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 2024-07-16.

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	17% 11% 4 8% 5% 5% 4 4 5% 6% 32%	89840
	Night	17% 12% 4 8% 5% 4 4 3 5% 6% 31%	89657
Transparency	Day	52% 6% 12% 19% 10% 1	89559
	Night	52% 6% 12% 19% 10% 1	89492
Seeing	Day	23% 17% 31% 18% 8% 2	14026
	Night	22% 14% 32% 23% 8% 1	23221

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	15% 18% 6% 14% 6% 4 4 3 4 5% 21%	2517
	11th to 20th	Day	13% 18% 8% 14% 6% 7% 4 4 4 4 20%	2520
	21st to end	Day	18% 20% 7% 14% 6% 5% 3 2 3 4 17%	3026
February	1st to 10th	Day	14% 17% 5% 12% 7% 5% 5% 4 6% 5% 21%	1710
	11th to 20th	Day	12% 16% 5% 10% 6% 4 4 4 4 5% 29%	1663
	21st to end	Day	19% 15% 4 9% 5% 5% 4 4 4 5% 25%	1531
March	1st to 10th	Day	19% 15% 5% 9% 5% 4 3 3 4 5% 28%	3234
	11th to 20th	Day	10% 17% 4 9% 5% 5% 5% 3 4 5% 30%	3480
	21st to end	Day	13% 9% 4 6% 4 3 3 3 4 5% 44%	3942
April	1st to 10th	Day	20% 13% 4 7% 5% 4 4 3 5% 6% 30%	2280
	11th to 20th	Day	21% 10% 3 6% 4 4 4 2 5% 5% 36%	2254
	21st to end	Day	22% 10% 4 8% 5% 5% 4 3 4 5% 31%	2417
May	1st to 10th	Day	21% 7% 2 8% 5% 4 5% 4 5% 6% 33%	4394
	11th to 20th	Day	17% 10% 3 8% 5% 4 4 4 5% 8% 33%	4398
	21st to end	Day	15% 6% 2 6% 5% 5% 4 4 6% 8% 40%	4837
June	1st to 10th	Day	19% 9% 2 8% 5% 5% 5% 4 6% 6% 29%	2660
	11th to 20th	Day	15% 6% 3 5% 5% 5% 5% 5% 8% 8% 36%	2660
	21st to end	Day	17% 7% 3 8% 5% 5% 5% 4 6% 7% 33%	2660
July	1st to 10th	Day	11% 7% 2 7% 6% 6% 5% 4 6% 8% 38%	2660
	11th to 20th	Day	12% 7% 3 6% 5% 6% 5% 4 7% 9% 37%	2593
	21st to end	Day	12% 6% 2 6% 5% 6% 6% 5% 7% 8% 36%	2744
August	1st to 10th	Day	12% 5% 3 8% 6% 5% 5% 5% 6% 8% 37%	2383
	11th to 20th	Day	13% 7% 2 7% 5% 6% 5% 4 6% 8% 37%	2303
	21st to end	Day	15% 6% 2 7% 5% 6% 4 5% 5% 7% 37%	2390
September	1st to 10th	Day	14% 7% 2 7% 5% 4 4 4 5% 7% 40%	2160
	11th to 20th	Day	13% 6% 3 6% 4 4 3 4 5% 7% 46%	2160
	21st to end	Day	18% 7% 3 6% 5% 4 4 4 4 7% 38%	1898
October	1st to 10th	Day	21% 9% 3 8% 5% 3 3 3 5% 6% 35%	1800
	11th to 20th	Day	23% 10% 2 8% 5% 6% 4 3 5% 7% 26%	1800
	21st to end	Day	26% 12% 4 8% 5% 5% 5% 2 4 6% 24%	1895
November	1st to 10th	Day	14% 9% 3 8% 4 5% 5% 5% 5% 7% 34%	1633
	11th to 20th	Day	17% 14% 5% 8% 5% 6% 4 3 4 5% 28%	1520
	21st to end	Day	21% 16% 5% 11% 5% 6% 4 3 4 6% 19%	1520
December	1st to 10th	Day	21% 19% 5% 10% 8% 5% 4 3 4 4 17%	1520
	11th to 20th	Day	19% 14% 6% 10% 5% 4 4 3 5% 5% 25%	1906
	21st to end	Day	26% 20% 4 13% 5% 3 3 2 3 3 19%	2772
January	1st to 10th	Night	17% 15% 9% 13% 6% 5% 4 3 5% 5% 19%	4429
	11th to 20th	Night	12% 15% 5% 13% 7% 6% 6% 4 5% 6% 20%	4440
	21st to end	Night	18% 17% 7% 12% 6% 6% 4 2 3 4 19%	4628
February	1st to 10th	Night	13% 17% 5% 10% 6% 6% 5% 4 6% 7% 23%	2470
	11th to 20th	Night	13% 13% 6% 9% 6% 5% 5% 3 5% 5% 30%	2403
	21st to end	Night	13% 14% 5% 7% 6% 5% 5% 4 5% 7% 28%	1922
March	1st to 10th	Night	19% 13% 4 9% 5% 4 4 2 4 5% 31%	3604
	11th to 20th	Night	15% 16% 4 9% 5% 5% 4 4 6% 6% 27%	3479
	21st to end	Night	14% 10% 4 7% 5% 4 3 3 5% 5% 40%	3690
April	1st to 10th	Night	20% 11% 4 6% 5% 3 4 4 4 6% 34%	1900
	11th to 20th	Night	18% 10% 3 5% 5% 5% 3 3 4 6% 38%	1875
	21st to end	Night	19% 9% 3 7% 4 4 4 5% 5% 6% 34%	1780
May	1st to 10th	Night	21% 8% 3 7% 4 4 4 3 5% 7% 33%	2532
	11th to 20th	Night	18% 8% 3 7% 4 4 4 3 6% 7% 37%	2495
	21st to end	Night	13% 6% 2 6% 4 4 5% 4 5% 8% 43%	2738
June	1st to 10th	Night	15% 7% 2 7% 6% 5% 5% 3 7% 8% 34%	1520
	11th to 20th	Night	12% 6% 3 6% 4 3 4 4 6% 7% 43%	1520
	21st to end	Night	17% 7% 3 7% 4 4 5% 3 7% 7% 36%	1520
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 4 5% 7% 47%	1520
	11th to 20th	Night	11% 5% 2 6% 4 5% 5% 3 7% 9% 43%	1488
	21st to end	Night	9% 5% 2 4 4 4 4 5% 7% 8% 47%	1574
August	1st to 10th	Night	10% 6% 3 6% 4 4 3 4 7% 7% 47%	1552
	11th to 20th	Night	9% 7% 2 7% 5% 4 5% 4 6% 8% 44%	1598
	21st to end	Night	11% 6% 2 7% 4 3 4 4 6% 7% 47%	1948
September	1st to 10th	Night	11% 6% 2 5% 5% 3 4 4 6% 7% 47%	1800
	11th to 20th	Night	14% 4 2 4 4 4 4 4 5% 5% 50%	1800
	21st to end	Night	17% 5% 2 6% 4 3 4 3 5% 6% 44%	2056
October	1st to 10th	Night	18% 8% 3 7% 4 4 4 3 5% 7% 38%	2160
	11th to 20th	Night	21% 11% 3 7% 6% 4 5% 3 5% 6% 31%	2160
	21st to end	Night	23% 13% 3 7% 6% 4 4 3 5% 6% 28%	2460
November	1st to 10th	Night	17% 11% 3 6% 4 4 5% 4 5% 7% 35%	2444
	11th to 20th	Night	17% 12% 4 8% 5% 4 5% 4 5% 6% 30%	2660
	21st to end	Night	20% 14% 5% 8% 5% 5% 5% 4 5% 6% 24%	2660
December	1st to 10th	Night	20% 18% 5% 11% 6% 5% 5% 4 5% 6% 18%	2660
	11th to 20th	Night	19% 17% 4 8% 6% 4 3 3 4 4 27%	3288
	21st to end	Night	28% 18% 4 12% 5% 4 3 2 4 4 16%	4884

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% 6% 13% 10% 6% 1	2518
	11th to 20th	Day	66% 6% 10% 11% 6% 1	2520
	21st to end	Day	73% 4 8% 10% 5% 1	3026
February	1st to 10th	Day	63% 6% 9% 16% 5% 1	1710
	11th to 20th	Day	58% 4 11% 16% 10% 1	1672
	21st to end	Day	61% 5% 11% 11% 11% 1	1531
March	1st to 10th	Day	59% 4 11% 14% 11% 1	3237
	11th to 20th	Day	55% 5% 13% 15% 10% 2	3480
	21st to end	Day	41% 5% 9% 25% 16% 4	3942
April	1st to 10th	Day	55% 5% 7% 18% 13% 3	2280
	11th to 20th	Day	49% 5% 7% 21% 16% 2	2254
	21st to end	Day	54% 5% 8% 15% 14% 4	2417
May	1st to 10th	Day	49% 7% 10% 22% 11% 2	4408
	11th to 20th	Day	49% 7% 14% 19% 10% 1	4402
	21st to end	Day	41% 6% 13% 22% 15% 2	4837
June	1st to 10th	Day	52% 7% 10% 18% 12% 1	2660
	11th to 20th	Day	42% 7% 13% 23% 12% 2	2660
	21st to end	Day	49% 6% 13% 19% 12% 1	2660
July	1st to 10th	Day	41% 7% 11% 26% 14% 1	2590
	11th to 20th	Day	40% 8% 16% 25% 11%	2518
	21st to end	Day	43% 8% 14% 24% 11%	2591
August	1st to 10th	Day	42% 9% 15% 24% 11%	2369
	11th to 20th	Day	44% 7% 16% 23% 9% 1	2303
	21st to end	Day	43% 7% 13% 25% 11% 1	2390
September	1st to 10th	Day	41% 6% 13% 27% 13% 1	2160
	11th to 20th	Day	37% 6% 13% 29% 14%	2160
	21st to end	Day	45% 6% 12% 28% 9%	1898
October	1st to 10th	Day	50% 6% 11% 21% 12% 1	1800
	11th to 20th	Day	57% 7% 13% 16% 6% 1	1800
	21st to end	Day	62% 5% 9% 16% 7%	1895
November	1st to 10th	Day	48% 6% 18% 20% 8% 1	1633
	11th to 20th	Day	57% 6% 12% 15% 8% 2	1520
	21st to end	Day	65% 6% 11% 10% 7% 1	1520
December	1st to 10th	Day	70% 5% 10% 9% 4	1520
	11th to 20th	Day	61% 5% 15% 12% 6%	1906
	21st to end	Day	72% 3 7% 13% 5%	2772
January	1st to 10th	Night	66% 6% 13% 12% 3	4429
	11th to 20th	Night	64% 6% 15% 11% 4 1	4440
	21st to end	Night	69% 4 10% 9% 6% 1	4628
February	1st to 10th	Night	58% 6% 13% 17% 6% 1	2470
	11th to 20th	Night	55% 5% 11% 20% 8% 1	2405
	21st to end	Night	54% 6% 11% 18% 10% 1	1922
March	1st to 10th	Night	55% 5% 9% 20% 10% 1	3614
	11th to 20th	Night	55% 6% 14% 17% 7% 1	3480
	21st to end	Night	44% 6% 9% 23% 16% 3	3690
April	1st to 10th	Night	51% 4 8% 20% 14% 2	1900
	11th to 20th	Night	48% 5% 8% 21% 16% 1	1875
	21st to end	Night	50% 7% 8% 17% 16% 3	1780
May	1st to 10th	Night	49% 6% 11% 20% 12% 2	2538
	11th to 20th	Night	46% 7% 12% 24% 9% 2	2496
	21st to end	Night	38% 7% 14% 22% 18% 2	2739
June	1st to 10th	Night	46% 7% 13% 21% 12% 1	1520
	11th to 20th	Night	37% 7% 13% 25% 17% 2	1520
	21st to end	Night	45% 6% 15% 20% 13% 1	1520
July	1st to 10th	Night	34% 6% 13% 25% 19% 2	1480
	11th to 20th	Night	36% 7% 13% 31% 12%	1440
	21st to end	Night	33% 8% 18% 28% 12% 1	1486
August	1st to 10th	Night	35% 7% 19% 27% 12% 1	1542
	11th to 20th	Night	36% 7% 17% 28% 11% 1	1598
	21st to end	Night	36% 7% 15% 29% 12% 1	1948
September	1st to 10th	Night	35% 8% 15% 27% 14% 1	1800
	11th to 20th	Night	34% 6% 15% 34% 11% 1	1800
	21st to end	Night	40% 6% 12% 32% 9% 1	2056
October	1st to 10th	Night	47% 6% 12% 21% 14%	2160
	11th to 20th	Night	53% 7% 14% 18% 7%	2160
	21st to end	Night	57% 5% 12% 15% 9%	2460
November	1st to 10th	Night	47% 6% 15% 21% 10% 1	2444
	11th to 20th	Night	53% 6% 12% 18% 9% 2	2660
	21st to end	Night	59% 7% 13% 15% 5%	2660
December	1st to 10th	Night	66% 7% 11% 12% 4	2660
	11th to 20th	Night	61% 5% 11% 15% 7% 1	3288
	21st to end	Night	73% 4 8% 12% 3	4884

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	24% 23% 33% 17% 4	180
	11th to 20th	Day	33% 17% 33% 12% 5%	180
	21st to end	Day	27% 20% 36% 12% 3 1	270
February	1st to 10th	Day	33% 26% 29% 11% 1	270
	11th to 20th	Day	28% 28% 39% 5%	270
	21st to end	Day	36% 19% 33% 11% 1	343
March	1st to 10th	Day	25% 24% 33% 15% 3	460
	11th to 20th	Day	25% 22% 35% 15% 3 1	460
	21st to end	Day	33% 21% 28% 12% 5% 1	506
April	1st to 10th	Day	30% 23% 28% 14% 5%	460
	11th to 20th	Day	32% 23% 22% 15% 6% 2	452
	21st to end	Day	30% 22% 30% 11% 7% 1	460
May	1st to 10th	Day	25% 17% 32% 18% 6% 3	463
	11th to 20th	Day	17% 18% 36% 20% 8% 2	487
	21st to end	Day	18% 16% 35% 19% 10% 3	529
June	1st to 10th	Day	24% 14% 34% 20% 7% 1	490
	11th to 20th	Day	15% 18% 32% 23% 10% 2	490
	21st to end	Day	20% 15% 35% 18% 9% 2	490
July	1st to 10th	Day	13% 9% 31% 31% 13% 3	485
	11th to 20th	Day	14% 9% 33% 26% 15% 3	465
	21st to end	Day	14% 10% 36% 26% 10% 3	488
August	1st to 10th	Day	14% 8% 30% 30% 13% 4	446
	11th to 20th	Day	14% 7% 28% 30% 16% 5%	439
	21st to end	Day	14% 10% 34% 23% 12% 6%	495
September	1st to 10th	Day	16% 11% 31% 22% 13% 6%	450
	11th to 20th	Day	14% 12% 25% 24% 14% 10%	450
	21st to end	Day	18% 14% 31% 20% 12% 5%	450
October	1st to 10th	Day	25% 19% 24% 15% 13% 3	450
	11th to 20th	Day	26% 22% 30% 16% 4 1	450
	21st to end	Day	32% 20% 27% 18% 4	453
November	1st to 10th	Day	25% 18% 32% 18% 7%	327
	11th to 20th	Day	30% 20% 33% 11% 4 2	180
	21st to end	Day	28% 24% 32% 11% 4 1	180
December	1st to 10th	Day	30% 22% 35% 11% 2 1	180
	11th to 20th	Day	33% 25% 30% 10% 1 1	180
	21st to end	Day	33% 24% 30% 9% 3 1	198
January	1st to 10th	Night	28% 19% 32% 18% 4	760
	11th to 20th	Night	29% 16% 33% 16% 5% 1	760
	21st to end	Night	21% 17% 35% 22% 5%	836
February	1st to 10th	Night	22% 21% 37% 18% 2	760
	11th to 20th	Night	21% 21% 41% 14% 3	743
	21st to end	Night	22% 22% 35% 18% 3	628
March	1st to 10th	Night	22% 19% 36% 21% 3	760
	11th to 20th	Night	25% 20% 30% 20% 4 1	760
	21st to end	Night	21% 16% 35% 19% 7% 1	836
April	1st to 10th	Night	25% 16% 34% 21% 3	760
	11th to 20th	Night	23% 22% 30% 19% 6% 1	751
	21st to end	Night	22% 15% 29% 24% 9%	640
May	1st to 10th	Night	24% 9% 27% 29% 9% 2	398
	11th to 20th	Night	16% 11% 34% 30% 7%	374
	21st to end	Night	13% 9% 38% 28% 12%	411
June	1st to 10th	Night	17% 8% 34% 32% 8%	380
	11th to 20th	Night	12% 13% 31% 31% 11% 1	380
	21st to end	Night	19% 7% 29% 35% 11%	380
July	1st to 10th	Night	7% 6% 33% 39% 15% 1	370
	11th to 20th	Night	11% 4 31% 38% 15% 1	360
	21st to end	Night	10% 4 32% 42% 11% 1	394
August	1st to 10th	Night	13% 3 27% 35% 21% 1	478
	11th to 20th	Night	12% 3 23% 37% 23% 2	534
	21st to end	Night	13% 6% 31% 29% 18% 3	760
September	1st to 10th	Night	13% 7% 33% 27% 18% 2	720
	11th to 20th	Night	15% 7% 24% 29% 21% 3	720
	21st to end	Night	20% 11% 23% 25% 18% 3	720
October	1st to 10th	Night	23% 12% 29% 21% 13% 2	720
	11th to 20th	Night	26% 16% 30% 20% 8% 1	720
	21st to end	Night	29% 15% 30% 20% 5%	792
November	1st to 10th	Night	24% 16% 31% 22% 7%	740
	11th to 20th	Night	25% 17% 33% 17% 8% 1	760
	21st to end	Night	30% 18% 27% 19% 5%	760
December	1st to 10th	Night	29% 19% 32% 17% 3	760
	11th to 20th	Night	26% 20% 34% 18% 1 1	760
	21st to end	Night	34% 16% 32% 15% 3	836

Page generated at 2024-07-13 10:19:53. Version ac07

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