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Source: https://outdoors.stackexchange.com/q/14655
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

<p>I've been learning about snow forecasting in the UK. I've learned an important feature is the atmosphere thickness, specifically the <strong>528 line</strong>:</p>

<blockquote>
  <p>Pressure decreases with altitude, and thickness measures the
  difference in height between two standard pressure levels in the
  atmosphere. It is proportional to the mean temperature of this layer
  of air, so is a useful way of describing the temperature of an
  airmass.</p>
  
  <p>Weather charts commonly show contour lines of 1,000-500 hPa thickness,
  which represent the depth (in decametres, where 1 dam = 10 m) of the
  layer between the 1,000 hPa and 500 hPa pressure levels. Cold, polar
  air has low thickness, and values of <strong>528 dam or less frequently bring
  snow to the UK</strong>. Conversely, warm, tropical air has high thickness, and
  values in excess of 564 dam across the UK often indicate a heatwave.</p>
</blockquote>

<p><a href="http://www.metoffice.gov.uk/guide/weather/symbols" rel="nofollow noreferrer">Taken from the UK met office</a></p>

<p>Looking at the <a href="http://www.metoffice.gov.uk/public/weather/surface-pressure/" rel="nofollow noreferrer">detailled pressure forcast</a> I can see this line isn't there today or tomorrow morning:</p>

<p><a href="https://i.stack.imgur.com/qO7AQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qO7AQ.png" alt="enter image description here"></a></p>

<p>But if I scroll the bar to tomorrow afternoon, it appears:</p>

<p><a href="https://i.stack.imgur.com/7UUTX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7UUTX.png" alt="enter image description here"></a>
<strong>(see the red dotted lines)</strong></p>

<p>Am I missing something here? This seems to be consistent when I've been checking this though the week. Why does this forecast only appear in advance?</p>

<p>I'm defineatly not imagining this, the line that appeared 22nd Dec in the above images is now not there until 23rd Dec.... I don't get what's going on here?</p>