Download Atmospheric Humidity

Chapter 3
Daily temperature variations



Properties of soil affect the rate of conduction from Earth to
atmosphere
Wind mixes energy into air column and can force
convection.
Nighttime cooling
 As sun lowers, the lower solar angle causes insolation to
be spread across a larger area
 Radiational cooling as infrared energy is emitted by the
Earth’s surface
 Radiation inversion (starts shortly after sunset): air near
ground much cooler than air above
 Thermal belt: warmer hillsides that are less likely to
experience freezing temperatures as in the valley below
 fruit trees planted on hillside rather than valley.
Daily temperature variations

Protecting crops from cold on cold
nights:
 Cover : prevent ground heat from being
radiated away
 Orchards heaters
 Fans: to mix cold air from ground with
warmer air above
 Sprinklers to keep the soil wet (has higher
thermal conductivity) heat is conducted
upward from subsurface soil more rapidly 
helps keep the surface air warmer.
The controls of temperature = the main
factors that cause variations in temperature
from one place to another.
Latitude: solar angle (intensity of incoming
solar radiation) and the length of daylight
hours
 Land & water distribution: water has high
specific heat  water heats/cools more
slowly than land
 Ocean currents: warm and cold currents
often influence air temperatures
 Elevation: cooling

Average air temperature near sea level in January (oF).
Average air temperature near sea level in July (oF).
Air temperature data

Daily, monthly, yearly temperature
 Daily range of temperature: maximum minus
minimum; greatest next to the ground,
becomes progressively smaller as we move
away from the surface. (clear vs. cloudy
days)
 Mean (monthly, annual, etc.): average of
temperature observations
 Maximum: highest temperature of time
period
 Minimum: lowest temperature of time period
Air temperature data

Special topic: What’s normal?
 Climate normal is the 30 year average for a
given temperature variable.

The use of temperature data
 Heating degree-day (subtract the mean
temperature of the day from 65°F):
people heat
when temperature below 65°F
 Cooling degree-day: people cool when
temperature above 65°F
 Growing degree-day: temperature above of
below base temperature for specific crop
Temperature
data for San
Francisco,
California
(37oN), and
Richmond,
Virginia (37oN)
— two cities
with the same
mean annual
temperature.
Mean annual total heating degree-days
across the United States (base 65oF).
Air temperature and human comfort

Body heats through metabolism (food  energy)

Body cools through emitting IR energy and evaporation of
perspiration
 wind-chill index (heat loss correlated with wind speed)
 Hypothermia (lowering of human body temperature below
its normal level)
Measuring air temperature

Thermometers: liquid-in-glass, maximum,
minimum, electrical resistance, bimetallic,
thermistors, etc.

ASOS (Automated Surface Observing System)

Infrared sensors (used by satellites)

Observation: Thermometers in the shade
 Radiant energy from the Sun in direct sunlight
increases the temperature recorded by a sensor.
 True air temperature measured in the shade.
Chapter 4
Circulation of Water in the
Atmosphere
A general definition of humidity is the amount
of water vapor in the air.
 Remember, humidity is not constant through
time or space,
 there is constant circulation of water through
the hydrologic cycle.
 Oceans occupy over 70% of the Earth’s
surface  we can think of this circulation as
beginning over ocean

The Hydrologic Cycle
Fig. 4-1, p. 90
The Many Phases of Water

Phase is related to molecular motion, an
increase or decrease in motion creates
a phase change.

Ice is the coolest/slowest phase

Water vapor is the warmest/fastest
phase
The higher the T, the
faster the molecules
move.
The molecules are closer
together, constantly
bumping into one another.
The molecules are in
an orderly pattern, locked
in a rigid position.
Evaporation, Condensation, & Saturation
Evaporation is the change of liquid into a
gas and requires heat.
 Sublimation: ice-to-vapor phase change and
requires heat.
 Condensation is the change of a gas into a
liquid and releases heat.

 Condensation nuclei

Saturation is an equilibrium condition in
which for each molecule that evaporates,
one condenses.
Water molecules at the surface of the water
are evaporating (changing from liquid into
vapor) and condensing (changing from
vapor into liquid). Since more molecules
are evaporating than condensing, net
evaporation is occurring.
When the number of water molecules
escaping from the liquid (evaporating)
balances those returning (condensing), the
air above the liquid is saturated with water
vapor.
Condensation is more likely to occur as the air cools.
In the warm air, fast-moving H2O vapor
molecules tend to bounce away after
colliding with nuclei.
In the cool air, slow-moving vapor
molecules are more likely to join together
on nuclei. The condensing of many
billions of water molecules produces tiny
liquid water droplets.
Humidity

Any of a number of ways of specifying the
amount of water vapor in the air.

Absolute humidity: mass of water
vapor/volume of air
 Water vapor density
 Not commonly used due to frequent change of volume (air
parcel rising and descending)
Absolute humidity 
mass of water vap or
volume of air
The water vapor content (humidity) inside this air parcel can be expressed in a
number of ways.
With the same amount of water vapor in a parcel of air, an increase in
volume decreases absolute humidity, whereas a decrease in volume
increases absolute humidity.
Humidity

Specific Humidity: mass of water vapor/mass of air
Specific humidity 

mass of water vap or
g kg
total mass of air
Mixing ratio: mass of water vapor/mass of dry air
mass of water vap or
g kg
Mixing ratio 
mass of dry air

Neither measurement changes with volume; they will
change only if we add or subtract water vapor.
The specific humidity does not change as air rises and
descends.