Daybis Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Daybis Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	11% 6% 2 6% 5% 4 5% 4 6% 8% 43%	131572
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 46%	131001
Transparency	Day	37% 6% 11% 21% 19% 5%	135312
	Night	36% 6% 10% 21% 20% 8%	135324
Seeing	Day	13% 8% 20% 24% 22% 13%	16949
	Night	14% 6% 21% 31% 23% 4	26708

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% 11% 4 7% 3 3 3 3 4 6% 40%	4908
	11th to 20th	Day	10% 4 3 6% 3 3 3 3 4 5% 56%	5310
	21st to end	Day	22% 10% 2 7% 5% 3 2 2 3 4 39%	6358
February	1st to 10th	Day	16% 8% 3 6% 4 3 4 4 5% 7% 41%	2100
	11th to 20th	Day	20% 9% 3 6% 4 4 3 3 4 5% 39%	2048
	21st to end	Day	18% 7% 3 7% 5% 3 3 2 4 5% 42%	1730
March	1st to 10th	Day	19% 9% 3 9% 4 4 4 3 5% 6% 34%	4580
	11th to 20th	Day	11% 6% 2 6% 4 5% 4 4 5% 6% 45%	4897
	21st to end	Day	12% 7% 3 8% 5% 4 4 3 5% 6% 41%	6084
April	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 7% 7% 46%	2520
	11th to 20th	Day	13% 6% 2 5% 3 4 4 3 5% 7% 47%	2494
	21st to end	Day	9% 5% 2 7% 5% 4 5% 3 6% 7% 46%	2520
May	1st to 10th	Day	14% 6% 3 7% 5% 4 5% 4 6% 8% 38%	5417
	11th to 20th	Day	12% 6% 2 8% 6% 5% 6% 4 7% 9% 34%	5787
	21st to end	Day	11% 6% 2 6% 4 5% 5% 5% 6% 9% 40%	6433
June	1st to 10th	Day	8% 5% 2 6% 6% 6% 6% 4 8% 10% 39%	2647
	11th to 20th	Day	5% 4 1 6% 5% 6% 6% 5% 8% 9% 45%	2800
	21st to end	Day	6% 3 2 4 6% 5% 7% 6% 7% 12% 43%	2800
July	1st to 10th	Day	5% 5% 2 7% 6% 6% 7% 6% 8% 10% 37%	6360
	11th to 20th	Day	6% 4 2 6% 5% 6% 7% 6% 10% 13% 35%	6131
	21st to end	Day	8% 6% 3 8% 6% 6% 7% 5% 9% 12% 31%	5932
August	1st to 10th	Day	3 3 1 5% 4 4 5% 4 6% 7% 59%	3905
	11th to 20th	Day	4 4 2 6% 5% 6% 5% 5% 8% 9% 46%	3849
	21st to end	Day	7% 3 2 5% 5% 5% 5% 4 6% 9% 48%	4312
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 6% 7% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 3 5% 6% 47%	2270
	21st to end	Day	10% 5% 2 5% 3 4 4 4 6% 8% 51%	2200
October	1st to 10th	Day	7% 4 2 6% 3 3 3 3 5% 5% 58%	2200
	11th to 20th	Day	9% 4 1 4 4 3 4 2 4 6% 59%	2019
	21st to end	Day	13% 4 2 5% 3 3 4 3 4 5% 54%	2200
November	1st to 10th	Day	9% 6% 2 4 3 3 3 3 4 5% 56%	1865
	11th to 20th	Day	11% 5% 2 6% 3 4 3 3 3 5% 56%	1890
	21st to end	Day	14% 6% 2 5% 4 3 4 3 4 7% 46%	1890
December	1st to 10th	Day	15% 9% 3 7% 4 3 4 3 4 5% 43%	3020
	11th to 20th	Day	14% 9% 4 8% 4 4 4 3 4 5% 42%	3463
	21st to end	Day	16% 7% 3 6% 4 4 4 3 4 5% 46%	4233
January	1st to 10th	Night	18% 9% 3 7% 3 3 3 2 4 5% 42%	8010
	11th to 20th	Night	14% 7% 3 7% 3 3 3 2 4 5% 50%	7680
	21st to end	Night	23% 8% 2 6% 4 3 3 2 4 4 41%	7807
February	1st to 10th	Night	17% 7% 3 6% 3 3 4 3 5% 6% 43%	2520
	11th to 20th	Night	21% 8% 2 6% 4 3 3 3 3 4 42%	2458
	21st to end	Night	19% 7% 3 6% 4 3 4 3 5% 5% 41%	2076
March	1st to 10th	Night	18% 10% 3 7% 4 4 4 3 4 6% 37%	5493
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 5% 7% 51%	5325
	21st to end	Night	17% 7% 2 6% 4 4 4 2 4 5% 45%	5085
April	1st to 10th	Night	12% 6% 2 6% 4 3 5% 4 5% 7% 47%	2100
	11th to 20th	Night	11% 5% 1 5% 3 3 4 4 4 7% 52%	2075
	21st to end	Night	11% 5% 2 5% 4 4 4 4 6% 7% 49%	2100
May	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 6% 9% 41%	4500
	11th to 20th	Night	18% 7% 2 8% 5% 4 5% 4 5% 7% 36%	4088
	21st to end	Night	9% 5% 2 7% 5% 5% 6% 5% 8% 9% 40%	4421
June	1st to 10th	Night	7% 5% 2 6% 5% 4 5% 4 6% 10% 45%	1747
	11th to 20th	Night	4 3 1 3 5% 4 4 5% 7% 10% 52%	1600
	21st to end	Night	4 4 2 5% 4 4 5% 5% 7% 11% 49%	1600
July	1st to 10th	Night	4 4 2 7% 5% 6% 6% 6% 9% 12% 38%	3618
	11th to 20th	Night	5% 4 2 5% 4 5% 5% 5% 10% 14% 41%	3840
	21st to end	Night	6% 8% 3 9% 6% 5% 6% 6% 9% 14% 30%	4915
August	1st to 10th	Night	3 2 1 3 3 3 4 4 6% 9% 64%	3244
	11th to 20th	Night	3 2 1 4 3 4 4 4 7% 12% 53%	3200
	21st to end	Night	5% 3 2 6% 4 4 6% 5% 7% 9% 50%	3586
September	1st to 10th	Night	9% 4 2 6% 5% 4 5% 4 6% 7% 49%	2000
	11th to 20th	Night	10% 5% 2 5% 4 4 3 4 7% 9% 46%	2110
	21st to end	Night	8% 5% 2 4 3 3 4 4 5% 7% 56%	2200
October	1st to 10th	Night	7% 3 1 4 3 3 3 3 5% 6% 62%	2200
	11th to 20th	Night	10% 5% 1 4 3 2 4 3 4 7% 57%	2376
	21st to end	Night	15% 6% 2 4 3 2 2 2 4 5% 56%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 5% 56%	2651
	11th to 20th	Night	12% 5% 2 4 4 3 3 2 4 6% 56%	2730
	21st to end	Night	14% 5% 2 5% 4 4 3 3 4 5% 50%	2730
December	1st to 10th	Night	20% 7% 2 5% 3 2 3 3 4 5% 46%	4380
	11th to 20th	Night	17% 8% 3 6% 4 3 3 2 4 4 47%	4952
	21st to end	Night	17% 8% 3 6% 3 4 4 3 5% 6% 42%	6944

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	47% 4 7% 26% 15% 2	4909
	11th to 20th	Day	30% 3 5% 24% 34% 4	5310
	21st to end	Day	51% 3 5% 15% 19% 7%	6358
February	1st to 10th	Day	42% 5% 9% 20% 19% 6%	2100
	11th to 20th	Day	48% 4 5% 20% 17% 7%	2058
	21st to end	Day	45% 5% 6% 14% 21% 9%	1730
March	1st to 10th	Day	51% 4 7% 13% 18% 7%	4580
	11th to 20th	Day	38% 5% 7% 16% 23% 11%	4897
	21st to end	Day	43% 5% 8% 21% 18% 5%	6086
April	1st to 10th	Day	35% 5% 11% 22% 20% 7%	2520
	11th to 20th	Day	35% 5% 7% 21% 24% 7%	2494
	21st to end	Day	35% 6% 10% 24% 22% 3	2520
May	1st to 10th	Day	42% 7% 11% 26% 11% 2	5452
	11th to 20th	Day	43% 8% 16% 22% 10%	5791
	21st to end	Day	38% 8% 13% 26% 14% 1	6447
June	1st to 10th	Day	36% 10% 13% 22% 17% 2	2647
	11th to 20th	Day	31% 9% 15% 24% 18% 3	2800
	21st to end	Day	32% 10% 17% 23% 15% 3	2800
July	1st to 10th	Day	37% 11% 17% 25% 8% 1	6291
	11th to 20th	Day	34% 12% 27% 23% 4 1	6056
	21st to end	Day	41% 11% 22% 22% 5%	5762
August	1st to 10th	Day	21% 8% 12% 31% 27% 1	3893
	11th to 20th	Day	30% 9% 16% 28% 15% 2	3885
	21st to end	Day	31% 8% 14% 20% 22% 4	4311
September	1st to 10th	Day	34% 10% 12% 20% 18% 5%	2400
	11th to 20th	Day	39% 6% 10% 17% 22% 6%	2270
	21st to end	Day	31% 6% 11% 23% 21% 9%	2200
October	1st to 10th	Day	19% 3 6% 15% 26% 30%	3590
	11th to 20th	Day	17% 2 4 15% 37% 24%	3259
	21st to end	Day	24% 3 5% 19% 35% 13%	3538
November	1st to 10th	Day	31% 4 8% 21% 27% 10%	1865
	11th to 20th	Day	32% 3 8% 19% 26% 11%	1890
	21st to end	Day	39% 5% 7% 17% 25% 7%	1890
December	1st to 10th	Day	44% 3 7% 22% 21% 3	3020
	11th to 20th	Day	45% 4 8% 18% 19% 6%	3460
	21st to end	Day	40% 4 8% 22% 24% 1	4233
January	1st to 10th	Night	44% 4 7% 24% 19% 2	8011
	11th to 20th	Night	38% 3 4 17% 29% 9%	7680
	21st to end	Night	48% 3 4 17% 21% 7%	7807
February	1st to 10th	Night	42% 3 7% 18% 21% 8%	2520
	11th to 20th	Night	47% 4 6% 18% 17% 9%	2459
	21st to end	Night	44% 5% 6% 15% 19% 10%	2076
March	1st to 10th	Night	49% 4 8% 13% 16% 10%	5493
	11th to 20th	Night	32% 5% 6% 18% 21% 18%	5325
	21st to end	Night	43% 3 7% 22% 19% 7%	5085
April	1st to 10th	Night	36% 6% 10% 20% 20% 9%	2100
	11th to 20th	Night	32% 4 8% 23% 23% 10%	2075
	21st to end	Night	34% 6% 11% 22% 22% 5%	2100
May	1st to 10th	Night	40% 7% 15% 21% 15% 2	4530
	11th to 20th	Night	47% 7% 11% 24% 10% 1	4089
	21st to end	Night	36% 9% 13% 22% 18% 2	4429
June	1st to 10th	Night	33% 9% 15% 21% 20% 3	1747
	11th to 20th	Night	24% 9% 15% 28% 18% 6%	1600
	21st to end	Night	28% 8% 19% 22% 19% 4	1600
July	1st to 10th	Night	32% 13% 23% 22% 10% 2	3580
	11th to 20th	Night	28% 13% 30% 24% 4 1	3795
	21st to end	Night	41% 12% 21% 20% 5% 1	4772
August	1st to 10th	Night	16% 7% 14% 33% 27% 3	3232
	11th to 20th	Night	21% 9% 22% 30% 15% 3	3230
	21st to end	Night	29% 8% 14% 21% 21% 6%	3586
September	1st to 10th	Night	32% 6% 14% 24% 18% 6%	2000
	11th to 20th	Night	32% 8% 13% 17% 23% 7%	2110
	21st to end	Night	28% 6% 8% 25% 22% 10%	2200
October	1st to 10th	Night	17% 3 7% 16% 27% 31%	3590
	11th to 20th	Night	19% 3 4 17% 29% 29%	3862
	21st to end	Night	22% 2 6% 19% 31% 20%	4252
November	1st to 10th	Night	30% 4 8% 18% 27% 12%	2651
	11th to 20th	Night	31% 4 8% 19% 27% 12%	2730
	21st to end	Night	37% 4 6% 21% 22% 10%	2730
December	1st to 10th	Night	41% 3 5% 22% 25% 4	4380
	11th to 20th	Night	42% 4 8% 18% 24% 4	4954
	21st to end	Night	41% 5% 8% 21% 22% 2	6944

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	24% 20% 31% 20% 4 1	330
	11th to 20th	Day	20% 21% 34% 16% 7% 2	330
	21st to end	Day	23% 22% 34% 17% 4 1	363
February	1st to 10th	Day	20% 15% 38% 18% 9% 1	330
	11th to 20th	Day	32% 18% 31% 15% 3 1	330
	21st to end	Day	27% 20% 33% 16% 5%	273
March	1st to 10th	Day	18% 18% 36% 21% 8%	330
	11th to 20th	Day	20% 16% 39% 20% 4 1	330
	21st to end	Day	17% 14% 33% 25% 11%	579
April	1st to 10th	Day	16% 17% 27% 26% 12% 2	540
	11th to 20th	Day	17% 12% 31% 25% 13% 2	532
	21st to end	Day	10% 14% 25% 31% 16% 4	540
May	1st to 10th	Day	17% 9% 20% 32% 17% 5%	507
	11th to 20th	Day	10% 6% 23% 30% 20% 12%	527
	21st to end	Day	10% 4 19% 30% 21% 15%	573
June	1st to 10th	Day	9% 3 15% 29% 27% 17%	577
	11th to 20th	Day	6% 2 11% 27% 32% 21%	730
	21st to end	Day	7% 2 10% 24% 35% 22%	730
July	1st to 10th	Day	7% 2 7% 21% 38% 25%	721
	11th to 20th	Day	7% 2 6% 20% 32% 34%	718
	21st to end	Day	6% 1 6% 20% 37% 30%	586
August	1st to 10th	Day	5% 2 6% 22% 36% 28%	526
	11th to 20th	Day	8% 1 8% 21% 33% 30%	519
	21st to end	Day	12% 2 7% 21% 30% 27%	583
September	1st to 10th	Day	9% 3 8% 25% 37% 18%	530
	11th to 20th	Day	11% 2 11% 20% 34% 23%	530
	21st to end	Day	10% 3 11% 30% 32% 15%	530
October	1st to 10th	Day	9% 3 17% 30% 29% 12%	530
	11th to 20th	Day	9% 7% 28% 27% 22% 7%	349
	21st to end	Day	17% 10% 23% 28% 16% 5%	363
November	1st to 10th	Day	15% 10% 23% 30% 17% 5%	330
	11th to 20th	Day	12% 10% 34% 33% 9% 2	330
	21st to end	Day	12% 18% 33% 21% 12% 3	330
December	1st to 10th	Day	22% 14% 36% 19% 8% 1	330
	11th to 20th	Day	16% 14% 36% 20% 12% 2	330
	21st to end	Day	21% 13% 35% 21% 8% 1	363
January	1st to 10th	Night	20% 12% 33% 28% 6%	1050
	11th to 20th	Night	20% 12% 33% 28% 7%	1050
	21st to end	Night	20% 12% 36% 29% 3	947
February	1st to 10th	Night	20% 10% 32% 31% 7%	840
	11th to 20th	Night	25% 13% 29% 29% 4	823
	21st to end	Night	21% 15% 34% 28% 3	692
March	1st to 10th	Night	18% 12% 30% 33% 7%	840
	11th to 20th	Night	16% 9% 36% 30% 8%	735
	21st to end	Night	15% 10% 32% 31% 12% 1	693
April	1st to 10th	Night	13% 11% 31% 32% 13%	630
	11th to 20th	Night	12% 7% 32% 34% 14% 2	623
	21st to end	Night	12% 7% 30% 35% 14% 2	630
May	1st to 10th	Night	11% 4 24% 39% 20% 2	603
	11th to 20th	Night	13% 4 17% 35% 27% 4	591
	21st to end	Night	9% 1 12% 38% 32% 7%	650
June	1st to 10th	Night	8% 3 8% 38% 37% 7%	600
	11th to 20th	Night	5% 1 8% 29% 46% 12%	600
	21st to end	Night	5% 1 8% 27% 47% 12%	600
July	1st to 10th	Night	4 1 5% 23% 51% 16%	585
	11th to 20th	Night	4 1 4 24% 46% 21%	582
	21st to end	Night	5% 1 5% 23% 49% 16%	657
August	1st to 10th	Night	5% 3 21% 51% 19%	597
	11th to 20th	Night	6% 1 3 24% 50% 16%	594
	21st to end	Night	8% 1 7% 22% 47% 14%	660
September	1st to 10th	Night	11% 2 7% 31% 43% 7%	600
	11th to 20th	Night	12% 2 10% 26% 43% 7%	710
	21st to end	Night	9% 3 11% 33% 39% 5%	800
October	1st to 10th	Night	8% 3 14% 34% 36% 6%	800
	11th to 20th	Night	13% 4 18% 40% 24% 1	800
	21st to end	Night	17% 5% 21% 32% 23% 2	880
November	1st to 10th	Night	13% 5% 17% 39% 22% 3	820
	11th to 20th	Night	12% 6% 29% 36% 15% 1	840
	21st to end	Night	17% 8% 25% 34% 14% 1	840
December	1st to 10th	Night	22% 9% 27% 33% 10%	840
	11th to 20th	Night	21% 8% 28% 32% 11%	840
	21st to end	Night	23% 10% 30% 29% 8%	1066

Page generated at 2026-04-29 01:21:48. Version ac07

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