Cap-Chat
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cap-Chat clear sky chart from 2005-11-06 to 2026-07-01.

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% 12% 4 9% 6% 5% 5% 4 5% 6% 27%	138140
	Night	16% 15% 5% 12% 8% 6% 5% 3 4 5% 20%	134462
Transparency	Day	38% 38% 9% 12% 4	141568
	Night	44% 37% 7% 9% 3	139125
Seeing	Day	23% 16% 27% 23% 9% 2	18400
	Night	24% 16% 24% 24% 10% 1	26092

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	13% 24% 9% 14% 10% 11% 6% 2 3 3 4	4093
	11th to 20th	Day	8% 22% 10% 23% 12% 5% 4 3 3 4 6%	4119
	21st to end	Day	3 8% 6% 14% 12% 16% 14% 9% 5% 5% 10%	5124
February	1st to 10th	Day	7% 14% 7% 12% 11% 8% 7% 5% 6% 5% 18%	1855
	11th to 20th	Day	11% 16% 7% 13% 7% 7% 6% 5% 5% 5% 17%	1843
	21st to end	Day	8% 14% 5% 9% 7% 7% 7% 5% 7% 8% 23%	1629
March	1st to 10th	Day	14% 16% 5% 11% 7% 8% 7% 4 5% 5% 19%	4752
	11th to 20th	Day	17% 18% 4 9% 6% 6% 7% 4 6% 6% 17%	5090
	21st to end	Day	19% 17% 6% 10% 8% 5% 3 3 4 5% 22%	5587
April	1st to 10th	Day	17% 16% 4 10% 6% 5% 4 3 5% 5% 25%	2551
	11th to 20th	Day	15% 13% 3 8% 5% 4 3 4 5% 7% 32%	2702
	21st to end	Day	21% 11% 3 7% 4 4 4 3 5% 6% 33%	2730
May	1st to 10th	Day	23% 11% 3 7% 4 4 4 3 5% 6% 29%	7872
	11th to 20th	Day	21% 9% 3 7% 4 5% 4 3 5% 8% 31%	8494
	21st to end	Day	22% 11% 3 7% 4 4 3 3 4 6% 32%	9716
June	1st to 10th	Day	20% 9% 3 6% 5% 5% 4 5% 6% 7% 31%	3150
	11th to 20th	Day	20% 7% 2 7% 4 4 4 4 6% 7% 35%	3150
	21st to end	Day	21% 7% 3 7% 5% 4 5% 4 6% 8% 30%	3150
July	1st to 10th	Day	17% 8% 2 7% 4 5% 5% 4 6% 8% 35%	6820
	11th to 20th	Day	15% 8% 2 7% 4 5% 5% 4 6% 8% 35%	6820
	21st to end	Day	12% 6% 2 5% 4 4 5% 4 6% 8% 45%	7324
August	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 6% 9% 41%	4282
	11th to 20th	Day	15% 7% 3 8% 5% 6% 5% 4 7% 8% 33%	4175
	21st to end	Day	15% 5% 2 5% 4 5% 4 4 5% 8% 42%	4675
September	1st to 10th	Day	19% 7% 3 6% 4 3 3 4 5% 7% 39%	2458
	11th to 20th	Day	22% 8% 2 5% 3 3 2 2 5% 6% 42%	2200
	21st to end	Day	18% 9% 2 7% 4 4 5% 3 6% 7% 36%	2200
October	1st to 10th	Day	20% 8% 3 6% 5% 4 4 3 5% 7% 34%	2026
	11th to 20th	Day	24% 12% 3 8% 5% 4 4 3 5% 6% 27%	2000
	21st to end	Day	24% 15% 5% 7% 4 3 4 3 6% 6% 22%	1983
November	1st to 10th	Day	22% 16% 6% 8% 5% 4 4 4 6% 5% 20%	1675
	11th to 20th	Day	22% 18% 8% 13% 5% 5% 3 2 4 5% 16%	1680
	21st to end	Day	21% 21% 8% 16% 6% 4 4 2 5% 4 10%	1624
December	1st to 10th	Day	12% 22% 11% 22% 9% 5% 3 2 3 4 6%	2350
	11th to 20th	Day	16% 15% 8% 22% 11% 7% 5% 2 2 3 9%	2688
	21st to end	Day	10% 16% 6% 20% 13% 8% 4 3 4 5% 11%	3553
January	1st to 10th	Night	14% 27% 7% 15% 12% 10% 3 2 3 3 4	8831
	11th to 20th	Night	9% 19% 7% 24% 11% 7% 5% 2 3 3 9%	8793
	21st to end	Night	3 11% 7% 17% 10% 12% 15% 6% 4 4 10%	9057
February	1st to 10th	Night	8% 14% 6% 12% 12% 10% 6% 4 5% 5% 19%	2763
	11th to 20th	Night	13% 18% 6% 11% 9% 8% 6% 4 5% 5% 15%	2663
	21st to end	Night	9% 16% 5% 11% 9% 6% 6% 4 5% 5% 24%	2176
March	1st to 10th	Night	14% 16% 4 12% 7% 7% 6% 3 4 4 21%	5308
	11th to 20th	Night	19% 21% 6% 11% 6% 6% 5% 4 4 4 15%	5089
	21st to end	Night	22% 20% 4 10% 7% 5% 3 2 4 5% 19%	5587
April	1st to 10th	Night	20% 15% 4 12% 7% 4 4 2 5% 5% 21%	2066
	11th to 20th	Night	19% 13% 3 6% 5% 4 4 4 5% 5% 31%	1868
	21st to end	Night	22% 12% 3 6% 4 4 4 2 4 6% 32%	1890
May	1st to 10th	Night	25% 10% 3 8% 5% 4 4 3 5% 5% 28%	5038
	11th to 20th	Night	23% 11% 4 8% 5% 5% 4 3 5% 6% 28%	4439
	21st to end	Night	23% 10% 3 8% 5% 5% 4 3 5% 6% 27%	4491
June	1st to 10th	Night	22% 8% 3 7% 5% 5% 5% 3 6% 8% 27%	1470
	11th to 20th	Night	23% 8% 3 6% 5% 4 4 4 5% 7% 32%	1470
	21st to end	Night	23% 8% 3 9% 6% 5% 4 4 5% 6% 28%	1470
July	1st to 10th	Night	18% 7% 3 9% 6% 6% 5% 5% 5% 6% 30%	3160
	11th to 20th	Night	13% 7% 3 8% 6% 5% 6% 5% 8% 9% 30%	3160
	21st to end	Night	14% 7% 3 7% 4 5% 4 4 7% 8% 36%	3531
August	1st to 10th	Night	16% 8% 3 8% 6% 5% 6% 5% 6% 9% 28%	2835
	11th to 20th	Night	14% 8% 3 8% 6% 6% 6% 5% 6% 8% 30%	2872
	21st to end	Night	18% 6% 2 5% 3 3 4 3 5% 6% 44%	3223
September	1st to 10th	Night	21% 6% 2 6% 4 5% 4 4 6% 6% 36%	1940
	11th to 20th	Night	22% 8% 2 7% 4 3 4 3 4 6% 38%	2200
	21st to end	Night	20% 7% 2 6% 4 4 3 3 4 6% 41%	2200
October	1st to 10th	Night	20% 9% 2 6% 4 4 4 3 6% 6% 34%	2373
	11th to 20th	Night	28% 11% 3 7% 5% 4 3 3 4 5% 28%	2400
	21st to end	Night	23% 16% 3 8% 5% 4 4 3 5% 6% 23%	2855
November	1st to 10th	Night	22% 18% 5% 9% 6% 4 4 3 5% 6% 18%	2843
	11th to 20th	Night	22% 22% 7% 9% 5% 4 4 3 4 4 17%	2940
	21st to end	Night	20% 22% 8% 14% 6% 4 4 3 4 4 12%	2993
December	1st to 10th	Night	11% 21% 12% 26% 8% 5% 3 2 3 3 6%	5060
	11th to 20th	Night	15% 17% 7% 19% 12% 7% 4 2 3 4 9%	5741
	21st to end	Night	13% 19% 6% 16% 13% 6% 4 3 4 5% 12%	7667

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	65% 27% 5% 2 1	4094
	11th to 20th	Day	69% 22% 5% 3 1	4145
	21st to end	Day	57% 28% 6% 6% 2	5124
February	1st to 10th	Day	28% 61% 3 6% 2 1	1855
	11th to 20th	Day	22% 62% 6% 7% 2	1851
	21st to end	Day	22% 63% 3 10% 2	1629
March	1st to 10th	Day	49% 32% 6% 9% 3	4756
	11th to 20th	Day	49% 32% 7% 9% 2	5090
	21st to end	Day	50% 29% 4 11% 5% 1	5587
April	1st to 10th	Day	24% 60% 3 8% 6%	2551
	11th to 20th	Day	23% 61% 3 9% 4	2702
	21st to end	Day	24% 60% 6% 7% 3	2730
May	1st to 10th	Day	44% 26% 10% 15% 5%	7884
	11th to 20th	Day	39% 26% 12% 16% 6%	8498
	21st to end	Day	43% 25% 10% 14% 8%	9717
June	1st to 10th	Day	21% 60% 5% 10% 3	3150
	11th to 20th	Day	22% 60% 5% 8% 4	3150
	21st to end	Day	21% 59% 5% 11% 3	3150
July	1st to 10th	Day	36% 27% 9% 22% 6%	6745
	11th to 20th	Day	36% 28% 18% 15% 3	6739
	21st to end	Day	25% 28% 16% 24% 7%	7114
August	1st to 10th	Day	24% 40% 14% 19% 2	4268
	11th to 20th	Day	32% 40% 15% 9% 4	4214
	21st to end	Day	22% 39% 15% 18% 6%	4675
September	1st to 10th	Day	17% 58% 10% 11% 3	2458
	11th to 20th	Day	23% 58% 7% 11% 2	2200
	21st to end	Day	18% 59% 6% 13% 4	2200
October	1st to 10th	Day	30% 39% 11% 17% 4	3299
	11th to 20th	Day	35% 39% 10% 15% 1	3240
	21st to end	Day	36% 38% 11% 11% 4	3183
November	1st to 10th	Day	33% 56% 6% 3 1	1675
	11th to 20th	Day	36% 56% 4 3 1	1680
	21st to end	Day	38% 57% 3 1 1	1624
December	1st to 10th	Day	58% 36% 4 2	2350
	11th to 20th	Day	54% 34% 5% 6% 1	2688
	21st to end	Day	52% 32% 6% 9% 2	3553
January	1st to 10th	Night	68% 25% 5% 2	8831
	11th to 20th	Night	65% 23% 6% 5% 1	8842
	21st to end	Night	61% 26% 5% 5% 2 1	9057
February	1st to 10th	Night	28% 61% 3 5% 3 1	2763
	11th to 20th	Night	28% 60% 4 7% 2	2666
	21st to end	Night	24% 61% 3 8% 4	2176
March	1st to 10th	Night	48% 30% 8% 9% 5%	5317
	11th to 20th	Night	55% 31% 5% 6% 4	5090
	21st to end	Night	52% 29% 5% 9% 4	5587
April	1st to 10th	Night	27% 60% 3 5% 4 1	2066
	11th to 20th	Night	25% 60% 3 7% 5%	1868
	21st to end	Night	26% 59% 3 7% 4	1890
May	1st to 10th	Night	43% 28% 8% 12% 8% 2	5044
	11th to 20th	Night	43% 27% 9% 13% 8% 1	4440
	21st to end	Night	45% 26% 8% 13% 8%	4492
June	1st to 10th	Night	21% 60% 4 9% 6%	1470
	11th to 20th	Night	24% 59% 4 8% 5%	1470
	21st to end	Night	23% 58% 4 9% 5% 1	1470
July	1st to 10th	Night	39% 28% 8% 19% 4	3125
	11th to 20th	Night	36% 30% 16% 17% 2	3118
	21st to end	Night	34% 28% 16% 15% 8%	3427
August	1st to 10th	Night	36% 39% 12% 11% 3	2825
	11th to 20th	Night	34% 39% 15% 9% 2	2899
	21st to end	Night	24% 38% 11% 17% 9%	3223
September	1st to 10th	Night	19% 58% 6% 11% 5%	1940
	11th to 20th	Night	24% 58% 7% 8% 3	2200
	21st to end	Night	17% 58% 5% 14% 6%	2200
October	1st to 10th	Night	33% 38% 7% 18% 5%	3880
	11th to 20th	Night	36% 37% 9% 15% 3	3899
	21st to end	Night	40% 38% 7% 12% 3	4605
November	1st to 10th	Night	34% 56% 5% 4 2	2843
	11th to 20th	Night	36% 56% 4 3 1	2940
	21st to end	Night	37% 56% 4 2 1	2993
December	1st to 10th	Night	57% 36% 4 2	5060
	11th to 20th	Night	52% 37% 5% 5%	5742
	21st to end	Night	52% 33% 6% 8% 1	7667

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% 22% 30% 20% 4 1	220
	11th to 20th	Day	28% 25% 25% 12% 8% 1	220
	21st to end	Day	16% 30% 28% 21% 4 2	242
February	1st to 10th	Day	17% 22% 32% 19% 7% 3	395
	11th to 20th	Day	17% 28% 31% 17% 7% 1	426
	21st to end	Day	17% 25% 32% 18% 7% 1	394
March	1st to 10th	Day	21% 21% 34% 19% 5% 1	540
	11th to 20th	Day	26% 20% 25% 20% 9% 1	540
	21st to end	Day	24% 24% 30% 16% 5% 2	594
April	1st to 10th	Day	25% 25% 28% 15% 5% 1	540
	11th to 20th	Day	23% 18% 31% 20% 6% 2	532
	21st to end	Day	26% 14% 28% 20% 9% 2	540
May	1st to 10th	Day	28% 12% 28% 22% 9% 1	622
	11th to 20th	Day	25% 12% 27% 25% 10% 2	774
	21st to end	Day	26% 16% 28% 21% 8% 1	845
June	1st to 10th	Day	23% 9% 29% 25% 12% 2	780
	11th to 20th	Day	25% 11% 25% 24% 14% 1	780
	21st to end	Day	23% 10% 25% 30% 10% 3	780
July	1st to 10th	Day	20% 11% 26% 29% 11% 2	720
	11th to 20th	Day	19% 9% 27% 31% 12% 1	718
	21st to end	Day	17% 8% 26% 33% 13% 2	753
August	1st to 10th	Day	17% 7% 28% 29% 15% 3	526
	11th to 20th	Day	15% 9% 24% 34% 16% 3	519
	21st to end	Day	18% 10% 25% 29% 14% 4	583
September	1st to 10th	Day	22% 13% 25% 27% 10% 2	530
	11th to 20th	Day	26% 14% 24% 23% 10% 3	530
	21st to end	Day	22% 19% 29% 18% 10% 2	530
October	1st to 10th	Day	22% 18% 29% 22% 7% 2	434
	11th to 20th	Day	34% 16% 20% 20% 7% 3	420
	21st to end	Day	32% 22% 23% 14% 9% 1	462
November	1st to 10th	Day	34% 24% 24% 16% 2	425
	11th to 20th	Day	34% 22% 23% 14% 6% 1	430
	21st to end	Day	34% 22% 27% 13% 4 1	374
December	1st to 10th	Day	24% 20% 30% 17% 8% 1	220
	11th to 20th	Day	28% 29% 26% 15% 3	220
	21st to end	Day	23% 25% 27% 20% 3 2	242
January	1st to 10th	Night	27% 20% 28% 18% 6% 1	950
	11th to 20th	Night	21% 24% 30% 18% 6% 1	943
	21st to end	Night	16% 24% 28% 23% 8% 1	924
February	1st to 10th	Night	14% 23% 32% 21% 9% 1	840
	11th to 20th	Night	23% 17% 27% 23% 8% 1	823
	21st to end	Night	16% 16% 30% 26% 10% 2	692
March	1st to 10th	Night	17% 20% 29% 24% 9% 1	840
	11th to 20th	Night	25% 18% 25% 23% 8% 1	840
	21st to end	Night	24% 17% 25% 24% 9% 1	924
April	1st to 10th	Night	28% 16% 26% 22% 8% 1	717
	11th to 20th	Night	26% 14% 22% 26% 10% 2	624
	21st to end	Night	29% 13% 20% 26% 10% 2	630
May	1st to 10th	Night	29% 11% 20% 26% 12% 2	630
	11th to 20th	Night	23% 9% 21% 30% 15% 2	547
	21st to end	Night	25% 9% 24% 31% 10% 1	455
June	1st to 10th	Night	23% 6% 20% 37% 14% 1	420
	11th to 20th	Night	27% 6% 17% 31% 18% 2	420
	21st to end	Night	24% 7% 20% 33% 15% 1	420
July	1st to 10th	Night	19% 6% 22% 39% 11% 3	390
	11th to 20th	Night	13% 5% 26% 37% 18% 1	388
	21st to end	Night	17% 6% 21% 32% 21% 2	438
August	1st to 10th	Night	18% 6% 19% 34% 19% 3	543
	11th to 20th	Night	14% 7% 16% 38% 23% 2	592
	21st to end	Night	19% 7% 21% 32% 19% 3	660
September	1st to 10th	Night	22% 11% 23% 28% 15% 1	603
	11th to 20th	Night	25% 10% 20% 29% 15% 2	800
	21st to end	Night	23% 12% 26% 28% 11% 1	800
October	1st to 10th	Night	25% 16% 22% 26% 9% 1	800
	11th to 20th	Night	34% 14% 18% 22% 10% 2	800
	21st to end	Night	33% 17% 21% 20% 9% 1	880
November	1st to 10th	Night	34% 23% 24% 15% 3 1	914
	11th to 20th	Night	34% 21% 21% 16% 6% 2	950
	21st to end	Night	31% 23% 25% 16% 5%	950
December	1st to 10th	Night	25% 22% 29% 17% 6% 1	950
	11th to 20th	Night	27% 27% 26% 14% 5% 1	950
	21st to end	Night	27% 22% 24% 20% 5% 2	1045

Page generated at 2026-06-28 02:10:48. Version ac07

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