Download PP 15-Weather - MIT

Weather
Weather is the current state of the atmosphere.
There are many factors that “cook” up our weather!
Factor #1: The Sun
Solar radiation is the major source of heat for our
Earth’s weather.
Lab: What’s the Angle?
Together, the sun angle and day length determine
the total amount of solar radiation incident at the
Earth.
Factor #2: The Greenhouse Effect
Greenhouse Effect
Factor #3: Earth’s Rotation
If the Earth did not rotate, air would sink south from
the cold polar regions and rise north from the
warmer tropics.
But the Earth does rotate causing a curvature in
the wind paths.
The Coriolis effect explains the direction of
movement of air masses.
Factor #4: Air Masses
Air masses are large bodies of air that takes the
characteristics of the area over which it forms.
Air masses are classified according to their source
region.
Symbol
Air Mass
Characteristics
cA
Continental arctic
Bitterly cold, dry
cP
Continental polar
Cold, dry
cT
Continental tropical
Hot, dry
mP
Maritime polar
Cool, moist
mT
Maritime tropical
Warm, moist
Each air mass has a different temperature
associated with it.
Fronts occur where two large air masses collide at
the earth's surface.
A front is the narrow region separating two air
masses of different densities.
Fronts are caused by winds moving one air mass
away from its birthplace.
In a cold front, cold dense air displaces warm air
forcing it up along a steep front.
Clouds, showers and sometimes thunderstorms are
associated with cold fronts.
In a warm front, advancing warm air displaces cold
air producing a gradual frontal slope.
A warm front is characterized by extensive
cloudiness and precipitation.
Stationary fronts occur when neither warm nor cold
air advances.
The two air masses reach a stalemate.
That is what stationary means - that neither front is
moving.
Temperatures remain stagnant and winds are gentle
to nil.
These type of conditions can last for days,
producing nothing but altocumulus clouds.
Occluded Front is when a cold front overtakes a
warm front.
Steady rains fall at an occluded front.
#1 = Cold Front
#2 = Warm Front
#3 = Occluded Front
#4 = Stationary Front
Lab: Air Masses
Lab: Fronts
Factor #5: Water Vapor
The air's capacity for water vapor increases as air
temperature increases.
Air with a temperature of 30°C can hold more than
three times as much water vapor as air at 10°C.
Relative humidity is the percentage of moisture the
air holds relative to the amount it could hold at a
particular temperature.
Factor #6: Air Pressure
A "high" is an area where the air's pressure is higher
than the pressure of the surrounding air.
A "low' is where the air pressure is lower.
Air descends in a high pressure system.
Air ascends in a low pressure system.
Differences in air pressure causes wind.
A trough is an elongated area of low atmospheric
pressure that can occur either at the Earth's surface
or at higher altitudes.
Cold air plunges south,
carving out trough.
Ahead of trough rising
air causes precipitation.
Measuring Weather Conditions
Station Reporting Symbol
The value highlighted in yellow located in the upper
left corner is the temperature in degrees Fahrenheit.
The reported temperature is 64 degrees.
Wind Speed
Anemometer
Wind Direction
Wind Vane
The symbol highlighted in yellow is known as a
"Wind Barb".
The wind barb indicates the wind direction and wind
speed.
Each short barb represents 5 knots, each long barb
10 knots.
A long barb and a short barb is 15 knots, simply by
adding the value of each barb together (10 knots + 5
knots = 15 knots).
The wind barb indicates winds from the Southeast.
Lab: Wind Speed
Lab: Jersey Winds
Lab: Doppler Effect
Air exerts pressure on a surface.
You don't feel the pressure because air is pushing
equally on all sides.
Air pressure is measured using a barometer.
Air pushes down on the mercury. The pressure
causes the mercury to rise up the vacuum tube. The
tube is calibrated in inches or millimeters.
The aneroid barometer does not use a liquid to
measure pressure.
It is not as accurate as a mercurial barometer.
Manometers indicate the difference between two
pressures when one side is open to atmosphere. If
a U-tube is filled to the half way point with water
and air pressure is exerted on one of the columns,
the fluid will be displaced.
The difference in height "h" which is the sum of the
readings above and below the half way point,
indicates the pressure in inches of water column.
The value highlighted in yellow located in the upper
right corner represents the last three digits of the
observed pressure reading in millibars (mb).
The black numbers are station reports of sea-level
pressure in millibars.
Isobars are normally drawn at 4 mb intervals, with
1000 mb being the base value.
If reported value less than 500: Initial 10 is missing.
Place it on left, then divide by 10.
For example: 027 becomes 1002.7 mb.
Lab: Isobars
At higher altitudes, air exerts less
pressure. Temperature can also affect air pressure
since air is a gas that expands when heated and
contracts when cooled. The pressures were
corrected to show what they would be at sea level.
A red "L" means low pressure (which can mean
storminess). Most fronts extend from low-pressure
centers.
A blue "H" means a center of high pressure (usually
calm, sunny weather).
Air descends in high pressure area (A) and rises in
low pressure area (B).
This combined with the Coriolis effect results in the
formation of rotating low or high pressure systems
in the atmosphere.
Winds flow clockwise around a high pressure center
in the northern hemisphere, while in the southern
hemisphere, winds flow counterclockwise around a
high.
Winds flow counterclockwise around a low pressure
center in the northern hemisphere, while in the
southern hemisphere, winds flow clockwise around
a low.
The value highlighted in yellow located in the lower
left corner is the dew point temperature in degrees
Fahrenheit.
The dew point temperature is the air temperature
where the moisture in the air begins to condense or
change from a vapor to a liquid.
The dew point temperature is always the coldest of
the three temperatures.
In order for a surface to collect dew out of the air,
the temperature of that surface must be at or below
the dew point temperature.
The reported dew point temperature is 58 degrees.
Humidity is the amount of moisture in the air.
Relative humidity is calculated using a
psychrometer sling.
It holds the two thermometers and provides a
mechanism for you to whip them around to
accelerate air exposure and get a more accurate
reading.
Relative humidity is determined by calculating the
difference between the two thermometers.
Lab: Relative Humidity
The symbol highlighted in yellow indicates the type
of weather occurring at the time the observation is
taken.
The symbol highlighted indicates fog was reported.
The symbol highlighted in yellow indicates the
amount of cloud cover occurring at the time the
observation is taken.
Weather Instruments
Instrument
What it
measures
Common Units of
Measure
Thermometer
Air temperature
Centigrade (Celsius), Fahrenheit
Weather Instruments
Instrument
What it
measures
Common Units of
Measure
Thermometer
Air temperature
Centigrade (Celsius), Fahrenheit
Barometer
Atmospheric
Pressure
Isobars, Millibars, Inches of
Mercury
Weather Instruments
Instrument
What it
measures
Common Units of
Measure
Thermometer
Air temperature
Centigrade (Celsius), Fahrenheit
Barometer
Atmospheric
Pressure
Isobars, Millibars, Inches of
Mercury
Psychrometer
Relative Humidity
Percentage (%)
Weather Instruments
Instrument
What it
measures
Common Units of
Measure
Thermometer
Air temperature
Centigrade (Celsius), Fahrenheit
Barometer
Atmospheric
Pressure
Isobars, Millibars, Inches of
Mercury
Psychrometer
Relative Humidity
Percentage (%)
Anemometer
Wind Speed
Kilometer per hour, Miles per hour
Lab: Making a Weather Map
Controlling
The
Weather
Drought Prevention
Cloud Seeding
Project STORMFURY
Project STORMFURY
Hail Cannons
A Hail Cannon is a shock wave generator used to
disrupt the formation of hailstones in the
atmosphere in their growing phase.
The device is repeatedly fired every 4 seconds over
the period when the storm is approaching and until
it has passed through the area.