CRUST Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CRUST Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	11% 9% 3 9% 5% 5% 5% 4 6% 8% 36%	130829
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 33%	130323
Transparency	Day	46% 4 14% 26% 9% 1	134284
	Night	49% 4 13% 23% 10% 1	134953
Seeing	Day	15% 9% 30% 29% 14% 3	17871
	Night	15% 7% 33% 33% 11% 1	27941

By Month

Percentage Cloud Cover NE (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	6% 11% 6% 19% 9% 8% 7% 4 4 6% 18%	4124
	11th to 20th	Day	4 11% 4 13% 10% 10% 13% 9% 7% 5% 13%	4328
	21st to end	Day	4 12% 7% 15% 7% 7% 6% 5% 8% 9% 22%	5280
February	1st to 10th	Day	8% 13% 6% 11% 8% 6% 6% 6% 7% 6% 23%	1890
	11th to 20th	Day	6% 12% 4 10% 9% 7% 8% 4 8% 7% 25%	1799
	21st to end	Day	6% 11% 4 13% 9% 7% 7% 4 6% 8% 26%	1575
March	1st to 10th	Day	10% 9% 3 8% 5% 5% 5% 3 7% 9% 35%	4690
	11th to 20th	Day	9% 9% 5% 12% 7% 5% 5% 5% 7% 8% 28%	4980
	21st to end	Day	16% 14% 5% 10% 5% 4 3 3 5% 6% 28%	5464
April	1st to 10th	Day	12% 13% 4 8% 6% 4 4 4 5% 6% 33%	2379
	11th to 20th	Day	17% 12% 4 8% 5% 4 4 4 6% 6% 30%	2572
	21st to end	Day	20% 10% 3 8% 5% 4 4 3 6% 5% 31%	2600
May	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 7% 47%	5967
	11th to 20th	Day	15% 9% 3 6% 4 4 4 4 6% 7% 38%	6319
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 8% 42%	7421
June	1st to 10th	Day	14% 6% 2 7% 4 4 4 3 7% 8% 41%	3000
	11th to 20th	Day	15% 6% 2 5% 4 5% 5% 4 6% 8% 41%	3000
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 8% 39%	3000
July	1st to 10th	Day	6% 4 2 6% 4 5% 5% 5% 8% 11% 45%	6799
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 9% 48%	6800
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 7% 10% 45%	7110
August	1st to 10th	Day	10% 4 2 6% 3 5% 5% 4 6% 8% 47%	4234
	11th to 20th	Day	11% 6% 2 5% 3 4 4 4 5% 10% 45%	4171
	21st to end	Day	10% 5% 2 5% 4 3 4 3 5% 9% 50%	4675
September	1st to 10th	Day	11% 5% 2 5% 4 4 3 3 7% 7% 49%	2445
	11th to 20th	Day	13% 7% 2 6% 4 3 4 4 6% 8% 44%	2200
	21st to end	Day	11% 6% 2 5% 5% 4 4 4 6% 8% 45%	2200
October	1st to 10th	Day	15% 9% 3 6% 4 4 4 2 4 6% 42%	2030
	11th to 20th	Day	17% 11% 3 7% 4 4 4 3 4 7% 37%	2000
	21st to end	Day	14% 13% 4 10% 4 4 5% 3 5% 6% 32%	2015
November	1st to 10th	Day	14% 13% 6% 11% 6% 4 4 3 4 5% 31%	1715
	11th to 20th	Day	12% 15% 5% 14% 6% 5% 4 3 4 5% 28%	1680
	21st to end	Day	10% 14% 5% 13% 7% 6% 5% 3 4 6% 27%	1680
December	1st to 10th	Day	10% 14% 5% 16% 10% 8% 6% 5% 5% 5% 16%	2403
	11th to 20th	Day	10% 17% 6% 11% 5% 6% 5% 4 6% 7% 23%	2709
	21st to end	Day	13% 14% 7% 19% 7% 6% 5% 3 5% 4 18%	3575
January	1st to 10th	Night	5% 11% 6% 17% 9% 9% 7% 6% 7% 8% 16%	8800
	11th to 20th	Night	4 13% 5% 13% 10% 9% 11% 8% 7% 5% 15%	8582
	21st to end	Night	5% 12% 6% 12% 6% 6% 7% 5% 8% 8% 27%	8934
February	1st to 10th	Night	7% 13% 5% 11% 6% 6% 7% 5% 6% 6% 27%	2730
	11th to 20th	Night	6% 10% 4 9% 7% 7% 6% 5% 8% 8% 31%	2599
	21st to end	Night	7% 12% 5% 10% 7% 5% 5% 4 7% 7% 31%	2073
March	1st to 10th	Night	6% 9% 4 9% 7% 5% 4 4 6% 7% 40%	5161
	11th to 20th	Night	8% 10% 4 11% 8% 6% 6% 4 5% 6% 31%	4979
	21st to end	Night	14% 15% 5% 11% 5% 4 3 3 4 6% 29%	5467
April	1st to 10th	Night	12% 10% 3 6% 5% 4 4 4 6% 7% 40%	2015
	11th to 20th	Night	17% 10% 4 7% 4 4 3 3 7% 5% 35%	1779
	21st to end	Night	17% 10% 3 7% 5% 3 4 4 4 6% 37%	1800
May	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 5% 7% 47%	3991
	11th to 20th	Night	14% 7% 2 6% 3 4 4 4 6% 7% 42%	3596
	21st to end	Night	13% 6% 2 5% 3 4 3 3 5% 7% 47%	3474
June	1st to 10th	Night	11% 5% 2 4 4 4 4 5% 7% 9% 46%	1400
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 4 6% 9% 45%	1400
	21st to end	Night	8% 5% 2 7% 4 3 4 5% 7% 10% 47%	1400
July	1st to 10th	Night	7% 4 1 4 3 4 4 5% 7% 10% 51%	3180
	11th to 20th	Night	4 3 1 5% 4 3 4 3 6% 10% 57%	3180
	21st to end	Night	7% 5% 2 6% 4 4 5% 4 7% 10% 45%	3763
August	1st to 10th	Night	6% 3 1 5% 4 4 5% 5% 7% 9% 52%	2926
	11th to 20th	Night	12% 3 2 5% 3 4 4 4 7% 8% 48%	2888
	21st to end	Night	10% 6% 3 6% 3 3 4 3 5% 6% 51%	3234
September	1st to 10th	Night	8% 4 2 3 3 4 4 3 5% 8% 56%	1950
	11th to 20th	Night	9% 5% 2 5% 4 4 3 4 6% 8% 52%	2200
	21st to end	Night	13% 5% 2 5% 3 4 3 3 5% 7% 51%	2200
October	1st to 10th	Night	15% 7% 2 6% 4 4 3 3 6% 8% 43%	2350
	11th to 20th	Night	16% 10% 3 6% 5% 3 4 4 4 7% 39%	2400
	21st to end	Night	17% 11% 3 6% 4 4 5% 4 5% 6% 34%	2824
November	1st to 10th	Night	10% 12% 5% 11% 5% 5% 4 4 5% 6% 32%	2801
	11th to 20th	Night	10% 14% 6% 12% 6% 5% 4 3 4 5% 29%	2940
	21st to end	Night	11% 12% 4 11% 6% 6% 5% 4 5% 6% 30%	2940
December	1st to 10th	Night	7% 14% 5% 17% 12% 8% 6% 4 6% 6% 16%	5004
	11th to 20th	Night	8% 16% 5% 15% 9% 7% 5% 4 5% 5% 21%	5718
	21st to end	Night	14% 15% 6% 17% 9% 6% 5% 4 4 4 17%	7645

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	69% 3 9% 15% 4	4125
	11th to 20th	Day	70% 3 10% 13% 4	4357
	21st to end	Day	58% 4 11% 20% 7%	5280
February	1st to 10th	Day	58% 4 14% 16% 8%	1890
	11th to 20th	Day	54% 4 12% 20% 8% 1	1808
	21st to end	Day	56% 5% 15% 18% 6%	1575
March	1st to 10th	Day	46% 3 16% 23% 11% 1	4692
	11th to 20th	Day	52% 4 14% 24% 5%	4980
	21st to end	Day	59% 3 11% 23% 4	5464
April	1st to 10th	Day	51% 5% 10% 22% 11% 1	2379
	11th to 20th	Day	54% 4 12% 19% 10% 1	2572
	21st to end	Day	53% 4 12% 21% 8% 1	2600
May	1st to 10th	Day	37% 3 13% 29% 16% 1	5978
	11th to 20th	Day	44% 4 11% 23% 17% 1	6323
	21st to end	Day	40% 4 9% 32% 14% 2	7421
June	1st to 10th	Day	40% 4 19% 27% 9% 1	3000
	11th to 20th	Day	40% 5% 17% 27% 10%	3000
	21st to end	Day	40% 6% 17% 29% 8% 1	3000
July	1st to 10th	Day	30% 7% 25% 30% 7%	6726
	11th to 20th	Day	32% 6% 19% 35% 8%	6719
	21st to end	Day	33% 7% 20% 31% 9%	6904
August	1st to 10th	Day	33% 6% 17% 28% 15% 1	4220
	11th to 20th	Day	35% 5% 16% 34% 10%	4210
	21st to end	Day	33% 4 20% 34% 9%	4674
September	1st to 10th	Day	34% 4 16% 34% 12% 1	2445
	11th to 20th	Day	39% 4 20% 29% 8% 1	2200
	21st to end	Day	37% 5% 16% 34% 8%	2200
October	1st to 10th	Day	33% 3 13% 34% 17% 1	3303
	11th to 20th	Day	42% 3 10% 29% 16%	3240
	21st to end	Day	49% 4 13% 26% 8%	3236
November	1st to 10th	Day	57% 4 13% 18% 8% 1	1715
	11th to 20th	Day	60% 5% 12% 17% 6% 1	1680
	21st to end	Day	59% 5% 15% 15% 5%	1680
December	1st to 10th	Day	71% 4 11% 10% 2 1	2403
	11th to 20th	Day	59% 3 16% 14% 8%	2710
	21st to end	Day	70% 3 8% 16% 3	3575
January	1st to 10th	Night	65% 4 12% 13% 5%	8800
	11th to 20th	Night	69% 3 9% 15% 5%	8628
	21st to end	Night	54% 4 13% 20% 8%	8934
February	1st to 10th	Night	56% 4 14% 18% 7%	2730
	11th to 20th	Night	49% 6% 11% 22% 12%	2600
	21st to end	Night	51% 5% 13% 22% 8% 1	2073
March	1st to 10th	Night	44% 4 16% 25% 9% 2	5172
	11th to 20th	Night	54% 4 11% 21% 10%	4980
	21st to end	Night	58% 4 11% 20% 7% 1	5467
April	1st to 10th	Night	44% 5% 11% 25% 13% 2	2015
	11th to 20th	Night	49% 3 11% 21% 13% 2	1779
	21st to end	Night	49% 4 11% 24% 10% 2	1800
May	1st to 10th	Night	37% 3 13% 26% 20% 1	4000
	11th to 20th	Night	41% 3 11% 23% 19% 3	3597
	21st to end	Night	37% 4 9% 29% 19% 2	3474
June	1st to 10th	Night	33% 5% 18% 31% 11% 2	1400
	11th to 20th	Night	35% 5% 17% 26% 14% 3	1400
	21st to end	Night	32% 7% 18% 34% 9% 1	1400
July	1st to 10th	Night	27% 8% 22% 34% 9% 1	3145
	11th to 20th	Night	24% 5% 19% 40% 11% 1	3138
	21st to end	Night	33% 7% 23% 26% 10% 1	3655
August	1st to 10th	Night	27% 5% 17% 35% 15% 1	2916
	11th to 20th	Night	31% 5% 18% 33% 12% 1	2915
	21st to end	Night	33% 4 17% 31% 13% 1	3234
September	1st to 10th	Night	27% 5% 16% 37% 14% 1	1950
	11th to 20th	Night	30% 6% 19% 32% 12% 1	2200
	21st to end	Night	34% 4 13% 34% 15% 1	2200
October	1st to 10th	Night	32% 3 12% 36% 15% 3	3844
	11th to 20th	Night	41% 3 11% 29% 15% 1	3899
	21st to end	Night	48% 4 12% 26% 10%	4562
November	1st to 10th	Night	52% 4 12% 21% 9% 2	2801
	11th to 20th	Night	58% 4 14% 17% 6% 1	2940
	21st to end	Night	55% 6% 17% 16% 6%	2940
December	1st to 10th	Night	69% 5% 13% 10% 2	5004
	11th to 20th	Night	65% 4 11% 15% 5%	5716
	21st to end	Night	71% 4 9% 13% 4	7645

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% 9% 39% 28% 9% 3	330
	11th to 20th	Day	15% 9% 33% 32% 11%	330
	21st to end	Day	16% 10% 34% 28% 10% 2	363
February	1st to 10th	Day	12% 16% 39% 26% 6% 1	330
	11th to 20th	Day	9% 14% 39% 23% 12% 2	315
	21st to end	Day	8% 7% 45% 34% 6%	249
March	1st to 10th	Day	14% 12% 35% 29% 9% 1	300
	11th to 20th	Day	16% 11% 33% 24% 12% 4	300
	21st to end	Day	17% 12% 32% 30% 7% 2	330
April	1st to 10th	Day	19% 15% 30% 26% 9% 1	479
	11th to 20th	Day	23% 14% 29% 25% 7% 1	690
	21st to end	Day	24% 9% 30% 24% 11% 1	700
May	1st to 10th	Day	18% 12% 26% 30% 12% 2	703
	11th to 20th	Day	18% 10% 29% 27% 12% 3	725
	21st to end	Day	17% 7% 26% 29% 18% 3	789
June	1st to 10th	Day	14% 9% 27% 30% 17% 4	730
	11th to 20th	Day	14% 10% 34% 27% 13% 2	730
	21st to end	Day	13% 8% 29% 27% 18% 5%	730
July	1st to 10th	Day	8% 3 24% 34% 22% 8%	721
	11th to 20th	Day	11% 5% 29% 27% 21% 6%	718
	21st to end	Day	5% 4 26% 36% 21% 8%	794
August	1st to 10th	Day	8% 5% 25% 32% 25% 5%	726
	11th to 20th	Day	13% 4 22% 32% 20% 9%	715
	21st to end	Day	11% 7% 27% 31% 20% 4	803
September	1st to 10th	Day	11% 8% 28% 32% 18% 3	575
	11th to 20th	Day	16% 7% 33% 33% 8% 3	330
	21st to end	Day	12% 10% 37% 23% 14% 5%	330
October	1st to 10th	Day	17% 8% 29% 31% 14% 2	330
	11th to 20th	Day	20% 14% 34% 25% 7% 1	330
	21st to end	Day	21% 13% 34% 22% 8% 2	363
November	1st to 10th	Day	21% 11% 29% 28% 10% 2	330
	11th to 20th	Day	20% 13% 35% 25% 7% 1	330
	21st to end	Day	17% 13% 36% 28% 6%	330
December	1st to 10th	Day	17% 9% 37% 27% 8% 2	330
	11th to 20th	Day	18% 14% 36% 27% 5%	330
	21st to end	Day	20% 9% 33% 28% 9% 3	363
January	1st to 10th	Night	10% 8% 38% 32% 11% 2	1050
	11th to 20th	Night	15% 9% 36% 33% 8%	1050
	21st to end	Night	14% 9% 38% 30% 9%	1155
February	1st to 10th	Night	13% 9% 33% 34% 9% 2	1050
	11th to 20th	Night	12% 8% 38% 30% 10% 2	1005
	21st to end	Night	12% 8% 38% 29% 10% 2	753
March	1st to 10th	Night	14% 8% 36% 30% 10% 2	715
	11th to 20th	Night	12% 9% 41% 31% 7% 1	600
	21st to end	Night	17% 8% 31% 30% 12% 2	660
April	1st to 10th	Night	17% 8% 34% 32% 8% 1	600
	11th to 20th	Night	22% 9% 32% 27% 8% 1	593
	21st to end	Night	21% 7% 28% 34% 10%	600
May	1st to 10th	Night	19% 6% 30% 35% 9% 1	600
	11th to 20th	Night	16% 6% 34% 32% 11% 1	591
	21st to end	Night	15% 5% 32% 33% 14%	650
June	1st to 10th	Night	12% 5% 30% 38% 14% 1	600
	11th to 20th	Night	12% 6% 31% 40% 12%	600
	21st to end	Night	9% 4 30% 40% 17% 1	600
July	1st to 10th	Night	6% 2 24% 44% 21% 2	585
	11th to 20th	Night	5% 3 30% 42% 18% 1	582
	21st to end	Night	6% 3 22% 47% 19% 2	657
August	1st to 10th	Night	7% 3 19% 48% 22% 1	597
	11th to 20th	Night	10% 4 23% 40% 21% 2	594
	21st to end	Night	11% 5% 27% 38% 17% 2	660
September	1st to 10th	Night	11% 3 29% 39% 17% 2	600
	11th to 20th	Night	12% 5% 31% 39% 13%	600
	21st to end	Night	14% 7% 31% 34% 14% 1	600
October	1st to 10th	Night	19% 8% 32% 33% 8%	750
	11th to 20th	Night	22% 10% 32% 28% 8%	800
	21st to end	Night	23% 10% 32% 29% 6%	1064
November	1st to 10th	Night	16% 8% 34% 32% 9%	1025
	11th to 20th	Night	17% 9% 36% 28% 8% 1	1050
	21st to end	Night	17% 11% 40% 26% 6% 1	1050
December	1st to 10th	Night	14% 8% 36% 32% 10%	1050
	11th to 20th	Night	17% 11% 36% 28% 7% 1	1050
	21st to end	Night	23% 8% 31% 28% 8% 1	1155

Page generated at 2026-02-13 00:42:16. Version ac07

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