Earth Science Data Visualizations – How to Read a Heat Map

This activity shows students how to read and interpret a common data representation, the heat map. Students will examine heat map representations of Earth science data over time, discuss trends and compare data sets in order to assess potential correlation.

• Download the appropriate data visualizations prior to class time. Familiarize yourself with the details of each heat map’s legend. If technology is available, have students watch the visualizations in pairs and write down observations, then compare with the rest of the class.

A heat map is a specialized chart that uses colors to represent data values in a table or physical map. Contrary to their name, heat maps don’t necessarily map temperature. Instead, they map the amount or concentration of certain measurements. Heat maps are mostly used to plot large and complex data such as Earth science measurements so they can be viewed as a whole data set at once. Heat maps can take the form of a rectangular chart, the cells of which contain numerical data. Or, more commonly in Earth science applications, heat maps are colors overlaid on a map of Earth. The colors represent variations in the data. The values the colors represent are indicated in the map legend and can take the form of a color gradient (range) or discrete colors.

Here are some examples of heat maps NASA uses to study changes on Earth:

• Land Surface Temperature
• Fire and Carbon Monoxide
• Chlorophyll and Sea Surface Temperature

Land Surface Temperature

Land surface temperature is how warm the “surface” of the Earth would feel to the touch in a particular location. From a satellite’s point of view, the surface is whatever it sees when it looks through the atmosphere to the ground. It could be snow and ice, the grass on a lawn, the roof of a building, or the leaves in the canopy of a forest. Thus, land surface temperature is not the same as the air temperature included in the daily weather report.

The land surface temperature maps used in this lesson were made using data collected during the daytime by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA’s Terra satellite. Temperatures represented on this map range from -25 degrees Celsius (deep blue) to 45 degrees Celsius (pinkish yellow). At mid-to-high latitudes, land surface temperatures can vary throughout the year, but equatorial regions tend to remain consistently warm. Antarctica and Greenland remain consistently cold. Elevation plays a clear role in temperatures, with mountain ranges like the North American Rockies cooler than other areas at the same latitude.

Scientists monitor land-surface temperature because the warmth rising off Earth’s landscapes influences (and is influenced by) our world’s weather and climate patterns. Scientists want to monitor how increasing atmospheric greenhouse gases affect land surface temperature and how rising land surface temperatures affect glaciers, ice sheets, permafrost, and the vegetation in Earth’s ecosystems. Commercial farmers may also use land surface temperature maps like these to evaluate water requirements for their crops during the summer, when they are prone to heat stress. Conversely, in winter, these maps can help citrus farmers determine where and when orange groves could have been exposed to damaging frost.

Fire and Carbon Monoxide

On Earth, something is always burning: wildfires started by lightning or people, controlled agricultural fires, or fossil fuels. When anything composed of hydrogen and carbon — whether it's vegetation, gasoline, or coal — burns completely in the presence of oxygen, the only end products are carbon dioxide and water vapor. But in most situations, burning is not complete, and fires or burning fossil fuels produce a mixture of gases, including carbon dioxide, methane and carbon monoxide.

The fire maps show the locations of actively burning fires around the world on a monthly basis, based on observations from the MODIS sensors on NASA's Terra and Aqua satellites. The colors are based on a count of the number (not size) of fires observed within a 1,000-square-kilometer area. White pixels show the high end of the count — as many as 100 fires in a 1,000-square-kilometer area per day. Yellow pixels show as many as 10 fires, orange shows as many as 5 fires, and red areas as few as 1 fire per day.

The carbon monoxide maps show the monthly averages of carbon monoxide at an altitude of about 12,000 feet, based on data from the Measurements of Pollution in the Troposphere, or MOPITT, sensor on NASA’s Terra satellite. Concentrations of carbon monoxide are expressed in parts per billion by volume (ppbv). A concentration of 1 ppbv means that for every billion molecules of gas in the measured volume, one of them is a carbon monoxide molecule. Yellow areas have little or no carbon monoxide, while progressively higher concentrations are shown in orange and red.

The comparison shows that fires and atmospheric carbon monoxide levels are very closely related for some regions and certain times of the year, but are less closely related in other places and times. For example, carbon monoxide concentrations across Africa and South America are directly correlated with fire counts there. When fire counts are high, carbon monoxide is high; when fire counts are low, carbon monoxide is low. These increases and decreases follow an obvious seasonal pattern linked to human cultural patterns of agricultural burning and land clearing.

In other parts of the world, however, carbon monoxide levels are elevated even during months when fire counts are low. About half way up the eastern coast of Asia, for example, a pocket of high carbon monoxide appears virtually year round, even when fires are not occurring nearby. Here, the carbon monoxide is part of the urban and industrial pollution generated in and around rapidly industrializing Beijing. A similar pattern exists over the United States, the North Atlantic, and Western Europe, which have relatively high (yellow) carbon monoxide concentrations even in December, January and February, when fire activity throughout the middle and high latitudes of the Northern Hemisphere is very low. That pattern suggests that the carbon monoxide is coming from the burning of fossil fuels (and also perhaps from wood-burning stoves or fireplaces).

Chlorophyll and Sea Surface Temperature

At the base of the ocean food web are single-celled algae and other plant-like organisms known as phytoplankton. Like plants on land, phytoplankton use chlorophyll and other light-harvesting pigments to carry out photosynthesis. Where phytoplankton grow depends on available sunlight, temperature and nutrient levels. Because cold waters tend to have more nutrients than warm waters, phytoplankton tend to be more plentiful where waters are cold.

The chlorophyll maps show milligrams of chlorophyll per cubic meter of seawater each month. Places where chlorophyll amounts were very low, indicating very low numbers of phytoplankton, are blue. Places where chlorophyll concentrations were high, meaning many phytoplankton were growing, are yellow. The observations come from the MODIS sensor on NASA's Aqua satellite. Land is dark gray, and places where MODIS could not collect data (because of sea ice, polar darkness or clouds) are light gray.

The sea surface temperature maps are based on observations by the MODIS sensors on NASA's Terra and Aqua satellites. The satellites measure the temperature of the top millimeter of the ocean surface. In this map, the coolest waters appear in blue (approximately -2 degrees Celsius), and the warmest temperatures appear in pink-yellow (35 degrees Celsius). Landmasses and the large area of sea ice around Antarctica appear in shades of gray, indicating no data were collected.

The highest chlorophyll concentrations, where tiny surface-dwelling ocean plants are thriving, are in cold polar waters or in places where ocean currents bring cold water to the surface, such as around the equator and along the shores of continents. The polar-equator difference intensifies in the hemisphere experiencing summer. For example, the difference in chlorophyll concentration between the Southern Ocean and equatorial ocean areas is bigger in December than September. In polar waters, nutrients accumulate in surface waters during the dark winter months when plants can’t grow. When sunlight returns in the spring and summer, the plants flourish in high concentrations.

Also, when surface waters are cold, it is easier for deeper water to rise to the surface, bringing nutrients to sunlit areas where phytoplankton can use them. When surface water is warm, cooler, nutrient-rich water is trapped below. Because the vertical layers of the ocean aren’t mixing, nutrients that have built up in deep waters can’t reach the surface. In places where ocean currents cause upwelling, sea surface temperatures are often cooler than nearby waters, and chlorophyll concentrations are higher. This connection is evident in multiple places. A band of cool, plant-rich waters circles the globe at the equator, with the strongest chlorophyll measurement appearing in the Atlantic Ocean and the open waters of the Pacific Ocean. In many coastal areas, the rising slope of the sea floor pushes cold water from the lowest layers of the ocean to the surface. The rising, or upwelling water carries iron and other nutrients from the ocean floor. Cold coastal upwelling and subsequent phytoplankton growth are most evident along the west coasts of North and South America and southern Africa. Following the density of phytoplankton gives scientists insight into the biodiversity of sea life, as organisms that eat the phytoplankton will be more abundant in these areas.

• What trends do you notice in the data comparisons?

Land surface temperature varies according to the season in each hemisphere. Land surface temperature has more variation at the poles than near the equator.

Fires influence CO concentrations, but not always in the immediate area of the fire. CO measurements are taken at 12,000 feet so atmospheric drift is a contributor to the mobilization of fire-induced CO as are other sources of CO such as the burning of fossil fuels.

Chlorophyll concentrations increase as sea surface temperature decreases.

• What advantage does a heat map have over other data representations?

A heat map allows us to look at a lot of data at a glance. Trends can easily be discerned and hypotheses formed that require more in-depth investigation of the data.

• How are color schemes selected for heat maps?

Sometimes they are random color gradients (ranges), but often they are colors associated with the represented phenomena.

Provide students with another heat map from NASA’s Earth Observatory or elsewhere on the internet and ask them to interpret what they are seeing, discerning correlation or potential causation when appropriate.

• View current data for:
  • land surface temperature
  • carbon monoxide
  • fires
  • chlorophyll
  • sea surface temperature
• Have students examine data sets from NASA’s Eyes on the Earth
• Graphing Sea Level Trends lesson
• Graphing Global Temperature Trends lesson
• Sea Level Rise lesson
• Research harmful algal blooms
• land surface temperature
• carbon monoxide
• fires
• chlorophyll
• sea surface temperature

• NASA’s Earth Observatory
• NASA’s Worldview
• Earth Vital Signs
• Earth Minute Videos