gistfile1.php

<?php
// Not tested, but it should work

function pg_adjust_main_query( $query ) {

  /**
   * If we're not on the blog posts index page,
   * or if we're not modifying the main query,
   * then bail out.
   */
  if ( ! $query->is_home() || ! $query->is_main_query() )
    return;

  $query->set( 'meta_key', 'YOUR_META_KEY_HERE' );
  $query->set( 'orderby', 'meta_value_num' );
}
add_action( 'pre_get_posts', 'pg_adjust_main_query' );