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 2024-11-12.

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%	104954
	Night	3 3 1 4 3 3 4 3 5% 7% 63%	102983
Transparency	Day	20% 2 5% 15% 28% 30%	104718
	Night	21% 2 5% 15% 29% 27%	102808
Seeing	Day	6% 8% 21% 31% 24% 10%	14493
	Night	5% 4 24% 40% 23% 4	24514

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 3 2 4 3 3 3 3 5% 5% 65%	2858
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 6% 71%	2860
	21st to end	Day	7% 5% 2 4 3 3 3 3 6% 7% 56%	3146
February	1st to 10th	Day	3 4 1 4 3 3 3 2 4 6% 69%	1710
	11th to 20th	Day	5% 2 1 3 3 3 3 3 4 6% 66%	1811
	21st to end	Day	3 4 1 3 3 2 3 3 4 4 69%	1693
March	1st to 10th	Day	3 4 1 4 3 3 3 2 4 6% 67%	3444
	11th to 20th	Day	6% 5% 1 4 3 3 4 3 4 6% 61%	3490
	21st to end	Day	1 3 1 4 4 4 5% 5% 7% 9% 57%	3825
April	1st to 10th	Day	1 2 1 3 3 3 2 2 5% 6% 73%	2231
	11th to 20th	Day	2 2 1 2 1 2 2 1 3 5% 79%	2251
	21st to end	Day	1 2 1 2 1 2 1 2 3 6% 79%	2280
May	1st to 10th	Day	1 1 1 1 1 2 2 3 3 85%	3800
	11th to 20th	Day	1 1 1 2 2 2 2 2 4 5% 76%	3915
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	4477
June	1st to 10th	Day	3 2 1 3 3 3 3 3 4 7% 67%	2470
	11th to 20th	Day	2 2 1 2 2 2 3 3 4 5% 74%	2470
	21st to end	Day	3 4 1 4 4 4 5% 4 7% 8% 56%	2470
July	1st to 10th	Day	3 2 1 4 4 4 4 4 7% 9% 57%	4110
	11th to 20th	Day	3 2 1 4 5% 5% 6% 5% 8% 10% 52%	4110
	21st to end	Day	4 4 2 6% 6% 7% 8% 8% 9% 12% 34%	4454
August	1st to 10th	Day	3 3 2 5% 4 5% 6% 5% 8% 10% 49%	4062
	11th to 20th	Day	3 2 1 4 4 3 5% 4 7% 9% 58%	3726
	21st to end	Day	3 3 1 5% 5% 6% 7% 6% 9% 10% 45%	4180
September	1st to 10th	Day	8% 4 2 5% 4 4 5% 4 7% 9% 49%	2280
	11th to 20th	Day	5% 3 1 3 3 3 4 3 5% 8% 61%	2219
	21st to end	Day	3 2 1 3 2 2 3 2 4 7% 70%	2026
October	1st to 10th	Day	2 1 1 2 2 1 2 2 4 5% 77%	3170
	11th to 20th	Day	3 2 1 3 2 2 2 3 3 5% 74%	3150
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	3438
November	1st to 10th	Day	2 2 1 3 2 3 2 3 3 6% 74%	1959
	11th to 20th	Day	2 3 1 3 2 3 3 2 4 6% 71%	1910
	21st to end	Day	5% 4 1 4 3 3 3 3 4 7% 62%	1900
December	1st to 10th	Day	5% 3 1 3 3 3 3 3 5% 7% 63%	1744
	11th to 20th	Day	5% 7% 2 5% 4 3 4 2 4 6% 58%	2169
	21st to end	Day	2 3 2 3 3 3 3 3 5% 8% 65%	3146
January	1st to 10th	Night	4 5% 2 4 3 2 3 3 5% 6% 64%	4097
	11th to 20th	Night	2 2 1 2 2 3 3 2 4 5% 74%	4110
	21st to end	Night	9% 6% 2 4 3 3 4 4 6% 6% 53%	4521
February	1st to 10th	Night	3 2 1 3 3 2 4 3 4 8% 67%	2470
	11th to 20th	Night	5% 5% 1 3 3 2 3 3 4 5% 67%	2255
	21st to end	Night	3 4 1 2 2 3 4 3 5% 6% 68%	1756
March	1st to 10th	Night	3 2 1 2 2 2 3 3 5% 7% 69%	3430
	11th to 20th	Night	6% 4 1 3 3 3 3 2 4 5% 66%	3490
	21st to end	Night	1 2 1 3 3 3 4 4 6% 8% 63%	3828
April	1st to 10th	Night	1 2 1 2 2 2 2 2 5% 6% 74%	1948
	11th to 20th	Night	2 3 1 3 2 2 2 2 4 5% 74%	1877
	21st to end	Night	1 2 1 2 2 2 2 2 4 5% 77%	1900
May	1st to 10th	Night	1 1 1 1 1 1 1 1 3 4 84%	3170
	11th to 20th	Night	2 2 1 3 3 3 2 3 4 5% 73%	2973
	21st to end	Night	1 1 2 1 1 1 1 2 3 86%	3110
June	1st to 10th	Night	3 2 1 3 3 3 4 4 4 6% 66%	1710
	11th to 20th	Night	2 2 1 3 3 3 3 2 5% 5% 72%	1710
	21st to end	Night	4 4 2 5% 5% 5% 6% 5% 7% 9% 50%	1710
July	1st to 10th	Night	3 4 2 5% 5% 5% 5% 6% 9% 12% 45%	2860
	11th to 20th	Night	2 3 1 5% 5% 6% 7% 7% 9% 13% 41%	2860
	21st to end	Night	4 5% 3 8% 6% 6% 7% 6% 9% 14% 30%	3109
August	1st to 10th	Night	2 4 3 6% 5% 6% 7% 7% 10% 14% 36%	2877
	11th to 20th	Night	3 4 2 5% 5% 6% 7% 6% 9% 13% 41%	3113
	21st to end	Night	4 5% 3 9% 7% 7% 8% 7% 9% 10% 32%	3487
September	1st to 10th	Night	5% 4 2 6% 5% 4 5% 5% 6% 9% 48%	1900
	11th to 20th	Night	4 3 1 4 3 4 4 3 6% 8% 58%	1956
	21st to end	Night	3 2 1 3 2 3 3 2 4 7% 69%	2152
October	1st to 10th	Night	3 2 1 2 2 2 2 2 4 6% 74%	3800
	11th to 20th	Night	3 2 1 3 2 3 2 2 3 4 75%	3778
	21st to end	Night	1 1 2 1 1 2 1 3 5% 82%	4120
November	1st to 10th	Night	2 1 2 2 2 3 3 4 5% 75%	2340
	11th to 20th	Night	2 2 1 3 2 2 3 2 4 6% 73%	2292
	21st to end	Night	6% 4 1 4 3 2 3 3 4 6% 64%	2280
December	1st to 10th	Night	5% 3 2 3 2 3 4 3 6% 7% 63%	2432
	11th to 20th	Night	4 3 2 5% 4 3 4 4 6% 7% 58%	3041
	21st to end	Night	5% 4 2 4 2 3 3 2 4 5% 65%	4521

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	22% 1 3 11% 44% 19%	2859
	11th to 20th	Day	15% 1 4 19% 44% 17%	2860
	21st to end	Day	28% 1 3 25% 28% 14%	3146
February	1st to 10th	Day	19% 1 4 16% 27% 34%	1710
	11th to 20th	Day	22% 1 3 14% 31% 29%	1821
	21st to end	Day	19% 1 2 9% 31% 38%	1693
March	1st to 10th	Day	21% 1 2 7% 38% 31%	3444
	11th to 20th	Day	26% 1 2 17% 30% 24%	3490
	21st to end	Day	24% 1 2 11% 39% 23%	3825
April	1st to 10th	Day	13% 1 2 10% 36% 38%	2231
	11th to 20th	Day	12% 1 1 7% 28% 51%	2254
	21st to end	Day	10% 1 1 10% 32% 46%	2280
May	1st to 10th	Day	8% 1 1 6% 23% 62%	3800
	11th to 20th	Day	12% 1 3 8% 28% 49%	3918
	21st to end	Day	6% 1 1 5% 15% 73%	4477
June	1st to 10th	Day	18% 1 2 9% 28% 42%	2470
	11th to 20th	Day	13% 1 1 6% 20% 59%	2470
	21st to end	Day	25% 3 6% 15% 20% 31%	2470
July	1st to 10th	Day	22% 4 12% 15% 21% 26%	4046
	11th to 20th	Day	24% 5% 11% 24% 29% 7%	4041
	21st to end	Day	34% 8% 18% 27% 11% 2	4314
August	1st to 10th	Day	26% 5% 12% 29% 22% 5%	4048
	11th to 20th	Day	21% 5% 9% 19% 34% 13%	3762
	21st to end	Day	28% 7% 14% 22% 20% 9%	4179
September	1st to 10th	Day	30% 3 8% 20% 23% 17%	2280
	11th to 20th	Day	22% 3 6% 17% 29% 23%	2219
	21st to end	Day	17% 2 3 15% 35% 28%	2026
October	1st to 10th	Day	11% 1 4 11% 25% 48%	3170
	11th to 20th	Day	15% 2 3 12% 32% 37%	3150
	21st to end	Day	7% 1 1 10% 32% 48%	3438
November	1st to 10th	Day	14% 1 4 16% 36% 29%	1959
	11th to 20th	Day	17% 2 3 17% 30% 31%	1910
	21st to end	Day	24% 2 3 17% 32% 22%	1900
December	1st to 10th	Day	22% 2 4 21% 32% 20%	1744
	11th to 20th	Day	30% 2 4 21% 28% 16%	2168
	21st to end	Day	20% 1 5% 16% 38% 21%	3146
January	1st to 10th	Night	23% 1 2 14% 43% 15%	4097
	11th to 20th	Night	15% 1 2 20% 47% 15%	4110
	21st to end	Night	33% 1 4 21% 29% 12%	4521
February	1st to 10th	Night	18% 2 3 17% 28% 32%	2470
	11th to 20th	Night	21% 1 2 16% 32% 29%	2255
	21st to end	Night	19% 1 2 12% 31% 34%	1756
March	1st to 10th	Night	17% 1 2 11% 44% 25%	3430
	11th to 20th	Night	24% 1 1 15% 37% 22%	3490
	21st to end	Night	20% 1 2 22% 36% 20%	3828
April	1st to 10th	Night	14% 1 2 11% 37% 34%	1948
	11th to 20th	Night	16% 1 1 8% 28% 46%	1877
	21st to end	Night	12% 2 1 11% 32% 41%	1900
May	1st to 10th	Night	9% 1 1 5% 24% 61%	3170
	11th to 20th	Night	16% 1 3 8% 23% 50%	2974
	21st to end	Night	7% 1 1 4 16% 70%	3110
June	1st to 10th	Night	20% 2 2 10% 26% 40%	1710
	11th to 20th	Night	15% 1 2 6% 21% 55%	1710
	21st to end	Night	29% 4 7% 14% 15% 31%	1710
July	1st to 10th	Night	27% 6% 14% 18% 17% 18%	2815
	11th to 20th	Night	30% 6% 13% 22% 23% 6%	2806
	21st to end	Night	39% 7% 21% 24% 7% 2	3011
August	1st to 10th	Night	32% 7% 15% 29% 11% 5%	2868
	11th to 20th	Night	30% 7% 12% 13% 31% 8%	3143
	21st to end	Night	41% 7% 15% 17% 15% 4	3487
September	1st to 10th	Night	31% 5% 10% 18% 18% 18%	1900
	11th to 20th	Night	23% 4 8% 18% 26% 21%	1956
	21st to end	Night	17% 2 6% 16% 32% 26%	2152
October	1st to 10th	Night	13% 2 5% 9% 28% 44%	3800
	11th to 20th	Night	13% 3 2 13% 28% 40%	3778
	21st to end	Night	8% 1 1 10% 30% 49%	4120
November	1st to 10th	Night	13% 2 3 15% 36% 31%	2340
	11th to 20th	Night	15% 1 2 17% 33% 31%	2292
	21st to end	Night	23% 1 3 17% 32% 23%	2280
December	1st to 10th	Night	22% 2 4 23% 28% 22%	2432
	11th to 20th	Night	26% 1 7% 20% 30% 15%	3041
	21st to end	Night	23% 1 3 17% 35% 21%	4521

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% 7% 27% 37% 13% 3	270
	11th to 20th	Day	6% 8% 26% 37% 19% 4	270
	21st to end	Day	5% 7% 28% 36% 16% 8%	297
February	1st to 10th	Day	4 11% 27% 33% 20% 4	270
	11th to 20th	Day	12% 8% 31% 36% 11% 2	270
	21st to end	Day	9% 27% 33% 22% 7% 1	348
March	1st to 10th	Day	9% 20% 32% 28% 9% 1	460
	11th to 20th	Day	10% 16% 28% 31% 12% 3	460
	21st to end	Day	8% 23% 34% 25% 8% 1	506
April	1st to 10th	Day	5% 16% 35% 28% 13% 4	460
	11th to 20th	Day	7% 18% 33% 28% 12% 2	452
	21st to end	Day	5% 15% 35% 27% 16% 3	460
May	1st to 10th	Day	3 13% 35% 30% 15% 3	463
	11th to 20th	Day	3 9% 28% 34% 21% 4	488
	21st to end	Day	2 8% 33% 36% 18% 3	529
June	1st to 10th	Day	5% 5% 21% 38% 24% 6%	490
	11th to 20th	Day	3 4 23% 39% 25% 7%	490
	21st to end	Day	4 3 11% 40% 31% 12%	490
July	1st to 10th	Day	6% 1 5% 30% 34% 24%	486
	11th to 20th	Day	6% 1 3 22% 39% 29%	484
	21st to end	Day	5% 1 3 17% 45% 29%	531
August	1st to 10th	Day	5% 2 2 20% 39% 32%	487
	11th to 20th	Day	4 3 5% 23% 41% 23%	477
	21st to end	Day	4 2 6% 22% 38% 28%	539
September	1st to 10th	Day	10% 2 6% 32% 36% 16%	490
	11th to 20th	Day	5% 1 11% 32% 36% 14%	490
	21st to end	Day	4 4 15% 37% 31% 10%	426
October	1st to 10th	Day	6% 3 15% 32% 30% 13%	300
	11th to 20th	Day	7% 6% 17% 32% 30% 8%	300
	21st to end	Day	4 5% 17% 32% 28% 14%	330
November	1st to 10th	Day	6% 5% 27% 36% 21% 5%	300
	11th to 20th	Day	3 5% 18% 39% 28% 7%	273
	21st to end	Day	9% 11% 23% 35% 18% 4	270
December	1st to 10th	Day	6% 6% 23% 36% 24% 5%	270
	11th to 20th	Day	9% 7% 30% 33% 17% 3	270
	21st to end	Day	8% 10% 30% 35% 14% 3	297
January	1st to 10th	Night	6% 5% 27% 41% 17% 4	760
	11th to 20th	Night	6% 4 29% 39% 18% 4	760
	21st to end	Night	10% 5% 26% 41% 15% 3	836
February	1st to 10th	Night	4 6% 32% 38% 16% 4	760
	11th to 20th	Night	7% 6% 34% 39% 11% 2	744
	21st to end	Night	6% 9% 45% 32% 8% 1	628
March	1st to 10th	Night	6% 8% 41% 34% 10% 1	760
	11th to 20th	Night	7% 7% 31% 39% 13% 2	760
	21st to end	Night	4 8% 38% 37% 11% 1	836
April	1st to 10th	Night	2 8% 41% 39% 8% 1	618
	11th to 20th	Night	3 9% 42% 34% 11% 1	563
	21st to end	Night	1 13% 39% 36% 10% 1	570
May	1st to 10th	Night	3 6% 38% 45% 7% 1	570
	11th to 20th	Night	4 4 32% 45% 14% 1	561
	21st to end	Night	3 5% 35% 46% 11%	617
June	1st to 10th	Night	5% 2 22% 52% 18% 1	570
	11th to 20th	Night	2 3 24% 53% 17% 1	570
	21st to end	Night	6% 2 13% 46% 32% 1	570
July	1st to 10th	Night	6% 2 6% 39% 42% 5%	555
	11th to 20th	Night	3 2 3 31% 53% 8%	552
	21st to end	Night	6% 1 2 27% 54% 10%	624
August	1st to 10th	Night	4 2 2 24% 56% 12%	567
	11th to 20th	Night	5% 2 4 31% 49% 9%	564
	21st to end	Night	5% 1 3 29% 54% 8%	627
September	1st to 10th	Night	6% 2 4 37% 49% 2	570
	11th to 20th	Night	6% 1 6% 46% 37% 4	626
	21st to end	Night	4 1 12% 45% 33% 5%	760
October	1st to 10th	Night	6% 2 16% 48% 25% 3	760
	11th to 20th	Night	4 2 18% 50% 22% 4	760
	21st to end	Night	3 3 20% 43% 25% 7%	836
November	1st to 10th	Night	3 2 20% 43% 24% 7%	780
	11th to 20th	Night	3 3 21% 44% 25% 3	764
	21st to end	Night	8% 4 24% 39% 19% 5%	760
December	1st to 10th	Night	6% 3 25% 43% 19% 4	760
	11th to 20th	Night	6% 4 28% 45% 14% 3	760
	21st to end	Night	8% 6% 26% 41% 16% 3	836

Page generated at 2024-11-08 20:45:40. Version ac07

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