Earth Science
Chapter 16
Section 4
A. Humidity:
Humidity is a measure of the amount of water vapor in the
air.
The percentage of water vapor in the air compared to the
maximum amount the air can hold is called the relative
humidity.
RH = Absolute humidity
Capacity
Measuring Relative Humidity:
Relative humidity can be measured with a psychrometer.
A psychrometer consists of two identical thermometers.
One has a muslin wick covering the bulb. The temperature
read by this thermometer is known as the wet-bulb
temperature.
The other thermometer reads the actual air temperature. This
thermometer reading is called the dry-bulb temperature.
The wet-bulb temperature is always lower than the dry-bulb
temperature because of evaporative cooling. Water draws
heat energy away from the wet bulb when it evaporates
causing a lower temperature reading.
When the wet-bulb and the dry-bulb temperatures are the
same no water is evaporating from the wet-bulb thermometer.
When this occurs the relative humidity is 100%. (the air is
saturated)
Using the sling psychrometer
1. Find the wet and dry-bulb temperatures.
2. Take the difference of the two readings. This is known as
the wet-bulb depression.
3. Using a psychometric chart.
Find the dry-bulb reading along the side.
Find the wet-bulb depression along the top.
Where the two numbers intersect is the RH in percent.
B. How Clouds form:
Clouds of all kinds form when water vapor in the air becomes
liquid water or ice crystals.
Water vapor becomes a liquid by a process called
condensation.
The temperature at which condensation takes place is called
the dew point. The temperature at which water vapor
changes to a liquid.
Clouds form whenever air is cooled to its dew point and
particles are present for the droplets to form on.
Air can rise through convection, frontal lifting or through
physical lifting.
Physical Lifting is also known as Orographic Lifting
This type of lifting occurs when air is forced upward by
topography, the physical relief of the land.
As winds sweep across the landscape, hills and valleys causing the
moving air to alternately ascend and descend, the resulting
expansional cooling and compressional warming of the air affects
the development of clouds and precipitation.
Example:
As air is forced to rise along windward slope (facing the wind), it
expands and cools, which increases its relative humidity, with this,
clouds and precipitation develop.
Meanwhile, on the mountain's leeward slope (down wind side ),
air descends and warms, which reduces its relative humidity, so
clouds and precipitation are less likely to form.
C. Types of Clouds:
Clouds are classified into three main types:
High Clouds
-Cirrus - Delicate streaks or patches
-Cirrostratus - Transparent thin white sheets
-Cirrocumulus – layers of small white puffs or ripples
Middle clouds
-Altostratus – Uniform white or gray sheet or layer
-Altocumulus – White or gray puffs or waves in patches or
layers
Low clouds
-Stratocumulus – patches or layers of large rolls or merged puffs
-Stratus – Uniform gray layer
-Nimbostratus – Uniform gray layer from which precipitation is
falling
Vertical development clouds
-Cumulus – Detached heaps or puffs with sharp outlines and flat
bases and slight vertical extent
-Cumulonimbus – Large puffy clouds of great vertical extent
with flattened tops, frequently anvil shaped
from which showers fall, with lightning and
thunder