If we're coding a season plugin with a calendar system, why not add a temperature system as well? The plugin comes with one of the most advanced temperature systems ever created for this game. It is designed to not affect gameplay too much so it isn't annoying. Each player has a personal temperature displayed on their action bar that shows them their personal temperature, air temperature can also be displayed. Air temperature differs per biome.
Lots of things change the players' temperature: season, biome, armour, weather, time, water, food, height, sprinting, block-light and more. Check out Modifiers for a full list. The temperature will be recalculated every 2 seconds by default.
The players will be punished for having a bad temperature. The effects start at -10°C by default (there is a setting for Fahrenheit) and at 50°C. The effects include hunger, slowness, freezing (with damage), healing getting disabled and burning. See Effects for details. Some effects are only visual, like the player starting to sweat when their temperature is above 40°C or the players' breath becoming visible if the air temperature is below 0°C.
Temperature is displayed on the action bar by default. It sits slightly above your hunger bar, like this:
You can easily configure the location of this action bar and its colours. The action bar changes colour and displays a warning when certain effects trigger. The temperature can be displayed somewhere else using PlaceHolderAPI. The same for the air temperature that is only displayed in /season by default.