title: Apply a threshold on a Pandas DataFrame column
body: <p>I have a Daframe that looks like this</p>
  
  <pre><code>In [52]: f
  Out[52]:
  Date
  2015-02-23 12:00:00    0.172517
  2015-02-23 13:00:00    0.172414
  2015-02-23 14:00:00    0.172516
  2015-02-23 15:00:00    0.173261
  2015-02-23 16:00:00    0.172921
  2015-02-23 17:00:00    0.172371
  2015-02-23 18:00:00    0.176374
  2015-02-23 19:00:00    0.177480
      ...
  </code></pre>
  
  <p>and I want to apply a threshold to the series so that is the values go below it I would just substitute the threshold's value to the actual one. </p>
  
  <p>I am trying to definte a boolean dataframe like</p>
  
  <p>Bool = f > Threshold </p>
  
  <p>but I am not sure how to go on. Thanks in Advance.</p>