Module 2 - Air and Humidity

1. Physical Science
Module 2
Air and Relative Humidity

2. Water/Air
Under usual Earth conditions, water exists in any
of three physical states:
 solid (ice, snow)
 liquid (which we typically call water)
 gas (humidity, water vapor [steam])

3. Humidity
 According to meteorologists, humidity can be
 Absolute or
 Relative
 Giving us the terms absolute humidity and relative
humidity
 In common usage, the adjective is often dropped, and
the speaker (or writer) just says "humidity," expecting us
to know of which type they are speaking
 Even some television and radio weathercasters,
including those formally trained in meteorology, often do
not clarify the difference when reporting the weather
conditions

4. Humidity
 Absolute humidity is defined as the ratio of the
mass of water vapor contained per volume of
moist air.
 Relative humidity, technically, is the ratio
between the partial pressure of water in the air
and the maximum possible vapor pressure of
water at a particular temperature.
 Relative humidity – is “Relative” to us…it’s what
we feel and what the meteorologist discusses on
the news channel

5. Relative Humidity
It is dimensionless
Relative humidity is usually what the
media announcers mean when they say
"humidity," and it is useful in determining
conditions for human comfort.
But, it can be confusing because its value
varies with air temperature.

6. Relative Humidity
For example, the morning may have a
relative humidity of 78%, which by
afternoon drops to 53% as the air
temperature rises. (Note: the absolute
humidity for that day remained essentially
constant, but the vapor pressure of water
increased with the temperature).

7. Relative Humidity
Similarly in winter, the outdoor relative
humidity may be 63%, but when outdoor
air permeates our warm homes and
offices, the relative humidity level may
drop to 35% or lower. (In this example, the
absolute humidity is quite low in the
outdoor air, but the vapor pressure of
water at cold temperatures is also low,
thus, the outdoor air is more humid,
relatively speaking.)

8. Short Movie on Humidity
http://glencoe.mcgraw-hill.com/sites/dl/free/007