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The Weather
The Isobars
Those plain lines that curve across the map are called
isobars (iso = equal, bar = pressure). They join together
places of equal atmospheric pressure. Some have numbers on
them showing this value in hectoPascals.
Atmospheric pressure on maps is
measured at sea level
Winds blow almost directly
along the isobars.
ANTI-CLOCKWISE
around HIGHS.
CLOCKWISE around LOWS
The Wind
The closer the isobars, the stronger the winds.
The Highs
• When isobars enclose an area of high pressure (above
1013 millibars. This is called a High, anticyclone or
Ridge and its centre is labelled on a weather map by
an 'H'.
•When a high is over us we can expect dry sunny
weather
The Lows
• Isobars make shapes and patterns. When they
enclose an area of low pressure this is called a Low,
cyclone, trough or depression and its centre is
labelled on a weather map with an 'L'.
•When a low is over us we can expect, cold rainy
weather.
What to do when trying to predict
the weather.
• Look if any anticyclones or cyclones are
present.
• Wind direction
• Wind speed
• Fronts
• Especially in the South Island think of the
southern Alps
•Write down the important features to look out for on this
map to forecast Gisborne’s weather?
•Write a weather forecast for Gisborne telling us the wind
direction/strength and the expected weather
Weather forecast for Gisborne
Sunday Max: 18°C Min: 14°C
Rain, heavy at times. Gusty south easterlies
Analysis Map v’s a
Prognosis Map
•
You can generally get two different
types of weather maps.
1. An analysis map
•
What has actually occurred. The current
situation
2. Prognosis
•
What the future holds for us
Fronts, Troughs and
Convergence Zones
• A front marks the boundary between
two air-masses, and appears on the
weather map as a line with triangles or
semicircles attached.
Cold Front
• Cold fronts push in underneath the
warmer air ahead of them, forcing the
warm air upwards and making cloud and
areas of rain. As a cold front passes by:
any rain clears but showers may appear,
humidity drops, air temperature usually
drops, pressure rises and the wind
changes direction.
Warm Front
• The advancing warm air rises over a
zone of retreating cooler air, making a
cloud bank that slopes forwards from
ground level upwards, often bringing
prolonged steady rain. As a warm front
passes by: any rain becomes patchy but
humidity remains high, air temperature
may rise a little, pressure steadies, and
the wind changes direction.
Occluded front
• Occurs when a cold front overtakes a
warm front, so that all that remains of
the original warm air is trapped above,
where it cools making dense cloud and
rain. As an occluded front passes by:
any rain becomes patchy, wind eases,
the rate of pressure fall may level out
but air temperature does not change
much.
Stationary front
• A front which has lost its impetus for
movement, so that neither air-mass is
making much progress. It takes a while for
a stationary front to pass by: any rain
clears only slowly and temperature and
pressure do not change much.
Weather in Darfield
• www.metservice.co.nz/default/index.php?a
lias=farmcanterburyplains
Weather from around the world
• http://www.bbc.co.uk/weather/world/