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 2024-11-30.

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	12% 9% 3 8% 5% 5% 5% 4 6% 7% 36%	101940
	Night	11% 10% 4 9% 6% 5% 5% 4 6% 7% 35%	97817
Transparency	Day	46% 4 14% 26% 9% 1	105413
	Night	48% 4 13% 24% 9% 1	102427
Seeing	Day	15% 9% 30% 29% 14% 3	16236
	Night	15% 7% 33% 33% 11% 1	26180

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% 13% 7% 17% 7% 7% 6% 4 5% 6% 23%	2218
	11th to 20th	Day	4 9% 4 16% 12% 13% 13% 6% 7% 5% 12%	2325
	21st to end	Day	7% 16% 9% 19% 6% 6% 4 3 4 5% 20%	2834
February	1st to 10th	Day	8% 14% 6% 11% 8% 5% 5% 6% 7% 6% 24%	1710
	11th to 20th	Day	6% 13% 4 10% 9% 7% 8% 4 8% 7% 24%	1664
	21st to end	Day	6% 12% 4 13% 8% 7% 7% 4 5% 7% 26%	1498
March	1st to 10th	Day	12% 12% 3 10% 6% 5% 5% 3 5% 7% 32%	3268
	11th to 20th	Day	9% 9% 5% 11% 7% 4 5% 5% 7% 8% 30%	3480
	21st to end	Day	16% 14% 5% 11% 6% 4 3 3 5% 6% 27%	3814
April	1st to 10th	Day	12% 13% 4 8% 6% 4 5% 4 5% 6% 33%	2259
	11th to 20th	Day	17% 12% 4 8% 5% 4 5% 4 6% 6% 30%	2442
	21st to end	Day	20% 9% 3 8% 5% 4 4 4 6% 6% 32%	2470
May	1st to 10th	Day	17% 8% 2 7% 5% 5% 5% 4 5% 7% 35%	4159
	11th to 20th	Day	15% 7% 3 7% 4 5% 4 5% 7% 8% 35%	4399
	21st to end	Day	19% 8% 2 7% 5% 4 4 3 5% 7% 35%	5166
June	1st to 10th	Day	14% 6% 2 7% 4 4 4 3 7% 8% 41%	2850
	11th to 20th	Day	15% 5% 2 5% 4 4 4 4 6% 8% 42%	2850
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 8% 40%	2850
July	1st to 10th	Day	7% 3 1 5% 4 5% 5% 5% 8% 12% 43%	4750
	11th to 20th	Day	5% 3 2 4 4 4 4 4 7% 10% 54%	4750
	21st to end	Day	5% 4 2 5% 4 5% 5% 5% 8% 10% 49%	4952
August	1st to 10th	Day	9% 4 2 6% 3 5% 5% 4 6% 8% 47%	4104
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 10% 45%	4041
	21st to end	Day	11% 6% 2 5% 4 3 4 3 5% 9% 50%	4532
September	1st to 10th	Day	11% 6% 2 5% 4 4 3 3 6% 7% 50%	2323
	11th to 20th	Day	13% 7% 2 6% 4 3 4 3 6% 8% 46%	2090
	21st to end	Day	12% 6% 2 6% 5% 4 4 4 6% 8% 44%	2090
October	1st to 10th	Day	16% 9% 3 7% 4 4 4 2 4 6% 41%	1929
	11th to 20th	Day	16% 11% 3 7% 4 4 4 3 4 7% 38%	1900
	21st to end	Day	14% 13% 4 11% 5% 4 5% 4 4 6% 32%	1914
November	1st to 10th	Day	14% 13% 5% 10% 6% 4 4 4 4 5% 31%	1632
	11th to 20th	Day	12% 16% 6% 14% 6% 4 4 3 4 5% 26%	1600
	21st to end	Day	11% 14% 5% 13% 8% 6% 5% 3 4 6% 26%	1597
December	1st to 10th	Day	9% 16% 5% 14% 8% 7% 5% 5% 5% 5% 20%	1359
	11th to 20th	Day	9% 14% 6% 13% 7% 7% 5% 4 6% 6% 23%	1679
	21st to end	Day	17% 13% 6% 13% 7% 7% 5% 3 5% 4 20%	2442
January	1st to 10th	Night	6% 12% 6% 14% 9% 9% 6% 5% 6% 6% 21%	4737
	11th to 20th	Night	5% 11% 6% 15% 11% 12% 9% 6% 6% 5% 13%	4640
	21st to end	Night	8% 16% 6% 15% 7% 6% 6% 4 5% 5% 21%	4833
February	1st to 10th	Night	7% 12% 5% 11% 6% 6% 7% 4 6% 7% 28%	2470
	11th to 20th	Night	7% 11% 4 9% 7% 7% 6% 4 7% 8% 30%	2403
	21st to end	Night	7% 12% 5% 10% 7% 5% 5% 4 6% 7% 31%	1973
March	1st to 10th	Night	7% 10% 5% 10% 8% 6% 5% 4 5% 6% 35%	3577
	11th to 20th	Night	8% 10% 4 10% 8% 5% 5% 4 5% 7% 35%	3479
	21st to end	Night	14% 15% 5% 11% 5% 4 3 3 4 6% 29%	3817
April	1st to 10th	Night	11% 10% 3 6% 5% 4 4 4 6% 7% 39%	1915
	11th to 20th	Night	17% 10% 4 7% 3 4 3 3 7% 5% 35%	1689
	21st to end	Night	17% 10% 3 7% 5% 3 4 3 4 6% 37%	1710
May	1st to 10th	Night	16% 8% 2 7% 5% 3 5% 4 6% 8% 36%	2789
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 6% 7% 42%	2506
	21st to end	Night	16% 7% 3 5% 3 4 3 3 5% 7% 44%	2418
June	1st to 10th	Night	11% 5% 2 4 4 4 4 6% 6% 9% 45%	1330
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 4 6% 9% 45%	1330
	21st to end	Night	7% 5% 2 7% 4 3 4 5% 7% 10% 47%	1330
July	1st to 10th	Night	8% 3 1 4 4 4 5% 5% 7% 10% 49%	2220
	11th to 20th	Night	3 3 1 4 3 3 4 3 5% 9% 63%	2220
	21st to end	Night	6% 3 1 5% 3 4 5% 4 8% 11% 49%	2611
August	1st to 10th	Night	5% 3 1 5% 4 4 5% 5% 7% 9% 52%	2836
	11th to 20th	Night	12% 3 2 5% 3 4 4 4 7% 8% 48%	2798
	21st to end	Night	10% 6% 2 6% 3 3 4 3 5% 6% 51%	3135
September	1st to 10th	Night	8% 4 2 3 3 4 4 3 5% 8% 56%	1852
	11th to 20th	Night	9% 5% 1 5% 3 3 3 4 6% 8% 53%	2090
	21st to end	Night	13% 5% 2 5% 3 4 4 3 4 7% 49%	2090
October	1st to 10th	Night	15% 7% 2 6% 4 4 3 3 6% 8% 42%	2232
	11th to 20th	Night	16% 10% 3 6% 5% 3 4 4 4 7% 39%	2280
	21st to end	Night	17% 11% 3 6% 4 4 5% 4 5% 6% 34%	2683
November	1st to 10th	Night	11% 13% 5% 11% 5% 5% 4 4 5% 6% 32%	2664
	11th to 20th	Night	11% 14% 6% 13% 6% 5% 5% 3 5% 5% 28%	2800
	21st to end	Night	11% 12% 4 11% 6% 6% 5% 4 5% 6% 30%	2799
December	1st to 10th	Night	9% 15% 5% 13% 8% 6% 5% 5% 7% 6% 22%	2818
	11th to 20th	Night	10% 15% 5% 14% 8% 7% 6% 4 5% 5% 22%	3518
	21st to end	Night	18% 14% 5% 12% 7% 5% 5% 4 4 5% 21%	5225

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	63% 5% 10% 18% 4	2219
	11th to 20th	Day	72% 3 11% 11% 2	2325
	21st to end	Day	68% 4 6% 16% 5%	2834
February	1st to 10th	Day	57% 5% 14% 17% 7%	1710
	11th to 20th	Day	56% 4 12% 18% 8% 1	1673
	21st to end	Day	57% 5% 14% 18% 6%	1498
March	1st to 10th	Day	53% 4 12% 25% 7%	3270
	11th to 20th	Day	51% 4 15% 26% 4	3480
	21st to end	Day	60% 3 11% 22% 5%	3814
April	1st to 10th	Day	51% 5% 10% 21% 12% 1	2259
	11th to 20th	Day	55% 4 12% 19% 10% 1	2442
	21st to end	Day	53% 4 13% 21% 7% 1	2470
May	1st to 10th	Day	47% 4 13% 25% 11% 1	4170
	11th to 20th	Day	45% 4 13% 26% 11% 1	4403
	21st to end	Day	48% 3 10% 30% 7% 1	5166
June	1st to 10th	Day	40% 4 19% 27% 10%	2850
	11th to 20th	Day	40% 5% 17% 27% 11%	2850
	21st to end	Day	40% 6% 17% 29% 8% 1	2850
July	1st to 10th	Day	31% 8% 23% 30% 9%	4676
	11th to 20th	Day	25% 6% 20% 38% 11% 1	4669
	21st to end	Day	28% 7% 19% 35% 9% 1	4794
August	1st to 10th	Day	33% 6% 17% 29% 14% 1	4090
	11th to 20th	Day	36% 5% 16% 33% 10%	4080
	21st to end	Day	34% 4 19% 34% 9%	4531
September	1st to 10th	Day	34% 4 15% 34% 12% 1	2323
	11th to 20th	Day	38% 4 19% 30% 8% 1	2090
	21st to end	Day	38% 5% 16% 33% 7%	2090
October	1st to 10th	Day	33% 3 12% 34% 16% 1	3202
	11th to 20th	Day	41% 3 10% 30% 16%	3140
	21st to end	Day	49% 4 13% 26% 8%	3135
November	1st to 10th	Day	56% 4 13% 18% 8% 1	1632
	11th to 20th	Day	61% 5% 12% 16% 6%	1600
	21st to end	Day	59% 6% 15% 15% 5%	1597
December	1st to 10th	Day	67% 4 12% 15% 3	1359
	11th to 20th	Day	61% 4 17% 14% 4	1680
	21st to end	Day	68% 3 7% 18% 4	2442
January	1st to 10th	Night	63% 5% 12% 16% 4	4737
	11th to 20th	Night	72% 3 9% 12% 4	4640
	21st to end	Night	65% 4 11% 16% 4	4833
February	1st to 10th	Night	54% 4 14% 19% 8%	2470
	11th to 20th	Night	51% 6% 11% 21% 11%	2404
	21st to end	Night	52% 5% 13% 21% 8% 1	1973
March	1st to 10th	Night	51% 4 12% 24% 9%	3588
	11th to 20th	Night	49% 4 13% 24% 10%	3480
	21st to end	Night	58% 4 11% 20% 7% 1	3817
April	1st to 10th	Night	43% 5% 11% 25% 14% 2	1915
	11th to 20th	Night	50% 3 11% 21% 13% 3	1689
	21st to end	Night	49% 4 11% 24% 10% 2	1710
May	1st to 10th	Night	46% 4 12% 26% 12% 1	2798
	11th to 20th	Night	41% 3 11% 27% 14% 3	2507
	21st to end	Night	41% 4 11% 32% 11% 1	2418
June	1st to 10th	Night	34% 5% 18% 31% 11% 2	1330
	11th to 20th	Night	35% 5% 16% 27% 15% 3	1330
	21st to end	Night	31% 7% 18% 34% 9% 1	1330
July	1st to 10th	Night	28% 9% 23% 30% 10% 1	2185
	11th to 20th	Night	19% 5% 15% 46% 14% 1	2178
	21st to end	Night	28% 6% 22% 31% 11% 1	2529
August	1st to 10th	Night	27% 5% 17% 35% 15% 1	2826
	11th to 20th	Night	31% 5% 18% 33% 12% 1	2825
	21st to end	Night	34% 4 17% 30% 14% 1	3135
September	1st to 10th	Night	27% 5% 15% 36% 15% 1	1852
	11th to 20th	Night	29% 6% 19% 32% 13% 1	2090
	21st to end	Night	35% 4 12% 33% 14% 1	2090
October	1st to 10th	Night	32% 3 12% 36% 15% 3	3726
	11th to 20th	Night	41% 3 11% 30% 15% 1	3779
	21st to end	Night	48% 4 11% 26% 11%	4421
November	1st to 10th	Night	52% 4 12% 21% 9% 2	2664
	11th to 20th	Night	60% 4 13% 16% 6% 1	2800
	21st to end	Night	55% 5% 17% 16% 6%	2799
December	1st to 10th	Night	62% 4 17% 14% 4	2818
	11th to 20th	Night	64% 4 14% 14% 3	3516
	21st to end	Night	66% 4 10% 15% 5%	5225

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% 8% 39% 28% 11% 3	270
	11th to 20th	Day	14% 8% 33% 32% 12%	270
	21st to end	Day	18% 7% 32% 29% 11% 2	297
February	1st to 10th	Day	12% 15% 39% 26% 6% 1	270
	11th to 20th	Day	10% 16% 40% 22% 11% 1	270
	21st to end	Day	9% 6% 44% 36% 6%	225
March	1st to 10th	Day	14% 12% 34% 30% 9% 1	270
	11th to 20th	Day	17% 10% 33% 24% 12% 4	270
	21st to end	Day	16% 12% 31% 31% 7% 3	297
April	1st to 10th	Day	19% 16% 31% 25% 9% 1	439
	11th to 20th	Day	24% 14% 28% 25% 8% 1	640
	21st to end	Day	24% 8% 31% 24% 12% 1	650
May	1st to 10th	Day	19% 12% 27% 29% 11% 2	653
	11th to 20th	Day	18% 9% 28% 28% 13% 3	675
	21st to end	Day	18% 7% 27% 28% 17% 3	734
June	1st to 10th	Day	13% 9% 27% 30% 16% 4	680
	11th to 20th	Day	15% 11% 34% 25% 13% 3	680
	21st to end	Day	13% 8% 29% 26% 18% 6%	680
July	1st to 10th	Day	8% 3 24% 33% 22% 9%	671
	11th to 20th	Day	11% 5% 28% 28% 22% 6%	668
	21st to end	Day	4 4 25% 36% 22% 8%	739
August	1st to 10th	Day	7% 5% 24% 33% 25% 5%	676
	11th to 20th	Day	13% 4 22% 32% 20% 9%	665
	21st to end	Day	12% 7% 27% 30% 20% 4	748
September	1st to 10th	Day	12% 8% 29% 33% 17% 2	533
	11th to 20th	Day	16% 6% 34% 33% 8% 3	300
	21st to end	Day	13% 11% 38% 23% 13% 3	300
October	1st to 10th	Day	18% 9% 28% 31% 13% 2	300
	11th to 20th	Day	17% 13% 34% 27% 8% 1	300
	21st to end	Day	19% 14% 35% 21% 8% 2	330
November	1st to 10th	Day	20% 10% 29% 29% 10% 2	300
	11th to 20th	Day	21% 13% 35% 24% 7%	300
	21st to end	Day	17% 13% 38% 27% 5%	299
December	1st to 10th	Day	16% 9% 37% 30% 7% 2	270
	11th to 20th	Day	16% 12% 38% 30% 4	270
	21st to end	Day	18% 8% 34% 27% 9% 2	297
January	1st to 10th	Night	10% 8% 37% 32% 11% 1	950
	11th to 20th	Night	15% 7% 35% 33% 8%	950
	21st to end	Night	15% 9% 36% 30% 9%	1045
February	1st to 10th	Night	13% 9% 32% 35% 10% 1	950
	11th to 20th	Night	12% 8% 38% 29% 10% 2	929
	21st to end	Night	12% 8% 38% 29% 11% 2	717
March	1st to 10th	Night	14% 8% 35% 30% 10% 2	679
	11th to 20th	Night	12% 8% 40% 32% 8% 1	570
	21st to end	Night	18% 8% 30% 30% 12% 2	627
April	1st to 10th	Night	17% 9% 34% 32% 8% 1	570
	11th to 20th	Night	22% 8% 32% 28% 9% 1	563
	21st to end	Night	21% 7% 28% 34% 11%	570
May	1st to 10th	Night	20% 6% 30% 34% 9% 1	570
	11th to 20th	Night	16% 6% 34% 32% 12% 1	561
	21st to end	Night	15% 6% 34% 32% 13%	617
June	1st to 10th	Night	12% 5% 30% 38% 14% 1	570
	11th to 20th	Night	12% 5% 32% 39% 12%	570
	21st to end	Night	8% 3 30% 41% 17% 1	570
July	1st to 10th	Night	6% 2 24% 45% 21% 2	555
	11th to 20th	Night	6% 3 29% 43% 18% 1	552
	21st to end	Night	6% 3 22% 47% 19% 2	624
August	1st to 10th	Night	7% 2 19% 49% 22% 1	567
	11th to 20th	Night	10% 4 23% 40% 21% 2	564
	21st to end	Night	11% 4 27% 38% 17% 2	627
September	1st to 10th	Night	11% 3 29% 39% 16% 2	570
	11th to 20th	Night	12% 4 31% 38% 14%	570
	21st to end	Night	14% 7% 32% 34% 12%	570
October	1st to 10th	Night	20% 8% 31% 32% 8%	712
	11th to 20th	Night	22% 9% 32% 29% 8%	760
	21st to end	Night	22% 10% 33% 28% 7%	1011
November	1st to 10th	Night	16% 8% 34% 32% 9%	975
	11th to 20th	Night	18% 9% 36% 29% 8% 1	1000
	21st to end	Night	17% 11% 40% 26% 6% 1	1000
December	1st to 10th	Night	14% 8% 35% 32% 10% 1	950
	11th to 20th	Night	17% 10% 36% 29% 8% 1	950
	21st to end	Night	23% 8% 31% 29% 8% 1	1045

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