Why does my 2000 ford expediton not start when it rains

Really bad spark plug wiring can leak enough voltage to make it start hard and maybe backfire.Along with old spark plugs,it might be enough to cause a no-start condition.Next time you get it started(on a dry night,Ha!), look at the engine while running it in a dark area away from street lamps.You may see or hear sparks jumping to ground instead of all the way thru the gap of the plug.
Don't wear any loose clothing or have any hoodie ties hanging from your person when you do this.When it's dark,it's easy enough to get too close to pulleys and other moving gotchas'.don-ohio