S290 Unit 4

4-1-S290-EPUnit 4 Basic Weather Processes
Unit 4
Basic Weather
Processes

Objectives
1. Describe the structure and composition of
the atmosphere.
2. Define weather and list its elements.
3. Describe the sun-earth radiation budget and
the earth’s heat balance.

Objectives
4. Describe factors affecting the temperature
of the earth’s surface and the lower
atmosphere.
5. Describe the greenhouse effect and its
influence on air temperature.
6. Describe temperature lag and the affect
daily and seasonal temperature lags have
on wildland fire behavior.

Our Atmosphere
• Encircling the earth
is a blanket of gases
bound to it by
gravity.
• Similar to its
oceans, the
atmosphere is in
constant motion.

Our “Thin” Atmosphere
The atmosphere extends hundreds of miles above
the earth’s surface. However, compared to the
diameter of the earth of nearly 8,000 miles, our
atmosphere is really quite thin.

Ninety-nine percent of
its gases lie within 18
miles of the earth’s
surface.
Because the upper
portion of the
atmosphere gradually
thins with increasing
altitude, it is impossible
to say exactly where it
ends and interplanetary
space begins.
18 miles

Our atmosphere can be
divided into many
layers based on its
change in temperature
with altitude.
On average,
temperature decreases
with increasing altitude
in the troposphere and
mesosphere, and
increases with altitude
in the stratosphere and
thermosphere.

The
Troposphere
The lowest layer
of the
atmosphere
varies in height
from 9 to 12 miles
above sea level
over the tropics,
to about 6 miles
above sea level
over the polar
regions.The Tropics
Polar Region
Troposphere

The Tropopause
This boundary
separates the
troposphere from the
stratosphere, and marks
the upper limit of nearly
all weather in our
atmosphere.
Because nearly all
weather occurs below
the tropopause, the
underlying troposphere
is often referred to as
the weathersphere.
Stratosphere
Tropopause
Troposphere
(weathersphere)

Nearly three-quarters
of all these atmospheric gases are
concentrated within the
troposphere.
Composition of the Atmosphere
The earth’s atmosphere is
principally composed of
gases and water vapor.

Nitrogen occupies 78 percent
and oxygen about 21 percent
of the total volume of dry
gases in the troposphere.
Dry Gases
The remaining 1 percent of
this volume includes
argon, neon, helium,
hydrogen, xenon and
carbon dioxide.

Water Vapor
• Is an extremely important element of
the atmosphere.
• Forms clouds that produce
precipitation.
• Stores and releases heat energy
called latent heat that is used to
power storms, such as
thunderstorms and hurricanes.

Water Vapor
The concentration of this
invisible gas varies greatly from
place to place, and from time to time.
Approximately half of all
water vapor is found within the
lowest 3 miles of the atmosphere;
in other words within the troposphere.

In tropical locations, water vapor may account for up to 4 percent of
the atmospheric gases.
In colder polar regions, its concentration may be a mere fraction of a
percent.

Wildland Fire Environmental Factors
• Wind, Stability, Temp, RH
• Fuel Moisture
• Fuel Temperature
• Fuel Characteristics
• Terrain
• Aspect
• Elevation

Of these three major components,
weather
is the most variable
over space and time.

Because of its
variability,
weather can be
difficult to predict,
particularly at
scales of less
than 50 miles
and greater than
24 hours.
0 50miles

What Is Weather?
It is the short-term variations of the atmosphere.
These variations include:
• Air pressure
• Air temperature
• Humidity
• Wind
• Clouds
• Precipitation
• Visibility
4-18-S290-EP

It cannot be
overemphasized:
A basic knowledge
and awareness of
weather is essential
for making critical
fire management
decisions.

According to the Standard Firefighting
Orders in the NWCG Fireline Handbook
All firefighters should “keep informed on fire
weather conditions and forecasts.”
Watch Out Situations
• Unfamiliar with weather and local factors
influencing wildland fire behavior
• Weather becoming hotter and drier
• Wind increases and/or changes direction

To understand
how weather can influence
wildland fire behavior,
we will begin with a discussion
of the weather element
atmospheric pressure.

Atmospheric Pressure
This downward force or weight
is the result of the pull of
gravity.PullPull
OfOf
GravityGravity
Or simply air pressure, is
defined as the amount of force
exerted by the weight of air
molecules on a surface area.
Top of atmosphere

50% of Total Weight
90% of Total Weight
99% of Total Weight
99.9% of Total Weight
Atmospheric pressure
always decreases with
increasing altitude.
In this figure, note that
50 percent of all gases are
concentrated within the
lowest 18,000 feet (3 miles)
of the atmosphere.
Millibar is the most
common pressure unit
used today.

Another common air pressure
unit used in aviation and on
television and radio
broadcasts is
inches of mercury.

Standard Atmospheric Pressure
At mean sea level, or the
average height of the ocean surface,
the average, or standard, value for
atmospheric pressure is
29.92 inches of mercury.
This value is equivalent to
1013.25 millibars.

Surface area
If we weigh a column of air with a
cross section of 1 square inch,
extending from sea level to the
top of the atmosphere,
it would weigh nearly
14.7 pounds per square inch
at its base.
This value also represents the
standard atmospheric pressure.

Measuring Air Pressure
Barometer – It is the instrument
used to measure air pressure.
More precisely, it is a calibrated
weather instrument used to
measure the weight of the
atmosphere on a surface area,
normally one square inch in size.
1 square inch

Two
Common
Types of
Barometers
The mercury
barometer – for
precision.
The aneroid
barometer – for
convenience.
Calibrated Scale
Partial
Vacuum
Vacuum
Mercury Barometer Aneroid Barometer

On average, air pressure decreases
approximately one inch of mercury every
1000 foot increase in elevation.

EXERCISE 1
Our Atmosphere

What Drives Our Weather?
The Sun – The principal
source of light and heat
energy
4-31-S290-EP

On a much
smaller scale,
heat also
originates from
large fires,
and other natural
and human related
heat-release
processes.

Solar and Terrestrial Radiation
Shortwave solar
radiation
Longwave terrestrial radiation
Shortwave solar
radiation travels
through the
atmosphere and
heats the earth’s
surface. This
heat is then
transferred to the
atmosphere as
longwave
terrestrial
radiation through
conduction and
convection.

Solar-Earth Radiation Budget

The
Earth’s
Heat
Balance
Any change
in this
equation will
cause the
earth to
either
warm or cool.
Incoming Solar Radiation
Outgoing
Terrestrial
Radiation
Incoming Solar
Radiation
Outgoing Earth
Radiation 4-35-S290-EP

Polar regions
lose more heat
than they gain.
Tropics gain
more heat than
they lose.
Latitudinal Distribution of Heat

Factors Affecting the Temperature of the
Earth’s Surface and Lower Atmosphere
• Solar angle and duration
• Atmospheric moisture and air
pollutants
• Surface properties of terrain and
vegetation

The Seasons in the Northern Hemisphere

Solar Angle and
Duration
Solar angle, length of
daylight, slope, aspect,
and shape of the
countryside all affect the
amount of solar radiation
striking a point on the
earth’s surface.
Basically, the higher the
solar angle and the longer
the daylight, the greater
the solar heating.

Seasonal
Change
In
Solar
Angle
At 40ºN
Latitude

Atmospheric Moisture and Air Pollutants
Clouds, water vapor
and air pollutants
absorb, reflect and
scatter both solar and
terrestrial radiation.
Their presence and
amount significantly
influence the
temperature of
the earth’s surface and
its atmosphere.

Heat Loss At Night
Cloudy nights tend to be
warmer than clear nights
because of the insulating
effect of cloud cover.
Clear nights tend to be
cooler than cloudy nights
because terrestrial heat is
allowed to escape freely
to space.

• Influence the amount
of heat absorbed and
reflected by the
terrain and
vegetation.
• Effect on surface air
temperature can be
quite dramatic.
Surface Properties of
Terrain and Vegetation

Surface Properties of
Terrain and Vegetation
• Color and texture
• Transparency
• Conductivity
• Specific heat
• Evaporation
• Condensation

Earth’s
Power Plant The difference in
surface air
temperature can
be quite large
because of these
properties, such
as the 30 degree
difference
between a
shoreline and a
rocky cliff just 20
miles apart.

Color and Texture
Rough textured,
irregular and dark
colored materials
are good absorbers
of solar radiation.
Whereas smooth,
uniform and light
colored materials
such as snow,
water and sandy
soils are
good reflectors
of solar radiation.

Albedo
Refers to the ability of a substance to reflect
light and heat energy.
Rough textured and dark colored materials,
such as tree bark, a rocky cliff, granite, a newly
plowed field, a forest canopy, and the surface of
a lake at high sun angle, all have a low albedo.
Smooth and light colored materials, such as a field
of snow, sandy soil, and the surface of a lake at
low sun angle, all have a high albedo.

Transparency
Affects the
distribution of light
and heat through a
substance.
Water allows solar
radiation to travel to
a much greater
depth than soil and
rock.
Soil will concentrate
heat within the top
layer.

Conductivity
The transfer of
heat between
molecules in
contact with one
another.

Conductors and Insulators
Materials that allow for the
efficient transfer of heat energy,
such as metal and granite, are
referred to as
conductors.
Materials that are poor
conductors of heat energy, such
as dry air, wood and water, are
referred to as
insulators.
metal
granite
sandstone
wet snow
ice at 0
o
C
mud
water
soil
wood
dry air

Specific Heat
The specific heat of a substance
refers to its capacity to ABSORB,
STORE and RELEASE heat
energy.
The greater the specific heat
capacity of a material, the
longer it will take for it to GAIN
and LOSE heat energy.
The specific heat capacity of all
materials is compared to that of
the most common element on
earth – WATER.

The
Specific Heat
Capacity
of
Common
Materials
water
ice at 0o
C
iron
gold
copper
brick
wood
glass
paper
dry air
cement
rocks
dry soil
For comparison, the
specific heat of water
is five times that of
rock.
Thus, water has the
capacity to store more
heat energy for longer
compared to that of
rocks.

Tropical and
Coastal Regions
Forest and
Woodland Regions
Plains and
Agricultural Regions
Mountain and
Plateau Regions
Semi-Arid and
Desert Regions
Polar and High
Altitude Regions
Regional Variation in Specific Heat
High Moderate
to High
Low Very LowLow to
Moderate
Moderate

Evaporation and condensation
have a large effect of the
heating and cooling
of nearly all objects,
especially the atmosphere.

Evaporation
It is the process where
liquid changes to vapor
or gaseous state.
During this process, heat
energy is removed from
the environment.
Thus, evaporation is a
cooling process.
Heat is removed from the
environment
vapor

Condensation
It is the process
where water vapor
changes to liquid.
During this
process, heat is
added to the
environment.
Thus,
condensation
is a
warming process.
Heat is
added
to the
environment
liquid state
(water droplets)
vapor
state
steam cloud
steam cloud

Evaporation and Condensation In the Environment
Heat added to the atmosphere during
this process is also essential for the
growth of clouds and thunderstorms.
Besides removing heat from the
environment, moisture is also added to
the atmosphere.

Greenhouse Effect
The ability of the atmosphere to retain
infrared radiation (heat energy)
through absorption by greenhouse gases,
such as water vapor, carbon dioxide,
methane and nitrous oxide.

Without these critical greenhouse
gases, the earth’s radiant heat would
escape to space without going into
heating the atmosphere.
On the other hand, should the atmosphere
possess too much of these greenhouse
gases, the earth would become
unbearably hot.

4-60-S290-EPUnit 4 Basic Weather Processes 4-60-S290-EP

Temperature Lag
The warmest and coldest times of the day and
year rarely coincide with the times of
maximum and minimum incoming solar
radiation (insolation).
This difference in time between the maximum
temperature and maximum insolation, and
minimum temperature and minimum insolation
is known as the
temperature lag.

The highest
temperatures
occur on average
3 to 5 weeks
after maximum
solar intensity
(highest solar angle)
around June 21st
.
Seasonal Temperature Lag
TemperatureLag
TemperatureLag
The lowest
temperatures occur
on average 3 to 5
weeks after minimum
solar intensity (lowest
solar angle) around
December 21st.

While the highest
daily temperatures
occurs roughly 2 to 4
hours after solar
noon.
During the summer
season, the lowest
daily temperatures
occur on average
1 to 2 hours after
sunrise.
Daily Temperature Lag

EXERCISE 2
Factors Affecting Air Temperature

Review Objectives
1. Describe the structure and composition of
the atmosphere.
2. Define weather and list its elements.
3. Describe the sun-earth radiation budget and
the earth’s heat balance.

Review Objectives
4. Describe factors affecting the temperature
of the earth’s surface and the lower
atmosphere.
5. Describe the greenhouse effect and its
influence on air temperature.
6. Describe temperature lag and the affect
daily and seasonal temperature lags have
on wildland fire behavior.

S290 Unit 4

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