Above the Clouds Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Above the Clouds 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 9% 6% 5% 5% 4 6% 7% 32%	149156
	Night	13% 9% 4 9% 6% 5% 5% 4 6% 7% 31%	141930
Transparency	Day	48% 7% 13% 20% 10% 1	152639
	Night	49% 7% 13% 19% 10% 1	146523
Seeing	Day	24% 17% 29% 19% 9% 3	17161
	Night	20% 14% 27% 29% 10% 1	26026

By Month

Percentage Cloud Cover NE (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	8% 10% 5% 20% 9% 6% 5% 4 5% 6% 22%	4661
	11th to 20th	Day	9% 13% 6% 14% 8% 5% 5% 4 5% 6% 24%	4650
	21st to end	Day	8% 11% 6% 11% 8% 6% 5% 5% 5% 7% 27%	5124
February	1st to 10th	Day	10% 11% 4 11% 6% 7% 5% 4 6% 7% 28%	1853
	11th to 20th	Day	9% 10% 5% 12% 9% 7% 5% 4 6% 7% 27%	2048
	21st to end	Day	13% 13% 4 10% 6% 5% 5% 4 5% 6% 29%	1730
March	1st to 10th	Day	15% 14% 5% 13% 6% 5% 5% 3 4 4 26%	4576
	11th to 20th	Day	12% 10% 4 9% 6% 5% 5% 4 6% 6% 33%	4747
	21st to end	Day	17% 11% 4 10% 5% 5% 5% 4 4 5% 30%	6102
April	1st to 10th	Day	17% 12% 4 9% 6% 4 4 3 4 6% 32%	2520
	11th to 20th	Day	15% 10% 3 7% 5% 6% 5% 4 5% 7% 33%	2494
	21st to end	Day	19% 9% 3 8% 5% 5% 4 4 5% 6% 31%	2709
May	1st to 10th	Day	25% 11% 3 8% 6% 5% 5% 4 5% 6% 22%	7675
	11th to 20th	Day	16% 9% 3 8% 6% 5% 5% 4 6% 7% 31%	8221
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 4 7% 8% 30%	9060
June	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 5% 7% 7% 30%	2940
	11th to 20th	Day	16% 7% 3 6% 5% 5% 6% 4 6% 8% 34%	2940
	21st to end	Day	16% 8% 3 8% 6% 6% 5% 4 7% 8% 29%	2940
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 8% 11% 36%	8280
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 4 8% 10% 41%	8280
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 7% 8% 39%	8993
August	1st to 10th	Day	12% 6% 3 8% 6% 7% 6% 5% 8% 8% 31%	6391
	11th to 20th	Day	13% 7% 2 7% 4 5% 6% 5% 7% 10% 35%	5770
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 11% 38%	6080
September	1st to 10th	Day	12% 5% 2 6% 4 3 4 4 7% 9% 45%	2520
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 49%	2296
	21st to end	Day	15% 7% 2 5% 4 3 4 3 7% 8% 41%	2200
October	1st to 10th	Day	18% 7% 2 7% 5% 4 5% 4 5% 6% 37%	2158
	11th to 20th	Day	13% 10% 3 8% 5% 5% 4 3 5% 8% 37%	1930
	21st to end	Day	21% 12% 3 7% 5% 5% 4 5% 4 5% 29%	1980
November	1st to 10th	Day	11% 9% 4 7% 6% 5% 5% 4 7% 6% 37%	1851
	11th to 20th	Day	13% 12% 4 7% 5% 4 5% 4 5% 6% 34%	1890
	21st to end	Day	16% 11% 4 10% 5% 4 4 4 6% 7% 28%	1753
December	1st to 10th	Day	11% 14% 6% 16% 8% 6% 5% 4 6% 6% 19%	2680
	11th to 20th	Day	16% 11% 4 9% 6% 5% 6% 4 6% 6% 28%	3066
	21st to end	Day	16% 13% 4 10% 6% 5% 6% 4 5% 4 27%	4048
January	1st to 10th	Night	10% 8% 6% 19% 8% 5% 5% 3 5% 6% 24%	8234
	11th to 20th	Night	8% 12% 6% 16% 8% 7% 6% 4 6% 6% 19%	8235
	21st to end	Night	10% 9% 4 11% 8% 8% 7% 5% 6% 6% 26%	9057
February	1st to 10th	Night	9% 12% 4 10% 6% 6% 5% 6% 7% 8% 27%	2763
	11th to 20th	Night	9% 8% 4 11% 8% 6% 6% 6% 7% 6% 29%	2458
	21st to end	Night	10% 11% 3 10% 8% 5% 5% 4 5% 7% 32%	2076
March	1st to 10th	Night	17% 12% 3 9% 6% 5% 5% 4 6% 6% 27%	5484
	11th to 20th	Night	11% 10% 4 9% 6% 6% 6% 4 5% 6% 33%	5414
	21st to end	Night	17% 13% 4 9% 5% 4 5% 3 5% 5% 30%	5072
April	1st to 10th	Night	16% 9% 3 9% 5% 5% 5% 4 5% 6% 32%	2100
	11th to 20th	Night	16% 10% 3 8% 5% 5% 5% 3 5% 6% 34%	2075
	21st to end	Night	17% 9% 4 7% 4 4 4 3 6% 6% 36%	1911
May	1st to 10th	Night	26% 12% 3 8% 5% 4 5% 3 5% 6% 23%	5259
	11th to 20th	Night	15% 9% 3 8% 6% 5% 5% 4 6% 7% 32%	4647
	21st to end	Night	17% 8% 3 7% 5% 5% 5% 4 6% 8% 32%	5114
June	1st to 10th	Night	16% 7% 2 6% 4 5% 6% 4 6% 9% 36%	1680
	11th to 20th	Night	17% 7% 3 5% 5% 3 4 5% 5% 8% 38%	1680
	21st to end	Night	16% 6% 2 7% 5% 5% 5% 4 6% 8% 37%	1680
July	1st to 10th	Night	9% 7% 2 6% 5% 5% 6% 4 7% 10% 38%	4680
	11th to 20th	Night	10% 5% 2 7% 4 5% 5% 4 6% 8% 43%	4680
	21st to end	Night	11% 6% 2 6% 4 4 4 4 7% 7% 44%	5084
August	1st to 10th	Night	14% 8% 3 8% 5% 5% 5% 4 6% 8% 35%	3711
	11th to 20th	Night	14% 6% 3 7% 5% 4 4 3 6% 8% 40%	4272
	21st to end	Night	9% 6% 2 6% 4 4 6% 4 8% 10% 42%	5045
September	1st to 10th	Night	11% 5% 2 5% 4 4 4 3 6% 8% 47%	2100
	11th to 20th	Night	11% 5% 2 6% 4 3 4 3 6% 5% 51%	2171
	21st to end	Night	13% 6% 2 5% 4 4 4 4 7% 7% 45%	2200
October	1st to 10th	Night	17% 7% 3 8% 5% 4 4 3 5% 8% 37%	2240
	11th to 20th	Night	15% 8% 3 5% 4 5% 5% 5% 6% 9% 35%	2470
	21st to end	Night	20% 8% 3 7% 5% 5% 5% 4 7% 7% 30%	2860
November	1st to 10th	Night	12% 9% 4 8% 5% 5% 5% 4 4 6% 36%	2667
	11th to 20th	Night	14% 8% 4 6% 5% 5% 5% 5% 6% 8% 34%	2730
	21st to end	Night	12% 10% 4 8% 6% 5% 5% 4 6% 8% 31%	2863
December	1st to 10th	Night	12% 12% 5% 16% 8% 7% 6% 4 6% 7% 19%	4720
	11th to 20th	Night	18% 11% 4 10% 6% 6% 5% 3 4 6% 28%	5348
	21st to end	Night	14% 15% 5% 12% 8% 6% 5% 3 4 5% 23%	7150

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	60% 7% 11% 18% 4	4662
	11th to 20th	Day	57% 7% 10% 14% 9% 2	4680
	21st to end	Day	51% 8% 10% 18% 11% 1	5124
February	1st to 10th	Day	53% 6% 13% 17% 9% 2	1853
	11th to 20th	Day	54% 7% 11% 18% 8% 2	2057
	21st to end	Day	55% 6% 12% 15% 12% 2	1730
March	1st to 10th	Day	60% 6% 9% 11% 9% 6%	4579
	11th to 20th	Day	49% 6% 15% 18% 9% 3	4747
	21st to end	Day	55% 6% 6% 14% 17% 3	6102
April	1st to 10th	Day	53% 5% 9% 19% 11% 3	2520
	11th to 20th	Day	49% 6% 11% 18% 15% 1	2494
	21st to end	Day	54% 5% 9% 17% 12% 2	2709
May	1st to 10th	Day	60% 7% 10% 15% 8% 1	7687
	11th to 20th	Day	49% 7% 16% 18% 9% 1	8225
	21st to end	Day	49% 8% 12% 23% 8% 1	9061
June	1st to 10th	Day	51% 7% 14% 18% 10%	2940
	11th to 20th	Day	45% 7% 13% 20% 13% 1	2940
	21st to end	Day	51% 8% 12% 19% 9% 1	2940
July	1st to 10th	Day	39% 8% 17% 25% 10%	8210
	11th to 20th	Day	33% 8% 17% 30% 11% 1	8205
	21st to end	Day	39% 8% 14% 27% 11%	8795
August	1st to 10th	Day	46% 11% 17% 17% 10% 1	6377
	11th to 20th	Day	41% 9% 19% 24% 7%	5809
	21st to end	Day	35% 11% 18% 24% 12% 1	6080
September	1st to 10th	Day	34% 7% 13% 31% 14% 1	2520
	11th to 20th	Day	36% 4 11% 29% 19% 1	2296
	21st to end	Day	39% 6% 15% 26% 12% 2	2200
October	1st to 10th	Day	42% 8% 12% 22% 15% 1	3509
	11th to 20th	Day	43% 4 13% 25% 13%	3120
	21st to end	Day	40% 7% 12% 28% 12% 1	3180
November	1st to 10th	Day	44% 7% 14% 22% 13%	1851
	11th to 20th	Day	49% 7% 12% 17% 14% 2	1890
	21st to end	Day	54% 6% 18% 17% 5%	1753
December	1st to 10th	Day	62% 9% 13% 12% 4	2680
	11th to 20th	Day	53% 9% 10% 18% 10% 1	3066
	21st to end	Day	58% 5% 8% 13% 13% 2	4048
January	1st to 10th	Night	58% 8% 14% 13% 7% 1	8234
	11th to 20th	Night	61% 8% 11% 11% 8% 2	8280
	21st to end	Night	51% 9% 10% 15% 12% 2	9057
February	1st to 10th	Night	51% 7% 14% 18% 9% 1	2763
	11th to 20th	Night	51% 7% 12% 20% 10% 1	2460
	21st to end	Night	50% 7% 14% 19% 8% 3	2076
March	1st to 10th	Night	53% 6% 11% 15% 12% 2	5494
	11th to 20th	Night	51% 6% 17% 15% 10% 2	5415
	21st to end	Night	53% 5% 7% 15% 16% 2	5072
April	1st to 10th	Night	51% 5% 10% 19% 13% 2	2100
	11th to 20th	Night	49% 5% 10% 22% 13% 1	2075
	21st to end	Night	48% 5% 10% 20% 14% 3	1911
May	1st to 10th	Night	60% 6% 11% 14% 9% 1	5265
	11th to 20th	Night	49% 8% 14% 21% 8% 1	4648
	21st to end	Night	48% 7% 13% 22% 9% 1	5115
June	1st to 10th	Night	44% 8% 15% 20% 13%	1680
	11th to 20th	Night	43% 8% 15% 20% 12% 2	1680
	21st to end	Night	45% 8% 14% 24% 9% 1	1680
July	1st to 10th	Night	40% 7% 20% 22% 10%	4640
	11th to 20th	Night	36% 9% 15% 26% 14%	4632
	21st to end	Night	36% 8% 16% 31% 9%	4967
August	1st to 10th	Night	47% 8% 17% 17% 9% 1	3701
	11th to 20th	Night	40% 8% 18% 24% 9%	4299
	21st to end	Night	32% 10% 22% 24% 11%	5045
September	1st to 10th	Night	34% 7% 17% 29% 13%	2100
	11th to 20th	Night	33% 6% 13% 30% 17% 1	2171
	21st to end	Night	37% 6% 15% 25% 15% 2	2200
October	1st to 10th	Night	46% 9% 13% 22% 9% 1	3656
	11th to 20th	Night	41% 6% 13% 24% 14% 2	4019
	21st to end	Night	41% 7% 13% 29% 9% 1	4610
November	1st to 10th	Night	47% 6% 12% 22% 11% 2	2667
	11th to 20th	Night	46% 7% 13% 21% 11% 2	2730
	21st to end	Night	50% 7% 16% 19% 7%	2863
December	1st to 10th	Night	61% 9% 13% 14% 3	4720
	11th to 20th	Night	55% 7% 9% 14% 13%	5348
	21st to end	Night	61% 6% 9% 14% 7% 2	7150

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% 24% 26% 19% 6% 1	220
	11th to 20th	Day	30% 23% 29% 12% 5% 1	220
	21st to end	Day	22% 27% 30% 14% 6% 1	242
February	1st to 10th	Day	31% 28% 28% 9% 3	318
	11th to 20th	Day	27% 24% 31% 16% 2	536
	21st to end	Day	27% 24% 30% 16% 4	446
March	1st to 10th	Day	29% 19% 32% 17% 3	540
	11th to 20th	Day	33% 22% 27% 13% 5%	540
	21st to end	Day	32% 24% 24% 12% 5% 3	594
April	1st to 10th	Day	37% 18% 26% 14% 5% 1	540
	11th to 20th	Day	37% 19% 25% 11% 7% 1	532
	21st to end	Day	29% 22% 29% 11% 7% 2	540
May	1st to 10th	Day	35% 19% 25% 12% 6% 2	543
	11th to 20th	Day	22% 19% 32% 19% 6% 2	567
	21st to end	Day	20% 19% 32% 18% 8% 2	617
June	1st to 10th	Day	22% 14% 36% 19% 7% 2	570
	11th to 20th	Day	20% 16% 33% 20% 10% 2	570
	21st to end	Day	21% 14% 34% 22% 8% 2	570
July	1st to 10th	Day	15% 12% 34% 25% 11% 3	565
	11th to 20th	Day	13% 11% 34% 28% 10% 4	564
	21st to end	Day	15% 10% 35% 25% 11% 4	620
August	1st to 10th	Day	10% 11% 32% 23% 18% 6%	566
	11th to 20th	Day	17% 12% 26% 23% 14% 9%	559
	21st to end	Day	13% 13% 27% 27% 14% 7%	627
September	1st to 10th	Day	15% 13% 27% 23% 14% 8%	570
	11th to 20th	Day	13% 12% 25% 20% 18% 12%	542
	21st to end	Day	19% 12% 28% 22% 12% 7%	530
October	1st to 10th	Day	23% 13% 24% 19% 16% 5%	530
	11th to 20th	Day	22% 17% 25% 22% 12% 2	491
	21st to end	Day	33% 15% 24% 19% 8% 1	462
November	1st to 10th	Day	26% 16% 24% 20% 12% 1	425
	11th to 20th	Day	27% 20% 25% 19% 9% 1	430
	21st to end	Day	34% 17% 26% 15% 7% 1	293
December	1st to 10th	Day	27% 20% 31% 15% 6% 1	220
	11th to 20th	Day	32% 23% 29% 15% 2	220
	21st to end	Day	32% 18% 26% 19% 5%	242
January	1st to 10th	Night	19% 19% 35% 23% 3	840
	11th to 20th	Night	23% 19% 29% 25% 4	840
	21st to end	Night	16% 17% 34% 28% 4 1	924
February	1st to 10th	Night	17% 19% 35% 26% 3	840
	11th to 20th	Night	20% 18% 36% 23% 3	823
	21st to end	Night	21% 17% 32% 26% 3	692
March	1st to 10th	Night	23% 15% 34% 26% 3	840
	11th to 20th	Night	24% 19% 27% 24% 5%	840
	21st to end	Night	27% 16% 27% 23% 7%	924
April	1st to 10th	Night	25% 19% 28% 23% 5%	840
	11th to 20th	Night	24% 20% 29% 21% 7%	831
	21st to end	Night	27% 15% 27% 24% 7%	651
May	1st to 10th	Night	27% 13% 29% 23% 8%	630
	11th to 20th	Night	17% 13% 29% 35% 6% 1	422
	21st to end	Night	18% 9% 28% 32% 12%	455
June	1st to 10th	Night	19% 9% 25% 34% 12% 1	420
	11th to 20th	Night	21% 8% 24% 35% 13%	420
	21st to end	Night	17% 6% 24% 39% 13% 1	420
July	1st to 10th	Night	15% 3 20% 46% 15% 2	410
	11th to 20th	Night	13% 3 25% 40% 17% 2	408
	21st to end	Night	11% 6% 21% 45% 17% 1	460
August	1st to 10th	Night	14% 4 17% 41% 24% 1	437
	11th to 20th	Night	15% 7% 20% 35% 20% 3	698
	21st to end	Night	13% 6% 18% 37% 23% 2	924
September	1st to 10th	Night	15% 6% 25% 33% 20% 2	840
	11th to 20th	Night	14% 7% 19% 30% 25% 4	813
	21st to end	Night	15% 7% 26% 33% 18% 2	800
October	1st to 10th	Night	22% 11% 17% 30% 17% 2	800
	11th to 20th	Night	21% 12% 22% 31% 13% 1	800
	21st to end	Night	26% 14% 26% 27% 7%	880
November	1st to 10th	Night	20% 17% 25% 29% 8%	820
	11th to 20th	Night	19% 18% 27% 26% 9% 1	840
	21st to end	Night	20% 18% 27% 28% 7%	840
December	1st to 10th	Night	24% 18% 31% 23% 4	840
	11th to 20th	Night	25% 18% 26% 27% 4	840
	21st to end	Night	27% 17% 30% 23% 4	924

Page generated at 2026-09-11 04:11:57. Version ac07

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