title: R - Changing the data in a data.frame
body: <pre><code>row.names   CLIMATE_IDENTIFIER  STN_ID  ENG_STN_NAME    DRY_BULB_TEMP   RELATIVE_HUMIDITY   DEW_POINT_TEMP  WIND_SPEED  WIND_DIRECTN_16 WIND_CHILL  LOCAL_YEAR  LOCAL_MONTH LOCAL_DAY   LOCAL_HOUR
  1   170 8203428 49109   MARTOCK 35M -1.4    86  -3.5    0   NA  12.24   2011    2   12  19
  2   199 8203428 49109   MARTOCK 35M -1.9    87  -3.7    0   NA  11.93   2011    2   12  19
  3   262 8203428 49109   MARTOCK 35M -0.3    94  -1.2    0   NA  12.93   2011    2   14  7
  4   263 8203428 49109   MARTOCK 35M 1.1 83  -1.4    0   NA  13.80   2011    2   14  13
  5   280 8203428 49109   MARTOCK 35M -0.7    90  -2.2    0   NA  12.68   2011    2   14  10
  6   299 8203428 49109   MARTOCK 35M 2.0 79  -1.3    0   NA  14.36   2011    2   14  12
  7   318 8203428 49109   MARTOCK 35M -1.6    94  -2.4    0   NA  12.12   2011    2   14  9
  8   347 8203428 49109   MARTOCK 35M 1.7 98  1.4 0   NA  14.17   2011    2   15  0
  9   402 8203428 49109   MARTOCK 35M 3.3 91  2.0 0   NA  15.17   2011    2   15  3
  10  419 8203428 49109   MARTOCK 35M 3.1 94  2.2 0   NA  15.04   2011    2   15  3
  11  567 8203428 49109   MARTOCK 35M -3.7    56  -11.2   0   NA  10.82   2011    2   16  18
  12  587 8203428 49109   MARTOCK 35M -4.9    60  -11.4   0   NA  10.07   2011    2   16  20
  </code></pre>
  
  <p>There's an example of my data. </p>
  
  <p>I'm looking to replace all values in the column <code>WIND_CHILL</code> with 
  <code>DRY_BULB_TEMP + ((-1.59+0.1345*DRY_BULB_TEMP)/5)*WIND_SPEED</code> but only when <code>WIND_SPEED &lt; 5</code>.</p>
  
  <p>Any ideas?</p>