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 2026-03-03.

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 6% 3 7% 5% 4 5% 4 6% 7% 51%	139818
	Night	2 6% 2 7% 5% 4 5% 4 6% 7% 52%	138889
Transparency	Day	29% 5% 11% 20% 21% 13%	134566
	Night	28% 5% 12% 23% 21% 10%	135146
Seeing	Day	9% 17% 33% 27% 11% 3	16820
	Night	7% 7% 26% 38% 19% 2	27289

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	1 8% 5% 14% 8% 7% 6% 4 6% 8% 33%	5318
	11th to 20th	Day	1 7% 3 14% 7% 5% 4 3 5% 4 49%	5340
	21st to end	Day	1 8% 5% 9% 4 3 4 3 5% 7% 51%	5844
February	1st to 10th	Day	2 13% 6% 13% 8% 5% 6% 4 4 5% 33%	1890
	11th to 20th	Day	3 13% 6% 12% 7% 5% 6% 5% 6% 6% 31%	1901
	21st to end	Day	1 13% 4 11% 8% 6% 6% 4 6% 8% 32%	1730
March	1st to 10th	Day	1 11% 5% 14% 7% 5% 6% 3 5% 6% 36%	4881
	11th to 20th	Day	1 6% 3 9% 6% 7% 6% 5% 8% 10% 39%	5000
	21st to end	Day	2 13% 6% 13% 7% 6% 6% 4 5% 6% 30%	5486
April	1st to 10th	Day	3 12% 4 9% 5% 4 6% 5% 7% 7% 38%	2233
	11th to 20th	Day	5% 12% 4 9% 6% 6% 5% 4 6% 6% 38%	2505
	21st to end	Day	4 11% 4 8% 6% 4 5% 5% 6% 6% 42%	2600
May	1st to 10th	Day	6% 9% 4 9% 5% 5% 5% 4 6% 6% 41%	5880
	11th to 20th	Day	4 6% 2 5% 4 4 5% 5% 7% 9% 50%	5841
	21st to end	Day	3 5% 2 5% 4 5% 5% 5% 7% 7% 50%	6423
June	1st to 10th	Day	3 3 2 5% 5% 4 5% 4 7% 8% 54%	2740
	11th to 20th	Day	3 4 2 4 3 4 3 3 4 6% 63%	2800
	21st to end	Day	1 2 1 4 3 3 4 3 5% 6% 68%	2682
July	1st to 10th	Day	2 2 1 3 2 3 3 3 4 6% 71%	5879
	11th to 20th	Day	2 3 1 3 4 4 6% 5% 8% 10% 55%	5880
	21st to end	Day	2 2 1 3 3 3 4 4 5% 7% 66%	6401
August	1st to 10th	Day	2 1 1 3 3 3 3 4 6% 8% 67%	5779
	11th to 20th	Day	1 2 1 3 2 2 4 3 6% 8% 69%	5798
	21st to end	Day	3 2 1 4 4 5% 5% 4 7% 8% 57%	6385
September	1st to 10th	Day	3 3 1 3 3 3 3 3 5% 8% 64%	2280
	11th to 20th	Day	4 4 1 4 3 4 4 4 6% 6% 61%	2200
	21st to end	Day	4 4 1 4 3 3 3 3 4 6% 65%	2200
October	1st to 10th	Day	4 5% 1 4 3 3 3 2 4 5% 66%	3412
	11th to 20th	Day	6% 6% 2 5% 3 3 4 4 5% 6% 57%	3250
	21st to end	Day	3 6% 2 7% 4 4 4 4 5% 6% 57%	3548
November	1st to 10th	Day	4 9% 3 8% 4 4 4 4 6% 6% 48%	1864
	11th to 20th	Day	6% 10% 2 7% 5% 4 5% 3 6% 7% 45%	1890
	21st to end	Day	3 8% 4 10% 5% 6% 5% 3 6% 7% 44%	1702
December	1st to 10th	Day	3 5% 2 7% 5% 4 4 3 4 7% 57%	2700
	11th to 20th	Day	2 8% 3 9% 6% 5% 6% 5% 9% 11% 36%	3106
	21st to end	Day	2 9% 5% 13% 6% 5% 6% 4 8% 8% 34%	4450
January	1st to 10th	Night	1 7% 3 12% 8% 7% 6% 4 7% 9% 35%	7605
	11th to 20th	Night	1 6% 3 11% 7% 5% 4 3 4 5% 51%	7650
	21st to end	Night	1 6% 4 8% 5% 4 4 3 6% 8% 51%	8370
February	1st to 10th	Night	2 10% 4 12% 6% 5% 6% 5% 6% 8% 36%	2730
	11th to 20th	Night	2 9% 4 10% 7% 5% 6% 5% 7% 6% 39%	2604
	21st to end	Night	1 10% 3 11% 6% 5% 5% 4 6% 9% 39%	2076
March	1st to 10th	Night	1 9% 4 12% 7% 6% 7% 5% 6% 7% 37%	5056
	11th to 20th	Night	1 6% 3 7% 5% 6% 7% 6% 8% 10% 42%	5000
	21st to end	Night	2 9% 4 9% 6% 6% 5% 4 7% 7% 42%	5489
April	1st to 10th	Night	4 12% 3 6% 4 4 4 4 5% 6% 46%	2166
	11th to 20th	Night	3 8% 2 6% 6% 5% 5% 5% 6% 7% 47%	1840
	21st to end	Night	5% 9% 3 6% 4 4 5% 5% 6% 8% 46%	1800
May	1st to 10th	Night	3 8% 3 7% 5% 5% 4 4 6% 6% 49%	4100
	11th to 20th	Night	2 3 1 5% 4 3 4 4 6% 8% 59%	4074
	21st to end	Night	3 4 2 5% 3 3 4 4 6% 8% 58%	4474
June	1st to 10th	Night	4 5% 1 5% 4 5% 4 3 5% 7% 57%	1650
	11th to 20th	Night	3 2 1 3 3 3 4 4 5% 7% 66%	1600
	21st to end	Night	2 2 1 3 3 3 4 3 5% 7% 69%	1709
July	1st to 10th	Night	1 2 1 4 4 4 4 3 4 4 70%	4100
	11th to 20th	Night	2 2 1 4 4 4 4 5% 7% 9% 58%	4100
	21st to end	Night	1 2 1 4 3 4 4 4 5% 6% 66%	4472
August	1st to 10th	Night	2 1 1 2 2 3 3 3 6% 7% 69%	4033
	11th to 20th	Night	2 2 1 4 3 3 4 4 5% 6% 66%	4051
	21st to end	Night	2 2 1 4 4 4 5% 5% 6% 8% 60%	4590
September	1st to 10th	Night	3 3 1 4 3 3 4 3 5% 6% 66%	2115
	11th to 20th	Night	4 3 1 3 3 3 5% 3 5% 7% 65%	2200
	21st to end	Night	4 3 1 4 2 2 2 2 3 6% 69%	2200
October	1st to 10th	Night	3 3 1 3 2 3 3 3 4 4 72%	3782
	11th to 20th	Night	4 5% 1 5% 3 4 4 4 6% 7% 57%	3898
	21st to end	Night	3 5% 2 5% 2 3 4 4 7% 7% 57%	4252
November	1st to 10th	Night	3 8% 3 6% 4 4 4 4 7% 8% 50%	2650
	11th to 20th	Night	4 7% 2 6% 3 5% 4 4 7% 8% 49%	2730
	21st to end	Night	2 8% 4 8% 5% 5% 5% 3 5% 8% 46%	2893
December	1st to 10th	Night	1 5% 2 7% 4 3 3 3 5% 8% 58%	4710
	11th to 20th	Night	2 7% 3 7% 4 5% 6% 5% 8% 10% 44%	5321
	21st to end	Night	2 9% 4 12% 7% 6% 6% 5% 6% 9% 34%	6799

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	42% 10% 17% 19% 11% 1	5321
	11th to 20th	Day	37% 3 12% 24% 17% 7%	5340
	21st to end	Day	30% 5% 17% 21% 21% 5%	5844
February	1st to 10th	Day	49% 6% 14% 18% 12% 2	1890
	11th to 20th	Day	49% 6% 14% 19% 11% 1	1910
	21st to end	Day	47% 6% 13% 20% 10% 2	1730
March	1st to 10th	Day	41% 6% 13% 21% 13% 6%	4881
	11th to 20th	Day	30% 10% 16% 28% 15% 1	5000
	21st to end	Day	48% 7% 21% 18% 6% 1	5486
April	1st to 10th	Day	40% 6% 15% 19% 15% 4	2233
	11th to 20th	Day	42% 6% 13% 21% 16% 2	2508
	21st to end	Day	38% 6% 11% 25% 19% 1	2600
May	1st to 10th	Day	35% 7% 10% 21% 21% 5%	5880
	11th to 20th	Day	25% 7% 9% 28% 27% 3	5844
	21st to end	Day	26% 6% 9% 25% 31% 3	6424
June	1st to 10th	Day	23% 5% 10% 23% 28% 9%	2740
	11th to 20th	Day	22% 3 7% 19% 29% 21%	2800
	21st to end	Day	15% 3 5% 12% 31% 34%	2682
July	1st to 10th	Day	14% 3 4 7% 17% 55%	5816
	11th to 20th	Day	17% 5% 8% 12% 27% 31%	5811
	21st to end	Day	14% 5% 6% 9% 25% 41%	6217
August	1st to 10th	Day	17% 4 7% 21% 25% 25%	4208
	11th to 20th	Day	14% 5% 7% 16% 33% 25%	4197
	21st to end	Day	15% 4 8% 15% 29% 29%	4604
September	1st to 10th	Day	18% 4 7% 17% 28% 27%	2280
	11th to 20th	Day	22% 5% 7% 18% 29% 20%	2200
	21st to end	Day	21% 3 6% 21% 34% 15%	2200
October	1st to 10th	Day	22% 3 6% 16% 34% 20%	3412
	11th to 20th	Day	26% 3 8% 30% 26% 7%	3250
	21st to end	Day	27% 3 12% 27% 27% 4	3548
November	1st to 10th	Day	33% 5% 14% 25% 18% 5%	1864
	11th to 20th	Day	35% 5% 15% 27% 17% 1	1890
	21st to end	Day	38% 5% 13% 26% 15% 4	1702
December	1st to 10th	Day	25% 6% 14% 38% 14% 2	2700
	11th to 20th	Day	34% 9% 19% 24% 13% 1	3104
	21st to end	Day	44% 7% 14% 17% 17% 2	4450
January	1st to 10th	Night	39% 9% 19% 21% 10% 2	7605
	11th to 20th	Night	35% 6% 9% 26% 18% 5%	7650
	21st to end	Night	29% 7% 15% 21% 24% 5%	8370
February	1st to 10th	Night	41% 8% 17% 21% 10% 2	2730
	11th to 20th	Night	40% 6% 14% 22% 16% 2	2605
	21st to end	Night	38% 6% 15% 25% 13% 4	2076
March	1st to 10th	Night	40% 6% 15% 22% 13% 5%	5056
	11th to 20th	Night	28% 9% 18% 28% 15% 1	5000
	21st to end	Night	36% 7% 20% 23% 13% 1	5489
April	1st to 10th	Night	35% 5% 15% 23% 18% 5%	2166
	11th to 20th	Night	31% 5% 16% 24% 21% 3	1840
	21st to end	Night	33% 6% 17% 25% 19% 1	1800
May	1st to 10th	Night	31% 6% 11% 24% 25% 3	4097
	11th to 20th	Night	20% 6% 10% 33% 27% 4	4075
	21st to end	Night	22% 4 10% 30% 33% 2	4474
June	1st to 10th	Night	25% 4 10% 25% 29% 8%	1650
	11th to 20th	Night	17% 3 8% 21% 30% 21%	1600
	21st to end	Night	14% 2 5% 11% 32% 35%	1709
July	1st to 10th	Night	16% 3 4 7% 15% 55%	4055
	11th to 20th	Night	19% 4 7% 10% 28% 33%	4046
	21st to end	Night	17% 3 6% 10% 26% 37%	4344
August	1st to 10th	Night	18% 4 8% 18% 27% 25%	2928
	11th to 20th	Night	20% 5% 8% 15% 28% 24%	2928
	21st to end	Night	14% 3 9% 14% 31% 29%	3302
September	1st to 10th	Night	18% 4 6% 19% 25% 28%	2115
	11th to 20th	Night	18% 4 9% 20% 28% 20%	2200
	21st to end	Night	18% 3 6% 21% 36% 16%	2200
October	1st to 10th	Night	16% 2 6% 20% 33% 22%	3782
	11th to 20th	Night	23% 4 9% 32% 25% 7%	3898
	21st to end	Night	23% 4 10% 31% 29% 4	4252
November	1st to 10th	Night	28% 6% 13% 27% 21% 5%	2650
	11th to 20th	Night	29% 5% 17% 28% 18% 2	2730
	21st to end	Night	34% 5% 13% 28% 16% 4	2893
December	1st to 10th	Night	24% 5% 12% 36% 21% 2	4710
	11th to 20th	Night	31% 7% 21% 28% 12% 1	5322
	21st to end	Night	42% 7% 17% 20% 13% 1	6799

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% 18% 36% 19% 12% 2	330
	11th to 20th	Day	9% 16% 35% 27% 12% 2	330
	21st to end	Day	6% 16% 38% 28% 10% 2	363
February	1st to 10th	Day	16% 22% 33% 23% 5% 1	330
	11th to 20th	Day	13% 24% 36% 22% 5% 1	386
	21st to end	Day	11% 28% 35% 19% 4 1	446
March	1st to 10th	Day	14% 23% 38% 18% 6% 1	509
	11th to 20th	Day	11% 25% 35% 22% 6% 1	500
	21st to end	Day	17% 29% 32% 19% 3	550
April	1st to 10th	Day	14% 24% 30% 22% 7% 2	500
	11th to 20th	Day	17% 28% 32% 16% 5% 1	492
	21st to end	Day	14% 27% 30% 22% 6% 1	500
May	1st to 10th	Day	13% 22% 35% 20% 8% 1	503
	11th to 20th	Day	12% 23% 31% 22% 9% 3	528
	21st to end	Day	11% 18% 31% 25% 12% 4	573
June	1st to 10th	Day	6% 15% 30% 31% 16% 3	670
	11th to 20th	Day	7% 16% 33% 28% 15% 1	730
	21st to end	Day	4 15% 34% 31% 13% 2	612
July	1st to 10th	Day	3 10% 37% 36% 12% 2	526
	11th to 20th	Day	2 7% 39% 34% 13% 4	524
	21st to end	Day	4 6% 34% 38% 14% 4	575
August	1st to 10th	Day	5% 11% 36% 31% 14% 3	527
	11th to 20th	Day	3 9% 33% 36% 15% 4	517
	21st to end	Day	5% 9% 31% 36% 14% 5%	583
September	1st to 10th	Day	4 11% 28% 33% 16% 7%	530
	11th to 20th	Day	7% 15% 31% 29% 13% 4	530
	21st to end	Day	7% 14% 24% 35% 16% 4	530
October	1st to 10th	Day	8% 17% 28% 25% 16% 6%	420
	11th to 20th	Day	12% 16% 27% 23% 16% 5%	330
	21st to end	Day	10% 15% 34% 28% 11% 1	363
November	1st to 10th	Day	14% 17% 27% 26% 12% 3	330
	11th to 20th	Day	12% 17% 29% 30% 8% 4	330
	21st to end	Day	9% 11% 31% 30% 16% 3	330
December	1st to 10th	Day	7% 17% 31% 29% 13% 3	330
	11th to 20th	Day	8% 16% 37% 32% 7% 1	330
	21st to end	Day	14% 21% 32% 25% 8% 1	363
January	1st to 10th	Night	8% 8% 33% 33% 16% 3	840
	11th to 20th	Night	7% 10% 30% 37% 14% 2	840
	21st to end	Night	6% 8% 31% 39% 13% 2	924
February	1st to 10th	Night	9% 8% 34% 38% 10% 1	840
	11th to 20th	Night	5% 10% 37% 35% 11% 1	824
	21st to end	Night	7% 9% 35% 38% 11%	692
March	1st to 10th	Night	8% 11% 33% 37% 10% 2	808
	11th to 20th	Night	6% 9% 29% 38% 16% 2	800
	21st to end	Night	8% 14% 31% 36% 11% 1	880
April	1st to 10th	Night	12% 11% 26% 37% 12% 1	766
	11th to 20th	Night	8% 10% 34% 35% 12%	593
	21st to end	Night	9% 9% 27% 36% 18% 2	600
May	1st to 10th	Night	8% 7% 29% 40% 16% 1	600
	11th to 20th	Night	7% 6% 22% 38% 24% 2	591
	21st to end	Night	8% 5% 24% 36% 23% 4	650
June	1st to 10th	Night	6% 8% 19% 41% 24% 2	600
	11th to 20th	Night	5% 11% 22% 43% 19%	600
	21st to end	Night	3 4 20% 41% 31% 2	600
July	1st to 10th	Night	3 4 15% 39% 36% 3	585
	11th to 20th	Night	4 3 15% 48% 28% 2	582
	21st to end	Night	3 1 11% 39% 45% 2	657
August	1st to 10th	Night	4 2 16% 47% 30% 1	597
	11th to 20th	Night	3 3 15% 44% 33% 2	594
	21st to end	Night	2 3 14% 45% 34% 2	660
September	1st to 10th	Night	4 3 14% 42% 32% 5%	715
	11th to 20th	Night	5% 6% 19% 38% 28% 5%	800
	21st to end	Night	6% 5% 17% 36% 30% 7%	800
October	1st to 10th	Night	7% 6% 22% 40% 23% 3	800
	11th to 20th	Night	8% 5% 22% 41% 21% 4	800
	21st to end	Night	7% 7% 25% 38% 19% 3	880
November	1st to 10th	Night	8% 8% 25% 38% 19% 2	820
	11th to 20th	Night	8% 5% 32% 39% 12% 4	840
	21st to end	Night	7% 9% 30% 36% 15% 3	1003
December	1st to 10th	Night	7% 8% 33% 38% 14% 1	1050
	11th to 20th	Night	10% 9% 32% 38% 10% 1	1050
	21st to end	Night	12% 11% 34% 34% 9% 1	1008

Page generated at 2026-02-27 23:40:21. Version ac07

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