Moist vs Adiabatic Rates: Difference and Comparison

The air in the atmosphere cools or warms at two different rates- moist and dry adiabatic lapse rates. The lapse rate is when an atmospheric variable tends to fall at a higher altitude.

While moist and dry adiabatic rates may seem synonymous, there are several points of difference between them.

Key Takeaways

  1. Moist adiabatic lapse rate refers to the rate at which saturated air cools as it rises, while the dry adiabatic lapse rate describes the cooling rate of unsaturated air.
  2. The moist adiabatic lapse rate is lower than the dry adiabatic lapse rate due to the release of latent heat during condensation.
  3. Both lapse rates are essential for understanding atmospheric stability, influencing weather patterns and cloud formation.

Moist vs Dry Adiabatic Rates 

The moist rate is slower than the adiabatic rate. It is the rate at which saturated air rises and cools, while the adiabatic rate is when unsaturated air rises and cools. The adiabatic rate assumes no heat is added or removed from the air as it grows, so the cooling rate is faster.

Moist vs Dry Adiabatic Rates

Moist adiabatic lapse rate relates to parcels of air that are saturated in nature. The sticky boxes of air expand at higher altitudes following a decrease in the pressure of the atmosphere.

The practical function of the saturated air parcels is to cool the clouds. They are also responsible for the occurrence of thunderstorms and the like. 

The dry adiabatic lapse rate is associated with unsaturated air. It refers to the rate at which a parcel of air cools or warms when it moves vertically.

According to estimates, the dry adiabatic lapse rate experiences a variation of  5.5 degrees Fahrenheit concerning a vertical movement of 1000 feet. 

Comparison Table

Parameters of ComparisonMoist Adiabatic RateDry Adiabatic Rate 
Parcel Of Air Dry Adiabatic Rate studies air parcels with little to no moisture content. Another name for moist adiabatic rate is saturated adiabatic rate.
Alternative Name 
An Adiabatic dry rate is responsible for stable conditions due to the absence of water content.The moist Adiabatic Rate is lower than Dry Adiabatic rate.
Relation The dry adiabatic rate is higher than dry adiabatic rate at Dry Adiabatic rate varies according to the heat capacity of air at a particular temperature and acceleration due to gravity.
Determining Factors Moist Adiabatic rate varies according to temperature. Moist Adiabatic rate is responsible for unstable conditions due to moisture. 
Effect on Atmosphere A dry Adiabatic rate is responsible for stable conditions due to the absence of water content.Dry Adiabatic rate is responsible for stable conditions due to the absence of water content.

What are Moist Adiabatic Rates?

Moist adiabatic lapse rate refers to the parcels of air that are already moist. Consequently, when such boxes of air rise, they become calmer and expand further.

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The moist adiabatic lapse rate is also known as the saturated adiabatic lapse rate. This type of air has a lapse rate of 6 degrees Celsius per 1000 meters.  

The parcel of air entailed in the moist adiabatic rate is heavier due to the moisture it contains. As a result, it rises at a relatively slower pace. As the parcel of air rises, it loses its internal heat.

The surge in the temperature arises due to the decrease in atmospheric pressure. To corroborate further, atmospheric pressure decreases due to the high altitude. 

Therefore, as the parcel of air undergoes expansion at higher altitudes, they are constantly at work and, eventually, lead to the cooling of clouds.

However, the moist lapse rate is less than the dry lapse rate because of the energy dispensed during condensation. When condensation is high, the wet adiabatic lapse rate is significantly fewer. 

In degrees Fahrenheit, the moist adiabatic lapse rate is around 3.3 degrees Fahrenheit for every change of 1000 feet in vertical movement.

It is possible to find the isobaric heat capacity of moist or saturated air using the specific enthalpy h for a saturated air parcel. 

What are Dry Adiabatic Rates?

The dry adiabatic lapse rate is the unsaturated lapse rate. In other words, the moisture content is absent in such air parcels. As the parcels of air travel a hundred metres, three degrees Celsius of cooling occurs.

For instance, when a parcel of air rises to 500 metres, it will gain 15 degrees Celsius of cooling. Conversely, when the mood moves downward, the average temperature is restored.

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Lapse rates are influenced by water content, sunlight falling on the Earth’s surface, and geographical features. The relative humidity in unsaturated air is less than 100%.

Because the moisture content in dry adiabatic air is less, it guarantees stable atmospheric conditions. 

When the rising air parcel has less moisture, condensation occurs at a nominally lower rate. As a result, the latent heat of condensation released is low.

In other words, there is less additional heat from inside. Here, latent heat refers to the heat absorbed during the movement from one phase to the next. Consequently, the fall in temperature with increasing altitudes is higher. 

The factor influencing dry adiabatic lapse rate is the heat capacity of air at a particular temperature and acceleration due to gravity.

The unsaturated adiabatic lapse rate is approximately 9.8 degrees Celsius per kilometre. The formation of clouds is attributed to the unsaturated parcel of air. 

Main Differences Between Moist and Dry Adiabatic Rates 

  1. Moist Adiabatic Rate studies the parcel of air that has high humidity. On the other hand, dry adiabatic rates learn the parcel of air with a relatively low or negligible humidity content.
  2. Moist Adiabatic and dry adiabatic rates are also known as saturated and unsaturated adiabatic lapse rates, respectively. 
  3. Moist Adiabatic rate is accountable for fluctuating atmospheric conditions due to water content. In contrast, the dry adiabatic rate does not play a prominent role in creating uncertain atmospheric conditions due to the absence of moisture. 
  4. The moist Adiabatic rate varies according to temperature and water content. On the other hand, the dry adiabatic rate depends on the acceleration due to gravity and atmospheric heat.
  5. While the moist adiabatic rate is approximated at 8 degrees Celsius per kilometre, the dry adiabatic rate is almost 4 degrees Celsius per kilometre.  
References
  1. https://www.pmfias.com/adiabatic-lapse-rate-latent-heat-condensation/ 
  2. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/adiabatic-lapse-rate 

Last Updated : 30 June, 2023

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