Basic Meteorology >> How to read Surface Maps

Basic Meteorology
	-- What you need to know to brief the current weather

Coast Guard Weather Briefer

Return of the BIG "W" by MSTCM Frank Prekel

The Third Class EPQ, 4.B.03 -- Analyze current(s), tides, and projected weather informatoin -- is within the Response Section. The intent of the qual is to bring basic weather knowledge back to the MST rading so that it can be better applied and anticipated during a "incident response," whether the incident be oil, chemical, or a WMD event.

MSTCM Frank Prekel
August 30, 2007

Note from the webmaster -- Give credit where credit is due, without reinventing the wheel, there are a few outstanding online learning guides. My ambitions here are solely to provide you with necessary training tool spread all over the internet.

This page provides atypical meteorology instruction. Normally this type of scientific instruction begins with a “parcel of air” and the processes this parcel will go through, such as temperature, moisture, pressure, and motion. However, with you as my audience in consideration, I’ll begin in reverse order, “beginning with the end in mind.” Concentrating first with providing you with what you will need to give an informative, yet concise weather brief to YOUR audience.
Basic Module (on this page)	Intermediate Module	Advanced Module
How to read 'surface' weather maps • Symbols - Surface Station Model - Upper Air Station Model - Map Symbols • Air Pressure and Wind - High - Low • Fronts and Air Masses - Stationary - Warm - Cold •Weather associated with each - Temperature difference - Dew Point difference	What is Weather? Fundementals • Atmospheric Composition • Atmospheric (Thermal) Structure • Heating Earth's Surface and Atmosphere • Global Circulation • Plus much more!	What 'causes' the weather? Meteorology Concepts • Dynamics (Atmospheric Motion) • Thermodynamics (Temperature & Moisture) • Using Skew-T P-Log

Basic

How to read ‘surface’ weather maps

Objective: Given a ‘surface’ weather map (a.k.a. surface chart), DETERMINE the weather conditions associated with the frontal boundary(ies) depicted, as well as be able to explain surface chart symbols / terminology associated with this chart.

Simple Surfac Map from NWS	More informative Surface Chart

Surface Maps by NWS	http://www.hpc.ncep.noaa.gov/html/sfc2.shtml
ONLINE locations for surface charts
METAR PLOT- multi-hour updates UNISYS SURFACE- hourly update http://adds.aviationweather.noaa.gov/progs/	http://www.hpc.ncep.noaa.gov/html/sfc2.shtml DIFAX- three hour update

ABOUT THE SURFACE CHART

If forecasting has to be performed from just one chart, many forecasters would choose the surface chart. It has a wealth of observations and it is the weather experienced by humans since it is at the surface. Unlike the upper air charts that only come out twice per day, the surface chart can be updated as much as multi-hourly, hourly or in three-hour increments.

Understanding pressure contour lines (isobars) is the key to interpreting the chart. Without understanding isobars; temperature advection, wind speed/direction and pressure distribution the surface chart can not be comprehended. Upper air charts are at a fixed pressure level (or rather called heights); temperature, dewpoints and wind are reported from that pressure level. This is not true for the surface chart with the exception of pressure. Temperature, dewpoint and wind information are gathered from observation stations just a few feet above the surface for all elevations. Pressure, however, is normalized so pressures at high elevation stations can be measured against pressures at low elevation stations. All stations, no matter the elevation, are given the station pressure the site would have if it were at sea level. In a place such as New Orleans, the surface pressure will be very close to the station pressure. But in a city like Denver, the station pressure may be 150 millibars less than surface pressure. Once each location has a sea level pressure, they can be compared to each other to determine where relative low and high pressures are distributed across the map.

Another huge difference between surface and upper air charts is the marking of fronts. They are marked on surface charts but not on upper air charts. Temperature advection and height contour kinking must be used to infer upper air fronts. With a large number of reporting stations at the surface, a fairly accurate position of fronts is possible. With far less reporting stations on upper level charts it is more difficult to obtain exact frontal position. Besides, there are only two upper air charts per day.

Fronts do not pass a reporting station at all levels in the atmosphere at the same time. The upper level front can pass BEFORE or AFTER the surface front. If the upper level winds are strong, the upper level front may advect out ahead of the surface front. Friction is strongest at the surface, which can impede surface front movement. In the case of a shallow polar front, the surface front will pass before the upper level front (i.e. 850 mb front).
www.theweatherprediction.com

Surface Station Model

Temp (F)
Weather
Dewpoint (F)

Pressure (mb)
Sky Cover
Wind (kts)

Data at Surface Station
Temp 45 °F, dewpoint 29 °F,
overcast, wind from SE at 15 knots,
weather light rain, pressure 1004.5 mb

Upper Air Station Model

Temp (C)

Dewpoint (C)

Height (m)

Wind (kts)

Data at Pressure Level - 850 mb
Temp -5 °C, dewpoint -12 °C,
wind from S at 75 knots,
height of level 1564 m

Note: Regarding weather charts; there are 6 standard charts, beginning at the surface and extending vertically into the atmosphere. They include

Pressure	Approximate Height		Approximate Temperature
Sea Level 1000mb 850 mb 700 mb 500 mb 300 mb 200 mb 100 mb	0 m 100 m 1500 m 3000 m 5000 m 9000 m 12000 m 16000 m	0 ft 300 ft 5000 ft 10000 ft 18000 ft 30000 ft 40000 ft 53000 ft	15 C 15 C 05 C -05 C -20 C -45 C -55 C -56 C	59 F 59 F 41 F 23 F -04 F -49 F -67 F -69F

Forecast Station Model

Temp (F)
Weather
Dewpoint (F)

PoP (%)
Sky Cover
Wind (kts)

Forecast at Valid Time
Temp 78 °F, dewpoint 64 °F,
scattered clouds, wind from E at 10 knots,
probability of precipitation 70% with rain showers

Map Symbols

Sky Cover	Wind	Fronts		Selected Weather Symbols
clear	Calm	cold front	warm front	Rain
1/8	1-2 knots (1-2 mph)	stationary front	occluded front	Rain Shower
scattered	3-7 knots (3-8 mph)	trough		Thunderstorm
3/8	8-12 knots (9-14 mph)	radar intensities		Drizzle
4/8	13-17 knots (15-20 mph)	tornado (T) #300		or Snow
5/8	18-22 knots (21-25 mph)	severe thunderstorm (S) #287		Snow Shower
broken	23-27 knots (26-31 mph)			Freezing Rain
7/8	48-52 knots (55-60 mph)			Freezing Drizzle
overcast	73-77 knots (84-89 mph)			Fog
obscured	103-107 knots (119-123 mph)			Haze
missing	Shaft in direction wind is coming from			Smoke
http://www.ametsoc.org/amsedu/dstreme/				Dust or Sand
http://www.ametsoc.org/amsedu/dstreme/				Blowing Snow

Surface level winds
http://www.rap.ucar.edu/weather/surface/

By The National Center for Atmospheric Research (NCAR)
Operated by the University Cooporation for Atmospheric Research

RAP Real Time Weather Data

Air Pressure and Wind

A barometer measures the air pressure. The air pressure is a function of how much air is pressing over an area. Air pressure will change rapidly with a change in altitude. Because of this rapid change with altitude, reported pressure is usually adjusted to sea level. Once all locations are adjusted to sea level then the air pressure can be compared between two places that even have different altitudes. Maps that show pressure adjusted to sea level are called isobaric maps. The isobars connect points of equal pressure. Where the isobars are close together the wind is stronger.

The English units of air pressure are inches of mercury. The metric version is millibars. The average sea level pressure is 29.92 inches of mercury and 1013 millibars. Using these average values it can be determined whether the pressure is above, significantly above, below or significantly below the average value.

English unit	Metric
29.92 inches of mercury	1013.25 Millibars

A few sea level pressure benchmark values follow:
1086 mb (32.08 inches of mercury): Highest Ever Recorded
1030 mb (30.42 inches of mercury): Strong High Pressure System
1013 mb (29.92 inches of mercury): Average Sea Level Pressure
1000 mb (29.54 inches of mercury): Typical Low Pressure System
980 mb (28.95 inches of mercury): CAT 1 Hurricane or a very intense mid-latitude cyclone
950 mb (28.06 inches of mercury): CAT 3 Hurricane
870 mb (25.70 inches of mercury): Lowest Ever Recorded (not including tornadoes)

High Pressure Centers
also known as anticyclones

A high pressure center is where the pressure has been measured to be the highest relative to its surroundings. That means, moving in any direction away from the "High" will result in a decrease in pressure. A high pressure center also represents the center of an anticyclone and is indicated on a weather map by a blue "H".

Winds flow clockwise around a high pressure center in the northern hemisphere, while in the southern hemisphere, winds flow counterclockwise around a high.

Sinking air in the vicinity of a high pressure center suppresses the upward motions needed to support the development of clouds and precipitation. This is why fair weather is commonly associated with an area of high pressure.

Low Pressure Centers
also known as cyclones

A low pressure center is where the pressure has been measured to be the lowest relative to its surroundings. That means, moving in any horizontal direction away from the "Low" will result in an increase in pressure. Low pressure centers also represent the centers of cyclones.

A low pressure center is indicated on a weather map by a red "L" and winds flow counterclockwise around a low in the northern hemisphere. The opposite is true in the southern hemisphere, where winds flow clockwise around an area of low pressure.

Rising motion in the vicinity of a low pressure center favors the development of clouds and precipitation, which is why cloudy weather (and likely precipitation) are commonly associated with an area of low pressure.

The below picture gives a visual dipictions of this "High" and "Low" pressure relationship. Remember the saying, "What goes up must come down?" That's exactly the process of weather.

Fronts and Air Masses

Fronts

A front is defined as the transition zone between two air masses of different density. Fronts extend not only in the horizontal direction, but in the vertical as well. Therefore, when referring to the frontal surface (or frontal zone), we referring to both the horizontal and vertical components of the front.

Stationary Front

When a warm or cold front stops moving, it becomes a stationary front. Once this boundary resumes its forward motion, it once again becomes a warm front or cold front. A stationary front is represented by alternating blue and red lines with blue triangles pointing towards the warmer air and red semicircles pointing towards the colder air.

Stationary Front

Warm Front -- transition zone from cold air to warm air

A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass. Warm fronts generally move from southwest to northeast and the air behind a warm front is warmer and more moist than the air ahead of it. When a warm front passes through, the air becomes noticeably warmer and more humid than it was before.

Warm Front

Symbolically, a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On colored weather maps, a warm front is drawn with a solid red line.

Warm air advancing

There is typically a noticeable temperature change from one side of the warm front to the other. In the map of surface temperatures below, the station north of the front reported a temperature of 53 degrees Fahrenheit while a short distance behind the front, the temperature increased to 71 degrees. An abrupt temperature change over a short distance is a good indication that a front is located somewhere in between.

Temperature difference

If warmer air is replacing colder air, then the front should be analyzed as a warm front. If colder air is replacing warmer air, then the front should be analyzed as a cold front. Common characteristics associated with warm fronts have been listed in the table below.

	Before Passing	While Passing	After Passing
Winds	south-southeast	variable	south-southwest
Temperature	cool-cold, slow warming	steady rise	warmer, then steady
Pressure	usually falling	leveling off	slight rise, followed by fall
Clouds	in this order: Ci, Cs, As, Ns, St, and fog; occasionally Cb in summer	stratus-type	clearing with scattered Sc; occasionally Cb in summer
Precipitation	light-to-moderate rain, snow, sleet, or drizzle	drizzle or none	usually none, sometimes light rain or showers
Visibility	poor	poor, but improving	fair in haze
Dew Point	steady rise	steady	rise, then steady

Table adapted from: Ahrens, (1994)

Finding Warm Fronts Using Wind Direction shift from east-southeast to south-southwest

Warm fronts are not always identifiable by simply examining the temperature field alone. Other fields must also be taken into consideration. For example, below is a surface weather map with an analyzed low pressure center (red "L") and associated cold front (blue line) and warm front (red line). The numbers are surface temperature reports and the symbols are wind barbs, indicating wind direction and wind speed.

surface chart of the US

At the time this map was generated, temperatures ahead of the warm front were in the 40's, while behind the front, temperatures were only slightly warmer (in the 50's). However, notice the change in wind direction (as indicated by the wind barbs) from one side of the warm front to the other. Winds ahead of the warm front were generally from the east, while behind the front, winds had shifted around and were blowing out of the south. This sudden shift in wind direction was the key indicator that a warm front was present.

Low pressure

A sudden change in wind direction is commonly observed with the passage of a warm front. Before the front arrives, winds ahead of the front (in the cooler air mass) are typically from the east, but once the front passes through, winds usually shift around to the south-southwest (in the warmer air mass).

Finding Warm Fronts Using Dew Points

higher dew point temperatures behind the warm front

Another indication of a possible frontal passage is a change in the air's relative humidity. The air mass behind a warm front is typically more moist than the air mass ahead of the front. The surface map below contains reports of temperature, dew point temperature, and wind barbs. Higher dew points indicate a higher moisture content of the air. Ahead of the warm front analyzed below, dew point temperatures were generally in the 40's, while behind the front, dew point values climbed into the 50's.

This increase in dew point temperature indicated that the air behind the warm front contained more moisture. A noticeable change in the air's relative humidity is commonly observed with the passage of a warm front. Before the front arrives, the air typically feels less humid than after the warm front passes through.

Cyclones and Associated Warm Front
leading edge of warmer air mass

Below is a simple model of a cyclone with a cold front extending to the south from the center of low pressure and a warm front extending to the east ahead of the storm.

At low levels, several air masses of distinctly different origin may be found in varying parts of the cyclone. The warm front marks the leading edge of warm moist air being pulled northward by southerly winds ahead (east) of the low.

Clouds and precipitation usually develop along and ahead of the warm front as warm moist air rides up and over the colder air ahead of it.

Cold Front - transition zone from warm air to cold air

A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. Cold fronts generally move from northwest to southeast. The air behind a cold front is noticeably colder and drier than the air ahead of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour.

Symbolically, a cold front is represented by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On colored weather maps, a cold front is drawn with a solid blue line.

There is typically a noticeable temperature change from one side of a cold front to the other. In the map of surface temperatures below, the station east of the front reported a temperature of 55 degrees Fahrenheit while a short distance behind the front, the temperature decreased to 38 degrees. An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in between.

If colder air is replacing warmer air, then the front should be analyzed as a cold front. On the other hand, if warmer air is replacing cold air, then the front should be analyzed as a warm front. Common characteristics associated with cold fronts have been listed in the table below.

Before Passing

While Passing

After Passing

Winds
south-southwest

gusty; shifting

west-northwest

Termperature
Warm

sudden drop

steadily dropping

Pressure
falling steadily

minimum, then sharp rise

rising steadily

Clouds
increasing: Ci, Cs and Cb

Cb

Cu

Precipitation
short period of showers

heavy rains, sometimes with hail, thunder and lightning

showers then clearing

Visibility
fair to poor in haze poor,

poor, followed by improving

good, except in showers

Dew Point
high; remains steady

sharp drop

lowering

Table adapted from: Ahrens, (1994)

Finding Cold Fronts Using Wind Direction
shift from south-southwest to west-northwest

Cold fronts are not always identifiable by simply examining the temperature field alone. Other fields must also be taken into consideration. For example, below is a surface weather map with an analyzed low pressure center (red "L") and associated cold front (blue line) and warm front (red line). The numbers are surface temperature reports and the symbols are wind barbs, indicating wind direction and wind speed.

At the time this map was generated, temperatures ahead of the cold front were in the 50's, while behind the front, temperatures were only slightly colder (in the 40's and 50's). However, notice the change in wind direction (as indicated by the wind barbs) from one side of the cold front to the other. Winds ahead of the cold front were generally from south-southwest, while behind the front, winds had shifted around and were blowing out of the west. This sudden shift in wind direction was the key indicator that a cold front was present.

A sudden change in wind direction is commonly observed with the passage of a cold front. Before the front arrives, winds ahead of the front (in the warmer air mass) are typically out of the south-southwest, but once the front passes through, winds usually shift around to the west-northwest (in the colder air mass).

Finding Cold Fronts Using Dew Points
lower dew point temperatures behind the cold front

Another indication of a possible frontal passage is a change in the air's relative humidity. The air mass ahead of a cold front is typically more moist than the air mass behind it. The surface map below contains reports of temperature, dew point temperature, and wind barbs. Higher dew points indicate a higher moisture content of the air. Ahead of the cold front analyzed below, dew point temperatures were generally in the 50's, while behind the front, dew point values dropped off into the 30's and 40's.

This decrease in dew point temperature indicated the presence of drier air behind the cold front. A noticeable change in the air's relative humidity is commonly observed with the passage of a cold front. Before the front arrives, the air typically feels more humid (in the warmer air mass), but once the front passes through, the humidity decreases and the air feels drier.

Cyclones and Associated Cold Front
leading edge of colder air mass

Below is a simple model of a cyclone with a cold front extending to the south from the center of low pressure and a warm front extending to the east ahead of the storm.

At low levels, several air masses of distinctly different origin may be found in varying parts of the cyclone. The cold front marks the leading edge of a colder and drier air mass being wrapped southeastward by north-northwesterly winds behind the low.

Precipitation Along a Cold Front
lifting the warm moist air ahead of it

The animation below is a sequence of vertical cross sections that depict the development of precipitation ahead of and along a cold front. The surging blue mass represents colder air behind the cold front (solid blue line) while the yellow shading indicates the warm moist air mass ahead of the front.

Animation by: Hall

As the front advances, the colder air lifts the warmer air ahead of it (red arrows). The air cools as it rises and the moisture condenses to produce clouds and precipitation ahead of and along the cold front. In contrast to lifting along a warm front, upward motions along a cold front are typically more vigorous, producing deeper clouds and more intense bands of showers and thunderstorms. However, these bands are typically quite narrow and move rapidly just ahead of the cold front.

A Closer Examination of the Animation:
Initially, the cold air mass wedges into the warmer air mass ahead of it, (separated from each other by the cold front). The lighter warm air is lifted upwards by the denser cold air and if enough water vapor condenses, clouds develop.

If condensation of water vapor persists, precipitation may develop, typically in a narrow band just ahead of the cold front.

Due to the steep slope of a cold front, vigorous rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms.