Day-to-day weather features such as fronts and troughs, highs and lows constantly wash against New Zealand in the global atmospheric circulation. Sometimes the weather in New Zealand is highly changeable, changing from heavy rain to blue skies with an almost daily frequency. Other times the weather can seem to be stuck one way or another for days. In this blog we discuss the reason for this: Planetary waves, also known as Rossby waves. We will also look at the tools forecasters have to analyse them, and the implications for New Zealand.