2 The temperature in the polar regions

Graphs are diagrams that show the relationship between two different quantities. The quantities and their values are displayed on two reference lines, called the axes, that are at right angles to one another and the axes are marked to show the range of possible values of the quantities. For example, in Figure 2 the axes are temperature and month. In graphs, the relationship is shown by a straight line or a curve of plotted points (the points may or may not be shown), so Figure 2 shows the relationship between mean atmospheric surface temperature and time given by the months of the year; in other words, how atmospheric surface temperature varies over the annual cycle. With this information, one can now find corresponding values for the two quantities that are plotted. As with all scientific diagrams, graphs follow a set of clear conventions, which are highlighted in Figure 3.

Figure 3 A version of Figure 2 with labels showing particular terms common to graphs.

The two quantities being described, mean atmospheric surface temperature and month, are represented by the vertical and horizontal axes, respectively. Each axis is labelled clearly to indicate what units are used, and has a marked scale. In this example, the vertical axis represents mean atmospheric surface temperature with units of degrees Celsius represented by °C; the scale ranges from −70 °C to +10 °C. The horizontal axis shows the months of the year. There are two lines plotted on the axes that represent the values for mean atmospheric surface temperature for two locations: one in the Northern Hemisphere and one in the Southern Hemisphere, for specific months. The circles and triangles on the lines represent the actual mean temperature at that month, and the lines join the data points to show how the mean temperature changes between them. For example, to find the atmospheric surface temperature in degrees Celsius corresponding to February in Antarctica, start by finding the point on the horizontal axis representing February, and ‘draw’ a line vertically upwards until it meets the plotted line for Antarctica. Then ‘draw’ a line horizontally from that intersection to meet the vertical axis and read off the corresponding temperature in degrees Celsius. You should find that the temperature in February in Antarctica is about −40 °C.

To work out a representative value for the temperature for each month, an average is used to give an idea of a ‘typical’ or expected value. There are different ways of expressing an average value but the most common is the mean (which is often referred to as the average, as done here). The average (or mean) is obtained by adding up all the values of a set of data and dividing by the number of items in the data set. In the example here, the temperatures in three successive Junes for the black line (Alert) in Figure 2 were 1.0 °C, 1.2 °C and 0.9 °C. The mean temperature is the total divided by the number of measurements, which is written = (1.0 + 1.2 + 0.9) ⁄ 3 = 1.0 °C. By looking at that mean of three years’ data you should be able to determine a more typical value for the temperature. The data points on both lines in Figures 2 and 3 represent the mean of over 50 years of temperature data.

The terms latitude and longitude are used to identify the position of particular places on the Earth. On most maps you can see vertical and horizontal lines to divide up regions. Lines that run from east to west in the maps are called lines of latitude, and the lines that run north to south are called lines of longitude. You can see this in Figure 4, where the vertical red lines are the lines of longitude, and the horizontal blue lines are the lines of latitude.

If you take a roughly spherical fruit such as an apple and hold it so that the stalk is at the top (equivalent to the North Pole), draw six equally spaced horizontal lines equivalent to some of the blue latitude lines in Figure 4, then slice along these lines, you can see that each slice will have the same width of skin because the blue latitude lines are equal distances apart. On the Earth, the distance between the middle (the Equator at 0° latitude) and the very ‘top’ (the North Pole at 90° latitude) is divided up into 90 lines called degrees of latitude, which are approximately 110 km apart. These 90° of latitude are the Northern Hemisphere and any latitude north of the Equator is labelled ‘N’ after the value. From the Equator to the South Pole there are also 90° of latitude and this is the Southern Hemisphere, which are labelled ‘S’ after the value. Throughout this course the term ‘high latitudes’ is used to denote latitudes greater than ~66° N or ~66° S, conversely latitudes in the range ~30° N to ~30° S are referred to as low latitudes.

If you take another apple and draw lines on it to match the red longitude lines in Figure 4, then cut the fruit along them, this time you will end up with wedges. If you think about the wedges and the skin width, the width of skin is greatest in the middle of the fruit (the Equator on Earth), whilst at both ends the width of the skin is reduced to zero. You can see this clearly in the view of the Earth from the North Pole in Figure 5.

Figure 4 The lines of longitude and latitude on planet Earth as viewed from above the Equator. The point labelled ‘A’ marks the location of Cairo, point ‘B’ the location of London and point ‘C’ the location of Rio de Janeiro.

Figure 5 The lines of longitude (in red) and latitude (in blue) on planet Earth as viewed from above the North Pole.

In Figure 4 you can see that the blue lines of latitude are always the same distance apart, whereas Figure 5 shows the distance between the red lines of longitude decreases towards the North (and also the South) Pole, where they actually meet. The Earth is divided into 360 of these lines, called degrees of longitude, and at the Equator the distance between each line is 110 km, just like the distance between degrees of latitude. However, the further one goes from the Equator, the shorter the distance between the lines of longitude becomes. In London, the distance between each line of longitude is about 70 km, and even if it were possible it would take a very long time to walk all 360 degrees of longitude from London and back again around the world. At the North Pole and South Pole you could do it in just seconds. For historical reasons, the 0° longitude line runs through Greenwich in London and, looking down from the North Pole in the direction of Greenwich, in a clockwise direction the degrees of longitude range from 0° to 180° (by convention this is the Western Hemisphere labelled ‘W’), and in an anticlockwise direction they range from 0° to 180° (the Eastern Hemisphere labelled ‘E’). (Note: At 0° longitude you can write either 0° E or 0° W.) If I gave you my location in degrees latitude and longitude you could see exactly where I was on Earth – for example, I am writing this from approximately 51.57° N 0° E, which is in London, England and 51.57 degrees of latitude north of the Equator, and in line with Greenwich.

The use of latitude and longitude was rare, except amongst sailors and the armed forces, until relatively recently. But this is changing, as ever more people are using electronic equipment such as mobile phones and in-car navigation, which routinely use satellites in a system called GPS (Global Positioning System) to determine location very accurately.

The distance between each degree of latitude is always the same wherever it is on the planet, whilst one degree of longitude varies depending on the latitude at which it is measured.