CAC
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CAC clear sky chart from 2005-11-06 to 2025-07-06.

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 3 1 3 3 3 4 3 5% 7% 65%	124103
	Night	3 3 1 3 3 3 3 3 5% 6% 65%	123655
Transparency	Day	20% 3 5% 12% 27% 32%	123871
	Night	20% 3 5% 13% 28% 31%	123475
Seeing	Day	6% 8% 19% 30% 26% 12%	15587
	Night	5% 3 18% 39% 29% 5%	25665

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 8% 2 4 3 3 3 3 4 6% 62%	4098
	11th to 20th	Day	3 3 1 2 2 2 2 2 3 4 75%	4100
	21st to end	Day	6% 6% 1 4 3 2 3 3 5% 7% 58%	4510
February	1st to 10th	Day	6% 5% 1 3 2 2 2 3 4 6% 64%	1800
	11th to 20th	Day	6% 5% 2 3 4 3 4 3 4 5% 63%	1928
	21st to end	Day	6% 5% 1 5% 3 2 2 2 3 5% 66%	1815
March	1st to 10th	Day	4 4 1 4 2 3 3 2 4 6% 67%	4954
	11th to 20th	Day	2 4 1 3 3 2 3 2 5% 6% 68%	5000
	21st to end	Day	2 6% 2 6% 3 3 4 3 4 7% 61%	5486
April	1st to 10th	Day	2 3 3 2 2 3 3 4 5% 71%	2245
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 4 79%	2371
	21st to end	Day	1 2 1 2 2 1 1 2 3 4 80%	2400
May	1st to 10th	Day	1 1 1 2 1 2 2 2 3 4 81%	5528
	11th to 20th	Day	1 1 1 2 2 2 2 2 4 5% 78%	5841
	21st to end	Day	1 1 1 2 1 2 2 2 3 4 82%	6423
June	1st to 10th	Day	2 2 1 2 2 3 3 3 5% 7% 71%	2600
	11th to 20th	Day	1 2 1 2 2 2 2 3 3 5% 77%	2600
	21st to end	Day	3 3 1 5% 5% 5% 5% 5% 7% 8% 54%	2600
July	1st to 10th	Day	2 2 1 4 5% 5% 5% 5% 7% 9% 55%	4176
	11th to 20th	Day	2 3 1 4 5% 6% 7% 7% 10% 12% 42%	4110
	21st to end	Day	5% 5% 2 7% 6% 7% 8% 7% 10% 13% 30%	4454
August	1st to 10th	Day	4 4 2 6% 5% 7% 8% 7% 10% 11% 37%	4092
	11th to 20th	Day	3 3 2 5% 5% 5% 6% 5% 9% 13% 43%	3846
	21st to end	Day	3 3 2 5% 5% 6% 7% 6% 9% 12% 43%	4180
September	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 4 7% 7% 48%	2214
	11th to 20th	Day	3 2 1 4 4 4 5% 4 6% 7% 60%	2090
	21st to end	Day	3 2 1 3 2 3 4 3 6% 7% 66%	2090
October	1st to 10th	Day	1 1 1 2 2 3 2 3 5% 6% 73%	3211
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 3 80%	3150
	21st to end	Day	1 1 1 1 1 2 1 2 4 84%	3438
November	1st to 10th	Day	2 1 1 3 2 2 2 2 4 7% 73%	1959
	11th to 20th	Day	2 2 1 3 2 2 3 3 4 6% 72%	2000
	21st to end	Day	6% 4 1 4 4 3 3 3 4 6% 62%	1925
December	1st to 10th	Day	4 2 1 2 1 2 3 3 4 5% 74%	2950
	11th to 20th	Day	4 6% 2 4 2 2 2 2 4 7% 65%	3409
	21st to end	Day	4 4 1 3 2 3 3 2 3 5% 69%	4510
January	1st to 10th	Night	4 5% 1 3 2 2 3 3 5% 6% 65%	5867
	11th to 20th	Night	2 2 1 2 2 1 2 2 3 5% 78%	5880
	21st to end	Night	9% 7% 2 4 3 3 4 3 6% 6% 54%	6468
February	1st to 10th	Night	4 4 1 3 3 3 3 3 4 6% 66%	2600
	11th to 20th	Night	5% 5% 1 5% 3 2 3 2 4 6% 63%	2354
	21st to end	Night	4 6% 2 4 3 3 3 3 5% 5% 63%	1815
March	1st to 10th	Night	3 4 1 3 2 2 3 3 5% 5% 68%	4940
	11th to 20th	Night	5% 4 1 2 2 2 3 3 5% 6% 67%	5000
	21st to end	Night	3 5% 2 5% 3 3 3 3 4 6% 64%	5489
April	1st to 10th	Night	2 3 1 2 2 2 2 3 4 5% 72%	2155
	11th to 20th	Night	2 3 1 2 2 1 2 3 4 4 75%	1977
	21st to end	Night	2 2 1 2 1 1 2 1 3 4 81%	2000
May	1st to 10th	Night	2 2 1 2 1 1 1 2 2 4 81%	4435
	11th to 20th	Night	1 2 1 3 2 2 2 2 3 5% 76%	4074
	21st to end	Night	1 1 1 2 1 2 2 1 3 4 83%	4474
June	1st to 10th	Night	2 3 1 2 2 2 3 3 4 7% 70%	1800
	11th to 20th	Night	2 2 1 2 2 2 3 2 3 4 77%	1800
	21st to end	Night	2 4 2 5% 4 5% 5% 4 7% 8% 54%	1800
July	1st to 10th	Night	2 3 1 4 5% 6% 6% 6% 9% 9% 49%	2905
	11th to 20th	Night	2 3 2 6% 6% 7% 10% 7% 10% 14% 33%	2860
	21st to end	Night	4 5% 3 9% 8% 8% 9% 7% 11% 15% 22%	3109
August	1st to 10th	Night	2 4 2 7% 6% 8% 8% 7% 10% 11% 35%	2847
	11th to 20th	Night	3 3 2 6% 5% 7% 8% 6% 10% 14% 36%	2991
	21st to end	Night	4 3 2 6% 6% 6% 8% 6% 9% 11% 39%	3487
September	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 4 8% 8% 45%	1962
	11th to 20th	Night	3 2 1 4 3 3 4 3 6% 6% 64%	2090
	21st to end	Night	3 2 1 4 2 2 3 3 4 5% 71%	2090
October	1st to 10th	Night	1 1 1 2 2 2 2 2 4 4 79%	3759
	11th to 20th	Night	2 1 1 2 2 2 2 2 4 5% 77%	3778
	21st to end	Night	1 1 1 1 1 2 2 3 4 84%	4120
November	1st to 10th	Night	2 1 1 2 2 2 2 2 4 6% 76%	2340
	11th to 20th	Night	2 2 1 3 2 2 3 3 5% 5% 72%	2400
	21st to end	Night	5% 4 1 5% 3 3 3 2 5% 6% 63%	2470
December	1st to 10th	Night	4 2 1 2 1 2 2 2 3 5% 77%	4240
	11th to 20th	Night	3 4 2 4 3 4 3 3 4 5% 66%	4811
	21st to end	Night	5% 4 1 3 3 2 3 3 4 5% 66%	6468

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	25% 2 2 9% 37% 25%	4099
	11th to 20th	Day	15% 1 1 10% 38% 34%	4100
	21st to end	Day	25% 2 3 14% 29% 28%	4510
February	1st to 10th	Day	22% 2 2 13% 26% 35%	1800
	11th to 20th	Day	26% 3 2 14% 29% 27%	1938
	21st to end	Day	24% 1 1 9% 27% 38%	1815
March	1st to 10th	Day	19% 2 1 6% 34% 38%	4954
	11th to 20th	Day	19% 1 2 14% 37% 26%	5000
	21st to end	Day	25% 2 1 10% 36% 27%	5486
April	1st to 10th	Day	16% 2 2 7% 38% 36%	2245
	11th to 20th	Day	13% 1 1 7% 28% 51%	2374
	21st to end	Day	11% 2 1 7% 34% 46%	2400
May	1st to 10th	Day	9% 2 1 5% 28% 55%	5528
	11th to 20th	Day	12% 2 2 7% 27% 51%	5844
	21st to end	Day	9% 1 1 4 15% 70%	6424
June	1st to 10th	Day	15% 2 2 7% 29% 45%	2600
	11th to 20th	Day	12% 2 1 5% 20% 59%	2600
	21st to end	Day	27% 5% 6% 11% 23% 28%	2600
July	1st to 10th	Day	25% 5% 13% 13% 18% 27%	4112
	11th to 20th	Day	29% 7% 18% 24% 18% 5%	4041
	21st to end	Day	39% 8% 23% 22% 6% 1	4314
August	1st to 10th	Day	35% 8% 18% 21% 15% 3	4078
	11th to 20th	Day	28% 6% 16% 27% 19% 4	3885
	21st to end	Day	30% 6% 16% 21% 18% 9%	4179
September	1st to 10th	Day	34% 5% 9% 16% 20% 16%	2214
	11th to 20th	Day	22% 4 6% 18% 28% 21%	2090
	21st to end	Day	18% 3 4 18% 34% 23%	2090
October	1st to 10th	Day	12% 4 4 11% 22% 47%	3211
	11th to 20th	Day	11% 2 2 10% 33% 42%	3150
	21st to end	Day	8% 1 1 8% 31% 51%	3438
November	1st to 10th	Day	13% 2 2 15% 36% 31%	1959
	11th to 20th	Day	15% 2 3 15% 34% 31%	2000
	21st to end	Day	24% 2 3 17% 28% 26%	1925
December	1st to 10th	Day	14% 1 2 13% 43% 27%	2950
	11th to 20th	Day	22% 1 3 13% 27% 34%	3408
	21st to end	Day	19% 1 2 15% 39% 23%	4510
January	1st to 10th	Night	19% 2 2 10% 46% 21%	5867
	11th to 20th	Night	12% 1 1 14% 39% 33%	5880
	21st to end	Night	29% 2 3 12% 28% 27%	6468
February	1st to 10th	Night	21% 2 3 13% 26% 36%	2600
	11th to 20th	Night	23% 2 3 15% 30% 27%	2355
	21st to end	Night	25% 1 2 8% 27% 37%	1815
March	1st to 10th	Night	19% 2 1 9% 33% 36%	4940
	11th to 20th	Night	19% 2 1 16% 36% 26%	5000
	21st to end	Night	23% 3 2 13% 35% 24%	5489
April	1st to 10th	Night	15% 3 2 12% 38% 31%	2155
	11th to 20th	Night	15% 1 1 10% 29% 44%	1977
	21st to end	Night	10% 2 2 9% 34% 44%	2000
May	1st to 10th	Night	9% 2 1 7% 35% 45%	4432
	11th to 20th	Night	13% 2 2 8% 25% 51%	4075
	21st to end	Night	8% 1 5% 18% 67%	4474
June	1st to 10th	Night	15% 2 2 7% 31% 43%	1800
	11th to 20th	Night	13% 2 1 4 22% 58%	1800
	21st to end	Night	26% 4 7% 12% 21% 31%	1800
July	1st to 10th	Night	28% 5% 14% 14% 17% 22%	2860
	11th to 20th	Night	35% 8% 18% 21% 11% 7%	2806
	21st to end	Night	45% 8% 26% 17% 3 1	3011
August	1st to 10th	Night	37% 8% 19% 18% 13% 5%	2838
	11th to 20th	Night	34% 6% 16% 20% 20% 4	3018
	21st to end	Night	36% 6% 21% 17% 13% 7%	3487
September	1st to 10th	Night	34% 7% 11% 15% 18% 15%	1962
	11th to 20th	Night	20% 4 6% 20% 28% 22%	2090
	21st to end	Night	16% 2 6% 17% 36% 22%	2090
October	1st to 10th	Night	10% 2 5% 9% 21% 52%	3759
	11th to 20th	Night	11% 2 3 13% 24% 46%	3778
	21st to end	Night	8% 1 2 9% 28% 52%	4120
November	1st to 10th	Night	12% 2 3 14% 36% 32%	2340
	11th to 20th	Night	15% 2 3 17% 32% 32%	2400
	21st to end	Night	24% 2 4 15% 29% 26%	2470
December	1st to 10th	Night	13% 1 2 13% 39% 31%	4240
	11th to 20th	Night	22% 1 3 12% 25% 36%	4811
	21st to end	Night	21% 1 2 13% 36% 26%	6468

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	11% 9% 23% 34% 20% 3	300
	11th to 20th	Day	9% 4 23% 38% 20% 5%	300
	21st to end	Day	6% 12% 24% 33% 21% 4	330
February	1st to 10th	Day	7% 9% 20% 34% 25% 5%	300
	11th to 20th	Day	10% 18% 29% 32% 11%	424
	21st to end	Day	12% 21% 29% 28% 8% 2	414
March	1st to 10th	Day	9% 18% 30% 31% 10% 2	500
	11th to 20th	Day	9% 18% 27% 24% 18% 4	500
	21st to end	Day	6% 21% 29% 28% 14% 3	550
April	1st to 10th	Day	5% 14% 30% 29% 18% 5%	500
	11th to 20th	Day	5% 17% 33% 28% 13% 4	492
	21st to end	Day	3 17% 29% 28% 16% 6%	500
May	1st to 10th	Day	5% 13% 30% 30% 15% 7%	503
	11th to 20th	Day	3 10% 26% 34% 21% 6%	528
	21st to end	Day	3 7% 29% 37% 20% 5%	573
June	1st to 10th	Day	4 3 22% 35% 27% 10%	530
	11th to 20th	Day	1 5% 18% 35% 31% 10%	530
	21st to end	Day	5% 2 11% 33% 35% 14%	530
July	1st to 10th	Day	5% 6% 27% 43% 19%	507
	11th to 20th	Day	4 4 20% 44% 28%	484
	21st to end	Day	7% 2 4 18% 37% 33%	531
August	1st to 10th	Day	6% 1 4 15% 38% 36%	487
	11th to 20th	Day	5% 1 5% 21% 43% 25%	477
	21st to end	Day	4 1 3 19% 38% 34%	539
September	1st to 10th	Day	8% 1 6% 30% 38% 18%	490
	11th to 20th	Day	4 2 8% 30% 40% 17%	490
	21st to end	Day	4 4 13% 32% 32% 15%	490
October	1st to 10th	Day	2 4 16% 30% 30% 19%	328
	11th to 20th	Day	4 5% 16% 28% 31% 16%	300
	21st to end	Day	2 5% 15% 32% 32% 14%	330
November	1st to 10th	Day	5% 7% 19% 38% 26% 6%	300
	11th to 20th	Day	3 5% 17% 30% 39% 6%	300
	21st to end	Day	11% 9% 19% 35% 23% 2	300
December	1st to 10th	Day	10% 6% 20% 35% 25% 4	300
	11th to 20th	Day	7% 6% 22% 39% 22% 3	300
	21st to end	Day	15% 9% 20% 33% 18% 3	330
January	1st to 10th	Night	7% 3 20% 45% 22% 4	800
	11th to 20th	Night	6% 3 21% 44% 21% 4	800
	21st to end	Night	11% 4 24% 40% 20% 3	880
February	1st to 10th	Night	6% 5% 22% 42% 21% 4	800
	11th to 20th	Night	8% 6% 25% 43% 16% 2	784
	21st to end	Night	7% 6% 33% 40% 12% 1	660
March	1st to 10th	Night	5% 7% 32% 41% 13% 2	800
	11th to 20th	Night	7% 7% 25% 35% 22% 4	800
	21st to end	Night	4 7% 24% 43% 18% 3	880
April	1st to 10th	Night	3 4 26% 47% 19% 2	755
	11th to 20th	Night	2 6% 31% 42% 17% 1	593
	21st to end	Night	2 4 29% 46% 16% 3	600
May	1st to 10th	Night	2 4 27% 47% 19% 1	600
	11th to 20th	Night	2 3 22% 47% 23% 2	591
	21st to end	Night	1 3 21% 50% 25% 1	650
June	1st to 10th	Night	4 2 12% 53% 27% 2	600
	11th to 20th	Night	2 1 13% 52% 30% 2	600
	21st to end	Night	3 2 9% 40% 44% 3	600
July	1st to 10th	Night	4 1 5% 32% 50% 9%	570
	11th to 20th	Night	6% 1 3 24% 58% 9%	552
	21st to end	Night	6% 2 3 20% 57% 12%	624
August	1st to 10th	Night	4 1 3 16% 59% 17%	567
	11th to 20th	Night	4 1 3 20% 57% 14%	564
	21st to end	Night	6% 1 4 19% 58% 13%	627
September	1st to 10th	Night	9% 1 6% 27% 50% 8%	632
	11th to 20th	Night	4 1 6% 34% 46% 9%	760
	21st to end	Night	4 2 11% 38% 39% 7%	760
October	1st to 10th	Night	2 2 14% 42% 35% 6%	760
	11th to 20th	Night	3 2 15% 43% 32% 6%	760
	21st to end	Night	2 2 14% 40% 33% 9%	836
November	1st to 10th	Night	3 2 18% 41% 30% 7%	780
	11th to 20th	Night	3 2 18% 43% 30% 4	800
	21st to end	Night	7% 4 26% 38% 21% 5%	800
December	1st to 10th	Night	9% 2 19% 44% 23% 4	800
	11th to 20th	Night	6% 6% 21% 42% 21% 3	800
	21st to end	Night	10% 4 21% 41% 19% 5%	880

Page generated at 2025-07-02 22:30:14. Version ac07

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