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 2025-02-25.

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	16% 11% 4 9% 6% 5% 5% 4 5% 6% 28%	108237
	Night	16% 15% 5% 12% 8% 6% 5% 3 4 5% 20%	110574
Transparency	Day	32% 46% 8% 10% 3	111714
	Night	40% 44% 6% 8% 3	115263
Seeing	Day	23% 16% 27% 23% 9% 2	16510
	Night	24% 16% 24% 24% 10% 1	24422

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	15% 27% 9% 14% 10% 8% 6% 2 3 3 3	3157
	11th to 20th	Day	8% 17% 11% 24% 14% 5% 4 3 3 4 7%	3156
	21st to end	Day	3 8% 4 12% 13% 17% 14% 8% 5% 5% 12%	3960
February	1st to 10th	Day	8% 15% 7% 12% 10% 8% 7% 5% 6% 5% 18%	1766
	11th to 20th	Day	11% 16% 7% 13% 7% 7% 6% 5% 5% 6% 17%	1753
	21st to end	Day	8% 13% 6% 9% 7% 7% 7% 5% 7% 8% 23%	1514
March	1st to 10th	Day	12% 13% 4 11% 6% 7% 6% 4 6% 6% 25%	3233
	11th to 20th	Day	19% 21% 4 10% 6% 5% 5% 3 4 4 17%	3480
	21st to end	Day	21% 17% 6% 8% 6% 4 3 2 3 4 26%	3816
April	1st to 10th	Day	17% 16% 4 10% 6% 5% 4 3 5% 5% 25%	2307
	11th to 20th	Day	13% 13% 4 8% 5% 4 4 4 5% 7% 33%	2442
	21st to end	Day	21% 11% 3 7% 4 3 4 3 5% 6% 33%	2470
May	1st to 10th	Day	18% 12% 4 8% 5% 4 4 4 5% 7% 30%	4229
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 6% 9% 33%	4546
	21st to end	Day	24% 10% 3 7% 4 4 4 4 5% 7% 29%	5185
June	1st to 10th	Day	21% 9% 3 6% 5% 4 4 4 6% 7% 30%	2850
	11th to 20th	Day	19% 6% 2 7% 4 4 4 4 6% 7% 36%	2850
	21st to end	Day	21% 7% 3 6% 5% 4 5% 4 6% 8% 31%	2850
July	1st to 10th	Day	16% 8% 2 7% 5% 4 4 3 6% 7% 37%	4760
	11th to 20th	Day	13% 7% 2 6% 4 5% 5% 4 7% 9% 39%	4760
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 45%	5086
August	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 6% 9% 40%	4151
	11th to 20th	Day	15% 7% 3 8% 5% 6% 5% 4 7% 8% 32%	4045
	21st to end	Day	15% 5% 2 5% 4 5% 4 4 5% 8% 42%	4532
September	1st to 10th	Day	20% 7% 3 6% 4 3 3 4 5% 7% 38%	2335
	11th to 20th	Day	23% 8% 2 5% 3 2 2 2 5% 5% 42%	2090
	21st to end	Day	19% 8% 2 7% 4 4 4 3 6% 6% 36%	2090
October	1st to 10th	Day	20% 8% 3 6% 5% 4 4 3 5% 7% 34%	1925
	11th to 20th	Day	24% 12% 3 8% 5% 4 4 3 5% 6% 27%	1900
	21st to end	Day	25% 15% 5% 7% 4 3 4 3 5% 6% 22%	1884
November	1st to 10th	Day	21% 16% 6% 8% 5% 5% 4 4 6% 5% 20%	1594
	11th to 20th	Day	22% 17% 7% 13% 5% 5% 3 3 4 5% 16%	1600
	21st to end	Day	21% 20% 7% 16% 6% 4 4 2 5% 4 11%	1547
December	1st to 10th	Day	12% 22% 11% 22% 9% 5% 4 2 3 4 6%	2280
	11th to 20th	Day	16% 15% 7% 22% 11% 7% 5% 2 2 3 10%	2618
	21st to end	Day	10% 16% 6% 20% 13% 7% 4 3 4 5% 11%	3476
January	1st to 10th	Night	15% 31% 8% 17% 12% 6% 3 2 2 2 3	6808
	11th to 20th	Night	9% 19% 8% 25% 12% 6% 4 2 3 3 9%	6747
	21st to end	Night	4 11% 6% 15% 11% 13% 14% 5% 4 5% 12%	6996
February	1st to 10th	Night	9% 14% 6% 12% 12% 9% 6% 4 5% 5% 19%	2632
	11th to 20th	Night	14% 18% 5% 11% 9% 8% 7% 4 5% 5% 15%	2533
	21st to end	Night	9% 16% 5% 11% 9% 6% 6% 4 5% 5% 24%	2034
March	1st to 10th	Night	10% 12% 4 12% 7% 8% 6% 4 5% 5% 27%	3607
	11th to 20th	Night	21% 24% 6% 11% 6% 5% 3 2 3 3 15%	3479
	21st to end	Night	23% 21% 4 7% 7% 4 3 3 3 4 21%	3816
April	1st to 10th	Night	19% 15% 4 11% 7% 4 4 2 5% 5% 22%	1870
	11th to 20th	Night	18% 12% 3 6% 5% 4 5% 4 5% 5% 33%	1688
	21st to end	Night	23% 11% 4 6% 4 4 4 3 4 6% 33%	1710
May	1st to 10th	Night	22% 12% 4 9% 6% 4 5% 4 6% 6% 23%	2715
	11th to 20th	Night	15% 11% 4 8% 6% 6% 5% 4 7% 7% 28%	2387
	21st to end	Night	24% 11% 3 9% 6% 5% 4 3 6% 7% 23%	2401
June	1st to 10th	Night	23% 8% 3 7% 5% 5% 5% 3 6% 8% 27%	1330
	11th to 20th	Night	21% 8% 2 6% 5% 4 4 4 5% 7% 33%	1330
	21st to end	Night	22% 8% 3 9% 5% 5% 4 4 5% 6% 28%	1330
July	1st to 10th	Night	19% 7% 3 8% 5% 5% 4 5% 5% 6% 33%	2210
	11th to 20th	Night	13% 8% 3 7% 6% 5% 6% 4 8% 8% 31%	2210
	21st to end	Night	14% 6% 2 6% 3 5% 4 4 7% 9% 40%	2461
August	1st to 10th	Night	17% 8% 3 8% 6% 5% 6% 5% 6% 9% 27%	2748
	11th to 20th	Night	14% 8% 3 8% 7% 6% 6% 5% 6% 8% 29%	2782
	21st to end	Night	18% 6% 2 4 3 3 4 3 5% 7% 45%	3124
September	1st to 10th	Night	21% 6% 2 6% 4 5% 3 4 5% 6% 36%	1843
	11th to 20th	Night	22% 8% 2 7% 4 3 4 3 4 6% 38%	2090
	21st to end	Night	20% 7% 2 7% 4 4 3 3 4 6% 40%	2090
October	1st to 10th	Night	20% 9% 2 6% 4 4 4 3 6% 6% 34%	2254
	11th to 20th	Night	29% 11% 3 7% 5% 4 3 3 4 5% 27%	2280
	21st to end	Night	24% 15% 3 8% 4 4 4 3 5% 6% 23%	2712
November	1st to 10th	Night	22% 18% 4 9% 6% 4 4 4 5% 6% 18%	2704
	11th to 20th	Night	22% 21% 6% 9% 5% 4 4 3 4 4 18%	2800
	21st to end	Night	20% 21% 8% 15% 6% 4 4 3 4 4 12%	2850
December	1st to 10th	Night	12% 21% 12% 26% 8% 4 3 2 3 3 6%	4910
	11th to 20th	Night	15% 17% 7% 19% 12% 7% 4 2 3 4 10%	5591
	21st to end	Night	13% 19% 6% 16% 13% 6% 4 3 4 5% 12%	7502

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	60% 34% 4 1 1	3158
	11th to 20th	Day	63% 28% 6% 3 1	3182
	21st to end	Day	51% 35% 6% 5% 2	3960
February	1st to 10th	Day	26% 64% 2 6% 2 1	1766
	11th to 20th	Day	21% 65% 6% 7% 2	1761
	21st to end	Day	19% 67% 2 9% 3	1514
March	1st to 10th	Day	32% 46% 7% 12% 2	3237
	11th to 20th	Day	46% 43% 4 4 2	3480
	21st to end	Day	41% 42% 3 10% 3 1	3816
April	1st to 10th	Day	20% 66% 2 6% 5%	2307
	11th to 20th	Day	18% 67% 3 8% 4	2442
	21st to end	Day	21% 66% 4 6% 2	2470
May	1st to 10th	Day	31% 45% 8% 12% 3	4243
	11th to 20th	Day	28% 45% 13% 11% 4	4550
	21st to end	Day	33% 45% 7% 10% 4	5185
June	1st to 10th	Day	19% 66% 4 8% 3	2850
	11th to 20th	Day	18% 66% 5% 7% 4	2850
	21st to end	Day	18% 65% 5% 10% 3	2850
July	1st to 10th	Day	30% 37% 8% 20% 5%	4685
	11th to 20th	Day	27% 38% 16% 15% 4	4679
	21st to end	Day	19% 39% 14% 22% 6%	4924
August	1st to 10th	Day	25% 41% 14% 18% 2	4137
	11th to 20th	Day	33% 42% 15% 7% 3	4084
	21st to end	Day	21% 40% 14% 18% 6%	4532
September	1st to 10th	Day	17% 61% 9% 10% 3	2335
	11th to 20th	Day	22% 60% 6% 10% 1	2090
	21st to end	Day	17% 62% 6% 12% 4	2090
October	1st to 10th	Day	29% 40% 10% 17% 4	3198
	11th to 20th	Day	34% 40% 10% 15% 1	3140
	21st to end	Day	36% 39% 10% 11% 4	3084
November	1st to 10th	Day	32% 59% 6% 3 1	1594
	11th to 20th	Day	34% 58% 4 3 1	1600
	21st to end	Day	35% 59% 3 1 1	1547
December	1st to 10th	Day	57% 37% 4 2	2280
	11th to 20th	Day	53% 35% 6% 6% 1	2618
	21st to end	Day	51% 33% 6% 9% 1	3476
January	1st to 10th	Night	64% 31% 3 1	6808
	11th to 20th	Night	61% 29% 6% 3 1	6796
	21st to end	Night	55% 32% 5% 4 3 1	6996
February	1st to 10th	Night	26% 63% 2 4 3 1	2632
	11th to 20th	Night	26% 63% 3 6% 2	2536
	21st to end	Night	22% 65% 3 7% 3	2034
March	1st to 10th	Night	32% 44% 7% 11% 5%	3616
	11th to 20th	Night	51% 41% 2 3 2	3480
	21st to end	Night	44% 41% 5% 7% 3	3816
April	1st to 10th	Night	22% 66% 2 5% 5% 1	1870
	11th to 20th	Night	20% 67% 2 6% 4	1688
	21st to end	Night	22% 65% 2 6% 4	1710
May	1st to 10th	Night	33% 47% 9% 7% 3	2721
	11th to 20th	Night	27% 46% 11% 12% 3	2388
	21st to end	Night	36% 45% 7% 8% 3	2402
June	1st to 10th	Night	19% 65% 3 8% 5%	1330
	11th to 20th	Night	19% 65% 4 6% 5%	1330
	21st to end	Night	19% 64% 4 9% 4	1330
July	1st to 10th	Night	32% 39% 7% 18% 4	2175
	11th to 20th	Night	31% 39% 14% 14% 2	2168
	21st to end	Night	24% 38% 15% 15% 9%	2383
August	1st to 10th	Night	36% 40% 12% 10% 2	2738
	11th to 20th	Night	34% 40% 15% 8% 2	2809
	21st to end	Night	23% 39% 11% 18% 9%	3124
September	1st to 10th	Night	19% 61% 5% 10% 5%	1843
	11th to 20th	Night	23% 60% 6% 7% 3	2090
	21st to end	Night	17% 61% 5% 12% 5%	2090
October	1st to 10th	Night	32% 39% 6% 18% 4	3761
	11th to 20th	Night	36% 38% 9% 15% 3	3779
	21st to end	Night	39% 39% 7% 12% 3	4462
November	1st to 10th	Night	32% 58% 5% 4 1	2704
	11th to 20th	Night	33% 59% 4 3 2	2800
	21st to end	Night	35% 59% 3 2 1	2850
December	1st to 10th	Night	56% 37% 4 2	4910
	11th to 20th	Night	51% 38% 5% 5%	5592
	21st to end	Night	51% 33% 6% 8% 1	7502

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% 23% 30% 20% 4 1	200
	11th to 20th	Day	27% 25% 26% 13% 9% 2	200
	21st to end	Day	17% 30% 26% 21% 4 2	220
February	1st to 10th	Day	18% 23% 34% 19% 5% 1	366
	11th to 20th	Day	18% 28% 31% 17% 5%	396
	21st to end	Day	16% 25% 32% 17% 8% 1	351
March	1st to 10th	Day	19% 19% 35% 20% 5% 1	460
	11th to 20th	Day	25% 20% 23% 22% 10% 1	460
	21st to end	Day	25% 26% 27% 16% 5% 2	506
April	1st to 10th	Day	24% 24% 28% 17% 6% 1	460
	11th to 20th	Day	20% 19% 32% 21% 7% 2	452
	21st to end	Day	27% 15% 31% 18% 7% 2	460
May	1st to 10th	Day	27% 13% 27% 23% 9% 1	534
	11th to 20th	Day	24% 12% 27% 25% 10% 2	674
	21st to end	Day	27% 17% 28% 21% 7% 1	735
June	1st to 10th	Day	24% 10% 30% 23% 12% 2	680
	11th to 20th	Day	24% 12% 25% 24% 14% 1	680
	21st to end	Day	23% 9% 25% 32% 9% 2	680
July	1st to 10th	Day	20% 11% 27% 28% 11% 2	670
	11th to 20th	Day	19% 10% 26% 32% 12% 1	668
	21st to end	Day	16% 8% 25% 34% 14% 2	700
August	1st to 10th	Day	19% 7% 29% 29% 13% 3	486
	11th to 20th	Day	15% 9% 24% 34% 14% 3	479
	21st to end	Day	18% 10% 25% 29% 14% 4	539
September	1st to 10th	Day	23% 13% 25% 27% 9% 2	490
	11th to 20th	Day	27% 13% 25% 23% 10% 2	490
	21st to end	Day	23% 18% 29% 19% 9% 2	490
October	1st to 10th	Day	23% 19% 29% 20% 7% 2	403
	11th to 20th	Day	33% 18% 22% 19% 7% 2	390
	21st to end	Day	33% 23% 22% 14% 8% 1	429
November	1st to 10th	Day	32% 25% 24% 17% 3 1	395
	11th to 20th	Day	33% 22% 24% 14% 6% 1	400
	21st to end	Day	33% 23% 27% 13% 3 1	347
December	1st to 10th	Day	24% 20% 32% 17% 7% 2	200
	11th to 20th	Day	27% 30% 27% 14% 3 1	200
	21st to end	Day	24% 25% 27% 20% 3 1	220
January	1st to 10th	Night	27% 21% 28% 18% 5% 1	900
	11th to 20th	Night	21% 24% 30% 18% 7% 1	894
	21st to end	Night	16% 24% 28% 23% 8% 1	880
February	1st to 10th	Night	14% 24% 33% 21% 7% 1	800
	11th to 20th	Night	24% 18% 27% 23% 7% 1	783
	21st to end	Night	16% 17% 29% 26% 10% 2	647
March	1st to 10th	Night	15% 20% 30% 24% 10% 1	760
	11th to 20th	Night	25% 18% 24% 23% 8% 1	760
	21st to end	Night	25% 16% 25% 24% 9% 1	836
April	1st to 10th	Night	27% 15% 26% 22% 9% 1	649
	11th to 20th	Night	24% 14% 22% 26% 12% 2	564
	21st to end	Night	29% 13% 20% 25% 10% 3	570
May	1st to 10th	Night	29% 10% 20% 28% 11% 2	570
	11th to 20th	Night	22% 8% 21% 31% 15% 2	495
	21st to end	Night	25% 9% 25% 31% 9% 1	411
June	1st to 10th	Night	24% 6% 19% 36% 14% 1	380
	11th to 20th	Night	25% 5% 17% 33% 18% 2	380
	21st to end	Night	23% 7% 20% 34% 14% 1	380
July	1st to 10th	Night	19% 6% 22% 39% 11% 3	370
	11th to 20th	Night	13% 6% 25% 38% 18% 1	368
	21st to end	Night	18% 7% 19% 32% 22% 2	416
August	1st to 10th	Night	19% 6% 19% 34% 18% 3	516
	11th to 20th	Night	15% 7% 16% 38% 22% 2	562
	21st to end	Night	19% 7% 21% 32% 19% 3	627
September	1st to 10th	Night	22% 10% 23% 28% 14% 1	573
	11th to 20th	Night	25% 10% 20% 28% 15% 1	760
	21st to end	Night	23% 12% 26% 28% 11% 1	760
October	1st to 10th	Night	26% 16% 22% 26% 9% 1	760
	11th to 20th	Night	35% 15% 18% 22% 9% 1	760
	21st to end	Night	34% 17% 21% 19% 8% 1	836
November	1st to 10th	Night	33% 23% 24% 16% 3 1	865
	11th to 20th	Night	33% 21% 22% 17% 6% 1	900
	21st to end	Night	31% 23% 24% 17% 5%	900
December	1st to 10th	Night	26% 22% 29% 17% 6% 1	900
	11th to 20th	Night	27% 27% 25% 15% 5% 1	900
	21st to end	Night	27% 22% 23% 21% 5% 2	990

Page generated at 2025-02-22 06:51:35. Version ac07

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