title: calculate number of frost change days (number of days) from the weather hourly data in r
body: <p>I have to calculate the following data <em>Number of frost change days</em>**(NFCD)**** as weekly basis.<br />
  That means the number of days in which minimum temperature and maximum temperature cross 0°C.
  Let's say I work with years 1957-1980 with hourly temp.</p>
  <p>Example data (couple of rows look like):</p>
  <pre><code>Date    Time (UTC)  temperature 
  1957-07-01  00:00:00    5
  1957-07-01  03:00:00    6.2
  1957-07-01  05:00:00    9
  1957-07-01  06:00:00    10
  1957-07-01  07:00:00    10
  1957-07-01  08:00:00    14
  1957-07-01  09:00:00    13.2
  1957-07-01  10:00:00    15
  1957-07-01  11:00:00    15
  1957-07-01  12:00:00    16.3
  1957-07-01  13:00:00    15.8
  </code></pre>
  <p>Expected data:</p>
  <pre><code>year month  week    NFCD 
  1957    7   1        1 
  1957    7   2        5 
  </code></pre>