Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather. University of Chicago PressChicago: 1999.
Weather maps are created by plotting or tracing the values of relevant quantities such as sea level pressure, temperature, and cloud cover onto a geographical map to help find synoptic scale features such as weather fronts.
The first weather maps in the 19th century were drawn well after the fact to help devise a theory on storm systems.Eric R. Miller. American Pioneers in Meteorology. Retrieved on 2007-04-18. After the advent of the telegraph, simultaneous surface weather observations became possible for the first time, and beginning in the late 1840s, the Smithsonian Institution became the first organization to draw real-time surface analyses. Use of surface analyses began first in the United States, spreading worldwide during the 1870s. Use of the Norwegian cyclone model for frontal analysis began in the late 1910s across Europe, with its use finally spreading to the United States during World War II.
Surface weather analyses have special symbols that show frontal systems, cloud cover, precipitation, or other important information. For example, an H may represent high pressure, implying clear skies and relatively warm weather. An L, on the other hand, may represent low pressure, which frequently accompanies precipitation. Various symbols are used not just for frontal zones and other surface boundaries on weather maps, but also to depict the present weather at various locations on the weather map. Areas of precipitation help determine the frontal type and location.
History of surface analysis
The use of weather charts in a modern sense began in the middle portion of the 19th century in order to devise a theory on storm systems.Human Intelligence.Francis Galton. Retrieved on 2007-04-18. The development of a telegraph network by 1845 made it possible to gather weather information from multiple distant locations quickly enough to preserve its value for real-time applications. The Smithsonian Institution developed its network of observers over much of the central and eastern United States between the 1840s and 1860s. The U.S. Army Signal Corps inherited this network between 1870 and 1874 by an act of Congress, and expanded it to the west coast soon afterwards.Frank Rives Millikan. Smithsonian Institution. Joseph Henry: Father of the Weather Service. Retrieved on 2006-10-22.
The weather data was at first less useful as a result of the different times at which weather observations were made. The first attempts at time standardization took hold in Great Britain by 1855. The entire United States did not finally come under the influence of time zones until 1905, when Detroit finally established standard time.WebExhibits. Daylight Saving Time. Retrieved on 2007-06-24. Other countries followed the lead of the United States in taking simultaneous weather observations, starting in 1873.NOAA. An Expanding Presence. Retrieved on 2007-05-05. Other countries then began preparing surface analyses. The use of frontal zones on weather maps did not appear until the introduction of the Norwegian cyclone model in the late 1910s, despite Loomis' earlier attempt at a similar notion in 1841.David M. Schultz. Perspectives on Fred Sanders's Research on Cold Fronts, 2003, revised, 2004, 2006, p. 5. Retrieved on 2006-07-14. Since the leading edge of air mass changes bore resemblance to the military fronts of World War I, the term "front" came into use to represent these lines.Bureau of Meteorology. Air Masses and Weather Maps. Retrieved on 2006-10-22.
Despite the introduction of the Norwegian cyclone model just after World War I, the United States did not formally analyze fronts on surface analyses until late 1942, when the WBAN Analysis Center opened in downtown Washington, D.C.Hydrometeorological Prediction Center. A Brief History of the Hydrometeorological Prediction Center. Retrieved on 2007-05-05. The effort to automate map plotting began in the United States in 1969,ESSA. Prospectus for an NMC Digital Facsimile Incoder Mapping Program. Retrieved on 2007-05-05. with the process complete in the 1970s. Hong Kong completed their process of automated surface plotting by 1987.Hong Kong Observatory. The Hong Kong Observatory Computer System and Its Applications. Retrieved on 2007-05-05. By 1999, computer systems and software had finally become sophisticated enough to allow for the ability to underlay on the same workstation satellite imagery, radar imagery, and model-derived fields such as atmospheric thickness and frontogenesis in combination with surface observations to make for the best possible surface analysis. In the United States, this development was achieved when Intergraph workstations were replaced by n-AWIPS workstations.Hydrometeorological Prediction Center. Hydrometeorological Prediction Center 1999 Accomplishment Report. Retrieved on 2007-05-05. By 2001, the various surface analyses done within the National Weather Service were combined into the Unified Surface Analysis, which is issued every six hours and combines the analyses of four different centers.David Roth. Hydrometeorological Prediction Center. Unified Surface Analysis Manual. Retrieved on 2006-10-22. Recent advances in both the fields of meteorology and geographic information systems have made it possible to devise finely tailored weather maps. Weather information can quickly be matched to relevant geographical detail. For instance, icing conditions can be mapped onto the road network. This will likely continue to lead to changes in the way surface analyses are created and displayed over the next several years.Saseendran S. A., Harenduprakash L., Rathore L. S. and Singh S. V. A GIS application for weather analysis and forecasting. Retrieved on 2007-05-05.
Station model used on weather maps
When analyzing a weather map, a station model is plotted at each point of observation. Within the station model, the temperature, dewpoint, wind speed and direction, atmospheric pressure, pressure tendency, and ongoing weather are plotted.National Weather Service. Station Model Example. Retrieved on 2007-04-29. The circle in the middle represents cloud cover; fraction it is filled in represents the degree of overcast.Dr Elizabeth R. Tuttle. Weather Maps. Retrieved on 2007-05-10. Outside the United States, temperature and dewpoint are plotted in degrees Celsius. The wind barb points in the direction from which the wind is coming. Each full flag on the wind barb represents of wind, each half flag represents . When winds reach , a filled in triangle is used for each of wind.American Meteorological Society. Selected DataStreme Atmosphere Weather Map Symbols. Retrieved on 2007-05-10. In the United States, rainfall plotted in the corner of the station model are in inches. The international standard rainfall measurement unit is the millimeter. Once a map has a field of station models plotted, the analyzing isobars (lines of equal pressure), isallobars (lines of equal pressure change), isotherms (lines of equal temperature), and isotachs (lines of equal wind speed) are drawn.CoCoRAHS. INTRODUCTION TO DRAWING ISOPLETHS. Retrieved on 2007-04-29. The abstract weather symbols were devised to take up the least room possible on weather maps.
Synoptic scale features
A synoptic scale feature is one whose dimensions are large in scale, more than several hundred kilometers in length.Glossary of meteorology. Synoptic scale. Retrieved on 2007-05-10. Migratory pressure systems and frontal zones exist on this scale.
Pressure centers
Centers of surface high- and low-pressure areas that are found within closed isobars on a surface weather analysis are the absolute maxima and minima in the pressure field, and can tell a user in a glance what the general weather is in their vicinity. Weather maps in English-speaking countries will depict their highs as Hs and lows as Ls,Weather Doctor. Weather's Highs and Lows: Part 1 The High. while Spanish-speaking countries will depict their highs as As and lows as Bs.Agencia Estatal de Meteorología. Meteorología del aeropuerto de La Palma..
Low pressure Low-pressure systems, also known as cyclones, are located in minima in the pressure field. Rotation is inward at the surface and counterclockwise in the Northern Hemisphere as opposed to inward and clockwise in the Southern Hemisphere due to the Coriolis force. Weather is normally unsettled in the vicinity of a cyclone, with increased cloudiness, increased winds, increased temperatures, and upward motion in the atmosphere, which leads to an increased chance of precipitation. Polar lows can form over relatively mild ocean waters when cold air sweeps in from the ice cap. The relatively warmer water leads to upward convection, causing a low to form, and precipitation usually in the form of snow. Tropical cyclones and winter storms are intense varieties of low pressure. Over land, thermal lows are indicative of hot weather during the summer.BBC Weather. Weather Basics - Low Pressure. Retrieved on 2007-05-05.
High pressure High-pressure systems, also known as anticyclones, rotate outward at the surface and clockwise in the northern hemisphere as opposed to outward and counterclockwise in the southern hemisphere. Under surface highs, sinking of the atmosphere slightly warms the air by compression, leading to clearer skies, winds that are lighter, and a reduced chance of precipitation.BBC Weather. High Pressure. Retrieved on 2007-05-05. The descending air is dry, hence less energy is required to raise its temperature. If high pressure persists, air pollution will build up due to pollutants trapped near the surface caused by the subsiding motion associated with the high.United Kingdom School System. Pressure, Wind and Weather Systems. Retrieved on 2007-05-05.
Fronts
Fronts in meteorology are boundaries between air masses that have different density, air temperature, and humidity. Strictly speaking, the front is marked at the warmer edge of a frontal zone where the gradient is very large. When a front passes over a point, it is marked by changes in temperature, moisture, wind speed and direction, a minimum of atmospheric pressure, and a change in the cloud pattern, sometimes with precipitation. Cold fronts develop where the cold air mass is advancing, warm fronts where the warm air is advancing, and a stationary front is not moving. Fronts classically wrap around low pressure centers as indicated in the image here depicted for the Northern Hemisphere. On a larger scale, the Earth's polar front is a sharpening of the general equator-to-pole temperature gradient, underlying a high-altitude jet stream for reasons of thermal wind balance. Fronts usually travel from west to east, although they can move in a north-south direction or even east to west (a "backdoor" front) as airflow wraps around a low pressure center. Frontal zones can be distorted by such geographic features as mountains and large bodies of water.
Cold front
A cold front is located at the leading edge of a sharp temperature gradient on an isotherm analysis, often marked by a sharp surface pressure trough. Cold fronts can move up to twice as quickly as warm fronts and produce sharper changes in weather since cold air is denser than warm air and rapidly lifts as well as pushes the warmer air. Cold fronts are typically accompanied by a narrow band of clouds, showers and thunderstorms. On a weather map, the surface position of the cold front is marked with a blue line of triangles (pips) pointing in the direction of travel, at the leading edge of the cooler air mass.
Warm front