C.J. Observatory & Retreat
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the C.J. Observatory & Retreat clear sky chart from 2005-11-06 to 2026-01-02.

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	3 2 1 3 3 3 3 3 5% 7% 66%	135688
	Night	3 3 1 4 3 3 4 3 5% 7% 64%	133171
Transparency	Day	18% 3 5% 14% 29% 31%	130672
	Night	20% 3 5% 14% 29% 29%	129190
Seeing	Day	6% 8% 21% 31% 24% 10%	15958
	Night	5% 4 24% 41% 23% 3	26010

By Month

Percentage Cloud Cover SW (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	3 5% 2 5% 3 3 3 3 4 5% 64%	4107
	11th to 20th	Day	2 1 1 1 2 2 2 2 3 5% 79%	4100
	21st to end	Day	5% 3 1 3 2 2 3 3 5% 7% 66%	4510
February	1st to 10th	Day	2 3 1 4 3 3 3 2 4 6% 69%	1800
	11th to 20th	Day	5% 2 1 3 3 3 3 3 4 6% 66%	1909
	21st to end	Day	3 4 1 3 3 2 3 2 4 4 70%	1780
March	1st to 10th	Day	2 3 1 3 3 3 2 2 4 6% 69%	4954
	11th to 20th	Day	4 3 1 3 2 3 4 3 5% 6% 66%	5000
	21st to end	Day	1 3 1 4 3 4 4 4 6% 8% 63%	5486
April	1st to 10th	Day	1 2 1 3 3 3 2 2 5% 6% 74%	2349
	11th to 20th	Day	2 2 1 2 1 2 2 1 3 5% 79%	2371
	21st to end	Day	1 2 1 2 1 2 1 2 3 5% 80%	2400
May	1st to 10th	Day	1 1 1 1 2 2 2 3 4 83%	5440
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 79%	5620
	21st to end	Day	1 1 1 1 2 2 2 3 4 83%	6423
June	1st to 10th	Day	3 2 1 3 3 3 3 3 4 7% 68%	2600
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 5% 76%	2600
	21st to end	Day	3 4 1 4 3 4 5% 4 7% 8% 56%	2600
July	1st to 10th	Day	3 2 1 4 4 4 4 4 6% 8% 59%	5879
	11th to 20th	Day	2 2 1 4 4 6% 6% 5% 8% 11% 52%	5880
	21st to end	Day	4 4 2 6% 5% 6% 8% 7% 9% 12% 37%	6401
August	1st to 10th	Day	2 2 1 4 3 4 5% 4 7% 10% 57%	5737
	11th to 20th	Day	2 2 1 4 4 4 5% 4 7% 10% 56%	5366
	21st to end	Day	3 2 1 4 4 5% 6% 5% 8% 10% 52%	5984
September	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 7% 9% 49%	2400
	11th to 20th	Day	5% 3 1 4 3 3 4 3 6% 8% 60%	2336
	21st to end	Day	3 2 1 3 2 2 3 2 4 7% 70%	2132
October	1st to 10th	Day	2 1 1 2 2 1 2 2 4 5% 77%	3270
	11th to 20th	Day	3 2 1 3 2 2 2 3 3 4 74%	3250
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	3548
November	1st to 10th	Day	2 2 1 3 2 3 2 3 3 6% 75%	2059
	11th to 20th	Day	2 3 1 3 2 2 3 3 4 6% 71%	2100
	21st to end	Day	5% 4 1 4 3 3 3 3 4 7% 63%	2100
December	1st to 10th	Day	6% 3 1 3 3 2 3 2 4 5% 68%	3089
	11th to 20th	Day	3 4 2 4 3 2 3 2 3 7% 68%	3499
	21st to end	Day	2 2 1 2 2 3 3 3 4 6% 72%	4609
January	1st to 10th	Night	4 7% 2 4 3 2 3 3 5% 6% 62%	5880
	11th to 20th	Night	2 2 2 2 2 2 2 3 5% 79%	5880
	21st to end	Night	6% 5% 1 3 2 2 3 3 5% 6% 62%	6468
February	1st to 10th	Night	2 2 1 3 2 2 4 3 4 8% 67%	2600
	11th to 20th	Night	5% 4 1 3 2 2 3 3 4 6% 67%	2376
	21st to end	Night	3 4 1 2 2 3 4 3 5% 6% 68%	1845
March	1st to 10th	Night	2 2 1 2 2 2 3 3 5% 6% 72%	4940
	11th to 20th	Night	5% 3 1 3 3 2 3 3 4 6% 68%	5000
	21st to end	Night	1 2 1 3 2 3 3 3 5% 7% 67%	5489
April	1st to 10th	Night	1 2 1 2 2 2 2 2 5% 6% 74%	2050
	11th to 20th	Night	2 3 1 3 2 2 2 2 4 5% 74%	1977
	21st to end	Night	1 2 1 2 2 2 2 2 4 5% 78%	2000
May	1st to 10th	Night	2 1 1 2 2 2 3 2 3 5% 77%	4540
	11th to 20th	Night	1 1 1 2 2 3 3 3 4 5% 75%	4278
	21st to end	Night	1 1 1 3 2 2 2 1 3 4 82%	4474
June	1st to 10th	Night	3 2 1 3 3 3 4 3 5% 6% 67%	1800
	11th to 20th	Night	2 2 1 3 3 3 3 2 5% 5% 73%	1800
	21st to end	Night	4 4 2 5% 5% 5% 6% 5% 6% 9% 50%	1800
July	1st to 10th	Night	3 3 2 4 5% 5% 5% 6% 9% 11% 47%	4100
	11th to 20th	Night	2 3 2 5% 5% 6% 7% 7% 10% 14% 38%	4100
	21st to end	Night	4 5% 3 9% 7% 7% 7% 6% 8% 13% 32%	4472
August	1st to 10th	Night	2 3 2 6% 5% 6% 7% 6% 9% 13% 41%	4075
	11th to 20th	Night	3 4 2 6% 6% 7% 8% 7% 10% 14% 33%	4483
	21st to end	Night	3 4 2 8% 6% 6% 7% 6% 9% 10% 39%	4993
September	1st to 10th	Night	5% 4 2 7% 5% 4 5% 5% 6% 9% 48%	2000
	11th to 20th	Night	4 3 1 4 4 4 4 3 6% 8% 57%	2059
	21st to end	Night	3 2 1 4 2 3 3 2 4 6% 68%	2266
October	1st to 10th	Night	3 2 1 2 2 2 2 2 4 6% 74%	3920
	11th to 20th	Night	3 2 1 3 2 3 2 2 3 4 75%	3898
	21st to end	Night	1 1 2 1 1 2 1 3 5% 83%	4252
November	1st to 10th	Night	2 1 2 2 2 3 3 4 5% 76%	2460
	11th to 20th	Night	3 2 1 3 2 2 3 2 4 6% 72%	2520
	21st to end	Night	5% 3 1 4 3 2 3 3 4 6% 65%	2520
December	1st to 10th	Night	3 2 1 2 1 2 2 2 4 5% 74%	4304
	11th to 20th	Night	2 2 1 3 3 3 4 3 5% 6% 67%	4941
	21st to end	Night	4 3 1 3 2 2 3 2 4 5% 70%	6611

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	24% 1 3 9% 43% 21%	4108
	11th to 20th	Day	11% 1 3 13% 44% 28%	4100
	21st to end	Day	20% 1 2 18% 31% 28%	4510
February	1st to 10th	Day	18% 1 3 15% 27% 35%	1800
	11th to 20th	Day	21% 2 3 15% 31% 28%	1919
	21st to end	Day	19% 1 2 8% 30% 39%	1780
March	1st to 10th	Day	17% 1 2 5% 35% 40%	4954
	11th to 20th	Day	20% 2 2 15% 30% 31%	5000
	21st to end	Day	21% 1 2 9% 36% 30%	5486
April	1st to 10th	Day	13% 1 2 10% 36% 38%	2349
	11th to 20th	Day	12% 1 1 7% 28% 52%	2374
	21st to end	Day	10% 1 1 10% 31% 48%	2400
May	1st to 10th	Day	8% 1 2 11% 30% 49%	5440
	11th to 20th	Day	10% 1 2 8% 26% 53%	5623
	21st to end	Day	8% 1 2 6% 14% 69%	6424
June	1st to 10th	Day	18% 1 2 9% 28% 42%	2600
	11th to 20th	Day	12% 1 1 6% 20% 60%	2600
	21st to end	Day	25% 3 6% 15% 20% 31%	2600
July	1st to 10th	Day	20% 5% 11% 17% 25% 22%	5816
	11th to 20th	Day	22% 6% 14% 26% 25% 6%	5811
	21st to end	Day	32% 8% 18% 23% 12% 7%	6217
August	1st to 10th	Day	26% 5% 12% 29% 22% 6%	4177
	11th to 20th	Day	21% 5% 9% 19% 33% 12%	3882
	21st to end	Day	28% 7% 14% 23% 21% 8%	4311
September	1st to 10th	Day	31% 3 8% 19% 23% 16%	2400
	11th to 20th	Day	23% 3 6% 17% 29% 22%	2336
	21st to end	Day	17% 2 4 17% 35% 27%	2132
October	1st to 10th	Day	11% 1 5% 11% 24% 48%	3270
	11th to 20th	Day	15% 2 3 11% 31% 37%	3250
	21st to end	Day	7% 1 1 10% 32% 49%	3548
November	1st to 10th	Day	13% 1 4 15% 36% 30%	2059
	11th to 20th	Day	17% 2 3 16% 31% 31%	2100
	21st to end	Day	23% 2 3 18% 32% 22%	2100
December	1st to 10th	Day	20% 1 4 17% 36% 21%	3089
	11th to 20th	Day	21% 1 3 14% 39% 23%	3498
	21st to end	Day	15% 1 4 17% 41% 22%	4609
January	1st to 10th	Night	25% 1 2 11% 44% 17%	5880
	11th to 20th	Night	12% 1 1 15% 46% 25%	5880
	21st to end	Night	24% 1 3 16% 28% 28%	6468
February	1st to 10th	Night	18% 2 3 17% 27% 34%	2600
	11th to 20th	Night	20% 2 3 16% 32% 27%	2376
	21st to end	Night	18% 2 2 12% 30% 36%	1845
March	1st to 10th	Night	14% 1 2 8% 39% 35%	4940
	11th to 20th	Night	19% 2 2 15% 36% 27%	5000
	21st to end	Night	18% 1 2 17% 36% 26%	5489
April	1st to 10th	Night	13% 1 2 11% 37% 34%	2050
	11th to 20th	Night	15% 1 1 7% 29% 46%	1977
	21st to end	Night	12% 2 1 11% 31% 44%	2000
May	1st to 10th	Night	13% 2 2 12% 25% 46%	4537
	11th to 20th	Night	13% 1 2 9% 24% 50%	4279
	21st to end	Night	10% 2 2 4 13% 70%	4474
June	1st to 10th	Night	19% 2 2 10% 26% 41%	1800
	11th to 20th	Night	15% 1 2 6% 21% 55%	1800
	21st to end	Night	30% 4 8% 14% 15% 30%	1800
July	1st to 10th	Night	26% 6% 14% 20% 20% 14%	4055
	11th to 20th	Night	30% 8% 17% 19% 21% 5%	4046
	21st to end	Night	40% 7% 19% 20% 7% 8%	4344
August	1st to 10th	Night	32% 7% 15% 29% 11% 6%	2959
	11th to 20th	Night	30% 7% 12% 13% 30% 7%	3243
	21st to end	Night	41% 6% 15% 18% 16% 4	3597
September	1st to 10th	Night	32% 5% 11% 18% 17% 18%	2000
	11th to 20th	Night	24% 4 8% 18% 25% 20%	2059
	21st to end	Night	18% 2 6% 16% 32% 25%	2266
October	1st to 10th	Night	14% 2 5% 9% 28% 43%	3920
	11th to 20th	Night	14% 3 2 13% 28% 40%	3898
	21st to end	Night	8% 1 1 10% 30% 49%	4252
November	1st to 10th	Night	12% 2 3 14% 36% 33%	2460
	11th to 20th	Night	16% 1 2 16% 32% 32%	2520
	21st to end	Night	22% 1 3 18% 32% 23%	2520
December	1st to 10th	Night	14% 1 3 22% 39% 20%	4304
	11th to 20th	Night	20% 1 4 13% 34% 28%	4941
	21st to end	Night	18% 1 2 17% 39% 23%	6611

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	12% 8% 26% 37% 14% 3	303
	11th to 20th	Day	6% 8% 28% 37% 17% 4	300
	21st to end	Day	5% 8% 30% 35% 16% 7%	330
February	1st to 10th	Day	4 11% 27% 34% 20% 4	300
	11th to 20th	Day	11% 10% 30% 36% 12% 2	300
	21st to end	Day	9% 26% 34% 22% 8% 2	379
March	1st to 10th	Day	10% 20% 33% 28% 9% 1	500
	11th to 20th	Day	11% 16% 28% 30% 11% 3	500
	21st to end	Day	8% 22% 35% 26% 9% 1	550
April	1st to 10th	Day	6% 16% 33% 28% 13% 4	500
	11th to 20th	Day	7% 19% 32% 26% 13% 2	492
	21st to end	Day	5% 14% 36% 28% 15% 2	500
May	1st to 10th	Day	4 13% 35% 31% 14% 3	503
	11th to 20th	Day	3 11% 29% 33% 20% 4	528
	21st to end	Day	2 8% 33% 36% 19% 3	573
June	1st to 10th	Day	5% 5% 22% 38% 24% 6%	530
	11th to 20th	Day	2 4 22% 39% 25% 8%	530
	21st to end	Day	4 2 12% 39% 31% 13%	530
July	1st to 10th	Day	6% 1 6% 29% 35% 24%	526
	11th to 20th	Day	6% 1 3 23% 39% 29%	524
	21st to end	Day	4 1 3 18% 44% 28%	575
August	1st to 10th	Day	4 2 2 20% 40% 32%	527
	11th to 20th	Day	4 3 5% 23% 41% 24%	517
	21st to end	Day	4 2 6% 23% 38% 28%	583
September	1st to 10th	Day	10% 2 6% 32% 35% 15%	530
	11th to 20th	Day	5% 2 11% 33% 35% 14%	530
	21st to end	Day	3 4 15% 38% 30% 10%	462
October	1st to 10th	Day	6% 4 15% 33% 29% 12%	330
	11th to 20th	Day	8% 5% 17% 33% 29% 8%	330
	21st to end	Day	3 5% 18% 33% 29% 12%	363
November	1st to 10th	Day	5% 4 26% 35% 23% 7%	330
	11th to 20th	Day	4 5% 18% 38% 27% 7%	330
	21st to end	Day	7% 11% 23% 36% 19% 4	330
December	1st to 10th	Day	5% 7% 24% 36% 23% 5%	330
	11th to 20th	Day	8% 6% 27% 33% 22% 5%	330
	21st to end	Day	8% 8% 29% 33% 17% 4	363
January	1st to 10th	Night	6% 5% 28% 41% 17% 4	804
	11th to 20th	Night	6% 3 31% 39% 18% 4	800
	21st to end	Night	9% 5% 28% 40% 14% 3	880
February	1st to 10th	Night	4 6% 33% 39% 15% 4	800
	11th to 20th	Night	7% 7% 34% 39% 11% 2	784
	21st to end	Night	5% 8% 45% 32% 8% 1	660
March	1st to 10th	Night	6% 9% 41% 34% 9% 1	800
	11th to 20th	Night	7% 7% 33% 38% 13% 2	800
	21st to end	Night	4 8% 38% 38% 11% 1	880
April	1st to 10th	Night	2 8% 42% 39% 8% 1	650
	11th to 20th	Night	3 9% 43% 33% 11% 1	593
	21st to end	Night	1 12% 40% 36% 9% 1	600
May	1st to 10th	Night	3 6% 38% 45% 7% 1	600
	11th to 20th	Night	3 5% 33% 43% 14% 1	591
	21st to end	Night	2 5% 34% 47% 12%	650
June	1st to 10th	Night	5% 2 23% 52% 17% 1	600
	11th to 20th	Night	2 3 24% 54% 17% 1	600
	21st to end	Night	6% 2 13% 46% 32% 1	600
July	1st to 10th	Night	6% 2 6% 40% 42% 4	585
	11th to 20th	Night	3 2 3 32% 52% 8%	582
	21st to end	Night	6% 1 2 28% 53% 9%	657
August	1st to 10th	Night	4 2 2 25% 56% 11%	597
	11th to 20th	Night	5% 2 5% 31% 49% 8%	594
	21st to end	Night	5% 1 3 29% 55% 7%	660
September	1st to 10th	Night	6% 2 4 38% 48% 2	600
	11th to 20th	Night	6% 1 6% 47% 36% 4	659
	21st to end	Night	4 1 11% 46% 33% 5%	800
October	1st to 10th	Night	6% 2 16% 49% 24% 3	800
	11th to 20th	Night	5% 2 18% 50% 21% 4	800
	21st to end	Night	2 3 20% 43% 24% 7%	880
November	1st to 10th	Night	3 2 20% 43% 25% 7%	820
	11th to 20th	Night	4 3 21% 44% 24% 3	840
	21st to end	Night	7% 4 26% 41% 18% 5%	840
December	1st to 10th	Night	5% 3 26% 44% 18% 3	840
	11th to 20th	Night	5% 4 27% 46% 15% 3	840
	21st to end	Night	8% 6% 27% 41% 15% 3	924

Page generated at 2025-12-29 23:14:07. Version ac07

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