'Permission denied' errors have been dealt with in version 2.5.3.0 onwards. The module is now automatically adjusting metar cache permissions to the extent needed and possible. Users should not need to modify the code.
If there are still problems, carefully examine the error message(s) on your screen and follow the instructions in the file DOCUMENTATION.TXT that comes with the downloaded package.

Dear John,
Thank you for this post. Yes the module is checking the 707 permission and sets it if it is different from that.
In the next release I may remove the automatic permission setting or add a config option to disable (or modify) it, so that users who have a similar setup like yours do not need to modify the code.

Just for anybody who like me could not get this module to work because of permission denied error. The server would not allow reading of a dir with 707 permission but every time I changed the 'metar_data' folder to 777 the module changed it back to 707. So I edited 'mod_mambweather.php' and replaced all references to 0707 with 0777

Works perfect now! Remember to backup and I feel this is specific to my kind of server setup so only use this solution if all other solutions fail.

Linux server cluster
PHP 5.2.6
Joomla 1.5.23