Passage: Weather is driven by air pressure, temperature and moisture differences between one place and another.  These differences can occur due to the sun's angle at any particular spot, which varies with latitude. The strong temperature contrast between polar and tropical air gives rise to the largest scale atmospheric circulations: the Hadley Cell, the Ferrel Cell, the Polar Cell, and the jet stream.  Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow.  Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year.  On Earth's surface, temperatures usually range ±40 °C (−40 °F to 100 °F) annually.  Over thousands of years, changes in Earth's orbit can affect the amount and distribution of solar energy received by the Earth, thus influencing long-term climate and global climate change.

Question: what causes extratropical cyclones?
instabilities of the jet stream flow