Climatology

Humidity

Refers to the amount of the moisture present in the air

Three ways in which the humidity can be measured:

a) Absolute Humidity

b) Specific Humidity

c) Relative Humidity

Absolute Humidity refers to the amount of moisture in grams

present in the unit volume of air.

The absolute humidity keeps on changing with the change in the

volume without any alteration in the amount of the moisture

present.

Specific Humidity refers to the amount of the moisture present in

per kilogram of air.

Specific Humidity is a more stable way of measuring the humidity

since this won’t change until there is an alteration in the amount

of the moisture

present.

Relative Humidity=

Moisture holding capacity of the air varies with the temperature.

Relative Humidity varies inversely with the moisture holding

capacity of the air.

The temperature at which the moisture holding capacity of the air

is exactly equal to the moisture present in the air is called the

Dew point.

Two processes of temperature change are:

a) Diabatic process

b) Adiabatic process

In diabatic process there is a uniform exchange of the heat of the

air with the surrounding environment.

This exchange of heat is responsible for the lowering of

temperature.

Diabatic process occurs only in those air masses which are either

stationary or have a horizontal movement.

Diabatic process of temperature change leads to the formation of

fog, dew or frost.

Four conditions necessary for the formation of the dew:

a) Longer winter nights: During

winter, especially during the nights, the earth loses radiation and

cools down to a relatively larger extent.

b) Absence of clouds: Clouds prevent the loss of radiation and

hence do not allow the surface of the earth to be cooled down

completely.

c) Relative humidity of the air should be high: Difference in the

moisture present in the air and the moisture holding capacity of

the air should be minimal.

d) Air should be calm: The movement of the air should be

minimal.

A number of microscopic droplets of water combine together to

form one drop of water which finally falls down in the form of

dew.

Four conditions necessary for the formation of the fog:

a) Longer winter nights

b) Absence of clouds

c) Relative humidity of the air should be high

d) Slight wind movement: Temperature of the air mass will reduce

beyond the saturation point. Thus moisture will transform into

macroscopic water droplets. But these water droplets do not

combine together to form drops of water. Instead they get

scattered in the environment with the moving wind in the form of

fog.

Four conditions necessary for the formation of the frost:

a) Longer winter nights: During winter, especially during the

nights, the earth loses radiation and cools down to a relatively

larger extent. In this case the temperature of the earth must fall

below zero degree centigrade.

b) Absence of clouds: Clouds prevent the loss of radiation and

hence does not allow the surface of the earth to be cooled down

completely.

c) High Relative Humidity: Air should be dry.

d) Air should be completely calm.

Frost gets formed when the dew point falls below the freezing

point.

Dry air ensures that the temperature at which the air gets

saturated is below the freezing point.

The temperature at which the moisture present becomes

equivalent to the moisture holding capacity of the air for the first

time will be below freezing point.

This leads to the formation of frost without any dew being formed

in the process.

Adiabatic process is responsible for rain, sleet, snow, hail or

cloudburst.

Note: An adiabatic process is any process occurring without the gain or

loss of heat within a system.

This process of temperature change is operative only in those air

masses which have a vertical movement.

It is the only process that is responsible for the formation of all

kinds of clouds.

This expansion leads to the decrease in the temperature of the air

parcel.

As an air starts sinking downwards, due to the increasing

atmospheric pressure, it contracts. This contraction leads to the

increase in the temperature of the air parcel.

Although upon expansion the temperature of the air parcel

decreases but the heat content of the air parcel remains

unaltered.

Upon contraction the temperature of the air parcel increases but

the heat content of the air parcel remains unaltered.

Note: Adiabatic Lapse Rate refers to the rate of decrease in

temperature with increase in altitude without exchange of heat.

A rising parcel of air cools under two different rates:

a) Dry Adiabatic Lapse Rate

b) Wet Adiabatic Lapse Rate

A rising air parcel cools at a fixed rate of 9.8 degree centigrade per

kilometer before reaching the condensation level.

For all practical purposes this temperature is considered to be 10

degree centigrade per kilometer.

This constant rate of cooling of the air parcel is referred to as the

Dry Adiabatic Lapse Rate.

The rate of cooling of the air parcel beyond the condensation

level is referred to as the Wet Adiabatic Lapse Rate.

This rate is not uniform

Note: The quantity of heat absorbed or released by a substance

undergoing a change of state, such as ice changing to water or water to

steam, at constant temperature and pressure is referred to as the

Latent Heat.

Two forces at play beyond the condensation level:

a) Expansion of the air mass and its gradual cooling.

b) Moisture holding capacity of the air is continuously

depreciating.

Release of latent heat of condensation reduces the rate of cooling

of the air parcel

Beyond condensation level:

Rate of cooling= 10o C/Km-Latent Heat of Condensation released

Rate of cooling beyond condensation level is always less than the

rate of cooling before the condensation level is reached, i.e. Wet

Adiabatic Lapse Rate is always less than Dry Adiabatic Lapse Rate.

Release of the latent heat of condensation beyond the

condensation level depends upon the initial amount of moisture

present in the air parcel.

There will be no condensation level and the rate of cooling will

remain uniform throughout if the moisture present in the air

parcel is zero.

The temperature of the air parcel if it has been brought down

from beyond the condensation level, is more than its initial

temperature.

- A sinking air parcel always gets heated at a fixed rate of ten

degree centigrade per kilometer.

Areas characterized by the sinking air parcel are devoid of the

rainfall.

Subtropical high pressure belt in both the hemispheres are prime

examples of areas with sinking air.

Latent Heat of condensation also depends upon the initial

temperature of the air.

- Initial temperature of the air determines the moisture

holding capacity of the air.

Thus Wet Adiabatic Lapse Rate depends upon:

a) initial moisture present in the air parcel

b) initial temperature of the air

Dry Adiabatic Lapse Rate is neither dependent on the initial

moisture present in the air parcel nor on the initial temperature

of the air.

A rising air parcel is characterized by Instability.

This condition is responsible for the cloud formation and the

rainfall.

A stationary air parcel is characterized by stability.

Rising of the air parcel will depend upon:

a) Environmental Lapse Rate

(refers to the rate of the decrease of temperature with

altitude in the stationary atmosphere at a given time

and location)

b) Triggering Effect:

- an intense heating will cause the air parcel to rise.

- a force uplift along an inclined topography by a horizontal

moving wind can also cause the air parcel to rise.

Normal Lapse Rate

Normal Lapse Rate refers to the temperature decrease in the

troposphere at a fixed rate of 6.5 degree centigrade per

kilometer.

Environmental Lapse Rate is the actual prevailing rate at which

the temperature is decreasing in the surrounding atmosphere at a

particular place and at a given time.

Environmental Lapse Rate changes with seasons and regions.

Thermal inversion occurs when a layer of warm air settles over a

layer of cooler air that lies near the ground.

Level of Inversion refers to the level beyond which temperature of

the air parcel increases.

Thermal Inversion is quite significant during winter season.

Wet adiabatic lapse rate is the rate at which the temperature of

the air parcel cools down.

The air parcel will rise if and only if the environmental lapse rate is

more than the dry adiabatic lapse rate.

The surrounding air will provide a buoyant force to the air parcel if

it is at a lower temperature than the air parcel.

If the environmental lapse rate is more than ten degree

centigrade, the air parcel will rise.

The air parcel will sink down if the surrounding air is at a lower

temperature than the air parcel.

Triggering Effect

The air will rise even faster after reaching the condensation level.

Four ways of depicting the ELR & DALR Note: ELR - Environmental Lapse Rate

DALR - Dry Adiabatic Lapse Rate

WALR - Wet Adiabatic Lapse Rate

1. Absolute Instability: ELR> DALR

2. Mechanical Instability: ELR>> DALR

3. Conditional Instability: WALR<ELR< DALR

4. Potential or Convectional Instability

Condition of absolute stability : ELR = DALR

Condition of absolute stability depicts thermal inversion. No rainfall can

occur under this condition.

Condition for no rainfall

Condition conducive for fog, dew and frost formation