AIG public observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AIG public observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	14% 9% 3 7% 5% 5% 5% 4 6% 7% 36%	148801
	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 37%	142353
Transparency	Day	31% 34% 8% 17% 10% 1	152439
	Night	32% 34% 7% 16% 10% 1	146787
Seeing	Day	18% 12% 26% 23% 15% 6%	17882
	Night	19% 9% 26% 29% 15% 2	27509

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% 16% 6% 10% 4 4 4 3 4 6% 26%	4691
	11th to 20th	Day	12% 13% 6% 15% 5% 5% 4 3 3 5% 31%	5025
	21st to end	Day	19% 12% 5% 10% 6% 6% 5% 4 4 5% 23%	5780
February	1st to 10th	Day	18% 13% 5% 9% 3 3 4 3 5% 5% 32%	2030
	11th to 20th	Day	16% 12% 5% 10% 5% 4 4 3 5% 5% 32%	2048
	21st to end	Day	17% 13% 6% 8% 4 3 3 3 4 5% 33%	1730
March	1st to 10th	Day	20% 13% 5% 8% 4 3 3 3 3 5% 33%	4580
	11th to 20th	Day	14% 8% 3 7% 4 4 4 3 6% 8% 40%	5086
	21st to end	Day	18% 10% 3 6% 4 4 5% 4 6% 6% 34%	5586
April	1st to 10th	Day	15% 10% 3 7% 4 4 5% 4 5% 7% 35%	2504
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 3 5% 6% 39%	2530
	21st to end	Day	17% 8% 2 7% 5% 4 5% 3 5% 7% 36%	2730
May	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 4 6% 7% 29%	7616
	11th to 20th	Day	18% 7% 3 6% 4 4 4 3 5% 7% 38%	7626
	21st to end	Day	17% 8% 3 8% 5% 6% 6% 5% 7% 9% 27%	8391
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	2760
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 6% 9% 42%	2940
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	2940
July	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 10% 40%	8280
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 11% 39%	8082
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 6% 7% 9% 33%	8340
August	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 10% 40%	6014
	11th to 20th	Day	11% 7% 3 8% 5% 5% 5% 5% 7% 10% 34%	5910
	21st to end	Day	5% 3 1 4 3 3 4 4 5% 9% 58%	6080
September	1st to 10th	Day	12% 4 2 5% 4 4 4 4 5% 6% 49%	2406
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 9% 47%	2233
	21st to end	Day	15% 6% 2 5% 4 3 3 3 5% 6% 48%	2200
October	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 4 5% 4 52%	2200
	11th to 20th	Day	13% 6% 2 6% 4 3 3 3 6% 6% 47%	2050
	21st to end	Day	19% 8% 3 6% 3 4 3 3 5% 6% 39%	2026
November	1st to 10th	Day	10% 7% 3 6% 4 3 4 3 3 6% 52%	1852
	11th to 20th	Day	15% 10% 4 8% 4 3 4 4 5% 6% 37%	1890
	21st to end	Day	18% 11% 5% 7% 4 3 3 3 4 5% 37%	1890
December	1st to 10th	Day	18% 11% 3 7% 4 3 3 3 3 5% 40%	3020
	11th to 20th	Day	18% 15% 4 9% 5% 5% 5% 4 4 4 27%	3463
	21st to end	Day	24% 17% 4 9% 5% 4 4 3 4 6% 20%	4272
January	1st to 10th	Night	15% 15% 5% 12% 6% 5% 5% 3 4 5% 23%	8232
	11th to 20th	Night	12% 16% 6% 15% 6% 5% 5% 3 3 4 25%	7941
	21st to end	Night	17% 13% 4 8% 5% 4 3 2 4 5% 33%	8401
February	1st to 10th	Night	20% 12% 4 9% 5% 4 4 3 5% 5% 29%	2588
	11th to 20th	Night	18% 11% 4 9% 5% 4 3 3 5% 6% 32%	2458
	21st to end	Night	20% 13% 4 8% 4 3 3 3 4 6% 32%	2076
March	1st to 10th	Night	20% 13% 2 6% 4 3 3 3 4 5% 37%	5493
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 7% 44%	5093
	21st to end	Night	17% 9% 3 8% 5% 4 5% 4 5% 6% 34%	5588
April	1st to 10th	Night	16% 9% 2 6% 4 4 4 4 6% 6% 37%	2132
	11th to 20th	Night	13% 9% 3 6% 5% 4 3 4 7% 7% 39%	2039
	21st to end	Night	17% 7% 3 6% 5% 4 4 4 6% 8% 37%	1890
May	1st to 10th	Night	20% 8% 3 6% 5% 4 4 4 6% 7% 32%	5237
	11th to 20th	Night	15% 7% 2 7% 4 5% 5% 4 6% 8% 36%	5234
	21st to end	Night	15% 8% 3 8% 5% 5% 6% 4 6% 8% 32%	5761
June	1st to 10th	Night	9% 5% 2 5% 5% 4 3 4 6% 9% 48%	1853
	11th to 20th	Night	8% 6% 2 5% 4 4 4 4 6% 11% 46%	1680
	21st to end	Night	7% 4 2 6% 5% 4 4 5% 7% 10% 46%	1680
July	1st to 10th	Night	9% 6% 2 6% 4 4 5% 3 6% 9% 46%	4708
	11th to 20th	Night	9% 6% 2 7% 5% 4 5% 4 7% 8% 44%	4878
	21st to end	Night	9% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	5736
August	1st to 10th	Night	11% 7% 2 7% 5% 4 4 4 5% 9% 42%	4135
	11th to 20th	Night	11% 6% 3 7% 4 5% 4 4 7% 8% 41%	4118
	21st to end	Night	3 3 1 4 3 3 3 3 6% 10% 60%	5046
September	1st to 10th	Night	12% 5% 1 4 3 3 4 3 5% 6% 53%	2191
	11th to 20th	Night	9% 4 2 5% 3 3 4 3 5% 8% 54%	2234
	21st to end	Night	15% 6% 2 5% 4 3 4 3 3 6% 48%	2200
October	1st to 10th	Night	8% 5% 1 5% 3 4 4 3 4 7% 56%	2200
	11th to 20th	Night	14% 6% 3 6% 4 3 4 3 5% 6% 47%	2348
	21st to end	Night	19% 8% 2 6% 3 4 3 3 4 6% 41%	2810
November	1st to 10th	Night	13% 7% 3 6% 4 3 3 3 5% 5% 48%	2666
	11th to 20th	Night	15% 8% 3 7% 4 3 4 3 5% 7% 40%	2730
	21st to end	Night	19% 12% 3 7% 5% 4 4 3 4 5% 34%	2730
December	1st to 10th	Night	19% 11% 4 7% 4 3 3 3 4 5% 37%	4380
	11th to 20th	Night	19% 14% 6% 9% 5% 4 4 3 4 5% 28%	4952
	21st to end	Night	23% 19% 6% 10% 5% 4 4 3 4 4 18%	6915

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	48% 22% 10% 15% 5%	4692
	11th to 20th	Day	45% 21% 6% 14% 12% 2	5025
	21st to end	Day	51% 23% 6% 13% 5% 3	5780
February	1st to 10th	Day	22% 59% 5% 9% 5%	2030
	11th to 20th	Day	23% 58% 3 9% 7%	2058
	21st to end	Day	20% 59% 4 10% 5% 1	1730
March	1st to 10th	Day	42% 28% 4 11% 11% 3	4580
	11th to 20th	Day	29% 29% 6% 23% 12% 1	5086
	21st to end	Day	33% 30% 10% 15% 10% 1	5586
April	1st to 10th	Day	22% 59% 5% 8% 4	2504
	11th to 20th	Day	19% 59% 5% 10% 5% 1	2530
	21st to end	Day	18% 60% 5% 11% 6% 1	2730
May	1st to 10th	Day	40% 26% 10% 15% 9%	7651
	11th to 20th	Day	36% 24% 9% 20% 11%	7630
	21st to end	Day	42% 26% 8% 14% 8% 2	8405
June	1st to 10th	Day	17% 60% 5% 9% 8% 1	2760
	11th to 20th	Day	18% 60% 4 9% 8%	2940
	21st to end	Day	17% 60% 7% 9% 7% 1	2940
July	1st to 10th	Day	31% 23% 10% 22% 13% 1	8211
	11th to 20th	Day	30% 24% 11% 24% 10%	8007
	21st to end	Day	37% 24% 9% 17% 12% 1	8156
August	1st to 10th	Day	28% 29% 12% 21% 8% 2	6001
	11th to 20th	Day	33% 30% 9% 22% 6%	5949
	21st to end	Day	15% 28% 14% 29% 14% 1	6079
September	1st to 10th	Day	14% 55% 4 14% 11% 2	2406
	11th to 20th	Day	14% 57% 6% 12% 10% 1	2233
	21st to end	Day	16% 58% 4 12% 9%	2200
October	1st to 10th	Day	12% 36% 4 24% 23% 1	3590
	11th to 20th	Day	15% 37% 5% 20% 21% 1	3316
	21st to end	Day	20% 35% 6% 17% 20% 2	3250
November	1st to 10th	Day	21% 54% 5% 14% 6% 1	1852
	11th to 20th	Day	21% 54% 2 11% 10% 1	1890
	21st to end	Day	26% 55% 5% 10% 4	1890
December	1st to 10th	Day	30% 35% 5% 19% 11%	3020
	11th to 20th	Day	42% 32% 5% 15% 6% 1	3460
	21st to end	Day	52% 27% 7% 9% 5%	4272
January	1st to 10th	Night	50% 23% 7% 12% 7%	8233
	11th to 20th	Night	51% 21% 5% 11% 9% 3	7941
	21st to end	Night	43% 23% 7% 18% 8% 1	8401
February	1st to 10th	Night	24% 59% 4 9% 4	2588
	11th to 20th	Night	23% 58% 4 10% 5%	2459
	21st to end	Night	21% 59% 4 8% 6% 1	2076
March	1st to 10th	Night	39% 29% 4 19% 7% 2	5493
	11th to 20th	Night	25% 30% 7% 22% 15% 1	5093
	21st to end	Night	37% 30% 5% 19% 9% 1	5588
April	1st to 10th	Night	22% 60% 5% 9% 4	2132
	11th to 20th	Night	19% 59% 5% 9% 6% 1	2039
	21st to end	Night	18% 59% 5% 12% 5% 1	1890
May	1st to 10th	Night	38% 27% 8% 18% 8% 1	5264
	11th to 20th	Night	34% 26% 10% 19% 10%	5235
	21st to end	Night	40% 26% 10% 15% 7% 2	5771
June	1st to 10th	Night	14% 59% 5% 10% 10% 1	1853
	11th to 20th	Night	16% 60% 4 9% 9% 2	1680
	21st to end	Night	14% 60% 8% 10% 7% 1	1680
July	1st to 10th	Night	28% 24% 11% 21% 14% 2	4670
	11th to 20th	Night	31% 24% 10% 24% 10%	4833
	21st to end	Night	31% 24% 12% 20% 12% 2	5606
August	1st to 10th	Night	32% 29% 10% 20% 8% 2	4124
	11th to 20th	Night	31% 31% 12% 19% 8%	4145
	21st to end	Night	13% 28% 14% 29% 16%	5046
September	1st to 10th	Night	12% 55% 7% 13% 11% 2	2191
	11th to 20th	Night	12% 57% 4 12% 13% 2	2234
	21st to end	Night	16% 57% 5% 10% 12% 1	2200
October	1st to 10th	Night	12% 35% 3 17% 32% 1	3590
	11th to 20th	Night	12% 36% 7% 16% 28% 2	3821
	21st to end	Night	19% 36% 5% 15% 22% 3	4536
November	1st to 10th	Night	22% 53% 5% 11% 8% 1	2666
	11th to 20th	Night	19% 54% 4 11% 9% 2	2730
	21st to end	Night	26% 54% 5% 10% 4	2730
December	1st to 10th	Night	32% 35% 7% 17% 9%	4380
	11th to 20th	Night	41% 32% 8% 13% 6%	4954
	21st to end	Night	55% 27% 6% 10% 3	6915

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	25% 21% 33% 17% 5%	330
	11th to 20th	Day	29% 27% 29% 12% 2	330
	21st to end	Day	30% 23% 31% 13% 2	363
February	1st to 10th	Day	27% 20% 35% 15% 3	330
	11th to 20th	Day	29% 25% 34% 9% 2 1	330
	21st to end	Day	28% 20% 41% 9% 2	273
March	1st to 10th	Day	25% 23% 37% 13% 2	330
	11th to 20th	Day	27% 18% 35% 14% 5% 1	536
	21st to end	Day	29% 19% 29% 16% 5% 1	594
April	1st to 10th	Day	23% 19% 30% 17% 10% 1	540
	11th to 20th	Day	22% 20% 27% 20% 8% 3	532
	21st to end	Day	22% 16% 34% 18% 8% 3	540
May	1st to 10th	Day	24% 15% 31% 19% 8% 3	543
	11th to 20th	Day	22% 11% 25% 25% 13% 5%	567
	21st to end	Day	17% 9% 23% 27% 16% 9%	617
June	1st to 10th	Day	13% 8% 23% 28% 20% 7%	600
	11th to 20th	Day	11% 6% 25% 33% 20% 6%	780
	21st to end	Day	10% 6% 24% 31% 20% 9%	780
July	1st to 10th	Day	12% 4 15% 25% 26% 18%	771
	11th to 20th	Day	8% 5% 19% 32% 23% 12%	700
	21st to end	Day	10% 5% 16% 33% 23% 13%	620
August	1st to 10th	Day	10% 4 17% 26% 27% 17%	566
	11th to 20th	Day	12% 4 17% 33% 23% 11%	559
	21st to end	Day	7% 6% 13% 27% 33% 13%	627
September	1st to 10th	Day	11% 6% 15% 28% 31% 9%	570
	11th to 20th	Day	10% 3 15% 24% 31% 17%	542
	21st to end	Day	18% 7% 19% 24% 24% 8%	530
October	1st to 10th	Day	10% 5% 22% 34% 22% 8%	530
	11th to 20th	Day	14% 15% 30% 24% 14% 3	530
	21st to end	Day	24% 12% 37% 19% 7% 1	409
November	1st to 10th	Day	15% 15% 35% 23% 9% 2	330
	11th to 20th	Day	22% 20% 31% 19% 6% 2	330
	21st to end	Day	27% 13% 36% 17% 6%	330
December	1st to 10th	Day	22% 22% 37% 14% 5% 1	330
	11th to 20th	Day	22% 19% 38% 15% 6%	330
	21st to end	Day	31% 18% 32% 15% 4	363
January	1st to 10th	Night	22% 17% 32% 23% 5%	1050
	11th to 20th	Night	28% 17% 33% 18% 3	932
	21st to end	Night	25% 16% 36% 21% 3	924
February	1st to 10th	Night	27% 13% 34% 24% 2	840
	11th to 20th	Night	25% 18% 33% 22% 2	823
	21st to end	Night	27% 15% 35% 23% 1	692
March	1st to 10th	Night	24% 15% 28% 28% 5%	840
	11th to 20th	Night	23% 14% 34% 23% 6% 1	840
	21st to end	Night	22% 13% 32% 27% 6%	924
April	1st to 10th	Night	20% 11% 35% 27% 7%	662
	11th to 20th	Night	20% 11% 29% 30% 9% 1	623
	21st to end	Night	19% 8% 35% 28% 8% 1	630
May	1st to 10th	Night	21% 6% 32% 29% 11% 1	630
	11th to 20th	Night	16% 7% 22% 35% 17% 3	621
	21st to end	Night	11% 4 21% 37% 24% 3	683
June	1st to 10th	Night	10% 3 20% 43% 22% 2	630
	11th to 20th	Night	10% 5% 16% 40% 27% 3	630
	21st to end	Night	7% 3 21% 40% 26% 3	630
July	1st to 10th	Night	11% 2 12% 32% 34% 9%	615
	11th to 20th	Night	7% 3 13% 37% 35% 5%	612
	21st to end	Night	12% 3 10% 39% 33% 4	690
August	1st to 10th	Night	8% 2 10% 36% 39% 4	627
	11th to 20th	Night	10% 1 11% 40% 33% 5%	624
	21st to end	Night	6% 2 9% 36% 38% 8%	693
September	1st to 10th	Night	14% 3 12% 34% 33% 5%	721
	11th to 20th	Night	9% 3 13% 31% 37% 6%	813
	21st to end	Night	18% 3 22% 30% 24% 2	800
October	1st to 10th	Night	10% 5% 20% 36% 24% 5%	800
	11th to 20th	Night	17% 9% 23% 34% 15% 2	800
	21st to end	Night	23% 9% 31% 26% 10% 1	880
November	1st to 10th	Night	16% 9% 28% 31% 15% 1	820
	11th to 20th	Night	20% 15% 29% 26% 10% 1	840
	21st to end	Night	26% 12% 26% 28% 8%	840
December	1st to 10th	Night	27% 10% 35% 23% 5%	840
	11th to 20th	Night	25% 13% 33% 23% 6% 1	840
	21st to end	Night	31% 14% 30% 21% 4	1050

Page generated at 2026-09-11 04:11:56. Version ac07

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