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ThetaE, ThetaW & derived parameters

The relationship between Theta E, Theta W and theoretical maximum temperature and snow probabilities (latter based on Bradbury, 1970).

 

Notes:

1. The relationship for day maximum temperature was developed specifically for SE England. In mid-winter in particular, at latitudes further north, the expected maxima will be lower. Also note that the figures are not necessarily based on a plentiful data-set over the whole range of expectations; for example, at the very high and very low end of the scale, the numbers of events used to find the theoretical 'base' maxima will have been much smaller than for the 'middle', or most-likely area of the data-set. With experience, you may indeed find your own figures are better than these, so keep a record of what happens in each occasion.
2. The figures will not apply over a full snow-cover, as insolation gives a different heating response over such surfaces.
3. The techniques use the data from 850 hPa (or mbar), which is very roughly at 1500m (or 5000 ft) amsl. Under conditions of strong anticyclonic subsidence, this level may not represent the air mass in the lowest 50 hPa of the atmosphere. Cold-undercutting (and indeed warming over sea surfaces) should be allowed for. Also, in cases of very high pressure (over 1035 hPa / mbar), the 850 hPa / mbar level will be even higher up, and will represent the near-surface even less.
4. Once you have found the basic working figure for maximum temperature, 'correct' for cloud cover using the table given. Note that basic figures & corrections are only given to the nearest 1degC and the results should not be used slavishly - arguments involving the odd degree between one situation and another are pointless given the approximations involved & the accuracy of the raw input data (whether model or ascent-based).
5. In the case of the snow risk, remember it is for mean sea level. You need to adjust for altitude using the adjusting table given.

Method of use:
(a): Find expected ThetaE (Equivalent Potential Temperature) over the area in question at around the middle of the day - from latest NWP output, or from radio-sonde ascents.
(b): Against the appropriate value of ThetaE in the table below, look up the value for that month - interpolating / adjusting as necessary.
(c): Look at the list of corrections given underneath and choose appropriate value: apply.
(d): For snow probabilities, find the base (i.e. msl) figure from the extreme right-hand column.
(e): Work out the altitude of the site in question, and using the table of adjustments, find the % risk of snow for that altitude. (IMPORTANT NOTE: In my experience, these figures are too pessimistic and should be used as a very rough guide only. I find that you need persistent, moderate or heavy precipitation before these figures are relevant. What I do is use the 50% equivalent isopleth at msl ... 3degC ThetaW / 16 degC ThetaE ... as a 'tracer' for 'snow-worthy' air at lower levels, then use other means to decide on the precise risk.)

 850 thetaE
(degC)
 850 thetaW
(degC)
 Jan.  Feb.  Mar.  Apr.  May  Jun.  Jul.  Aug.  Sep.  Oct.  Nov.  Dec.  Snow
Prob. (at msl)
 54  18  19  20  21  23  25  26  26  25  24  22  20  19  
 46  16  17  19  19  21  23  25  25  23  23  21  19  17  
 40  14  16  17  18  20  22  23  23  22  21  19  17  16  
 36  12  15  15  17  19  21  21  21  21  19  17  15  15  -
 32  11  13  15  15  18  20  21  21  20  19  17  15  14  -
 30  10  13  14  15  17  19  20  20  19  18  16  14  13  -
 26  8  11  13  13  15  17  19  19  17  17  15  13  11  1%
 22  6  9  10  11  13  15  16  16  15  14  12  10  9  14%
 18  4  8  9  10  12  14  15  15  14  13  11  9  8  34%
 14  2  6  7  8  10  12  13  13  12  11  9  7  6  78%
 10  0  4  5  6  8  10  11  11  10  9  7  5  4  96%
 8  -1  3  4  5  7  9  10  11  10  8  6  4  3  100%
 6  -3  1  3  4  6  8  9  10  9  7  5  3  2  100%
 4  -4  -1  1  1  4  6  7  7  6  5  3  1  1  100%
 2  -5  -3  -1  0  2  4  5  5  4  3  1  0  -1  100%
 0  -6  -5  -3  -1  0  2  3  3  2  1  -1  -2  -3  

Corrections to be applied (for the maximum temperature figures):

A day of heavy overcast with precipitation: apply correction to above of at least -2 to -3degC, and for air-masses laden with cloud and having high low-level humidity (spring / summer), then the correction may be as high as -5degC.
A day of bright, virtually uninterrupted sunshine (but no snow cover) apply + 1degC. On days of strong sunshine over a dry ground (in a relatively warm air-mass for the season), then correction is at least +2degC, and may be up to +3degC.

Use the following table to adjust the % snow risk (at mean sea level) in the above to the required elevation:

 Elevation (in metres)  % PROB at msl >>>>  20%  40%  60%  80%
 50m    28%  50%  68%  84%
 100m    39%  59%  73%  89%
 150m    50%  68%  78%  93%
 200m    59%  73%  83%  98%
 250m    66%  78%  88%  100%
 300m    72%  83%  92%  100%
 350m    77%  88%  98%  100%