Colony Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Colony Skies Observatory clear sky chart from 2005-11-06 to 2024-12-02.

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 7% 3 8% 5% 4 5% 4 6% 7% 49%	105169
	Night	3 7% 3 8% 5% 5% 5% 4 6% 7% 49%	103200
Transparency	Day	31% 5% 11% 20% 20% 13%	104934
	Night	31% 5% 13% 21% 19% 10%	103024
Seeing	Day	9% 17% 33% 28% 11% 3	15188
	Night	7% 7% 25% 39% 20% 2	25564

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	2 9% 6% 17% 9% 6% 6% 4 6% 6% 29%	2858
	11th to 20th	Day	1 12% 6% 19% 9% 6% 5% 3 6% 5% 28%	2860
	21st to end	Day	2 13% 8% 14% 5% 4 3 3 3 6% 38%	3146
February	1st to 10th	Day	2 14% 6% 14% 8% 5% 6% 4 4 5% 32%	1710
	11th to 20th	Day	3 14% 6% 12% 7% 5% 5% 4 6% 6% 32%	1715
	21st to end	Day	2 15% 5% 12% 9% 6% 6% 4 6% 7% 28%	1570
March	1st to 10th	Day	1 10% 6% 16% 8% 6% 6% 4 6% 7% 31%	3376
	11th to 20th	Day	2 7% 4 9% 6% 6% 5% 4 7% 9% 41%	3490
	21st to end	Day	3 16% 7% 14% 7% 6% 6% 4 6% 7% 22%	3825
April	1st to 10th	Day	3 12% 4 9% 5% 4 6% 4 7% 7% 36%	2121
	11th to 20th	Day	6% 12% 4 9% 6% 6% 5% 4 6% 6% 37%	2378
	21st to end	Day	4 11% 4 8% 6% 4 5% 5% 6% 6% 42%	2470
May	1st to 10th	Day	6% 11% 5% 12% 6% 6% 6% 4 6% 6% 34%	4110
	11th to 20th	Day	4 5% 2 5% 4 4 4 3 6% 9% 55%	4071
	21st to end	Day	5% 6% 2 6% 5% 5% 5% 5% 7% 8% 47%	4477
June	1st to 10th	Day	3 3 2 6% 5% 4 5% 4 7% 8% 53%	2603
	11th to 20th	Day	4 5% 2 4 3 4 4 3 5% 6% 62%	2660
	21st to end	Day	1 2 1 4 3 2 4 3 6% 6% 68%	2548
July	1st to 10th	Day	1 2 1 2 2 3 3 3 4 6% 74%	4110
	11th to 20th	Day	2 3 1 4 4 4 6% 5% 9% 10% 51%	4110
	21st to end	Day	3 3 1 4 4 4 4 4 6% 8% 60%	4454
August	1st to 10th	Day	3 2 1 4 4 4 4 5% 7% 9% 59%	4092
	11th to 20th	Day	2 2 1 3 3 3 4 4 6% 8% 65%	4028
	21st to end	Day	2 2 1 3 2 3 3 3 5% 7% 68%	4464
September	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 7% 65%	2166
	11th to 20th	Day	4 4 1 3 3 4 4 4 6% 6% 61%	2090
	21st to end	Day	4 4 1 4 3 3 3 2 4 6% 65%	2090
October	1st to 10th	Day	4 5% 1 4 3 3 3 2 4 5% 66%	3308
	11th to 20th	Day	7% 6% 2 5% 3 3 4 4 5% 6% 57%	3150
	21st to end	Day	3 6% 2 7% 4 4 4 4 5% 5% 56%	3438
November	1st to 10th	Day	4 10% 3 8% 4 4 4 4 6% 6% 46%	1774
	11th to 20th	Day	5% 10% 3 7% 5% 4 5% 3 6% 7% 46%	1800
	21st to end	Day	3 9% 4 10% 5% 6% 5% 3 6% 7% 43%	1621
December	1st to 10th	Day	4 9% 3 8% 8% 5% 6% 4 4 8% 39%	1528
	11th to 20th	Day	3 11% 4 11% 7% 6% 7% 5% 7% 8% 30%	1926
	21st to end	Day	2 9% 5% 14% 6% 5% 5% 4 6% 7% 36%	3032
January	1st to 10th	Night	2 8% 4 15% 9% 7% 5% 3 5% 7% 34%	4097
	11th to 20th	Night	2 11% 5% 16% 11% 7% 5% 3 5% 6% 29%	4110
	21st to end	Night	2 11% 6% 13% 7% 4 5% 4 6% 8% 35%	4521
February	1st to 10th	Night	2 10% 5% 12% 7% 5% 6% 5% 6% 8% 35%	2470
	11th to 20th	Night	2 10% 5% 10% 7% 5% 6% 5% 6% 6% 39%	2350
	21st to end	Night	1 11% 3 12% 7% 5% 6% 4 6% 9% 36%	1884
March	1st to 10th	Night	1 9% 3 11% 7% 7% 7% 5% 7% 7% 35%	3496
	11th to 20th	Night	1 7% 2 7% 4 4 6% 5% 7% 10% 47%	3490
	21st to end	Night	2 12% 6% 13% 7% 7% 5% 4 6% 5% 33%	3828
April	1st to 10th	Night	4 13% 3 6% 4 4 4 4 5% 6% 45%	2058
	11th to 20th	Night	3 8% 2 6% 6% 5% 5% 5% 6% 6% 46%	1747
	21st to end	Night	5% 10% 3 6% 4 4 5% 4 6% 7% 47%	1710
May	1st to 10th	Night	3 10% 3 9% 7% 6% 5% 5% 7% 6% 38%	2860
	11th to 20th	Night	3 4 1 5% 4 4 4 4 6% 7% 59%	2834
	21st to end	Night	5% 5% 2 5% 4 4 5% 4 6% 9% 53%	3110
June	1st to 10th	Night	4 5% 1 5% 4 5% 4 3 5% 7% 56%	1568
	11th to 20th	Night	3 2 1 3 3 3 4 4 5% 7% 65%	1520
	21st to end	Night	2 2 1 3 3 3 3 3 5% 7% 68%	1623
July	1st to 10th	Night	1 2 1 3 3 3 3 3 4 5% 72%	2860
	11th to 20th	Night	2 3 1 4 4 4 4 5% 7% 10% 55%	2860
	21st to end	Night	2 3 1 5% 4 4 5% 4 6% 7% 60%	3109
August	1st to 10th	Night	2 2 1 3 3 5% 5% 4 7% 8% 61%	2847
	11th to 20th	Night	2 2 1 4 4 4 4 4 6% 7% 62%	2811
	21st to end	Night	1 2 1 3 3 2 3 3 5% 7% 71%	3201
September	1st to 10th	Night	3 3 1 4 3 3 3 3 5% 6% 66%	2009
	11th to 20th	Night	4 3 1 3 3 3 4 3 5% 7% 65%	2090
	21st to end	Night	5% 3 1 4 2 2 2 2 3 5% 70%	2090
October	1st to 10th	Night	3 3 1 3 2 3 3 3 4 4 72%	3666
	11th to 20th	Night	3 5% 1 5% 3 4 4 4 7% 7% 57%	3778
	21st to end	Night	3 5% 2 5% 2 3 4 4 7% 7% 57%	4120
November	1st to 10th	Night	3 8% 3 6% 4 4 4 4 6% 8% 49%	2521
	11th to 20th	Night	4 8% 3 6% 3 5% 4 4 6% 8% 49%	2600
	21st to end	Night	2 9% 4 9% 6% 5% 5% 3 5% 8% 46%	2755
December	1st to 10th	Night	2 9% 3 8% 6% 5% 5% 5% 6% 8% 43%	2687
	11th to 20th	Night	3 10% 4 9% 5% 5% 6% 5% 7% 10% 35%	3271
	21st to end	Night	2 10% 4 14% 7% 6% 5% 4 6% 8% 32%	4649

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	49% 9% 15% 16% 10% 1	2859
	11th to 20th	Day	56% 6% 16% 14% 7% 2	2860
	21st to end	Day	45% 8% 20% 17% 9% 2	3146
February	1st to 10th	Day	51% 6% 14% 17% 11% 1	1710
	11th to 20th	Day	49% 6% 14% 19% 11% 1	1724
	21st to end	Day	50% 7% 14% 18% 10% 1	1570
March	1st to 10th	Day	44% 7% 15% 21% 11% 2	3376
	11th to 20th	Day	33% 9% 17% 29% 10% 1	3490
	21st to end	Day	55% 5% 19% 15% 5% 1	3825
April	1st to 10th	Day	42% 7% 15% 19% 14% 4	2121
	11th to 20th	Day	44% 6% 13% 20% 16% 1	2381
	21st to end	Day	38% 6% 11% 25% 18% 1	2470
May	1st to 10th	Day	42% 8% 10% 21% 16% 4	4110
	11th to 20th	Day	23% 6% 8% 33% 27% 3	4074
	21st to end	Day	30% 6% 10% 27% 24% 3	4477
June	1st to 10th	Day	24% 6% 10% 24% 27% 9%	2603
	11th to 20th	Day	23% 3 7% 20% 29% 19%	2660
	21st to end	Day	15% 3 5% 12% 30% 35%	2548
July	1st to 10th	Day	11% 2 4 8% 16% 58%	4046
	11th to 20th	Day	19% 6% 8% 14% 23% 31%	4041
	21st to end	Day	17% 6% 7% 12% 24% 34%	4314
August	1st to 10th	Day	18% 4 7% 21% 25% 24%	4078
	11th to 20th	Day	14% 5% 7% 16% 33% 25%	4067
	21st to end	Day	14% 4 8% 15% 29% 30%	4463
September	1st to 10th	Day	18% 3 7% 16% 28% 28%	2166
	11th to 20th	Day	21% 5% 7% 18% 28% 21%	2090
	21st to end	Day	20% 3 6% 20% 36% 15%	2090
October	1st to 10th	Day	22% 3 6% 15% 35% 20%	3308
	11th to 20th	Day	27% 3 8% 30% 26% 7%	3150
	21st to end	Day	27% 3 12% 27% 26% 4	3438
November	1st to 10th	Day	34% 5% 14% 24% 18% 5%	1774
	11th to 20th	Day	35% 5% 15% 27% 18% 1	1800
	21st to end	Day	39% 5% 12% 24% 15% 4	1621
December	1st to 10th	Day	40% 9% 19% 19% 10% 4	1528
	11th to 20th	Day	46% 8% 15% 20% 11% 1	1924
	21st to end	Day	43% 5% 16% 16% 18% 2	3032
January	1st to 10th	Night	47% 6% 15% 21% 9% 1	4097
	11th to 20th	Night	54% 10% 14% 15% 6% 2	4110
	21st to end	Night	44% 10% 19% 17% 8% 2	4521
February	1st to 10th	Night	43% 9% 17% 21% 9% 1	2470
	11th to 20th	Night	41% 5% 15% 21% 16% 1	2351
	21st to end	Night	41% 6% 16% 23% 12% 2	1884
March	1st to 10th	Night	39% 7% 18% 23% 13%	3496
	11th to 20th	Night	27% 8% 22% 30% 13% 1	3490
	21st to end	Night	47% 6% 14% 21% 10% 2	3828
April	1st to 10th	Night	36% 5% 16% 22% 16% 5%	2058
	11th to 20th	Night	32% 6% 16% 24% 21% 2	1747
	21st to end	Night	33% 5% 16% 25% 19% 1	1710
May	1st to 10th	Night	38% 7% 11% 23% 20% 2	2860
	11th to 20th	Night	20% 6% 12% 32% 23% 6%	2835
	21st to end	Night	25% 5% 13% 33% 23% 1	3110
June	1st to 10th	Night	25% 4 10% 25% 28% 7%	1568
	11th to 20th	Night	18% 3 8% 21% 31% 20%	1520
	21st to end	Night	14% 2 6% 12% 31% 35%	1623
July	1st to 10th	Night	14% 2 5% 9% 12% 58%	2815
	11th to 20th	Night	20% 4 7% 12% 27% 30%	2806
	21st to end	Night	22% 3 8% 13% 24% 30%	3011
August	1st to 10th	Night	18% 4 9% 18% 27% 24%	2838
	11th to 20th	Night	20% 6% 8% 15% 28% 24%	2838
	21st to end	Night	13% 2 9% 14% 32% 30%	3201
September	1st to 10th	Night	18% 4 6% 18% 25% 29%	2009
	11th to 20th	Night	18% 4 9% 20% 28% 21%	2090
	21st to end	Night	18% 3 5% 20% 37% 16%	2090
October	1st to 10th	Night	16% 2 6% 20% 33% 22%	3666
	11th to 20th	Night	23% 4 9% 32% 26% 7%	3778
	21st to end	Night	23% 4 10% 31% 28% 4	4120
November	1st to 10th	Night	29% 6% 13% 26% 21% 5%	2521
	11th to 20th	Night	30% 5% 17% 28% 18% 3	2600
	21st to end	Night	35% 5% 12% 27% 17% 4	2755
December	1st to 10th	Night	36% 7% 18% 24% 12% 3	2687
	11th to 20th	Night	40% 8% 19% 21% 12%	3272
	21st to end	Night	45% 7% 18% 16% 12% 1	4649

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	13% 16% 32% 20% 15% 3	270
	11th to 20th	Day	10% 14% 37% 26% 11% 1	270
	21st to end	Day	6% 16% 33% 31% 12% 2	297
February	1st to 10th	Day	14% 23% 35% 22% 6% 1	270
	11th to 20th	Day	12% 21% 37% 23% 6% 2	320
	21st to end	Day	12% 29% 36% 19% 3 1	382
March	1st to 10th	Day	15% 22% 39% 18% 6%	460
	11th to 20th	Day	10% 24% 36% 23% 6% 1	460
	21st to end	Day	17% 29% 32% 19% 3	506
April	1st to 10th	Day	15% 23% 31% 22% 7% 2	460
	11th to 20th	Day	18% 28% 32% 15% 5% 1	452
	21st to end	Day	15% 25% 30% 23% 7% 1	460
May	1st to 10th	Day	14% 23% 37% 18% 7% 1	463
	11th to 20th	Day	11% 22% 31% 24% 10% 3	488
	21st to end	Day	11% 18% 31% 23% 12% 4	529
June	1st to 10th	Day	6% 14% 31% 30% 16% 3	623
	11th to 20th	Day	7% 15% 33% 28% 15% 1	680
	21st to end	Day	4 14% 35% 31% 14% 2	568
July	1st to 10th	Day	3 10% 37% 36% 12% 2	486
	11th to 20th	Day	3 8% 38% 34% 13% 5%	484
	21st to end	Day	4 4 33% 40% 15% 4	531
August	1st to 10th	Day	5% 9% 35% 32% 15% 3	487
	11th to 20th	Day	3 10% 33% 36% 15% 4	477
	21st to end	Day	5% 9% 31% 36% 14% 5%	539
September	1st to 10th	Day	4 11% 28% 33% 17% 7%	490
	11th to 20th	Day	8% 15% 31% 30% 12% 4	490
	21st to end	Day	7% 14% 24% 36% 15% 4	490
October	1st to 10th	Day	8% 16% 27% 26% 17% 6%	386
	11th to 20th	Day	10% 15% 27% 24% 18% 6%	300
	21st to end	Day	9% 16% 35% 28% 11% 1	330
November	1st to 10th	Day	14% 18% 27% 25% 12% 4	300
	11th to 20th	Day	11% 17% 29% 30% 8% 4	300
	21st to end	Day	10% 11% 29% 30% 17% 3	300
December	1st to 10th	Day	7% 15% 32% 30% 12% 4	273
	11th to 20th	Day	7% 15% 39% 31% 7% 1	270
	21st to end	Day	12% 19% 34% 26% 8% 1	297
January	1st to 10th	Night	9% 7% 31% 33% 18% 3	760
	11th to 20th	Night	8% 9% 28% 37% 15% 3	760
	21st to end	Night	7% 8% 28% 39% 14% 3	836
February	1st to 10th	Night	8% 8% 33% 39% 11% 2	760
	11th to 20th	Night	5% 9% 35% 37% 12% 1	744
	21st to end	Night	7% 9% 33% 38% 12%	628
March	1st to 10th	Night	8% 10% 32% 38% 10% 2	760
	11th to 20th	Night	6% 8% 29% 39% 16% 2	760
	21st to end	Night	8% 14% 29% 37% 11% 1	836
April	1st to 10th	Night	13% 12% 26% 38% 11% 1	728
	11th to 20th	Night	9% 10% 33% 35% 12%	563
	21st to end	Night	9% 9% 26% 35% 18% 2	570
May	1st to 10th	Night	8% 7% 30% 39% 15% 1	570
	11th to 20th	Night	7% 5% 22% 39% 25% 2	561
	21st to end	Night	8% 6% 24% 36% 23% 4	617
June	1st to 10th	Night	6% 8% 18% 41% 24% 2	570
	11th to 20th	Night	5% 11% 22% 42% 19%	570
	21st to end	Night	2 4 20% 41% 31% 2	570
July	1st to 10th	Night	3 3 15% 38% 37% 3	555
	11th to 20th	Night	4 2 14% 48% 29% 3	552
	21st to end	Night	3 1 10% 38% 46% 2	624
August	1st to 10th	Night	4 2 13% 47% 32% 1	567
	11th to 20th	Night	3 3 15% 44% 34% 2	564
	21st to end	Night	3 3 15% 44% 34% 2	627
September	1st to 10th	Night	4 4 15% 41% 32% 5%	679
	11th to 20th	Night	5% 6% 18% 38% 29% 5%	760
	21st to end	Night	6% 5% 16% 36% 30% 7%	760
October	1st to 10th	Night	8% 6% 21% 39% 24% 3	760
	11th to 20th	Night	7% 4 21% 42% 22% 4	760
	21st to end	Night	8% 7% 25% 37% 19% 3	836
November	1st to 10th	Night	8% 8% 25% 37% 19% 3	780
	11th to 20th	Night	9% 5% 31% 38% 13% 4	800
	21st to end	Night	7% 9% 30% 36% 15% 3	955
December	1st to 10th	Night	7% 7% 32% 38% 14% 1	960
	11th to 20th	Night	10% 9% 30% 39% 10% 1	950
	21st to end	Night	12% 9% 33% 36% 8% 1	912

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