How to read a surface weather map


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We are surrounded by more weather data than we could ever use. Our phones send us data on command. Social networks are full of analyzes from meteorologists around the world. Even the weather segment of broadcast news turns into a college-level weather course when active weather looms on the horizon. But among all the data we deal with, an old-fashioned surface map is still the best way to keep tabs on weather conditions anywhere.

Meteorologists have maps for every occasion, from watching for a winter storm that pours fresh powder on the slopes to tracking thunderstorms that can crush a camping trip. However, most weather maps are dedicated to showing us a single variable, such as current temperatures or winds aloft. But it’s the basic surface map, with its station plots and wind barbs, that reveals the most information in the quickest way before going out. Yet detailed surface weather maps are only useful to the extent of your ability to understand what they are telling you. Here’s how to read them.

Station plots

Station plots in the Southeastern United States April 3, 2020 (NOAA / WPC)

Scientists needed a way to convey as much information as possible without making a map completely unreadable. Enter the train station layout, an ingenious way to print a lot of information in a small space. It takes a bit of decryption to figure it out, but once you get the hang of it, you can get the gist of the weather for a specific location in just a few minutes.

A station ground
A station ground

The base station graph shows temperature, dew point, wind direction, wind speed, cloud cover, precipitation, and air pressure, all of which are measured at weather stations around the world. That’s a huge amount of useful information packed into a small graph, and some maps have even more data, such as visibility and cloud heights and types.

A station plot always shows the current temperature at the top left, the dew point at the bottom left, and the atmospheric pressure at the top right. The temperature and dew point units depend on the source that mapped; most surface cards issued in the United States use Fahrenheit, while organizations in most other countries use Celsius.

Air pressure

Hurricane Katrina made landfall on the northern Gulf Coast on August 29, 2005
Hurricane Katrina made landfall on the northern Gulf Coast on August 29, 2005 (NOAA / WPC)

Air pressure readings are important because they tell you how the atmosphere is moving around you. The high pressure promotes calm skies perfect for hiking, while the low pressure signals unstable conditions and precipitation. Strong winds result from rapid pressure changes over short distances.

Decoding atmospheric pressure on weather maps requires memorization and contextual clues to translate. Most station plots describe atmospheric pressure as a set of three numbers, such as 994 or 112. This number indicates the last three digits of a station’s atmospheric pressure reading to the tenth of a millibar ( mb) nearest; the last number actually comes after the decimal point. For example, 994 would indicate a pressure reading of 999.4 millibars, while 112 would convert to 1011.2 millibars. (Meteorologists put a nine in front of larger numbers and a ten in front of smaller numbers.) It is helpful to keep in mind that it is rare to see air pressure greater than 1040 millibars outside of a wave. intense cold. Equally rare is air pressure below 980 millibars outside the eye of a hurricane or northeast.

Wind speed and direction

Wind barbs
Wind barbs

Wind barbs are the most visible part of a station layout. They show you the speed and direction of the winds at the station. The point always points in the direction from which the wind blows: if the point points towards the southeast, it means that the wind is blowing from the southeast.

A wind beard contains half lines, full lines and flags to indicate wind speed. Speeds are always displayed in knots (kts), regardless of the units used for temperatures and dew points. (One knot is approximately equal to 1.151 mph.) A half line represents five knots, a full line represents ten knots, and flags are reserved for 50 knot increments.

The above wind barbs show winds blowing from the northwest at 35 knots (graph A) and from the southeast at 75 knots (graph B). If the winds are calm, a thin circle will appear around the station plot (plot C).

Cloud cover and precipitation

A station plot icon indicates cloud cover by the amount of shading within the center point of the plot. A hollow circle indicates clear skies, while a solid circle indicates cloudy conditions that could spoil a hiking trip. Each successive quarter shadow of the point indicates scattered (25% filled), partly cloudy (50%) and mostly cloudy (75%) clouds.

Symbols representing the type and intensity of precipitation will appear directly to the left of a station plot. Solid dots represent rain, while asterisks represent snow, with the intensity indicated by the use of two symbols (light precipitation), three symbols (moderate precipitation) or four symbols (heavy precipitation). A thunderstorm is indicated by a long letter-shaped arrow R. While these are some of the more common symbols, there are many more for types of precipitation., including freezing drizzle and stormy snow.

Pressure systems and isobars

Some surface maps are analyzed by computer programs or human forecasters to add additional information that helps you understand current conditions and predict what is to come. The most common value-added analyzes on surface maps are isobars, pressure systems, and limits.

The strongest depression on record in Minnesota occurred on October 27, 2011.
The strongest depression on record in Minnesota occurred on October 27, 2011.
(NOAA / WPC)

Isobars

We’re used to seeing a blue H on an anticyclone and a red one THE on a low pressure system. Wind usually blows from areas of high pressure to areas of low pressure. But even without those letters printed on the map, it’s usually easy to spot these features using isobars, lines drawn on a map that connect areas with air pressure readings as high as they are low.

These solid concentric lines are useful for identifying pressure centers and getting a general idea of ​​where the wind is, making them especially useful for outdoor enthusiasts about to embark on the next adventure. Isobars that are tightly packed show larger pressure changes over short distances, indicating rapidly changing weather conditions and gusty winds.

The historic super storm of March 1993
The historic super storm of March 1993 (NOAA / WPC)

Facades

A front is the boundary between two different air masses; they are ubiquitous on weather forecasts. The difference between the two regions can be subtle or dramatic: a strong cold front, for example, can drop temperatures by several tens of degrees in a matter of minutes.

Cold fronts show cooler, drier air entering warmer, more humid air, which can produce powdery snow. They are usually shaded in blue, with triangular flags pointing in the direction of forward movement of the forehead. Warm fronts exist along the leading edge of warmer, humid air pushing into cooler, drier air. They can sometimes bring stormy weather and are usually marked with a red line with semicircles pointing in the direction of movement of the forehead.

A stationary front is located along the edge of colder and warmer air masses that do not advance towards each other. These are indicated by alternating flags and semicircles (alternating in the same colors described above), each pointing to their respective air mass.

Occluded fronts are shown in purple on weather maps. An occlusion occurs when cold air passes warm air near the center of a low-pressure system, pinching a region of warm air above the earth’s surface. A “triple point”, where a cold front, a warm front and an occluded front meet, can sometimes serve as a hotbed for severe thunderstorms, which can ruin an afternoon race.

A line dries up over Texas and Oklahoma on May 31, 2013, the day the largest tornado on record hit El Reno, Oklahoma.
A line dries up over Texas and Oklahoma on May 31, 2013, the day the largest tornado on record hit El Reno, Oklahoma. (NOAA / WPC)

Dry lines, shown on surface maps in beige, with hollow semicircles pointing in the direction of movement, are a type of front that is typically only seen on the southern plains. They form a dividing line between the humid air coming from the Gulf of Mexico and the extremely dry air of the desert region to the west. A dry line is moving east in great leaps during the afternoon due to daytime warming. These fronts may serve as a catalyst for intense spring tornado outbreaks in Texas and Oklahoma.

Resources

With these basics in mind, the best place to download reliable and up-to-date weather maps is the US National Weather Service. Just be sure to save them on your phone or mark the direct link in your phone’s browser before you go. The agency’s products are free for public use, and the entire organization costs each taxpayer just over a dollar a year. Various branches of the NWS publish a multitude of area maps covering the United States. The Weather Prediction Center primarily manages daily weather, such as fronts and temperatures, while the Storm Prediction Center manages severe thunderstorms and fire weather forecasts. The Ocean Prediction Center offers detailed analyzes of weather conditions in the Atlantic, Pacific and Arctic Oceans.

International meteorological organizations also provide fantastic services for their coverage areas including Environment Canada, UK Met Office and Australian Meteorological Bureau. Private companies and academic institutions also publish weather maps, such as the College of DuPage’s robust surface analyzes.

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Dale D. Schrum

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