The Clear Nights Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Clear Nights Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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 6% 5% 5% 5% 4 6% 8% 41%	149008
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 43%	143044
Transparency	Day	38% 8% 12% 19% 17% 6%	152783
	Night	37% 7% 12% 19% 19% 7%	147328
Seeing	Day	15% 10% 22% 24% 19% 9%	17601
	Night	15% 9% 22% 30% 22% 4	27564

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	16% 7% 2 6% 4 3 4 3 5% 4 44%	5258
	11th to 20th	Day	10% 7% 2 4 2 2 2 2 4 6% 58%	5280
	21st to end	Day	21% 9% 3 7% 4 4 3 3 4 5% 37%	6019
February	1st to 10th	Day	15% 7% 2 7% 5% 4 5% 4 5% 7% 41%	2100
	11th to 20th	Day	20% 8% 1 6% 3 3 4 3 5% 5% 42%	2048
	21st to end	Day	17% 7% 2 7% 4 4 4 3 5% 4 42%	1730
March	1st to 10th	Day	20% 8% 3 7% 4 4 3 3 5% 5% 36%	4783
	11th to 20th	Day	9% 5% 2 5% 3 3 4 4 6% 7% 52%	5090
	21st to end	Day	15% 6% 3 7% 4 3 4 3 4 5% 46%	5586
April	1st to 10th	Day	11% 6% 2 5% 5% 4 3 4 5% 6% 48%	2320
	11th to 20th	Day	11% 5% 1 6% 3 3 4 3 5% 7% 51%	2494
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 5% 7% 50%	2695
May	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	7616
	11th to 20th	Day	14% 6% 3 7% 5% 5% 6% 5% 7% 9% 34%	7626
	21st to end	Day	13% 7% 3 8% 5% 5% 6% 5% 7% 8% 34%	8391
June	1st to 10th	Day	10% 6% 2 8% 6% 6% 7% 6% 8% 9% 33%	2730
	11th to 20th	Day	6% 4 2 6% 6% 6% 7% 5% 8% 11% 41%	2730
	21st to end	Day	6% 5% 2 6% 5% 6% 8% 6% 8% 10% 39%	2730
July	1st to 10th	Day	7% 5% 2 8% 6% 7% 7% 7% 9% 12% 30%	7680
	11th to 20th	Day	6% 4 2 7% 6% 6% 7% 6% 10% 13% 33%	7680
	21st to end	Day	7% 6% 2 8% 7% 7% 8% 7% 10% 12% 26%	8340
August	1st to 10th	Day	10% 6% 2 8% 6% 7% 7% 7% 10% 13% 26%	6014
	11th to 20th	Day	5% 3 2 5% 4 5% 5% 5% 7% 10% 49%	5586
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 6% 9% 44%	6033
September	1st to 10th	Day	12% 5% 3 6% 5% 5% 6% 4 7% 8% 39%	2310
	11th to 20th	Day	14% 6% 1 6% 4 4 5% 5% 7% 8% 41%	2310
	21st to end	Day	14% 5% 2 6% 4 4 5% 3 5% 5% 48%	2310
October	1st to 10th	Day	13% 6% 1 5% 3 3 3 3 4 7% 53%	2310
	11th to 20th	Day	11% 4 1 4 3 2 3 2 5% 6% 60%	2182
	21st to end	Day	12% 4 1 4 3 4 3 3 4 5% 56%	2280
November	1st to 10th	Day	13% 5% 2 4 4 3 4 3 4 5% 54%	1919
	11th to 20th	Day	13% 5% 2 3 4 3 4 3 4 6% 55%	1890
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 7% 47%	1890
December	1st to 10th	Day	16% 6% 3 6% 3 3 3 3 5% 6% 47%	3020
	11th to 20th	Day	16% 7% 3 5% 4 3 3 2 4 5% 46%	3463
	21st to end	Day	21% 7% 2 5% 3 3 3 3 4 6% 43%	4565
January	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 5% 6% 47%	7635
	11th to 20th	Night	12% 6% 2 6% 4 3 4 3 4 6% 50%	7680
	21st to end	Night	19% 10% 3 7% 4 3 3 3 4 5% 40%	8146
February	1st to 10th	Night	13% 6% 2 6% 5% 4 3 3 5% 6% 45%	2520
	11th to 20th	Night	20% 7% 3 6% 4 3 3 3 4 6% 43%	2458
	21st to end	Night	16% 6% 2 6% 3 5% 4 3 4 6% 44%	2076
March	1st to 10th	Night	20% 8% 3 6% 4 3 4 3 5% 6% 37%	5288
	11th to 20th	Night	10% 4 2 4 3 3 4 4 6% 7% 53%	5090
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 6% 47%	5588
April	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 6% 7% 48%	2310
	11th to 20th	Night	11% 5% 2 4 4 3 3 4 5% 7% 53%	2081
	21st to end	Night	7% 5% 2 6% 3 4 3 3 6% 8% 53%	1922
May	1st to 10th	Night	12% 7% 2 6% 4 5% 5% 3 6% 10% 38%	5237
	11th to 20th	Night	14% 5% 2 6% 5% 4 5% 4 6% 9% 38%	5234
	21st to end	Night	14% 8% 2 6% 4 4 5% 5% 6% 10% 36%	5761
June	1st to 10th	Night	7% 6% 2 7% 6% 5% 5% 5% 8% 10% 39%	1890
	11th to 20th	Night	6% 5% 2 6% 4 5% 5% 4 8% 12% 44%	1890
	21st to end	Night	5% 5% 2 6% 5% 5% 5% 6% 9% 11% 41%	1890
July	1st to 10th	Night	8% 6% 3 8% 6% 6% 6% 5% 10% 13% 31%	5278
	11th to 20th	Night	6% 5% 2 6% 6% 6% 7% 6% 10% 14% 30%	5277
	21st to end	Night	7% 7% 3 8% 6% 6% 7% 6% 9% 13% 29%	5736
August	1st to 10th	Night	9% 7% 3 8% 6% 7% 7% 6% 9% 12% 26%	4135
	11th to 20th	Night	5% 3 2 4 4 5% 4 4 8% 13% 48%	4442
	21st to end	Night	8% 6% 2 6% 4 5% 5% 4 7% 8% 45%	5046
September	1st to 10th	Night	8% 6% 2 6% 5% 5% 6% 5% 7% 10% 39%	2310
	11th to 20th	Night	12% 5% 2 7% 4 4 5% 4 5% 6% 45%	2310
	21st to end	Night	12% 5% 2 5% 5% 4 4 4 6% 8% 46%	2310
October	1st to 10th	Night	12% 5% 2 5% 3 4 4 3 5% 6% 51%	2310
	11th to 20th	Night	11% 4 1 4 2 3 3 3 4 5% 61%	2436
	21st to end	Night	11% 4 1 4 4 3 3 3 4 5% 57%	2737
November	1st to 10th	Night	13% 5% 2 4 3 3 3 3 4 5% 55%	2596
	11th to 20th	Night	12% 6% 2 4 4 3 2 2 4 5% 55%	2730
	21st to end	Night	13% 5% 2 6% 4 3 4 2 5% 5% 50%	2730
December	1st to 10th	Night	14% 7% 2 5% 3 2 3 3 4 5% 52%	4380
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 5% 7% 48%	4952
	21st to end	Night	20% 6% 3 6% 4 4 4 3 4 6% 42%	6633

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	42% 5% 7% 20% 21% 6%	5259
	11th to 20th	Day	29% 3 4 19% 31% 14%	5280
	21st to end	Day	50% 2 5% 14% 20% 9%	6019
February	1st to 10th	Day	42% 5% 8% 18% 20% 6%	2100
	11th to 20th	Day	44% 4 6% 19% 21% 6%	2058
	21st to end	Day	44% 5% 7% 15% 20% 9%	1730
March	1st to 10th	Day	48% 5% 8% 12% 22% 7%	4783
	11th to 20th	Day	31% 5% 8% 15% 22% 20%	5090
	21st to end	Day	41% 4 6% 20% 19% 9%	5586
April	1st to 10th	Day	36% 5% 9% 22% 20% 8%	2320
	11th to 20th	Day	32% 4 8% 21% 25% 10%	2494
	21st to end	Day	32% 7% 9% 24% 24% 3	2695
May	1st to 10th	Day	44% 7% 12% 19% 16% 2	7651
	11th to 20th	Day	43% 9% 15% 20% 11% 2	7630
	21st to end	Day	46% 10% 11% 17% 15% 1	8405
June	1st to 10th	Day	42% 11% 13% 19% 12% 2	2730
	11th to 20th	Day	34% 10% 18% 23% 14% 1	2730
	21st to end	Day	35% 12% 17% 21% 12% 3	2730
July	1st to 10th	Day	38% 15% 23% 19% 4 1	7615
	11th to 20th	Day	35% 16% 28% 17% 3 1	7611
	21st to end	Day	43% 16% 19% 17% 6%	8156
August	1st to 10th	Day	41% 15% 20% 20% 3	6001
	11th to 20th	Day	26% 10% 20% 30% 12% 1	5624
	21st to end	Day	34% 9% 15% 25% 15% 3	6033
September	1st to 10th	Day	39% 9% 15% 17% 16% 4	2310
	11th to 20th	Day	38% 8% 12% 19% 19% 3	2310
	21st to end	Day	37% 5% 11% 21% 20% 6%	2310
October	1st to 10th	Day	27% 4 7% 14% 34% 13%	3700
	11th to 20th	Day	17% 2 5% 16% 30% 30%	3461
	21st to end	Day	24% 3 6% 18% 36% 13%	3618
November	1st to 10th	Day	34% 4 7% 15% 28% 12%	1919
	11th to 20th	Day	31% 3 7% 19% 27% 12%	1890
	21st to end	Day	36% 5% 8% 17% 26% 7%	1890
December	1st to 10th	Day	39% 5% 6% 18% 25% 8%	3020
	11th to 20th	Day	41% 4 9% 18% 21% 7%	3460
	21st to end	Day	42% 4 8% 22% 22% 3	4565
January	1st to 10th	Night	38% 5% 8% 22% 22% 6%	7636
	11th to 20th	Night	36% 3 6% 19% 27% 10%	7680
	21st to end	Night	48% 3 5% 14% 24% 7%	8146
February	1st to 10th	Night	40% 4 7% 19% 21% 10%	2520
	11th to 20th	Night	44% 4 5% 17% 20% 9%	2459
	21st to end	Night	41% 5% 7% 16% 19% 13%	2076
March	1st to 10th	Night	47% 5% 8% 15% 15% 10%	5288
	11th to 20th	Night	29% 5% 9% 13% 26% 19%	5090
	21st to end	Night	38% 4 7% 20% 21% 11%	5588
April	1st to 10th	Night	34% 7% 9% 21% 20% 10%	2310
	11th to 20th	Night	30% 4 7% 22% 26% 11%	2081
	21st to end	Night	30% 5% 13% 23% 23% 6%	1922
May	1st to 10th	Night	39% 8% 16% 18% 15% 4	5264
	11th to 20th	Night	41% 7% 14% 24% 12% 1	5235
	21st to end	Night	42% 9% 14% 18% 15% 2	5771
June	1st to 10th	Night	37% 10% 17% 20% 15% 2	1890
	11th to 20th	Night	31% 10% 18% 28% 11% 2	1890
	21st to end	Night	32% 10% 22% 20% 14% 3	1890
July	1st to 10th	Night	39% 14% 24% 18% 4 1	5235
	11th to 20th	Night	37% 18% 28% 13% 3 1	5226
	21st to end	Night	43% 14% 22% 14% 6% 2	5606
August	1st to 10th	Night	44% 14% 19% 19% 5%	4124
	11th to 20th	Night	25% 10% 24% 28% 12% 1	4470
	21st to end	Night	33% 9% 14% 24% 18% 2	5046
September	1st to 10th	Night	36% 10% 17% 18% 15% 4	2310
	11th to 20th	Night	38% 9% 10% 21% 18% 4	2310
	21st to end	Night	36% 8% 11% 18% 20% 7%	2310
October	1st to 10th	Night	30% 5% 8% 13% 29% 14%	3700
	11th to 20th	Night	17% 2 4 15% 31% 31%	3883
	21st to end	Night	22% 3 5% 20% 31% 19%	4349
November	1st to 10th	Night	33% 4 8% 15% 28% 13%	2596
	11th to 20th	Night	32% 3 6% 17% 27% 14%	2730
	21st to end	Night	36% 5% 6% 18% 26% 9%	2730
December	1st to 10th	Night	35% 4 6% 19% 28% 8%	4380
	11th to 20th	Night	37% 4 11% 17% 24% 7%	4954
	21st to end	Night	44% 3 6% 24% 20% 3	6633

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	23% 25% 37% 14% 1	330
	11th to 20th	Day	24% 25% 29% 16% 5% 1	330
	21st to end	Day	24% 26% 35% 11% 4 1	363
February	1st to 10th	Day	20% 22% 39% 15% 4	330
	11th to 20th	Day	37% 16% 32% 14% 1	330
	21st to end	Day	26% 21% 36% 15% 2	273
March	1st to 10th	Day	21% 18% 31% 23% 6% 1	416
	11th to 20th	Day	22% 15% 32% 23% 8% 1	540
	21st to end	Day	17% 19% 34% 23% 7% 1	594
April	1st to 10th	Day	16% 16% 30% 25% 11% 1	540
	11th to 20th	Day	13% 16% 27% 27% 15% 2	532
	21st to end	Day	12% 13% 24% 29% 19% 4	540
May	1st to 10th	Day	12% 13% 29% 27% 13% 6%	543
	11th to 20th	Day	14% 8% 25% 24% 21% 9%	567
	21st to end	Day	15% 6% 20% 26% 22% 11%	617
June	1st to 10th	Day	11% 4 19% 25% 28% 12%	570
	11th to 20th	Day	7% 4 16% 28% 24% 20%	570
	21st to end	Day	7% 2 14% 31% 27% 19%	570
July	1st to 10th	Day	7% 2 10% 23% 35% 23%	565
	11th to 20th	Day	7% 1 8% 21% 37% 25%	564
	21st to end	Day	5% 3 10% 24% 39% 19%	620
August	1st to 10th	Day	12% 4 10% 28% 25% 20%	566
	11th to 20th	Day	9% 2 8% 22% 38% 21%	559
	21st to end	Day	15% 3 10% 28% 29% 16%	627
September	1st to 10th	Day	11% 3 12% 27% 31% 16%	570
	11th to 20th	Day	16% 2 11% 26% 33% 12%	570
	21st to end	Day	14% 3 17% 30% 26% 10%	570
October	1st to 10th	Day	18% 6% 20% 29% 21% 7%	570
	11th to 20th	Day	12% 9% 23% 28% 22% 6%	570
	21st to end	Day	14% 10% 24% 29% 19% 5%	615
November	1st to 10th	Day	21% 7% 27% 26% 15% 4	397
	11th to 20th	Day	15% 16% 40% 21% 6% 2	330
	21st to end	Day	16% 17% 33% 22% 10% 2	330
December	1st to 10th	Day	27% 19% 29% 18% 7%	330
	11th to 20th	Day	26% 18% 26% 22% 8% 1	330
	21st to end	Day	27% 17% 31% 18% 5% 1	363
January	1st to 10th	Night	23% 19% 29% 23% 6%	840
	11th to 20th	Night	20% 17% 30% 26% 8%	840
	21st to end	Night	22% 16% 35% 23% 4	924
February	1st to 10th	Night	18% 15% 29% 32% 6%	840
	11th to 20th	Night	24% 16% 31% 27% 2	823
	21st to end	Night	20% 17% 34% 26% 3	692
March	1st to 10th	Night	16% 13% 31% 32% 7%	840
	11th to 20th	Night	18% 13% 32% 28% 9%	840
	21st to end	Night	16% 13% 31% 30% 10%	924
April	1st to 10th	Night	13% 13% 31% 30% 12%	840
	11th to 20th	Night	14% 12% 25% 31% 18% 1	629
	21st to end	Night	9% 10% 23% 36% 22% 1	630
May	1st to 10th	Night	9% 7% 26% 37% 19% 2	630
	11th to 20th	Night	11% 4 22% 34% 24% 5%	621
	21st to end	Night	14% 3 12% 32% 33% 6%	683
June	1st to 10th	Night	9% 2 11% 38% 37% 5%	630
	11th to 20th	Night	7% 2 14% 29% 36% 12%	630
	21st to end	Night	7% 2 9% 33% 41% 9%	630
July	1st to 10th	Night	8% 2 7% 25% 44% 14%	615
	11th to 20th	Night	7% 1 4 23% 50% 14%	612
	21st to end	Night	6% 1 8% 28% 47% 11%	690
August	1st to 10th	Night	10% 2 6% 26% 44% 12%	627
	11th to 20th	Night	10% 1 5% 26% 47% 10%	624
	21st to end	Night	16% 1 10% 26% 37% 10%	693
September	1st to 10th	Night	9% 2 12% 28% 40% 10%	840
	11th to 20th	Night	13% 2 12% 28% 41% 4	840
	21st to end	Night	14% 3 13% 31% 34% 4	840
October	1st to 10th	Night	15% 3 16% 34% 25% 6%	840
	11th to 20th	Night	12% 5% 22% 33% 24% 3	840
	21st to end	Night	13% 8% 22% 34% 22% 2	913
November	1st to 10th	Night	15% 8% 22% 33% 21% 1	820
	11th to 20th	Night	15% 13% 27% 31% 13% 2	840
	21st to end	Night	15% 15% 26% 31% 13% 1	840
December	1st to 10th	Night	20% 13% 29% 28% 10% 1	840
	11th to 20th	Night	21% 11% 28% 30% 10% 1	840
	21st to end	Night	26% 13% 25% 29% 7%	924

Page generated at 2026-10-26 04:37:51. Version ac07

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