Giraffes and planets

Giraffes are covered in patches.  The patches are brown and the space in between the patches is white.  Everyone knows this.

Here is something you probably didn’t know: you can see those patches in infrared light.  In simple terms, infrared light measures heat (at least at the temperatures we are used to in everyday life).  A hot potato is brighter in infrared light than a cool potato.  You get the idea: bright = hot.

A giraffe in visible light (left) and in infrared light (right). The color bar shows how the brightness corresponds to temperature (in degrees Fahrenheit). Credit: NASA.

Giraffes use their patches to cool off.  They have a system of blood vessels under each patch.  They use their patches as “thermal windows” to get rid of extra heat.  A giraffe only heats up a fraction of its body (the patches) but it cools off efficiently.  Why does this work?  Because the amount of energy that an object radiates depends very sensitively on its temperature.  If you double the temperature of a potato you make it 16 times brighter!  A potato can cool off twice as fast by doubling the temperature on just one sixteenth of its surface.  Likewise, a giraffe cools off faster by increasing the temperature by a few degrees under its patches.

Planets also use hot zones to cool off.  The hottest places on Earth — deserts — emit the most energy.  This image shows that the Sahara desert emits way more than its share of heat:

A map of the energy emitted by the Earth in infrared light. Redder means The top panel is the average during July 2001 and the bottom panel is an average over the entire year of 2011. From Gomez-Leal et al 2012.

The Sahara desert helps Earth keep cool!  It’s not the only thing emitting infrared energy of course: how a planet cools is a very complex process.  But this is important to know.  A planet with completely uniform temperature is not very good at cooling off.  But a planet with a few hot spots (like deserts) can beam a lot of excess heat into space and keep cool.  This is exactly what happens (with a few more details like humidity and clouds included) in 3-dimensional climate simulations.

There you have it: giraffes and planets have something in common.  And deserts actually keep the planet cool!

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