Understanding the long range forecast

Resolution counts

Weather forecasting is basically an ‘initial value problem.’ This means that if weather models could capture the current weather perfectly i.e. correctly initialise the current state of the atmosphere, then in theory, they could forecast the future weather accurately well beyond the present limit of a week or so. After all, the atmosphere is a fluid, and follows strict laws of physics.

What makes a rainbow?

An explanation of atmospheric optical phenomena

Here at MetService, people often send us photos of interesting clouds, unusual weather, and also atmospheric optical phenomena. Atmospheric optics is the branch of physics which describes how light interacts with the Earth’s atmosphere, to create a wide range of visual spectacles. Things such as rainbows, ice haloes, and crepuscular rays all come under atmospheric optics, along with many others. These can be observed all around New Zealand under the right conditions.

5 years on from the 'big snow' of August 2011

The photos in this blog were taken by MetService staff in August 2011.

Image by Ian Attwood

Photo taken by Ian in Karori, a western suburb of Welington.

We’ve just come out of a cold, snowy spell – the most significant of this winter so far. Ski fields nationwide are well topped-up, the Tararua range has been textured with snow after a pretty bare winter, and parts of the central North Island continue to suffer the effects of substantial snowfalls and drifts.

It's a bird! It's a plane! It's radar interference!

Weather radars are used all around the world to detect rain, hail and snow, as it happens. However, there are times when the radar network can pick up other things as well. While you generally won't see aeroplanes on our radar images (the radar beam is usually beneath the flight path of most flights, and also our radars are calibrated to primarily pick up smaller objects, whereas radars at airports are calibrated differently to primarily pick up aeroplane-sized objects), there is a myriad of other things that can cause interference on our radar images, which I will explain in this blog.

What is a fresh wind?

An explanation of wind speeds and the Beaufort Scale

Sometimes in MetService forecasts, you will see a forecast for “fresh northerlies”. But what exactly does the word ‘fresh’ mean? For many people, the word ‘fresh’ carries connotations of cool or clean air (eg the phrase ‘fresh air’). However, the word ‘fresh’ also has a more technical definition, that comes from the Beaufort Scale.

Mean Wind Speeds vs. Gust Speeds

A classic cold front on Sunday 24 April 2016

By meteorologist Emma Blades

As a cold front swept up the North Island on Sunday 24 April 2016 it was like a blanket had lifted leaving blue skies behind. In the classic cold-front scenario, showers usually follow the main rain band. But because the flow was west to southwest behind the front, the South Island was sheltering the North Island from the showers, resulting in clear skies behing the front in this case.

Planetary Waves and the Hovmöller Diagram

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.