Marble Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Marble Lake 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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	131261
	Night	15% 12% 4 9% 5% 5% 4 3 5% 6% 31%	129564
Transparency	Day	53% 3 11% 21% 11% 1	134778
	Night	53% 3 11% 21% 11% 1	134121
Seeing	Day	24% 15% 32% 19% 8% 2	15955
	Night	23% 12% 31% 24% 9% 1	25277

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	13% 15% 5% 13% 8% 5% 5% 4 6% 6% 21%	4661
	11th to 20th	Day	11% 21% 10% 16% 6% 6% 4 3 4 3 16%	4650
	21st to end	Day	10% 16% 7% 18% 7% 7% 6% 3 4 5% 18%	5641
February	1st to 10th	Day	13% 16% 6% 13% 6% 5% 4 4 5% 5% 22%	1890
	11th to 20th	Day	11% 16% 5% 11% 6% 5% 5% 3 4 5% 28%	1798
	21st to end	Day	18% 16% 4 9% 6% 5% 4 3 4 6% 24%	1599
March	1st to 10th	Day	18% 14% 5% 10% 5% 4 4 3 4 5% 28%	4677
	11th to 20th	Day	10% 15% 4 8% 6% 6% 6% 3 6% 7% 30%	4980
	21st to end	Day	17% 13% 4 7% 4 3 3 2 3 5% 38%	5638
April	1st to 10th	Day	20% 13% 3 8% 5% 4 4 3 4 6% 31%	2400
	11th to 20th	Day	20% 11% 3 7% 4 3 3 3 4 6% 36%	2374
	21st to end	Day	21% 10% 4 7% 6% 4 4 3 4 5% 32%	2555
May	1st to 10th	Day	22% 9% 3 8% 5% 4 4 4 5% 6% 30%	6286
	11th to 20th	Day	16% 9% 3 8% 5% 4 5% 4 5% 7% 34%	6318
	21st to end	Day	19% 9% 2 7% 5% 5% 5% 4 5% 7% 33%	6949
June	1st to 10th	Day	19% 10% 3 7% 5% 5% 5% 4 6% 6% 30%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 5% 7% 8% 36%	2800
	21st to end	Day	17% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	2800
July	1st to 10th	Day	13% 7% 2 7% 6% 5% 5% 4 6% 9% 35%	6360
	11th to 20th	Day	10% 6% 2 6% 4 5% 6% 4 6% 8% 42%	6360
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	6926
August	1st to 10th	Day	15% 8% 3 8% 4 5% 5% 4 6% 8% 35%	4303
	11th to 20th	Day	16% 7% 2 7% 5% 4 4 4 7% 8% 35%	4174
	21st to end	Day	13% 6% 2 6% 4 5% 5% 4 7% 9% 39%	4350
September	1st to 10th	Day	14% 6% 2 7% 5% 5% 4 4 5% 7% 41%	2400
	11th to 20th	Day	11% 6% 3 6% 4 4 4 3 5% 8% 47%	2400
	21st to end	Day	19% 6% 2 7% 4 5% 4 3 5% 6% 38%	2102
October	1st to 10th	Day	20% 8% 3 7% 5% 4 4 3 4 7% 35%	2000
	11th to 20th	Day	20% 10% 3 9% 6% 5% 5% 3 4 8% 27%	2000
	21st to end	Day	25% 11% 3 8% 5% 4 4 4 4 5% 25%	2119
November	1st to 10th	Day	15% 10% 4 8% 6% 5% 4 4 5% 7% 32%	1797
	11th to 20th	Day	15% 14% 5% 9% 6% 5% 4 3 4 5% 28%	1680
	21st to end	Day	21% 16% 5% 10% 6% 5% 4 3 4 5% 19%	1680
December	1st to 10th	Day	20% 20% 6% 11% 6% 5% 4 3 4 5% 16%	2680
	11th to 20th	Day	16% 19% 6% 12% 5% 4 3 2 4 4 23%	3066
	21st to end	Day	22% 19% 5% 11% 5% 3 3 2 3 5% 22%	4048
January	1st to 10th	Night	13% 14% 8% 15% 7% 6% 4 3 4 5% 20%	8234
	11th to 20th	Night	10% 19% 7% 15% 8% 6% 5% 3 5% 5% 16%	8235
	21st to end	Night	11% 15% 7% 13% 7% 6% 5% 4 4 5% 23%	8539
February	1st to 10th	Night	12% 16% 5% 9% 6% 5% 6% 4 6% 6% 24%	2730
	11th to 20th	Night	12% 13% 5% 11% 6% 5% 5% 4 5% 5% 28%	2599
	21st to end	Night	12% 15% 4 9% 6% 4 5% 4 6% 7% 28%	2030
March	1st to 10th	Night	18% 14% 4 9% 5% 5% 4 3 4 4 29%	5148
	11th to 20th	Night	10% 12% 4 8% 5% 5% 6% 5% 8% 9% 29%	4979
	21st to end	Night	18% 10% 3 8% 5% 4 4 3 5% 6% 34%	5294
April	1st to 10th	Night	19% 12% 3 7% 5% 4 4 3 5% 6% 32%	2000
	11th to 20th	Night	18% 9% 3 7% 5% 3 4 3 4 7% 37%	1975
	21st to end	Night	18% 9% 3 8% 4 4 4 4 5% 6% 34%	1860
May	1st to 10th	Night	24% 9% 3 6% 4 5% 5% 4 5% 7% 29%	3652
	11th to 20th	Night	15% 7% 3 7% 4 4 4 2 5% 7% 42%	3575
	21st to end	Night	18% 8% 2 6% 4 4 5% 4 5% 6% 36%	3926
June	1st to 10th	Night	14% 9% 2 7% 5% 6% 5% 4 7% 8% 34%	1600
	11th to 20th	Night	12% 5% 2 6% 4 4 5% 4 5% 9% 43%	1600
	21st to end	Night	16% 7% 3 7% 5% 4 4 4 5% 8% 37%	1600
July	1st to 10th	Night	11% 7% 2 7% 5% 5% 5% 4 7% 8% 38%	3600
	11th to 20th	Night	12% 6% 3 5% 4 4 3 3 5% 7% 48%	3600
	21st to end	Night	9% 5% 2 5% 4 4 4 3 6% 7% 50%	3926
August	1st to 10th	Night	12% 6% 2 6% 5% 4 3 3 6% 8% 45%	2842
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 44%	2875
	21st to end	Night	11% 5% 2 6% 4 4 5% 5% 6% 9% 44%	3515
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 6% 8% 47%	2000
	11th to 20th	Night	11% 4 1 5% 4 4 4 5% 5% 6% 51%	2000
	21st to end	Night	18% 6% 2 6% 4 3 3 4 6% 5% 43%	2292
October	1st to 10th	Night	16% 8% 2 7% 4 4 4 4 5% 6% 40%	2400
	11th to 20th	Night	20% 10% 3 8% 5% 4 4 3 6% 6% 32%	2400
	21st to end	Night	21% 11% 3 8% 5% 5% 4 3 5% 7% 29%	2720
November	1st to 10th	Night	15% 12% 4 7% 4 4 5% 4 5% 6% 34%	2720
	11th to 20th	Night	17% 12% 4 8% 5% 4 5% 4 5% 6% 30%	2940
	21st to end	Night	20% 14% 4 9% 5% 5% 5% 4 5% 6% 23%	2940
December	1st to 10th	Night	18% 18% 5% 11% 6% 5% 5% 4 6% 7% 17%	4720
	11th to 20th	Night	20% 16% 6% 10% 5% 4 4 3 4 5% 23%	5348
	21st to end	Night	24% 18% 4 11% 6% 4 3 2 3 4 19%	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	64% 2 10% 17% 6% 1	4662
	11th to 20th	Day	73% 2 7% 10% 8% 1	4680
	21st to end	Day	69% 2 7% 12% 8% 1	5641
February	1st to 10th	Day	63% 2 8% 19% 7% 1	1890
	11th to 20th	Day	60% 2 8% 18% 11% 1	1807
	21st to end	Day	64% 2 9% 13% 11% 1	1599
March	1st to 10th	Day	60% 2 10% 12% 15% 2	4680
	11th to 20th	Day	55% 2 15% 15% 11% 2	4980
	21st to end	Day	52% 1 5% 21% 18% 3	5638
April	1st to 10th	Day	56% 2 6% 18% 14% 3	2400
	11th to 20th	Day	51% 2 7% 21% 17% 1	2374
	21st to end	Day	56% 2 7% 17% 15% 3	2555
May	1st to 10th	Day	56% 2 7% 20% 11% 3	6297
	11th to 20th	Day	52% 2 10% 19% 16% 1	6322
	21st to end	Day	51% 2 11% 22% 12% 2	6949
June	1st to 10th	Day	54% 2 8% 20% 14% 1	2800
	11th to 20th	Day	44% 4 11% 25% 13% 2	2800
	21st to end	Day	49% 2 12% 21% 13% 1	2800
July	1st to 10th	Day	45% 4 13% 26% 12% 1	6290
	11th to 20th	Day	38% 4 13% 29% 16%	6285
	21st to end	Day	35% 4 12% 37% 11%	6728
August	1st to 10th	Day	47% 3 14% 24% 11%	4289
	11th to 20th	Day	46% 3 13% 29% 8% 1	4213
	21st to end	Day	40% 4 12% 33% 11% 1	4350
September	1st to 10th	Day	42% 3 12% 30% 13% 1	2400
	11th to 20th	Day	37% 2 12% 33% 16% 1	2400
	21st to end	Day	48% 2 12% 26% 12%	2102
October	1st to 10th	Day	44% 4 11% 31% 10%	3260
	11th to 20th	Day	49% 2 13% 23% 12% 1	3240
	21st to end	Day	55% 3 14% 21% 7%	3396
November	1st to 10th	Day	51% 3 13% 24% 8% 1	1797
	11th to 20th	Day	58% 3 11% 16% 10% 1	1680
	21st to end	Day	68% 3 10% 11% 8% 1	1680
December	1st to 10th	Day	71% 2 13% 10% 3	2680
	11th to 20th	Day	67% 2 10% 13% 7%	3066
	21st to end	Day	68% 2 7% 16% 8%	4048
January	1st to 10th	Night	68% 2 9% 14% 7%	8234
	11th to 20th	Night	70% 2 9% 12% 7%	8280
	21st to end	Night	63% 2 8% 13% 13% 1	8539
February	1st to 10th	Night	59% 3 9% 21% 7% 1	2730
	11th to 20th	Night	58% 2 9% 20% 10% 1	2601
	21st to end	Night	56% 3 9% 21% 10% 1	2030
March	1st to 10th	Night	60% 2 6% 17% 12% 4	5158
	11th to 20th	Night	51% 3 16% 19% 9% 2	4980
	21st to end	Night	52% 2 7% 21% 16% 2	5294
April	1st to 10th	Night	53% 2 7% 21% 14% 2	2000
	11th to 20th	Night	49% 2 7% 23% 17% 1	1975
	21st to end	Night	51% 3 8% 18% 17% 3	1860
May	1st to 10th	Night	56% 2 8% 19% 11% 4	3658
	11th to 20th	Night	44% 2 13% 24% 15% 2	3576
	21st to end	Night	49% 2 12% 21% 15% 1	3927
June	1st to 10th	Night	48% 3 11% 24% 14% 1	1600
	11th to 20th	Night	39% 3 13% 26% 18% 2	1600
	21st to end	Night	46% 3 13% 24% 13% 1	1600
July	1st to 10th	Night	43% 3 16% 21% 16% 1	3560
	11th to 20th	Night	36% 4 12% 34% 14%	3552
	21st to end	Night	33% 4 12% 36% 14%	3809
August	1st to 10th	Night	38% 3 14% 28% 14% 2	2832
	11th to 20th	Night	39% 3 16% 31% 10% 1	2902
	21st to end	Night	36% 5% 15% 31% 11% 1	3515
September	1st to 10th	Night	34% 4 15% 31% 15% 1	2000
	11th to 20th	Night	34% 3 13% 37% 13% 1	2000
	21st to end	Night	43% 3 11% 30% 12% 1	2292
October	1st to 10th	Night	39% 3 11% 29% 18%	3920
	11th to 20th	Night	45% 2 16% 25% 11%	3899
	21st to end	Night	49% 4 14% 25% 8%	4380
November	1st to 10th	Night	50% 3 13% 24% 10% 1	2720
	11th to 20th	Night	55% 3 11% 20% 10% 1	2940
	21st to end	Night	62% 3 12% 18% 5%	2940
December	1st to 10th	Night	67% 3 14% 13% 3	4720
	11th to 20th	Night	64% 3 8% 15% 9%	5348
	21st to end	Night	70% 2 6% 13% 9%	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	23% 16% 38% 19% 4	220
	11th to 20th	Day	32% 15% 34% 15% 4	220
	21st to end	Day	27% 21% 36% 13% 3 1	338
February	1st to 10th	Day	30% 25% 29% 13% 3	330
	11th to 20th	Day	31% 23% 37% 7% 1	315
	21st to end	Day	36% 16% 36% 11% 1	363
March	1st to 10th	Day	27% 21% 32% 16% 3	500
	11th to 20th	Day	28% 19% 34% 16% 3 1	500
	21st to end	Day	35% 17% 29% 12% 6%	550
April	1st to 10th	Day	29% 23% 29% 14% 5%	500
	11th to 20th	Day	33% 20% 25% 14% 7% 2	492
	21st to end	Day	29% 18% 34% 11% 7% 1	500
May	1st to 10th	Day	27% 15% 32% 18% 6% 3	503
	11th to 20th	Day	19% 13% 37% 21% 7% 3	527
	21st to end	Day	20% 14% 33% 21% 9% 3	573
June	1st to 10th	Day	24% 12% 35% 22% 7% 1	530
	11th to 20th	Day	15% 16% 32% 24% 10% 2	530
	21st to end	Day	20% 13% 34% 21% 9% 2	530
July	1st to 10th	Day	13% 9% 31% 30% 14% 3	525
	11th to 20th	Day	14% 9% 32% 29% 13% 3	524
	21st to end	Day	13% 11% 35% 28% 10% 3	576
August	1st to 10th	Day	15% 6% 29% 33% 15% 4	526
	11th to 20th	Day	16% 5% 26% 30% 18% 6%	519
	21st to end	Day	15% 8% 33% 26% 12% 5%	583
September	1st to 10th	Day	17% 11% 28% 24% 14% 6%	530
	11th to 20th	Day	12% 7% 25% 26% 18% 12%	530
	21st to end	Day	22% 10% 29% 21% 13% 5%	530
October	1st to 10th	Day	26% 13% 28% 16% 13% 4	530
	11th to 20th	Day	26% 19% 31% 18% 4 2	530
	21st to end	Day	31% 16% 31% 18% 4	538
November	1st to 10th	Day	30% 16% 30% 18% 5%	371
	11th to 20th	Day	29% 16% 38% 10% 5% 1	220
	21st to end	Day	33% 21% 30% 12% 4	220
December	1st to 10th	Day	31% 16% 39% 10% 3	220
	11th to 20th	Day	36% 20% 31% 11% 1 1	220
	21st to end	Day	34% 19% 30% 12% 5%	242
January	1st to 10th	Night	27% 16% 35% 18% 4	840
	11th to 20th	Night	29% 16% 30% 19% 5% 1	840
	21st to end	Night	22% 16% 34% 22% 6% 1	924
February	1st to 10th	Night	23% 18% 37% 20% 3	840
	11th to 20th	Night	21% 17% 42% 17% 3	804
	21st to end	Night	23% 18% 36% 20% 3	660
March	1st to 10th	Night	23% 17% 36% 21% 3	800
	11th to 20th	Night	26% 16% 33% 21% 4 1	800
	21st to end	Night	24% 13% 35% 19% 8% 1	880
April	1st to 10th	Night	27% 14% 33% 22% 4	800
	11th to 20th	Night	25% 18% 30% 19% 7% 1	791
	21st to end	Night	24% 12% 29% 26% 9% 1	660
May	1st to 10th	Night	25% 8% 25% 32% 8% 2	432
	11th to 20th	Night	18% 10% 32% 31% 9%	394
	21st to end	Night	15% 8% 35% 29% 13% 1	433
June	1st to 10th	Night	19% 6% 31% 35% 9% 1	400
	11th to 20th	Night	14% 10% 31% 32% 12% 1	400
	21st to end	Night	19% 7% 27% 36% 11%	400
July	1st to 10th	Night	8% 5% 32% 38% 15% 1	390
	11th to 20th	Night	13% 3 28% 39% 16% 1	388
	21st to end	Night	10% 5% 29% 42% 13% 1	438
August	1st to 10th	Night	12% 3 24% 37% 22% 2	550
	11th to 20th	Night	13% 3 23% 36% 23% 3	594
	21st to end	Night	14% 6% 28% 32% 18% 2	835
September	1st to 10th	Night	12% 6% 33% 28% 19% 2	800
	11th to 20th	Night	14% 5% 22% 29% 25% 5%	800
	21st to end	Night	22% 9% 23% 27% 18% 3	800
October	1st to 10th	Night	22% 10% 28% 24% 15% 2	800
	11th to 20th	Night	28% 12% 30% 22% 8% 1	800
	21st to end	Night	28% 13% 31% 23% 5%	880
November	1st to 10th	Night	26% 14% 30% 23% 7% 1	820
	11th to 20th	Night	26% 15% 33% 18% 8% 1	840
	21st to end	Night	32% 17% 26% 20% 5%	840
December	1st to 10th	Night	32% 15% 31% 18% 4	840
	11th to 20th	Night	28% 18% 34% 18% 2 1	840
	21st to end	Night	35% 14% 32% 16% 4	924

Page generated at 2026-02-13 01:42:17. Version ac07

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