Crystal Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Crystal Creek Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	13% 7% 2 7% 4 4 4 4 6% 7% 42%	130994
	Night	13% 7% 2 6% 4 4 4 3 5% 7% 46%	131009
Transparency	Day	39% 5% 11% 23% 18% 4	134688
	Night	37% 5% 10% 23% 20% 6%	135392
Seeing	Day	17% 11% 24% 24% 17% 7%	17068
	Night	17% 10% 25% 28% 17% 3	26938

By Month

Percentage Cloud Cover SE (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	16% 11% 3 9% 3 3 3 2 4 5% 40%	4691
	11th to 20th	Day	13% 6% 2 7% 3 3 3 2 3 4 54%	5284
	21st to end	Day	20% 8% 2 6% 4 4 3 3 4 5% 41%	5980
February	1st to 10th	Day	20% 9% 4 6% 5% 3 3 2 4 5% 39%	2100
	11th to 20th	Day	21% 10% 3 5% 3 4 3 3 4 5% 39%	2048
	21st to end	Day	20% 8% 3 7% 4 3 4 2 4 7% 38%	1730
March	1st to 10th	Day	17% 12% 3 8% 5% 4 5% 3 4 4 34%	4580
	11th to 20th	Day	12% 7% 3 7% 4 3 4 3 4 6% 47%	4899
	21st to end	Day	12% 7% 3 7% 5% 5% 4 3 5% 6% 44%	6067
April	1st to 10th	Day	14% 7% 3 5% 4 5% 3 4 6% 5% 45%	2412
	11th to 20th	Day	17% 6% 2 6% 4 3 3 3 5% 6% 45%	2374
	21st to end	Day	12% 7% 2 7% 4 5% 4 4 6% 8% 42%	2400
May	1st to 10th	Day	16% 7% 2 7% 5% 4 6% 5% 7% 9% 32%	5492
	11th to 20th	Day	15% 8% 3 9% 5% 5% 6% 4 6% 9% 30%	5861
	21st to end	Day	17% 7% 3 7% 5% 5% 4 4 6% 8% 35%	6468
June	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 9% 40%	2800
	11th to 20th	Day	5% 4 2 5% 4 5% 6% 5% 8% 11% 46%	2800
	21st to end	Day	5% 4 2 5% 5% 6% 6% 6% 7% 10% 45%	2800
July	1st to 10th	Day	5% 5% 3 7% 6% 5% 7% 6% 8% 10% 38%	6360
	11th to 20th	Day	7% 4 2 6% 5% 6% 7% 6% 9% 13% 33%	6360
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 8% 11% 33%	6216
August	1st to 10th	Day	3 3 2 4 4 4 5% 4 7% 8% 57%	3905
	11th to 20th	Day	7% 4 2 6% 4 5% 5% 4 7% 10% 46%	3849
	21st to end	Day	6% 4 2 5% 5% 4 5% 3 6% 9% 52%	4312
September	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 8% 45%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 6% 8% 45%	2281
	21st to end	Day	11% 5% 2 5% 4 4 4 3 5% 7% 51%	2200
October	1st to 10th	Day	9% 5% 2 4 4 4 3 3 4 5% 58%	2180
	11th to 20th	Day	9% 4 2 5% 3 3 3 4 4 5% 59%	2000
	21st to end	Day	17% 5% 2 4 3 3 3 3 4 3 52%	2170
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 2 4 5% 55%	1852
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 5% 7% 52%	1890
	21st to end	Day	17% 8% 2 4 3 4 3 4 5% 6% 44%	1890
December	1st to 10th	Day	15% 8% 3 8% 4 3 3 2 3 5% 46%	3020
	11th to 20th	Day	21% 9% 3 6% 4 3 3 3 3 4 41%	3253
	21st to end	Day	14% 10% 3 7% 4 4 5% 3 6% 6% 36%	4070
January	1st to 10th	Night	18% 9% 3 9% 3 3 4 2 4 5% 41%	8232
	11th to 20th	Night	13% 7% 3 7% 3 3 3 3 3 4 52%	7703
	21st to end	Night	22% 9% 3 6% 5% 3 3 2 3 5% 40%	8222
February	1st to 10th	Night	18% 10% 3 7% 4 3 3 2 3 5% 39%	2520
	11th to 20th	Night	20% 9% 3 6% 4 4 3 2 4 4 40%	2458
	21st to end	Night	19% 7% 3 5% 5% 3 4 4 4 5% 40%	2076
March	1st to 10th	Night	19% 9% 3 7% 4 4 4 3 4 6% 38%	5493
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 7% 51%	5311
	21st to end	Night	14% 7% 2 6% 4 4 4 3 4 5% 45%	5102
April	1st to 10th	Night	13% 6% 2 6% 4 4 3 3 6% 7% 46%	2010
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 4 7% 49%	1975
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 6% 46%	2000
May	1st to 10th	Night	17% 9% 3 7% 4 4 3 3 5% 7% 36%	4402
	11th to 20th	Night	16% 9% 3 8% 5% 5% 4 3 5% 6% 36%	4032
	21st to end	Night	13% 7% 3 7% 5% 5% 5% 4 7% 8% 37%	4383
June	1st to 10th	Night	8% 6% 2 5% 4 5% 5% 5% 7% 11% 44%	1600
	11th to 20th	Night	3 4 1 3 4 4 4 5% 7% 10% 56%	1600
	21st to end	Night	3 4 2 4 4 4 4 4 6% 10% 55%	1600
July	1st to 10th	Night	5% 4 2 7% 4 5% 6% 5% 7% 12% 44%	3618
	11th to 20th	Night	4 4 2 5% 3 4 4 5% 8% 15% 47%	3617
	21st to end	Night	8% 6% 3 7% 5% 5% 5% 4 7% 13% 39%	4628
August	1st to 10th	Night	2 2 1 3 3 3 3 3 6% 7% 67%	3244
	11th to 20th	Night	4 3 1 4 4 4 4 4 8% 12% 54%	3200
	21st to end	Night	4 3 1 4 3 4 4 4 8% 9% 55%	3586
September	1st to 10th	Night	7% 5% 3 6% 4 4 4 4 6% 7% 52%	2000
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 5% 7% 51%	2097
	21st to end	Night	9% 5% 2 4 3 3 3 3 5% 6% 56%	2200
October	1st to 10th	Night	9% 3 1 4 3 2 2 3 4 6% 63%	2215
	11th to 20th	Night	10% 5% 2 4 2 3 3 2 4 5% 58%	2400
	21st to end	Night	16% 7% 2 4 3 3 3 3 2 5% 52%	2668
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 5% 55%	2666
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 5% 7% 53%	2730
	21st to end	Night	16% 7% 2 4 3 3 4 3 4 5% 48%	2730
December	1st to 10th	Night	18% 7% 2 6% 3 3 3 2 4 5% 47%	4380
	11th to 20th	Night	20% 9% 3 6% 3 3 3 2 4 4 44%	5161
	21st to end	Night	18% 8% 2 6% 4 5% 5% 4 6% 6% 36%	7150

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	48% 3 9% 25% 14% 1	4692
	11th to 20th	Day	36% 3 4 23% 31% 3	5284
	21st to end	Day	47% 2 6% 19% 21% 5%	5980
February	1st to 10th	Day	49% 3 6% 22% 17% 3	2100
	11th to 20th	Day	50% 3 6% 20% 17% 5%	2058
	21st to end	Day	48% 3 8% 15% 18% 7%	1730
March	1st to 10th	Day	53% 4 5% 15% 13% 9%	4580
	11th to 20th	Day	40% 4 7% 19% 21% 9%	4899
	21st to end	Day	42% 4 8% 24% 21% 1	6069
April	1st to 10th	Day	40% 6% 8% 23% 19% 4	2412
	11th to 20th	Day	40% 4 8% 21% 21% 6%	2374
	21st to end	Day	39% 6% 10% 25% 18% 2	2400
May	1st to 10th	Day	47% 8% 12% 21% 12% 1	5527
	11th to 20th	Day	48% 8% 16% 18% 10% 1	5865
	21st to end	Day	45% 7% 11% 22% 13% 2	6482
June	1st to 10th	Day	37% 9% 13% 21% 18% 2	2800
	11th to 20th	Day	30% 8% 15% 26% 18% 3	2800
	21st to end	Day	31% 9% 15% 24% 18% 3	2800
July	1st to 10th	Day	36% 7% 17% 26% 12% 1	6291
	11th to 20th	Day	37% 9% 20% 29% 5% 1	6285
	21st to end	Day	42% 8% 20% 24% 5%	6037
August	1st to 10th	Day	22% 7% 17% 30% 22% 2	3893
	11th to 20th	Day	32% 7% 16% 26% 17% 2	3885
	21st to end	Day	29% 5% 11% 25% 26% 3	4311
September	1st to 10th	Day	35% 8% 11% 25% 16% 5%	2400
	11th to 20th	Day	37% 6% 11% 20% 22% 5%	2281
	21st to end	Day	32% 5% 10% 23% 23% 7%	2200
October	1st to 10th	Day	19% 3 6% 15% 38% 20%	3557
	11th to 20th	Day	19% 3 5% 21% 37% 16%	3240
	21st to end	Day	27% 3 4 26% 32% 8%	3484
November	1st to 10th	Day	33% 4 7% 22% 28% 7%	1852
	11th to 20th	Day	33% 3 10% 20% 25% 8%	1890
	21st to end	Day	41% 3 8% 20% 24% 4	1890
December	1st to 10th	Day	43% 3 4 27% 21% 2	3020
	11th to 20th	Day	49% 3 5% 21% 17% 4	3250
	21st to end	Day	47% 5% 11% 25% 12% 1	4070
January	1st to 10th	Night	48% 3 10% 22% 15% 2	8233
	11th to 20th	Night	37% 4 5% 19% 30% 5%	7703
	21st to end	Night	49% 2 6% 19% 18% 6%	8222
February	1st to 10th	Night	48% 3 8% 18% 18% 3	2520
	11th to 20th	Night	49% 3 7% 19% 17% 5%	2459
	21st to end	Night	46% 4 7% 16% 20% 7%	2076
March	1st to 10th	Night	48% 4 7% 15% 18% 7%	5493
	11th to 20th	Night	34% 5% 7% 21% 18% 16%	5311
	21st to end	Night	41% 4 8% 21% 22% 3	5102
April	1st to 10th	Night	38% 5% 11% 22% 20% 5%	2010
	11th to 20th	Night	35% 4 8% 22% 23% 8%	1975
	21st to end	Night	36% 6% 11% 25% 19% 3	2000
May	1st to 10th	Night	48% 7% 12% 19% 13% 2	4432
	11th to 20th	Night	48% 6% 15% 21% 9% 1	4033
	21st to end	Night	42% 8% 12% 23% 13% 2	4391
June	1st to 10th	Night	32% 9% 14% 23% 21% 2	1600
	11th to 20th	Night	21% 8% 15% 30% 23% 3	1600
	21st to end	Night	23% 7% 17% 29% 20% 5%	1600
July	1st to 10th	Night	31% 9% 19% 27% 12% 2	3580
	11th to 20th	Night	25% 7% 24% 35% 6% 2	3572
	21st to end	Night	36% 8% 24% 24% 7%	4495
August	1st to 10th	Night	15% 6% 12% 40% 25% 1	3232
	11th to 20th	Night	22% 7% 23% 28% 18% 2	3230
	21st to end	Night	23% 6% 15% 24% 27% 4	3586
September	1st to 10th	Night	31% 7% 10% 25% 21% 7%	2000
	11th to 20th	Night	32% 7% 12% 20% 26% 4	2097
	21st to end	Night	28% 5% 9% 26% 24% 8%	2200
October	1st to 10th	Night	17% 3 6% 16% 37% 21%	3618
	11th to 20th	Night	19% 3 4 16% 36% 22%	3899
	21st to end	Night	26% 2 6% 27% 24% 14%	4304
November	1st to 10th	Night	32% 4 6% 24% 26% 8%	2666
	11th to 20th	Night	31% 4 7% 22% 25% 10%	2730
	21st to end	Night	39% 4 7% 21% 24% 6%	2730
December	1st to 10th	Night	41% 3 7% 26% 21% 2	4380
	11th to 20th	Night	45% 2 6% 21% 22% 3	5163
	21st to end	Night	46% 5% 10% 24% 12% 3	7150

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	26% 23% 34% 14% 3	330
	11th to 20th	Day	31% 23% 34% 9% 3 1	330
	21st to end	Day	27% 27% 33% 11% 2	363
February	1st to 10th	Day	30% 21% 33% 12% 4 1	330
	11th to 20th	Day	40% 23% 28% 6% 2 1	330
	21st to end	Day	34% 26% 29% 10% 2	273
March	1st to 10th	Day	30% 21% 34% 11% 4	330
	11th to 20th	Day	30% 22% 35% 12% 1	330
	21st to end	Day	27% 18% 37% 14% 5%	575
April	1st to 10th	Day	27% 16% 30% 21% 7%	504
	11th to 20th	Day	24% 16% 32% 19% 7% 2	492
	21st to end	Day	21% 16% 30% 22% 9% 2	500
May	1st to 10th	Day	21% 17% 30% 21% 9% 2	503
	11th to 20th	Day	17% 9% 29% 26% 14% 5%	527
	21st to end	Day	13% 7% 27% 29% 16% 9%	582
June	1st to 10th	Day	10% 5% 22% 30% 22% 10%	730
	11th to 20th	Day	7% 3 19% 33% 27% 11%	730
	21st to end	Day	5% 4 20% 34% 25% 11%	730
July	1st to 10th	Day	8% 2 13% 31% 32% 13%	721
	11th to 20th	Day	8% 2 12% 28% 34% 16%	718
	21st to end	Day	7% 2 12% 32% 32% 15%	706
August	1st to 10th	Day	6% 4 13% 29% 35% 13%	526
	11th to 20th	Day	11% 3 12% 30% 32% 11%	519
	21st to end	Day	11% 2 11% 28% 32% 16%	583
September	1st to 10th	Day	9% 2 15% 35% 30% 9%	530
	11th to 20th	Day	12% 2 13% 28% 31% 15%	530
	21st to end	Day	15% 5% 15% 32% 22% 10%	530
October	1st to 10th	Day	9% 5% 22% 32% 25% 6%	510
	11th to 20th	Day	9% 12% 37% 28% 11% 2	330
	21st to end	Day	26% 12% 33% 21% 7% 1	363
November	1st to 10th	Day	21% 12% 28% 28% 10% 2	330
	11th to 20th	Day	12% 19% 38% 25% 4 1	330
	21st to end	Day	25% 18% 28% 17% 11% 2	330
December	1st to 10th	Day	31% 18% 30% 17% 4 1	330
	11th to 20th	Day	28% 18% 29% 18% 6% 1	330
	21st to end	Day	30% 16% 37% 14% 2	363
January	1st to 10th	Night	25% 18% 31% 22% 4	1050
	11th to 20th	Night	24% 21% 32% 19% 4	1050
	21st to end	Night	25% 15% 37% 21% 2	1092
February	1st to 10th	Night	27% 15% 34% 20% 4	840
	11th to 20th	Night	30% 17% 31% 21% 2	823
	21st to end	Night	27% 19% 32% 20% 2	692
March	1st to 10th	Night	22% 16% 31% 27% 4	840
	11th to 20th	Night	24% 13% 35% 23% 4 1	734
	21st to end	Night	20% 15% 32% 24% 9%	693
April	1st to 10th	Night	17% 14% 35% 25% 9%	603
	11th to 20th	Night	20% 10% 34% 26% 9% 1	593
	21st to end	Night	15% 10% 35% 27% 11% 1	600
May	1st to 10th	Night	15% 9% 28% 32% 14% 2	600
	11th to 20th	Night	14% 7% 28% 32% 16% 4	591
	21st to end	Night	8% 4 22% 37% 24% 5%	650
June	1st to 10th	Night	11% 3 16% 34% 30% 6%	600
	11th to 20th	Night	4 2 17% 34% 37% 5%	600
	21st to end	Night	5% 1 13% 37% 36% 8%	600
July	1st to 10th	Night	4 3 10% 30% 41% 11%	585
	11th to 20th	Night	6% 2 11% 26% 42% 13%	582
	21st to end	Night	5% 1 9% 28% 44% 12%	657
August	1st to 10th	Night	5% 1 6% 30% 47% 10%	597
	11th to 20th	Night	7% 2 8% 37% 38% 8%	594
	21st to end	Night	8% 2 10% 30% 40% 10%	660
September	1st to 10th	Night	8% 2 12% 36% 38% 5%	600
	11th to 20th	Night	10% 3 13% 31% 35% 7%	697
	21st to end	Night	12% 4 16% 33% 31% 4	800
October	1st to 10th	Night	10% 4 19% 33% 31% 4	800
	11th to 20th	Night	13% 8% 24% 34% 20% 2	800
	21st to end	Night	20% 9% 25% 30% 14% 1	880
November	1st to 10th	Night	14% 8% 26% 33% 17% 1	820
	11th to 20th	Night	15% 9% 33% 31% 10%	840
	21st to end	Night	24% 10% 27% 27% 10% 1	840
December	1st to 10th	Night	25% 13% 30% 27% 6%	840
	11th to 20th	Night	23% 15% 28% 27% 6%	940
	21st to end	Night	27% 15% 30% 24% 4	1155

Page generated at 2026-03-30 01:18:42. Version ac07

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