Difference Between Moist and Adiabatic Rates

The air in the atmosphere usually cools or warms at two different rates- moist and dry adiabatic lapse rates. The lapse rate is the rate at which 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.

Moist vs Dry Adiabatic Rates 

The main difference between moist and dry adiabatic rates is that the moist adiabatic rate is usually less than the dry adiabatic rate because the rising moist air condenses the water vapours. This process involves the release of heat which is then utilised by the surrounding air. However, this does not happen in the case of dry air due to the absence of moisture. Consequently, it cools faster than saturated or moist air. 

Moist and Dry Adiabatic Rates

Moist adiabatic lapse rate relates to parcels of air that are saturated in nature. The moist parcels of air usually expand at higher altitudes following a decrease in the pressure of the atmosphere. The practical function performed by the saturated parcels of air 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 Between Moist and Dry Adiabatic Rates 

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

What is Moist Adiabatic Rates?

Moist adiabatic lapse rate refers to the parcels of air that are already moist. Consequently, when such parcels of air rise, they become cooler and expand further. The moist adiabatic lapse rate is also known as the saturated adiabatic lapse rate. This type of air usually 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, the decrease in atmospheric pressure occurs 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 in comparison to the dry lapse rate because of the energy dispensed during condensation. When condensation is high, the moist adiabatic lapse rate is significantly fewer. 

In degree 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 by using the specific enthalpy h for a saturated parcel of air. 

What is Dry Adiabatic Rates?

The dry adiabatic lapse rate is the unsaturated lapse rate. In other words, the moisture content is absent in such parcels of air. 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 air moves downward, the normal temperature is restored. 

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 usually guarantees stable conditions in the atmosphere. 

When the rising parcel of air has less moisture, condensation takes place 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 estimated at approximately 9.8 degrees Celsius per kilometre. The formation of clouds is usually 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 study the parcel of air that has a relatively low or negligible humidity content.
  2. Moist Adiabatic rate and dry adiabatic rate are also known as saturated and unsaturated adiabatic lapse rates respectively. 
  3. Moist Adiabatic rate is generally accountable for fluctuating atmospheric conditions due to the presence of 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.  

Conclusion

To conclude, the moist and adiabatic lapse rates have considerable differences. Lapse rates imply the heating or cooling of parcels of air. Usually, the saturated moist adiabatic rate is fewer when the condensation is higher. The moist adiabatic rate is subject to variations due to varying temperatures. 

Unsaturated or dry adiabatic rate experiences a higher fall in temperature than the normal parcel of air due to less additional heat from inside. Thus, moist and adiabatic lapse rates vary on factors, such as the type of parcel of air, averaged temperature change, and determining factors.  

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 
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