By default, your Flight Conditions are set to Clear Skies. To set your Flight Conditions to Live Weather by default you need to go to the World Map.
Open Flight Conditions at the top right of the screen.
The first thing you will see is the Weather & Time information. Clear Skies is the Presets by default.
You can start by changing the presets from Clear Skies to Live Weather.
Once done. You can go back to the World Map menu and set your flight. You can easily verify if the Live Weather is enabled before a Flight by looking if the Live Icon appears in the Flight Conditions box.