Specific humidity

Relative humidity

Saturation mixing ratio

Vapour pressure

Mixing ratioAbsolute humidity

Outline

I• Introduction

II• Measurement Used to Determine Atmospheric Humidity

III• Effect of Humidity

IV• Instruments Used to Measure Humidity

V

• Relationship of Humidity to other Meteorological Parameters

VI• Humidity on Land and Oceans

IntroductionAtmospheric humidity is a measurable quantity of the moisture content found in the Earth’s atmosphere.

Humidity is the amount of water vapour in the atmosphere. The main sources of water vapour in the lower atmosphere are evaporation and transpiration.

Atmospheric water vapour accounts for only about 1/1000th of the total amount of water in the global hydrological cycle.

The total volume of water in the atmosphere is about 1.3 x 1013 m3 , the overwhelming majority of which is the vapor phase (Air Resources Laboratory, 2008)

Oceans contain about 1.35 x 1018 m3 of water (Air Resources Laboratory, 2008)

Atmospheric water vapour is one of the most important factors in determining Earth’s weather and climate, because of its role as greenhouse gas and because of the large amounts of energy involved as water changes between the gaseous (vapor) phase and liquid and solid phases

Measurements

Various ways of expressing humidity

Absolute humidity (g of vapour/m3 of air)– expresses the vapourcontent of the air using the mass of water vapour contained in a givenvolume of air. Absolute humidity changes when the volume changes,even though the mass of water vapour has not changed.

Specific humidity ( g of vapour/kg. of air)– measures the water vapourcontent of the air using mass of water for a given mass of air. Specifichumidity doesn’t change as the air parcel expands or is compressed.

Vapour Pressure – measures the water vapour content of the air usingpartial pressure of the water vapour in the air.

Measurements

Measurements

Mixing Ratio (g of water vapour/kg. of dry air) -refers the ratio of the mass of water vapour to themass of dry air in the sample. It is not affected bychanges in pressure and temperature.

Saturation mixing ratio - refers to the mass of watervapour that can held in a kilogram of dry air atsaturation. Saturation can be generally defined as thecondition of liquid water or the deposition of ice at agiven temperature and pressure.

Relative humidity – the most commonly used tomeasure humidity. It is the amount of water in the airrelative to the saturation amount the air can hold at agiven temperature multiplied by 100.

Temperature

Degrees

Celsius

Vapor (g) per

Kilogram of Dry

Air

50 88.12

40 49.81

30 27.69

20 14.85

10 7.76

0 3.84

Fig. 1 Illustration on how relative humidity changes in a parcel of air with an increase in air temperature

Note:

The more watervapour in the air, thehigher the relativehumidity is at a giventemperature.

Air with a relativehumidity of 50%contains half of watervapour it could holdat a particulartemperature.

Measurements

Factors

Change the amount of water vapour available –Example: Lake – relative humidity can be increased by evaporation

from the surface of the lake.

Water vapour Humidity

Change the temperature of the air -Example: No water vapour is added.

Air temperature Relative humidity

Note:

Relative humidity varies considerably, tending to be the highest close todawn when the air temperature is at its lowest, and decreasing as theair temperature rises. The decrease in the relative humidity towardsmidday tends to be the largest in summer. In areas with high humiditylevels, the transmission of solar radiation is reduced because ofatmospheric absorption and scattering. High humidity reducesevaporation of water and sweat. Consequently, high humidityaccompanied by high ambient temperature causes a lot of discomfort.(http://mnre.gov.in)

Effects of Humidity

Transpiration of water vapour through leaf stomata depends on the gradient ofmoisture between the leaf interior (which is saturated) and the overlying air, aswell as the availability of moisture in the soil. The lower the atmospheric humidity,the greater the transpiration rate. The transpiration rate is determined by a balancebetween the amount of energy available to convert water from the liquid to vapourphase and the moisture gradient.Transpiration rates also depend on the resistance to water movement between theleaf and the air.

Atmospheric humidity affects the organisms by modifying its water content. Someforms are hygroscopic and absorb water from moist air. Different studies showedthat specific organisms gains or lose weight depending on the humidity’s in acertain environment.

Animals

Plants

HumansHumans too are very sensitive to humidity because our skin relies on the air toget rid of moisture.

So if the air is at 100% relative humidity, sweat will not evaporate into the air. Asa result, we feel much hotter than the actual temperature when the relativehumidity is high. If the relative humidity is low, we can feel much cooler than theactual temperature because our sweat evaporates easily, cooling us off.

For example, if the air temperature is 75 degrees Fahrenheit (24 degrees Celsius)and the relative humidity is zero percent, the air temperature feels like 69degrees Fahrenheit (21 C) to our bodies. If the air temperature is 75 degreesFahrenheit (24 C) and the relative humidity is 100 percent, we feel like it's 80degrees (27 C) out. People tend to feel most comfortable at a relative humidity ofabout 45 percent (www.science.howstuffworks.com). High humidity causeshealth problems such as sunstroke and heatstroke.

Instruments

Sling psychrometer- often used to determine relative humidity. Consists of thermometers (wet bulb and dry bulb).

The difference between the dry and wet bulb reading will give with the aid of psychometric table, the dew point temperature and the relative humidity. Performed near saturation but under read at lower humidities

Hygrothermograph – records both relative humidity and temperature on graph paper in the same manner as the thermograph and barograph do.

Hair hygrometers – it works on the fact that hair changes its length whenhumidity varies. This device usually consists of a number of human or horse hairsconnected to the mechanical lever system. When humidity increases the length ofthe hair becomes longer. This device is less accurate than the psychrometer.

Satellites – on a global scale, humidity is being measured using remotely placedsatellites. These satellites are able to detect the concentration of water in thetroposphere at altitudes between 4 and 12 kilometers. Satellites that can measurewater vapour have sensors that are sensitive to infrared radiation.

Relationship of humidity to

other Meteorological

ParametersWeather Parameters Relationship Remarks

Air pressure Inversely proportional More humidity in the air, the lighter the air pressure and When less humidity is in the air, there is more air pressure. (Givoni, 1976).

Air temperature Directly related to solar radiation

Higher the temperature, the higher is the amount of energy in the air.(Bharat and Sheeba, 2011)

Precipitation Directly proportional As precipitation increases, humidity also increases.

Temperature Inversely proportional Temperature changes; relative humidity also changes but in opposite direction.

Soil temperature Indirect and minimal Air temperature was the factor that influenced most the soil temperature.(Salamene et. al. 2010)

Wind Less affected Air pressure differences between different locations will cause air pressure differences which in turn produce air movement (wind)

Climate Variables and their

Correlation

Humidity – Ocean and Land

Earth has become more humid in recent decades.