My car won't start when it's foggy outside?

I have a Ford Fiesta 1.3 from 2001 which i've had for just over a year and it's always started fine, but in the last couple of weeks I haven't been able to get it start when there's moisture outside. I've replaced the coil pack a few days ago and thought that it had solved the problem as we had very heavy rain and the car still started fine. But I went to it this morning(it was very foggy) and it's not starting again. I'm not sure if it could have anything to do with the battery as it seems to run down fast when i'm trying to start it. It will turn over but it just won't start and I only got two tries out of it this morning before the battery died.
I;m thinking of trying new HT leads next and probably a new battery.
Does anyone have any other ideas before I go spending more money.

HT leads and distributor cap most likely. They can be suspect after about 10 years no matter what the mileage. When they are dry they still work, but in damp they collect enough water to short out.

Battery also sounds like it's on it's way out, but get the leads first as that's most likely what the actual problem is.

Honestly, sometimes the air can be so dense with water, the air/watet mixture with gas won't allow the engine to ignite. This can be especially true with smaller engines. Try replacing the air filter with a thicker one and see if it helps.

Get the battery checked for sure, it sounds like it's on the way out and may not have enough kick to deal with damp in the system.
New HT leads and distributor cap will certainly help. Winter's on the way and the first frost will probably kill your battery.

Spray with WD40

It sounds like you need new spark plug wires. You can probably verify this before you spend money on a new set of spark plug wires. When you are having the problem, have someone crank the car over while you observe the spark plug wires. If you see sparks between the spark plug wires and the meral parts of the engine, you know the wires are bad. It is easier to do this at night. The other way you could do it is wait until the car is running. While it is running, spray the plug wires with plain water from a spay bottle. If that makes the car stall out or run bad, you again know that the spak plug wires are the problem.

As far as the car only being able to cranked a short time before the battery goes dead: That probably means that your battery is on its way out. The best way to verify that is to perform a load test on the battery. Some auto parts retailers will load test your battery for free. You can probably make a few phone calls and find an auto parts retailer who will do this.

To find out where the charge is leaking go out when it's dark and have a friend crank it. You'll see sparks where it's bad. I'd guess wd40 would help but best to diagnose the problem than replace things at random.

#1. Take the 12 volt battery to any store that sells new ones *other than Auto Zone. Ask for a free 15 second battery *load test to see what the reserve amperage capacity is. Amperage spins starter motors not voltage.

#2. When was the last tome that the fuel filter was changed. It should be done every 30,000 miles. Ask a Ford Dealer to test the fuel filter on the fuel rail pressure fitting. This is a very inexpensive test BUT the fuel filter must be clean to get an accurate fuel pressure reading.

If the battery, fuel filter and electric fuel pump are OK you may have a faulty idle air control valve and or crankshaft position sensor. Both of these parts are inexpensive.