4. Which one has a higher density: a parcel of dry air or moist air at constant

Question

4. Which one has a higher density: a parcel of dry air or moist air at constant pressure and temperature? Approach this question based on the following facts: 1) the density of moist air, pm, can be expressed as the sum of the partial contributions from dry air and from water vapour, pm - 2) the ideal gas law for moist air can be expressed as pPm Rm T , where Rm denotes the 'effective gas constant' of moist air, which is not really constant, since it depends on humidity according to Rm can be expressed as a function of mixing ratio and R Let's compare two air parcels at the same pressure p and temperature T, one dry, the other moist with mixing ratio w. [E.g., make a table with two columns comparing the dry and moist cases.] a) Generally, under which conditions will the moist air parcel have a lower density than the dry air parcel? The observed surface temperature and water vapour mixing ratio and air pressure at Key West. Florida, are 26.0 °C, 16.1 g/kg. and p = 1010 hPa respectively. Calculate the air density at these conditions and compare it to the air density of dry air at the same pressure and temperature, b)

Explanation / Answer

4. Moist air is less dense than dry air at constant temperature and pressure. This is because moist air is laden with water vapour. Water vapour is less dense than oxygen. And when water vapour displaces other gases to reside in air, thus making up the moist air, then the moist air is less dense. The mixing ratios and partial contributions from dry air and moist air as indicated in the formulas given clearly indicate that dry air is denser than moist air.

Related Questions

Navigate

• Browse (All)
• Browse 4
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.