The document discusses various methods of measuring humidity. It describes common humidity measurements like relative humidity, specific humidity, and dew point temperature. It then explains different types of instruments that can measure humidity, including psychrometers, hair hygrometers, capacitive hygrometers, resistive hygrometers, chilled mirror hygrometers, humidity cards, and gravimetric hygrometers. Each works on a different principle like evaporative cooling, expansion/contraction of materials, changes in electrical properties, or absorption of moisture. The document provides details on the principles, components, and workings of these various humidity measuring instruments.

1. Devanshi Maharana
2022-11-128

2. 1. Evaporation from water bodies
2. Evaporation of water from moist
soil surface.
3. Transpiration through vegetation
INTRODUCTION:
•Atmosphere contains some amount
of moisture, which varies with
latitudes, seasons and time of the
day
•Moisture gets into atmosphere by:

3. What is HUMIDITY?
•Humidity is the amount of water vapour content of air suspended in the
atmosphere at any place and time
•Since water vapour is dry like any other gas, it has nothing to do with wetness
or dryness of atmosphere
•Although there is spatial and temporal variation in the quantity of water
vapour present in the atmosphere, the total amount of water vapour remains
nearly constant.
•Among various components of atmosphere, water vapour constitutes only a
small fraction varying from 0-4% by volume.

4. HYDROLOGICAL CYCLE
•The entire process of maintaining constant
water vapour content of the atmosphere is
known as the hydrological cycle
•Even though the water vapour content of the
atmosphere is on the average constant ,there are
local changes because of seasonal and local
variations
•The hydrological cycle involves evaporation ,
condensation and precipitation
•The cyclic movement is mainly responsible for
the distribution of moisture over the earths
surface.
•It is close to all weather phenomena.

5. TRANSFER OF HUMIDITY
LAND SEA MOISTURE EXCHANGE
•Continous exchange of moisture taking place between the continents and the ocean
•Amount of moisture gained through precipitation on the continents is more than
lost through evapo transpiration
•Ocean loose more moisture through evaporation than receive through precipitation
•This can be seen in a daily phenomenon in the form of land breeze and sea breeze.
•The phenomenon of el nino and la nina is also due to this.
LAND SEA MOISTURE EXCHANGE
MERIDIONAL MOISTURE EXCHANGE

6. Winds carry
moisture to
continents
Water vapour
condenses and
precipitates
Water returns to
ocean by Surface
run off or ground
water seepage
Off shore moisture
laden winds from
lands moves to
oceans and
precipitates
Evaporation from
ocean

8. MERIDIONAL MOISTURE EXCHANGE
•Transfer of moisture between different latitude belts on the earth
•Why it happens?
Precipitation exceeds evaporation Evaporation exceeds precipitation
40°-70° North and South latitude
(middle latitude zones)
10°-40° North and south
latitude(equatorial zones)
Gain of energy through latent heat of
condensation
Energy transformed into latent heat of
evaporation
•Latitudinal imbalance produced requires a meridional exchange of moisture as well as
energy between dry and wet zones.
•This naturally upsets the latitudinal heat balance.
•However planetary winds, cyclones, anticyclones, and atmospheric disturbances play a
significant role in removing the latitudinal imbalances

10. MEASUREMENT OF HUMIDITY
1. ABSOLUTE HUMIDITY
2. SPECIFIC HUMIDITY
3. RELATIVE HUMIDITY
4. MIXING RATIO
5. VAPOUR PRESSURE
6. DEW POINT TEMPERATURE

11. ABSOLUTE HUMIDITY
•The weight of water vapour per unit volume of air.
•Expressed as grams of water vapour per cubic metre of air(g/cm3).
•It is seldom used by meteorologists because it varies with expansion and
contraction of air even if the amount of water vapour is constant.
Temperature (°C) Water vapour(g/m3)
0 4.80
10 9.41
20 17.31
30 30.4
40 51.2
50 83.0

12. SPECIFIC HUMIDITY
•Weight of water vapour per weight of a given mass of air(including water
vapour)
•Expressed as grams of water vapour per kilogram of air (g/kg)
•Most commonly used as it represents quantity of water vapour(more constant
property)
•It changes only as the quantity of water vapour undergoes change
•Specific humidity is not affected by changes in pressure or temperature of air
•It is directly proportional to the vapour pressure of air and inversely
proportional to air pressure

13. CONTD..
•Maximum amount of specific humidity is found in the equatorial zones and
minimum at the poles.
•Capacity of air depends on its temperature.
•Amount of water vapour present near the equator is much more than polar
region
•Specific humidity in a particular region is much higher during summer than in
winter.
•It is also higher on the oceans than the continents.

14. Relative Humidity Specific Humidity (g/kg)
10% 1.7
20% 3.5
30% 5.3
40% 7.1
50% 9.0
60% 11.0
70% 13.1
80% 15.3
90% 17.7
100% 20.2

15. RELATIVE HUMIDITY
•It is the ratio of the amount of water vapour in the
air to the amount of air can hold at that
temperature.(or its capacity)
•RH is the ratio of the air’s water vapour content to
its water vapour capacity
•RH= Vapour pressure*100
Saturation vapour pressure
•RH is expressed in terms of percentages
•RH increases with the increase in evaporation, as
vapour pressure is directly proportional to the
amount of water vapour present in the air.
•RH= e/es
(e-vapour pressure;es-saturation vapour pressure)

17. MIXING RATIO
•Weight of water vapour per unit weight of dry air
•Expressed in grams of water vapour per kilogram of dry air(not the
moist air i.e dry air +water vapour)
•This value is used in weather forecasting analysis
•Equation: mixing ratio = 622*e/(p-e)
w- mixing ratio
e-vapour pressure
p-total air pressure

18. •It is the part of total atmospheric pressure
which is attributable to its water vapour
contents
•Water vapour that rises up into the
atmosphere , leads to a small increase in
pressure in the air
•This increase is the result of the motion of
the water vapour molecules supplied to air
through evaporation
VAPOUR PRESSURE

19. SATURATED VAPOUR PRESSURE
•Saturated vapor pressure refers to the
maximum pressure of the water vapor that
can exist in equilibrium with its liquid phase,
at a particular temperature
•At this point, the rate of evaporation of the
liquid is equal to the rate of condensation of
the vapor
•The saturation vapour pressure is a function
of temperature.
•They are logarithmically related
Log10es = 0.02604T+0.82488
Log10es = 0.02604T+0.82488

20. •The dew point temperature is the temperature at
which air becomes saturated with water vapor,
leading to the formation of dew, fog, or clouds
• At this point, the air cannot hold any more
moisture, and any additional moisture will
condense into water droplets.
•The dew point temperature is an important metric
for meteorologists, as it helps to determine the
likelihood of precipitation, as well as the potential
for fog or dew formation
•Higher dew point temperatures indicate higher
humidity levels, while lower dew point
temperatures indicate drier air.
DEW POINT TEMPERATURE

21. INSTRUMENTS MEASURING HUMIDITY

22. •The Chinese used charcoal and a lump of earth:-the dry weight of the
charcoal and a lump of earth was compared to their damp weight after
exposure to the air
•Based on the differences in weight, the humidity level was calculated
•In 1480, Leonardo da Vinci invented the first crude hygrometer
•Francesco Folli created a more practical version of the device during the
1600s
• Robert Hooke improved several meteorological instruments including the
hygrometer
•In 1783, Swiss physicist and geologist Horace Benedict de Saussure invented
the first hygrometer that measured humidity utilizing human hair
HISTORY OF INSTRUMENTS

23. STATION HYGROMETER
Working principle
concept of evaporative cooling.
•The wet bulb thermometer has a wick
wrapped around its bulb which is saturated
with distilled water
•When air is passed over this wet wick, some
of the water evaporates, which causes the
temperature of the wet bulb thermometer to
drop due to the cooling effect of evaporation
•The dry bulb thermometer, on the other
hand, measures the ambient air temperature

24. •The amount of cooling of the wet bulb
thermometer is dependent on the amount
of moisture in the air
•If the air is already saturated with
moisture, the evaporation of water from
the wet bulb will be slower, and therefore
the temperature difference between the
two thermometers will be lower
•On the other hand, if the air is dry, the
water will evaporate more quickly, leading
to a greater temperature difference
between the two thermometers

25. HAIR HYGROMETER
Working principle:
Expansion and contraction of hair
with change in moisture content
•A bunch of hair is used in a socket.
•On moisture absorption, the hair
strands expand
•Slight change increase in the volume
of hair is magnified by a lever
mechanism.

26. •A pointer attached to the lever mechanism
moves on a graduated scale from 1-100 %
•The pointer on the contraction or
expansion of hair points at the relative
humidity directly.
HAIR HYGROMETER

27. HAIR HYGROGRAPH
Working principle:
based on the fact that human hair
changes its length and shape as the
moisture content in the air changes
•It measures the continuous changes
in RH on graph paper throughout the
day.
•A recording mechanism is used for
that.
•A band of hair is fixed on the levers
and slight increase in volume is
transmitted to pen arm.
•A caliberated chart is wrapped around
a rotating drum which works on clock
mechanism.

28. •The X-axis represents time and Y-axis
represents relative humidity
•This instrument is place in a double
stevenson screen
WHY IS DEOILED HUMAN HAIR USED?
•When air is humid ,the space between
the cells absorb water vapour and hair
thickens and lengthens.
•The contraction and expansion is thus
used to measure the moisture content.

29. Working principle
similar to that of a regular
psychrometer, with the difference
being the method of air circulation
•As the sling psychrometer is swung
around in the air, the wet bulb
thermometer is moistened with
distilled water, and the air passes
over the wet wick, causing the water
to evaporate and cool the
thermometer
Sling Psychrometer

30. •The dry bulb thermometer, on the other
hand, measures the ambient air
temperature
•The amount of cooling of the wet bulb
thermometer is dependent on the amount
of moisture in the air, which can be used to
determine the relative humidity
•Once the readings from both
thermometers are stabilized, the
temperature difference between the two
thermometers is used to calculate the
relative humidity of the air using
psychrometric tables or formulas

31. ASSMANN PSYCHROMETER(Aspiration psychrometer)
Working principle:
its working is based on the concept of
evaporative cooling, which is used to determine
the humidity of the air
•Two thermometers i.e wet and dry
thermometers are enclosed in a double walled
radiation screen.
•Nickel plated coaxial tubes are are used to
minimise the radiation effects.

32. •To ensure proper evaporation from the
wick of the wet bulb ,the psychrometer is
aspirated by a clock work motor housed in
the casing.
•The wet bulb is kept on the downstream
side of the dry bulb i.e near the fan
•This is done to prevent the cold air from
the wet bulb coming in contact with the dry
bulb
•The temperature difference is used to
calculate the relative humidity of the air
using psychrometric tables.

33. HYGROMETRIC TABLES

34. Working principle: When the humidity or
moisture content in the surrounding
environment changes, it affects the dielectric
constant of the dielectric material between the
sensor plates which in turn affects the
capacitance.
•The capacitive hygrometer measures the
capacitance of the sensor, which is the ability of
the sensor to store electrical charge
•The capacitance is typically measured using an
AC signal, which is applied to the sensor plates.
CAPACITATIVE HYGROMETER

35. •The dielectric constant is a measure of the
ability of a material to store electrical charge,
and it changes with humidity
•Materials with dielectric constant ranging
from 2-15 are used
•As the humidity increases, the dielectric
constant of the humidity-sensitive material
also increases, resulting in an increase in
capacitance
C=1.418 RH+ 29.139
Advantages
high accuracy, fast response time, and wide
measurement range.

36. Working principles
changes in electrical resistance with varying moisture
levels
•A resistive hygrometer is constructed by placing the
moisture-sensitive material between two conductive
electrodes
•Moisture-sensitive material such as ceramic,
polymer, or organic materials that is used as the
sensing element and connected in a circuit to
measure the resistance changes
•Salts and polymers with conductive properties are
typical materials
RESISTIVE HYGROMETER

37. •When the humidity level changes, the
sensing material absorbs or releases
moisture, which leads to a change in its
electrical resistance
•This resistance change can be measured
using a Wheatstone bridge circuit or other
types of circuitry to determine the
humidity level

38. Working principle
It is based on the fact that the dew point
temperature of a gas is directly related to its
moisture content
When the gas temperature is lowered to the
dew point temperature, the moisture in the
gas condenses on a surface, forming dew or
frost.
•In this method, a small mirror is cooled
using a thermoelectric cooler or a
refrigeration unit.
DEW POINT HYGROMETERS -CHILLED MIRROR HYGROMETER

39. •Cooling methods, including dry ice,
compressed carbon dioxide, mechanical
refrigeration, and liquid air
•The mirror is exposed to the air being
measured, and the temperature of the
mirror is gradually lowered until moisture
starts to condense on its surface
•The temperature of the mirror at which
condensation occurs is measured using a
sensor
•This temperature corresponds to the dew
point temperature of the air being
measured

40. OPTICAL HYGROMETER
Working principle
•Absorption or reflection of light depends upon
the amount of water molecules in the air
•The amount of light that is scattered by tiny
water droplets or ice crystals in the air is
measured
•Types of optical hygrometers
-chilled mirror dew point hygrometer
-infrared absorption hygrometer
-Raman scattering hygrometer

41. •The easiest method of measuring moisture is with
humidity cards
•These cards are designed to indicate the humidity
range by changing colors to indicate whether the
humidity is less than the range, within it, or more
than the top margin
•Cobalt chlorine solutions and various additives are
utilized to cover a wide range of moisture conditions
•These cards help measure humidity in containers
and packaging when observed through transparent
windows
• A relative humidity card can measure from 5
percent to 95 percent
HUMIDITY CARDS

42. Working principle
hygroscopic materials tend to absorb moisture
from the surrounding air until they reach a
state of equilibrium
measuring the change in weight of a
hygroscopic material as it absorbs or releases
water vapour
•Components of a gravimetric hygrometer
include a sample chamber, a hygroscopic
material, a balance, and a temperature and
pressure sensor
GRAVIMETRIC HYGROMETER

43. •The sample chamber is used to hold the gas or air sample to be
measured
•The hygroscopic material is usually a salt or a silica gel that has a high
affinity for water vapor.
•The balance is used to measure the weight of the hygroscopic material
before and after exposure to the gas or air sample
•Weight of the hygroscopic material is then measured using the balance,
and the change in weight is used to calculate the amount of moisture
present in the gas or air sample
•Gravimetric hygrometers are highly accurate and expensive, and they are
used to set standards throughout the United States, United Kingdom,
European Union, and Japan