Haz-Mat
Incident Considerations

Chapter 3
– Weather Considerations
03-1
Weather Considerations

Session objectives:
– The student will identify how various
meteorological factors may influence a
hazardous materials incident
03-2
Weather Considerations

Response personnel must have a basic
understanding of:
– daily weather patterns
– irregular weather events that may occur in
the area
03-3
Components That Affect Local
Weather
Temperature
 Relative humidity
 Atmospheric pressure
 Air density

03-4
Temperature

Temperature affects the following:
– volatilization of a chemical from a liquid to
a gas
– vapor pressure
– dispersion of a vapor or a gas
– surface temperatures
– reactivity and/or stability of a material
03-5
Humidity
Humidity affects vapor production from
a liquid
 As the humidity rises:

– vaporization of water solubles will increase
– vaporization of non-water solubles will
decrease

Water-reactive liquids may fume
vigorously
03-6
Precipitation

Precipitation can aggravate a haz mat
incident:
– may cause water-reactive chemicals to
react violently
– some products are designed to become
activated by water
– consider protective measures to maintain
control of a spill during impending rain
03-7
Wind
Wind will affect the dispersion of a
vapor or gas
 Low wind
vs.
High wind

03-8
Strategic Considerations
– Can evacuations be performed safely?
– Will in-place protection be effective?
– How large an area is affected?
03-9
Strategic Considerations
– How much larger will the incident be when
resources are in place?
– How steady is the wind?
– Will the wind change significantly ?
– How will topographical features impact the
incident?
03-10
Atmospheric Stability

There are two types of days:
– Stable
– Unstable
03-11
Stable Conditions

Conditions constant and unchanging
– atmospheric inversion layer
– smog build-up
– cloud cover greater than 50%

Wind speeds are low
03-12
Unstable Days
Conditions are changing and variable
 Characterized by:

– bright sunshine
– moderate afternoon winds
– cumulus development if moisture is
present
– dust devils, straight upward movement of
smoke and windy afternoons
03-13
Daily Wind Patterns
Winds blow up-canyon/onshore during
the day
 Winds blow down-canyon/offshore at
night

03-14
Events That Cancel Daily
Patterns
Gradient winds
 Foehn winds
 Frontal winds

03-15
Gradient Winds

Free-flowing prevailing winds
– at an elevation where they are not
influenced by topography
– usually 2000 feet or more above the
surface
– Gradient winds are a predominant element
much of the time
03-16
Foehn Winds

Pattern Common to California
» example: Santa Ana winds

They are characterized by:
– strong gusty winds
– low humidity
– blowing out of the east or north east
– generally last 2 or 3 days
03-17
Frontal Winds
Associated with the passing of a major
weather system
 Can cause a 180 degree shift in wind
direction as the front passes

03-18
Predicting The Effect Of Weather
Time of day the release occurs
 Projected duration of the incident
 Anticipated changes in the weather
 Be aware of local weather patterns

03-19
Sources of Weather Information
Local newspaper
 The National Weather Service
 Municipal airports
 Some fire stations
 Continue to evaluate the forecast
against current conditions

03-20
Weather Monitoring Equipment
A piece of barrier tape tied to a pole
 Electronic weather equipment hooked
into a computer
 Assign a person the responsibility of
monitoring the weather

03-21