What is lapse rate? | adiabatic lapse rate( ALR ) | DALR,WALR,ELR ~ Civil Engineer Knowledge

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1. What is lapse rate ?

Ans. Lapse rate is the change of air temperature with height. So,the tendency of atmosphere to resist or enhance particle motion is termed as stability. Here, we may assume the lapse rate alone as a indicator of the stability condition of the atmosphere.

There are mainly three categories of lapse rate-

a. Unstable lapse rate

b. Neutral lapse rate

c. Stable lapse rate

2. What is Adiabatic Lapse Rate ( ALR )?

Ans. The internet decrease of temperature with height which orchids in the rising parcel of air mass is called adiabatic lapse rate. Here, cooling process is termed as adiabatic.

3. What is Dry adiabatic lapse rate(DALR)?

Ans. DALR is the rate of a temperature decrease with height for a parcel of dry or unsaturated air rising under adiabatic condition. Unsaturated air has less than 100% relative humidity.Where actual temperature is higher than dew Point. So, adiabatic means that no heat transfer occure or out of parcel.

4. Mechanism of dry adiabatic lapse rate (DALR).

Ans. At higher altitudes pressure is lower. When air parcel rises due to lower pressure during expansion it passes surrounding air, doing work. Because, the parcel does work but gains no heat it loses its internal energy for that its temperature decrease. The rate of temperature decrease is 9.8 degree Celsius per 1000 m for dry adiabatic lapse rate.

DALR = 9.8°C/ 1000 m.

5.What is wet adiabatic lapse rate (WALR)?

Ans. When the air is saturated with water vapours and the situated adiabatic lapse rate applies.

WALR = 6°C/ 1000 m

6. What is environmental lapse rate (ELR)?

Ans. This is the rate of decrease of temperature with altitude in the stationary atmosphere.

There are many condition like-

(i) where, environmental lapse rate(ELR) is greater than dry adiabatic lapse rate(DALR) then this condition is called super adiabatic condition.Here, atmosphere is unstable.

(ELR)> (DALR) = super adiabatic condition

This is to say, at the same height drop in temperature is more in ELR than DALR. In this case warmer air will continue to lift up. Where parcel of cooler rear wheel continue how to come down. In this condition environmental is unstable and the dispersion pollutants will be Rapid.

(ii) Where, environmental lapse rate is less than dry adiabatic lapse rate. So, this condition is called sub adiabatic condition. In this condition, the atmosphere will stable.

(ELR)< (DALR) = sub adiabatic condition and the atmosphere will be stable

(iii) When the environmental lapse rate equal to dry adiabatic lapse rate. Therefore, the atmosphere will be neutral.

(iv) Negative lapse rate happend because temperature of environment increase with altitude. This condition also called inversion condition. As a result, atmosphere will be very stable.

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