How are these maps made?

What is a millibar?

Chart of Earth's atmospheric layers

What do you mean by "total column water vapor", and why are you measuring it in units of millimeters?

This map shows the total amount of water vapor present in the atmospheric column above each point of the Earth's surface. If all the water vapor in the column were forced to fall as rain, the depth of the resulting puddle on the surface at that point is equal to the value shown on the map. Fifty millimeters is about 2.5 inches.

The atmosphere can hold very little water vapor at extremely cold temperatures, which results in the strong correspondence between dry areas in this map and cold areas in the temperature maps. The large area of maximum water vapor in the neighborhood of the equator is the Intertropical Convergence Zone or ITCZ, a region of strong convection and powerful thunderstorms.

What do the colors mean?

The color bar is the key to reading the map. The bar will tell you the lightest beige color in the map shows where water vapor amounts are close to 10 mm. Green is approximately 40 mm, and dark blue tops out at 70 mm.

Why are there "stripes" of color in the real-time map?

The AIRS instrument on the Aqua satellite orbits Earth from pole to pole. Imagine if you held a beach ball and tried wrapping a ribbon around the ball, going from top to bottom then bottom to top. Depending on the width of the ribbon, it can take many loops to cover the entire ball. AIRS orbits Earth 15 times a day--not enough orbits for AIRS to completely cover the globe. The black areas between the colored stripes show where AIRS has not yet retrieved data. We call these areas "gores". The stripes in the map shows where AIRS collected data in it's orbital path. We call this stripe a "swath".

> View an Aqua satellite orbit animation

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What do the black areas in the map signify?

In addition to the gores explained in the paragraph above, note that AIRS was designed to retrieve data in clear skies or skies with speckled or thin clouds. So black areas can also show where dense clouds have collected.

AIRS, the Atmospheric Infrared Sounder, is an instrument on an Earth-orbiting NASA satellite called Aqua.