How to set HSTS header from .htaccess only on HTTPS

Apparently there is a HTTPS environment variable available that can be used easily. For people with the same question:

Header set Strict-Transport-Security "max-age=31536000" env=HTTPS

To build on nielsr's answer, I used the following in the .htaccess to meet the secure deployment recommendations at https://hstspreload.org which will hardcode the domain into the Chrome browser. Be aware this will enforce HSTS across your subdomains, and that inclusion in the preload list cannot easily be undone, so rtfm.

<IfModule mod_headers.c>
Header set Strict-Transport-Security "max-age=31536000; includeSubDomains; preload" env=HTTPS
</IfModule>