Relative humidity

I have no clue about the humidity vs. TMCC signal but the humidifier is a must for Northeast layouts in basements to control the humidity levels and prevent rust.

A good earth ground can depend on the moisture content of the soil.  We have these issues with our mobile units sometimes.  Rare but we do.  I have heard others having signal issues during long dry spells.

The grounding properties of soil thru out the US varies greatly. The Northeast has some of the best. Here in Virginia which is mid Atlantic the soil is not so good.
So what should you do? Well the best way to insure a good ground is to drive ground rods. I know it's a PITA but hey you asked. For dry conditions to really affect your ground you'd have to have marginal conditions to start out with but if you install more than one rod it will usually take care of it in all but the most extreme cases

 
Ground rods should be 5/8 diameter copper clad and 8 feet long(Lowes,Home Depot Electriacl supply). You should never drive the ground rod up against the house where they back filled the basement
It needs to be in undisturbed earth. So come out from the foundation about 4 feet with the first one. I recommend 2 in undisturbed earth the first 4 feet away from the house and the second at least 10 feet from the first.
Your wire should be number 4 solid copper and clamps should be the acorn style and NOT the 2 bolt style used for water pipes.

Laughing about this but really not.
To drive the rods , you'll probably have to stand on a step ladder for the first series of whacks of coarse you'll be using a sledge hammer. You'll need a partner to steady the rod for you. Don't get the moron of the year award by getting your partner to hold the rod with their hand(unless you don't like them and just want to break their arm)
Drill a 3/4 hole in a 4 foot long piece of 2x4 slide it over the rod and use it to steady with. That way if you miss the rod you'll hit the 2x4 instead of their arm.
I've driven thousands of ground rods over the years and you don't know how many times I've seen one of the guys on my crew go to the hospital cause the idiot tried to hold a rod for his buddy with his hand.

David

You can also buy a steel post driver from a farm store and eliminate the second man. They're pretty cheap but you'll still need a sledge hammer for the last foot or so.

Ron

Another option to the post driver is to rent a heavy duty hammer drill.
I use a Makita heavy duty drill That can drill or act as a jack hammer.
It has a selector on the side of it for drilling or hammering only.
I use a ground rod driving bit to put the rods in. It doesn't spin the rods it merely
Hammers them in ,in about 3 or 4 minutes it'll put an 8 foot rod in the ground.

Word of caution . This is a law in all 50 states you have to call your state underground utility locating service hot line. It doesn't matter if you've been living there since before houses were invented and don't think there's any thing in your way . It's the law.
Here in Virginia it's called Miss Utility and it's FREE. You call 48 hours prior to doing any digging and they will mark the ground with paint to show you where the power,phone ,natural gas ,cable TV, water and sewer are.
The 800 number for your state should be in the front in your phone book.
The concept is simple .Call and you'll see where the utility's are and no worries
Don't call and hit a cable and get out your wallet cause you will be billed for fixing it.

David