StarHub Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the StarHub Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	12% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	130285
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	130865
Transparency	Day	40% 6% 12% 20% 17% 5%	133884
	Night	39% 6% 11% 19% 19% 6%	135335
Seeing	Day	16% 10% 24% 25% 18% 8%	17126
	Night	16% 8% 25% 33% 16% 2	27249

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	11% 5% 2 5% 3 3 3 2 3 5% 56%	5258
	11th to 20th	Day	14% 5% 2 5% 3 3 3 2 3 4 58%	5280
	21st to end	Day	15% 10% 3 8% 4 4 4 3 5% 5% 40%	5780
February	1st to 10th	Day	16% 7% 3 6% 4 4 3 3 4 6% 44%	1890
	11th to 20th	Day	20% 6% 2 6% 4 3 3 2 4 5% 45%	1961
	21st to end	Day	16% 7% 2 6% 5% 4 4 3 5% 7% 41%	1787
March	1st to 10th	Day	21% 7% 2 6% 4 4 4 3 5% 6% 38%	4957
	11th to 20th	Day	10% 6% 2 4 3 4 3 3 4 6% 54%	4980
	21st to end	Day	13% 5% 2 5% 4 4 4 3 4 5% 51%	5465
April	1st to 10th	Day	11% 5% 2 4 4 5% 3 4 5% 7% 51%	2392
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 6% 8% 51%	2374
	21st to end	Day	10% 5% 2 5% 4 4 4 4 5% 7% 48%	2400
May	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 11% 35%	5465
	11th to 20th	Day	17% 8% 3 7% 5% 5% 6% 4 7% 9% 28%	5861
	21st to end	Day	11% 5% 2 5% 5% 5% 6% 4 6% 9% 41%	6433
June	1st to 10th	Day	10% 7% 2 7% 6% 6% 5% 4 8% 9% 36%	2780
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 9% 41%	2800
	21st to end	Day	8% 6% 2 6% 6% 6% 6% 5% 8% 11% 37%	2663
July	1st to 10th	Day	7% 6% 2 9% 6% 7% 7% 6% 10% 13% 28%	5900
	11th to 20th	Day	8% 6% 3 9% 7% 7% 7% 6% 10% 13% 25%	5900
	21st to end	Day	9% 7% 3 8% 7% 7% 6% 6% 8% 11% 29%	6424
August	1st to 10th	Day	21% 11% 4 9% 6% 6% 5% 5% 7% 8% 18%	4234
	11th to 20th	Day	10% 7% 3 8% 6% 5% 6% 5% 6% 10% 34%	3866
	21st to end	Day	7% 4 2 7% 5% 5% 6% 4 8% 10% 42%	4312
September	1st to 10th	Day	11% 5% 3 6% 6% 5% 5% 5% 7% 8% 38%	2400
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 5% 8% 37%	2397
	21st to end	Day	17% 8% 4 9% 4 4 4 3 6% 7% 35%	2086
October	1st to 10th	Day	15% 5% 3 6% 4 4 4 3 4 7% 46%	2000
	11th to 20th	Day	14% 6% 1 5% 3 3 4 3 4 5% 53%	2000
	21st to end	Day	12% 4 2 6% 3 4 4 3 4 5% 54%	2200
November	1st to 10th	Day	13% 6% 2 6% 3 3 3 3 4 6% 51%	2057
	11th to 20th	Day	15% 5% 1 5% 4 3 4 2 5% 5% 51%	2016
	21st to end	Day	12% 6% 2 6% 3 2 3 2 6% 6% 52%	1811
December	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 4 5% 52%	2680
	11th to 20th	Day	14% 8% 2 6% 4 3 3 3 4 6% 46%	3070
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 7% 46%	4406
January	1st to 10th	Night	10% 7% 2 5% 3 3 3 3 5% 7% 52%	7635
	11th to 20th	Night	11% 5% 2 6% 4 3 3 3 4 5% 55%	7680
	21st to end	Night	16% 9% 3 7% 4 3 3 3 4 6% 43%	8401
February	1st to 10th	Night	18% 7% 2 5% 3 3 4 3 5% 6% 44%	2730
	11th to 20th	Night	19% 8% 3 5% 4 3 4 3 5% 5% 42%	2542
	21st to end	Night	16% 6% 3 5% 4 4 4 3 5% 6% 44%	2015
March	1st to 10th	Night	19% 8% 3 6% 4 4 3 3 4 5% 40%	4941
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 8% 50%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 3 4 6% 50%	5467
April	1st to 10th	Night	12% 5% 2 6% 4 4 4 3 6% 7% 48%	2006
	11th to 20th	Night	9% 6% 2 6% 3 4 4 3 6% 7% 49%	1975
	21st to end	Night	10% 4 2 5% 4 3 4 4 5% 8% 51%	2000
May	1st to 10th	Night	9% 6% 3 8% 5% 5% 5% 4 7% 10% 37%	4472
	11th to 20th	Night	17% 7% 3 8% 5% 4 5% 4 6% 8% 33%	4032
	21st to end	Night	13% 6% 2 6% 4 4 4 4 7% 9% 40%	4421
June	1st to 10th	Night	8% 6% 2 6% 5% 6% 5% 5% 8% 10% 39%	1610
	11th to 20th	Night	9% 6% 3 7% 5% 4 6% 4 7% 11% 38%	1600
	21st to end	Night	8% 5% 3 7% 5% 5% 6% 6% 8% 11% 38%	1727
July	1st to 10th	Night	7% 6% 2 8% 5% 5% 5% 6% 8% 14% 34%	4058
	11th to 20th	Night	9% 7% 3 8% 6% 6% 7% 7% 9% 13% 25%	4057
	21st to end	Night	11% 8% 3 9% 6% 6% 6% 5% 7% 11% 29%	4427
August	1st to 10th	Night	17% 10% 4 9% 6% 5% 6% 4 6% 9% 25%	2915
	11th to 20th	Night	10% 7% 3 9% 6% 5% 6% 5% 6% 9% 33%	3135
	21st to end	Night	7% 4 2 6% 5% 5% 6% 5% 8% 9% 43%	3586
September	1st to 10th	Night	11% 5% 2 5% 5% 5% 5% 3 6% 10% 43%	2000
	11th to 20th	Night	15% 6% 2 7% 5% 4 5% 4 6% 8% 40%	2000
	21st to end	Night	18% 7% 3 7% 6% 5% 4 3 5% 6% 35%	2310
October	1st to 10th	Night	13% 6% 2 5% 3 3 3 4 5% 6% 49%	2400
	11th to 20th	Night	14% 5% 2 4 3 4 3 2 4 6% 54%	2400
	21st to end	Night	11% 4 2 5% 3 3 4 3 4 5% 55%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 6% 52%	2461
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 5% 50%	2599
	21st to end	Night	12% 5% 2 5% 3 4 3 3 5% 7% 51%	2804
December	1st to 10th	Night	11% 6% 2 5% 3 3 3 2 4 5% 55%	4720
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 4 6% 42%	5345
	21st to end	Night	12% 6% 2 5% 3 3 4 3 5% 7% 49%	6774

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	33% 2 9% 23% 25% 7%	5259
	11th to 20th	Day	33% 3 3 14% 34% 13%	5280
	21st to end	Day	48% 3 4 17% 18% 10%	5780
February	1st to 10th	Day	42% 4 10% 20% 19% 6%	1890
	11th to 20th	Day	44% 3 7% 17% 23% 5%	1970
	21st to end	Day	44% 5% 9% 19% 18% 7%	1787
March	1st to 10th	Day	47% 4 7% 14% 19% 9%	4957
	11th to 20th	Day	32% 4 7% 16% 26% 16%	4980
	21st to end	Day	37% 4 8% 22% 23% 7%	5465
April	1st to 10th	Day	33% 5% 8% 22% 24% 8%	2392
	11th to 20th	Day	31% 5% 11% 24% 21% 8%	2374
	21st to end	Day	34% 7% 12% 23% 20% 4	2400
May	1st to 10th	Day	41% 8% 19% 22% 9% 1	5497
	11th to 20th	Day	50% 7% 14% 19% 10% 1	5865
	21st to end	Day	37% 8% 13% 24% 16% 2	6447
June	1st to 10th	Day	42% 9% 15% 19% 14% 1	2780
	11th to 20th	Day	37% 9% 17% 22% 14% 2	2800
	21st to end	Day	39% 11% 16% 22% 11% 2	2663
July	1st to 10th	Day	44% 9% 21% 17% 7% 1	5835
	11th to 20th	Day	48% 11% 27% 12% 3	5831
	21st to end	Day	46% 9% 17% 20% 7%	6240
August	1st to 10th	Day	61% 8% 17% 10% 4	4221
	11th to 20th	Day	44% 8% 12% 24% 11% 1	3902
	21st to end	Day	36% 7% 15% 28% 14% 1	4312
September	1st to 10th	Day	41% 10% 13% 18% 14% 4	2400
	11th to 20th	Day	46% 5% 11% 17% 17% 3	2397
	21st to end	Day	49% 6% 10% 18% 15% 3	2086
October	1st to 10th	Day	36% 5% 7% 24% 22% 6%	3260
	11th to 20th	Day	23% 3 5% 20% 23% 26%	3240
	21st to end	Day	27% 3 4 25% 29% 12%	3538
November	1st to 10th	Day	36% 3 8% 19% 27% 7%	2057
	11th to 20th	Day	36% 4 7% 21% 25% 7%	2016
	21st to end	Day	33% 4 8% 25% 25% 6%	1811
December	1st to 10th	Day	35% 4 8% 22% 23% 9%	2680
	11th to 20th	Day	41% 4 11% 20% 18% 6%	3066
	21st to end	Day	39% 3 12% 25% 20% 2	4406
January	1st to 10th	Night	33% 3 11% 20% 28% 5%	7636
	11th to 20th	Night	33% 3 5% 14% 33% 11%	7680
	21st to end	Night	44% 3 5% 18% 22% 8%	8401
February	1st to 10th	Night	42% 4 8% 20% 20% 6%	2730
	11th to 20th	Night	44% 4 9% 16% 19% 6%	2544
	21st to end	Night	41% 4 9% 17% 20% 8%	2015
March	1st to 10th	Night	47% 4 6% 14% 21% 8%	4941
	11th to 20th	Night	34% 5% 8% 12% 24% 16%	4980
	21st to end	Night	36% 5% 9% 24% 20% 6%	5467
April	1st to 10th	Night	36% 5% 12% 18% 22% 8%	2006
	11th to 20th	Night	33% 7% 11% 23% 19% 7%	1975
	21st to end	Night	31% 7% 13% 28% 17% 4	2000
May	1st to 10th	Night	41% 8% 19% 21% 10% 2	4502
	11th to 20th	Night	48% 7% 12% 19% 12% 1	4033
	21st to end	Night	39% 8% 15% 24% 13% 1	4429
June	1st to 10th	Night	37% 10% 17% 21% 12% 2	1610
	11th to 20th	Night	38% 10% 17% 24% 11% 1	1600
	21st to end	Night	37% 10% 18% 23% 10% 1	1727
July	1st to 10th	Night	40% 9% 27% 18% 5% 1	4015
	11th to 20th	Night	47% 12% 27% 10% 3	4006
	21st to end	Night	48% 9% 16% 19% 7%	4297
August	1st to 10th	Night	56% 8% 22% 13% 3	2904
	11th to 20th	Night	46% 8% 19% 20% 7% 1	3164
	21st to end	Night	34% 8% 14% 30% 11% 2	3586
September	1st to 10th	Night	36% 9% 17% 18% 16% 4	2000
	11th to 20th	Night	42% 9% 12% 19% 16% 3	2000
	21st to end	Night	49% 6% 11% 19% 14% 2	2310
October	1st to 10th	Night	39% 5% 6% 20% 25% 4	3920
	11th to 20th	Night	23% 3 6% 17% 24% 27%	3899
	21st to end	Night	25% 3 6% 27% 27% 12%	4252
November	1st to 10th	Night	35% 4 6% 21% 27% 7%	2461
	11th to 20th	Night	37% 4 8% 16% 26% 9%	2599
	21st to end	Night	33% 4 8% 23% 25% 7%	2804
December	1st to 10th	Night	33% 4 8% 14% 32% 10%	4720
	11th to 20th	Night	44% 3 13% 15% 18% 7%	5348
	21st to end	Night	35% 4 12% 26% 20% 2	6774

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	21% 24% 39% 11% 5%	330
	11th to 20th	Day	18% 26% 32% 21% 3	330
	21st to end	Day	25% 20% 35% 15% 5%	363
February	1st to 10th	Day	20% 21% 38% 18% 3 1	330
	11th to 20th	Day	32% 16% 34% 15% 3	449
	21st to end	Day	22% 15% 35% 21% 7%	446
March	1st to 10th	Day	20% 16% 33% 22% 7% 1	508
	11th to 20th	Day	21% 19% 36% 17% 6% 1	500
	21st to end	Day	19% 17% 32% 23% 9% 1	550
April	1st to 10th	Day	18% 18% 31% 21% 9% 3	500
	11th to 20th	Day	14% 12% 31% 26% 12% 5%	492
	21st to end	Day	13% 10% 29% 26% 19% 4	500
May	1st to 10th	Day	15% 12% 34% 21% 14% 5%	503
	11th to 20th	Day	16% 8% 21% 30% 18% 7%	527
	21st to end	Day	16% 5% 18% 27% 21% 12%	573
June	1st to 10th	Day	13% 2 21% 32% 24% 8%	530
	11th to 20th	Day	11% 4 16% 31% 23% 17%	530
	21st to end	Day	8% 2 15% 31% 28% 16%	530
July	1st to 10th	Day	10% 2 10% 30% 31% 17%	525
	11th to 20th	Day	9% 3 9% 27% 35% 16%	524
	21st to end	Day	7% 2 13% 31% 35% 12%	576
August	1st to 10th	Day	18% 4 12% 25% 26% 16%	526
	11th to 20th	Day	14% 2 15% 21% 28% 19%	519
	21st to end	Day	12% 3 12% 28% 27% 17%	583
September	1st to 10th	Day	12% 3 18% 26% 22% 18%	530
	11th to 20th	Day	19% 4 13% 26% 25% 12%	530
	21st to end	Day	22% 4 22% 23% 22% 8%	530
October	1st to 10th	Day	17% 5% 23% 23% 22% 10%	530
	11th to 20th	Day	14% 10% 23% 31% 17% 5%	530
	21st to end	Day	13% 9% 25% 30% 19% 5%	583
November	1st to 10th	Day	18% 8% 28% 30% 13% 4	535
	11th to 20th	Day	19% 11% 31% 29% 9% 1	496
	21st to end	Day	15% 14% 28% 26% 15% 2	351
December	1st to 10th	Day	20% 15% 39% 17% 9%	220
	11th to 20th	Day	18% 15% 34% 22% 10% 2	220
	21st to end	Day	17% 18% 39% 15% 8% 2	327
January	1st to 10th	Night	19% 15% 32% 28% 5%	840
	11th to 20th	Night	17% 14% 32% 30% 7%	840
	21st to end	Night	20% 10% 39% 27% 4	924
February	1st to 10th	Night	20% 12% 35% 28% 5%	840
	11th to 20th	Night	24% 16% 36% 23% 1	823
	21st to end	Night	20% 15% 35% 27% 3	692
March	1st to 10th	Night	15% 12% 36% 30% 6%	809
	11th to 20th	Night	17% 13% 38% 25% 7% 1	800
	21st to end	Night	17% 11% 37% 29% 6%	880
April	1st to 10th	Night	14% 14% 38% 30% 5%	800
	11th to 20th	Night	12% 10% 36% 31% 10%	791
	21st to end	Night	13% 6% 34% 38% 9% 1	800
May	1st to 10th	Night	12% 9% 31% 38% 9%	795
	11th to 20th	Night	16% 5% 23% 40% 16%	591
	21st to end	Night	12% 2 14% 39% 30% 2	650
June	1st to 10th	Night	9% 3 20% 47% 20%	410
	11th to 20th	Night	10% 4 13% 47% 25% 3	400
	21st to end	Night	7% 2 15% 41% 33% 2	527
July	1st to 10th	Night	10% 2 10% 41% 34% 4	585
	11th to 20th	Night	7% 2 8% 36% 40% 7%	582
	21st to end	Night	9% 1 11% 44% 32% 4	657
August	1st to 10th	Night	16% 1 9% 36% 35% 4	597
	11th to 20th	Night	13% 3 9% 35% 35% 5%	752
	21st to end	Night	13% 1 10% 34% 36% 6%	880
September	1st to 10th	Night	13% 1 14% 31% 34% 6%	800
	11th to 20th	Night	18% 2 15% 34% 29% 3	800
	21st to end	Night	22% 3 16% 34% 23% 2	800
October	1st to 10th	Night	17% 3 17% 32% 27% 4	800
	11th to 20th	Night	16% 6% 25% 33% 19% 2	800
	21st to end	Night	14% 5% 24% 36% 18% 3	880
November	1st to 10th	Night	15% 5% 23% 37% 16% 3	820
	11th to 20th	Night	19% 9% 26% 32% 12% 2	840
	21st to end	Night	15% 9% 28% 34% 13% 1	840
December	1st to 10th	Night	21% 9% 29% 35% 7%	840
	11th to 20th	Night	18% 9% 29% 35% 9%	840
	21st to end	Night	16% 12% 30% 34% 8%	924

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