ATMOSPHERIC HUMIDITY
ABE 110 Plant Climate - rbsaludes
Humidity
• refers to the amount of moisture
(water vapor) in the air
• water vapor exerts weight and
pressure in air parcel
• air can hold up certain amount of
water vapor depending on the
temperature
ABE 110 Plant Climate - rbsaludes
Role of Humidity in Plants and Animals
• influences the drying capacity of air
• influences rate of transpiration in plants
• affects phyto-pathogen growth and
development
• influences thermal comfort (evaporative
cooling)
• primary source of water (dew formation)
ABE 110 Plant Climate - rbsaludes
Atmospheric moisture content can be
expressed in several ways:
1. Absolute humidity
2. Mixing Ratio
3. Specific Humidity
4. Vapor Pressure
5. Relative Humidity
6. Dew-Point Temperature
7. Wet-bulb Temperature
ABE 110 Plant Climate - rbsaludes
ABSOLUTE HUMIDITY
•mass of water vapor per volume of air (g/m3)
•measured by shining electromagnetic radiation
across a path of humid air
•attenuation of the signal is a measure of the
number of water vapor molecules along the
path
MIXING RATIO
•mass of water vapor per mass of dry air (g/kg)
SPECIFIC HUMIDITY
•mass of water vapor per mass of air including its
moisture (almost identical to mixing ratio at low
water contents), g/kg
Mixing ratio and specific humidity are not measured
directly but are extremely useful for meteorological
conditions
VAPOR PRESSURE
• partial pressure exerted by water vapor in the
atmosphere (Pa or mb)
• moist air behaves like an ideal gas and obeys
Dalton’s Law
P
Pd
ea
P = Pd + e a
pressure of mixture of gases
partial pressure of dry air
partial pressure of water vapor in mixture
• amount of water in air is directly related to the
partial pressure of water vapor
SATURATION VAPOR PRESSURE (es)
• upper limit of vapor pressure at a particular
temperature
• atmosphere can hold
large mass of vapor
(higher es) at warmer
temperature
SVP as a function of temperature or the
dew-point curve (Stull, 2000)
Saturation vapour pressure can be calculated from
the air temperature using Tetens Formula:
es = 0.6108 e
where
es
T
⎡ 17.27T ⎤
⎢ T + 237.3 ⎥
⎣
⎦
saturation vapour pressure at the air
temperature
T [kPa],
air temperature [°C],
ACTUAL VAPOR PRESSURE (ea)
• actual vapor pressure exerted by water in an
unsaturated air
• directly measures the actual amount of water in
air
• ea < es (unsaturated air) and ea = es (saturated
air)
• difference between saturation vapor pressure (es)
and actual vapor pressure (ea) is called the vapor
pressure deficit
• vapor pressure deficit accurately measures the
evaporative capacity of air
RELATIVE HUMIDITY (RH)
• does not indicate the actual amount of moisture
in the air
• ratio of the actual vapor pressure (ea) to the
saturation vapor pressure (es), %
⎛ ea ⎞
RH = ⎜⎜ ⎟⎟ x100
⎝ es ⎠
• RH is affected by water vapor content of air
and air temperature
DEW POINT TEMPERATURE (Tdp)
•temperature to which air must be cooled to become
saturated at constant vapor pressure
• air temperature and dew point temperature are equal at
100% RH (saturated air) – condensation occurs
•saturation vapor pressure at dew point temp equals
ambient vapor pressure
•dew point is also a good indicator of dryness of air
•the drier the air, the larger the difference between the air
temperature and dew point temperature
WET - BULB TEMPERATURE (Twb)
• temperature to which the latent heat of vaporization
will cool the ambient air
• measured simultaneously with dry-bulb
temperature (air temperature) and use
psychrometric table/chart to convert to relative
humidity
• when air temperature equals wet-bulb temperature,
the relative humidity is 100%
RH
Sample Problem:
When air temperature is 23°C and ambient
vapour pressure is 1.1 kPa, graphically find the
relative humidity, dew point temperature, and
wet-bulb temperature.
Ans. RH = 39% (0.39)
Dew point temp = 8°C
Wet-bulb temp = 14.5°C
Daily variation in RH, es, and air temperature
Relative Humidity
4
12
Time, hours
22
Elevation, m
Typical midday profile of vapor pressure and
relative humidity over an irrigated crop
Vapor pressure, kpa
Relative humidity, %
PROFILE CHARACTERISTICS OF VAPOR
PRESSURE
• nearly isohumic (no change in vapor pressure
with height) in early morning
• steep lapse profile during midday due to
higher evapotranspiration and increase in
turbulence
• humidity gradients become moderate in the
early evening due to decrease in
evapotranspiration and turbulence
• inverted profile at night due to dew formation
Mean daily pattern of vapor pressure over and
within a dry bean crop
above canopy
Vapor pressure, kPa
within canopy
4
12
Time, hrs
22
MEASUREMENT OF ATMOSPHERIC HUMIDITY ABE 110 Plant Climate - rbsaludes
Unit of Measurement
Parameter Vapor Pressure Vapor Concentration Moisture Content Mixing ratio Relative Humidity ABE 110 Plant Climate - rbsaludes
Unit Pa kg/m3 g/kg g/kg percent Methods of Measuring Air Humidity
1. Thermodynamic method (e.g. psychrometer)
2. Mechanical method - change in physical
dimensions of hygroscopic substances (e.g. Hair
hygrometer)
3. Change in chemical and electrical properties of
material due to absorption or adsorption
4. Condensation method (e.g. dew hygrometer)
5. Spectral absorption of water vapor
ABE 110 Plant Climate - rbsaludes
PSYCHROMETER
Measures relative humidity
by getting the wet bulb and
dry bulb temperature and
Wet Bulb Depression: The
difference between the dry
bulb and wet bulb temperature
Relative Humidity can be
determined from the wet bulb
depression using a
psychrometric table
ABE 110 Plant Climate - rbsaludes
HAIR HYGROMETER measures relative humidity by means of the variation in
length of a strand of human hair
Elongation of hair as a function of
moisture condition of air is non
linear. This non-linearity is
corrected by mechanical linkage
between hair and the recorder
pen in the hygrograph
ABE 110 Plant Climate - rbsaludes
DEW POINT HYGROMETER The dew point hygrometer
uses a cooled mirror as a
sensor; in this case, the
temperature of the mirror is
monitored to determine the
temperature at which dew
first appears
ABE 110 Plant Climate - rbsaludes
Capacitance and Resistance Hygrometers
A thin film capacitor
hygrometer measures
humidity by detecting the
change in capacitance of a
thin polymer film (linear
response)
Resistance hygrometer works on
the principle that electrical
resistance varies in a material that
absorbs moisture (logarithmic
ABE 110 Plant Climate - rbsaludes
response)
Spectral absorption of water vapor
a.Infrared gas analyzer (IRGA)
• higher absorption of IR at higher water
vapor concentration of air
b. Lyman – alpha hygrometer
• far violet (121.56 nm) is strongly
absorbed by water vapor
ABE 110 Plant Climate - rbsaludes