fronts and air masses

1. FRONTS AND AIR
MASSES
GROUP 1 - METEOROLOGY
CONCORD TECHNICAL INSTITUTE

2. OBJECTIVES :
• Explain the characteristics of the principal word air masses.
• Derived air masses.
• Discuss the various air masses.
• Elaborate on the characteristics of the principle world air masses.

3. AIR MASS

4. AIR MASS
• An air mass is a large volume of air in the atmosphere that is mostly
uniform in temperature and moisture. Air masses can extend thousands of
kilometers in any direction, and can reach from ground level to the
stratosphere—16 kilometers (10 miles) into the atmosphere.
• Air masses form over large surfaces with uniform temperatures
and humidity, called source regions. Low wind speeds let air
remain stationary long enough to take on the features of the source
region, such as heat or cold. When winds move air masses, they carry
their weather conditions (heat or cold, dry or moist) from the source
region to a new region. When the air mass reaches a new region, it might
clash with another air mass that has a different temperature and humidity.
This can create a severe storm.

5. • Meteorologists identify air masses according to where they form over
Earth. There are four categories for air masses: arctic, tropical, polar,
and equatorial. Arctic air masses form in the Arctic region and are very
cold. Tropical air masses form in low-latitude areas and are moderately
warm. Polar air masses take shape in high-latitude regions and are
cold. Equatorial air masses develop near the Equator and are warm.
• Air masses are also identified based on whether they form over land or
water. Maritime air masses form over the water and are humid. Continental
air masses form over land and are dry.
• Therefore, an air mass that develops over northern Canada is called a
continental polar air mass and is cold and dry. One that forms over the
Indian Ocean is called a maritime tropical air mass and is warm and humid.

6. Air masses are classified on weather maps using two or three letters.
•A lowercase letter describes the amount of moisture in the air mass: m for
maritime (moist) and c for continental (dry).
•An uppercase letter describes the heat of the air mass: E for equatorial, T for
tropical, M for monsoon, P for polar, A for Arctic or Antarctic, and S for superior—a
unique situation with dry air formed by a powerful downward motion of the
atmosphere.
•A lowercase letter describes the relationship between the air mass and the
earth: k signifies that the air mass is colder than the ground below it,
while w describes an air mass that is warmer than the ground below it.

9. WEATHER FRONTS

12. WEATHER FRONTS
• Weather fronts mark the boundary or transition zone between two air masses and
impact the weather.
• A weather front is a boundary between two air masses. It can be thought of as the
frontline in a battle, where the warm air represents one side and its 'enemy,’ and the
cold air, the other.

13. • Across a front, there can be large temperature variations, as warm air comes into
contact with cooler air. The temperature difference can indicate the 'strength' of
a front. For example, if very cold air comes into contact with warm tropical air,
the front can be 'strong' or 'intense'. However, if there is little difference in
temperature between the two air masses, the front may be 'weak.’

14. WEATHER FRONTS AND THE UK WEATHER
• The UK has unique weather because we are an island nation, with a large ocean to
the west of us, a large landmass to the east of us and because of our position north
of the equator. These factors mean that we experience a large number of frontal
systems and their associated weather.
• Most of the frontal systems that affect the UK are formed out over the Atlantic
Ocean and move in a west to east direction. Met Office surface pressure charts
show forecast pressure and weather fronts, out to five days ahead, for Europe and
the northwest Atlantic. Three of the most common weather fronts displayed on
these maps are explained below.

15. COLD FRONTS
• A cold front is symbolized on a weather map as a line with triangles. The triangles
can be thought of as icicles. Cold fronts are often colored blue.
• The presence of a cold front means that cold air is advancing and pushing
underneath warmer air. This is because the cold air is 'heavier,' or denser than the
warm air. Cold air is thus replacing warm air at the surface. The tips of the 'icicles'
indicate the direction of movement of the cold air.

16. STATIONARY FRONTS
• A stationary front may form when a cold or warm front slows down or grows over
time from underlying surface temperature differences, like a coastal front. Winds on
the cold air and warm air sides often flow nearly parallel to the stationary front,
often in opposite directions along either side of the stationary front. A stationary
front usually remains in the same area for hours to days and may undulate as
atmospheric waves move eastward along the front.

17. • Stationary fronts may also change into a cold or warm front and may form one
or more extratropical or mid-latitude cyclones at the surface when atmospheric
waves aloft are fiercer, cold or warm air masses advance fast enough into other
air masses at the surface. For instance, when a cold air mass traverses
sufficiently quickly into a warm air mass, the stationary front changes into a cold
front.

18. WARM FRONTS
• A warm front is symbolized on a weather map as a line with semicircles. The
semicircles can be thought of as half-suns. Warm fronts are often colored red.
• The presence of a warm front means that warm air is advancing and rising over
cold air. This is because warm air is 'lighter,' or less dense than cold air. Warm air
is replacing cooler air at the surface. The edges of the 'suns' indicate the
direction of movement of the warm air.

19. OCCLUDED FRONTS
• An occluded front is symbolized on a weather map as a line with both semicircles
and triangles. They are often colored purple.
• These are slightly more complex than cold or warm fronts. The word occluded
means 'hidden' and an occlusion occurs when the cold front 'catches up' with the
warm front. The warm air is then lifted from the surface, and therefore hidden.
Occlusion can be thought of as having the characteristics of both warm and cold
fronts.