4-1-S290-EP Unit 4 Basic Weather Processes Unit 4 Basic Weather Processes
4-2-S290-EPUnit 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.
4-3-S290-EPUnit 4 Basic Weather Processes
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.
4-4-S290-EPUnit 4 Basic Weather Processes
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.
4-5-S290-EPUnit 4 Basic Weather Processes
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.
4-6-S290-EPUnit 4 Basic Weather Processes
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
4-7-S290-EPUnit 4 Basic Weather Processes
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.
4-8-S290-EPUnit 4 Basic Weather Processes
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
4-9-S290-EPUnit 4 Basic Weather Processes
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)
4-10-S290-EPUnit 4 Basic Weather Processes
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.
4-11-S290-EPUnit 4 Basic Weather Processes
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.
4-12-S290-EPUnit 4 Basic Weather Processes
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.
4-13-S290-EPUnit 4 Basic Weather Processes
Water Vapor
The concentration of thisinvisible 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.
4-14-S290-EPUnit 4 Basic Weather Processes
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.
4-15-S290-EPUnit 4 Basic Weather Processes
Wildland Fire Environmental Factors
• Wind, Stability, Temp, RH
• Fuel Moisture• Fuel Temperature• Fuel Characteristics
• Terrain• Aspect• Elevation
4-16-S290-EPUnit 4 Basic Weather Processes
Of these three major components, weather
is the most variable over space and time.
4-17-S290-EPUnit 4 Basic Weather Processes
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
4-18-S290-EPUnit 4 Basic Weather Processes
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
4-19-S290-EPUnit 4 Basic Weather Processes
It cannot be overemphasized:
A basic knowledge and awareness of
weather is essentialfor making criticalfire management
decisions.
4-20-S290-EPUnit 4 Basic Weather Processes
According to the Standard Firefighting Orders in the NWCG Fireline Handbook
All firefighters should “keep informed on fireweather 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
4-21-S290-EPUnit 4 Basic Weather Processes
To understandhow weather can influence
wildland fire behavior, we will begin with a discussion
of the weather element atmospheric pressure.
4-22-S290-EPUnit 4 Basic Weather Processes
Atmospheric Pressure
This downward force or weight is the result of the pull of
gravity. PullPullOfOf
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
4-23-S290-EPUnit 4 Basic Weather Processes
50% of Total Weight
90% of Total Weight
99% of Total Weight
99.9% of Total WeightAtmospheric pressure always decreases with
increasing altitude.
In this figure, note that50 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.
4-24-S290-EPUnit 4 Basic Weather Processes
Another common air pressure unit used in aviation and on
television and radio broadcasts is
inches of mercury.
4-25-S290-EPUnit 4 Basic Weather Processes
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.
4-26-S290-EPUnit 4 Basic Weather Processes
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.
4-27-S290-EPUnit 4 Basic Weather Processes
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
4-28-S290-EPUnit 4 Basic Weather Processes
Two CommonTypes of
Barometers
The mercurybarometer – for
precision.
The aneroid barometer – for convenience.
Calibrated Scale
PartialVacuum
Vacuum
Mercury Barometer Aneroid Barometer
4-29-S290-EPUnit 4 Basic Weather Processes
On average, air pressure decreases approximately one inch of mercury every
1000 foot increase in elevation.
4-31-S290-EPUnit 4 Basic Weather Processes
What Drives Our Weather?
The Sun – The principal source of light and heat energy
4-31-S290-EP
4-32-S290-EPUnit 4 Basic Weather Processes
On a much smaller scale,
heat also originates from
large fires, and other natural
and human related heat-releaseprocesses.
4-33-S290-EPUnit 4 Basic Weather Processes
Solar and Terrestrial Radiation
Shortwave solarradiation
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.
4-35-S290-EPUnit 4 Basic Weather Processes
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
4-36-S290-EPUnit 4 Basic Weather Processes
Polar regions lose more heat than they gain.
Tropics gainmore heat than they lose.
Latitudinal Distribution of Heat
4-37-S290-EPUnit 4 Basic Weather Processes
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
4-39-S290-EPUnit 4 Basic Weather Processes
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.
4-41-S290-EPUnit 4 Basic Weather Processes
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.
4-42-S290-EPUnit 4 Basic Weather Processes
Heat Loss At Night
Cloudy nights tend to bewarmer than clear nightsbecause of the insulating
effect of cloud cover.
Clear nights tend to be cooler than cloudy nightsbecause terrestrial heat isallowed to escape freely
to space.
4-43-S290-EPUnit 4 Basic Weather Processes
• Influence the amount of heat absorbed and reflected by the terrain and vegetation.
• Effect on surface air temperature can be quite dramatic.
Surface Properties ofTerrain and Vegetation
4-44-S290-EPUnit 4 Basic Weather Processes
Surface Properties of Terrain and Vegetation
• Color and texture• Transparency• Conductivity• Specific heat• Evaporation• Condensation
4-45-S290-EPUnit 4 Basic Weather Processes
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.
4-46-S290-EPUnit 4 Basic Weather Processes
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.
4-47-S290-EPUnit 4 Basic Weather Processes
AlbedoRefers 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.
4-48-S290-EPUnit 4 Basic Weather Processes
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.
4-49-S290-EPUnit 4 Basic Weather Processes
Conductivity
The transfer of heat between molecules in contact with one another.
4-50-S290-EPUnit 4 Basic Weather Processes
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 0oC
mud
water
soil
wood
dry air
4-51-S290-EPUnit 4 Basic Weather Processes
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 allmaterials is compared to that of the most common element on earth – WATER.
4-52-S290-EPUnit 4 Basic Weather Processes
The Specific Heat
Capacityof
CommonMaterials
water
ice at 0oC
iron
gold
copper
brick
wood
glass
paper
dry air
cementrocks
dry soil
For comparison, the specific heat of wateris five times that of rock.
Thus, water has the capacity to store more heat energy for longer compared to that ofrocks.
4-53-S290-EPUnit 4 Basic Weather Processes
Tropical and Coastal Regions
Forest andWoodland Regions
Plains and Agricultural Regions
Mountain andPlateau Regions
Semi-Arid andDesert Regions
Polar and HighAltitude Regions
Regional Variation in Specific Heat
High Moderate to High
Low Very Low Low to Moderate
Moderate
4-54-S290-EPUnit 4 Basic Weather Processes
Evaporation and condensationhave a large effect of the
heating and cooling of nearly all objects,
especially the atmosphere.
4-55-S290-EPUnit 4 Basic Weather Processes
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
4-56-S290-EPUnit 4 Basic Weather Processes
CondensationIt 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)
vaporstate
steam cloudsteam cloud
4-57-S290-EPUnit 4 Basic Weather Processes
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.
4-58-S290-EPUnit 4 Basic Weather Processes
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.
4-59-S290-EPUnit 4 Basic Weather Processes
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 atmospherepossess too much of these greenhouse
gases, the earth would become unbearably hot.
4-61-S290-EPUnit 4 Basic Weather Processes
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.
4-62-S290-EPUnit 4 Basic Weather Processes
The highest temperatures
occur on average 3 to 5 weeks
after maximum solar intensity
(highest solar angle) around June 21st.
Seasonal Temperature Lag
Tem
per
atu
re L
ag
Tem
per
atu
re L
ag
The lowest temperatures occur
on average 3 to 5 weeks after minimum solar intensity (lowest solar angle) around
December 21st.
4-63-S290-EPUnit 4 Basic Weather Processes
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
4-65-S290-EPUnit 4 Basic Weather Processes
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.
4-66-S290-EPUnit 4 Basic Weather Processes
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.