What causes the veins in hands to enlarge and when lifting them up, the veins go back to normal?

this only happens sometimes, but I can actually see the veins in my hands enlarge. when I hold my hands up in the air, the veins return to normal. when i return my hands back down, the veins become enlarged again.

it only happens sometimes, but I can actually see the veins in my hands enlarge (very big). when I hold my hands up in the air, the veins return to normal. when i return my hands back down, the veins become enlarged again. what does it mean if they get tingly?

Gravity and poor circulation. Nothing to worry about unless they start to swell up a lot or get numb or tingly.
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If it is getting tingly you need to see your doctor. It feels like pins & needles kind of poking you all over or like when you keep it bent for too long and it falls asleep.

gravity, its easier harder for blood to flow upwards so the veins appear smaller

Veins are more noticeable when you are warmer. The blood flows more freely as the veins enlarge. When you put your arms up, gravity takes over and the blood has to work harder to get to your hands. Tingly would just mean your arm is becoming numb from being up in the air.

You can look this up in a physiology or anatomy book also. But anyway, veins have valves, which stop backflow of the blood in them (arteries don't have valves except in the heart). When your hands are above your heart level, the blood flows back down due to gravity, making your veins look like they shrink. When your hands are below the level of your heart, the blood stops for a moment and pools in your veins and behind the valves. This process is normal. You can even do an experiment where you actually find a valve on your arm or hand and press gently on it; that part of the vein shouldn't get large when you put your hands down. But as mentioned, a textbook would describe this tiny (and fun) experiment much better than I can.

Gravity