If you were looking at radar imagery overnight Thursday 21 February 2013 or this morning (Friday 22 February 2013), you could be forgiven for thinking that there was quite a lot of light precipitation over the northern half of the North Island and west of Auckland.

MetService radar imagery from 7:00pm Thu-21-Feb 2013 to 11:20am Fri-22-Feb 2013

Except for parts of Gisborne and Hawkes Bay, we know that there was almost no precipitation over the North Island in the period covered by the radar imagery above.

So what was all that yellow radar echo over parts of the central North Island and around Waikato and Auckland?

We’re not completely sure, but we strongly suspect the echo is swarms of insects. To show up in radar imagery like this, they must have some of the characteristics of precipitation particles: similar size, fairly numerous, enough water content (or perhaps coating).

The overnight period Thursday 21 February / Friday 22 February 2013 isn't the only time lately radar echoes like this have been observed over the North Island. They've shown up, to a greater or lesser extent, most evenings in February. The weather over the North Island has been settled -- and very dry -- during this time.

Interestingly, there is quite a lot of literature on the subject of radar entomology.