Soil as a Three Phase medium

The space occupied by bulk soil can be categorized as volumes of:

a. Solids (mineral and organic matter),

b. liquids (water and solutes),

c. gases (soil air and water vapor).

In terms of particle size the solid phase could be classified

according USDA as

a. gravel (< 2mm),

b. sand (2mm - 50 micron),

c. silt (50 - 2 micron), and

d. clay (< 2 micron).

The solid matter constitutes 50% and the pore space (air and water)

50% of the soil volume. The proportions of air and water can vary

interdependently, with an increase in one being associated with a

decrease in the other.

Soil Functions

a. A medium for plant growth - anchors roots provides

nutrients and water

b. A hydrologic buffer - regulates water flow in the landscape

c. A chemical reactor: absorbs, releases, and transforms

inorganic and biochemical compounds (e.g. nutrients,

pesticides, minerals, heavy metals)

d. A habitat for organisms: micro-organisms are responsible for

most chemical transformations, while macro-organisms are

responsible for most physical transformations.

e. Moderates soil temperature

f. General hydrologic exchange (evapotranspiration)

g. Controls water infiltration into groundwater

Soil water potential

Definition:

Total soil water potential is defined as the amount of work per unit

quantity of pure water that must be done by external forces to

transfer reversibly and isothermally an infinitesimal amount of

water from the standard state to the soil at the point under

consideration.

a. Soil water status is related to energy and the forces that hold

and move water within the soil.

b. Three major forces are involved in the movement of soil

water, namely, Gravitational, Matric, and Osmotic (solute)

potential comprise the total soil water potential and give a

measure of the differences in energy status between soil

water and pure, standing water.

c. Standing water has a potential of zero.

d. Soil water movement occurs when there is a difference in

total potential between two points in the soil.

e. The direction of water movement, like energy transfers, will

be in the direction of the point having the lowest potential.

f. A dry soil absorbs water from a wet soil and soil water

moves toward an absorbing plant root.

g. water potential tells you how much energy will be released

when the water moves

h. water potential tells you how much energy will have to be

exerted to move the water , e.g. water down in soil being

taken up by plant roots water always flows from high to low

potential

  Components Total Soil Water Potential:

Ψt = Ψg + Ψm + Ψs + Ψp + ……

 Ψg - Gravitational Potential

Ψs - Solute (or Osmotic) Potential

Ψm - Matric potential

Ψp - Pressure Potential

Ψg = gh the Gravitational Potential g is acceleration due to

gravity and h is the height of the soil water above reference

elevation.

Gravitational potential

a. Gravitational potential is that portion the total soil water

potential due to differences in elevation of the reference

surface of pool water and that of the soil water from the

upper rooting zones following heavy precipitation or

irrigation.

b. It causes vertical and lateral infiltration or subsurface runoff.

c. Gravitational potential describes the force gravity has on

water.

d. The greater the height of water above a given reference point,

the greater the gravitational potential.

e. Gravitational potential is measured as the height above the

reference point. If the reference is the soil surface and there

are 10 cm of water sitting on the soil surface, then the

gravitational potential is equal to 10 cm.

f. Gravitational potential is responsible for water movement

under saturated conditions.

g. it plays an important role in removing excess water from the

upper rooting zones following heavy precipitation or

irrigation to ensure sufficient aeration in the plant rooting

system

h. recharging the groundwater reservoir

Osmotic potential - Osmotic potential is the difference in energy

between pure water and water containing dissolved salts.

a. Water flows from areas of pure water to areas of salty water.

b. Osmotic potential is expressed as a negative potential,

because the energy level of pure water is zero.

c. Movement of water into plant roots is greatly influenced by

osmotic potential, but within the bulk of the soil, osmotic

potential has little influence on water movement.

d. In soils, water will move from a wet zone (high potential) to

a dry zone (low potential).

e. Plant root potential is normally lower than the soil water

potential; therefore, water moves from the soil to the root.

f. Ψs the Solute (or Osmotic) potential depends on soil

solution and concentration of solutes which include inorganic

salts and organic compounds.

g. The greater the concentration of solutes the greater the

osmotic potential (the more negative the potential)

h. Any increase in solute concentration of soil water will result

in a lowering of soil water potential and a reduction in the

amount of water available for plant uptake.

i. In saline soils osmotic potential may control the movement of

water from soil into plant roots and microorganisms.

j. High salt concentrations in the soil surrounding the root zone

of a plant may lead to a water stress.

Explain. Salts in the soil water solution can reduce

evapotranspiration by making soil water less available for plant

root uptake. Salts have affinity for water and hence additional

force is required for crop to extract water from saline soil. The

presence of salts in the soil water reduces the total potential

energy of soil water. In addition some salts cause toxic effects in

plants and can reduce plant metabolism and growth.

Ψp Pressure Potential it is a hydrostatic pressure and applies only

to saturated zoneor to saturated soils

Ψm Matric potential

i. Matric potential represents that portion of the total soil water

potential due to the attractive forces between water molecules

and soil solids through capillarity and adsorptive forces.

ii. Matric potential describes the surface attraction of soil

particles for water. If a dry soil is adjacent to a pool of water,

the soil will absorb the water. Because free, standing water

(as in a pool of water) has an energy level of zero, the matric

potential of the soil must be less than zero.

iii.The matric potential is a measure of the water retaining

capacity of the soil and is affected by soil texture and soil

water content.

iv.Matric potential is responsible for soil water movement in

unsaturated conditions.

v. The matric potential is low in wet soils, in fact zero in

saturated soils.

vi.Results in a negative energy potential and has a negative

value

vii. Affected by differences in adhesion and soil porosity

referred to as suction or tension always has a negative value

free water has a water potential of zero in soil,

viii. Matric potential ψ is always negative (unless the soil is

saturated, then ψ = 0)

ix. the more negative the ψ value, the more suction it has to the

colloids, the harder it is for a plant to take it up don’t forget:

– a low ψ is a very big negative number (e.g -2000) – a high

ψ is a very small negative number (e.g. -10)

Water Content and Water Potential

Soil moisture storage - How the soil holds water

Soils hold water in two ways;

a. as a thin film an individual soil particles

b. as water stored in the pores of the soil. (Water held

/stored as a thin film on individual soil particles is said

to be in adsorption)

Soil water potential or soil water tension is measured either in

centibars or kilopascals. Soil moisture tension is the physical

force holding moisture in the soil, measured in centi bars or kPa of

soil water tension. The amount of moisture in the soil is not as

important as how difficult it is for the plant to extract it from the

soil. Soil moisture tension has to be overcome for the plant to

move water into its root system. Different soil types will have

different tensions for the same volumetric moisture content.

Moisture retention curves

A plant grows at 20% moisture content on a typical sand. What is

the moisture tension? How about for other soils?

Soil water definitions

These are the terms most commonly used when working with soil

water.

Saturation water content- when all the available pore space is

filled with water, the soil is said to be saturated all soil pores are

filled with water. Water at the saturation point in soils is held at a

tension of 0 MPa (0 bars)

Field capacity is the amount of water remaining in the soil a few

days after having been wetted and after free drainage has ceased.

The matric potential at this soil moisture condition is around - 1/10

to – 1/3 bar. The volumetric soil moisture content remaining at

field capacity is different for different soil types. For example,

about 15 to 25% for sandy soils, 35 to 45% for loam soils, and 45

to 55% for clay soils.

Permanent Wilting Point

Permanent wilting point is the water content of a soil when most

plants growing in that soil wilt and fail to recover their turgor upon

rewetting. The matric potential at this soil moisture condition is

commonly estimated at -15 bar. Most agricultural plants will

generally show signs of wilting long before this moisture potential

or water content is reached.

Oven-dry - The oven-dry condition is the reference state used as

the basis for expressing most soil characteristics. If soil is placed in

an oven and dried at 105oC, additional water will be removed.

Available Water Capacity

Available water (holding) capacity is the portion of water that can

be absorbed by plant roots. By definition it is the amount of water

available, stored, or released between field capacity and the

permanent wilting point water contents.

Soil Moisture Storage

Soil moisture storage refers to the amount of water held in the soil

at any particular time. The amount of water in the soil depends on

soil properties like soil texture and organic matter content. The

maximum amount of water the soil can hold is called the field

capacity. Soil moisture storage falls between 0 and the field

capacity.

Change in Soil Moisture Storage (∆ ST .

The change in soil moisture storage is the amount of water

that is being added to or removed from what is stored.

Total Available Soil Moisture (TAM) The quantity of water

(cm/m or mm/m) that a plant is able to extract from a soil,

calculated as FC (%Volume )−PWP (%Volume )× Depth of soil

Depletion factor (p) is the fraction of the available soil moisture

below which there will be moisture tress in the crop.

Readily Available soil moisture (RAM) is the fraction p of

total available soil moisture that a crop can extract from the

soil without suffering water stress.RAM=p TAM

Irrigation Depth or Depth of water; dw. dw=volumetric water content×depth of soil

Water in a field is usually expressed in terms of the average water

depth only. For example we say, the water depth in the field is 50

mm. We also say that the rainfall amount is 100 mm. For example

if a field is to be irrigated, we say the irrigation application should

be 85 mm.

Soil water balance - Soil water balance of the root zone

Evapotranspiration can also be determined by measuring the

various components of the soil water balance. The method consists

of assessing the incoming and outgoing water flux into the crop

root zone over some time period.

Irrigation (I) and rainfall (P) add water to the root zone. Part of I

and P might be lost by surface runoff (RO) and by deep percolation

(DP) that will eventually recharge the water table. Water might

also be transported upward by capillary rise (CR) from a shallow

water table towards the root zone.

Soil evaporation and crop transpiration deplete water from the root

zone. If all fluxes other than evapotranspiration (ET) can be

assessed, the evapotranspiration can be deduced from the change in

soil water content (D SW) over the time period:

ET = I + P - RO - DP + CR ± D SW

Content

Methods for determining soil moisture content

Soil moisture content of a soil is the amount of water stored in the

soil’s pores. Knowledge of the soil water content is important, for

example, in budgeting water, planning drainage lines or irrigation.

The techniques are

i. Gravimetric method

ii. Neutron Scattering technique

iii. Gamma-ray attenuation technique

iv. Gamma-ray backscattering technique

v. Electrical Resistance

vi. Thermal Conductivity

vii. Capacitance method

viii. Time Domain Reflectometry

Gravimetric method

Description: The oven-drying technique is probably the most

widely used of all gravimetric methods for measuring soil moisture

and is the standard for the calibration of all other soil moisture

determination techniques. This method involves removing a soil

sample from the field and determining the mass of water content in

relation to the mass of dry soil. Although the use of this technique

ensures accurate measurements, it also has a number of

disadvantages: laboratory equipment, sampling tools, and 24 hours

of drying time are required. In addition, it is a destructive test in

that it requires sample removal. Measurements will become

inaccurate because of field variability from one site to another. The

standard method is to dry the sample in an oven at 105 ºC for 24

hours

Measured Parameter: Mass wetness or water content by mass

Response Time: 24 hours

Volume of sample cylinder = 134.5 cm3

Mass of container = 120 g

Mass of wet soil sample + container = 365.19 g

Mass of dry soil + container = 313.41 g

Mass of wet soil = 365.19 – 120 = 245.19 g

Mass of dry soil = 313.41 – 120.0 = 193.41 g

Mass of water/moisture = 245.19 – 193.41 = 51.78g

Disadvantages:

a. Destructive test

b. Time consuming

c. Inapplicable to automatic control

d. Must know dry bulk density and transform data to volume

moisture content

Advantages:

a. Ensures accurate measurements

b. Not dependent on salinity and soil type

c. Easy to calculate

The soil water content (wetness) can be expressed in terms of

either mass or volume ratios or fractions.

Volumetric water content θv; volume wetness or volume

fraction of water θv Volumetric water content is the volume of

water in a given volume of soil.

θV =V w

V s+V w+V a

It is a dimensionless ratio of the water volume Vw relative to the

total bulk soil volume, Vt

V t=V s+V w+V a

Gravimetric water content; Mass wetness or dry mass fraction

(θg) : mass of water per unit mass of dry soil; This is the mass of

water relative to the mass of dry soil particles

θg=wet weight− dry weight

dry weight= weight loss by drying

weight of dried sample

θg in % water by weight=wet soil weight−dry soil weight

dry soil weight×100

Bulk Density

Dry Bulk Density= Mass of soilTotal Volume of soil

=M s

V a+V w+V s

Wet Bulk Density=mass of soil+waterTotal Volume

=M s+M w

V a+V w+V s

Importance of Soil Bulk Density

Bulk Density can influence the soils physical, chemical, and

biological properties.

Soil texture, Soil structure and arrangement of aggregates are some

of the physical properties of soil that affect or influence its water

holding capacity, drainage, and gas exchange.

As soil bulk density increases, the pore space in the soil decreases.

As pore space is reduced, especially macropores, the capacity for

gases to enter and exit the soil is also reduced. High bulk density

slows drainage of water. If surplus water cannot be moved through

the soil in a timely manner, crops or their roots can be injured or

killed.

Physical resistance of highly compacted soil can impede root

penetration. Roots tend to follow paths of least resistance and grow

where conditions for their survival are most favourale. Where pore

space is limited and bulk density density is high, root growtrh can

be expected to be impaired. Bulk density is also used as an index

of soil compactness, since a greater bulk density implies that the

solid phase is a larger proportion of the total volume

General Soil Properties by Type

Soil TypeDry bulk density

(g/cm3)

Sand 1.54

coarse sandy

loam1.47

Loam 1.36

fine silt loam 1.25

Clay 1.10

Particle Density

Particle Density is the mass per unit volume of soil particles,

usually expressed in grams per cm3 of soil particles.

Instead of particle density, the term specific gravity is often used.

Average specific organic matter is 1.47; sand 2.66; clay 2.75. For

the soil as a whole the particle density varies from 2.6 to 2.9(2.65

in average)

Particle density= Mass of soil particleVolume of particle

=M s

V s

Porosity

Soil porosity is the total pore space of the soil. To calculate the soil

porosity its particle density and bulk density have to be known.

Total porosity or the total pore spae, of a soil is calculated from the

dry bulk density and particle density

Use of bulk density to estimate soil porosity

porosity( f )=1−ρdb

ρpd 

Example:Find the porosity of soil having a dry bulk density of 1.28 g/cm3

and particle density 2.65 g/cm3

To convert gravimetric to volumetric water content, using the soil

bulk densityθV

θg=

V w M s

V t M w

with M s

V t=ρb

and M w

V w=ρw

θV =θg ρb

ρw

θV =θg

ρb

ρw

where ρdb=

Mass of soil solidsTotal Volume(V a+V w+V s )

Air-filled porosity

Air-filled porosity fa is useful for many soil-related investigations

and has been found to be a good indicator of soil biological and

chemical activities.f a=f −θV

where f is the total porosity

f =1−( Dry bulk density ( ρb )Particle density ( ρp ) )

Methods for expressing soil moisture content

There are several ways to express soil water concentration, these are:

a. Gravimetric terms as gravimetric moisture content or mass

wetness

b. Volumetric terms as volumetric moisture content or volume

wetness

c. Depth of water, similar to expressing amount of rainfall,

evapotranspiration, runoff, infiltration and irrigation water

amount (mm depth of water)

d. Moisture tension in kPa

Bulk density1. wet bulk density

ρwb=mws

V T

2. dry bulk density ρdb=

mds

V T

Porosity1. φ= volume of soil pores

total volume of soil=

V V

V T

2. φ=(1−ρdb

ρpd)

Gravimetric moisture content – dry basis θg=

mass of water (g)mass of dry soil (g)

θg %= mass of water (g)

mass of dry soil(g)× 100

     Gravimetric moisture content – wet basis      θg=

mass of water (g)mass of moist soil (g)

  θg=

mass of water (g)mass of dry soil (g )¿

+mass of water (g)¿

To convert between dry and wet basis:

θgd=mw

mds= mass of water

mas of dry soil

θws=mw

mws= mass of water

mass of wet soil

θgws=mw

mw+m ds

                                                

  θgds=mw

mds                                        

θgws(mw+mds)=mw

θgws× mw+θgws × mds=mw

mw (1−θgws )=mds×θgws

mw

mds=θgds=

θgws

(1−θgws)

Volumetric moisture content

 θv=volumeof watervolume of soil

θv=θg×ρdb

ρwater

where ρdb isthe dry bulk density of soil∧¿

ρwater is the density of w ater all measured∈g /cm3 

Depth of water dw - Equivalent surface depth of water

Equivalent surface depth of water expresses the water in a soil

sample as if it were removed and set on top of the sample.

dw (mm

)=θVm(meter )of water

m (meter ) depthof soil

dw (mmm

)=θV1000 mmof water

m depth of soil - expressed as mm of water per m depth of

soil

dw (mmcm

)=θV1000 mm of water

100 cmdepth of soil - expressed as mm of water per cm depth

of soil

If θV =(FC−PWP)

dw( mmcm )=(FC−PWP)× 1000 mm of water

m depthof soil

dw(mmcm )=(FC−PWP)× 1000 mm of water

100 cm depth of soil

Irrigation water management

Allowable depletion AD is the maximum moisture deficit that

should occur before water is applied. Plant water stress will occur

if SWD exceeds allowable depletion. Field soils are generally at

water content between the FC and WP. Commonly used

terminology in irrigation management is soil water depletion

(SWD) or soil water deficit (SWD). Soil water depletion (SWD)

refers to the amount of available water that has been removed and

that is θFC−θVi. Moisture remaining is how much of the available

water remains and that is θVi−θPWP

Often the depleted and remaining water are expressed as a fraction

or percentage. The equations for determining the fraction of

available water depleted and the fraction of available water

remaining are as follows:

fractionof available water remaining , f R=θVi−θPWP

θFC−θPWP

fractionof available water depleted , f D=θFC−θVi

θFC−θPWP

It is very useful in irrigation management to know the depth of

water required to fill a layer of soil to field capacity.

This depth is equal to SWD hence, SWD=f D× (θFC−θPWP ) ×depth of Soil(L)

SWD=(θFC−θVi )

(θFC−θPWP )× (θFC−θPWP) × depthof Soil(L)

SWD=( θFC−θVi ) ×depth of Soil( L)

A sample of silt loam has a volumetric water content at field

capacity as 36% and that at permanent wilting point as 16%. The

moisture content at the time of sampling was 26% and the soil

depth was 0.915 m. Estimate the

a. available water capacity

b. fraction of available water depleted

c. fraction of available water remaining

d. soil moisture depleted at the time of sampling

e. soil moisture remaining at the time of sampling

Management allowable depletion

Plants can remove only a portion of the available water before

growth and yield are affected. This portion is the readily available

water (RAW) and for most crops ranges between 40 and 65

percent of the total available water in the crop root zone. The

readily available water (RAW) can be calculated by: RAW=MAD × TAW

Where MAD is the management allowed deficiency or the portion

(decimal) of the total available water that management determines

can be removed from the crop root zone without adversely

affecting yield and/or economic return.

Sample questions1. An undisturbed soil core is 10 cm in diameter and 10

cm in length. The wet soil mass is 1320 g. After oven

drying the core, the dry soil mass is 1100 g. The

mineral density of the soil is 2.6 g cm-3. Calculate:

i. Dry soil bulk density

ii. Water content on a mass basis

iii. Water content on a volume basis

iv. Soil porosity

v. Equivalent depth of water (cm) contained in a 1

m soil profile, if the undisturbed core is

representative of the 1 m soil depth

 

2. Consider a 1.2 m depth soil profile with 3 layers. The dry

bulk density of each layer (top, center, and bottom) is 1.20,

1.35, and 1.48 g/cm3. The top 30-cm layer has a water

content of 0.12 g/g, the center 50-cm layer has a water

content of 0.18 g/g, and the bottom 40 cm layer has a water

content of 0.22 g/g.

i. What is the total amount of water in the whole

profile in mm?

ii. How much water (mm) do you need to apply to

bring the 1.2 m soil profile to a volumetric water

content of 0.35 cm3 / cm3 ?

3. An undisturbed soil sample with a volume of 80 cm3 is taken

from an irrigated field. The mass of the soil sample after

drying is 100 grams.

i. What is the soil bulk density?

ii. What is the porosity?

4. How much water is in the top 20 cm of soil which has a

volumetric moisture content of 0.2?

5. How much water is in the top 15 cm of soil that has a volume

water concentration of 20%?

6. A soil sample weighed 230 g in a moisture container. The

mass of the moisture box was 78g. After drying at 105 C to a

constant mass, the soil and box weighed 204 g. The soil

sample filled a 1000 cm3 container as it was taken from the

field. Find the moisture percentage in the soil by mass and by

volume.

7. The following data presents a soil sample taken from a site in

Ghana:

a. mass of soil at field capacity 85g,

b. mass of soil at permanent wilting point 71 g,

c. air-dry mass 64 g and

d. oven dry mass 58g.

Find

i. % moisture content by mass at field capacity

ii. % moisture content at permanent wilting point

iii.Available moisture content

8. A cube of soil measures 10 cm× 10 cm× 10 cm and has a total mass

of 1990g of which 280g is water. Assume the density of

water is 1.00g/cc and particle density is 2.65 g/cc. Find the

depth of water, water holding capacity and aeration porosity

of the soil.

Moisture in the atmosphere – Humidity

Humidity and vapour pressure

According to Dalton’s Law the total air pressurePtotal = Pwater vapour + POxygen + PNitrgen + POthers

Pwater vapour = partial water vapour pressure

e.g. Moist Air

The total pressure P of moist air is the sum of (1) the partial

pressure Pd of the dry air and (2) Pe of the water vapour (vapour

pressure)

The total pressure of the air is the sum:

P = Pdry air + Pwater vapour,

Where, the second term Pwater vapour is the vapour pressure.

The maximum possible vapour pressure is the saturation vapour

pressure. This is the vapour pressure in equilibrium with a liquid

water surface. The saturation vapour pressure depends on

temperature as shown in the Figure 1 below.

Temperature (˚C)

i. The vapour pressure cannot remain greater than the

saturation vapour pressure. If it becomes greater, then

water condenses to liquid until the vapour pressure is

reduced to the saturation vapour pressure.

ii. When saturated air is heated without change in the vapour

pressure, the relative humidity is reduced.

iii. Under a given temperature a fixed volume of air has the

capacity to absorb only a certain quantity of water vapour.

When this capacity is reached, the air is said to be

saturated with water vapour.

Vapour pressure

Vapour pressure increases with temperature

a. At higher temperature more molecules have the necessary

kinetic energy to escape the attractive forces of the liquid phase

b. The more molecules in the vapour phase, the higher the vapour

pressure

c. At 100°C the vapour pressure of water is 760 mmHg (1 atm.) or

equal to the atmospheric pressure on the liquid (in an open

container)

Dew Point (Temperature)

The dewpoint temperature is the temperature at which the air can

now longer hold all of its water vapour, and some of the water

vapour must condense into liquid water. Dew point is the

temperature at which water vapour saturates from an air mass into

liquid or solid usually forming rain, snow, frost, or dew. Dew point

normally occurs when a mass of air has a relative humidity of

100%. This happens in the atmosphere as a result of cooling

through a number of different processes.

a. Dew point temperature is defined as the temperature to which

the air would have to cool (at constant pressure and constant

water vapor content) in order to reach saturation.

b. At 100% relative humidity, the dewpoint temperature and

real temperature are the same, and clouds or fog can begin to

form.

c. While relative humidity is a relative measure of how humid

the air is, the dewpoint temperature is an absolute measure of

how much water vapour is in the air (how humid it is).

d. The higher the dew points, the higher the moisture content of

the air at a given temperature.

e. A state of saturation exists when the air is holding the

maximum amount of water vapour possible at the existing

temperature and pressure.

f. Condensation is the formation of liquid drops from water

vapour. Another way in which condensation occurs is during

the formation of dew.

In meteorology, vapour pressure, dew point temperature and

relative humidity are common expressions to indicate air

humidity.

Relative Humidity (%) = Actual vapour presure at a given temperature

Saturated Vapour pressure at the same temperature× 100

Relative humidity =Actual Vapour amount

Saturation Vapour amount× 100

a. Relative humidity is the ratio of the actual amount of

moisture in the atmosphere to the amount of moisture the

atmosphere can hold.

b. Specific Humidity or Moisture Content of Air. Specific

humidity or moisture content of air is the ratio of the mass of

water to the mass of dry air in a given volume of moist air

c. Specific Volume - The specific volume, v, of a system is the

volume occupied by unit mass of the system. The

relationship between the specific volume and density is:

d. Percentage Saturation - Percentage saturation is defined as

the ratio of the specific humidity of air to the specific

humidity of saturated air at the same temperature Other

measurements

e. Absolute Humidity- The ratio of the mass of water vapor to

the volume occupied by a mixture of water vapor and dry air.

f. Specific Humidity- The mass of water vapor per unit mass

of air, including the water vapor.

g. Mixing Ratio- mass of water vapor/mass of dry air.

h. Saturation Mixing Ratio- mass of water vapor when a

parcel is saturated/mass of dry air in the parcel.

i. A relative humidity of 100% means the air can hold no more

water (rain or dew is likely)

j. A relative humidity of 0% indicates there is no moisture in

the atmosphere.

k. Mold and condensation problems occur when the relative

humidity is too high.

l. Warm air can hold more moisture than cold air. So, if warm

air and cold air contain the same amount of moisture, the

warm air will have a lower relative humidity.

m.Two conditions cause the relative humidity to rise: when the

temperature falls or when moisture is added to the air.

n. Relative humidity, combined with air temperature, can be

used to estimate the actual amount of moisture in the

atmosphere, sometimes referred to as precipitable water.

o. Water vapor acts as a green house gas by trapping infrared

radiation reflected from the earth.

p. Desert temperatures can become much lower at night, as

there is little moisture in the air to trap the heat.

q. The effect of moisture in gases also plays a very significant

role in corrosion phenomena which can result in damage and

loss of not only unprotected metals, like iron and steel

structural components, but also improperly treated or stored

steel and other metal products.

We have to remember that there are only two ways of increasing

the relative humidity:

1. Cooling the air so it becomes closer to the dew point

(temperature)

2. Adding water vapor to the air

Relative humidity is used by

a. meteorologists to help predict the weather

b. pathologists to predict disease development on plants, and

c. agricultural scientists to estimate evapotranspiration

Measurement of relative humidity

a. Direct measurement using

i. Hair hygrometers which uses human hair as a sensing

element. Hair changes in length in proportion to

humidity of the air. The response to changes in

humidity is slow and is not very dependable at very

high relative humidities. Hair hygrometers work on the

fact that hair changes its length when humidity varies.

This device usually consists of a number of human or

horse hairs connected to a mechanical lever system.

When humidity increases the length of the hairs

becomes longer. This change in length is then

transmitted and magnified by the lever system into a

measurement of relative humidity.

ii. Electric hygrometers are based on substances whose

electrical properties change as a function of their

moisture content. As the humidity of the air around the

sensor increases, its moisture increases, proportionally

affecting the sensor's electrical properties. These

devices are more expensive than wet- and dry-bulb

psychrometers, but their accuracy is not as severely

affected by incorrect operation. Sensors lose their

calibration if allowed to become contaminated, and

some lose calibration if water condenses on them.

b. Indirect measurement - psychrometer

Psychrometer is a particular kind of hygrometer. It consists of two

matched thermometers one of which measures air temperature Td

called the dry-bulb. The other thermometer is covered with a wet

muslin cloth and reads the wet-bulb temperature Tw. The difference

in temperatures Td – Tw is proportional to humidity.

Wet bulb depression – the difference between the dry bulb and

the wet bulb temperature readings is called the wet bulb

depression.

a. The wet bulb depression increases as the relative humidity

decreases

b. The lesser the wet bulb depression, the wetter the air

c. The greater the wet bulb depression the drier the air

Relative humidity can be determined from wet bulb and dry bulb

temperatures. Dry bulb temperature is the actual air temperature,

while wet bulb temperature can be determined by using a shoe lace

to cover the bulb of a thermometer.

Saturation vapour pressure over liquid water

The saturation vapour pressure es(T) in pascals over liquid water at

temperature T is given by the empirical formula:

es(T) = 611.2exp[17.67T/(T + 243.5)] Pa,

where T is in degrees Celsius, and -30°C < T < 35°C.

Latent heat of vaporization of liquid water

The latent heat L of vaporization of liquid water in kJ/kg at

temperature T is given by the empirical formula:

L = 2501 - 2.375T kJ/kg,

where T is in degrees Celsius, and 0°C < T < 40°C.

A. Sling Psychrometer

The sling psychrometer consists of a dry and wet-bulb

thermometer. The term bulb refers to that portion of the glass tube

where the mercury is stored. The dry and wet bulbs are exactly

alike in construction.

The only difference is that the wet-bulb has a piece of muslin cloth

or wick wrapped around its bulb and which is dipped in water

shortly before the psychrometer is read.

The weather observer first wets the cloth cladding the wet-bulb,

whirls the psychrometer a few times, then reads the wet-bulb. He

reads the dry-bulb last. Normally, the wet-bulb's reading will be

lower than the dry-bulb's. The dry-bulb reading is the air

temperature. The difference between the dry and the wet-bulb

readings will give, with the aid of a psychrometric table, the dew

point temperature and the relative humidity. (Dew point is the

temperature at which the water vapor will condense while relative

humidity is the ratio of the amount of water vapor actually present

in the air to the maximum amount of water vapor the air can hold

at a given temperature).

B. Hygrometer

The other instrument used to measure humidity is the Hygrometer.

The hygrometer is less accurate than the psychrometer. It uses

human hair from which the oil has been removed by using ether.

The hair becomes longer as the relative humidity of the air

increases. This change can be made to move an indicator needle

which moves over a scale, the graduations of which reads from 0%

to 100%.

C. Hygrothermograph

The hygrothermograph records both relative humidity and

temperature on graph paper in the same manner as the thermograph

and barograph do.