<div class="eI0">
  <div class="eI1">Model:</div>
  <div class="eI2"><h2><a href="http://en.wikipedia.org/wiki/European_Centre_for_Medium-Range_Weather_Forecasts" target="_blank">ECMWF</a>: Global weather forecast model from the "European Centre for Medium-Range Weather Forecasts". ECMWF is now running its own Artificial Intelligence/Integrated Forecasting System (AIFS) as part of its experiment suite. These machine-learning-based models are very fast, and they produce a 10-day forecast with 6-hourly time steps in approximately one minute. </h2></div>
 </div>
 <div class="eI0">
  <div class="eI1">Updated:</div>
  <div class="eI2">4 times per day, from 3:30, 09:30, 15:30 and 21:30 UTC</div>
 </div>
 <div class="eI0">
  <div class="eI1">Greenwich Mean Time:</div>
  <div class="eI2">12:00 UTC = 14:00 CEST</div>
 </div>
 <div class="eI0">
  <div class="eI1">Resolution:</div>
  <div class="eI2">0.25&deg; x 0.25&deg;</div>
 </div>
 <div class="eI0">
  <div class="eI1">Parameter:</div>
  <div class="eI2">Wet bulb potential temperature (&#952;w) in C</div>
 </div>
 <div class="eI0">
  <div class="eI1">Description:</div>
  <div class="eI2">

The ThetaW map - updated every 6 hours - shows the modelled wet bulb potential temperature at the 850hPa level. 
The theta w (&#952;w) areas are encircled by isotherms - lines connecting locations with equal wet bulb potential temperature. 
When an air parcel, starting from a certain pressure level, is lifted dry adiabatically until
saturation and subsequently is brought to a level of 1000 hPa along a saturated adiabat it
reaches what is called the saturated potential wet-bulb temperature: &#952;w.
As long as an air parcel undergoes an adiabatisch process, be it either dry or saturated, and
in both descending and ascending motions &#952;w does not change. Even when precipitation is
evaporating adiabatically &#952;w does not change, therefore &#952;w is "conservative".<BR>
An air mass is defined as a quantity of air with a horizontal extent of several hundred or
thousand kilometres and a thickness of several kilometres, which is homogeneous in thermal
characteristics. Such an air mass may form when air has been over an extensive and
homogeneous part of the Earth's surface during a considerable amount of time. This is the
so-called source area. In due time, by means of radiative exchange processes and contact
with the Earth's surface, an equilibrium develops which is evident from the fact that &#952;w has
approximately the same value in the entire air mass both horizontally and vertically, Hence &#952;w
can be used to characterise an air mass, with both sensible and latent heat are accounted
for.<BR>
Depending on possible source areas several main air mass types can be distinguished: polar
air (P), midlatitude air (ML) and (sub)tropical air (T). Also, but these are less important arctic
air (A) and equatorial air (E). These five main types can be subdivided in continental air (c)
and maritime air (m).<BR>
<BR>
Table 1: Characteristic values for &#952;w at 850 hPa (in &deg;C) for various air masses.
<TABLE CELLSPACING=0 COLS=4 BORDER=1>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT><B>Summer</B></TD>
			<TD ALIGN=LEFT><BR></TD>
			<TD ALIGN=LEFT><B>Winter</B></TD>
			<TD ALIGN=LEFT><BR></TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>cA &lt; 7 </TD>
			<TD ALIGN=LEFT>mA &lt; 9</TD>
			<TD ALIGN=LEFT>cA &lt; -5 </TD>
			<TD ALIGN=LEFT>mA &lt; -7</TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>cP 7 - 12 </TD>
			<TD ALIGN=LEFT>mP 6 - 12</TD>
			<TD ALIGN=LEFT>CP -6 &ndash; 2</TD>
			<TD ALIGN=LEFT>mP -3 - 5</TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>CML 11 &ndash; 16</TD>
			<TD ALIGN=LEFT>mML 11 - 16</TD>
			<TD ALIGN=LEFT>CML 1 &ndash; 8</TD>
			<TD ALIGN=LEFT>mML 3 - 9</TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>cT 15 - 19 </TD>
			<TD ALIGN=LEFT>mT 14 - 19</TD>
			<TD ALIGN=LEFT>CT 8 &ndash; 14</TD>
			<TD ALIGN=LEFT>mT 8 - 16</TD>
		</TR>
		<TR>
			<TD HEIGHT=18 ALIGN=LEFT>cE &gt; 17 </TD>
			<TD ALIGN=LEFT>mE &gt; 18</TD>
			<TD ALIGN=LEFT>cE &gt; 14 </TD>
			<TD ALIGN=LEFT>mE &gt; 16</TD>
		</TR>
	</TBODY>
</TABLE>
<BR>
If the &#952;w distribution is considered on a pressure surface, preferably 850 hPa, then extensive
areas with a small or no gradient can be observed. These areas of homogeneous &#952;w values
may be associated with air masses. Often various homogeneous areas are separated from
one another by relatively narrow transformation zones displaying a strong gradient. Here
frontal zones intersect with the pressure surface. Generally speaking a surface front is
located where at 850 hPa the 'warm boundary' of the zone with the large &#952;w gradient is
present.(Source: <a href="http://www.maq.wur.nl/UK/" target="_blank">Wageningen University</a>)

    
  </div>
 </div>
 <div class="eI0">
  <div class="eI1">NWP:</div>
  <div class="eI2">Numerical weather prediction uses current weather conditions as input into mathematical models of the atmosphere to predict the weather. Although the first efforts to accomplish this were done in the 1920s, it wasn't until the advent of the computer and computer simulation that it was feasible to do in real-time. Manipulating the huge datasets and performing the complex calculations necessary to do this on a resolution fine enough to make the results useful requires the use of some of the most powerful supercomputers in the world. A number of forecast models, both global and regional in scale, are run to help create forecasts for nations worldwide. Use of model ensemble forecasts helps to define the forecast uncertainty and extend weather forecasting farther into the future than would otherwise be possible.<br>
<br>Wikipedia, Numerical weather prediction, <a href="http://en.wikipedia.org/wiki/Numerical_weather_prediction" target="_blank">http://en.wikipedia.org/wiki/Numerical_weather_prediction</a>(as of Feb. 9, 2010, 20:50 UTC).<br>
</div></div>
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