Wet Adiabatic Lapse Rate: Detailed Insight And Facts


The term wet in wet adiabatic lapse rate indicates the presence of water content in the adiabatic system. Lapse indicates a decrease in certain quantity.

The rate by which the temperature decreases with altitude when the surrounding air is moist (or wet) is called as wet adiabatic lapse rate. Lapse refers to the a decreasing quantity which in this case is temperature.

What is moist lapse rate?

Moist lapse rate refers to the rate of decrease in temperature with altitude when the surrounding air is wet or moist. Presence of water molecules make it wet.

The water content in the atmosphere affects the process of convection as vapours of water has latent heat of vapourisation. Unlike dry adiabatic lapse rate, wet adiabatic lapse rate varies with temperature.

Types of lapse rate

Lapse rates are of three types, these types differ from each other mainly because of presence/absence of moisture in the surrounding system. Some do not vary with time or temperature and some vary with temperature.

The three types of lapse rates are as follows-

  • Dry adiabatic lapse rate– The term dry refers to absence of water content in surrounding air. Dry adiabatic lapse rate has constant value and does not change with temperature.
  • Wet or moist adiabatic lapse rate– Wet or moist represents water content in the atmosphere. The wet or moist adiabatic lapse rate depends on the temperature.
  • Environmental lapse rate– The rate by which temperature changes with altitude within 11km or troposphere is called as the environmental lapse rate. This particular lapse rate is a very dynamic quantity and varies from day to day.

Difference between dry adiabatic and wet adiabatic rate

The primary difference between dry adiabatic lapse rate and wet adiabatic lapse rate is the presence of water content in one and absence of same in other.

The detailed differences between dry adiabatic rate and wet adiabatic rate is given in the table given below-

Dry adiabatic rateWet adiabatic rate
The surrounding air has no moisture or water content in it.Atmosphere or surrounding air has water content in it.
Dry adiabatic rate does not change with temperature.Wet adiabatic rate changes with temperature.
Specific humidity of air is zero.Specific humidity of air is greater than zero.
Table: Difference between dry adiabatic rate and wet adiabatic rate

What is saturated vapour?

The maximum amount of vapour that a given quantity of air can hold at a specific temperature and pressure is called as saturated vapour. The mixture cannot hold more amount of vapour at the same temperature and pressure.

We cannot add more water vapour to that packet of air at that particular temperature or pressure. To add more water content, one needs to increase the temperature or pressure. By doing that, the capacity of the mixture to hold vapour will increase.

What is relative humidity?

The term relative represents a comparison or ratio between two things.

Relative humidity is the ratio of actual vapour present in air to the maximum amount of vapour the mixture can have.

Mathematically, relative humidity can be represented as-

Where,
phi is relative humidity

p is partial pressure of vapour

p* is the pressure of vapour at saturation

What is specific humidity?

Specific humidity can be defined as the mass of water vapour present in the air-vapour mixture.

Mathematically, specific humidity can be represented as-

ω = 0.622 x Pg/P-Pg

where,

omega is specific humidity

pg is pressure of vapour

p is the atmospheric pressure

wet adiabatic lapse rate
Image: Hygrothermograph for humidity and temperature recording
Image credits: José Cruz/ABr, UmidaderelativaCC BY 3.0 BR

Benefits of specific humidity

The dermatologists have confirmed that humidity in air beneficial for our skin health and body health.

The benefits of specific humidity are-

  • Humid air makes the skin healthier and fresh.
  • Maintains a good oxygen intake inside the body.
  • Reduces fatigue and helps in high blood circulation.
  • Reduces the chances of respiratory infection.

Benefits of relative humidity

The benefits of relative humidity are as follows-

  • Helps in maintaining the correct temperature of the system.
  • Increases efficiency of the air conditioner and saves energy.
  • Helps reduce energy costs by maintaining the correct temperature of the system.

What is unsaturated air?

Unsaturated air is polar opposite of saturated air. It is the state of air where one can add more vapour to the air-vapour mixture.

As, the vapour inside the unsaturated air has not reached the maximum limit, we can add more vapour to the unsaturated air mixture without increasing or decreasing temperature and pressure.

Why is dry adiabatic rate higher than saturated adiabatic?

Adiabatic cooling of rising parcel of saturated air in which condensation takes place results in greater energy release at higher temperatures.

The energy release due to latent heat of vaporization moderates the adiabatic cooling process and the rate of cooling below dry adiabatic rate varies with temperature. Thus, the dry adiabatic rate is higher than saturated adiabatic rate.

What is normal 1000 meter lapse rate?

As discussed in above section, dry adiabatic lapse rate is a constant quantity and does not change with temperature.

The dry adiabatic lapse rate is simply the adiabatic lapse rate for an atmosphere which lacks moisture or water content.

The atmosphere may contain water vapour but does not have any liquid moisture. The constant value of dry adiabatic lapse rate is 9.8 Degree celsius/1000m.

What is positive lapse rate and negative lapse rate?

The lapse rate can be positive, zero or negative according to temperature variation with elevation.

The lapse rate follows a sign convention. It is positive when the temperature reduces with increasing altitude and considered negative when the temperature increases with increasing elevation.

As lapse refers to a decreasing quantity so it follows a positive sign when the specific quantity decreases and vice versa for increasing quantity. Lapse rate is considered zero when the temperature remains constant with elevation.

What do you mean by degree of saturation?

The term degree of saturation is usually used while dealing with refrigerants.

It is defined as the ratio of actual humidity ratio to the humidity ratio of saturated mixture at the same temperature and pressure.

Mathematically,

where,

phi is degree of saturation

pvs is saturation pressure of vapour

P is pressure of atmosphere

Abhishek

Hi ....I am Abhishek Khambhata, have pursued B. Tech in Mechanical Engineering. Throughout four years of my engineering, I have designed and flown unmanned aerial vehicles. My forte is fluid mechanics and thermal engineering. My fourth-year project was based on the performance enhancement of unmanned aerial vehicles using solar technology. I would like to connect with like-minded people.

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