Good Hope Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Good Hope Observatory clear sky chart from 2005-11-06 to 2024-01-13.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	17% 10% 4 6% 6% 3 5% 5% 5% 7% 33%	74564
	Night	17% 10% 4 6% 5% 3 4 4 5% 6% 34%	75152
Transparency	Day	50% 7% 11% 19% 11% 2	74285
	Night	50% 6% 11% 19% 12% 2	74950
Seeing	Day	24% 14% 24% 22% 12% 5%	13722
	Night	23% 11% 24% 27% 13% 2	23802

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	18% 15% 8% 10% 9% 3 4 4 4 5% 21%	1706
	11th to 20th	Day	23% 17% 6% 10% 6% 4 4 3 4 4 19%	1638
	21st to end	Day	21% 18% 7% 10% 7% 3 3 3 3 4 21%	1782
February	1st to 10th	Day	23% 14% 5% 8% 8% 3 4 3 4 5% 23%	1620
	11th to 20th	Day	22% 14% 6% 10% 6% 3 4 3 4 6% 22%	1600
	21st to end	Day	21% 15% 6% 8% 6% 3 4 3 4 4 27%	1504
March	1st to 10th	Day	17% 13% 5% 6% 5% 2 3 4 4 6% 35%	1980
	11th to 20th	Day	20% 13% 4 6% 6% 2 4 4 4 6% 30%	1980
	21st to end	Day	21% 12% 4 6% 7% 2 4 5% 5% 6% 28%	2178
April	1st to 10th	Day	17% 10% 5% 6% 5% 3 5% 4 5% 7% 33%	2043
	11th to 20th	Day	17% 8% 3 6% 4 3 5% 4 5% 6% 38%	2159
	21st to end	Day	21% 8% 3 5% 6% 3 4 4 5% 7% 33%	2340
May	1st to 10th	Day	20% 8% 3 6% 7% 4 5% 5% 5% 8% 29%	2340
	11th to 20th	Day	21% 7% 3 7% 6% 4 5% 4 5% 7% 31%	2301
	21st to end	Day	13% 5% 3 6% 7% 4 5% 5% 6% 9% 37%	2531
June	1st to 10th	Day	14% 5% 3 5% 6% 3 5% 6% 7% 9% 38%	2461
	11th to 20th	Day	13% 6% 3 6% 7% 3 6% 5% 6% 9% 36%	2520
	21st to end	Day	14% 6% 2 5% 6% 4 6% 6% 6% 10% 36%	2397
July	1st to 10th	Day	12% 5% 3 7% 5% 4 6% 5% 7% 9% 36%	2340
	11th to 20th	Day	12% 6% 3 6% 6% 4 6% 6% 6% 9% 37%	2340
	21st to end	Day	14% 7% 3 7% 6% 4 6% 6% 6% 8% 33%	2547
August	1st to 10th	Day	11% 7% 3 6% 6% 4 6% 7% 8% 11% 31%	2327
	11th to 20th	Day	14% 7% 2 6% 6% 3 5% 5% 7% 11% 33%	2303
	21st to end	Day	12% 6% 2 5% 5% 4 5% 5% 7% 9% 39%	2453
September	1st to 10th	Day	15% 5% 2 5% 4 3 3 5% 5% 7% 46%	2120
	11th to 20th	Day	12% 6% 3 6% 4 3 4 5% 6% 8% 43%	1980
	21st to end	Day	18% 7% 3 6% 5% 3 4 6% 5% 6% 38%	1980
October	1st to 10th	Day	17% 6% 3 5% 5% 3 3 4 4 7% 43%	1823
	11th to 20th	Day	19% 8% 2 6% 5% 3 4 4 4 6% 40%	1800
	21st to end	Day	24% 8% 3 4 5% 2 4 4 4 6% 37%	1980
November	1st to 10th	Day	14% 8% 3 4 5% 2 5% 4 4 6% 47%	1800
	11th to 20th	Day	16% 11% 4 6% 6% 3 4 4 6% 8% 33%	1710
	21st to end	Day	19% 13% 5% 7% 5% 2 4 4 4 6% 31%	1523
December	1st to 10th	Day	25% 15% 7% 8% 5% 3 5% 4 4 5% 20%	1520
	11th to 20th	Day	22% 17% 7% 8% 5% 2 3 3 3 5% 26%	1910
	21st to end	Day	22% 20% 6% 7% 6% 3 5% 4 5% 7% 17%	3028
January	1st to 10th	Night	19% 17% 6% 9% 6% 3 5% 4 3 4 22%	2459
	11th to 20th	Night	23% 16% 7% 10% 8% 4 4 4 3 4 20%	2367
	21st to end	Night	19% 18% 5% 9% 6% 3 4 3 3 4 24%	2574
February	1st to 10th	Night	20% 15% 5% 9% 6% 3 4 4 4 5% 25%	2340
	11th to 20th	Night	21% 14% 5% 9% 7% 4 5% 4 4 4 24%	2243
	21st to end	Night	23% 14% 5% 7% 5% 2 4 5% 4 5% 26%	1750
March	1st to 10th	Night	18% 11% 4 6% 6% 2 4 3 4 5% 35%	1980
	11th to 20th	Night	23% 10% 4 6% 4 2 5% 4 4 6% 31%	1980
	21st to end	Night	20% 11% 4 7% 5% 2 4 4 5% 5% 33%	2178
April	1st to 10th	Night	20% 9% 4 5% 5% 3 4 4 4 7% 34%	1916
	11th to 20th	Night	17% 7% 2 4 5% 3 4 5% 6% 8% 38%	1766
	21st to end	Night	16% 7% 3 8% 5% 2 5% 3 5% 6% 39%	1620
May	1st to 10th	Night	19% 7% 4 6% 5% 4 5% 5% 5% 7% 33%	1620
	11th to 20th	Night	19% 7% 4 6% 5% 3 4 5% 6% 9% 32%	1592
	21st to end	Night	11% 6% 2 6% 5% 3 5% 6% 6% 8% 42%	1744
June	1st to 10th	Night	11% 7% 3 4 4 3 5% 5% 6% 10% 42%	1490
	11th to 20th	Night	10% 6% 3 5% 6% 4 5% 6% 7% 9% 38%	1440
	21st to end	Night	13% 4 3 4 5% 2 5% 6% 6% 9% 43%	1553
July	1st to 10th	Night	11% 6% 3 6% 7% 3 5% 6% 6% 8% 39%	1620
	11th to 20th	Night	10% 5% 3 5% 6% 3 5% 5% 6% 9% 42%	1620
	21st to end	Night	11% 6% 3 5% 5% 4 5% 6% 8% 9% 40%	1771
August	1st to 10th	Night	9% 5% 2 6% 7% 4 7% 6% 8% 9% 37%	1611
	11th to 20th	Night	12% 8% 2 4 6% 4 6% 5% 6% 9% 40%	1598
	21st to end	Night	11% 5% 3 5% 4 2 4 5% 5% 9% 47%	1885
September	1st to 10th	Night	12% 6% 3 5% 5% 4 5% 4 5% 7% 44%	1837
	11th to 20th	Night	12% 4 2 4 4 3 4 4 5% 7% 50%	1980
	21st to end	Night	15% 6% 2 5% 4 3 5% 5% 5% 6% 43%	1980
October	1st to 10th	Night	14% 5% 3 6% 5% 2 4 4 5% 8% 44%	2137
	11th to 20th	Night	16% 6% 2 5% 5% 3 5% 4 5% 6% 43%	2160
	21st to end	Night	25% 6% 2 5% 4 2 4 4 4 5% 39%	2376
November	1st to 10th	Night	13% 8% 3 6% 4 3 4 5% 4 5% 46%	2274
	11th to 20th	Night	19% 12% 4 6% 4 2 4 5% 5% 6% 33%	2470
	21st to end	Night	19% 11% 4 8% 7% 4 4 4 4 5% 32%	2653
December	1st to 10th	Night	23% 16% 7% 9% 5% 3 4 3 4 5% 22%	2660
	11th to 20th	Night	21% 16% 7% 8% 6% 2 4 3 4 4 25%	3285
	21st to end	Night	21% 17% 6% 8% 7% 3 4 4 4 5% 21%	4623

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	67% 6% 9% 13% 5% 1	1707
	11th to 20th	Day	70% 4 9% 13% 4	1638
	21st to end	Day	68% 4 8% 13% 7% 1	1782
February	1st to 10th	Day	65% 5% 8% 17% 4	1620
	11th to 20th	Day	65% 4 10% 15% 7% 1	1609
	21st to end	Day	62% 4 8% 15% 9% 1	1504
March	1st to 10th	Day	51% 4 10% 22% 11% 1	1980
	11th to 20th	Day	55% 5% 9% 19% 10% 1	1980
	21st to end	Day	57% 5% 8% 18% 11% 2	2178
April	1st to 10th	Day	51% 7% 10% 17% 12% 3	2043
	11th to 20th	Day	46% 6% 11% 16% 18% 3	2159
	21st to end	Day	50% 7% 12% 19% 11% 2	2340
May	1st to 10th	Day	54% 7% 11% 17% 9% 1	2340
	11th to 20th	Day	53% 7% 12% 17% 9% 2	2305
	21st to end	Day	42% 10% 16% 22% 10% 1	2531
June	1st to 10th	Day	41% 9% 14% 21% 14% 1	2461
	11th to 20th	Day	45% 9% 13% 20% 12% 1	2520
	21st to end	Day	42% 8% 14% 23% 11% 1	2397
July	1st to 10th	Day	42% 10% 12% 20% 13% 2	2275
	11th to 20th	Day	41% 9% 12% 21% 14% 2	2271
	21st to end	Day	45% 8% 12% 21% 11% 1	2405
August	1st to 10th	Day	43% 11% 15% 23% 8% 1	2314
	11th to 20th	Day	44% 8% 14% 24% 11%	2303
	21st to end	Day	38% 10% 14% 23% 12% 3	2453
September	1st to 10th	Day	37% 7% 11% 21% 21% 3	2120
	11th to 20th	Day	37% 7% 11% 26% 16% 3	1980
	21st to end	Day	44% 7% 11% 19% 17% 1	1980
October	1st to 10th	Day	42% 6% 12% 23% 16% 2	1823
	11th to 20th	Day	46% 6% 9% 25% 14%	1800
	21st to end	Day	49% 4 8% 22% 14% 2	1980
November	1st to 10th	Day	40% 4 10% 25% 17% 4	1800
	11th to 20th	Day	49% 6% 9% 18% 15% 3	1710
	21st to end	Day	55% 5% 9% 17% 13%	1523
December	1st to 10th	Day	68% 5% 8% 14% 4 1	1520
	11th to 20th	Day	63% 3 9% 17% 7% 1	1906
	21st to end	Day	66% 6% 11% 13% 4	3028
January	1st to 10th	Night	67% 4 8% 13% 7% 1	2459
	11th to 20th	Night	70% 5% 9% 12% 4	2367
	21st to end	Night	65% 4 9% 15% 5% 2	2574
February	1st to 10th	Night	62% 4 11% 15% 7%	2340
	11th to 20th	Night	64% 4 9% 15% 8%	2245
	21st to end	Night	60% 7% 9% 13% 9% 2	1750
March	1st to 10th	Night	52% 4 9% 24% 10% 2	1980
	11th to 20th	Night	54% 6% 10% 18% 11% 1	1980
	21st to end	Night	54% 5% 10% 17% 11% 4	2178
April	1st to 10th	Night	51% 5% 9% 20% 13% 2	1916
	11th to 20th	Night	41% 8% 13% 18% 15% 5%	1766
	21st to end	Night	45% 7% 12% 20% 14% 3	1620
May	1st to 10th	Night	50% 7% 11% 17% 11% 3	1620
	11th to 20th	Night	48% 9% 13% 16% 12% 2	1593
	21st to end	Night	38% 9% 17% 24% 11% 1	1745
June	1st to 10th	Night	38% 9% 12% 25% 15% 2	1490
	11th to 20th	Night	40% 11% 14% 20% 13% 2	1440
	21st to end	Night	35% 11% 15% 22% 15% 2	1553
July	1st to 10th	Night	41% 11% 13% 19% 14% 2	1575
	11th to 20th	Night	36% 9% 14% 23% 16% 2	1566
	21st to end	Night	37% 10% 15% 23% 15% 1	1672
August	1st to 10th	Night	39% 10% 18% 21% 11% 1	1600
	11th to 20th	Night	39% 10% 14% 23% 14% 1	1598
	21st to end	Night	33% 8% 13% 27% 15% 4	1885
September	1st to 10th	Night	39% 6% 13% 19% 18% 4	1837
	11th to 20th	Night	31% 8% 14% 24% 21% 2	1980
	21st to end	Night	40% 6% 13% 20% 19% 1	1980
October	1st to 10th	Night	38% 7% 12% 23% 17% 3	2137
	11th to 20th	Night	41% 7% 9% 25% 17% 2	2160
	21st to end	Night	48% 4 8% 22% 15% 2	2376
November	1st to 10th	Night	40% 6% 11% 19% 18% 6%	2274
	11th to 20th	Night	52% 5% 8% 18% 15% 3	2470
	21st to end	Night	56% 4 9% 18% 11% 1	2653
December	1st to 10th	Night	66% 5% 8% 15% 4 1	2660
	11th to 20th	Night	63% 4 10% 15% 7%	3288
	21st to end	Night	66% 6% 9% 14% 6%	4623

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	32% 29% 22% 12% 4 1	270
	11th to 20th	Day	36% 26% 30% 7% 1	246
	21st to end	Day	39% 27% 24% 6% 3	264
February	1st to 10th	Day	38% 24% 30% 7% 1	240
	11th to 20th	Day	47% 25% 21% 6%	240
	21st to end	Day	40% 24% 28% 8%	222
March	1st to 10th	Day	33% 18% 30% 15% 3	420
	11th to 20th	Day	31% 24% 25% 14% 6% 1	420
	21st to end	Day	33% 16% 31% 15% 5%	462
April	1st to 10th	Day	30% 21% 23% 20% 6%	420
	11th to 20th	Day	27% 17% 25% 18% 9% 5%	412
	21st to end	Day	29% 19% 26% 16% 7% 2	420
May	1st to 10th	Day	28% 18% 27% 14% 7% 5%	423
	11th to 20th	Day	24% 13% 28% 22% 9% 4	447
	21st to end	Day	15% 11% 24% 26% 16% 8%	485
June	1st to 10th	Day	16% 8% 25% 30% 16% 6%	450
	11th to 20th	Day	18% 8% 26% 25% 16% 7%	450
	21st to end	Day	14% 10% 30% 28% 14% 4	450
July	1st to 10th	Day	15% 5% 20% 26% 19% 15%	445
	11th to 20th	Day	11% 6% 21% 33% 20% 9%	444
	21st to end	Day	16% 6% 22% 28% 20% 9%	488
August	1st to 10th	Day	14% 4 17% 31% 21% 12%	446
	11th to 20th	Day	18% 7% 15% 29% 21% 10%	439
	21st to end	Day	13% 4 21% 30% 20% 12%	495
September	1st to 10th	Day	16% 6% 18% 28% 21% 11%	450
	11th to 20th	Day	13% 3 17% 30% 25% 12%	450
	21st to end	Day	21% 10% 23% 27% 14% 5%	450
October	1st to 10th	Day	18% 9% 24% 23% 21% 5%	450
	11th to 20th	Day	21% 14% 26% 24% 13% 2	450
	21st to end	Day	29% 17% 25% 19% 10% 1	495
November	1st to 10th	Day	20% 12% 27% 25% 12% 4	398
	11th to 20th	Day	30% 20% 23% 19% 6% 1	270
	21st to end	Day	28% 19% 36% 12% 3 1	181
December	1st to 10th	Day	31% 26% 27% 11% 3 2	180
	11th to 20th	Day	33% 24% 22% 17% 3 1	180
	21st to end	Day	38% 21% 22% 14% 5%	270
January	1st to 10th	Night	32% 21% 27% 17% 3	760
	11th to 20th	Night	35% 16% 27% 20% 2	729
	21st to end	Night	31% 20% 30% 18% 2	792
February	1st to 10th	Night	30% 16% 30% 22% 2	720
	11th to 20th	Night	31% 21% 32% 15% 1	703
	21st to end	Night	34% 17% 32% 16% 2	592
March	1st to 10th	Night	28% 17% 29% 24% 3	720
	11th to 20th	Night	31% 15% 29% 18% 6% 1	720
	21st to end	Night	27% 14% 31% 22% 5% 1	792
April	1st to 10th	Night	27% 16% 28% 24% 6%	720
	11th to 20th	Night	20% 14% 30% 24% 11% 1	702
	21st to end	Night	17% 12% 31% 32% 8%	540
May	1st to 10th	Night	23% 9% 28% 30% 10% 1	540
	11th to 20th	Night	20% 8% 29% 30% 11% 1	531
	21st to end	Night	12% 5% 22% 32% 25% 4	584
June	1st to 10th	Night	12% 4 23% 40% 21% 1	410
	11th to 20th	Night	14% 3 22% 37% 22% 2	360
	21st to end	Night	13% 3 22% 41% 20% 1	473
July	1st to 10th	Night	14% 3 15% 36% 28% 4	525
	11th to 20th	Night	12% 2 15% 34% 33% 4	522
	21st to end	Night	13% 3 15% 36% 29% 3	591
August	1st to 10th	Night	10% 3 13% 39% 33% 3	537
	11th to 20th	Night	15% 3 10% 38% 30% 4	534
	21st to end	Night	13% 4 12% 34% 31% 6%	697
September	1st to 10th	Night	15% 3 15% 34% 28% 5%	720
	11th to 20th	Night	15% 3 16% 31% 32% 4	720
	21st to end	Night	17% 7% 21% 31% 22% 2	720
October	1st to 10th	Night	18% 7% 18% 30% 23% 4	720
	11th to 20th	Night	19% 10% 27% 31% 12% 1	720
	21st to end	Night	29% 11% 20% 30% 10% 1	792
November	1st to 10th	Night	17% 9% 25% 32% 16% 1	740
	11th to 20th	Night	29% 12% 25% 25% 8% 1	760
	21st to end	Night	26% 14% 27% 25% 7% 1	760
December	1st to 10th	Night	33% 18% 25% 19% 4	760
	11th to 20th	Night	27% 19% 26% 24% 4	760
	21st to end	Night	30% 15% 27% 26% 2	836

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