Climate Diagram

 

Climate diagrams: a method to illustrate climate dynamics across biomes

Prepared by:  María Fernanda Barberena, Universidad del Turabo, Puerto Rico and Jorge R. Ortiz-Zayas, Universidad de Puerto Rico en Río Piedras

I.  Introduction

Historically, ecologists have been interested in studying how climate conditions influence the development of specific types of vegetation in particular areas of the World.  These areas are called biomes.  Some of the most important biomes are: the tundra, the tropical rainforest, the deserts, the grasslands, the taiga, the temperate deciduous forest, the temperate rainforest, and scrub forest (or  chaparral).  The climatic conditions in a biome affect the vegetation because the relationship between temperature and humidity determines the amount of available water for plants. It is important to highlight that this relationship changes with elevation because as we go up mountains, the air becomes more cold and dense making it less able to store water, thus becoming saturated with water faster than warm air from lower elevations. So to compare the vegetation between areas of different elevations and to be able to determine their dry and wet seasons, we need graphic tools that remove the effect of elevation, one of these tools are climate diagrams.

Climate diagrams are diagrams that summarize trends in temperature and precipitation for at least 30 years. They allow to establish the relationship between temperature and precipitation and to determine the length of dry, wet, and extremely wet periods.

These diagrams are also called Walter Lieth diagrams or ombrothermic diagrams because in the same diagram the researcher plots months of the year in the X-axis, average monthly temperature in one Y-axis graph (Y1) and total monthly precipitation in the other Y-axis (Y2). The X-axis should start with the coldest month, as a consequence for the Northern hemisphere the X-axis should start with January, while for the Southern hemisphere it should start with July.  Temperature should be expressed in degrees centigrade and precipitation should be expressed in millimeters.  It is VERY important to observe the scales of the axes because they MUST respect the following relationship: Precipitation (P) = Temperature (T) x 2.  As an example, a 10 ° C average temperature in the Y1-axis must be equal to a 20 mm of monthly precipitation in the Y2-axis).  Usually, the precipitation curve is represented in blue and the temperature curve in red.

The relationship between the scales of the axes is based on the Gaussen-Bognouls aridity index, which states that when

P >3T  --> wet period

3T > P > 2T --> semi wet period

P < 2 T --> arid period

This means that when the curve of precipitation falls below the temperature curve there is a dry period, when the precipitation curve runs above the temperature curve there is a wet period, and when the precipitation curve exceeds 100 mm there is an excess water period. In the Walter-Lieth diagrams, the dry period is illustrated by a dotted area; the wet period is illustrated by using vertical lines and the period with excess water in black. When there are months where the temperature falls below 0, these are freezing months and the corresponding months should be illustrated by blackening the X-axis along the segment corresponding to these months.

II. Materials
Graph paper
Blue and red pencils
Data on average monthly temperature and total monthly rainfall
Calculator

III.  Procedure

1-     On graph paper, draw the x-axis marking each of the months starting with the coldest month.  Remember to start in January if you are plotting data collected in the northern hemisphere or in July if the data is from the southern hemisphere.

2-     Label the months on the x-axis.

3-     Draw the second Y-axes: the Y1-axis corresponds to the temperature and the Y2-axis corresponds to the precipitation.  (Remember to pay attention to scale: in the Y1-axis, the 10 ° C line in the temperature axis must correspond to the 20 mm rainfall scale on the Y2-axis

4-     For each month, draw a red dot to show average monthly temperature. Once the twelve dots are drawn, link them with a red line.

5-     Similarly, draw a blue dot to show total monthly precipitation for each month. Once the twelve dots are drawn, link them with a blue line.

  1. NOTE OF CAUTION: These charts should ideally be constructed using mean monthly values collected over at least 30 years. Therefore, to calculate the data for the Y-axes (the red and blue dots):

i.     Temperature: Suppose you have daily temperature from 1957 to 2001. To calculate the value for January temperature: average monthly temperature: calculate average temperature for each January: January 1957, January 1958, January 1959, ... January 2001. Then you calculate the overall average for January, taking into account all the averages you calculated for each January. This overall average is the value to be placed in the diagram to show average temperature for January. Follow the same procedure for each of the month.  Draw a red line to connect the dots.                             

ii.     Precipitation: Suppose you have daily rainfall from 1957 to 2001. To calculate the value for January precipitation: total monthly precipitation: sum the precipitation amount for every day for January 1957; sum the precipitation amount for every day for January 1958, January 1959... January 2001. Then you calculate average total precipitation for January, taking into consideration each of the monthly totals you previously calculated.  This average is the value to be placed in the diagram to show monthly precipitation for January.   Follow the same procedure for each of the months.  Draw a blue line to connect the dots. 

iii.     In addition to the averages, you may include a measure of variation (such as standard deviation) and also outlier values that represent months with extreme values (such as months either very dry or very wet).

6-     Now you can determine dry and wet periods:

  1. When the temperature line runs above the precipitation line there is a dry season and the area between the lines should be illustrated by filling it with dots
  2. When the temperature line runs below the precipitation line there is a wet period and the area between the lines should be illustrated by filling it with vertical lines
  3. When precipitation exceeds 100 mm, there is a period with excess water and the area should be illustrated by coloring it in black.

As shown in Figure 1, the diagram should also include the following information:
a. Station Name: located in the upper left side of axis-Y1
b. Height in meters: left middle side (next to name)
c. Average annual temperature (° C) right middle side (next to height)
d. Average total precipitation (mm): upper right (next to the axis-Y2)
e. Absolute maximum temperature (° C): upper right side of axis-Y1
f. Average daily maximum temperature of hottest month (° C): upper right side of axis 1 under "e"
g. Number of years that were included (first for temperature, second for precipitation) below the station name
h. Average daily maximum temperature of the coldest month (° C): lower right side of axis-Y1
i.  Absolute minimum temperature (° C): lower right side of axis-Y1 under "h"

 

 

Figure 1.  Climate diagram of the subtropical dry forest of Aguirre, Puerto Rico.


IV.  Practice exercise

 

  1. Build a climate diagram for the cities of Portland (Oregon) and San Juan (Puerto Rico).  Historic temperature and rainfall data for these cities are available in Table 1.    You can use the blank graphs at the end of this protocol to build your climate diagrams.
  2. In which biomes are these two cities located?
  3. Identify dry, wet and very wet months in each city.
  4. Answer the following questions:
    1. Where would you expect to find the most exuberant vegetation? Why?
    2. What are some possible adaptations of plants to adapt to life in each biome?
    3. What possible behavioral adaptations should humans have to make to live in each biome to be able to deal with winter and summer climates?

Table 1. Mean monthly temperature  (T) and total monthly rainfall (P) for two cities:  San Juan, Puerto Rico and Portland, Oregon.  Data from www.worldclimate.com

Month

Portland Intl. Airport, Oregon

LMM Airport, San Juan, P.R.

T (ºC)

P, (mm)

T  (ºC)

P, (mm)

Jan

4.2

133.1

25

75.3

Feb

6.4

105.2

25

55.7

Mar

8.5

92

25.5

58.8

Apr

10.5

60.7

26.3

94.9

May

13.9

52.8

27.1

155.6

Jun

17.5

37.8

27.9

112.3

Jul

20.1

14.8

28.1

114.6

Aug

20.3

23.1

28.1

133.2

Sep

17.3

41.3

28

135.9

Oct

12.5

75.7

27.7

139.9

Nov

7.8

134.6

26.6

148.2

Dec

4.5

148.8

25.6

118.2

Temperature data for Portland are based on data collected from 1961 to 1990.  Rainfall data for Portland were collected from 1938 to 1995.  Monthly temperature data for San Juan were collected from 1961 to 1990.  Rainfall data for San Juan were collected from 1955 to  1995.  The San Juan weather station is located at 18.43° N 66.00° W at an elevation of 2m above mean sea level.   The weather station in Portland is located at 45.60°N 122.60°W a and at 6m above mean sea level.

 

 

 

 

 

 

For additional information please visit http://www.usf.uos.de/projects/climate/cd/doc/index.htm where you can have access to the Climate Diagram World Atlas of H. Lieth , J Berlekamp, S.Fuest y S. Riediger.