Astronight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astronight Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	149077
	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	142650
Transparency	Day	43% 6% 12% 22% 15% 2	152612
	Night	43% 5% 12% 21% 16% 2	147204
Seeing	Day	23% 17% 32% 18% 7% 2	17530
	Night	21% 14% 33% 24% 7% 1	26340

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	11% 7% 3 9% 6% 5% 5% 4 5% 6% 39%	4661
	11th to 20th	Day	15% 8% 3 5% 3 3 4 3 4 5% 46%	4650
	21st to end	Day	14% 8% 3 4 3 3 4 3 5% 6% 46%	5124
February	1st to 10th	Day	17% 8% 3 5% 4 4 5% 5% 5% 6% 37%	1946
	11th to 20th	Day	13% 9% 2 7% 4 4 5% 4 6% 7% 39%	2048
	21st to end	Day	18% 10% 3 7% 5% 4 4 3 5% 5% 37%	1730
March	1st to 10th	Day	21% 11% 2 6% 3 4 4 3 5% 5% 35%	4576
	11th to 20th	Day	16% 9% 3 6% 4 4 5% 4 5% 6% 39%	4888
	21st to end	Day	26% 9% 2 6% 4 4 4 4 5% 6% 31%	5903
April	1st to 10th	Day	19% 10% 3 6% 4 4 4 3 4 6% 36%	2520
	11th to 20th	Day	18% 7% 2 5% 3 5% 6% 3 5% 6% 38%	2515
	21st to end	Day	21% 6% 3 7% 5% 4 4 3 5% 6% 37%	2730
May	1st to 10th	Day	27% 9% 2 6% 4 4 4 3 5% 6% 28%	7639
	11th to 20th	Day	19% 8% 3 7% 4 4 4 3 5% 6% 36%	7780
	21st to end	Day	20% 7% 2 7% 4 5% 5% 4 6% 8% 33%	9060
June	1st to 10th	Day	18% 5% 2 6% 5% 5% 5% 4 6% 8% 35%	2940
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	2940
	21st to end	Day	17% 6% 3 6% 5% 5% 5% 4 6% 8% 36%	2940
July	1st to 10th	Day	18% 6% 2 6% 4 4 5% 4 6% 9% 35%	8280
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 7% 11% 45%	8280
	21st to end	Day	15% 6% 2 6% 4 3 4 3 5% 8% 44%	8993
August	1st to 10th	Day	12% 6% 2 7% 5% 6% 6% 5% 7% 10% 34%	6093
	11th to 20th	Day	14% 6% 3 6% 4 4 5% 4 6% 8% 42%	5891
	21st to end	Day	11% 4 2 6% 4 5% 5% 5% 7% 9% 43%	6080
September	1st to 10th	Day	14% 5% 2 4 5% 3 4 4 6% 6% 46%	2515
	11th to 20th	Day	15% 5% 2 4 3 4 3 3 4 6% 51%	2310
	21st to end	Day	18% 6% 3 5% 4 4 3 3 5% 7% 42%	2310
October	1st to 10th	Day	20% 5% 2 6% 3 4 4 3 5% 5% 43%	2310
	11th to 20th	Day	16% 6% 2 5% 4 3 4 3 5% 7% 46%	1981
	21st to end	Day	21% 6% 3 6% 3 4 4 3 5% 5% 41%	1980
November	1st to 10th	Day	15% 6% 2 5% 4 4 3 3 4 5% 49%	1851
	11th to 20th	Day	17% 6% 2 5% 3 3 3 3 4 5% 48%	1890
	21st to end	Day	19% 7% 2 5% 3 3 4 3 5% 6% 42%	1890
December	1st to 10th	Day	15% 8% 2 5% 4 4 3 3 4 6% 47%	2719
	11th to 20th	Day	20% 5% 3 5% 3 3 3 3 4 5% 47%	3066
	21st to end	Day	20% 10% 2 6% 4 4 4 3 4 5% 38%	4048
January	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 6% 45%	8234
	11th to 20th	Night	15% 10% 3 9% 5% 4 4 3 4 5% 38%	8235
	21st to end	Night	14% 10% 3 6% 4 5% 5% 4 5% 5% 41%	9057
February	1st to 10th	Night	16% 9% 2 6% 5% 4 5% 4 5% 6% 39%	2670
	11th to 20th	Night	14% 8% 3 6% 4 4 4 3 6% 7% 40%	2458
	21st to end	Night	18% 9% 2 6% 4 3 4 3 4 6% 40%	2076
March	1st to 10th	Night	24% 8% 2 6% 3 4 4 3 4 5% 37%	5484
	11th to 20th	Night	16% 10% 3 7% 5% 5% 5% 3 4 5% 38%	5289
	21st to end	Night	26% 8% 2 6% 4 4 4 3 5% 5% 33%	5268
April	1st to 10th	Night	20% 7% 3 6% 5% 4 4 4 5% 6% 36%	2100
	11th to 20th	Night	21% 8% 2 6% 4 4 4 3 4 5% 40%	2054
	21st to end	Night	20% 8% 2 6% 5% 3 4 3 5% 5% 39%	1890
May	1st to 10th	Night	29% 9% 3 6% 4 4 4 3 4 6% 29%	5259
	11th to 20th	Night	18% 8% 3 7% 4 5% 4 5% 6% 8% 33%	5122
	21st to end	Night	21% 8% 3 8% 4 5% 5% 3 5% 7% 32%	5114
June	1st to 10th	Night	17% 8% 2 6% 5% 4 4 3 4 5% 40%	1680
	11th to 20th	Night	18% 7% 3 5% 5% 4 4 4 6% 7% 38%	1680
	21st to end	Night	16% 6% 3 6% 5% 4 5% 4 6% 9% 38%	1680
July	1st to 10th	Night	14% 7% 3 7% 5% 4 4 4 6% 9% 37%	4680
	11th to 20th	Night	9% 6% 2 6% 4 4 5% 3 6% 9% 46%	4680
	21st to end	Night	13% 6% 3 6% 3 3 4 4 5% 7% 47%	5084
August	1st to 10th	Night	14% 6% 2 7% 5% 5% 6% 4 7% 9% 36%	3995
	11th to 20th	Night	12% 6% 3 8% 4 5% 5% 4 7% 8% 39%	4137
	21st to end	Night	12% 4 2 5% 4 4 4 4 6% 9% 47%	5045
September	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 5% 6% 48%	2105
	11th to 20th	Night	13% 6% 1 5% 3 4 3 3 4 6% 52%	2310
	21st to end	Night	19% 6% 3 5% 4 3 3 4 5% 6% 42%	2310
October	1st to 10th	Night	18% 6% 2 6% 4 4 4 3 4 5% 45%	2310
	11th to 20th	Night	17% 4 2 5% 3 3 4 4 6% 7% 46%	2483
	21st to end	Night	22% 7% 2 6% 4 4 4 3 5% 6% 40%	2860
November	1st to 10th	Night	15% 5% 2 5% 4 3 3 4 4 6% 48%	2667
	11th to 20th	Night	18% 6% 2 5% 3 3 4 3 5% 5% 47%	2730
	21st to end	Night	16% 7% 2 5% 5% 4 4 3 4 6% 44%	2730
December	1st to 10th	Night	17% 8% 2 5% 4 4 4 3 5% 5% 41%	4676
	11th to 20th	Night	22% 7% 3 6% 3 3 3 3 4 5% 41%	5348
	21st to end	Night	17% 10% 3 6% 5% 4 4 2 4 5% 39%	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	44% 4 11% 25% 14% 1	4662
	11th to 20th	Day	41% 5% 10% 22% 19% 4	4680
	21st to end	Day	38% 4 10% 25% 21% 2	5124
February	1st to 10th	Day	46% 6% 12% 19% 14% 3	1946
	11th to 20th	Day	43% 7% 12% 21% 14% 3	2057
	21st to end	Day	48% 5% 11% 20% 14% 3	1730
March	1st to 10th	Day	51% 4 9% 15% 11% 10%	4579
	11th to 20th	Day	43% 7% 14% 23% 11% 2	4888
	21st to end	Day	52% 4 7% 15% 15% 7%	5903
April	1st to 10th	Day	49% 5% 9% 17% 16% 4	2520
	11th to 20th	Day	44% 7% 11% 19% 17% 3	2515
	21st to end	Day	47% 6% 7% 18% 18% 4	2730
May	1st to 10th	Day	56% 6% 12% 15% 11% 1	7651
	11th to 20th	Day	48% 6% 14% 19% 11% 1	7784
	21st to end	Day	48% 7% 11% 21% 12% 1	9061
June	1st to 10th	Day	46% 8% 12% 21% 13%	2940
	11th to 20th	Day	41% 8% 13% 19% 16% 3	2940
	21st to end	Day	44% 8% 13% 22% 12% 1	2940
July	1st to 10th	Day	44% 7% 14% 21% 13% 1	8210
	11th to 20th	Day	32% 6% 16% 30% 15% 1	8205
	21st to end	Day	40% 5% 11% 26% 17% 2	8795
August	1st to 10th	Day	44% 8% 19% 19% 9% 1	6079
	11th to 20th	Day	39% 6% 15% 30% 9% 1	5930
	21st to end	Day	35% 7% 14% 27% 15% 2	6080
September	1st to 10th	Day	37% 6% 11% 24% 20% 1	2515
	11th to 20th	Day	35% 4 9% 26% 23% 3	2310
	21st to end	Day	41% 6% 9% 22% 20% 2	2310
October	1st to 10th	Day	42% 5% 9% 22% 20% 3	3700
	11th to 20th	Day	32% 5% 8% 28% 24% 3	3184
	21st to end	Day	34% 4 8% 31% 20% 3	3180
November	1st to 10th	Day	38% 5% 12% 26% 17% 2	1851
	11th to 20th	Day	39% 4 9% 23% 22% 3	1890
	21st to end	Day	43% 5% 14% 23% 13% 2	1890
December	1st to 10th	Day	40% 4 14% 28% 13% 1	2719
	11th to 20th	Day	40% 3 14% 19% 19% 5%	3066
	21st to end	Day	48% 5% 8% 20% 14% 5%	4048
January	1st to 10th	Night	40% 3 13% 28% 14% 2	8234
	11th to 20th	Night	48% 4 11% 18% 14% 5%	8280
	21st to end	Night	44% 5% 9% 18% 19% 4	9057
February	1st to 10th	Night	45% 6% 12% 21% 14% 2	2670
	11th to 20th	Night	43% 5% 11% 22% 15% 3	2460
	21st to end	Night	45% 5% 9% 23% 15% 3	2076
March	1st to 10th	Night	48% 4 9% 16% 15% 7%	5494
	11th to 20th	Night	46% 7% 16% 17% 12% 2	5290
	21st to end	Night	52% 4 6% 12% 20% 6%	5268
April	1st to 10th	Night	47% 6% 11% 17% 15% 3	2100
	11th to 20th	Night	46% 5% 9% 19% 18% 3	2054
	21st to end	Night	46% 5% 12% 16% 17% 4	1890
May	1st to 10th	Night	56% 5% 12% 14% 11% 1	5265
	11th to 20th	Night	48% 7% 15% 19% 10% 1	5123
	21st to end	Night	52% 6% 12% 21% 8% 1	5115
June	1st to 10th	Night	46% 6% 11% 24% 13% 1	1680
	11th to 20th	Night	43% 8% 14% 19% 14% 2	1680
	21st to end	Night	42% 9% 14% 21% 13% 1	1680
July	1st to 10th	Night	44% 6% 13% 21% 15%	4640
	11th to 20th	Night	34% 6% 17% 24% 18% 1	4632
	21st to end	Night	38% 5% 11% 29% 17% 1	4967
August	1st to 10th	Night	45% 7% 17% 18% 12% 1	3985
	11th to 20th	Night	40% 9% 15% 26% 10% 1	4164
	21st to end	Night	33% 7% 16% 28% 17% 1	5045
September	1st to 10th	Night	36% 6% 12% 26% 19% 1	2105
	11th to 20th	Night	33% 5% 11% 23% 26% 2	2310
	21st to end	Night	41% 6% 9% 23% 18% 2	2310
October	1st to 10th	Night	39% 5% 15% 23% 16% 2	3700
	11th to 20th	Night	27% 5% 11% 27% 27% 3	4019
	21st to end	Night	35% 4 11% 31% 17% 2	4610
November	1st to 10th	Night	37% 6% 11% 23% 20% 3	2667
	11th to 20th	Night	39% 4 10% 23% 20% 3	2730
	21st to end	Night	41% 5% 14% 24% 14% 2	2730
December	1st to 10th	Night	45% 4 11% 27% 12% 1	4676
	11th to 20th	Night	46% 5% 14% 14% 20% 2	5348
	21st to end	Night	46% 5% 8% 22% 16% 3	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	28% 26% 30% 14% 3	220
	11th to 20th	Day	34% 23% 31% 10% 2	220
	21st to end	Day	26% 26% 35% 10% 3	242
February	1st to 10th	Day	31% 20% 34% 14% 1	428
	11th to 20th	Day	29% 26% 32% 11% 2	536
	21st to end	Day	27% 20% 37% 12% 3	446
March	1st to 10th	Day	30% 21% 35% 11% 3	540
	11th to 20th	Day	29% 25% 31% 11% 3 1	540
	21st to end	Day	33% 23% 28% 10% 5% 1	594
April	1st to 10th	Day	31% 25% 25% 12% 5% 1	540
	11th to 20th	Day	31% 25% 28% 11% 5%	532
	21st to end	Day	27% 19% 34% 15% 4 1	540
May	1st to 10th	Day	32% 22% 30% 10% 4 1	543
	11th to 20th	Day	23% 18% 39% 15% 4 1	567
	21st to end	Day	21% 15% 38% 20% 5% 1	617
June	1st to 10th	Day	20% 14% 37% 20% 8% 2	570
	11th to 20th	Day	17% 18% 36% 20% 6% 1	570
	21st to end	Day	18% 15% 36% 21% 8% 2	570
July	1st to 10th	Day	18% 10% 31% 29% 10% 2	565
	11th to 20th	Day	14% 9% 33% 29% 13% 2	564
	21st to end	Day	13% 9% 36% 30% 10% 2	620
August	1st to 10th	Day	11% 9% 34% 30% 13% 2	566
	11th to 20th	Day	17% 9% 28% 29% 14% 4	559
	21st to end	Day	12% 11% 35% 25% 14% 4	627
September	1st to 10th	Day	18% 11% 31% 23% 14% 4	570
	11th to 20th	Day	19% 13% 26% 21% 13% 8%	570
	21st to end	Day	21% 13% 32% 20% 9% 5%	570
October	1st to 10th	Day	25% 15% 25% 21% 10% 4	570
	11th to 20th	Day	22% 18% 27% 22% 10% 2	479
	21st to end	Day	28% 15% 32% 18% 7% 1	462
November	1st to 10th	Day	22% 16% 33% 19% 8% 1	425
	11th to 20th	Day	29% 20% 30% 15% 5% 1	430
	21st to end	Day	30% 19% 30% 15% 5% 1	430
December	1st to 10th	Day	24% 24% 33% 15% 4	246
	11th to 20th	Day	35% 25% 30% 9% 2	220
	21st to end	Day	25% 26% 30% 14% 4 2	242
January	1st to 10th	Night	22% 20% 39% 17% 2	840
	11th to 20th	Night	27% 19% 34% 17% 2	840
	21st to end	Night	20% 20% 38% 19% 3	924
February	1st to 10th	Night	22% 17% 38% 21% 2	840
	11th to 20th	Night	22% 19% 40% 17% 2	823
	21st to end	Night	25% 18% 35% 19% 3	692
March	1st to 10th	Night	26% 18% 35% 19% 3	840
	11th to 20th	Night	22% 20% 36% 17% 4	840
	21st to end	Night	27% 19% 32% 18% 4	924
April	1st to 10th	Night	25% 19% 33% 19% 3	840
	11th to 20th	Night	26% 19% 33% 17% 5%	810
	21st to end	Night	23% 16% 37% 21% 2	630
May	1st to 10th	Night	28% 14% 33% 20% 4	630
	11th to 20th	Night	20% 14% 37% 22% 5%	587
	21st to end	Night	19% 10% 35% 30% 6%	455
June	1st to 10th	Night	20% 7% 34% 31% 7% 1	420
	11th to 20th	Night	17% 11% 35% 29% 7% 1	420
	21st to end	Night	19% 5% 35% 34% 8%	420
July	1st to 10th	Night	15% 5% 30% 41% 8% 1	410
	11th to 20th	Night	13% 4 28% 42% 12%	408
	21st to end	Night	13% 4 30% 42% 10%	460
August	1st to 10th	Night	12% 5% 28% 42% 12% 1	565
	11th to 20th	Night	12% 6% 30% 38% 13% 1	641
	21st to end	Night	14% 6% 26% 38% 14% 2	924
September	1st to 10th	Night	15% 5% 34% 32% 13%	840
	11th to 20th	Night	16% 8% 26% 29% 19% 3	840
	21st to end	Night	22% 7% 27% 30% 13% 2	840
October	1st to 10th	Night	22% 11% 27% 26% 13% 2	840
	11th to 20th	Night	20% 12% 31% 26% 11% 1	813
	21st to end	Night	25% 13% 33% 24% 5%	880
November	1st to 10th	Night	19% 14% 33% 26% 8% 1	820
	11th to 20th	Night	22% 18% 33% 20% 6% 1	840
	21st to end	Night	24% 19% 33% 20% 4 1	840
December	1st to 10th	Night	25% 18% 36% 18% 4	840
	11th to 20th	Night	25% 17% 32% 22% 3 1	840
	21st to end	Night	25% 18% 35% 19% 3	924

Page generated at 2026-10-11 05:06:29. Version ac07

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