Anticyclones are usually associated with light winds and sunny skies but they also have some undesirable attributes, especially when they get blocked and linger. In the last FMC Bulletin, we examined how a High created gale force winds in the Tararua Range, contrary to what a basic reading of the weather map may have suggested (see 8 below).
It was just one of 10 reasons to ‘Hate a High’.
1. Near the centre are dead winds and usually an area of low cloud causing dull days called ‘anticyclonic gloom’, or dirty air which may turn into fog.
2. Round the rim, winds are strong. If the central pressure is over 1030 ‘it’s going to get dirty’ so look for a gale somewhere on the outside of the High.
3. Highs intensify the trade winds in the tropics. It may take about a week for a High to travel eastwards past New Zealand, and during this time the stronger trade winds tend to give night-time rain to the eastern side of the larger tropical Islands. In Fiji this is sometimes called Bogi Walu.
4. The bigger the Highs are, the slower they move, blocking the fronts and lows that are trying to follow them. When this block is released, the western (back) end of a high may become a breeding ground for storms.
5. Intensifying Highs tend to squash together the isobars between themselves and any nearby low pressure centres, creating ‘squash zones’.
6. A deepening low-pressure system and an intense or lingering anticyclone get together like the arms of an eggbeater and create a zone of enhanced wind and rain.
7. As air flows around a High, it spins out across the isobars and speeds up until it is as much as 20% more than that indicated by the isobar-spacing.
8. If a mountain range blocks the air flowing around a High, the air tends to squeeze around the mountains rather than flow over them. This splits the wind flow over New Zealand into rivers of wind and puddles of calm. Sometimes a narrow gap is made just above the mountains through which pent-up air may be suddenly released at a rapid rate.
9. In winter and spring a High may bring unwelcome frost.
10. In summer and autumn a High may allow sea breezes to converge and, if it is cold enough aloft, this can form thunderstorms and hail.
Bob McDavitt retired from the MetService in 2012. FMC thanks Bob for his valued contribution as the Bulletin weather columnist over many years. This column was originally published in the November 2009 FMC Bulletin.