Discussion based on plots created as part of
Lab Activity #1:
Seasonal Temperature Change
Questions to Ask about a Plot before You Try to Read and
Before you begin to describe or interpret
any plot of data, such as the global temperature and temperature
difference plots in Lab Activity #1, there are several types of
questions that you should try to answer about the plot:
- Questions about metadata (that is, data or information
about the data):
- What quantity(ies) is (are) being plotted?
- What units are used to express the data values?
- Have "raw" data (observations or model
output) been analyzed or processed before being plotted
(e.g., averaged over time or space)?
- When were the data recorded?
- Where were the data recorded?
- (A secondary, but sometimes valuable, question to ask might be,
what instruments were used to record the data, or what numerical
model calculated the data, if a forecast or analysis model
- Questions about the plot (that is, the method
and conventions used to visualize the data):
- How are the data plotted or visualized?
- What conventions have been adopted (stated and/or
unstated) to implement the plotting/visualization method?
- On a graph of one quantity plotted as a function
of the other, which axis corresponds to the dependent
variable and which to the independent variable? What
are the vertical and horizontal scales (range of
values) on each axis? Are colors assigned to plotted
curves or symbols assigned to data points, and if so,
what do the colors or symbols represent?
a color-shaded plot or a color-filled contour plot,
what colors have been assigned to what (range of)
data values? What are the maximum and minimum data
values (or range of values) represented?
Plots of Monthly Mean Surface TemperatureData
(created using My World GIS software)
For the two temperature
plots in Lab Activity #1, some responses (and some actual answers)
to these questions would be:
- What data are plotted? Some sort of temperature
is plotted. A description of the data available through My World
GIS says the following:
"Temperature is measured directly at thousands of
weather stations around the world, as well as by ships, airplanes,
and satellites. The temperature data set in [My World GIS] was
created by combining measurements from all these sources and
using a complex computer program to calculate probable temperatures
in locations for which there are no measurements available."
It adds, "Data for some grid cells are based on inferences
and interpolations between observations and might be inaccurate
for some cells."
Whatever the source, the
temperatures are averaged temporally (that is, over time) over
a full month and spatially over 2.5° latitude ×
2.5° longitude "cells".
They are expressed in Kelvins.
- When were the data recorded? In January and
in July, 1982. We don't know how frequently the temperatures were
measured over the course of each day during those two months, but
the answer would vary depending on the individual sources of data
that went into the overall data set.
- Where were the data recorded? First, the data
are supposed to represent temperature at the earth's surface.
Whether that means air temperature close to the earth's surface
or the temperature of the surface itself (water, rock, soil, sand,
etc.), isn't clear. Second, the data were recorded globally,
though with incomplete coverage because there are some parts of
the earth where temperatures aren't recorded by one means or
- How were the data plotted? The visualization
is a color-shaded plot. A table of colors (20 in this case) is
defined, and these colors are assigned to span a specified range
of temperatures so that each color in the table represents a
particular sub-range of the full range of temperatures. For the
monthly-average surface temperatures, a color table consisting
of 20 colors, from dark blue at one end of the color table and
ranging through progressively lighter shades of blue, then
transitioning abruptly to shades of yellow, then orange, and then
red. The 20 colors are assigned to a range of temperatures from
220K (–53°C or –63°F) at the dark blue end
of the color table to 320K (47°C or 117°F) at the dark
red end. Each color represents the same increment of temperature,
which is (320K – 220K)/20 = 5K. Since an increment of
temperature in Kelvins and in degrees Celsius (degrees centigrade)
are equal, each color represents an increment of 5°C, too,
which is 5°C × (9°F/5°C) = 9°F.
As a result of the nature of the data plotted (spatially
averaged values over 2.5°× 2.5° latitude/longitude
cells, using color shading as the visualization method), when you
zoom in closely on the plot you see "pixelation", in
this case the individual square cells over which the temperatures
are averaged, each a single color.
Note, though, that in three
dimensions on a globe, while it is true that latitude lines are
evenly spaced and parallel, longitude lines are, in contrast,
most widely spaced at the equator and converge at the poles. (At
the equator, 1° of latitude and longitude span the same
distance—namely, 111 km. At progressively higher latitudes,
1° of longitude spans a progressively smaller distance, while
1° of latitude spans the same distance everwhere.)
In the plots below, the map projection separates the longitude
lines at the poles and spreads them out until they all become
parallel to each other, creating a rectangular map in two dimensions
in which each 2.5° latitude × 2.5° longitude grid
cell appears as a square and all of the cells are the same size.
The distance from north to south across each cell is the same for
all cells (about 277 km), but the distance across each cell from
east to west varies from a maximum of about 277 km at the equator
to about 11 km across the center of the row of grid cells that
join at the poles (each of which is centered at 87.75°
latitude). Hence, the true size of the grid cells is largest at
the equator and smallest at the poles, though you wouldn't guess
that looking at the uniformly sized grid cells in the map projection
The varying grid cell sizes has implications for how we calculate
global averages using these data sets, which we'll learn more
Features of plots of
average surface temperature for a selected year (e.g., 1982):
- Temperatures are generally warmest in the low latitudes
(which are defined to be between 30°S and 30°N latitude)
and decrease progressively across the middle latitudes (between
30° and 60°N and between 30°S and 60°S latitude)
to their coldest values in the high latitudes (between 60°
and 90°N and between 60° and 90°S). (This is a
qualitative description of the broadest, global aspects
of the temperature pattern.)
- There are some large temperature differences between
places at the same time of year (e.g.,
differences > 83°C [~150 °F]
in January and > 111°C [~200 °F] in July). (Note
that a difference or change in temperature of 1K is the same
as a difference or change in temperature of 1°C, which
is the same as a difference or change in temperature of
1.8°F = 9/5°F.) (This begins to quantify
some aspects of the temperature pattern, which can add value
to the qualitative description.)
- One of a number of secondary aspects of the pattern
is that although the hottest temperatures generally lie within
the low latitudes, they lie north of the equator in July but
tend to lie south of the equator in January (a distinction that
is most obvious over land).
Plots of Monthly-Average Surface
|January 1982 ||July 1982
Plot of the Difference between Montly-Average
Surface Temperatures from Different Months
It is not always easy to describe differences between plots of
similar quantities, such as monthly-average surface temperatures
in January and July, simply by placing them side by side. One,
possibly more effective way to look for differences between them
might be to make similar plots for the intervening months and animate
the series of monthly plots. Another, often very powerful method
is simply to subtract the values on one plot from the values in the
corresponding cells on the other, and plot the difference. Lab
Activity #1 does this for you, and the answers to some of the
questions raised about the individual January and July plots of
monthly-average surface temperature, are different:
- What data are plotted? The July minus January
difference in monthly-average surface temperatures.
- When were the data recorded? Same answer as for the
individual monthly-average data plots.
- Where were the data recorded? Same answer as
for the individual monthly-average data plots.
- How were the data plotted? The visualization
is a color-shaded plot. A color table containing 60 colors,
starting with dark blue and transitioning through progressively
lighter shades of blue, to white, and then to light red through
progressively darker reds. These colors are assigned to values
of July minus January temperature differences ranging from
–60K to 60 K (which is the same as –60°C to
60°C, since differences or changes in
temperature are the same whether expressed in Kelvins or °C).
(In °F, the range of temperature differences would be 9/5
larger: –108°F to 108°F.) Each individual color
therefore represents an increment of 2K or 2°C (or 3.6°F).
Features of the patterns shown on plot of
minus January difference in monthly-average surface temperature
(see a plot below):
Plot of July minus January temperature difference in
- July – January (that is, "July minus January")
monthly-average temperature differences are opposite in sign in
N. and S. Hemispheres (positive differences, shown in red, imply
that it is warmer in July than in January; red colors appear
almost exclusively in the N. Hem.; negative differences, shown
in blue, imply that it is warmer in January than in July, and
blue colors appear almost exclusively in the S. Hem.). In 1982,
a more or less typical year, in the polar regions in both hemispheres
the differences between July and January approached 60°C
- July – January temperature differences are smallest
in the low latitudes and largest at higher latitudes (though there
seems to be a relatively abrupt increase in the size of the
differences near the Antarctic coastline and around the edge of
the Arctic Ocean, and these aren't always land/water boundaries).
- July – January temperature differences
are generally small over oceans and are much larger over land.
Notable exceptions to this observation: (a) in the low latitudes,
where temperature differences are small over land, and (b) at the
highest latitudes where they are often large over oceans.
- July – January temperature differences over land seem
larger farther inland than near most coasts, especially the
western coasts at midlatitudes.
- Other, even smaller or more subtle features. For example,
large lakes and inland seas (e.g., Great Lakes in the U.S.; Caspian
Sea in western Asia) show smaller July – January temperature
differences than surrounding, land-based areas.(The resolution
of the data is barely high enough to tell, though!)
Red colors represent positive values (July warmer than January),
blue colors represent negative value (July colder than January).
White and near-white colors represent very small temperature
difference between July and January.