Why clouds can move one way, but the rain on radar moves another

A really good question came up yesterday:“Why do the clouds look like they’re streaming one way, but the rain on radar looks like it’s moving another?”

If you’ve ever watched a wet-season low on satellite and radar and thought “that doesn’t match”, you’re not imagining it. It’s actually a clue about what’s happening at different heights in the atmosphere.

What a low is doing down low (near the ground)

A low is simply an area of lower air pressure — meaning there’s a bit less “air weight” sitting over that spot.

Nature hates a gap, so air from surrounding areas flows in to fill it.

In the southern hemisphere, that inflow spirals in clockwise. So instead of air going straight into the centre, it wraps in like a tightening spiral. That’s the low-level circulation you often see hinted at in the wind streamlines.

Why that creates rain bands

As air converges at the surface (piling in), it can’t just keep stacking up at ground level forever.

It’s forced upward.

When air rises, it cools. Cooling air can’t hold as much water vapour, so that moisture condenses into cloud. Keep that going long enough and you build rain bands and thunderstorms.

That’s why lows often have spiral-shaped rain areas:

• The low keeps feeding moisture inward

• Convergence keeps forcing it up

• Rain keeps regenerating in bands that curve around the circulation

What a low is doing up high (and why cloud can drift away)

Now jump higher up in the atmosphere.

Air that rises through storms can’t rise forever, so it spreads outward at upper levels. That’s called outflow — the system “venting” at the top.

This matters because it helps the system keep going. Think of it like taking the lid off a pot:

• If the top can vent, more air can rise underneath

• If the top is blocked, the system struggles

Here’s the key point for the cloud-vs-rain question:The winds up high can be a totally different direction (and usually faster) than the winds near the surface.

So the high cloud shield can be blown away one way, while the low-level rain bands are still being wrapped around and drawn in toward the low.

Why radar can look “backwards” compared to satellite

Radar mainly shows what’s happening in the lower part of the atmosphere (where the rain actually is). Satellite (especially water vapour / high cloud) often highlights what’s happening higher up.

So you can get situations like this:

• The overall rain band shape curls in toward the low (because of the low-level inflow)

• Individual rain cells within that band can be “steered” along by winds above the surface

• The high cloud can streak off in yet another direction because upper winds are doing their own thing

That’s why radar sometimes looks like rain is drifting away from the centre even though the band is still feeding inward:the band is being rebuilt on the inflow side while older parts of the band get carried along and decay.

What today’s images show with the current system

Looking at your two frames (mid-morning and early afternoon):

• There’s a broad low circulation sitting over the Gulf Country / NT–QLD border region, with a clear clockwise inflow pattern

• Rain and storms are wrapped around the system, including heavier bursts where bands tighten into the low near the Gulf communities

• The Top End is active too, with storms/heavier rain around Darwin / Arnhem Land and nearby waters feeding in from the west/northwest

• There’s also a separate active area along the North QLD coast and adjacent waters — deep tropical moisture focusing into coastal convergence zones (common when an inland low is part of the broader monsoon pattern)

So if you’re watching cloud movement on satellite and thinking “everything is streaming one way”, that’s likely the upper-level flow.But the rain bands on radar are being shaped by the lower-level inflow into the low, which curls and wraps around the circulation.

Infamous lows and ex-tropical cyclones: when the “rain engine” gets stuck on

Most wet-season lows are normal business. The trouble starts when a low:

• Moves slowly or stalls

• Stays connected to deep tropical moisture

• Keeps reloading the same areas for days

A few classic examples that remind us what a “stuck rain engine” can do:

• 1974 Brisbane floods — Tropical Cyclone Wanda

• 2010–11 QLD floods — La Niña season + repeated heavy rain (TC Tasha part of the early setup)

• 2013 — ex-Tropical Cyclone Oswald (tropical low)

• 1998 Townsville — “Night of Noah”

• 2019 North QLD floods (Townsville region) — monsoon trough + prolonged heavy rain

The lessons learned (and why this helps us prepare)

These events keep repeating the same ingredients, so it pays to watch the right things:

• Don’t fixate on cyclone category or wind alone. Flood risk is more about rainfall persistence, catchment wetness, and how slow the system is moving

• A low can be “just a low” and still be dangerous if it sits in one spot

• Ignoring warnings doesn’t just risk your life — it risks the lives of the emergency crews who may have to come and get you.

• The biggest preventable tragedies keep coming back to driving into floodwater

Practical take for today

• Use radar and satellite together: radar for where it’s raining now, satellite for the moisture feed and cloud structure

• If you’re travelling anywhere near active bands: have a Plan B and don’t push through water on roads — it’s not worth it