Why spring does not always mean the end of winter

Last week, intense snowstorms killed many lambs on high-country farms of the North Island.

Hundreds of people shivered in their homes without power for days when powerlines were brought down by blizzard-like conditions, which also stranded motorists on the Desert Rd and Napier-Taupo highway.

To quote MetService weather ambassador Bob McDavitt, these storms were "very late, seeing that we were well past the equinox (September 21) and we are now just over two months away from the longest day, or midsummer (December 21)".

Another MetService forecaster, Peter Smith said, "Cold snaps like this were quite common in spring. Spring is a season where you can have quite changeable, unsettled weather, so it's not unheard of to have this sort of cold snap".

Two questions spring to mind: why during longer days with more sunshine hours do we get such cold snaps, and where does the heat from the warmer August and September months go?

While there is still considerable public debate regarding climate change, scientists do have a good understanding of the dynamic nature of the Earth's atmospheric circulation, which brings about such contrasting weather events.

The atmosphere is our protective blanket, nurturing life on Earth. It acts as a protector from the hostile environment of empty space and high-energy ultraviolet and other ionising radiation streaming earthwards from nuclear fusion reactions in the sun.

The atmosphere is dynamic and transports water from the oceans to the land, acting as a big condenser in a huge solar-powered still. Salt water is purified through evaporation, leaving the salt behind in the ocean, and subsequently condenses as rain.

Fortunately, the surface air, known as the troposphere, is homogeneous, mixing easily on a global scale. However, this mixing can cause high winds, from mixing different air masses, causing other atmospheric phenomena, such as lightning and thunderstorms, which we call weather.

Weather starts with wind, which is caused by different air masses with contrasting temperatures rising from different parts of the Earth, the hot equator contrasting with the cold North and South poles. There are also the dark, cool side and sunny, warm side of the Earth, which change every 12 hours. The atmosphere containing the envelope of gases surrounding the Earth is divided into different regions, depending on altitude.

Atmospheric chemistry and behaviour varies a great deal with altitude. The hot equatorial air rises to the top of the troposphere (the lowest layer in the atmosphere), which extends up to about 16km above the Earth's surface.

The air cools as it rises, moving towards the two tropics in the two hemispheres in what are called Hadley Cells.

When the cooler air reaches the surface at the tropics, some of it travels along the Earth's surface back towards the equator, replacing the rising equatorial air. Air at the equator's surface does not move much – known as the Doldrums – with the consequence that the northern and southern-hemisphere air masses do not mix easily. At the tropics, the high, cooler equatorial moisture-laden air brings high rainfall.

The condensation of water in the form of rain releases stored latent heat energy, which warms the surface tropical air, causing it to flow along the Earth's surface away from the tropics towards the poles, meeting colder air, which causes it to rise again.

The air in the troposphere around the Earth is therefore homogeneous, keeping our atmosphere constant and relatively stable.

More locally, the recent burst of cold air from the Southern Ocean was a reminder that spring is not necessarily the end of winter in New Zealand.

A high-pressure system to the south of New Zealand, cold Southern Ocean air trapped in a cold front, and a low-pressure system over the North Island reminded us that spring is not always a smooth transition between winter and summer.

David Shillington is head of Universal College of Learning's School of Applied Health Sciences.

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