Atmospheric Moisture 24.1

Three States of Water: Solid (ice) Liquid (water) Gas (vapor) Water and the Hydrosphere Three States of Water: Solid (ice) Liquid (water) Gas (vapor) Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

Hydrosphere: Total realm of water at Earth’s surface. Water and the Hydrosphere Hydrosphere: Total realm of water at Earth’s surface. Oceans Ice Surface water Groundwater Atmospheric water Soil moisture Biota Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

Distribution of water in the hydrosphere Water and the Hydrosphere Distribution of water in the hydrosphere Oceans: 97.5% Fresh water: 2.5% Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

Hydrologic Cycle: Water and the Hydrosphere Water moves among the ocean, atmosphere and land Evaporation Precipitation Transpiration from plants Runoff Sinks into soil Recharge of groundwater Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

Water and the Hydrosphere Precipitation: Particles of liquid water or ice that fall from the atmosphere and may reach the ground. Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

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 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 + liquid water uptake - water vapor loss 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 Texas rice plants Provides 44 kJ/mol of energy loss Called latent heat of vaporization 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)

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 Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

Expressed as a percentage: 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. Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

Relative Humidity changes when: Atmosphere gains or loses water vapor Evaporation Temperature changes Lower temperature relative humidity rises Raise temperature relative humidity decreases Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

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%.

Hair Hygrometer 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 What is humidity? What is relative humidity? How relative humidity is measured? What is a Hygrometer? How Wet- and Dry Bulb Psychrometer and Hair Hygrometer measure relative 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 Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

Humidity Dew-point temperature: temperature at which air with a given humidity will reach saturation when cooled without changing its pressure Visualizing Physical Geography Copyright © 2008 John Wiley and Sons Publishers Inc.

Dew Point Temperature The temperature at which air, when cooled without changing pressure or water content, reaches saturation Air is seldom saturated Typically only find saturation at night (low temperature) Air typically does not cool below the dew point Takes significant amount of energy Latent heat of cooling