Transcript Intro. To Weather Forecasting - EHS

Weather
Variables and Forecasting
Modified from National Weather Service www.srh.noaa.gov/shv
Air Masses
A large body of air with
uniform temperature and
humidity conditions
inherited from a "source
region.”
Source regions = areas
where air stagnates,
taking on the properties
of the underlying region.
4 Principal Air Mass Types that affect North America:
P = Polar - Located poleward of 60° Lat.
c = Continental – From over large land masses
m = Maritime – From over oceans
T = Tropical - Located between 25° N & S
Air Masses are separated by FRONTS
Meteorology –
primarily concerned with Largescale weather systems, such as
cyclones
Major fronts
Movement of the jet streams
Weather Forecasting requires
that we examine multiple
variables over space and time.
Pressure and Wind
• High Pressure: air moves
downward, clockwise (Nhemisphere) and away
• Low Pressure: air moves
upward, counter clockwise (Nhemisphere) and towards
• Isobars: Lines of equal pressure
• Pressure Gradient: the
difference in the pressures
– where the isobars are close
together the gradient is strong
– Strong pressure gradient force =
High wind velocities
Origin of Wind
How Do Clouds Form?
 Air must be forced upward
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Orographic Lift
Frontal Wedging
Convergence
Convective Lift
 If upward moving air contains moisture,
that moisture will cool as it is lifted
upward in the atmosphere.
 When air cools to its dew point, the
moisture will condense and form clouds.
Cloud Formation
Convective Lift:
• Warm air rises because it is
•
•
•
•
less dense
As air rises it gradually cools
Moisture in the air will
condenses as the
temperature approaches
dewpoint
Water droplets bond onto
condensation nuclei.
These nuclei will collide with
other nuclei, eventually
forming a cloud.
Cloud Formation
Orographic Lift
Types of Clouds
Low clouds: Consist of stratoform (flat) clouds, or
cumuloform (puffy) clouds, lying on or just above the
surface (up to ~10,000 ft).
Stratus:
Cumulus:
Types of Clouds (Cont’d)
Middle Clouds: Consist of altoform (meaning a middle
cloud) type clouds, extending from 10,000 – 20,000 feet
above the surface.
Altostratus:
Altocumulus:
Types of Clouds (Cont’d)
High Clouds: Consists of mainly ice crystals suspended
above 20,000 feet from the surface. These clouds are
wispy/crisp/featherlike in appearance, and do not produce
precipitation.
Cirrus:
Cirrostratus:
Cirrocumulus:
Altocumulus-Lenticularis
Formation of Precipitation
 Collision & Coalescence process
 water droplets continue to grow, forming
clouds.
 If enough lift is present in the atmosphere
updrafts are created, forcing the cloud to
grow taller.
 The water droplets continue to grow inside of
clouds, until they become too heavy to be
suspended by the updraft.
 It is then that these droplets fall down to the
earth as rain.
Types of Fronts
Cold Front: A transition zone where cold/dry, and
stable air, replaces warm/moist, and unstable air.
It is depicted by a blue line with triangles pointing
towards the direction of movement.
Colder
27
Temperatures
33
54
Warmer
Temperatures
62
Cold Front (Vertical Profile)
Advancing
Cold air
Rising
Warm
air
Warm Front
Warm Front: Warm, moist air replaces cold, dry air. Slower moving
than cold fronts, these fronts are depicted as the red line with
semicircles pointing towards the direction of movement.
45
Warm
Front
Colder
Temperatures
38
55
Warmer
Temperatures
62
Warm Front (Vertical Profile)
Warm
air
Cold air
Stationary Front
A boundary that has essentially no movement, but separates warm, moist air
from cold, dry air. It is depicted by an alternating red and blue line
containing blue triangles and red semicircles. The triangles point towards
the warmer air, while the semicircles point towards the cooler air.
Cold
Air
Warm
Air
Mapping Exercise
 Draw and analyze a surface weather map
for pressure and temperature, and draw
the fronts that are indicated by these two
variables.
 Discuss our results.
Pressure/Front Map
Temperature/Front Map