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 2024-12-06.

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% 8% 3 8% 5% 4 5% 4 6% 7% 38%	101982
	Night	11% 9% 3 8% 5% 4 4 4 5% 7% 39%	97810
Transparency	Day	43% 6% 13% 25% 11% 1	105585
	Night	43% 5% 13% 25% 13% 2	102349
Seeing	Day	15% 9% 31% 28% 14% 3	16137
	Night	14% 9% 32% 32% 11% 1	26066

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	12% 16% 3 15% 9% 7% 6% 4 5% 5% 19%	2218
	11th to 20th	Day	5% 9% 6% 15% 7% 7% 8% 6% 8% 6% 22%	2569
	21st to end	Day	6% 14% 12% 13% 7% 5% 5% 4 6% 7% 21%	3135
February	1st to 10th	Day	8% 12% 5% 10% 6% 6% 5% 4 6% 8% 30%	1710
	11th to 20th	Day	6% 12% 4 10% 7% 7% 6% 5% 5% 7% 30%	1664
	21st to end	Day	9% 12% 4 10% 7% 6% 4 4 6% 9% 29%	1459
March	1st to 10th	Day	11% 13% 4 10% 6% 5% 4 3 4 5% 34%	3402
	11th to 20th	Day	9% 10% 4 7% 4 4 5% 4 5% 7% 41%	3480
	21st to end	Day	22% 13% 4 12% 8% 5% 4 3 4 5% 21%	3814
April	1st to 10th	Day	17% 10% 3 7% 4 3 4 4 5% 7% 36%	2232
	11th to 20th	Day	18% 14% 4 7% 5% 3 4 3 4 6% 33%	2378
	21st to end	Day	23% 10% 3 6% 4 4 4 3 5% 6% 31%	2470
May	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 4 6% 7% 37%	4109
	11th to 20th	Day	13% 7% 2 6% 4 4 3 3 6% 6% 46%	4399
	21st to end	Day	17% 6% 2 6% 4 5% 5% 4 7% 9% 35%	4824
June	1st to 10th	Day	11% 6% 3 5% 4 4 5% 4 6% 8% 44%	2748
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	2850
	21st to end	Day	9% 6% 2 6% 3 4 5% 4 7% 9% 44%	2850
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 7% 5% 7% 10% 39%	4750
	11th to 20th	Day	5% 3 1 4 3 4 4 4 6% 8% 60%	4644
	21st to end	Day	5% 4 2 5% 5% 5% 5% 5% 7% 8% 49%	4576
August	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 6% 9% 44%	4104
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 7% 10% 40%	4041
	21st to end	Day	9% 5% 2 6% 4 4 4 4 5% 8% 49%	4473
September	1st to 10th	Day	10% 4 2 5% 3 4 3 3 5% 8% 52%	2280
	11th to 20th	Day	10% 4 2 7% 4 4 5% 4 6% 7% 47%	2142
	21st to end	Day	15% 6% 1 4 3 3 3 3 4 7% 49%	2068
October	1st to 10th	Day	16% 8% 2 5% 4 4 4 3 6% 7% 42%	1900
	11th to 20th	Day	14% 11% 3 6% 4 3 4 3 5% 6% 42%	1900
	21st to end	Day	17% 10% 3 8% 4 3 4 3 5% 6% 38%	2058
November	1st to 10th	Day	14% 11% 3 8% 6% 4 3 3 5% 7% 37%	1629
	11th to 20th	Day	10% 12% 5% 14% 7% 6% 4 4 3 5% 30%	1600
	21st to end	Day	13% 10% 5% 9% 6% 4 4 4 7% 6% 32%	1600
December	1st to 10th	Day	8% 13% 4 12% 8% 5% 7% 5% 6% 8% 25%	1560
	11th to 20th	Day	9% 14% 7% 12% 6% 5% 5% 4 5% 6% 28%	1904
	21st to end	Day	21% 15% 4 15% 5% 4 3 2 4 6% 21%	2442
January	1st to 10th	Night	8% 12% 5% 14% 10% 6% 4 4 5% 6% 24%	4737
	11th to 20th	Night	5% 10% 5% 13% 10% 9% 10% 6% 7% 6% 18%	4393
	21st to end	Night	7% 18% 10% 13% 6% 5% 4 4 5% 6% 23%	4532
February	1st to 10th	Night	6% 11% 4 10% 6% 5% 5% 5% 7% 8% 33%	2470
	11th to 20th	Night	6% 9% 4 9% 5% 6% 6% 5% 6% 7% 38%	2403
	21st to end	Night	10% 9% 4 8% 6% 5% 5% 5% 6% 7% 35%	2013
March	1st to 10th	Night	8% 8% 3 7% 5% 3 5% 3 5% 7% 47%	3454
	11th to 20th	Night	8% 8% 3 8% 5% 4 5% 4 6% 8% 42%	3480
	21st to end	Night	23% 13% 3 10% 5% 3 3 3 4 5% 27%	3817
April	1st to 10th	Night	14% 10% 3 6% 5% 4 4 2 6% 7% 40%	1948
	11th to 20th	Night	17% 11% 4 7% 3 4 3 4 5% 5% 37%	1750
	21st to end	Night	18% 10% 3 7% 4 4 3 4 5% 4 39%	1710
May	1st to 10th	Night	17% 7% 2 5% 4 3 3 2 5% 6% 45%	2796
	11th to 20th	Night	12% 7% 2 5% 3 4 4 4 6% 6% 46%	2506
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 8% 40%	2743
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 49%	1427
	11th to 20th	Night	9% 5% 2 6% 5% 3 5% 4 6% 8% 49%	1330
	21st to end	Night	9% 4 2 4 3 4 3 3 5% 8% 55%	1330
July	1st to 10th	Night	10% 5% 1 5% 4 4 5% 4 8% 9% 43%	2220
	11th to 20th	Night	3 2 1 4 3 5% 4 3 5% 7% 63%	2313
	21st to end	Night	5% 3 2 4 3 3 4 4 7% 9% 55%	2971
August	1st to 10th	Night	11% 6% 2 6% 3 4 4 4 5% 6% 50%	2836
	11th to 20th	Night	13% 4 1 5% 3 3 4 4 5% 9% 49%	2798
	21st to end	Night	9% 4 2 5% 4 3 3 3 5% 8% 54%	3192
September	1st to 10th	Night	8% 4 2 4 3 3 3 3 5% 7% 58%	1900
	11th to 20th	Night	9% 5% 1 6% 4 3 4 4 4 7% 54%	2036
	21st to end	Night	10% 7% 1 5% 3 3 3 3 4 6% 55%	2092
October	1st to 10th	Night	14% 6% 2 5% 3 3 3 4 5% 7% 48%	2280
	11th to 20th	Night	15% 9% 2 5% 4 3 5% 3 5% 6% 45%	2280
	21st to end	Night	16% 11% 3 7% 4 3 3 3 5% 5% 39%	2536
November	1st to 10th	Night	11% 8% 3 7% 5% 4 5% 4 6% 7% 40%	2669
	11th to 20th	Night	12% 11% 4 11% 5% 4 5% 3 4 5% 36%	2800
	21st to end	Night	11% 10% 4 8% 6% 4 4 4 5% 6% 37%	2800
December	1st to 10th	Night	8% 10% 5% 11% 8% 6% 5% 3 6% 7% 30%	2737
	11th to 20th	Night	11% 10% 4 12% 7% 6% 6% 5% 6% 6% 27%	3286
	21st to end	Night	19% 15% 6% 12% 6% 5% 4 3 5% 5% 20%	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	67% 5% 9% 14% 4	2219
	11th to 20th	Day	58% 6% 13% 18% 5%	2569
	21st to end	Day	63% 7% 12% 14% 4	3135
February	1st to 10th	Day	52% 5% 12% 23% 9%	1710
	11th to 20th	Day	52% 5% 15% 20% 7% 1	1673
	21st to end	Day	53% 5% 13% 19% 10% 1	1459
March	1st to 10th	Day	50% 6% 11% 24% 9% 1	3402
	11th to 20th	Day	42% 5% 11% 29% 11% 2	3480
	21st to end	Day	66% 5% 9% 16% 4	3814
April	1st to 10th	Day	47% 6% 11% 23% 11% 2	2232
	11th to 20th	Day	53% 5% 10% 18% 12% 1	2378
	21st to end	Day	54% 5% 12% 21% 8% 1	2470
May	1st to 10th	Day	42% 6% 12% 24% 15% 1	4147
	11th to 20th	Day	38% 6% 10% 30% 14% 2	4403
	21st to end	Day	42% 9% 12% 28% 9% 1	4837
June	1st to 10th	Day	36% 7% 15% 29% 12% 1	2748
	11th to 20th	Day	43% 7% 13% 21% 14% 2	2850
	21st to end	Day	33% 8% 17% 29% 12% 1	2850
July	1st to 10th	Day	36% 9% 18% 23% 13%	4676
	11th to 20th	Day	21% 6% 17% 37% 17% 2	4563
	21st to end	Day	29% 8% 16% 32% 14% 1	4432
August	1st to 10th	Day	36% 6% 13% 31% 13% 2	4090
	11th to 20th	Day	38% 7% 15% 31% 8%	4080
	21st to end	Day	33% 7% 17% 29% 13% 1	4472
September	1st to 10th	Day	31% 5% 14% 37% 13% 1	2280
	11th to 20th	Day	34% 7% 17% 31% 11%	2142
	21st to end	Day	37% 4 12% 33% 13% 2	2068
October	1st to 10th	Day	32% 3 8% 32% 23% 1	3160
	11th to 20th	Day	37% 4 13% 22% 22% 2	3140
	21st to end	Day	45% 4 15% 26% 9%	3374
November	1st to 10th	Day	47% 5% 12% 21% 12% 3	1629
	11th to 20th	Day	57% 5% 11% 18% 9% 1	1600
	21st to end	Day	51% 5% 15% 20% 7% 2	1600
December	1st to 10th	Day	57% 6% 15% 15% 7%	1560
	11th to 20th	Day	56% 6% 15% 18% 4	1901
	21st to end	Day	66% 4 11% 13% 6%	2442
January	1st to 10th	Night	60% 5% 12% 18% 5%	4737
	11th to 20th	Night	62% 6% 11% 15% 5% 1	4393
	21st to end	Night	60% 5% 11% 17% 6% 1	4532
February	1st to 10th	Night	47% 5% 13% 21% 11% 2	2470
	11th to 20th	Night	44% 6% 14% 23% 11% 2	2404
	21st to end	Night	47% 7% 12% 19% 13% 2	2013
March	1st to 10th	Night	36% 6% 13% 27% 16% 2	3454
	11th to 20th	Night	38% 6% 10% 31% 14% 1	3480
	21st to end	Night	60% 4 8% 20% 6% 1	3817
April	1st to 10th	Night	44% 5% 12% 26% 12% 1	1948
	11th to 20th	Night	49% 4 6% 23% 16% 2	1750
	21st to end	Night	48% 5% 9% 22% 14% 2	1710
May	1st to 10th	Night	40% 5% 10% 27% 15% 3	2823
	11th to 20th	Night	36% 8% 9% 24% 20% 3	2507
	21st to end	Night	40% 8% 13% 25% 13% 1	2751
June	1st to 10th	Night	31% 6% 15% 31% 15% 2	1427
	11th to 20th	Night	32% 6% 16% 26% 16% 3	1330
	21st to end	Night	28% 6% 15% 30% 18% 3	1330
July	1st to 10th	Night	35% 9% 19% 22% 15% 1	2185
	11th to 20th	Night	20% 6% 13% 38% 20% 3	2271
	21st to end	Night	24% 7% 17% 35% 15% 3	2876
August	1st to 10th	Night	34% 5% 14% 28% 16% 4	2826
	11th to 20th	Night	33% 6% 16% 31% 12% 1	2825
	21st to end	Night	29% 6% 18% 31% 14% 2	3192
September	1st to 10th	Night	26% 6% 15% 35% 17% 1	1900
	11th to 20th	Night	30% 6% 16% 31% 17%	2036
	21st to end	Night	31% 4 12% 34% 18% 1	2092
October	1st to 10th	Night	24% 3 6% 36% 26% 4	3800
	11th to 20th	Night	32% 4 13% 28% 21% 2	3779
	21st to end	Night	42% 4 12% 28% 13% 1	4172
November	1st to 10th	Night	43% 5% 14% 23% 13% 2	2669
	11th to 20th	Night	51% 5% 12% 20% 12% 1	2800
	21st to end	Night	47% 5% 16% 22% 8% 2	2800
December	1st to 10th	Night	53% 6% 15% 19% 7%	2737
	11th to 20th	Night	55% 6% 15% 17% 6% 1	3288
	21st to end	Night	66% 5% 10% 14% 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	15% 9% 35% 26% 14% 2	270
	11th to 20th	Day	13% 11% 34% 35% 7%	270
	21st to end	Day	14% 10% 36% 26% 11% 2	297
February	1st to 10th	Day	13% 14% 43% 26% 3	270
	11th to 20th	Day	16% 13% 40% 25% 6% 1	270
	21st to end	Day	16% 10% 40% 28% 5%	225
March	1st to 10th	Day	16% 19% 37% 20% 8% 1	270
	11th to 20th	Day	18% 9% 36% 26% 10% 1	270
	21st to end	Day	27% 9% 26% 22% 12% 3	297
April	1st to 10th	Day	25% 13% 31% 24% 7%	412
	11th to 20th	Day	28% 13% 24% 25% 9% 2	576
	21st to end	Day	24% 10% 34% 20% 11% 2	650
May	1st to 10th	Day	19% 10% 37% 24% 9% 2	653
	11th to 20th	Day	19% 9% 28% 26% 15% 3	675
	21st to end	Day	17% 7% 32% 28% 13% 3	734
June	1st to 10th	Day	10% 8% 26% 32% 21% 3	680
	11th to 20th	Day	15% 8% 34% 30% 11% 2	680
	21st to end	Day	12% 8% 33% 30% 14% 4	680
July	1st to 10th	Day	7% 4 24% 33% 25% 7%	671
	11th to 20th	Day	7% 7% 25% 32% 22% 7%	668
	21st to end	Day	6% 4 25% 38% 21% 6%	739
August	1st to 10th	Day	9% 4 21% 37% 25% 4	676
	11th to 20th	Day	11% 6% 22% 33% 22% 7%	665
	21st to end	Day	9% 6% 26% 33% 23% 3	715
September	1st to 10th	Day	11% 8% 29% 31% 19% 3	490
	11th to 20th	Day	11% 4 33% 31% 16% 6%	352
	21st to end	Day	20% 13% 29% 19% 14% 5%	300
October	1st to 10th	Day	20% 8% 29% 25% 15% 3	300
	11th to 20th	Day	15% 15% 39% 23% 7% 1	300
	21st to end	Day	22% 15% 37% 18% 6% 1	330
November	1st to 10th	Day	23% 12% 30% 26% 9%	300
	11th to 20th	Day	17% 14% 46% 19% 4	300
	21st to end	Day	20% 15% 38% 21% 6%	300
December	1st to 10th	Day	12% 10% 46% 24% 8% 1	285
	11th to 20th	Day	19% 14% 36% 27% 4	270
	21st to end	Day	30% 10% 33% 23% 3 1	297
January	1st to 10th	Night	11% 11% 39% 30% 8% 2	950
	11th to 20th	Night	14% 11% 37% 29% 8% 1	950
	21st to end	Night	16% 11% 36% 29% 8%	1045
February	1st to 10th	Night	12% 10% 39% 30% 8% 1	950
	11th to 20th	Night	11% 11% 41% 29% 7% 1	929
	21st to end	Night	15% 10% 34% 30% 10% 1	757
March	1st to 10th	Night	16% 10% 38% 27% 9%	594
	11th to 20th	Night	15% 10% 39% 29% 6% 1	570
	21st to end	Night	21% 10% 28% 28% 12% 1	627
April	1st to 10th	Night	18% 9% 37% 29% 6%	570
	11th to 20th	Night	22% 11% 30% 28% 8% 1	563
	21st to end	Night	20% 9% 32% 29% 9% 1	570
May	1st to 10th	Night	17% 11% 33% 31% 8% 1	570
	11th to 20th	Night	17% 7% 31% 32% 12% 1	561
	21st to end	Night	14% 8% 25% 39% 12% 2	617
June	1st to 10th	Night	11% 7% 27% 39% 16% 1	570
	11th to 20th	Night	11% 3 33% 42% 11%	570
	21st to end	Night	10% 5% 25% 45% 15% 1	570
July	1st to 10th	Night	5% 3 20% 46% 23% 2	555
	11th to 20th	Night	5% 4 27% 43% 20% 1	552
	21st to end	Night	5% 2 24% 46% 21% 3	624
August	1st to 10th	Night	8% 3 20% 45% 22% 2	567
	11th to 20th	Night	12% 2 17% 41% 26% 3	564
	21st to end	Night	9% 4 23% 40% 22% 2	627
September	1st to 10th	Night	9% 5% 26% 41% 17% 2	570
	11th to 20th	Night	10% 6% 30% 34% 18% 2	570
	21st to end	Night	12% 9% 33% 33% 12% 1	572
October	1st to 10th	Night	17% 9% 33% 30% 11% 1	760
	11th to 20th	Night	18% 12% 33% 29% 8% 1	760
	21st to end	Night	22% 12% 31% 29% 6%	864
November	1st to 10th	Night	15% 11% 34% 30% 9%	975
	11th to 20th	Night	17% 12% 37% 25% 8% 1	1000
	21st to end	Night	16% 14% 38% 26% 7%	1000
December	1st to 10th	Night	13% 11% 38% 30% 8% 1	978
	11th to 20th	Night	19% 9% 35% 29% 7% 1	950
	21st to end	Night	23% 11% 33% 27% 6%	1045

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