Other concepts used for the characterization of atmospheric humidity (Chap. 4)
are the specific humidity, q, defined as the mass of water vapour per unit mass of
humid air and the absolute humidity v, defined as the mass of vapour of water per
unit volume of humid air.
To analyse the dynamics of the evaporation process, consider the simple case of
an open cup with water, the level of which level falls during the night because of
evaporation, due the liquid molecules changing into the vapour phase. This process
can be explained in terms of kinetic energy.
The molecules of a liquid move relative to one another in a totally random
fashion. The water molecules remain in the liquid state due to attractive forces,
which keep them in this state. A molecule in the surface area of the water, with a
certain velocity, may momentarily leave the liquid and then eventually be
recaptured by attractive forces of other molecules of water if its velocity is not
excessive. Otherwise, the molecule will escape from the liquid medium into the gas
phase. Only molecules with appropriate velocity and kinetic energy will escape into
the gaseous phase, due an increase in the ambient temperature. This confirms the
observable reality that evaporation increases with temperature.
As higher velocity molecules escape into the atmosphere, the average velocity,
kinetic energy, and temperature of the remaining liquid molecules will decrease. It
can be thus anticipated that evaporation is a process of internal cooling of the
system. In the case of water, the temperature drop caused by evaporation is due to
the release of the latent heat of vaporization, L, for example 2.44 MJkg
−1 at 25 °C.
An example of this is the cooling sensation experienced by a person after intense
perspiration followed by a slight breeze, or after a hot shower.
Consider a capped bottle, partially filled with water under vacuum. The water
molecules with the highest velocity will migrate to the gas phase. Some of them,
due to their random and disorderly movement, collide with the surface of the liquid
and are reintegrated into the liquid phase, via condensation process which is the
reverse of evaporation. The number of water-vapor molecules thus increases until
the number of molecules of water that evaporate is equal to the number of
molecules of water-vapour that condense, wherein under these conditions, the
vapour pressure is the saturation pressure.
The saturation vapor pressure does not depend on the volume of the container. If
the volume above the liquid is suddenly reduced, the density of the molecules in the
vapor phase temporarily increases, as well as the intensity of impacts/shocks of
these molecules on the surface of the liquid until a new equilibrium state is reached,
occurring at the same pressure and temperature. The vapor pressure depends on the
ambient temperature. Under higher temperatures, more molecules have enough
kinetic energy to separate from the liquid phase into the vapor phase, so that
equilibrium is reached at a higher vapor pressure.
In real situations, the evaporation of liquids like water occurs in the atmosphere
and not in a vacuum. As in a vacuum, the equilibrium is reached when the number
of molecules that evaporate is equal to the number of molecules that condense.
In the case of water vapor, the number of molecules in the condensation and
evaporation is not affected by the presence of air, although collisions with air
Annex A2: Basic Topics on Laws of Motion and Evaporation
359
are the specific humidity, q, defined as the mass of water vapour per unit mass of
humid air and the absolute humidity v, defined as the mass of vapour of water per
unit volume of humid air.
To analyse the dynamics of the evaporation process, consider the simple case of
an open cup with water, the level of which level falls during the night because of
evaporation, due the liquid molecules changing into the vapour phase. This process
can be explained in terms of kinetic energy.
The molecules of a liquid move relative to one another in a totally random
fashion. The water molecules remain in the liquid state due to attractive forces,
which keep them in this state. A molecule in the surface area of the water, with a
certain velocity, may momentarily leave the liquid and then eventually be
recaptured by attractive forces of other molecules of water if its velocity is not
excessive. Otherwise, the molecule will escape from the liquid medium into the gas
phase. Only molecules with appropriate velocity and kinetic energy will escape into
the gaseous phase, due an increase in the ambient temperature. This confirms the
observable reality that evaporation increases with temperature.
As higher velocity molecules escape into the atmosphere, the average velocity,
kinetic energy, and temperature of the remaining liquid molecules will decrease. It
can be thus anticipated that evaporation is a process of internal cooling of the
system. In the case of water, the temperature drop caused by evaporation is due to
the release of the latent heat of vaporization, L, for example 2.44 MJkg
−1 at 25 °C.
An example of this is the cooling sensation experienced by a person after intense
perspiration followed by a slight breeze, or after a hot shower.
Consider a capped bottle, partially filled with water under vacuum. The water
molecules with the highest velocity will migrate to the gas phase. Some of them,
due to their random and disorderly movement, collide with the surface of the liquid
and are reintegrated into the liquid phase, via condensation process which is the
reverse of evaporation. The number of water-vapor molecules thus increases until
the number of molecules of water that evaporate is equal to the number of
molecules of water-vapour that condense, wherein under these conditions, the
vapour pressure is the saturation pressure.
The saturation vapor pressure does not depend on the volume of the container. If
the volume above the liquid is suddenly reduced, the density of the molecules in the
vapor phase temporarily increases, as well as the intensity of impacts/shocks of
these molecules on the surface of the liquid until a new equilibrium state is reached,
occurring at the same pressure and temperature. The vapor pressure depends on the
ambient temperature. Under higher temperatures, more molecules have enough
kinetic energy to separate from the liquid phase into the vapor phase, so that
equilibrium is reached at a higher vapor pressure.
In real situations, the evaporation of liquids like water occurs in the atmosphere
and not in a vacuum. As in a vacuum, the equilibrium is reached when the number
of molecules that evaporate is equal to the number of molecules that condense.
In the case of water vapor, the number of molecules in the condensation and
evaporation is not affected by the presence of air, although collisions with air
Annex A2: Basic Topics on Laws of Motion and Evaporation
359