9.3: Map Analysis

Analyzing Weather Maps

Surface observations, as described above, are plotted on a map as station plot models, as shown in Figure 9.3.1. The raw data provides the prevailing wind direction in different parts of the US, which areas are experiencing widespread rain, and which areas are overcast. From the wind direction patterns, you can even infer centers of high and low pressure. However, much of this information is unclear without some analysis of the map. Map analysis can be done by hand or by computer and involves drawing contours (isopleths, lines of equal value) to connect areas of constant air pressure and temperature at the surface. Aloft, contours may be drawn to show areas of constant height, constant humidity, constant wind speed, and other parameters of interest at constant pressure levels. Map analysis also includes drawing and labeling atmospheric boundaries, such as fronts or dry lines, to show the locations and movement of air masses. Here, we will focus on lines of constant pressure (isobars) and lines of constant temperature (isotherms).

Figure 9.3.1: A surface weather map with numerous meteorological station models, each representing observations from an individual surface weather station. At each station, numerical values and symbols indicate air temperature, dew point, sea-level pressure, wind speed and direction, cloud cover, present weather, and recent pressure change. The dense distribution of station plots illustrates detailed, real-time surface weather conditions across the region, supporting weather monitoring, forecasting, and synoptic analysis. (CC-BY-NC-SA 4.0; Roland Stull via Practical Meteorology) .

Isopleths

Figure 9.3.2 shows a simplified example of a surface temperature map. It shows a uniform grid of equally spaced temperature values in °C. Values range from the low single digits at one edge of the grid to the high twenties and low thirties at the opposite edge. The top row contains values between 2 and 5 degrees, while successive rows show progressively higher temperatures. Mid-grid values are mostly between 20 and 30 degrees, and the highest values, around 29 to 33 degrees, appear clustered near one side of the grid. When analyzing temperature on such a map, you draw isopleths of constant temperature, called isotherms. The main purpose is to separate areas of warmer air from those of cooler air and to determine how quickly temperature changes across a horizontal distance, known as the temperature gradient. Isotherms are typically drawn every 5°C on the 5’s – for example, 5°C, 10°C, 15°C, 20°C, 25°C, and so on. Standard conventions are used for contouring on different map surfaces; for example, isotherms are usually dashed and shown in red.

Observing the temperature field in Figure 9.3.2, you can see that many values do not align exactly with the lines you need to draw. Many values lie in between contours, such as 4°C and 16°C. You will need to interpolate data, meaning you will have to infer a value based on the data around it. Starting in the upper-left corner, imagine drawing your 5°C isotherm. Five lies much closer to 4 than 16, so you will start your isotherm close to 4. 5°C will also lie between 3 and 15°C, but slightly farther away from the 3 value, so you will slope your isotherm down slightly. 5 °C will also lie between 2 and 7°C, so you will continue your isotherm to the right. However, it will slope down even further, because 5 lies closer to 7 than to 2. Your isotherm will then cross directly through the 5°C data point, slope back upward slightly between 3 and 6, cross directly through the middle between 4 and 6, and then back up through 5 in the upper right-hand corner.

Figure 9.3.2: A simplified surface temperature map, with values in degrees Celsius arranged in evenly spaced rows and columns. The X and Y axes represent east-west and north-south extents on a map. The temperature values show a smooth increase across the region, indicating a spatial temperature gradient. (CC-BY-NC-SA 4.0; Roland Stull, via Practical Meteorology).

The complete contoured example of this temperature field is in Figure 9.3.3. The atmosphere is a continuous fluid, so any fields that you are contouring (pressure, temperature) will never have values that jump or suddenly end. Contours will either be closed (both ends connect) or extend to the edge of the map. They will never cross or end suddenly. If two contours crossed, it would mean that at some point, one air temperature would differ from another, which is impossible. Areas where isotherms are close together indicate a strong temperature gradient, which may indicate a frontal zone.

Figure 9.3.3: The smooth contour lines connect locations with equal temperature, illustrating gradual temperature changes across the region. A labeled cool region corresponds to lower-temperature values, grouped by higher-frequency isotherms, while a labeled warm region encloses the highest temperatures, including a closed contour around values near 30 degrees and above. (CC-BY-NC-SA 4.0; Roland Stull, via Practical Meteorology).

Isobars

Pressure fields are analyzed by drawing isobars, or lines of constant air pressure. The surface mean sea level pressure is analyzed in much the same way as the isotherms above. However, the conventions are different. Typically, pressure is analyzed every 4 mb and centered on 1000 mb. Isobars are typically drawn as solid black lines. The mean sea-level pressure field is analyzed to identify areas of high and low pressure, as shown in Figure 9.3.4. Areas where isobars are drawn close together indicate strong pressure gradient forces. A stronger pressure gradient indicates higher wind speeds. As you will learn later, winds tend to flow counterclockwise around areas of lower pressure and clockwise around areas of high pressure in the Northern Hemisphere.

Figure 9.3.4: Surface weather map of continental North America at 7 am (EST) on July 21, 2010. A low-pressure trough spans the length of California, while high-pressure regions exist off the coast of South Carolina and north of Montana. (Public Domain, National Center for Environmental Prediction via Wikimedia Commons).

Figure 9.3.4 displays numerous surface weather station models, each encoding observed conditions such as air temperature, dew point, wind, cloud cover, sea-level pressure, and present weather. Curved contour lines represent lines of equal sea-level pressure, illustrating the overall pressure pattern across the region. Labeled pressure systems identify areas of relatively higher pressure and lower pressure, each associated with surrounding isobars. Frontal boundaries are drawn using standard meteorological symbols to indicate cold, warm, and stationary fronts, highlighting zones of air-mass interaction. The combined features depict a snapshot of atmospheric conditions used for synoptic-scale weather analysis and forecasting at a specific observation time.

Because weather information is shared across continents and country borders, it is important to maintain standards so that everyone speaks the same language. Weather is one of the greatest international collaborations ever. If it weren’t for China and Russia sharing their weather information with the US, our long-range forecasts would be more inaccurate because we wouldn’t know the initial conditions of the atmosphere upstream of the North American continent.