[brstamper (OP)]

Is static cling generally worse in summer than in winter because summer air is moister and contain more ions? []

[DoctorBeaver]

Is the air a factor?

[graham.d]

I think the moisture content is on the right lines but I'm not sure I agree with the premise that static is worse in summer. It is often worse inside heated buildings when the external temperature is below freezing. This is because in these conditions the moisture is frozen out of air that is subsequently drawn into buildings. This air, when warmed, has a very low dew point and the humidity is extremely low. In a normal environment, it is the presence of moisture in the air that reduces static by allowing built up charge to leak away.

[JimBob]

I agree with Graham - the important factor is humidity.

[rosalind dna]

Or that in the summer time if/when we have a heatwave that the
air is drier so creates static.
Making things like newly washed or ironed clothes stick to my hands, but I don't know about anyone else's experiences there

[lyner]

This is a well known phenomenon and 'its obvious' why, until you start to think about it. Here's a possible explanation. When the air is near saturation, there are always small droplets of water forming and then evaporating. A droplet contains many millions of atoms and can easily gain or lose an electron - it requires much less energy than is needed to ionise a molecule. If a droplet touches a charged surface, then it can pick up charge and then (same charge) will be repelled, carrying the charge with it.* This can happen much easier with water droplets than with air molecules.  Higher humidity - more drops - quicker static discharge.
When an object becomes charged, there may be several hundred or even a couple of  thousand volts involved on a dry day. On a wet day there may be only a very few volts.

*There is a terriffic demo in which you connect two metal plates (say 10 cm diameter to a high voltage source; one to the + and one to the - terminal.  You suspend a light (polystyrene) ball, coated in graphite (a conductor) on a light string between them- like a pendulum. You nudge the ball against one plate and it charges up and gets repelled / attracted to the other plate, where it dumps its charge and picks up the other charge, moving back to the first plate and so on for ever. You have a tiny current, effectively, flowing in pulses. It could be the same effect with water drops.