Split Rock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Split Rock Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	16% 11% 4 9% 5% 5% 4 4 5% 6% 31%	131880
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 31%	130267
Transparency	Day	51% 5% 11% 20% 11% 1	135397
	Night	52% 5% 11% 19% 11% 1	134837
Seeing	Day	23% 18% 31% 18% 8% 2	16195
	Night	22% 14% 32% 23% 8% 1	25509

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% 6% 4 5% 6% 20%	4661
	11th to 20th	Day	11% 22% 9% 14% 6% 6% 4 3 3 4 17%	4650
	21st to end	Day	10% 16% 7% 17% 7% 7% 5% 3 4 5% 18%	5598
February	1st to 10th	Day	13% 16% 5% 12% 6% 5% 4 4 6% 5% 22%	1890
	11th to 20th	Day	12% 16% 5% 10% 7% 4 4 4 5% 5% 28%	1843
	21st to end	Day	19% 15% 4 10% 6% 6% 4 3 4 5% 24%	1687
March	1st to 10th	Day	18% 14% 5% 10% 5% 4 4 3 5% 5% 28%	4753
	11th to 20th	Day	11% 14% 4 9% 5% 5% 6% 4 5% 7% 30%	5090
	21st to end	Day	18% 12% 4 7% 4 3 3 3 3 4 38%	5773
April	1st to 10th	Day	20% 13% 4 7% 5% 4 4 3 4 6% 30%	2520
	11th to 20th	Day	22% 11% 3 7% 3 4 3 2 4 5% 35%	2446
	21st to end	Day	21% 10% 4 7% 5% 5% 4 3 4 5% 32%	2544
May	1st to 10th	Day	23% 8% 3 8% 5% 4 4 4 5% 6% 29%	6300
	11th to 20th	Day	17% 9% 3 8% 5% 5% 4 4 5% 7% 33%	6318
	21st to end	Day	19% 9% 3 6% 5% 5% 4 4 6% 7% 33%	6949
June	1st to 10th	Day	19% 9% 2 8% 5% 5% 5% 4 6% 6% 30%	2800
	11th to 20th	Day	14% 6% 3 5% 5% 5% 5% 5% 7% 8% 37%	2800
	21st to end	Day	17% 7% 3 8% 5% 5% 5% 4 6% 7% 34%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 6% 5% 4 6% 9% 35%	6360
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 9% 41%	6360
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 7% 9% 44%	6926
August	1st to 10th	Day	15% 8% 3 8% 5% 4 5% 4 5% 7% 36%	4335
	11th to 20th	Day	17% 7% 3 7% 4 5% 4 4 6% 8% 35%	4174
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	4327
September	1st to 10th	Day	14% 7% 2 7% 5% 4 4 4 5% 7% 41%	2400
	11th to 20th	Day	11% 5% 3 6% 4 4 4 4 5% 7% 48%	2400
	21st to end	Day	19% 6% 3 6% 5% 4 4 4 4 7% 38%	2108
October	1st to 10th	Day	20% 8% 3 7% 4 3 3 3 5% 6% 36%	2000
	11th to 20th	Day	21% 10% 2 9% 5% 6% 4 3 5% 7% 29%	2000
	21st to end	Day	25% 11% 3 8% 5% 5% 5% 3 4 6% 25%	2105
November	1st to 10th	Day	15% 9% 3 9% 5% 5% 5% 5% 5% 6% 33%	1809
	11th to 20th	Day	16% 14% 6% 8% 5% 6% 4 4 4 5% 28%	1680
	21st to end	Day	21% 16% 6% 11% 5% 5% 4 3 4 6% 19%	1680
December	1st to 10th	Day	21% 21% 5% 11% 7% 5% 4 3 4 4 17%	2680
	11th to 20th	Day	17% 19% 6% 13% 5% 4 3 3 4 4 23%	3066
	21st to end	Day	23% 19% 5% 11% 5% 3 3 2 3 5% 22%	4048
January	1st to 10th	Night	14% 14% 9% 15% 7% 6% 4 3 4 5% 20%	8234
	11th to 20th	Night	11% 19% 7% 15% 8% 6% 5% 3 4 5% 17%	8235
	21st to end	Night	11% 15% 7% 13% 6% 6% 5% 3 5% 5% 23%	8581
February	1st to 10th	Night	12% 16% 5% 10% 6% 6% 5% 4 6% 7% 24%	2730
	11th to 20th	Night	13% 13% 6% 10% 6% 5% 5% 3 6% 5% 29%	2663
	21st to end	Night	12% 15% 5% 8% 6% 5% 5% 5% 6% 7% 27%	2118
March	1st to 10th	Night	20% 14% 4 9% 5% 5% 4 2 3 5% 29%	5304
	11th to 20th	Night	11% 13% 3 8% 5% 5% 5% 5% 7% 8% 29%	5089
	21st to end	Night	18% 10% 4 7% 5% 4 4 3 5% 5% 35%	5401
April	1st to 10th	Night	20% 11% 4 7% 5% 3 4 4 4 6% 32%	2100
	11th to 20th	Night	19% 11% 3 5% 6% 5% 3 3 4 5% 37%	2036
	21st to end	Night	19% 9% 3 7% 4 4 4 5% 5% 6% 34%	1873
May	1st to 10th	Night	24% 8% 3 7% 4 4 4 4 6% 7% 29%	3626
	11th to 20th	Night	15% 7% 3 7% 4 3 3 3 5% 7% 42%	3575
	21st to end	Night	18% 8% 2 6% 4 4 5% 4 5% 7% 35%	3926
June	1st to 10th	Night	14% 7% 2 7% 6% 5% 5% 3 7% 8% 35%	1600
	11th to 20th	Night	12% 7% 3 6% 4 4 4 4 6% 7% 43%	1600
	21st to end	Night	17% 7% 3 7% 4 4 5% 3 7% 7% 36%	1600
July	1st to 10th	Night	12% 6% 3 7% 6% 6% 5% 4 6% 8% 37%	3600
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 7% 47%	3600
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 7% 50%	3926
August	1st to 10th	Night	12% 6% 2 6% 4 4 3 4 6% 8% 45%	2811
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 8% 45%	2875
	21st to end	Night	12% 5% 2 6% 4 4 5% 4 7% 8% 44%	3538
September	1st to 10th	Night	11% 5% 2 5% 5% 3 4 4 5% 7% 49%	2000
	11th to 20th	Night	12% 4 2 4 4 4 4 4 5% 5% 52%	2000
	21st to end	Night	18% 6% 2 6% 4 3 4 3 5% 6% 43%	2286
October	1st to 10th	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	2400
	11th to 20th	Night	20% 11% 3 7% 5% 4 5% 3 5% 6% 32%	2400
	21st to end	Night	22% 12% 3 7% 5% 4 4 3 5% 6% 30%	2734
November	1st to 10th	Night	16% 11% 3 6% 4 4 5% 4 5% 6% 35%	2708
	11th to 20th	Night	17% 13% 4 8% 5% 4 5% 4 5% 6% 30%	2940
	21st to end	Night	20% 15% 5% 8% 5% 5% 5% 3 5% 5% 23%	2940
December	1st to 10th	Night	19% 18% 5% 11% 6% 5% 5% 4 5% 6% 17%	4720
	11th to 20th	Night	21% 16% 5% 10% 6% 4 4 3 4 4 24%	5348
	21st to end	Night	25% 18% 4 11% 6% 4 3 2 3 4 20%	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	63% 5% 10% 15% 6% 1	4662
	11th to 20th	Day	71% 4 7% 9% 8% 1	4680
	21st to end	Day	68% 4 7% 11% 8% 1	5598
February	1st to 10th	Day	62% 6% 8% 16% 6% 1	1890
	11th to 20th	Day	59% 4 11% 16% 10% 1	1852
	21st to end	Day	62% 5% 10% 12% 10% 1	1687
March	1st to 10th	Day	58% 4 10% 11% 14% 2	4756
	11th to 20th	Day	53% 5% 15% 15% 10% 2	5090
	21st to end	Day	50% 4 6% 20% 17% 3	5773
April	1st to 10th	Day	55% 4 7% 17% 13% 3	2520
	11th to 20th	Day	51% 5% 7% 20% 16% 2	2446
	21st to end	Day	54% 5% 8% 16% 14% 3	2544
May	1st to 10th	Day	54% 6% 9% 18% 10% 3	6311
	11th to 20th	Day	49% 6% 11% 17% 15% 1	6322
	21st to end	Day	49% 6% 12% 21% 11% 2	6949
June	1st to 10th	Day	52% 7% 9% 18% 12% 1	2800
	11th to 20th	Day	42% 7% 13% 23% 13% 2	2800
	21st to end	Day	48% 6% 13% 20% 12% 1	2800
July	1st to 10th	Day	44% 6% 13% 25% 11% 1	6290
	11th to 20th	Day	36% 7% 13% 27% 16%	6285
	21st to end	Day	34% 7% 12% 36% 11%	6728
August	1st to 10th	Day	46% 6% 14% 23% 11%	4321
	11th to 20th	Day	45% 6% 13% 27% 8% 1	4213
	21st to end	Day	40% 6% 13% 30% 10% 1	4327
September	1st to 10th	Day	41% 6% 12% 29% 12% 1	2400
	11th to 20th	Day	35% 6% 13% 31% 15%	2400
	21st to end	Day	46% 6% 12% 26% 10%	2108
October	1st to 10th	Day	44% 5% 11% 30% 9%	3260
	11th to 20th	Day	47% 5% 13% 22% 12% 1	3240
	21st to end	Day	54% 5% 14% 20% 7%	3382
November	1st to 10th	Day	49% 6% 16% 20% 8% 1	1809
	11th to 20th	Day	57% 5% 12% 16% 9% 1	1680
	21st to end	Day	66% 6% 10% 10% 7%	1680
December	1st to 10th	Day	70% 5% 13% 8% 3	2680
	11th to 20th	Day	65% 4 10% 13% 6%	3066
	21st to end	Day	67% 3 7% 15% 8%	4048
January	1st to 10th	Night	67% 5% 10% 12% 6%	8234
	11th to 20th	Night	68% 4 10% 10% 7%	8280
	21st to end	Night	63% 5% 7% 12% 13% 1	8581
February	1st to 10th	Night	57% 6% 12% 17% 7% 1	2730
	11th to 20th	Night	56% 5% 11% 19% 8% 1	2665
	21st to end	Night	55% 6% 11% 18% 10% 1	2118
March	1st to 10th	Night	59% 4 7% 15% 11% 4	5314
	11th to 20th	Night	48% 7% 17% 17% 8% 2	5090
	21st to end	Night	50% 5% 8% 21% 14% 2	5401
April	1st to 10th	Night	52% 4 8% 20% 13% 2	2100
	11th to 20th	Night	49% 5% 8% 21% 16% 1	2036
	21st to end	Night	50% 7% 8% 17% 16% 3	1873
May	1st to 10th	Night	54% 5% 9% 18% 10% 3	3632
	11th to 20th	Night	41% 6% 14% 23% 14% 2	3576
	21st to end	Night	47% 6% 13% 18% 14% 1	3927
June	1st to 10th	Night	46% 7% 12% 21% 13% 1	1600
	11th to 20th	Night	37% 7% 13% 25% 17% 2	1600
	21st to end	Night	45% 6% 15% 21% 13% 1	1600
July	1st to 10th	Night	43% 6% 15% 20% 15% 1	3560
	11th to 20th	Night	35% 6% 12% 33% 14%	3552
	21st to end	Night	33% 6% 12% 35% 14%	3809
August	1st to 10th	Night	37% 6% 15% 27% 13% 2	2801
	11th to 20th	Night	38% 6% 15% 31% 10% 1	2902
	21st to end	Night	35% 7% 16% 30% 11% 1	3538
September	1st to 10th	Night	34% 7% 15% 30% 14% 1	2000
	11th to 20th	Night	32% 5% 14% 36% 12% 1	2000
	21st to end	Night	41% 6% 11% 29% 11% 1	2286
October	1st to 10th	Night	39% 4 11% 28% 17%	3920
	11th to 20th	Night	43% 5% 17% 24% 10%	3899
	21st to end	Night	49% 6% 15% 23% 8%	4407
November	1st to 10th	Night	48% 6% 14% 22% 10% 1	2708
	11th to 20th	Night	53% 6% 12% 19% 9% 1	2940
	21st to end	Night	60% 7% 12% 15% 5%	2940
December	1st to 10th	Night	66% 6% 13% 11% 3	4720
	11th to 20th	Night	64% 4 8% 14% 9%	5348
	21st to end	Night	69% 4 6% 11% 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% 21% 38% 15% 3	220
	11th to 20th	Day	35% 16% 33% 10% 5%	220
	21st to end	Day	25% 24% 35% 12% 3 1	330
February	1st to 10th	Day	32% 25% 29% 12% 2	330
	11th to 20th	Day	28% 28% 38% 6%	330
	21st to end	Day	35% 19% 35% 10% 1	403
March	1st to 10th	Day	26% 24% 33% 14% 3	540
	11th to 20th	Day	28% 22% 32% 14% 3 1	540
	21st to end	Day	34% 21% 28% 12% 5% 1	594
April	1st to 10th	Day	31% 25% 27% 13% 4	540
	11th to 20th	Day	33% 23% 23% 14% 6% 1	516
	21st to end	Day	30% 22% 31% 11% 7% 1	500
May	1st to 10th	Day	25% 18% 31% 17% 5% 3	503
	11th to 20th	Day	16% 18% 37% 19% 8% 2	527
	21st to end	Day	20% 15% 34% 19% 9% 3	573
June	1st to 10th	Day	24% 15% 35% 19% 6% 1	530
	11th to 20th	Day	14% 18% 34% 23% 10% 2	530
	21st to end	Day	19% 15% 36% 18% 8% 2	530
July	1st to 10th	Day	14% 9% 32% 30% 12% 3	525
	11th to 20th	Day	13% 11% 33% 26% 14% 2	524
	21st to end	Day	13% 12% 36% 26% 10% 3	576
August	1st to 10th	Day	15% 8% 30% 31% 14% 3	526
	11th to 20th	Day	15% 7% 28% 29% 16% 5%	519
	21st to end	Day	14% 11% 34% 24% 12% 5%	583
September	1st to 10th	Day	16% 12% 31% 23% 12% 6%	530
	11th to 20th	Day	12% 11% 23% 26% 17% 11%	530
	21st to end	Day	19% 13% 31% 21% 12% 5%	530
October	1st to 10th	Day	23% 19% 26% 16% 13% 3	530
	11th to 20th	Day	24% 23% 32% 16% 4 1	530
	21st to end	Day	30% 21% 27% 18% 3	531
November	1st to 10th	Day	27% 19% 31% 17% 6%	383
	11th to 20th	Day	27% 24% 33% 10% 5% 1	220
	21st to end	Day	34% 22% 30% 10% 3	220
December	1st to 10th	Day	31% 20% 36% 10% 3	220
	11th to 20th	Day	34% 25% 29% 9% 2 1	220
	21st to end	Day	33% 22% 31% 10% 4	242
January	1st to 10th	Night	26% 19% 34% 17% 3	840
	11th to 20th	Night	30% 17% 33% 16% 4	840
	21st to end	Night	21% 18% 35% 22% 5%	924
February	1st to 10th	Night	21% 20% 37% 20% 2	840
	11th to 20th	Night	22% 20% 41% 15% 3	823
	21st to end	Night	22% 22% 35% 19% 2	692
March	1st to 10th	Night	24% 19% 35% 20% 2	840
	11th to 20th	Night	25% 20% 30% 20% 3 1	840
	21st to end	Night	23% 17% 35% 18% 7% 1	924
April	1st to 10th	Night	26% 17% 34% 20% 3	840
	11th to 20th	Night	24% 21% 30% 19% 6% 1	816
	21st to end	Night	22% 15% 30% 24% 8%	673
May	1st to 10th	Night	25% 9% 27% 29% 9% 1	419
	11th to 20th	Night	15% 11% 35% 31% 7%	394
	21st to end	Night	14% 8% 37% 28% 11%	433
June	1st to 10th	Night	17% 9% 35% 32% 8%	400
	11th to 20th	Night	13% 13% 32% 31% 10% 1	400
	21st to end	Night	19% 7% 29% 35% 10%	400
July	1st to 10th	Night	7% 7% 33% 38% 14% 1	390
	11th to 20th	Night	10% 6% 29% 39% 14% 1	388
	21st to end	Night	10% 5% 32% 42% 11% 1	438
August	1st to 10th	Night	13% 3 27% 36% 20% 1	532
	11th to 20th	Night	13% 3 24% 36% 22% 2	594
	21st to end	Night	13% 6% 31% 30% 17% 2	845
September	1st to 10th	Night	13% 7% 33% 28% 17% 2	800
	11th to 20th	Night	14% 6% 22% 30% 24% 4	800
	21st to end	Night	21% 11% 23% 26% 17% 3	800
October	1st to 10th	Night	21% 11% 30% 23% 14% 2	800
	11th to 20th	Night	25% 16% 30% 20% 8% 1	800
	21st to end	Night	27% 16% 31% 21% 5%	880
November	1st to 10th	Night	25% 17% 31% 21% 6%	820
	11th to 20th	Night	25% 17% 33% 17% 7% 1	840
	21st to end	Night	30% 19% 27% 19% 5%	840
December	1st to 10th	Night	29% 19% 31% 18% 3	840
	11th to 20th	Night	27% 20% 34% 18% 1	840
	21st to end	Night	34% 15% 32% 15% 3	924

Page generated at 2026-04-14 02:44:28. Version ac07

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