Read the Sky: Clouds and Weather

Clouds are the atmosphere showing its working, and they were the original forecast tool long before models and phone apps turned up.

They still work, because a cloud tells you what the air is doing at a height you have no other way of measuring.

High clouds

High cloud sits roughly 5 to 13 km up (16,000 to 43,000 ft) in our latitudes, cold enough to be made entirely of ice crystals.

Cirrus, the thin wispy filaments old hands call mare's tails, means fine weather right now and nothing that should change your plans.

What matters is the trend, because cirrus that thickens and spreads across the sky is the leading edge of an approaching front.

Once it starts filling in, expect a change within 24 to 48 hours.

Cirrostratus is a milky veil over the whole sky, which is cirrus that has thickened and joined up as the front closes in.

The giveaway is a halo ring around the sun or moon, caused by light bending through all those ice crystals.

See a halo and you are usually looking at rain inside 12 to 24 hours.

Mid clouds

Mid cloud lives between 2 and 7 km (6,500 to 23,000 ft) and is a mix of water droplets and ice.

Altostratus is a grey or blue-grey sheet where the sun looks like it is behind frosted glass, dim but with no halo.

The missing halo is the useful bit, because it tells you the cloud has thickened well past cirrostratus and the front is close.

Rain is not far behind, usually hours rather than a day.

Altocumulus turns up as patches or rows of white and grey lumps, the mackerel sky, and on its own it is harmless.

The exception is altocumulus castellanus, which carries little turret-shaped tops and tells you the mid-level air is unstable.

Turrets in the morning sky are a solid hint that thunderstorms will fire in the afternoon.

A halo around the moon buys you about half a day before the rain arrives.

Low clouds

Low cloud runs from the surface up to about 2 km (6,500 ft), and it is what decides whether your morning is grey or golden.

Nimbostratus is a dark featureless grey layer with no visible base, and if steady rain is falling this is the cloud doing it.

It sits at the end of the warm-front sequence, so the rain has already arrived and will hang about for hours.

Stratocumulus forms low lumpy rolls and patches, and it is the standard winter morning cloud right along the southern coast.

Expect a grey start and maybe light drizzle, rarely anything serious, and it often lifts or breaks up by the middle of the day.

Clouds that grow upwards

These ones are built by surface heat and instability rather than a front sliding over, so they change fast through the day.

Cumulus is the classic fluffy white cloud with a flat base and cauliflower top, and small scattered ones mean fair weather.

They grow as the ground heats up, so watch their height rather than how many there are.

Cumulus that keeps stacking vertically after midday is telling you the atmosphere has more energy than it is letting on.

Cumulonimbus is the big one, a tower that can punch 12 km (39,000 ft) up with a flat anvil spreading out from the summit.

That anvil is ice being sheared off the top of the storm, and it is the clearest visual warning the sky gives you.

Underneath sits heavy rain, lightning, savage gusts and sometimes hail, so if you see an anvil on the horizon start heading in.

Quick reference

• Cirrus (wispy, high): fine now, change in 24 to 48 hours
• Cirrostratus (halo): front approaching, rain in 12 to 24 hours
• Altostratus (grey veil, no halo): rain very soon
• Altocumulus (mackerel sky): harmless, unless it has turrets
• Nimbostratus (dark, low, no base): raining now and set in
• Stratocumulus (low lumpy rolls): grey morning, possible drizzle
• Cumulus (small, fluffy): fair weather
• Cumulus building tall: storm under construction
• Cumulonimbus (anvil top): thunderstorm, get off the water

The two sequences to know

Warm front, which runs cirrus, then cirrostratus with a halo, then altostratus, then nimbostratus and steady rain, over 12 to 24 hours.

Every step is the same cloud deck getting thicker and lower as the sloping front edge slides over the top of you.

Afternoon storm, which opens with small cumulus in the morning, towers after midday, cumulonimbus by early afternoon and a storm around 3 to 4pm.

That one is common inland through summer and close to daily on the Queensland coast in the build-up.

The old sayings

Red sky at night, sailor's delight, holds up reasonably well, because a red sunset means dust and stable air away to your west.

Red sky in the morning, sailor's warning, is the same logic reversed, with moisture-laden air sitting to the east of you.

Using it on the water

Cloud reading covers now and the next few hours, which happens to be the window a forecast handles worst.

Run the two together: take the broad setup off a synoptic chart, then let the sky confirm or contradict the timing.

If the sky is running ahead of the forecast, back the sky, and check the QLD warnings when storms are in the mix.

Common questions

How do I tell cirrostratus from altostratus?

Look for the halo, because cirrostratus is ice and bends light into a ring, while thicker altostratus dims the sun and shows nothing.

Does a mackerel sky mean rain?

Not on its own, but if the altocumulus carries turret tops in the morning, the mid-level air is unstable and storms are likely later.

How far off is a cumulonimbus I can see?

An anvil is often visible 50 km (30 miles) or more away, so a storm on the horizon can still reach you within a couple of hours.

Why is my coastal winter morning always grey?

Stratocumulus, which forms in a shallow moist layer under stable air and generally breaks up once the surface warms through the morning.