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 2025-12-03.

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	16% 10% 3 8% 5% 4 5% 4 5% 7% 32%	126412
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 32%	124265
Transparency	Day	50% 5% 10% 21% 12% 2	130030
	Night	52% 5% 10% 19% 12% 2	128716
Seeing	Day	24% 14% 24% 21% 12% 5%	16255
	Night	23% 11% 24% 28% 13% 1	26265

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	17% 20% 9% 17% 7% 3 3 2 3 4 15%	4056
	11th to 20th	Day	16% 22% 8% 12% 9% 5% 4 2 3 3 16%	4060
	21st to end	Day	19% 18% 5% 11% 6% 4 4 3 5% 5% 22%	4466
February	1st to 10th	Day	23% 14% 5% 9% 6% 4 4 2 4 5% 24%	1800
	11th to 20th	Day	22% 15% 5% 11% 5% 4 3 2 4 6% 22%	1783
	21st to end	Day	20% 16% 5% 10% 4 3 3 3 4 3 28%	1678
March	1st to 10th	Day	18% 17% 4 6% 4 3 3 3 4 6% 33%	4935
	11th to 20th	Day	14% 9% 3 6% 4 4 4 3 5% 7% 41%	4980
	21st to end	Day	18% 10% 3 9% 6% 4 5% 4 5% 6% 29%	5465
April	1st to 10th	Day	18% 12% 4 7% 5% 4 4 3 5% 7% 31%	2271
	11th to 20th	Day	16% 9% 2 7% 4 5% 4 4 5% 6% 38%	2403
	21st to end	Day	20% 9% 3 6% 5% 5% 4 3 6% 7% 33%	2600
May	1st to 10th	Day	18% 9% 3 9% 6% 6% 6% 4 7% 8% 25%	5862
	11th to 20th	Day	18% 8% 3 8% 5% 5% 6% 4 6% 7% 30%	5861
	21st to end	Day	17% 7% 2 7% 5% 5% 5% 4 6% 9% 32%	6433
June	1st to 10th	Day	14% 6% 2 7% 5% 4 5% 5% 7% 9% 37%	2735
	11th to 20th	Day	13% 6% 3 7% 6% 5% 6% 4 7% 9% 35%	2800
	21st to end	Day	13% 6% 2 5% 5% 5% 6% 4 6% 10% 37%	2663
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	5900
	11th to 20th	Day	12% 7% 3 7% 5% 6% 6% 5% 8% 10% 31%	5900
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 5% 6% 8% 33%	6424
August	1st to 10th	Day	11% 8% 3 8% 5% 5% 6% 5% 8% 9% 31%	4234
	11th to 20th	Day	11% 6% 3 8% 5% 4 5% 5% 7% 9% 36%	4174
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 7% 10% 48%	4449
September	1st to 10th	Day	14% 5% 2 6% 3 3 4 4 5% 7% 47%	2354
	11th to 20th	Day	11% 6% 2 7% 4 4 4 4 5% 8% 44%	2200
	21st to end	Day	19% 8% 3 7% 4 4 5% 4 5% 6% 36%	2200
October	1st to 10th	Day	16% 6% 3 6% 4 3 3 3 5% 6% 46%	2025
	11th to 20th	Day	17% 8% 2 7% 4 4 4 3 4 6% 41%	2000
	21st to end	Day	21% 9% 2 5% 4 3 4 3 4 6% 38%	2200
November	1st to 10th	Day	13% 8% 2 5% 4 4 5% 3 4 6% 46%	1994
	11th to 20th	Day	16% 13% 3 7% 5% 4 4 3 6% 7% 32%	1890
	21st to end	Day	18% 16% 5% 8% 4 3 3 3 4 6% 29%	1683
December	1st to 10th	Day	19% 13% 6% 19% 5% 4 4 3 4 4 21%	2616
	11th to 20th	Day	23% 16% 6% 9% 4 3 3 3 4 5% 24%	2990
	21st to end	Day	19% 17% 5% 10% 5% 4 5% 4 5% 6% 18%	4328
January	1st to 10th	Night	16% 20% 8% 18% 5% 3 3 2 3 3 19%	5889
	11th to 20th	Night	17% 17% 7% 14% 8% 5% 3 3 3 4 18%	5900
	21st to end	Night	21% 14% 5% 10% 6% 4 3 2 3 3 27%	6490
February	1st to 10th	Night	20% 16% 4 10% 6% 4 4 3 4 5% 27%	2600
	11th to 20th	Night	21% 15% 5% 10% 5% 5% 4 3 4 4 24%	2500
	21st to end	Night	21% 14% 5% 9% 4 3 4 4 4 6% 26%	1950
March	1st to 10th	Night	21% 12% 4 6% 4 4 4 3 4 5% 34%	4918
	11th to 20th	Night	13% 11% 4 6% 3 4 4 3 5% 7% 40%	4980
	21st to end	Night	15% 10% 3 8% 5% 4 5% 4 5% 6% 34%	5467
April	1st to 10th	Night	20% 11% 4 6% 5% 4 3 4 4 6% 33%	2128
	11th to 20th	Night	17% 7% 2 5% 4 4 4 4 6% 8% 37%	1964
	21st to end	Night	15% 7% 3 8% 5% 4 4 3 6% 7% 39%	1800
May	1st to 10th	Night	24% 11% 4 8% 5% 4 4 3 4 7% 24%	4033
	11th to 20th	Night	16% 8% 3 7% 4 5% 5% 4 6% 8% 33%	4032
	21st to end	Night	20% 11% 3 8% 5% 4 4 3 5% 6% 30%	4421
June	1st to 10th	Night	12% 7% 3 5% 4 4 4 4 6% 9% 41%	1655
	11th to 20th	Night	10% 7% 3 7% 5% 5% 6% 4 7% 9% 38%	1600
	21st to end	Night	12% 6% 2 5% 4 4 6% 4 7% 9% 42%	1727
July	1st to 10th	Night	11% 8% 3 7% 5% 4 5% 5% 6% 8% 37%	4058
	11th to 20th	Night	13% 9% 4 7% 6% 5% 5% 4 6% 9% 32%	4057
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 4 8% 8% 35%	4427
August	1st to 10th	Night	12% 8% 3 8% 5% 6% 6% 4 7% 9% 32%	2915
	11th to 20th	Night	13% 9% 2 7% 5% 5% 5% 4 5% 7% 38%	2875
	21st to end	Night	7% 4 2 6% 5% 4 5% 4 6% 9% 46%	3416
September	1st to 10th	Night	12% 6% 3 6% 4 5% 4 3 5% 7% 45%	2042
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 5% 7% 51%	2200
	21st to end	Night	16% 7% 2 7% 4 5% 4 4 5% 6% 40%	2200
October	1st to 10th	Night	14% 5% 2 6% 4 3 4 3 5% 7% 46%	2375
	11th to 20th	Night	15% 6% 2 6% 5% 4 4 3 5% 6% 44%	2400
	21st to end	Night	23% 7% 2 6% 4 3 4 3 4 5% 40%	2640
November	1st to 10th	Night	13% 8% 3 6% 4 4 3 4 4 5% 46%	2520
	11th to 20th	Night	19% 13% 3 7% 3 3 5% 3 4 5% 33%	2730
	21st to end	Night	18% 13% 3 9% 6% 5% 4 3 4 5% 30%	2933
December	1st to 10th	Night	17% 14% 5% 18% 5% 4 4 3 4 5% 22%	4609
	11th to 20th	Night	21% 16% 7% 10% 4 4 3 3 4 4 24%	5205
	21st to end	Night	18% 17% 6% 10% 6% 5% 4 3 5% 5% 22%	6609

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	75% 4 8% 9% 4	4057
	11th to 20th	Day	76% 3 5% 9% 7%	4060
	21st to end	Day	66% 4 8% 14% 8% 1	4466
February	1st to 10th	Day	63% 5% 8% 17% 6%	1800
	11th to 20th	Day	64% 4 10% 14% 8% 1	1792
	21st to end	Day	61% 4 9% 16% 10% 1	1678
March	1st to 10th	Day	53% 4 6% 22% 11% 4	4935
	11th to 20th	Day	43% 4 8% 30% 13% 3	4980
	21st to end	Day	55% 4 7% 20% 13% 2	5465
April	1st to 10th	Day	53% 7% 10% 16% 12% 3	2271
	11th to 20th	Day	46% 5% 11% 17% 17% 3	2403
	21st to end	Day	49% 6% 11% 21% 11% 2	2600
May	1st to 10th	Day	56% 5% 11% 18% 8% 2	5897
	11th to 20th	Day	52% 7% 10% 22% 9% 1	5865
	21st to end	Day	46% 7% 13% 21% 11% 2	6447
June	1st to 10th	Day	41% 9% 14% 20% 14% 1	2735
	11th to 20th	Day	45% 9% 13% 21% 12% 1	2800
	21st to end	Day	42% 8% 14% 23% 12% 1	2663
July	1st to 10th	Day	39% 6% 10% 25% 17% 2	5835
	11th to 20th	Day	47% 5% 13% 25% 9% 1	5831
	21st to end	Day	48% 4 11% 19% 17% 1	6240
August	1st to 10th	Day	46% 6% 14% 26% 7%	4221
	11th to 20th	Day	43% 4 11% 28% 13%	4213
	21st to end	Day	31% 6% 14% 30% 17% 2	4449
September	1st to 10th	Day	36% 6% 10% 21% 23% 3	2354
	11th to 20th	Day	37% 6% 10% 27% 17% 3	2200
	21st to end	Day	48% 7% 11% 18% 16% 1	2200
October	1st to 10th	Day	32% 3 7% 26% 28% 3	3298
	11th to 20th	Day	40% 3 10% 24% 16% 7%	3240
	21st to end	Day	34% 3 9% 31% 20% 4	3538
November	1st to 10th	Day	41% 4 10% 25% 17% 4	1994
	11th to 20th	Day	50% 5% 9% 18% 15% 3	1890
	21st to end	Day	58% 5% 9% 17% 12%	1683
December	1st to 10th	Day	69% 3 9% 15% 3	2616
	11th to 20th	Day	65% 2 11% 16% 6%	2986
	21st to end	Day	65% 5% 10% 15% 5%	4328
January	1st to 10th	Night	72% 3 7% 13% 4 1	5889
	11th to 20th	Night	72% 3 8% 11% 5% 1	5900
	21st to end	Night	64% 3 8% 17% 8% 1	6490
February	1st to 10th	Night	61% 4 11% 15% 9%	2600
	11th to 20th	Night	64% 4 9% 14% 9%	2502
	21st to end	Night	59% 7% 9% 14% 9% 2	1950
March	1st to 10th	Night	54% 4 7% 23% 11% 3	4918
	11th to 20th	Night	44% 4 7% 27% 14% 3	4980
	21st to end	Night	50% 4 10% 20% 14% 2	5467
April	1st to 10th	Night	53% 5% 9% 19% 13% 2	2128
	11th to 20th	Night	42% 8% 12% 19% 15% 4	1964
	21st to end	Night	44% 6% 12% 21% 15% 2	1800
May	1st to 10th	Night	60% 6% 10% 13% 9% 2	4060
	11th to 20th	Night	47% 9% 12% 22% 9% 1	4033
	21st to end	Night	52% 7% 13% 19% 7% 2	4429
June	1st to 10th	Night	39% 8% 12% 24% 16% 2	1655
	11th to 20th	Night	40% 11% 14% 21% 13% 2	1600
	21st to end	Night	36% 10% 15% 21% 16% 2	1727
July	1st to 10th	Night	45% 6% 10% 23% 15% 2	4015
	11th to 20th	Night	49% 5% 15% 22% 9% 1	4006
	21st to end	Night	45% 5% 12% 19% 17% 1	4297
August	1st to 10th	Night	47% 6% 16% 23% 7% 1	2904
	11th to 20th	Night	45% 5% 9% 23% 18% 1	2902
	21st to end	Night	35% 5% 11% 32% 16% 2	3416
September	1st to 10th	Night	39% 6% 12% 19% 21% 5%	2042
	11th to 20th	Night	32% 7% 12% 24% 22% 3	2200
	21st to end	Night	44% 6% 12% 19% 18% 1	2200
October	1st to 10th	Night	33% 4 8% 27% 23% 5%	3882
	11th to 20th	Night	37% 4 8% 23% 20% 8%	3899
	21st to end	Night	35% 2 6% 28% 25% 4	4252
November	1st to 10th	Night	40% 5% 11% 20% 19% 6%	2520
	11th to 20th	Night	53% 4 7% 18% 14% 3	2730
	21st to end	Night	57% 4 9% 18% 10% 1	2933
December	1st to 10th	Night	66% 3 10% 16% 4 1	4609
	11th to 20th	Night	65% 2 12% 15% 6%	5208
	21st to end	Night	65% 4 9% 15% 7%	6609

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	34% 29% 21% 11% 4 1	300
	11th to 20th	Day	39% 26% 27% 7% 1	300
	21st to end	Day	36% 30% 24% 7% 3	330
February	1st to 10th	Day	36% 24% 30% 8% 2	300
	11th to 20th	Day	48% 24% 21% 7% 1	300
	21st to end	Day	38% 25% 29% 8%	277
March	1st to 10th	Day	34% 18% 28% 15% 4	500
	11th to 20th	Day	31% 24% 25% 13% 5% 1	500
	21st to end	Day	32% 17% 32% 15% 4	550
April	1st to 10th	Day	32% 22% 22% 18% 5%	500
	11th to 20th	Day	29% 18% 25% 17% 8% 4	492
	21st to end	Day	27% 18% 28% 18% 7% 2	500
May	1st to 10th	Day	27% 18% 28% 16% 7% 5%	503
	11th to 20th	Day	23% 15% 27% 22% 10% 4	527
	21st to end	Day	17% 12% 25% 24% 14% 7%	573
June	1st to 10th	Day	15% 10% 26% 28% 16% 5%	530
	11th to 20th	Day	18% 7% 25% 26% 17% 7%	530
	21st to end	Day	13% 9% 29% 28% 15% 6%	530
July	1st to 10th	Day	15% 4 21% 26% 20% 13%	525
	11th to 20th	Day	12% 6% 22% 32% 19% 9%	524
	21st to end	Day	17% 6% 21% 27% 21% 8%	576
August	1st to 10th	Day	14% 4 19% 31% 20% 11%	526
	11th to 20th	Day	17% 6% 16% 29% 21% 10%	519
	21st to end	Day	11% 4 20% 31% 22% 11%	583
September	1st to 10th	Day	15% 6% 18% 29% 21% 11%	530
	11th to 20th	Day	12% 2 16% 31% 25% 12%	530
	21st to end	Day	23% 9% 23% 27% 13% 4	530
October	1st to 10th	Day	17% 9% 23% 23% 22% 6%	530
	11th to 20th	Day	21% 13% 26% 24% 13% 3	530
	21st to end	Day	27% 17% 25% 19% 10% 1	583
November	1st to 10th	Day	19% 14% 29% 23% 12% 3	472
	11th to 20th	Day	32% 20% 25% 18% 5% 1	330
	21st to end	Day	34% 18% 32% 11% 4 1	221
December	1st to 10th	Day	31% 25% 28% 11% 3 1	204
	11th to 20th	Day	35% 23% 22% 17% 3 1	200
	21st to end	Day	37% 21% 22% 15% 5%	300
January	1st to 10th	Night	32% 21% 27% 17% 3	800
	11th to 20th	Night	35% 17% 28% 19% 2	800
	21st to end	Night	30% 19% 31% 17% 2	880
February	1st to 10th	Night	30% 16% 30% 23% 2	800
	11th to 20th	Night	33% 21% 31% 15% 1	783
	21st to end	Night	32% 17% 33% 17% 2	660
March	1st to 10th	Night	29% 16% 28% 24% 3	800
	11th to 20th	Night	31% 16% 28% 18% 6% 1	800
	21st to end	Night	27% 16% 32% 21% 4	880
April	1st to 10th	Night	27% 16% 29% 23% 5%	800
	11th to 20th	Night	20% 15% 30% 24% 10% 1	780
	21st to end	Night	17% 12% 32% 32% 8%	600
May	1st to 10th	Night	24% 9% 27% 30% 10% 1	600
	11th to 20th	Night	19% 8% 29% 31% 12% 1	591
	21st to end	Night	14% 5% 22% 31% 24% 4	650
June	1st to 10th	Night	12% 4 23% 40% 20% 1	455
	11th to 20th	Night	14% 3 22% 38% 21% 2	400
	21st to end	Night	13% 3 20% 40% 22% 2	527
July	1st to 10th	Night	13% 3 15% 37% 28% 4	585
	11th to 20th	Night	13% 2 14% 34% 32% 4	582
	21st to end	Night	13% 2 14% 37% 30% 3	657
August	1st to 10th	Night	10% 3 14% 40% 31% 3	597
	11th to 20th	Night	15% 3 11% 37% 30% 4	594
	21st to end	Night	12% 4 12% 35% 32% 6%	775
September	1st to 10th	Night	15% 3 15% 36% 27% 5%	800
	11th to 20th	Night	14% 3 15% 32% 33% 4	800
	21st to end	Night	18% 7% 21% 32% 20% 2	800
October	1st to 10th	Night	17% 7% 18% 30% 25% 4	800
	11th to 20th	Night	18% 9% 27% 32% 13% 1	800
	21st to end	Night	27% 10% 20% 31% 11% 1	880
November	1st to 10th	Night	18% 10% 25% 33% 15% 1	820
	11th to 20th	Night	30% 12% 25% 24% 8% 1	840
	21st to end	Night	27% 14% 27% 24% 7% 1	840
December	1st to 10th	Night	33% 17% 26% 19% 4	809
	11th to 20th	Night	28% 19% 26% 24% 4	800
	21st to end	Night	29% 15% 26% 27% 2	880

Page generated at 2025-11-30 01:00:55. Version ac07

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