Chapter 12
Meteorology is the study of atmospheric phenomena.
• Weather – the current state of the atmosphere
• Climate – long-term variations in weather for a particular area;
average weather over a long period of time
A Question of Balance
• The surface of the Earth heats up unevenly by the sun since it is
curved and sits on its axis
• Those areas that receive the majority of the sun’s radiation over the
year are warmer
An air mass is a large body of air
that takes on the characteristics of
the area over which it forms.
• The area from which air
masses form comes from is
called a source region
• Air masses are either
Maritime (form over water) or
Continental (form over land)
• Air masses are also either
tropical (warm) or polar (cold)
• So when they are classified
they are labeled as “maritime
tropical,” which tells us where
they form and their
temperature
Air Mass Modification – when air
mass moves, it starts to acquire
some of the characteristics of the
new surface beneath it. Essentially,
they transfer heat from one location
to another
Coriolis Effect – the rotation
of the Earth causes moving
particles such as air to be
deflected to the right in
the northern hemisphere
and to the left in the
southern hemisphere
Trade Winds
• occurs between the equator
and 30oN and S latitude
Prevailing Westerlies
• flows between 30o and 60o
N and S of the Equator
Polar Easterlies
• between 60o and the N and
S poles
Jet Stream
• narrow bands of fast, highaltitude, westerly winds
• Narrow region separating two air masses of different densities.
• Typically, air masses have different temperature and moisture.
Types of Fronts:
1.
Cold Front – Where cool dense air meets warm air, moving it steeply
upward
•
•
•
2.
Fastest of all front systems
Move furthest while maintaining
identity
Can result in clouds, showers and storms
Warm Front – Where warm air replaces cooler air and gradually rises
•
•
•
Tend to move slowly
Less violent than cold front
Can result in extensive
cloudiness and precipitation
3. Stationary Front – When a front does not move, or barely
moves as the differences between the air masses are small. Air
masses “stall”
•
Patterns similar to a warm front
4. Occluded Front – When a rapidly moving cold air mass
overtakes a warm front wedging the warm air upward
•
Can cause precipitation
Rising air is associated with low pressure
Sinking air is associated with high pressure
This movement combined with the Coriolis effect results in rotating pressure systems.
High-Pressure Systems
Sinking air reaches the surface and spreads away from the center
The Coriolis effect causes it to spin in a clockwise direction in the northern
hemisphere
Associated with fair weather
Low-Pressure Systems
As air rises, there is a flow of air into the center to replace it.
The Coriolis effect causes it to rotate counter-clockwise in the northern hemisphere
Associated with clouds and precipitation
Thermometer – device used to measure
temperature
Barometer – device used to measure air
pressure
Anemometer – device used to measure
wind speed
Hygrometer - device used to measure
relative humidity
Ceilometer – device used to measure the
height of cloud layers and estimates the
amount of sky covered by clouds
To make accurate forecasts,
meteorologists must gather
atmospheric data at heights up
to 30,000m
At present, the instrument of
choice is a balloon-borne
package of sensors called a
Radiosonde
• Sensors measure temperature, air
pressure, and humidity.
• Readings are constantly sent back
by radio signal, for tracking
• Provide accurate snapshots of
atmospheric conditions
• Very expensive
The term radar stands for “radio detecting and ranging”
Weather Radar – radio waves detect where rain is
falling at any given point.
• range of 400km in diameter
Doppler Radar
• Doppler Effect – the change in wave frequency that
occurs in energy as the energy moves toward or
away from an observer.
• Used to plot the speed at which raindrops move
toward or away from a radar station
• Provides good estimation of wind speeds
associated with precipitation areas
• Provides a distinct advantage over conventional
radar systems
One of the main uses of
satellites in orbit around Earth
is to observe weather.
Mounted cameras take photos of
Earth at regular intervals.
Able to track clouds, but not
necessarily precipitation
Infrared Imagery
• Detects differences in thermal
energy, which are used to map
either cloud cover or surface
temperatures
• Objects that radiate warmth at
slightly different frequencies show
up as different colors
• Allows meteorologists to determine
the temperature of a cloud, which is
useful in detecting strong
thunderstorms and their potential to
produce severe weather
Station Model – a record of weather data for a particular site at
a particular time
• Allows meteorologists to fit a large amount of data into a small space
• Gives meteorologists a uniform way of communicating weather data
(Ex)
Isobars – pressure
Isotherms - temperature
• Meteorologists use isobars, isotherms, and station
model data to analyze current weather conditions
before they can move on to forecasting.
• Isotherms help identify frontal systems
• Isobars help identify pressure systems
Short-Term Forecast:
• Digital Forecast – relies on
numerical data from computers,
satellites, and models
• Analog Forecast –involves
comparing current weather
patterns to patterns that took
place in the past (assuming that
the weather will behave in a
similar fashion)
• The more data collected, the more
accurate the short-term forecast
will be
Long-Term Forecast:
• Usually can’t be trusted if they
are more than three days out