Why does my car only squeal when it is cold or wet?

For a week or two, usually yes — if the squeal fades within 60 seconds of starting. That's almost always a loose or slightly worn belt, which is annoying but not dangerous in the short term. But if the squeal persists past warm-up, gets louder during acceleration, or appears when you add steering effort, you have a failing component — a tensioner, a pulley, or an accessory — that can leave you stranded. A snapped serpentine belt while driving means the immediate loss of power steering assist, alternator charging, and water-pump cooling all at once, potentially in the middle of a highway, so a persistent or worsening squeal is not something to ride out for months. The clearest safety line is the burning-rubber smell: a continuous squeal paired with a hot-rubber smell means the belt is slipping badly enough to overheat and can snap, so that combination means stop driving and fix it. So the rule of thumb is that a brief cold-start squeal that fades is safe to drive on while you plan the fix, but a squeal that persists, worsens under load, or smells like hot rubber should be handled within a week rather than ignored.

The range runs from $0 to well over $1,500 depending on what's actually squealing, which is exactly why the free tests matter before you spend. At the free end, the squeal just needs warm-up or, on some older cars, a tensioner adjustment. The common real fixes are a serpentine belt replacement at $80-$200 with labor, a tensioner at $150-$400, and a bad idler or accessory pulley at $200-$500. At the expensive end, a failing power steering pump is $300-$700 and an AC compressor is $400-$1,500. The most costly trap is mistaking a failing accessory — an AC compressor, an alternator, or a power steering pump — for a simple belt: the belt screams because the bearing inside the accessory is seizing, so replacing the belt alone fixes nothing and the new belt squeals again within days. That's why the point of diagnosing first is to tell a belt problem (cheap) from a bearing or accessory problem (expensive) before authorizing anything — a $7 can of belt dressing or a spray of water is enough to separate the two.

You can, but only as a temporary diagnostic test, not a real fix. Belt-dressing spray (about $7) makes a worn belt grip the pulleys better for a few days, so it's genuinely useful for one thing: if spraying it makes the squeal disappear, the belt is your problem and needs replacement, while if spraying it doesn't help, the issue is a pulley or an accessory rather than the belt itself. Used that way, it saves you from replacing the wrong part. What you shouldn't do is treat belt dressing as a permanent solution, because it accelerates belt wear and can drip into and contaminate other components, turning a diagnostic shortcut into a new problem. The cleaner version of the same test is a light mist of plain water on the running belt: if the squeal briefly stops or changes, it's the belt; if water makes no difference, it's a bearing. So spray to diagnose, not to fix — then replace the belt if the spray confirmed it, and look at the pulleys and accessories if it didn't.