Tag Archives | SST

Painting Temperatures by Number

By on June 21, 2011 in Design

Today is the Solstice, and pretty soon the dog days of sumer will be here. (Though, for those of us in the Northeast, we’ve already seen our fair share.) As things begin to heat up, I figured it would be appropriate to highlight some of the ways oceanographers visualize sea surface temperature data.

Temperature is a scalar variable, and its values are represented by continuous real numbers. Other variables types include categorical, vector or arrays (like RGB images), but we’ll save those for another day.

Temperature data collected at a singular point can easily be represented as numerical value (i.e. 76 degrees C), or if collected over a period of time, a time-series graph displaying temperature values over time can be easily drawn. Satellites, though, collect data over a large spatial area, meaning the dataset is inherently 3-dimensional. Each data point consists of a latitude (X position), longitude (Y position) and temperature (Z) value for each data point. The visualization challenge is figuring out how to display this 3D dataset on a 2D graph.

The most common solution is to create a pseudo-color image based on the temperature data. The trick is choosing the best colors to represent temperature values. The following are just a few examples.

The Scientist’s Rainbow

Perhaps the most common colormap, the rainbow (know as Jet in Matlab parlance) is often a scientist’s default choice. Like a Swiss Army knife, it is a good all-purpose tool. It is especially handy in helping subtle features stand out.

Sea Surface Temperature for June 2, 2011 from RU COOL

This Mid Atlantic image demonstrates how the rainbow palette allows you to easily distinguish small features within the range of a degree, even though the temperatures in the entire image range from 41 to 80 degrees Fahrenheit.

As useful as it is, the rainbow does have some major weaknesses. It is practically useless for colorblind viewers. In addition, it is important to keep in mind that non-scientists often associate rainbow representations with temperature, and only temperature. There is something intuitive about the red=hot and blue=cold extremes. Therefore, when creating images for public audiences it is unwise to use rainbow coloring for anything other than temperature, unless you enjoy confusing users.

An Alternate Choice

Sea Surface Temperature for May 2011 from NASA Earth Observatory

Given an infinite number of available colors (or at least 4 billion on modern displays), it’s easy to create an alternative color map. But balancing the need to highlight nuances in a dataset with accessibility for colorblind users while also keeping an image aesthetically pleasing is no small task.

Luckily, the artists at NASA’s Earth Observatory have come up with good alternatives over the years.

Hurricane Ready Temperatures

Hurricane-Ready Waters in the Atlantic, July 28, 2007

There is also no reason that colors need to be evenly distributed. If there is a scientific need to represent a dataset in a particular way, then adjusting the color map to highlight the message that needs to be communicated is certainly appropriate.

For instance, Tropical Storms develop best over waters that are above 80 degrees Fahrenheit. This image from NASA’s Earth Observatory, clearly shows those portions of the Atlantic Ocean that are ripe for hurricane development. For an alternate take, check out this recent image from NOAA’s Environmental Visualization Lab.

Temperature Anomalies

Sea Surface Temperature Anomaly for May 2011 from NASA Earth Observatory

Finally, sometimes it’s not the temperature value you want to represent, but the difference between the current temperature and a historical average. This is referred to as an anomaly, and it is often graphed slightly differently as it is a divergent dataset. Anomalies are typically represented as positive and negative differences, colored using red and blue respectively. Differences close to zero are left white.

This SST Anomaly image is from May 2011. These images are used to track El Nino and La Nina patterns, among other processes.

Well, that’s a quick roundup of some common examples. Do you have your own favorite way of coloring spatial maps of temperature or other datasets?

For more, check out the excellent primer Use of Color in Data Visualization by Robert Simmon.

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Sea Surface Temperature

By on July 6, 2010 in Better Know a Dataset

In the first of several series I hope to start on this blog, let start off with our first installment of Better Know a Dataset. Given that New Jersey is currently in the midst of a sweltering heat wave (record highs around 103 °F were set it many places today), it seems only appropriate that we should start with temperature.

Sea Surface Temperature, or SST for short, is the most common ocean data measurement you will run across. Temperature is the easiest variable to measure in the ocean (and anywhere else for that matter), and as a result you can easily find oodles of temperature data online, often spanning back several decades.

Surface actually is a relative term, as it depends on how the temperature is measured. Surface buoys typically include a temperature probe underneath the surface, about 3 feet down. This insures the temperature measurement reflects the water temperature only, and does not bob in and out of the water.

But perhaps the most popular SST datasets are those measured from satellites, like the AVHRR sensors on NOAA’s POES satellites. The advantage of satellite SST data is that it can map out entire regions of the ocean’s surface, rather than just one point. The disadvantages are that satellites may not pass over a region every day, they have a lower spatial resolution (typically 1km^2) whereas a buoy can measures small scale changes in temperature, and if clouds are present most satellite sensors can not see through them.

Satellites are able to record the temperature of the ocean’s surface by measuring the emitted infrared radiation from the ocean. In theory, this measurement is from the top few micrometers of the surface. In actual practice, given that the ocean is a very dynamic place with a lot of wind, waves and mixing, the top few micrometers are often essentially the same as the top few feet or more. However, on a calm sunny day the skin temperature may be several degrees higher than the true surface layer. In oceanography, nothing is ever easy.

The image above, from the Rutgers Coastal Ocean Observation Lab, is a quintessential example of a SST image. It shows snapshot of sea surface temperature that was recorded by the satellite NOAA-15 as it flew over the Mid Atlantic earlier today. The white area on the left is the East Coast of the United States. You can see Cape Cod near the top right and Cape Hatteras in the bottom left. The recognizable coastlines of New Jersey, Long Island and the Chesapeake Bay are also clearly visible.

The temperature data in this image is colored using the typical scientist’s palette, that is, the color scaling uses the full rainbow of colors to show a full range in temperatures. While this is not a very intuitive color scale (a topic we’ll delve into in future posts), it is one that is commonly used by scientists to extract as much detail from the data as possible.

Red and orange areas depict the warmest areas while blue and purple areas are the coolest. Many web sites use a static color map throughout the year. However, due to the extreme range in ocean temperatures over the course of a year in the Mid-Atlantic and the desire of scientists, fisherman and others, to be able to extract all the features possible out of these images, Rutgers constantly changes the scales on their color map to keep up with the changing temperatures. This means you need to constantly check the color scale, as red areas on one image may not be the same temperature as red areas on another image.

There are several notable features in this image.

  • The white and purple patchy areas on the right are actually clouds that are obscuring the ocean surface and somewhat corrupting the data. (Scientists would flag this data as questionable and not use it in their research.)
  • The red ribbon of warm water toward the bottom of the image is the Gulf Stream.
  • Above the Gulf Stream and in the midst of the clouds, you can see a warm core eddy, that shows up as a red circle. This eddy was originally part of the Gulf Stream before it split off and started heading back towards the coast. The eddy itself rotates in a clockwise direction.
  • Off the coasts of Delaware, New Jersey and Long Island the temperatures are somewhat cooler than further offshore. This is due to coastal upwelling. Over the past week, winds in the area have been from the Southwest, which when combined with the forcing from the Earth’s rotation, drives surface waters offshore and allows cooler waters to come to the surface. Upwelling also brings nutrient-laden bottom waters to the sunny surface where phytoplankton can have a feeding frenzy.

As you can see, sea surface temperature maps provide a rich dataset to analyze ocean currents and features that impact the biology and chemistry in the ocean.

Of course, they can also help you find a cool beach on a hot day.

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