~ by Gord May
As published in “Good Old Boat” magazine Issue 45 (Nov./Dec. 2005)
(Copyright Gordon A. May 2005, All Rights Reserved)
Going cruising? Here’s how to read the weather maps
“If previous surface charts
are available for the last day or two, you will be able to judge the movement of weather
systems over time, based upon the principle of continuity.”
“High-altitude clouds moving from left to right indicate the weather may worsen; from right to left it may improve. If the clouds move toward or away from you, the weather may stay about the same.”
“A veering wind
suggests fair or improving weather in the Northern Hemisphere, due to clockwise rotation of high-pressure areas.”
Ode to the Weatherman
And in the dying embers,
These are my main regrets:
When I’m right, no one remembers;
But when I’m wrong, no one forgets.
The weather affects every person every day; but few people are more affected by weather than the mariner. An unexpected change in winds, seas, or visibility can reduce the efficiency of marine
operations and threaten the safety
of a vessel and its crew. It behooves the prudent mariner to become familiar with reading weather maps, and the interpretation of what their content portends for future weather conditions.
Illustration 1: Simplified surface map
Surface maps depict the large-scale elements of the weather. These elements include high- and low-pressure systems, cold and warm fronts, and precipitation areas. Current
surface maps are updated every hour. Forecast
surface maps are updated once each day.
High and low pressure centers are indicated by a large block H and L, respectively, together with a set of digits identifying the estimated value of the central pressure. On some charts
, the H is colored blue, while the L is drawn in red.
A high-pressure system is an area of relative pressure maximum that has diverging winds and (in the Northern Hemisphere) a clockwise rotation. Fair weather is typically associated with high pressure.
A low-pressure system is an area of relative pressure minimum that has converging winds and rotates (in the Northern Hemisphere) in a clockwise direction. Stormy weather is often associated with low-pressure systems.
In the Southern Hemisphere, the rules listed here must be reversed.
A low-pressure trough that contains significant weather phenomena (such as precipitation and distinct wind
shifts) may be identified on the map by a thick brown dashed line running along the axis of the trough. On some maps this trough line may have the abbreviation, “TROF.” Surface fronts are generally found under trough regions.
A trough is an elongated area of low atmospheric pressure that can occur either at the Earth’s surface or at higher altitudes.
Upper-level troughs influence many surface weather features, including the formation and movement of surface low-pressure areas and the locations of clouds and precipitation.
Precipitation tends to fall to the east of the trough axis while colder, drier air tends to prevail to the west of the trough. This happens because air rises to the east of troughs. As air rises, it cools, and its humidity begins condensing into clouds and precipitation. Air sinks on the west side of troughs, which inhibits clouds and precipitation.
On weather maps of the Northern Hemisphere, troughs are shown by upper-air winds or jet streams, blowing south and then turning back to the north.
A high-pressure ridge is an elongated area of high atmospheric pressure. They occur both at the Earth’s surface and at higher altitudes. Upper-level ridges can have a major impact on the weather at the surface. Sunny, dry weather usually prevails to the east of the upper-level ridge axis while cloudy, wet weather can dominate the weather picture to the west of the upper-level ridge axis. Air tends to sink to the east of the ridge axis, which inhibits clouds and precipitation. On the other hand, air tends to rise to the west of the ridge axis, which can lead to the formation of clouds and precipitation.
On Northern Hemisphere weather maps, upper-air ridges are shown by the path of upper altitude winds, or the jet stream, turning to flow northward and then back to the south.
Illustration 2 Front symbols
The surface analysis may include one or more color-coded lines to identify a front. A front is defined as the transition zone between air masses having dissimilar thermal and moisture properties. Usually, these transition zones are only 50 to 100 kilometers wide, a sufficiently small horizontal distance to permit
their representation as lines on a large-scale surface-analysis chart. Fronts are classified according to their movement and can be represented graphically on a surface analysis chart.
Warm fronts — A red line with red half-moons pointing in the direction of air flow indicates a warm front, which is the leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. With the passage
of a warm front (generally at 10 to 15 knots), the temperature and humidity increase, the pressure rises and, although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage
. Precipitation — in the form of rain, snow, or drizzle — is generally found ahead of the surface front, as well as strong winds, convective showers, and thunderstorms. Fog
is common in the cold air ahead of the front. Although clearing usually occurs after passage, some conditions may produce fog
in the warm air.
As the warm front approaches, winds blow from the east or southeast and pressure drops steadily. Cirrus clouds are sighted first, followed by cirrostratus, altostratus, and finally nimbostratus. Cloud cover gets progressively greater, from a few tenths coverage with cirrus, to completely overcast with the coming of the nimbostratus clouds. Gentle precipitation begins as the nimbostratus clouds move overhead.
As the warm front passes, temperatures rise, precipitation ceases and winds shift to the south or southwest. Further, the sky clears and the pressure steadies. Later, with the approach of the cold front, cumulonimbus clouds fill much of the sky and bring the likelihood of heavy precipitation and the possibility of hail and tornado activity. The passage of the front is accompanied by a drop in temperature, clearing skies, a wind shift to the northwest, and rising pressure. Fair weather can probably be expected for the next day or two.
Cold fronts — Cold fronts are depicted by a blue line with blue barbs pointing in the direction of the cold-air flow. A cold front is the leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. Generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually clocking from the southwest to the northwest in the Northern Hemisphere).
Precipitation is generally at and/or behind the front and, with a fast-moving system (up to 30 knots), a squall line may develop ahead of the front (weather deteriorates with rain, strong winds, and thunderstorms).
As the low approaches, cool temperatures are the rule
, and winds are easterly because the warm sector of the cyclone is to the south. (A cyclone is just a meteorologist’s word for a pressure system centered on a low core
. Most cyclones are not capable of sending you and Toto to Oz.) The pressure drops and the sky becomes increasingly overcast. Further, precipitation is to be expected, and if it is winter or early spring, possibly snow, sleet, or glaze. As the front becomes occluded and slowly passes, winds shift from the north or northeast to the northwest. The sky begins to clear and the barometric tendency rises. Temperatures, however, remain cool or cold.
Clouds that are moving in a direction that differs from the way the wind is blowing indicate a condition known as wind shear. This sometimes indicates the arrival of a cold front.
Stationary fronts — A line with alternating red warm-front symbols and blue cold-front symbols pointing in opposite directions symbolizes little frontal movement.
When warm and cold air of equal pressure are next to each other, no movement will take place. Stationary fronts usually produce weather similar to a warm front but milder.
Occluded fronts — A front with purple (combined red and blue) half-moons and barbs on the same side, pointing toward the direction of frontal motion indicates an occluded front.
Often, in the later stages of a storm’s life cycle, a frontal occlusion occurs. This occurs when the air in the warm sector of the storm is lifted off the ground. Two types of occluded fronts exist.
The first is a cold occlusion, which occurs when the air behind the front is colder than the air ahead of the front. In this situation, the coldest air undercuts the cool air ahead of the front and the occluded front acts very similar to a cold front.
The second type is a warm occlusion, which occurs when the air behind the front is warmer than the air ahead of the front. In this situation, the cool air is lighter than the coldest air ahead of the front. As a result, the cool air rises up and over the coldest air at the surface and the occluded front acts very similar to a warm front.
In both types of occlusions, the occluded front has well-defined vertical boundaries between the coldest air, the cool air, and the warm air. Many weather textbooks state that occluded fronts occur when the cold front catches up with and overtakes the warm front, but many scientists disagree. They say that frontal occlusions occur when storms redevelop farther back into the cold air. In most cases, storms begin to weaken after a frontal occlusion occurs.
Illustration 3 Isobars
Isobars are thin solid lines depicting the features of the horizontal pressure field at mean sea level. These lines are called isobars and connect all points having the same sea level-corrected barometric pressure. By meteorological tradition, the isobar spacing is at 4 mb intervals, centered upon 1000 mb; that is, 996, 1000, 1004 mb, and so forth.
Isobars with the lowest value will encircle the region with the lowest point in the pressure field, while the closed isobar with the largest value isolates the highest sea-level pressure. The packing of the isobars reveals how rapidly the pressure varies with distance in the horizontal direction. A tighter packing indicates a much more rapid horizontal variation of air pressure.
The isobar pattern is also useful for visualizing the near-surface wind regimes. The winds tend to parallel the isobars, with low pressure to the left of the wind flow in the Northern Hemisphere; a slight cross-isobar deflection of the winds toward lower pressure is often seen. As a result, winds appear to spiral in toward a surface low-pressure center in a counterclockwise fashion and spiral around a high-pressure cell in a clockwise outflow. Additionally, where the isobars are packed more closely, the wind speed tends to be greater.
If previous surface charts are available for the last day or two, you will be able to judge the movement of weather systems over time, based upon the principle of continuity. Hence, you can make a reasonable short-range weather forecast
based upon the movement of the low- and high-pressure centers.
Surface station models
Illustration 4, Station plot
The location of each reporting station has been printed on the base maps as a small circle. The weather data from each reporting station are plotted around these circles on these base maps in a particular systematic fashion (convention) called a “station model.”
Here’s where you’ll find the data:
Temperature (F) is plotted upper left
Present weather symbol, center left
Dewpoint (F), lower left
Pressure (0.1 mb-coded), upper right as last 3 digits
Cloud cover, center circle. White fill indicates percentage of cloud cover
Winds are shown in the wind barb
Dew points indicate the amount of moisture in the air. The higher the dew points, the higher the moisture content of the air at a given temperature. Dew-point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water-vapor content) to reach saturation.
Relative humidity can be inferred from dew-point values. When air temperature and dew-point temperatures are very close, as shown in the illustration, the air has a high relative humidity. The opposite is true when there is a large difference between air and dew-point temperatures, which indicates air with lower relative humidity. Weather conditions at locations with high dew-point temperatures (65 or greater) are likely to be uncomfortably humid.
Humidity is required for thunderstorms to grow, for two reasons. First, the humidity in the air condenses to form the water
drops and ice crystals that make up a cloud and the rain that begins falling if the water
drops or ice crystals grow large enough. Second, humidity also makes the air more unstable.
If the reported value is greater than 500, the initial 9 is missing. Place it on left, then divide by 10. For example: 827 becomes 982.7 mb.
If the reported value is less than 500, the initial 10 is missing. Place it on left, then divide by 10. For example: 027 becomes 1002.7 mb.
Cloud cover and stability
In addition to coverage; cloud shape, size, height, color and sequence may foretell what’s to come. Stand with your back to the wind (true, not apparent) and watch which way the clouds are moving. High-altitude clouds moving from left to right indicate the weather may worsen; from right to left it may improve. If the clouds move toward or away from you, the weather may stay about the same.
Ed. Gord, does this hold for the Northern Hemisphere only?)
OOPS ~ This is reversed in the Southern Hemisphere .
"Buys-Ballot law" (also called ‘baric wind law’): Describes the relationship of the horizontal wind direction to the pressure distribution. In the Northern Hemisphere, if one stands with one's back to the true wind, the pressure on one's left is lower than the pressure on one's right. It is reversed in the Southern Hemisphere.
In other words, wind travels counterclockwise around low pressure zones in the Northern Hemisphere. It is approximately true in the higher latitudes of the Northern Hemisphere, and is reversed in the Southern Hemisphere, but the angle between barometric gradient and wind is not a right angle in low latitudes.
This rule is a combination of 2 laws:
Pressure gradient force: Air moves from high to low pressure systems.
Coriolis force: Air doesn't follow a straight line. As air moves from high to low pressure in the northern hemisphere, it is deflected to the right by the Coriolis force.
In the southern hemisphere, air moving from high to low pressure is deflected to the left by the Coriolis force.
The amount of deflection the air makes is directly related to both the speed at which the air is moving and its latitude. Therefore, slowly blowing winds will be deflected only a small amount, while stronger winds will be deflected more. Likewise, winds blowing closer to the poles will be deflected more than winds at the same speed closer to the equator. The Coriolis force is about zero at the equator. So, in simple terms, as air begins flowing from high to low pressure, the Earth rotates under it, making the wind follow a curved path. In the Northern Hemisphere, the wind turns to the right of its direction of motion. In the Southern Hemisphere, it turns to the left
E. & O. E.