Fishermen Help

How to Read Sea Surface Temperature Images

(printable version)

Now we're going to give you all the information you need to know to become an expert at reading these images. There are 3 simple steps:

1. Image Navigation,
2. Is that a cloud?
3. Where are the fish?

1) Navigation - One of the most important things to remember in life is not to believe everything you read. Well, with satellite images, you shouldn't believe everything you see. The satellites are 500 miles up in space and moving at speeds of around 20,000 mph. Because of this excessive speed and some tilt and roll in the satellite, the image isn't exactly in the right spot most of the time. How will you know that? You must look at the image very carefully.

This SST image is not quite right. If you look at arrow A, you will see the coastline in the Cape May area. Now look carefully at the image itself. Notice that the deep red pattern is the same shape as the coast, but doesn't match up. This deep red area (hot temperature) is land. You can see that the coastline near point B should be matched up with the image near point C.

Now let's move the coastline to the correct spot.

Now you can see what it should look like. Notice that when we move the coast, the Latitude/longitude grid also moves. As a web site user, you can't move the coast, but you can now estimate how much error there is in the image navigation when planning your boat trip.

A good thing to note: Land appears much warmer than the coastal water in this image because the satellite pass was taken on a hot summer afternoon. Land heats up a great deal on a hot day while water does not. Just think about when you walk across a beach on a hot day. The sand can burn your feet, but when you run down to the water, it's about the same temperature all day long. The opposite is true with night time images. Land can get very cool at night, while the ocean water will not change more than 4 or 5 degrees over a day.

These two images are a good example of this diurnal (night/day) heat difference.

2) Is that a cloud? As I've already said, the satellites are 500 miles up in space. These satellites cannot see through clouds, so when there is a cloud above the earth, the satellite sees the temperature of the cloud and not the earth below it. Clouds are high up in the sky and are almost always cooler than the earth below. The farther you go up in altitude, the cooler it gets. Just ask a mountain climber.

So, knowing that clouds are cool, look at the image below. The areas marked A, B, and C are definitely clouds. They are so cold, they are off the temperature scale (white). Meanwhile, all of the ocean around them is green and yellow. Unless something very strange is going on, white areas in our imagery are RARELY water.

There is one other way of identifying a cloud. Clouds move rapidly over the earth at speeds between 15 to 50 mph. Water currents move at less than 3 mph. So, the temperature patterns in the ocean should not change quickly. If you can compare two images that are less than 24 hours apart, you will know if a pattern is a cloud because it will not be in one of the two images. For example, look at these two images above, specifically the areas marked D and E. In image 1, there is a cloud there (blue/purple), while in image 2 it has moved off to the east (marked cloud).

THE TEMPERATURE EXCEPTION - Upwelling - Sometimes during the summer, water from the ocean floor rises quickly from the bottom and can cool water temperatures 15 degrees in one day. These two images show one such upwelling event. They are only 20 hours apart, but the coastal water temperature dropped 10 degrees. In our area, these upwellings only occur along the coast. Find out more on Upwelling.

3) Where are the fish? Well, we aren't biologists here, in fact, most of us don't fish at all. But, many of the people who have used our web site over the years are fishermen and fishing boat captains. Everything we are about to tell you, is based on what we have learned from them.

On any day of the year, there is plenty of algae floating on the ocean surface. These plants are food for small fish. The small fish are food for medium size fish. Finally, the medium fish are food for the big boys (Tuna, Mahimahi, etc). So, if you think about it, generally speaking, wherever there's a lot of seaweed at the surface, you will probably find some sort of big fish down below. The tough part is finding these seaweed lines.

When you see huge temperature differences in an image, there are usually surface currents pushing toward each other in an area we call a convergence zone. The surface water, and everything in it, pushes together. When two water masses push towards one another, it's kind of like two conveyor belts facing each other (figure 2). But seaweed and algae float so they begin to pile up at the surface in large lines (Figure 2). Got it? The bottom line: Temperature fronts generally equal some fish.

All of our Sea Surface Temperature (SST) images come from NOAA/AVHRR satellites, which you can learn more about.