How do you get the weather module to recognise SVG files?
I don't like to, but I hacked the module: weather_theme.inc. I altered:

$image = $custom_path . $symbol . '.svg';


Here's the full function:

/**
 * Returns the &lt;img&gt; tag for the weather image to use for the current condition.
 * 
 * @param $symbol
 *   The weather condition number from yr.no
 * @param $condition
 *   The translated condition text.
 * 
 * @return $string
 *   Ready formatted HTML &lt;img&gt; tag
 */
function weather_format_image($symbol, $condition) {
  // Support a custom image directory. If the variable is not set or the specified
  // file is not available, fall back to the default images of the module.
  $custom_path = file_default_scheme() . '://' . variable_get('weather_image_directory', '') . '/';
  // Construct the filename
  $image = $custom_path . $symbol . '.png';
  if (!is_readable($image)) {
    $default_path = drupal_get_path('module', 'weather') . '/images/';
    $image = $default_path . $symbol . '.png';
  }
  $size = getimagesize($image);
  // Prepare the &lt;img&gt; tag
  return theme('image', array(
    'path' =&gt; $image,
    'width' =&gt; $size[0],
    'height' =&gt; $size[1],
    'alt' =&gt; $condition,
    'title' =&gt; $condition,
    'attributes' =&gt; array('class' =&gt; 'weather-image')
  ));
}