The Weather System is one of the most advanced parts of the game. It basically define the temperature of a region, and how your HUD will react to this.
How It Works
When a player join the game and the account is created, the players receives two values, that defines the range of temperatures that don't affect the Comfort. This range is from 16C to 25C. For the game the SL default temperature is 20C. So, whenever the player goes that have the default temperature, the player will be inside its "Comfort Zone".
But the temperature in a place can be changed, as we will see, and when the temperature is outside the player's comfort zone, the HUD will react, and the Comfort will start to drop, also the HUD will show an icon right to the status bars, with a sun when it is hot, and a snowflake when is cold.
The drop rate of the Comfort will increase when the difference between the player comfort zone and ambient temperature is higher. For example, your comfort will drop much faster if you are in a local with -5C than in one with 3C. To reduce this drop, you can use a Freeze or Warm Protection consumable. When you are in a place inside your comfort zone, your Comfort will start to recover very slowly. To recover it quickly, use a Comfort Recovery consumable.
Another way to reduce the drop rate is being near an Ambient Modifier.
Setting Weather for a Region
When a specific temperature for a region isn't found, the HUD will determine it is the default one, 20C. But, in some cases, a region can have a different temperature. A specific region temperature is defined in the game server by the staff, and can be request only by the owner of the region. For those that want to change the temperature in their lands, the parcel and area thermostats are the available options.