Chapter 24 – Weather
A large body of air with similar temperature and moisture is an air mass. An air mass that forms
over continents has low humidity and the symbol "c" for continental. An air mass that forms
over oceans has high humidity and the symbol "m" for maritime. An air mass that forms near
the poles has cold temperatures and the symbol "P" for polar. An air mass that forms near the
equator has high temperatures and the symbol "T" for tropical. When describing air masses,
the lowercase humidity symbol precedes the uppercase temperature symbol such as cP for
continental polar, mP for maritime polar, cT for continental tropical, and mT for maritime
tropical.
The boundary between air masses is called a front.
Cold air moving into warmer air is called a cold front. Because cold air is more dense than warm
air, the cold air pushes and lifts the warm air creating a vertical front. The clouds formed from
this movement are vertically developed such as cumulus and cumulonimbus. The precipitation
is often heavy and over an isolated area. Thunderstorms, tornadoes, and violent weather are
associated with this type of front. The map symbol is triangles, pointing in the direction the
cold air is moving.
Warm air moving into colder air is called a warm front. Because the less dense warm air cannot
push the cold air out of the way, it overrides the cold air in a less steep, horizontal boundary.
Therefore, the cloud types produced are horizontally developed clouds such as cirrus,
cirrostratus, altostratus, stratocumulus, stratus, and nimbostratus. The precipitation is
generally a long, steady amount spread over a large area. The map symbol is half-circles which
point in the direction the warm air is moving.
When a cold or warm front stalls, usually due to high or low pressure centers in the local area, it
becomes a stationary front. Stationary fronts typically have the same weather as warm fronts.
The map symbol for a stationary front is triangles on one side and half-circles on the other side,
with the symbols pointing in the direction that the air was moving before it became stationary.
An occluded front forms when a fast-moving cold front overtakes a slower-moving warm front.
The cold front lifts the warm front off the surface so the map symbol has the triangles and halfcircles on the same side pointing in the direction the cold and warm air is moving.
A strong storm with heavy rain, thunder, lightning, and strong winds is a thunderstorm. There
are three stages: 1st. cumulus stage – warm, moist air rises and condenses
2nd. mature stage – strong updrafts develop and downdrafts form as precipitation drags the air
down 3rd. dissipation stage – downdrafts stop updrafts and water vapor supply is cut off.
Cumulonimbus clouds develop areas of distinct electrical charge. When opposite charges are
equaled – either from cloud to ground, ground to cloud, or cloud to cloud – lightning occurs.
The released electricity heats the air and causes it to expand rapidly, causing thunder.
Some cumulonimbus clouds produce tornadoes. Most tornadoes in the United States occur in
late spring or early summer, from Texas through the Midwest, when warm, humid air from the
Gulf meets cool, dry air from the west. In addition to the conditions which produce
thunderstorms, for tornadoes to develop there needs to be wind shear (changing wind
direction with height). Tornadoes typically last minutes to hours, usually have paths less than
100m wide, and have winds that can exceed 400 km/h.
A tropical storm with winds more than 120 km/h that spiral inward to a low pressure center is a
hurricane. The calm center of the hurricane is called the eye. The strongest winds of the storm
are found just outside the eye in the eyewall. The deadliest force of a hurricane is often its
storm surge as the low pressure of the storm causes sea levels to rise and large waves.
Hurricanes are categorized on the Safir-Simpson scale from a Category 1 (least damage) to a
Category 5 (catastrophic damage). Hurricanes can last up to weeks and are often 700km in
diameter. In the Pacific these storms are called typhoons.
The instrument which measures temperature is the thermometer. The instrument which
measures wind speed is the anemometer. The instrument which measures wind direction is the
weather vane or wind sock. The instrument which measures air pressure is the barometer. The
instrument which measures humidity is the psychrometer or hygrometer. A weather balloon
carrying a thermometer, hygrometer, and barometer which is sent high into the atmosphere is
called a radiosonde.
Weather maps plot information in a standard format using data displayed on and around a
station model (the location where the data came from). Wind direction is shown by an arrow
and the wind speed by the number of feathers on the arrow. Temperature is displayed on the
top left of the station model, without a degree sign or units. Air pressure is shown on the top
right in millibars and just lists the last three digits without a decimal or unit. Sky conditions are
shown by shading in a percentage of the circle to represent percent cloud coverage. Also
plotted on some models are precipitation and cloud types.
A watch is given when conditions are favorable for severe weather. A warning is given when
severe weather has been spotted or is expected within 24 hours.