When someone tells you their laptop only plays up when it's raining, you brace yourself. In fifteen-odd years of repairs, you hear a lot of things that sound like they couldn't possibly be true — and sometimes they aren't. But sometimes, the pattern is real, and finding out why is what makes this job genuinely interesting.

This one came in on a grey Tuesday in February. The customer — let's call her Margaret — had a mid-2019 MacBook Pro that had been behaving strangely for about six weeks. It would freeze. Sometimes it would shut itself off completely. The screen would occasionally flicker before going black. She'd tried restarting it, which usually worked fine, and life went on.

The thing she'd noticed — and half-apologised for mentioning — was that it seemed to happen more on wet days. She commuted by train a couple of times a week, and on dry mornings the MacBook sat in her bag without complaint. On rainy mornings, she said, there was a decent chance it would misbehave by lunchtime.

What She Thought Was Wrong

Margaret had done what most of us do: she'd Googled it. The internet had helpfully suggested that her MacBook might have a failing logic board, that humidity could affect electronics, or that her battery was dying and needed replacing. She'd come in expecting to hear that it needed a new battery at minimum, and possibly something more serious and expensive.

She wasn't wrong that humidity can affect electronics. But the truth turned out to be both simpler and more specific than that.

What the Diagnostics Showed

The first thing we do with any Mac that's behaving erratically is run Apple's built-in hardware diagnostics, check the system logs for crash reports, and have a look inside if anything in the history suggests liquid. The diagnostics came back clean — no memory errors, no obvious hardware failures flagged. The crash logs were interesting, though: the shutdowns were thermal events. The machine was overheating and cutting out to protect itself.

That moved the investigation in a clear direction. We opened it up.

Inside, the MacBook was in decent condition overall — no signs of a serious liquid spill, no corrosion on the logic board. But when we pulled the bottom case off and had a proper look, two things stood out. First, the fan was absolutely caked in lint and fine grey dust — the kind that builds up over a few years of daily use, especially in a bag. Second, and this was the key detail: the thermal paste between the processor and its heatsink had dried out almost completely. It had shrunk away from the edges and gone powdery.

On its own, either of those things will cause a Mac to run hotter than it should. Together, they were a reliable recipe for the machine to hit its thermal limit and shut down under any meaningful load.

So Why Did Rain Make It Worse?

This is the part Margaret found surprising — and honestly, it's the part that makes this case worth writing about.

On dry days, she walked about ten minutes to the station and the MacBook sat in a warm bag. By the time she got to her desk, the machine was at room temperature and the fan — clogged as it was — could just about cope with a normal workload.

On wet days, she was rushing, often damp when she got on the train, and she'd sit with the MacBook open on her lap for the forty-minute journey to work — running her emails, sometimes with a video call already underway before she reached the office. The machine was being asked to do more, sooner, before it had settled, in a slightly more humid environment that meant the already-poor thermal paste was conducting even less efficiently. The margin between "just about coping" and "thermal shutdown" was razor thin — and on rainy days, that margin disappeared.

It wasn't magic. It wasn't weather sensitivity. It was a chain of small, compounding problems that only tipped over the edge under specific conditions.

The Fix

The repair itself was straightforward:

  • Full strip-down and thorough clean of the fan and heatsink fins — a surprising amount of compressed-air-resistant compacted dust came out
  • Old thermal paste removed carefully from the processor die and heatsink
  • Fresh, quality thermal paste applied correctly
  • Reassembled, tested under sustained load for over an hour

Temperatures under full load dropped by nearly 25°C compared to what we'd measured before the clean. The fan — which had been spinning at maximum speed and barely keeping up — settled down to a sensible pace. Margaret picked it up the next day, and as luck would have it, it was raining.

She messaged a week later to say it hadn't skipped a beat.

The Practical Lesson

There are a couple of things worth taking away from this one.

The first is that intermittent problems almost always have a consistent cause — even if that cause only shows itself under particular conditions. "It only does it sometimes" is frustrating to hear, but it's not a dead end. The conditions matter enormously, and describing them carefully (like Margaret did) is genuinely useful for diagnosis.

The second is about thermal paste and fan maintenance. On any laptop that's three or four years old and used daily, the thermal paste is probably due for replacement. Most manufacturers use compound that degrades over time, and a dried-out application is far more common than people realise. It's also one of the most cost-effective services we offer — the difference in temperatures, fan noise, and general responsiveness can be dramatic.

If your Mac or laptop feels warmer than it used to, if the fan runs more often or more loudly, or if you're getting unexpected shutdowns, don't immediately assume the worst. It might be nothing more than dust and dried-out paste — a morning's work, not a new machine.

Have a look at our Mac repairs page if you want to know more about what's involved, or take a peek at our laptop repair service if you're on a Windows machine with the same symptoms. The underlying physics is the same.