Probably you are running your site with PHP 7, and the latest versions want numerics to be really numeric.
Go to the funtion stripunits (around line 1140)
//=========================================================================
// strip trailing units from a measurement
// i.e. '30.01 in. Hg' becomes '30.01'
function strip_units ($data) {
preg_match('/([\d\,\.\+\-]+)/',$data,$t);
return $t[1];
}
And change the line with return $t[1];
to return (float) $t[1];
It could result in problems at some other place, so if this is the only warning, change your line 491 from
if ( (strip_units($avgspd) + strip_units($gstspd) < 0.1 ) and
to
if ( ( (float) strip_units($avgspd) + (float) strip_units($gstspd) < 0.1 ) and
By adding the (float) before a numeric value, PHP will discard all other characters so that only the numeric values is processed.
Wim