Why do white scratch lines appear appear on dry skin?

Interestingly both the above answers are correct, but I think what you're referring to is that the scratching dislodges dead layers of skin resulting in the white lines, rather than dermatographism (literally "skin writing") where there would be white lines that turn read and you can actually write something on the skin.

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RE:

Why do white scratch lines appear appear on dry skin?

In winter when your skin is a little dry and you care not to apply a moisturizer, and you hadn't taken a bath since eons 'cause well, you don't want to.

Then things get a little itchy. And like a good scratcher, you scratch. And there they are! The pretty white lines. What is the...

It's our natural mechanism telling us to respect our body.

The pretty white lines following the itching process are warning signals from the skin.

The message:

The skin is wanting in oil, the outer portion is getting dried, and the dead cells are accumulating on it!

Act on this warning immediately!

Solution:

Answer is blowing in the wind, man!

Go for a shower, exercise daily, apply moisturizer, and above all stick to this schedule on a daily basis!

Logic:

It's only when u respect your body that others will respect this wonderfully well oiled physique that has all the solutions in it.

Strategy:

Go to bed early, rise early, drink lots of potable water, do the necessary daily ablution processes, take bath, and practice yoga.

While you do the Yogic processes, go for the breathing (pranayama) and the meditation as well as the physical exercises (asanas).

Always create a positive (protective) insulation around you.

You'll be the winner!

when the skin looks discolored, crackly, whiter than the rest of the skin. Use lotion whenever you want really, if your skin looks like i described above, definitely use it, but if you suffer from dry skin, use lotion at least once every hour to avoid that look. And where healing lotion (i.e. Nivea) during the night (you can put the lotion on and then wear a pair of old socks to keep the lotion in through the night, or purchase a pair of gloves specifically for that case, they sell them at wal-mart).

Just lack of hydrants in your skin. Hydrants are contents in the skin that keep your skin moistened and soft. Moisturizers are one of them. White lines are created by scratching because layer of the skin is no more soft and flexible, instead its hard and fragile, so it is breaking apart.

So that is it!

Now go and take bath and use Dove soap because its 30% moisturizing cream!! Ha Ha Ha......

I would think that "belledam" is probably spot-on with her answer.

I, personally, have always wondered why scar tissue absorbs just enough skin pigmentation to avoid being totally white, but is never as dark as the surrounding skin - even in winter when no one has any suntan.

First of all, I dont know the answer to your question but I can tell you it doesnt happen only on dry skin, it happens on all skin, though on darker skin it will be more visible.

Its dead skin.