clarify default temperature to ambient and describe how to set ambient temp/gradient

Signed-off-by: Ashton Larkin <ashton@openrobotics.org>

Author: adlarkin

tutorials/03_thermal_camera.md

@@ -125,11 +125,33 @@ Here's a description of the elements in this plugin (if the plugin isn't used, t
 ## Assigning a temperature to a model
 
 Now that we have set up our thermal camera, we'll need to assign temperatures to models in the environment.
-If a model doesn't have a temperature associated with it, then the thermal camera cannot detect it.
+If a model doesn't have a temperature associated with it, then the model's temperature is set to the ambient temperature.
+There will be some variation in the model's temperature based on the model's color and the world's temperature gradient.
 
 The thermal camera can support objects that either have a uniform or varying surface temperature.
 We will go over each approach in the following subsections.
 
+### Setting atmospheric temperature properties
+
+It is recommended to specify the ambient temperature and world's temperature gradient, since this dictates how objects with no specified temperature appear.
+Here's an example that sets the ambient temperature to 300 kelvin, and the temperature gradient to 0.1 Kelvin/meter:
+
+```xml
+<atmosphere type="adiabatic">
+  <temperature>300</temperature>
+  <!--
+    This is a more exaggerated temperature gradient, which produces a
+    temperature range of ~11.5 kelvin for objects in the thermal camera
+    view that don't have a user-specified temperature.
+    Typical temperature gradient is -0.0065 K/m which produces a
+    temperature range of 0.75 kelvin.
+  -->
+  <temperature_gradient>0.1</temperature_gradient>
+</atmosphere>
+```
+
+The default values for ambient temperature and temperature gradient can be found in the [SDFormat Specification](http://sdformat.org/spec?ver=1.7&elem=world#world_atmosphere).
+
 ### Objects with a uniform temperature
 
 Here's an example of a box model that has a uniform temperature assigned to it: