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NAMIBIA METEOROLOGICAL SERVICE
SELECTED AERODROMES IN NAMIBIA.
Occasional paper
By
Olga M. Karunga
Meteorological Technician
and
Zilore Mumba
Chief Meteorological Technician
November 2001
Table of Contents
Abstract
| Figure 1. | Mean Minimum and Maximum Temperatures at Keetmanshoop: 1a) Minimum, and 1b) Maximum Temperature frequency curves. 1c) Mean minimum, and 1d) Mean Maximum Temperature curves |
| Figure 2. | Mean Minimum and Maximum Temperatures at Walvis Bay: 2a) Minimum, and 2b) Maximum Temperature frequency curves. 2c) Mean minimum, and 2d) Mean Maximum Temperature curves |
| Figure 3. | Mean Minimum and Maximum Temperatures at Windhoek: 3a) Minimum, and 3b) Maximum Temperature frequency curves. 3c) Mean minimum, and 3d) Mean Maximum Temperature curves |
| Figure 4. | Mean Minimum and Maximum Temperatures at Grootfontein: 4a) Minimum, and 4b) Maximum Temperature frequency curves. 4c) Mean minimum, and 4d) Mean Maximum Temperature curves |
| Figure 5. | Surface Pressure Curves at Windhoek and Grootfontein: 5a) Windhoek Occurrence Frequency, and 5b) Grootfontein Occurrence Frequency curves. 5c) Windhoek Mean, and 5d) Grootfontein Mean Monthly Pressure curves |
OCCURRENCE FREQUENCIES OF MINIMUM AND MAXIMUM TEMPERATURES AT SELECTED AERODROMES IN NAMIBIA.
By
Olga M. Karunga
Meteorological Technician
and
Zilore Mumba
Chief Meteorological Technician
Abstract
The frequency of occurrence of minimum and maximum temperatures,
and surface pressure has been analysed at selected stations
in Namibia. Only data for the latest 7 years, 1994-2000 for
temperature, and 4 years, 1997-200 for pressure were used.
Frequencies of minimum and maximum temperatures clearly highlight the occurrence of very low temperatures in winter in southern and central Namibia, and the occurrence of very high temperatures, associated with Bergwind conditions in early winter on the central coast, which situations are cannot be depicted by mean curves. The availability of pressure data was limited to Windhoek in central Namibia and Grootfontein in the north.
It is proposed in this study to present an alternative method which may provide more information on temporal variations, as well as highlight certain peculiarities, specific to given locations.
2. OBJECTIVE
The objective of this study is to provide statistical mean data which can serve as;
3. DATA USED
The data used was supplied by the Climate section of the Namibian Meteorological service.
The data comprised of daily surface pressure for Windhoek airport and Grootfontein and daily maximum and minimum temperatures at four locations, namely Keetmanshoop, Walvis Bay, Windhoek airport and Grootfontein. Data for seven years, 1994-2000 was used for temperatures and four years, 1997-2000 for pressures.
4. METHODOLOGY
The first step in the computation was the determination of the lowest and highest values in the entire record of each station. Based on these two values the station data record was divided into categories of five degree (or 5 hPa) intervals, e.g. Windhoek minimum temperatures ranged from -6 to 23.3 degrees Celsius. This gave 7 categories as follows: -10 to -5, -5 to 0, 0 to 5, 5 to 10, 10 to 15, 15 to 20 and 20 to 25. The number of days temperature fell within a given range were totalled on a month by month basis. From monthly totals for each range and the total number of days temperature was available in the particular month (i.e. for 7 Januarys, etc.), percentage of occurrence of temperature was calculated for each temperature h range.
5. RESULTS
The results are presented in the frequency of occurrence graphs which follow with the discussion below. For each station, the minimum temperatures are given on the left and maximum temperature on the right. At the bottom are graphs with several curves per graph. The various curves on each graph represent the following: The top most curve represents the highest maximum temperature ever recorded at a particular station during the entire period; The second and third graphs (from top) represent the mean maximum and mean minimum temperatures respectively; and the bottom most curve represents the highest maximum temperature ever recorded at a particular station during the entire period.
6. DISCUSSION
6.1 TEMPERATURES Keetmanshoop
a) Minimum The main features of the annual minimum temperature curve for Keetmanshoop are the high temperatures near 20 degrees C during the peak southern summer months of January/February, then a gradual decrease in temperature to the lowest values near 8 degrees C in the peak winter months of June, July and August. However of interest is the high occurrence (30%) of below period average temperatures (~2 degrees C) in July. Synoptic observation suggests that this may be attributed to the effect of cold fronts which regularly affect the subcontinent, the post frontal pressure build up being responsible for advection of cold polar air into southern parts of Namibia. Clearly the advantage of the frequency occurrence over the mean curve is in providing insights into the effects of synoptic systems at work.
b) Maximum The maximum temperature curve similarly shows the annual march of temperature, with the highest temperatures near 35 degrees C in January/February and the lowest near 25 degrees in July. The effects of the cold frontal passage are not as evident in the maximum temperature as in the minimum temperature curves. A final remark about the annual temperature march is that the occurrence of the highest temperatures in the peak summer months ( which is similarly the case at Walvis Bay and Windhoek, but no so at Grootfontein) is suggestive of the drier nature of the Namibian climate over those areas. Increased cloudiness and rainfall in the peak summer months of December January and February would have the effect of moderating temperatures, so that the highest temperatures would be expected to occur in the dry months just before the rain season.
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Table 1. Monthly Mean & Extreme Minimum and Maximum Temperatures at Keetmanshoop
Walvis Bay
a) Minimum
Not unlike the minimum temperature curve for Keetmanshoop, Walvis Bay experiences the highest minimum temperatures of the year (~16 degrees C) during the peak southern summer months of January/February, then a gradual decrease in temperature to the lowest values near 9 degrees C in August. However the influence of cold fronts, associated with very low temperatures at Keetmanshoop does not appear to reach as far north as Walvis Bay.
b) Maximum
The maximum temperature curve shows high mean temperatures of 24 degrees C in January, but there is a steady increase through February and March to the highest mean temperatures of about 27 degrees C in April. There is a slight drop from April to May, whence it remains constant through May and June, before dropping to the lowest mean values of the year (21 degrees C) in August and September, and then a steady rise through the remainder of the year to 24 degrees C in December.
The frequency curve for Walvis Bay reveals another feature, peculiar to the region, and suggestive of a particular synoptic regime. This is the occurrence of very high temperatures, experienced when an easterly current blows perpendicular to the coast over this region. These situation is known locally as Bergwind conditions. In fact the highest temperatures at Walvis Bay occur in June and July, when the long term mean averages 32 degrees C, but under Bergwind conditions the daily maximum is in the upper thirties.
Windhoek
a) Minimum
The main features of the annual minimum temperature curve for Windhoek are the high temperatures near 17 degrees C during the peak southern summer months of January/February, then a gradual decrease in temperature to the lowest values near 5 degrees C in the peak winter months of June, July. However of interest is the high occurrence (30%) of below period average temperatures (~2 degrees C) in July.
This is similarly attributed to the effect of cold fronts whose influence extends this far north.
b) Maximum
The highest mean maximum temperatures occur in October, November and December, the highest being 31 degrees C in December, and the lowest mean maximum near 20 degrees in July. The effects of the cold frontal passage are also not directly evident in the maximum temperature curves.
The occurrence of the highest mean temperatures in October-December is a expected, due to the moderating effects of clouds and rainfall during the potentially hottest peak southern summer months of December,-February.
Grootfontein
a) Minimum
Not unlike the mean minimum temperature curve for Windhoek, Grootfontein experiences the high mean temperatures (~18 degrees C) during the peak southern summer months of December/January, then there is a gradual decrease in temperature to the lowest values near 3 degrees C in the peak winter months of June/July.
b) Maximum
The highest mean maximum temperatures occur in October, November and December, the highest being 32 degrees C in October, and the lowest mean maximum near 23 degrees in July.
Similarly as for Windhoek, the occurrence of the highest mean temperatures in October-December is a expected, due to the moderating effects of clouds and rainfall during the potentially hottest peak southern summer months of December,-February.
Windhoek and Grootfontein
The main feature of the annual pressure curves for both Windhoek and Grootfontein are
the lowest mean pressures near 830 (and 850) hPa during January, increasing to the
highest mean pressures of 835 (and (855) hPa in June/July.
7. CONCLUSIONS
Frequency occurrence curves clearly provide more information than mean curves. Frequency of occurrence of minimum and maximum temperatures in Namibia reveal certain characteristics of the meteorological situations, which are not evident from mean temperature curves. These are the occurrence of very low temperatures in southern and central parts of Namibia during winter, associated with the advection of cold polar air after the passage of a cold front. The other feature is the occurrence of high temperatures over the central coast at the start of winter, associated with subsiding winds blowing down the high inland plateau at right angles to the coast, known locally as Bergwind conditions.
This study has shown that the use of combination of monthly mean and frequency occurrence curves provides more information than mean curves alone. Frequency occurrence analysis would also assist determine synoptic regimes that would be worthy of investigation.
8. ACKNOWLEDGEMENTS
The authors would like to thank the Deputy Director of the Namibia
Meteorological Service , for allowing publication of this work. The
authors would also like to thank the Chief Meteorological Technician,
Climate, and his staff for providing the data used in this study.
Figure 2a. Minimum Temperatures Occurrence Frequencies at Walvis Bay |
Figure 2b. Maximum Temperatures Occurrence Frequencies at Walvis Bay |
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Table 3. Monthly Mean & Extreme Minimum and Maximum Temperatures at Windhoek
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Table 4. Monthly Mean & Extreme Minimum and Maximum Temperatures at Grootfontein
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Keetmanshoop
Minimum Temperature Frequencies (%) in Each Temperature Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
25_30 1.9 5.7 .5 .0 .0 .0 .0 .0 .0 .0 .0 1.5
20_25 37.8 46.6 26.5 10.8 .0 .0 .0 .0 .6 8.3 10.2 22.6
15_20 48.3 44.6 55.3 42.0 14.6 1.0 1.4 5.6 23.9 36.1 42.4 55.4
10_15 11.5 3.1 17.2 30.1 48.8 26.1 18.7 28.7 40.3 34.3 39.5 19.5
5_10 .5 .0 .5 17.0 28.3 47.3 43.5 43.8 27.8 19.4 7.8 1.0
0_5 .0 .0 .0 .0 8.3 25.1 35.5 21.3 7.4 1.9 .0 .0
-5_0 .0 .0 .0 .0 .0 .5 .9 .6 .0 .0 .0 .0
Keetmanshoop
Maximum Temperature Frequencies (%) in Each Temperature Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
40_45 2.4 1.6 .5 .0 .0 .0 .0 .0 .0 .0 1.4 2.6
35_40 44.5 60.6 31.2 2.3 .0 .0 .0 2.3 5.0 12.5 24.5 49.7
30_35 45.0 30.6 49.3 46.3 9.7 .5 .0 9.6 31.7 48.1 48.1 38.5
25_30 7.6 5.2 16.7 39.0 50.2 33.5 17.8 41.8 40.6 31.5 17.3 8.2
20_25 .5 2.1 1.9 10.7 27.1 43.8 47.4 25.4 16.1 7.4 5.8 1.0
15_20 .0 .0 .5 1.7 11.1 17.7 24.4 16.4 6.1 .5 2.9 .0
10_15 .0 .0 .0 .0 1.9 3.9 10.3 4.5 .6 .0 .0 .0
5_10 .0 .0 .0 .0 .0 .5 .0 .0 .0 .0 .0 .0
Walvis Bay
Minimum Temperature Frequencies (%) in Each Temperature Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
25_30 .0 .0 .0 1.4 .0 .0 .0 .0 .0 .0 .0 .0
20_25 1.2 2.7 2.5 8.1 7.9 2.2 1.7 1.3 .0 .0 .0 .0
15_20 73.2 78.9 48.5 21.4 11.8 16.3 21.1 3.2 .7 1.4 7.7 38.4
10_15 25.6 18.4 48.0 59.5 46.6 32.0 28.9 26.6 62.2 75.4 84.7 60.5
5_10 .0 .0 1.0 9.5 33.1 47.2 43.3 62.3 36.4 23.2 7.7 1.1
0_ 5 .0 .0 .0 .0 .6 2.2 5.0 6.5 .7 .0 .0 .0
Walvis Bay
Maximum Temperature Frequencies (%) in Each Temperature Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
40_45 .0 .0 1.6 3.9 .0 .0 .0 .0 .0 .0 .0 .0
35_40 .0 .0 1.6 10.0 10.1 3.4 1.1 5.2 .0 .5 .5 .0
30_35 3.0 2.1 8.1 11.1 12.4 26.3 32.4 5.2 1.4 1.4 2.9 .5
25_30 51.2 58.9 53.5 38.3 18.5 13.4 10.6 7.8 7.7 9.8 24.4 36.3
20_25 45.8 39.0 35.1 31.1 44.9 33.0 26.3 33.8 52.1 72.0 67.5 62.2
15_20 .0 .0 .0 5.6 14.0 22.3 27.9 46.1 38.7 16.4 4.8 1.0
10_15 .0 .0 .0 .0 .0 1.7 1.7 1.9 .0 .0 .0 .0
Hosea Kutako
Minimum Temperature Frequencies (%) in Each Temperature Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
20_25 12.9 9.2 2.3 .0 .0 .0 .0 .0 .5 .5 8.1 13.0
15_20 73.3 82.1 65.3 13.0 .5 .0 .0 .5 12.4 36.3 50.5 69.0
10_15 12.4 8.2 31.0 68.1 31.5 1.9 3.3 14.4 49.5 54.0 32.9 17.1
5_10 1.4 .5 1.4 17.4 53.1 47.3 38.0 56.7 29.5 8.4 8.6 .9
0_ 5 .0 .0 .0 1.4 13.1 45.4 48.4 24.7 6.7 .9 .0 .0
-5_0 .0 .0 .0 .0 .0 1.9 5.3 9.9 3.7 1.4 .0 .0
-10_-5 .0 .0 .0 .0 .0 .0 .5 .0 .0 .0 .0 .0
Hosea Kutako
Maximum Temperature Frequencies (%) in Each Temperature Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
35_40 3.7 1.0 .5 .0 .0 .0 .0 .0 .0 .0 .0 3.2
30_35 45.2 43.9 33.8 3.8 .0 .0 .0 .5 21.9 61.8 66.7 63.6
25_30 44.7 49.0 56.9 83.7 34.7 3.8 1.9 37.2 63.3 36.9 29.5 28.6
20_25 5.1 6.1 8.8 11.5 56.3 74.5 64.4 53.0 10.0 1.4 3.8 4.6
15_20 1.4 .0 .0 1.0 6.6 19.2 29.6 7.0 4.8 .0 .0 .0
10_15 .0 .0 .0 .0 2.3 1.9 3.7 2.3 .0 .0 .0 .0
5_10 .0 .0 .0 .0 .0 .5 .5 .0 .0 .0 .0 .0
Grootfontein
Minimum Temperature Frequencies in Each Temperature Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
25_30 .9 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0 .0
20_25 20.3 9.7 10.1 .5 .0 .0 .0 .0 3.9 13.6 23.9 21.9
15_20 76.5 86.7 80.2 53.6 8.9 .0 .9 7.2 43.5 71.8 65.1 75.8
10_15 2.3 3.6 9.7 43.1 55.6 26.8 28.1 49.0 45.4 14.1 11.0 2.3
5_10 .0 .0 .0 2.9 31.3 58.9 52.5 38.5 5.8 .5 .0 .0
0_ 5 .0 .0 .0 .0 3.3 12.0 16.6 5.3 1.4 .0 .0 .0
-5_.0 .0 .0 .0 .0 .9 2.4 1.8 .0 .0 .0 .0 .0
Grootfontein
Maximum Temperature Frequencies (%) in Each Temperature Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
35-40 8.8 4.1 4.6 .5 .0 .0 .0 .0 1.0 12.1 13.8 12.0
30-35 37.8 44.9 41.9 29.5 4.7 .0 .0 10.7 66.3 75.3 67.6 59.0
25-30 43.8 44.4 46.5 66.7 72.0 40.7 31.6 68.4 27.9 9.3 17.1 22.6
20-25 9.7 6.6 6.5 3.4 20.6 54.1 57.0 19.4 4.8 2.8 1.4 6.0
15-20 .0 .0 .5 .0 2.3 4.8 10.4 1.5 .0 .5 .0 .5
10-15 .0 .0 .0 .0 .5 .5 1.0 .0 .0 .0 .0 .0
Windhoek
Surface Pressure Frequencies (%) in Each Pressure Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
840_845 .0 .0 .0 .0 .0 .0 3.3 2.5 .0 .0 .0 .0
835_840 .8 2.7 5.0 36.2 48.2 63.4 74.4 68.3 37.8 22.1 9.2 .8
830_835 66.9 80.2 86.0 61.2 45.9 35.7 22.3 29.2 58.0 76.2 75.0 90.2
825_830 32.3 17.1 9.1 2.6 5.9 .9 .0 .0 4.2 1.6 15.8 8.9
Grootfontein
Surface Pressure Frequencies (%) in Each Pressure Range by Month
1 2 3 4 5 6 7 8 9 10 11 12
860_865 .0 .0 .0 .0 .0 .0 4.9 .0 .0 .0 .0 .0
855_860 .8 .0 .8 15.8 36.1 55.8 65.0 58.1 11.7 5.8 2.5 .8
850_855 33.9 53.6 66.1 82.5 58.8 41.7 29.3 41.9 79.2 82.6 50.4 61.8
845_850 63.7 44.6 32.3 1.7 5.0 2.5 .8 .0 9.2 11.6 46.2 37.4
840_845 1.6 1.8 .8 .0 .0 .0 .0 .0 .0 .0 .8 .0
Grootfontein
Surface Pressure (hPa) Extremes (with Date) and Means by Month
1 2 3 4 5 6 7 8 9 10 11 12
855.5 853.9 855.0 858.0 859.4 858.7 861.4 859.5 859.4 856.3 859.3 859.5
10/00 17/00 5/98 20/99 24/97 18/99 21/98 22/98 25/99 30/00 3/00 26/00
849.2 849.8 850.5 853.3 854.0 855.0 855.8 855.0 852.6 851.8 850.2 850.6
844.0 844.6 840.6 848.7 847.2 848.3 849.3 850.4 847.0 848.2 844.8 847.2
12/00 14/98 28/99 17/98 30/97 10/98 14/97 16/99 12/97 18/98 22/00 30/98
Hosea Kutako
Surface Pressure (hPa) Extremes (with Date) and Means by Month
1 2 3 4 5 6 7 8 9 10 11 12
835.0 835.3 836.0 839.0 839.5 839.7 841.7 841.6 838.6 837.8 839.8 835.8
10/00 28/00 15/00 11/97 24/97 18/99 21/98 3/98 26/99 4/00 15/97 20/98
830.6 831.6 832.1 834.3 834.3 835.5 836.3 835.9 834.0 833.6 832.0 831.9
827.0 826.6 829.0 828.2 825.4 829.5 830.0 830.1 827.5 829.6 826.5 828.0
20/00 22/97 28/99 13/99 30/97 5/00 14/97 16/99 30/97 8/99 25/98 23/97