8.4: Map Analysis

Surface observations as described above are plotted on a map in the form of station plot models, as in the figure below. When looking at the map of raw data below, you can get an idea of the prevailing wind direction in different parts of the US, which areas are experiencing widespread rain, which areas are overcast, and from the wind direction patterns you can even infer centers of high and low pressure. However, much of this information is not very clear without doing some analysis on the map. Map analysis can either be done by hand or by computer, and involves the drawing of contours, or 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 boundaries in the atmosphere, such as fronts or dry lines to show the locations and movement of air masses. Fronts will be covered in a later chapter. Here, we will focus on lines of constant pressure, isobars, and lines of constant temperature, isotherms.

Map with station plot models (CC BY-NC-SA 4.0).

Isopleths

The below figure shows a grid of equally spaced temperature values in °C. When analyzing the temperature on a map, you draw isopleths of constant temperature: isotherms. The main purpose of this is to separate areas of warmer air from cooler air, and to get an idea of how quickly the temperature changes across a horizontal distance, which is also 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. You will not need to know these conventions for this class, but it may still come in handy to be aware.

In looking at the below temperature field, you can see that there are many values that do not line up 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 where a datapoint is based on the data around it. Starting from the upper left corner, imagine that you are going to draw your 5°C isotherm. Five lies much closer to 4 than 16, so you will start your isotherm close to the 4. 5°C will also lie in between 3 and 15 °C, but slightly farther away from the 3 value, so you will slope your isotherm down slightly. Five degrees centigrade will also lie in 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 and slope back upward slightly between the 3 and 6 and directly through the middle between the 4 and 6 and back up through the 5 in the upper right hand corner.

Blank temperature field (CC BY-NC-SA 4.0).

The complete contoured example of this temperature field is given below. 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 will connect) or they will extend to the edge of the map. They will never cross or end suddenly. If two contours were to cross, it would mean that one place has two different air temperatures at one time, which is impossible. Areas where isotherms are close together indicate a strong temperature gradient, which may be indicative of a frontal zone.

Completed temperature field with isotherms (CC BY-NC-SA 4.0).

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 in order to identify areas of high and low pressure, as shown in the figure below. Areas where isobars are drawn close together indicate places where the pressure gradient force is strong. A stronger pressure gradient force is indicative of stronger 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.

Surface map with isobars and station plot models (Public Domain).

Because weather information is shared across continents and across country borders, it is important to maintain standards such that everyone effectively 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.