Is using thermal during the day similar or almost the same as using it at night? Considering a new ATN thor 4 640 2.5-25 scope.

Similar, but you're definitely going to have a different dynamic with the sun beating down on things.

I use mine in the daytime often.

It is difficult to see temperature differences that subtend a small part of the entire temp range of the view.
For example, when the hot sun shines on rock, and shaded, cool areas, or water or metal that reflects the sky, appears in the same view. Wide temp range, lower granularity of temp resolution.
Same effect makes it difficult to see birds and animals in tree branches when the very cold sky shows in the field of view (day OR night) - the mass of tree branches go super white and the sky is black. Look at a lower angle so as to include more tree than sky so the contrast changes, and you can resolve animals better.
It's because the sporting thermals automatically adjust the overall scene thermal gain so that the hottest thing in the view is white and the coldest thing is black, or vice versa.
So, when you are seeking, say a deer that has a surface temp of 88 degrees in the circumstances, against a background with a temp of 89 degrees, he will be tougher to find if the total scene includes much hotter or much colder things, and harder still with BOTH extremes pulling away the granularity of small differences.

The thermals can distinguish small fractions of a degree differences, but the display will show such a tiny difference only when the difference between the highest and lowest temp areas in the whole scene are nearly the same. The more thermally smooth the whole background is, the more obvious even a tiny warmer or cooler speck becomes.

In many circumstances, you must scan the scene slowly to include or eliminate hot or cold objects in your view in order to see what you are looking for in the scene. Or get closer so the total scene excludes extreme temp objects, including out near the edges of the view. And don't forget that the symbology in the screen covers areas that the sensor is looking at. Something affecting performance may be hidden by the battery meter or other on-screen stuff, or barely off screen parts of the scene that the display doesn't show you.
Remove the hottest and coldest things to eliminate the distraction to the gain control subsystem.
The sky is always the first thing I avoid in tough viewing conditions, and then I work on eliminating the other extremes.
A clear sky adds way too much to the temp range.

I have used a card to block the top half of the view in my thermal scope. Out in front of the thermal. To block most of the sky from the view. Imagine aiming the crosshair on a small target near the top of a rocky hill. The crosshair is in the center of the view, and you cannot change that. Having the sky above it causes the small target to wash out and become hard to see amongst the hot rocks. Block the sky, it changes the dynamic range, and you can see the target.

That's why animals are easier to see when the sun isn't warming up the scene. Without the hot sunshine, animals often are THE hottest thing. And it's why spotting critters in the limbs is tough. Compared to the sky, even frozen limbs are hot white and the critters are just barely "whiter" because the sky is 50-ish degrees colder than the limbs. You must reduce the amount of the cold sky in your view.

I had two deer in the yard once. Sunny day. One in the grass. One on the gravel 5 feet away from the other deer. Neither farther than 20 feet from me. With the 1X thermal, looking down at both, the one in the grass entirely disappeared except his eye and his tail. He was GONE as far as the thermal was concerned. I knew he was there but could just barely see him. The other deer was obvious in the thermal. When I shifted the view to exclude the gravel, the deer in the grass was obvious in thermal.

So...Yes. Thermals work the same in daytime as at night. Using the thermal in the daytime, especially in summer, will make you better at using it at night.

Wow!! excellent write up. Thanks so much.

I have used mine before dark, but not all day. I can only assume solar heating changes the picture

See...13'ers are not all bad. Well i am but he certainly is not. Great write up! Ty.