ATMOSPHERIC MOISTURE24.1

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Water and the Hydrosphere

Three States of Water:

•Solid (ice)

•Liquid (water)

•Gas (vapor)

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c.Hydrosphere: Total realm of water at Earth’s surface.

•Oceans

•Ice

•Surface water

•Groundwater

•Atmospheric water

•Soil moisture

•Biota

Water and the Hydrosphere

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Water and the Hydrosphere

Distribution of water in the hydrosphere

•Oceans: 97.5%

•Fresh water: 2.5%

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Water and the Hydrosphere

Hydrologic Cycle:Water moves among the ocean, atmosphere and land

• Evaporation

• Precipitation

• Transpiration from plants

• Runoff

• Sinks into soil

• Recharge of groundwater

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Precipitation: Particles of liquid water or ice that fall from the atmosphere and may reach the ground.

Water and the Hydrosphere

•Humidity refers to the amount of moisture (water vapor) in the surrounding air.

•Relative Humidity is a measure of the amount of moisture in the air compared with the amount of moisture the air can hold.

•Relative humidity is expressed as a percentage of how much moisture the air could possibly hold at the temperature it happens when you measure it.

•The "wetter or damper" you feel,, the higher is the relative humidity. If you feel the air is dry around you, the relative humidity is low.

•We are very sensitive to humidity. Sweating keeps our body cool and maintain its current temperature.

•If the air is at 90% relative humidity, sweat will not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high.

•If the relative humidity is low, we can feel much cooler than the actual temperature because our sweat evaporates easily, cooling the body.

HUMIDITY Organisms need to remain in highly hydrated

state Why? Most (all?) biochemical reaction require water

to occur Water budget for organism

a. + liquid water uptakeb. - water vapor loss

i. breathing, sweating, evapotranspiration Water loss generally viewed as bad However, many cases where vapor loss is

beneficial or essential for survival

ORGANISM SURVIVAL THROUGH VAPOR TRANSPORT

Vapor loss or latent heat flux cools organism Can allow survival of organism in harsh

environment

a. Texas rice plants Provides 44 kJ/mol of energy loss

a. Called latent heat of vaporization

b. 580 x energy required to raise 1 mol water 1oC Amount of cooling defined by: Difference in water vapor concentrations Conductance to water vapor from the surface to

the air Remember Flux = conductance (Conc, surface –

Conc, air)

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Humidity

Humidity: the amount of water vapor in the air

The maximum quantity of moisture that can be held in the air depends on air temperature

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Humidity

Relative Humidity: compares the amount of water vapor present in the air to the maximum amount that the air can hold at that temperature

Expressed as a percentage:

At 100% relative humidity, air is saturated.

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Humidity

Relative Humidity changes when:

1. Atmosphere gains or loses water vapor

•Evaporation

2. Temperature changes

•Lower temperature relative humidity rises

•Raise temperature relative humidity decreases

HOW RELATIVE HUMIDITY (RH) IS MEASURED?

•Humidity is measured by means of a hygrometer.

•There are different types of hygrometers.

•The most common hygrometers are Wet- and Dry- Bulb Psychrometer and Hair Hygrometer.

WET- AND DRY BULB PSYCHROMETER

It consists of two identical mercury thermometers, one of which has a wet cotton or linen wick

around its bulb.

•Evaporating water from the wick absorbs heat from the thermometer bulb, causing the thermometer reading to drop.

•The difference between dry-bulb and wet-bulb temperatures is compared on psychrometric charts.

WET - AND DRY BULB PSYCHROMETER

If the difference between wet and dry bulb is 6º F and the temperature is 72º F (dry bulb), then the RH is 54%.

•Humidity stretches

the hair (human /

animal / synthetic)

while dryness

shortens it.

•The hygrometer has the job of stretching a hair between a fixed

and a movable point

to measure humidity

over time.

Hair Hygrometer

•Other kinds of hygrometers use materials with electrical resistance that varies with the amount of moisture absorbed.

•With such hygrometers, a measurement of electrical resistance can be calibrated as a humidity measurement.

QUESTIONS

1. What is humidity?2. What is relative humidity?3. How relative humidity is measured? 4. What is a Hygrometer?

5. How Wet- and Dry Bulb Psychrometer and Hair Hygrometer measure relative humidity?

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Humidity

Specific Humidity: actual quantity of water held by a parcel of air

•Grams of water vapor per kilogram of air (g/kg)

•Highest in equatorial zones

•Lowest near poles

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Dew-point temperature: temperature at which air with a given humidity will reach saturation when cooled without changing its pressure

Humidity

DEW POINT TEMPERATURE

The temperature at which air, when cooled without

changing pressure or water content, reaches saturation

Air is seldom saturateda. Typically only find saturation at night (lowb. temperature)c. Air typically does not cool below the dew

pointd. Takes significant amount of energye. Latent heat of cooling