We are surrounded by more weather data than we could ever use. Our phones chirp data points at us on command. Analyzes by meteorologists from all over the world can be found on social media. Even the weather segment on the broadcast news turns into a college-level meteorology course when active weather is on the horizon. Of all the data we process, an old-fashioned surface map is still the best way to keep an eye on weather conditions anywhere.
Weather forecasters have maps for every occasion, from monitoring a winter storm that throws fresh powder on the slopes to tracking thunderstorms that can crash a camping trip. Most weather maps, however, only show us one variable, such as current temperatures or winds at higher altitudes. But it's the basic surface map, with its station plots and wind spikes, that reveals most of the information in the fastest possible way before we go outside. However, detailed surface weather maps are only as useful as your ability to understand what they are telling you. Here's how to read it.
(Photo: NOAA / WPC)
Scientists needed a way to convey as much information as possible without making a map completely illegible. Enter the station plot, an ingenious way to print a lot of information in the smallest of spaces. Understanding takes a little bit of decoding, but once you get the hang of it, you can get an overview of the weather for a particular location in minutes.
The basic station diagram shows temperature, dew point, wind direction, wind speed, cloud cover, precipitation and barometric pressure, all of which are measured at weather stations around the world. That's a tremendous amount of useful information in one small graph – and some maps contain even more data like visibility, cloud heights and types.
A station diagram always shows the current temperature at the top left, the dew point at the bottom left and the air pressure at the top right. The units for temperature and dew point depend on the source from which the map was created. Most surface maps issued in the United States use Fahrenheit, while organizations in most other countries use Celsius.
(Photo: NOAA / WPC)
Barometric pressure readings are important as they tell you how the atmosphere is moving around you. High pressure promotes calm skies perfect for hiking, while low pressure signals unexplained conditions and rainfall. Strong winds result from rapid pressure changes over short distances.
Decoding barometric pressure on weather maps requires memorization and the contextual clues for translation. In most station plots, barometric pressure is represented as a set of three numbers; B. 994 or 112. This number displays the last three digits of a station's air pressure reading to the nearest tenth of a millibar (mb). The last number actually comes after the decimal point. For example, 994 would display a pressure value of 999.4 millibars while 112 would convert to 1011.2 millibars. (Meteorologists place a nine before larger numbers and a ten before smaller numbers.) It is helpful to note that it is rare for an air pressure greater than 1,040 millibars to occur outside of a deep cold snap. It's just as unusual to see air pressure less than 980 millibars outside the eye of a hurricane or a northeast coast.
Wind speed and wind direction
Wind spikes are the most visible part of a station property. They tell you the speed and direction of the winds at the station. The barb always points in the direction from which the wind is blowing: if the barb is pointing southeast, it means that the wind is blowing from the southeast.
A wind hook contains half lines, full Lines and flags to indicate wind speeds. The speeds are always displayed in knots (kn), regardless of the units used for temperatures and dew points. (One knot is approximately 1.151 miles per hour.) Half a line represents five knots, a full line represents ten knots, and flags are reserved for increments of 50 knots.
The wind spikes above show winds blowing from the northwest at 35 knots (plot A) and from the southeast at 75 knots (plot B). When the wind is calm, a thin circle appears around the station plot (plot C).
Cloud cover and precipitation
A station plot symbol conveys the cloud cover by the amount of shading within the center of the plot. A hollow circle indicates a clear sky, while a solid circle indicates cloudy conditions that could ruin a backpacking trip. Each successive quarter shade of the point indicates scattered clouds (25 percent filled), partly cloudy (50 percent), and mostly cloudy (75 percent).
Precipitation type and intensity symbols are displayed directly to the left of a station plot. Fixed points represent rain, while asterisks represent snow. The intensity is 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 arrow shaped like the letter R. While these are some of the most common symbols, there are many others for precipitation types, including freezing drizzle and thunderstorms.
Pressure systems and isobars
Some surface maps are analyzed by computer programs or human forecasters to add additional information that will help you understand current conditions and predict what is to come. The most common value added analyzes on surface maps are isobars, pressure systems and boundaries.
(Photo: NOAA / WPC)
We are used to seeing a blue H over a high pressure center and a red L over a low pressure system. The wind generally blows from areas of high pressure to areas of low pressure. But even without these letters on the map, it's usually easy to identify these features using isobars – lines drawn on a map connecting areas where air pressure is equal or low.
Those solid concentric lines are helpful for identifying pressure centers and getting a general idea of where it is windy. This makes them especially useful for outdoor enthusiasts looking to head out on their next adventure. Tightly packed isobars show larger changes in pressure over short distances, which indicates rapidly changing weather conditions and gusty winds.
(Photo: NOAA / WPC)
A front is the boundary between two different air masses; They are omnipresent in weather forecasts. The difference between the two regions can be subtle or dramatic – a strong cold front, for example, can lower temperatures by dozens of degrees in just a few minutes.
Cold fronts show cooler, drier air pushing into warmer, more humid air that powder snow can create. They are usually shaded in blue with triangular flags pointing in the direction of the forward movement of the front. At the leading edge of warmer, more humid air, there are warm fronts that invade cooler, drier air. They can sometimes bring stormy weather and are usually indicated by a red line with semicircles pointing in the direction of frontal movement.
A stationary front lies on the edge of cooler and warmer air masses that do not come together. These are marked by alternating flags and semicircles (alternating in the colors described above), which each point to their respective air mass.
Concealed fronts are displayed in purple on weather maps. Occlusion occurs when cold air overtakes warm air near the center of a low pressure system and traps an area of warm air above the surface of the earth. A "triple point" – where a cold front, a warm front, and an obscured front meet – can sometimes serve as a focus for severe thunderstorms that can ruin an afternoon run.
(Photo: NOAA / WPC)
Dry lines, which are shown on surface maps in beige with hollow semicircles in the direction of movement, are a kind of front that is typically only seen in the southern plains. They are a dividing line between humid air from the Gulf of Mexico and extremely dry air from the desert region in the west. A dry line moves east in great jumps in the afternoon due to warming during the day. These fronts can act as a catalyst for intense spring tornado outbreaks in Texas and Oklahoma.
With these basics in mind, the United States Weather Service is the best place to download reliable, up-to-date weather maps. Just save them to your phone or bookmark the direct link in your phone's browser before you set off. 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 issue a variety of surface maps covering the United States. The weather forecast center mainly handles everyday weather such as fronts and temperatures, while the storm forecast center handles severe thunderstorms and fire weather outlooks. The Ocean Prediction Center provides detailed weather analysis across the Atlantic, Pacific and Arctic.
International weather organizations also offer fantastic services for their coverage areas, including Environment Canada, the UK's Met Office and the Australian Bureau of Meteorology. Private companies and academic institutions also issue weather maps, such as the College of DuPage's robust surface analysis.
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Main photo: NOAA / Unsplash
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