean reaches coast

2. Air rises on windward side of mountains and is cooled at dry adiabatic lapse rate.  When cooling   is sufficient, condensation occurs and clouds form.

3. Cooling occurs at wet adiabatic rate and precipitation occurs.  Air passes over summit and starts down the leeward slope.

   4.  Air warms on descent and is rid of moisture by end of descent