HOW TO READ A WEATHER MAP by David Dorn
"Weather Maps may seem confusing when you first see them. But you can learn the basics easily. The
information they contain helps us to understand the current weather picture and to make future weather
predictions. Just looking out your window only tells you the immediate weather in your local spot, but it
cannot tell you what changes to expect like the coming of a storm, or the possibility of surf arriving in a
few days. Weather Maps look at a large area and show the large weather systems that are moving toward
your area. Come with me as we take a look at How to read a weather map". David DornInstructor/Author.
Weather maps contain information about atmospheric pressure, fronts, storms, and wind speeds and wind
direction. The areas of different atmospheric pressure are what drive our winds, and in turn generate surf.
It is nice to know if the wind is going to be too light or way too strong before we venture out onto the
water, or before we plan a weekend camping trip. Weather Maps also show the positions of severe storms,
like hurricanes, and typhoons that are especially important to island, and coastal communities and
mariners.
How they are made: Weather maps use a collection of weather observations that are made at many
locations at the same time. They are all put down on a single map to create a snapshot of an area too
large to normally observe. Each weather station, or data collection point transmits its data to a central
point, usually the national weather service. Each surface station reports the atmospheric pressure,
temperature, humidity, cloud cover, and weather condition for their particular location. Meteorologists
combine the data and create the map to show the area of interest. By connecting the location of each
surface station that reported the same barometric pressure, with a line (isobar), a pattern emerges that is
useful in predicting weather. Air temperature is also measured, and edges of warm and cold air masses
are marked on the Map. A cold front or warm front will be marked on the map. A weather map uses
specific symbols to indicate specific features.
The most obvious feature of a weather map is the squiggly lines. These lines are called
isobars. The isobars are lines that connect areas of equal atmospheric pressure. The word
isobar, comes from iso=equal and bar=pressure. Anywhere on one line will have the same
pressure. The pressure is usually noted on every other isobar so you can see the pattern of
pressure distribution. The point with the highest pressure is called a High. It is usually
marked with a capital letter “H”. this High pressure center is surrounded by concentric
circles of isobars, the isobars closest to the center of the High will have a higher number,
than the ones further out. The area with the lowest pressure on the map is called a “low”,
this is designated by a capital “L”.
Atmospheric pressure is measured at the earth surface, and is know as surface pressure. The difference in
surface pressure is measured with an instrument called a barometer, and is sometimes called barometric
pressure. Pressure changes with altitude. Pressure readings taken at any altitude, like from a surface
station in Denver (5000ft), will be corrected to sea level, for a common frame of reference. Pressure also
changes with temperature. Hot air has lower density and rises creating lower surface pressure. Cooler
denser air descends creating a higher surface pressure. Low pressure cells are areas of converging air that
rises upward an the center. Lows are usually associated with unstable conditions and stormy weather.
High pressure areas, “Highs” are associated with milder, more stable weather.
The atmospheric pressure will attempt to reach an equilibrium, so air will flow from a high pressure area
toward a low pressure area. The horizontal flow of air is called wind. The Between the lows and highs are
where the winds are blowing, The wind blows from High to Low, but not directly. Because of the earth's
rotation (Corriolis force) the wind will blow more parallel to the isobars. The wind blows clockwise around
a High in the northern hemisphere. The Wind blows counterclockwise around a Low in the northern
hemisphere. In the southern hemisphere the wind rotational direction is reversed. When you have a lot of
lines close together is means strong winds. Areas with widely spaced isobars will have little or no wind.
The horizontal movement of air at the surface are the surface winds
that we measure.
The rotation of winds around the pressure cells
caused by the earths rotation, called the Corriolis
force.
Also when there is a very low pressure low, next to a high pressure high, the difference creates stronger
winds than normal. The greater the difference between the high and the low, the greater the Pressure
Gradient Force (PGF) that drives the wind will be. More isobars (lines) packed close together means strong
winds. Weather systems generally move across the map from west to east. The exceptions are hurricanes
and tropical storms that often move towards the west.
Frontal Systems: The areas between hot and cold air masses are called fronts. Fronts are located on the
transition between different air masses. Fronts are significant because they influence the vertical
movement of air, and the formation of clouds, rain, and storms. The four types of fronts are Warm front,
Cold front, Occluded Front, and Stationary Front.
Warm Fronts are the leading edge of a warm air mass that
generally brings light rain and gradually deteriorating weather.
They appear on the weather map as a red line with semi
circles on the side the front is moving towards. Cold Fronts are
the leading edge “front” of a cold air mass that brings about
abrupt weather change and can bring sudden heavy rain and
the possibility of thunderstorms. Cold fronts are indicated by a
blue line with blue triangles indicating the direction it is
travelling. The Occluded Front is caused when a cold front
overtakes a warm front, they can bring about thunderstorms
and a variety of weather conditions. The occluded front is indicated by a line with triangles and semi
circles on the same side indicating the direction it is travelling. The Stationary Fronts are non-moving
fronts that stay in a particular area for an extended period of time and can bring extended bad weather
and persistent protracted rains and floods. Stationary fronts are indicated by a red or blue line with
triangles and semicircles on opposite sides.
Storms that are large enough will be named, and their progress will be tracked across the map. Special
attention is paid to storm tracking to give the population the maximum amount of time to prepare for
them. Severe weather is responsible for many natural disasters, like high winds, huge waves, and flooding
rains. Intense severe winds and thunderstorm activity can also be anticipated. The storm's anticipated
track is often indicated with an arrow. The ITCZ (Inter-Tropical Convergence Zone) is located near the
equator, and is an area on no wind doldrums. The ICTZ has a special symbol of a stripped red line.
Hurricanes are severe low pressure storms with dangerously
high wind speeds; they cover hundreds of square miles and can
last for weeks. Hurricanes tend to be fuelled by warm water and
meteorologists use computer models to help them predict their
behavior. Hurricanes usually start out as Tropical Storms which
are less severe. The hurricane is dangerous to ships at sea, and
can generate huge waves that can batter a coastline thousands
of miles away. When the hurricane makes landfall, the waves
and wind combine to create an especially destructive storm.
Hurricanes can also cause coastal flooding, by creating wind
driven waves that pile up on shore, and by creating a higher
than normal sea level by lowering the atmospheric pressure to
an extremely low level. When a hurricane, moves over land it will lose its power and eventually dissipate.
When a hurricane moves into the western pacific it is called a Typhoon, A Typhoon is exactly the same
thing by a different name. In the southern hemisphere the Hurricane is called a “Cyclone”.
Wind Barbs are little arrows that indicate
the wind's direction and strength. The
number of "tail feathers" indicates how
strong the wind is. The arrows are
oriented to indicate the direction the
wind is blowing. and on the back of the
arrow the are different symbols to
indicate the wind strength. A short tail
feather means 5knots of wind, a longer
feather has the value of 10 knots of
wind. The feathers can be combined to
indicate other wind speeds. A short
feather plus a long feather combines to
indicate 15knots of wind. A triangle
represents 50 knots. See the diagram
below for some of the different wind
barbs. A knot, is a nautical mile per
hour. 1 knot = 1.152 miles per hour.
1 Knot = 1 Nautical Mile per hour
1 Nautical mile = 6076.12 ft. = 1852 m
1 Statute mile = 1760 yards = 5280 feet
Cloud cover symbols are
given by surface stations to
describe the local cloud
cover. A all black circle
symbol describes a clear
sky, the all white circle
symbol means overcast sky
(white=cloud). A quarter
white circle means 25
percent cloud cover. Half
white circle means 50
percent cloud cover etc.
Cloud cover symbols can
also be combined with a wind barb to describe the
local weather at the station.
Current Weather Map (North Pacific)