In the days following Wednesday 6 July 2011, stormy westerly conditions affected New Zealand. In this blog, we'll look at why.
The "Long Waves"
Below is the mean sea level analysis - the weather map - for 6am Sunday 10 July. In between big highs over the mid South Pacific and south of western Australia is a really large trough; it's the area shaded light blue. The weather map looked like this, more or less, since Wednesday 6 July: that is, the big features on it aren't moving much.