When Precipitation Patterns Change

Part A: What is Drought?

In Lab 3, you learned to interpret climographs to understand a location's normal climate. Another way that climographs can be used is to plot current conditions over a background of the average conditionsthis provides a graphic way to see how the current year compares to the long term average. These dynamic graphs indicate if current conditions are abnormally hot, cold, wet, or dry.

  1. Click the thumbnail image at right to see a larger view of a climograph for San Antonio, Texas. The graph shows conditions for January through mid-July of 2008.
  2. Examine the graph to interpret the conditions in San Antonio. The background colors (pale red for temperatures and light green for accumulated precipitation) show the average conditions compiled from many years of data. The brighter red and green lines show daily temperatures and accumulated rainfall through July of 2008.

    Checking In

    • What does the graph indicate about San Antonio's temperature?
      The temperature was above average during January but has been in the normal range since then.
    • What does the cumulative rainfall graph indicate?
      Rainfall has been below normal all year. The cumulative total for 2008 is roughly one third of the normal total for the end of July.
  3. Explore weather and climate conditions for stations in the United States via data located at NOAA's Southern Regional Climate Center (SRCC). Use the drop-down menus on the left of the screen to explore climate data in your own geographic area or other areas of interest.
  4. To generate temperature and accumulated precipitation maps for any region of the country, start at the Select a Station link above, and type in the name of your station. On the map that appears, click the map icon and then click 'more.' You can switch between the tabs to see the station information, annual summaries, and climate normals. Use the pull down menu to change to another year of interest.
  5. The National Weather Service (NWS) provides local climate records and summaries. Use the following instructions to locate a climograph for your area of interest. Note: instructions will vary by climate office; not all climate offices offer these types of graphs. A few that do include: Cleveland, Ohio, and Burlington, VT.
    • Go to the NWS home page Weather.gov, enter your city and state and click Go.
    • On the page that opens, click the forecast office title on the upper-left of the page. This will take you to the local climate office page.
    • Scroll down the menu list on the left-hand side of the page and click the link for 'Local' under the 'Climate' header.
    • On the page that opens choose the Local Data/Records tab. Look at the list of choices and locate the Climate graphs.

    Stop and Think

    1. List 5 cities or locations for which you examined dynamic climographs or accumulated precipitation maps. Include the date range that you observed. Tell whether each location is wetter than normal, about normal, or drier than normal. Explain your reasoning.

Defining drought

The word "drought" means different things to different people. What visions does the term bring to your mind?

A drought-stunted soybean plant withers in the Arkansas sun. Photo courtesy of USDA Natural Resources Conservation Service.

This rangeland in Hawaii is starting to show the stress of drought. Photo courtesy of USDA Natural Resources Conservation Service.

This lake near San Luis Obispo, California contains only a small bit of water following several years of drought. Photo courtesy of USDA Natural Resources Conservation Service.

Parched land, dried crops, dust storms, and starving livestock are some of the scenes that people associate with the term drought. Unlike most hazardous weather conditions, drought is not always obvious. Drought can be years in the making, as moisture in the soil evaporates and surface water sources disappear due to the lack of rain.

  1. Read the information at What is drought? to come up with your own meaningful definition of drought. Discuss your definition with a lab partner to see if it can be improved.

    Stop and Think

    2. Write a definition for drought, in your own words.
  2. Learn all about drought at the UNL Drought for Kids page.
  3. Find out what the how drought is studied by reading the links on the, How Do People Study Drought? page.
  4. Learn about the physical processes that cause or contribute to drought in Earth Observatory's North American Drought Article. Read the information about each contributing factor and view the animations about soil moisture (on the second page of the article). The animations will help you to visualize the feedback loop that exists among rainfall, soil moisture, and temperature.
  5. Checking In

    • What are some of the indicators that drought is present?
      Indicators of drought include soil moisture that is below normal, lower-than-normal rainfall or snowpack, and decreased water levels in streams and reservoirs.
    • The 3 main contributors to drought are high temperatures, low soil moisture content, and atmospheric circulation patterns that keep rain away from an area. Tell how each of these factors promotes drought.
      Higher surface temperatures result in an increase in evaporation of water. This leads to less moisture being available on the surface. If soils are wet, then much of the heat from incoming sunlight is used to evaporate the water they contain, so temperatures are kept cooler. If soil is dry, then there is little or no water available to evaporate and the land surface gets hotter and drier. Air circulation patterns are strongly affected by sea surface temperatures: air rises over areas of warm ocean water, pulling dry air across land.

Is it a drought today?

drought timeline
Based on the signs and symptoms of a drought, scientists classify drought as in one of three stages. Listed in increasing in severity, the three stages are: meteorological drought, agricultural drought, and hydrological drought.
  1. Examine the diagram to see the signs of meteorological, agricultural, and hydrological droughts.
  2. Interpret the chart to answer the following questions.
    • What are the causes of soil water deficiency?
    • If an area is experiencing reduced streamflow, which stage of drought is occurring?