Sibley State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sibley State Park clear sky chart from 2005-11-06 to 2026-03-18.

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% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	131556
	Night	10% 9% 3 9% 5% 5% 5% 4 6% 7% 38%	130202
Transparency	Day	44% 5% 13% 25% 11% 1	135120
	Night	44% 5% 12% 24% 12% 1	134717
Seeing	Day	15% 9% 31% 28% 14% 3	17839
	Night	14% 9% 33% 32% 11% 1	27906

By Month

Percentage Cloud Cover SE (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	8% 13% 4 15% 12% 8% 8% 4 6% 6% 16%	4124
	11th to 20th	Day	5% 9% 6% 13% 7% 8% 10% 6% 9% 7% 21%	4799
	21st to end	Day	3 9% 8% 12% 7% 5% 6% 5% 6% 8% 33%	5812
February	1st to 10th	Day	8% 12% 5% 10% 7% 6% 5% 4 6% 8% 30%	1890
	11th to 20th	Day	7% 12% 3 10% 7% 6% 6% 5% 5% 7% 31%	1844
	21st to end	Day	9% 12% 4 10% 7% 6% 4 4 6% 9% 30%	1608
March	1st to 10th	Day	11% 10% 3 8% 4 4 4 3 5% 7% 42%	4998
	11th to 20th	Day	9% 9% 3 7% 4 4 5% 4 5% 8% 40%	5057
	21st to end	Day	20% 12% 4 10% 7% 5% 4 3 4 6% 24%	5462
April	1st to 10th	Day	17% 10% 3 7% 4 3 3 4 5% 7% 37%	2350
	11th to 20th	Day	17% 14% 4 7% 5% 4 4 3 4 6% 32%	2505
	21st to end	Day	23% 10% 3 6% 4 4 4 3 5% 6% 32%	2600
May	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 49%	5903
	11th to 20th	Day	18% 8% 2 5% 3 3 3 2 5% 5% 44%	6319
	21st to end	Day	17% 7% 2 6% 4 5% 5% 4 7% 8% 36%	6935
June	1st to 10th	Day	11% 6% 3 5% 4 4 5% 4 6% 8% 44%	2893
	11th to 20th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	3000
	21st to end	Day	10% 6% 2 6% 3 4 5% 4 7% 9% 44%	3000
July	1st to 10th	Day	7% 4 2 5% 4 5% 6% 5% 7% 10% 45%	6800
	11th to 20th	Day	7% 4 2 4 3 4 4 3 6% 8% 55%	6655
	21st to end	Day	7% 5% 2 6% 4 4 5% 5% 7% 9% 46%	6589
August	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 9% 44%	4234
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 6% 10% 40%	4171
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 5% 8% 49%	4614
September	1st to 10th	Day	10% 4 2 5% 3 4 3 3 5% 8% 51%	2400
	11th to 20th	Day	10% 4 2 7% 4 4 5% 4 6% 7% 46%	2254
	21st to end	Day	15% 6% 1 5% 3 3 3 3 4 7% 50%	2177
October	1st to 10th	Day	15% 8% 2 5% 4 4 4 3 6% 7% 43%	2000
	11th to 20th	Day	14% 11% 3 6% 4 3 4 3 5% 6% 41%	2000
	21st to end	Day	17% 11% 3 8% 4 3 4 2 5% 6% 38%	2166
November	1st to 10th	Day	13% 11% 3 8% 5% 4 3 3 5% 7% 37%	1712
	11th to 20th	Day	9% 12% 5% 14% 6% 6% 4 4 3 5% 31%	1680
	21st to end	Day	13% 9% 5% 10% 6% 4 4 4 6% 6% 32%	1680
December	1st to 10th	Day	6% 10% 5% 13% 11% 7% 7% 5% 7% 8% 22%	2690
	11th to 20th	Day	9% 15% 8% 11% 6% 5% 5% 4 5% 6% 27%	3060
	21st to end	Day	17% 18% 4 20% 6% 4 3 2 3 6% 17%	3575
January	1st to 10th	Night	7% 10% 5% 13% 11% 8% 7% 4 6% 7% 24%	8799
	11th to 20th	Night	4 9% 5% 12% 9% 8% 11% 8% 7% 6% 22%	8156
	21st to end	Night	4 11% 7% 9% 5% 5% 6% 4 6% 7% 35%	8402
February	1st to 10th	Night	6% 12% 4 10% 6% 6% 5% 4 6% 8% 33%	2730
	11th to 20th	Night	6% 8% 4 9% 5% 6% 6% 5% 6% 7% 38%	2663
	21st to end	Night	9% 9% 4 8% 6% 5% 5% 5% 6% 7% 35%	2218
March	1st to 10th	Night	9% 7% 2 6% 5% 3 4 3 6% 7% 48%	5079
	11th to 20th	Night	9% 7% 2 7% 4 4 5% 5% 7% 8% 41%	5057
	21st to end	Night	21% 11% 4 10% 6% 4 4 3 4 5% 29%	5467
April	1st to 10th	Night	14% 10% 3 5% 5% 4 4 2 6% 7% 40%	2050
	11th to 20th	Night	16% 11% 4 7% 3 4 4 4 5% 6% 37%	1843
	21st to end	Night	19% 10% 3 7% 4 4 3 4 5% 5% 37%	1800
May	1st to 10th	Night	12% 5% 2 4 3 3 3 2 5% 6% 55%	4012
	11th to 20th	Night	13% 8% 3 6% 3 4 4 3 5% 5% 47%	3596
	21st to end	Night	17% 9% 2 7% 4 4 4 4 5% 7% 37%	3942
June	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 8% 49%	1502
	11th to 20th	Night	10% 5% 2 6% 4 3 5% 4 6% 8% 48%	1400
	21st to end	Night	10% 4 2 5% 3 4 3 3 5% 8% 54%	1400
July	1st to 10th	Night	8% 4 1 5% 4 4 4 4 7% 10% 48%	3178
	11th to 20th	Night	6% 2 1 4 3 4 4 3 6% 8% 58%	3309
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 7% 8% 48%	4266
August	1st to 10th	Night	11% 6% 2 6% 3 4 4 4 5% 6% 49%	2926
	11th to 20th	Night	13% 4 1 5% 3 3 4 4 5% 9% 49%	2888
	21st to end	Night	9% 4 2 5% 4 3 3 3 5% 8% 54%	3293
September	1st to 10th	Night	8% 4 2 4 3 4 3 3 5% 7% 57%	2000
	11th to 20th	Night	9% 5% 1 6% 4 3 5% 4 4 7% 53%	2143
	21st to end	Night	9% 6% 1 5% 3 3 3 2 4 6% 56%	2202
October	1st to 10th	Night	14% 6% 2 5% 3 3 3 4 5% 7% 49%	2400
	11th to 20th	Night	15% 9% 2 5% 4 3 4 3 5% 6% 45%	2400
	21st to end	Night	16% 11% 3 8% 4 3 3 3 5% 5% 39%	2670
November	1st to 10th	Night	11% 8% 3 7% 5% 5% 5% 4 6% 7% 41%	2806
	11th to 20th	Night	11% 11% 4 10% 5% 4 5% 3 4 6% 36%	2940
	21st to end	Night	11% 10% 4 9% 6% 4 4 4 5% 6% 37%	2940
December	1st to 10th	Night	5% 7% 5% 12% 10% 8% 6% 4 6% 8% 29%	4720
	11th to 20th	Night	10% 11% 6% 14% 8% 6% 5% 4 5% 6% 25%	5360
	21st to end	Night	16% 18% 6% 17% 6% 5% 4 3 4 5% 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	67% 5% 10% 11% 6%	4125
	11th to 20th	Day	59% 4 11% 18% 8%	4799
	21st to end	Day	50% 5% 14% 21% 10%	5812
February	1st to 10th	Day	53% 4 12% 22% 8% 1	1890
	11th to 20th	Day	51% 5% 14% 22% 7% 1	1853
	21st to end	Day	51% 5% 14% 20% 10%	1608
March	1st to 10th	Day	42% 5% 15% 25% 11% 2	4998
	11th to 20th	Day	42% 4 12% 29% 11% 2	5057
	21st to end	Day	61% 5% 10% 19% 5%	5464
April	1st to 10th	Day	47% 6% 11% 23% 12% 1	2350
	11th to 20th	Day	53% 5% 11% 18% 12% 1	2505
	21st to end	Day	53% 5% 11% 21% 9% 1	2600
May	1st to 10th	Day	35% 5% 12% 29% 19% 1	5941
	11th to 20th	Day	41% 4 8% 23% 15% 8%	6323
	21st to end	Day	43% 7% 10% 31% 9% 1	6949
June	1st to 10th	Day	36% 6% 15% 29% 12% 1	2893
	11th to 20th	Day	44% 7% 13% 20% 14% 1	3000
	21st to end	Day	33% 8% 16% 29% 12% 1	3000
July	1st to 10th	Day	31% 8% 18% 31% 11%	6726
	11th to 20th	Day	26% 5% 16% 37% 14% 1	6574
	21st to end	Day	32% 7% 16% 31% 13% 1	6403
August	1st to 10th	Day	36% 6% 13% 30% 13% 2	4220
	11th to 20th	Day	38% 7% 15% 31% 9%	4210
	21st to end	Day	33% 7% 17% 30% 13% 1	4613
September	1st to 10th	Day	31% 5% 15% 36% 12% 1	2400
	11th to 20th	Day	36% 7% 17% 30% 10%	2254
	21st to end	Day	36% 4 11% 34% 14% 1	2177
October	1st to 10th	Day	32% 4 8% 32% 23% 1	3260
	11th to 20th	Day	38% 4 12% 22% 22% 2	3240
	21st to end	Day	46% 4 15% 26% 9%	3482
November	1st to 10th	Day	48% 5% 12% 21% 12% 3	1712
	11th to 20th	Day	56% 5% 11% 19% 9% 1	1680
	21st to end	Day	51% 5% 15% 20% 7% 2	1680
December	1st to 10th	Day	59% 6% 14% 16% 5%	2690
	11th to 20th	Day	58% 5% 14% 17% 6%	3057
	21st to end	Day	71% 3 10% 11% 5%	3575
January	1st to 10th	Night	59% 5% 13% 16% 7%	8800
	11th to 20th	Night	61% 4 9% 16% 10% 1	8156
	21st to end	Night	47% 4 11% 27% 10%	8402
February	1st to 10th	Night	48% 5% 14% 20% 11% 2	2730
	11th to 20th	Night	44% 5% 13% 24% 12% 2	2664
	21st to end	Night	46% 6% 13% 20% 13% 2	2218
March	1st to 10th	Night	35% 5% 13% 31% 15% 2	5079
	11th to 20th	Night	38% 4 14% 30% 13% 1	5057
	21st to end	Night	59% 4 9% 19% 9% 1	5467
April	1st to 10th	Night	44% 5% 12% 26% 12% 1	2050
	11th to 20th	Night	48% 5% 7% 23% 16% 2	1843
	21st to end	Night	49% 5% 9% 21% 14% 2	1800
May	1st to 10th	Night	30% 5% 11% 31% 20% 3	4039
	11th to 20th	Night	38% 6% 7% 20% 21% 8%	3597
	21st to end	Night	44% 6% 10% 27% 11% 1	3950
June	1st to 10th	Night	32% 5% 15% 31% 15% 2	1502
	11th to 20th	Night	33% 6% 17% 25% 16% 3	1400
	21st to end	Night	30% 6% 15% 29% 18% 3	1400
July	1st to 10th	Night	30% 7% 18% 27% 17% 1	3145
	11th to 20th	Night	24% 5% 16% 37% 15% 2	3270
	21st to end	Night	33% 7% 16% 31% 11% 2	4141
August	1st to 10th	Night	35% 5% 14% 27% 16% 3	2916
	11th to 20th	Night	34% 6% 16% 31% 12% 1	2915
	21st to end	Night	28% 6% 18% 32% 14% 2	3293
September	1st to 10th	Night	26% 6% 15% 35% 16% 1	2000
	11th to 20th	Night	30% 6% 16% 30% 17%	2143
	21st to end	Night	31% 4 11% 35% 18% 1	2202
October	1st to 10th	Night	24% 3 6% 36% 26% 4	3920
	11th to 20th	Night	33% 4 12% 27% 21% 2	3899
	21st to end	Night	43% 4 12% 27% 13% 1	4306
November	1st to 10th	Night	42% 5% 14% 24% 13% 2	2806
	11th to 20th	Night	50% 5% 12% 20% 12% 1	2940
	21st to end	Night	48% 5% 16% 21% 8% 2	2940
December	1st to 10th	Night	52% 5% 17% 19% 6%	4720
	11th to 20th	Night	59% 5% 12% 19% 5%	5362
	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	15% 9% 34% 28% 12% 3	330
	11th to 20th	Day	13% 11% 37% 33% 6%	330
	21st to end	Day	12% 12% 39% 25% 10% 2	363
February	1st to 10th	Day	15% 15% 42% 25% 4 1	330
	11th to 20th	Day	15% 11% 39% 25% 8% 2	330
	21st to end	Day	15% 10% 42% 28% 5%	273
March	1st to 10th	Day	16% 17% 38% 21% 7% 1	330
	11th to 20th	Day	19% 12% 34% 25% 9% 1	321
	21st to end	Day	26% 10% 28% 22% 12% 3	330
April	1st to 10th	Day	24% 12% 32% 25% 6%	450
	11th to 20th	Day	27% 13% 26% 25% 8% 1	623
	21st to end	Day	24% 10% 34% 19% 11% 1	700
May	1st to 10th	Day	18% 10% 35% 24% 11% 3	703
	11th to 20th	Day	20% 9% 27% 26% 15% 3	725
	21st to end	Day	17% 6% 31% 30% 13% 3	789
June	1st to 10th	Day	10% 8% 26% 31% 21% 3	730
	11th to 20th	Day	17% 8% 33% 30% 11% 2	730
	21st to end	Day	13% 8% 34% 29% 13% 4	730
July	1st to 10th	Day	7% 4 23% 33% 25% 7%	721
	11th to 20th	Day	8% 7% 26% 32% 22% 6%	718
	21st to end	Day	6% 4 25% 38% 21% 6%	794
August	1st to 10th	Day	10% 5% 21% 36% 24% 4	726
	11th to 20th	Day	11% 6% 22% 33% 22% 7%	715
	21st to end	Day	9% 5% 25% 33% 23% 4	768
September	1st to 10th	Day	11% 8% 29% 30% 18% 4	530
	11th to 20th	Day	11% 4 31% 33% 16% 5%	384
	21st to end	Day	19% 12% 27% 20% 16% 6%	330
October	1st to 10th	Day	18% 8% 29% 27% 14% 3	330
	11th to 20th	Day	17% 15% 39% 21% 7% 1	330
	21st to end	Day	24% 15% 36% 18% 6% 1	363
November	1st to 10th	Day	22% 13% 32% 25% 8%	330
	11th to 20th	Day	16% 15% 45% 20% 4	330
	21st to end	Day	19% 16% 38% 21% 6% 1	330
December	1st to 10th	Day	13% 11% 43% 25% 8% 1	330
	11th to 20th	Day	18% 15% 36% 27% 4	330
	21st to end	Day	29% 9% 36% 21% 4 1	363
January	1st to 10th	Night	10% 11% 39% 30% 8% 2	1050
	11th to 20th	Night	13% 12% 38% 29% 7% 1	1050
	21st to end	Night	14% 11% 38% 29% 7%	1155
February	1st to 10th	Night	12% 10% 40% 29% 8%	1050
	11th to 20th	Night	11% 11% 42% 29% 7% 1	1029
	21st to end	Night	14% 10% 36% 29% 9% 1	834
March	1st to 10th	Night	15% 10% 38% 28% 8%	656
	11th to 20th	Night	16% 11% 39% 28% 5% 1	621
	21st to end	Night	21% 10% 29% 28% 12% 1	660
April	1st to 10th	Night	18% 10% 36% 30% 6%	600
	11th to 20th	Night	21% 12% 31% 28% 8% 1	593
	21st to end	Night	21% 9% 32% 29% 8% 1	600
May	1st to 10th	Night	16% 10% 33% 31% 9% 1	600
	11th to 20th	Night	18% 7% 31% 32% 11% 1	591
	21st to end	Night	15% 8% 25% 39% 12% 2	650
June	1st to 10th	Night	11% 7% 27% 38% 16% 1	600
	11th to 20th	Night	12% 3 32% 42% 11%	600
	21st to end	Night	11% 6% 25% 43% 15% 1	600
July	1st to 10th	Night	5% 3 20% 46% 24% 2	585
	11th to 20th	Night	5% 4 26% 44% 19% 1	582
	21st to end	Night	6% 2 23% 46% 20% 3	657
August	1st to 10th	Night	8% 4 21% 45% 21% 2	597
	11th to 20th	Night	12% 3 17% 41% 25% 3	594
	21st to end	Night	8% 4 23% 40% 23% 2	660
September	1st to 10th	Night	10% 5% 26% 40% 17% 2	600
	11th to 20th	Night	10% 6% 30% 35% 18% 2	600
	21st to end	Night	11% 9% 31% 34% 13% 1	602
October	1st to 10th	Night	16% 9% 33% 30% 11% 1	800
	11th to 20th	Night	19% 12% 33% 29% 8% 1	800
	21st to end	Night	22% 12% 31% 29% 6%	910
November	1st to 10th	Night	15% 11% 35% 29% 9%	1025
	11th to 20th	Night	16% 13% 37% 25% 8% 1	1050
	21st to end	Night	16% 14% 38% 25% 7%	1050
December	1st to 10th	Night	12% 10% 40% 29% 8% 1	1050
	11th to 20th	Night	18% 11% 36% 28% 6% 1	1050
	21st to end	Night	22% 10% 34% 27% 6% 1	1155

Page generated at 2026-03-15 01:23:55. Version ac07

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