Reading the Weather Forecast for Stargazing

In New Zealand the weather decides more observing sessions than any telescope does. We live on narrow islands in the middle of a big ocean, under the westerlies, with mountain ranges that make their own weather. Forecasts here are good, but reading them for astronomy is a slightly different skill from reading them for a barbecue.

Start with the right forecast

The MetService rural forecasts are often more useful to an astronomer than the town forecasts. They describe the weather across a whole region, include hourly model data for wind, cloud and rain, and give a better feel for what is happening away from the city where you will probably be observing. The national overview is worth a glance first to see the big picture: where the highs and lows sit, and which way fronts are moving.

NIWA publishes climate summaries and longer-range outlooks that help with planning trips weeks ahead. They won't tell you about a particular night, but they will tell you whether a month is likely to be drier or cloudier than usual.

Cloud is not a single thing

"Partly cloudy" covers a lot of ground. For astronomy it helps to think about three kinds:

• Low cloud (stratus, stratocumulus) is thick and blocks everything. It often forms overnight near coasts and in valleys.
• Middle cloud (altocumulus, altostratus) usually arrives ahead of fronts. A thin sheet can be bright enough to wash out faint objects while still showing the Moon and planets.
• High cloud (cirrus) is the sneaky one. It can leave the sky looking clear to a casual glance while killing contrast on galaxies and nebulae. Halos around the Moon are a sure sign. Our page on sun halos explains the ice-crystal optics behind them.

Satellite imagery and cloud charts on forecast sites show whether a clear gap is widening or closing. Watching the loop for a few hours before dark tells you more than any single snapshot.

Seeing versus transparency

Astronomers use two words for sky quality, and they don't always go together.

• Transparency is how clear the air is: how faint a star you can see, how dark the background is. Dry, clean air after a front brings excellent transparency.
• Seeing is how steady the air is. Turbulence makes stars twinkle and planets boil at high magnification. Steady air often comes with calm, slightly hazy high-pressure conditions.

So a crisp, sparkling night after a cold southerly might be wonderful for the Milky Way and terrible for Jupiter. A still, slightly murky night under a high might be the reverse. Match your targets to the night.

Wind

Wind matters twice: at ground level it shakes telescopes and chills observers, and higher up it drives turbulence that ruins seeing. Strong jet stream winds over the country usually mean twinkling stars and poor planetary views, even when the ground is calm. Upper-level wind or jet stream charts, which several forecast and astronomy weather sites publish, are the place to check this.

The nor'west arch

Anyone in Canterbury, Otago or Marlborough knows the nor'west arch: a band of cloud over the Southern Alps with a strip of clear sky beneath it, formed as moist westerly air is lifted over the ranges and dries out descending the eastern side. East of the mountains, a nor'wester can bring clear but very turbulent, gusty conditions. The sky may look inviting, but seeing tends to be poor and the wind makes observing hard. On the West Coast at the same time it is usually raining.

Southerly clearances

A southerly change sweeps up the country behind a cold front, often with a band of showers. Once it passes, the air can be exceptionally clean and transparent, with stars right down to the horizon. These are the nights for wide-field Milky Way viewing and astrophotography, provided you have warm clothes. Expect cold temperatures, lingering showers on south-facing coasts and some wind, especially early in the clearance.

They are also good aurora nights, because the southern horizon is often clear. If the space weather looks active, combine the two: our guide to reading an aurora forecast covers that side of things.

Sea fog and valley fog

Coastal observers know the frustration of a clear forecast followed by fog rolling in from the sea after dark. Sea fog forms when moist air moves over cooler water, and it is common on some coasts in spring and early summer. Inland, still clear nights let cold air drain into valleys and pool, forming radiation fog by the early hours. If your site is low-lying, a short drive uphill can put you above it.

Dew

Clear, calm, humid nights bring heavy dew. If the forecast shows a small gap between the overnight temperature and the dew point, expect optics to fog up unless you use dew shields or heaters.

Planning around the Moon

The Moon is the one part of the forecast that is perfectly predictable. A bright Moon washes out faint objects, so plan deep-sky sessions around new moon, and save the brighter phases for the Moon itself, planets and double stars. Our Moon page has more on what to look for when it is up.

A simple routine

1. A week ahead, note the Moon phase and check longer-range outlooks.
2. Two or three days ahead, look at the national pressure map for highs and fronts.
3. On the day, check rural cloud and wind forecasts and satellite imagery.
4. At dusk, step outside and look for cirrus, haloes and twinkling.

Forecasts will still surprise you. That is part of observing in New Zealand, and the nights that clear unexpectedly are often the most memorable.