Transcript Weather Forecasting

Weather
Forecasting
Predicting Changes in
our Atmosphere
What Causes Changes in the
Weather ?
• Air Masses / Air Movement
• Fronts
• Pressure Systems
Weather Forecasting
Because much of modern society
depends on weather conditions,
knowing current weather patterns and
methods used to determine the future
state of the atmosphere is important.
This attempt to predict the future weather
is called weather forecasting.
Forecasting is based on probability of
occurrence.
Weather Maps
Station models are used to record
the atmospheric variables
recorded at different locations.
These variables are:
amount of cloud cover
barometric pressure
temperature
dewpoint
wind direction
wind speed
Using Isolines
Isolines are lines connecting points of
equal value.
Weather maps use different types of
isolines:
Isobars show barometric pressure.
Isotherms show temperatures.
Synoptic Weather Maps
Synoptic weather maps provide a
synopsis, or summary of weather
conditions all over the United States at
the indicated hour.
What is an Air Mass?
Air mass = a large body of air in the
troposphere having similar
pressure, wind and temperature
characteristics throughout.
Air Mass Characteristics
Air masses have characteristics that are
determined by the part of the Earth’s surface
that the air mass forms over.
These areas are called source regions.
Examples: Forming over land = dry air mass
Forming over water = moist air mass
Forming over equator = warm air mass
Forming over the poles = cold air mass.
Source
Regions
Air masses move and swirl over the
surface of the Earth in different
directions and at different speeds and
often bump into one another.
Rain, thunderstorms, snow, tornadoes
and all weather-related events can
result when air masses meet.
What is a Weather Front?
• Front = the boundary formed as two air
masses meet and collide.
Types of Fronts
• Warm Front = forms when a warm air mass
slides over a departing cold air mass. As
warm air rises the temperature drops and
rain or precipitation often forms.
• Cold Front = forms as a cold air mass
overtakes and moves underneath a warm air
mass. The warm air is forced upward rapidly
and a cold front forms. Cold fronts usually
bring violent weather.
Types of Fronts (cont.)
• Stationary Front = occurs when a cold air
mass meets a warm air mass and neither air
mass moves. Precipitation sometimes forms
between these two fronts.
• Occluded Front = forms when two cold air
masses meet and force a warm air mass
between them to rise completely off the
ground. The weather in an occluded front is
difficult to predict because there are three air
masses involved.
What is a Pressure System?
• Pressure = as particles move and collide
they exert pressure;
• Differences in air pressure have a great
affect on the weather.
• When particles are more densely packed
together they exert high pressure,
– High air pressure usually means clear, fair
weather.
• When they are less densely packed they
exert less (low) pressure.
– Low air pressure usually means clouds and rainy
weather.
High Pressure System
• Cold air is dense and it sinks.
• As the air sinks it warms up.
• Warm air holds more water vapor.
– The total amount of water vapor remains the same.
• As the air warms the relative humidity
decreases as droplets in the clouds evaporate.
• A high pressure system usually means fair
weather as the moisture in the air is evaporated
so few clouds form.
Low Pressure System
• Warm air has low density and it rises (forced
upwards by surrounding denser air)
• As the air rises it cools
• As the air cools the relative humidity increases –
eventually reaching the dew point
– At the dew point condensation takes place and clouds form
• Low pressure systems usually form along fronts
where warm air and cold air meet
• Low pressure systems cause most of the weather in
the US.
• A low pressure system leads to precipitation in the
form of rain, snow, hail, sleet.
Moving Fronts
• When we witness a change in the
weather from day to day it is due to the
movement of air masses. It is the
movement and collision of air masses
that causes the weather to change.
What is Severe Weather?
• Severe weather are events that fall outside
of the ordinary weather patterns.
• Severe weather can include the following:
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Thunderstorms
Lightning
Tornadoes
Hurricanes
Floods
Droughts
Blizzards
Fog
Ice Storms
Thunderstorms
• Thunderstorms are formed by rapid upward
movement of warm moist air.
• Thunderstorms can occur within a warm
moist air mass.
• Thunderstorms most often occur at cold
fronts.
• Thunderstorms account for the greatest
number of weather-related fatalities in the US
due to the flooding, lightning and tornadoes
that can be associated with them.
How Thunderstorms Form
• As warm moist air is forced upward it cools
and the water vapor condenses forming
cumulus clouds. (These clouds can be very
tall – up to 10km high)
• As the drips fall they collide and combine to
form larger drops
• The falling drops create a downward
movement of air that spreads out as it hits
the surface causing winds
– If the winds accompanying a thunderstorm are
greater than 54mph then forcasters classify it as a
severe thunderstorm (these usually contain hail).
Lightning
Hurricanes
• Hurricanes are the largest storms that occur on
Earth.
• Hurricanes are very large, swirling, low pressure
systems that from over tropical waters (water
temperature must be at least 80 degrees F).
• Hurricanes can last for days due to the constant
supply of energy from the warm waters
• To be classified as a hurricane there must be
sustained winds of greater than 74mph.
– Greatest winds recorded are 236mph; Category 5 “Mitch” in
Nov 1998
– 1900 Galveston TX was most deadly hurricane ever with
8000 deaths.
Eye of the Hurricane
The eye of a hurricane is the relatively
calm area in the middle of the storm.
The eye has the lowest pressure of the
storm and the eye wall is where the
strongest winds occur.
A storm surge is a dome of water that
can raise the water level higher than
high tide where the storm lands.
How Hurricanes Form
• Hurricanes form over warm tropical oceans where
two opposing winds meet and begin to swirl
• A low pressure forms in the middle of the swirling
winds and begins rotating
• Warm moist air is forced upwards in the center
• The dropping pressure in the center pulls more air
toward the center creating increasing winds and
lower pressure. This cycle of increasing strength
continues as long as the storm remains over warm
water.
• Hurricanes weaken as they hit land because there is
no longer a supply of energy (from the warm water)
available.
Life Cycle of a Hurricane
• Life-cycle of a hurricane = form first as
a low pressure system, then grow into
a tropical depression (<31mph winds),
then grow into a tropical storm (3274mph winds), then grow into a
hurricane (>74mph winds).
• Out of the 10 tropical storms per year
average, 6 will develop into hurricanes
and 2 will strike the US.
Key Hurricane Alerts
Tropical Storm Watch: tropical storm
conditions are possible in your area w/in
the next 36 hours.
Tropical Storm Warning: tropical storm
conditions are expected in your area w/in
the next 24 hours.
Hurricane Watch: hurricane conditions are
possible in your area w/in 36 hours.
Hurricane Warning: hurricane conditions
are expected in your area in 24 hours or
less.
Hurricane Strength
The Saffir – Simpson Scale
Category
1
Atm.
Wind Speed
Pressure
(mph)
(mb.)
>980
74 – 95
Storm
Surge
(feet)
4–5
2
965 -979
96 – 110
6–8
3
945 -964
111 – 130
9 – 12
4
920 -944
131 – 155
13 – 18
5
<920
>155
>18
Hurricane Origins
Tornadoes
• Tornadoes are violent funnel-shaped storms with whirling
winds that move in narrow paths over land.
• Tornadoes form from severe thunderstorms
• Tornado producing thunderstorms involve the rapid upward
movement of warm moist air
• The upward moving air begins to rotate (why the rotation starts
is still not understood)
• As the speed of the rotation increases more warm moist air is
drawn into the low pressure at the center
• A funnel shaped cloud extends from the bottom of the cloud
sometimes touching ground
– As it touches ground it picks up dirt and debris that give it
the characteristic dark gray, black color.
• Tornadoes are one of the most destructive types of storms.
Strength of Tornadoes:Fujita Scale
Scale
Wind Speed
(mph)
Expected Damage
F0
<72
Light Damage: broken branches,
billboards.
F1
72 – 112
Moderate Damage: mobile homes
overturned, roofs.
F2
113 – 157
Considerable Damage: roofs off,
trees snapped, uprooted.
F3
158 – 206
Severe Damage: trains overturned,
cars thrown.
F4
207 – 260
Devastating Damage: houses
leveled, large missles.
F5
>260
Incredible Damage: houses lifted,
trees debarked, auto missles.
Tornado Alley
Floods
Droughts
Blizzards
Fog
Ice Storms
What is Climate?
• Climate is the state of the atmosphere of a
given region averaged over time.
– Climate is how hot/cold, wet/dry a place is
in general.
– Climate is what it is usually like in a place.
– Example: Florida’s climate is usually
sunny and hot.
The Hydrologic Cycle
• The hydrologic cycle, or water cycle, is the
basis for the Earth’s weather.