Less hot water during summer days

Hi all.

So I've noticed pretty consistently over the past couple years that our showers in the summer seem to be a lot cooler than the ones in the winter. The thermostat on the water heater hasn't been adjusted, but it's a distinct difference in temperature. It varies day to day exactly how much cooler the water gets, but sometimes it's pretty drastic. For example, there's usually steam in our shower when I turn the handle 80% towards the hot. This morning it was turned all the way on hot and it was barely tepid.

It's most noticeable when the weather is particularly warm (we're in North Carolina, so it's in the 80s out from now to September). We have a direct vent gas water heater. Any suggestions on what's going on or how to fix it without jacking the thermostat up?

First try to see if it's the same through out the house.
You can use a kitchen food thermometer.
The digital ones at the grocery store work pretty good.
What type of shower valve do you have?
Thermostatic?
Pressure balanced?
2 handle?

Could be the valve and not the WH