> If it's not humidity on those cold days, what is it?
Sunny vs. cloudy? Little wind vs. more wind? Fog? Rain? Drizzle? You don't mention any of these other obvious factors. Are they really all exactly the same on both types of days?
At 45 degrees F, the air is much colder than you are. Under those conditions, as I've already noted, your body is trying to retain heat, not shed it. More humidity, if anything, will increase your body's ability to retain heat; so a more humid cold day, if anything, should feel warmer than a less humid cold day. However, as I've already noted, the amount of water vapor the air can hold at a cold temperature is not very large anyway, so there is very little difference between "more humid" and "less humid".
To put some numbers to this, I'll use figures from the Engineering Toolbox[1] to compare 41 degrees F at 100% relative humidity with 104 degrees F at 25% relative humidity (which is a typical condition for a hot desert day). I pick those temperatures because they have entries in the table on the page I linked to.
41 degrees F, 100% RH: 6.82 g of water vapor per cubic meter of air
104 degrees F, 25% RH: 0.25 x 51.1 = 12.775 g of water vapor per cubic meter of air.
In other words, there is close to twice the water vapor in the air on a "dry heat" desert day than on a humid cold day. And if we want to compare with a hot, humid, day, let's pick 86 degrees F at 90% RH (typical for, say, southern Texas or southern Florida in the late spring--not even summer):
86 degrees F, 90% RH: 0.9 x 30.4 = 27.36 g of water vapor per cubic meter of air.
So the difference between "dry heat" and "hot and humid" is at least (remember that I picked a significantly less severe hot and humid condition) twice the total amount of water vapor in 100% saturated air at 45 degrees F. And the difference in water vapor in the air between the two conditions you describe on 45 degree days will be considerably less than that total amount of 6.82 g; at 45 degrees F, most days are going to have fairly high RH simply because there is always some water vapor in the air, and even "some" is enough to get a fairly high RH. So even if the humidity effect were in the direction you describe (which, as I said above, it isn't), it would be too small at 45 degrees F to matter.
In short, I just don't see humidity as a reasonable explanation for what you describe at 45 degrees F. I would be looking at the other factors I listed.
Comments
> If it's not humidity on those cold days, what is it?
Sunny vs. cloudy? Little wind vs. more wind? Fog? Rain? Drizzle? You don't mention any of these other obvious factors. Are they really all exactly the same on both types of days?
At 45 degrees F, the air is much colder than you are. Under those conditions, as I've already noted, your body is trying to retain heat, not shed it. More humidity, if anything, will increase your body's ability to retain heat; so a more humid cold day, if anything, should feel warmer than a less humid cold day. However, as I've already noted, the amount of water vapor the air can hold at a cold temperature is not very large anyway, so there is very little difference between "more humid" and "less humid".
To put some numbers to this, I'll use figures from the Engineering Toolbox[1] to compare 41 degrees F at 100% relative humidity with 104 degrees F at 25% relative humidity (which is a typical condition for a hot desert day). I pick those temperatures because they have entries in the table on the page I linked to.
41 degrees F, 100% RH: 6.82 g of water vapor per cubic meter of air
104 degrees F, 25% RH: 0.25 x 51.1 = 12.775 g of water vapor per cubic meter of air.
In other words, there is close to twice the water vapor in the air on a "dry heat" desert day than on a humid cold day. And if we want to compare with a hot, humid, day, let's pick 86 degrees F at 90% RH (typical for, say, southern Texas or southern Florida in the late spring--not even summer):
86 degrees F, 90% RH: 0.9 x 30.4 = 27.36 g of water vapor per cubic meter of air.
So the difference between "dry heat" and "hot and humid" is at least (remember that I picked a significantly less severe hot and humid condition) twice the total amount of water vapor in 100% saturated air at 45 degrees F. And the difference in water vapor in the air between the two conditions you describe on 45 degree days will be considerably less than that total amount of 6.82 g; at 45 degrees F, most days are going to have fairly high RH simply because there is always some water vapor in the air, and even "some" is enough to get a fairly high RH. So even if the humidity effect were in the direction you describe (which, as I said above, it isn't), it would be too small at 45 degrees F to matter.
In short, I just don't see humidity as a reasonable explanation for what you describe at 45 degrees F. I would be looking at the other factors I listed.
[1] https://www.engineeringtoolbox.com/maximum-moisture-content-...