Otter Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Otter Creek Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	13% 8% 3 7% 5% 5% 4 4 6% 7% 38%	126349
	Night	14% 9% 3 7% 4 4 4 3 5% 7% 40%	123434
Transparency	Day	28% 38% 7% 16% 10% 1	129991
	Night	29% 38% 6% 15% 11% 1	127839
Seeing	Day	18% 11% 26% 24% 15% 7%	16658
	Night	18% 9% 25% 28% 17% 2	26349

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	14% 13% 9% 8% 3 4 5% 3 6% 5% 29%	3617
	11th to 20th	Day	14% 10% 4 14% 5% 4 4 2 4 5% 34%	3641
	21st to end	Day	21% 10% 6% 6% 4 4 4 3 4 6% 32%	4604
February	1st to 10th	Day	18% 12% 4 9% 4 3 3 2 4 7% 34%	2000
	11th to 20th	Day	18% 9% 4 8% 5% 4 3 3 5% 6% 35%	1948
	21st to end	Day	18% 10% 3 8% 5% 4 3 4 6% 6% 33%	1650
March	1st to 10th	Day	19% 12% 4 6% 4 4 3 3 3 5% 37%	4480
	11th to 20th	Day	12% 9% 3 6% 4 3 4 3 5% 7% 44%	4837
	21st to end	Day	14% 8% 3 8% 5% 4 4 4 6% 7% 38%	5645
April	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 4 6% 8% 39%	2400
	11th to 20th	Day	14% 9% 2 7% 4 5% 4 3 4 7% 41%	2374
	21st to end	Day	18% 6% 2 6% 5% 5% 4 3 6% 6% 38%	2535
May	1st to 10th	Day	19% 9% 3 8% 5% 6% 5% 4 6% 8% 26%	5862
	11th to 20th	Day	16% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	5861
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 5% 7% 10% 34%	6433
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 6% 4 6% 7% 44%	2688
	11th to 20th	Day	6% 5% 2 7% 5% 5% 5% 5% 6% 11% 43%	2800
	21st to end	Day	7% 6% 2 5% 5% 4 5% 5% 8% 9% 44%	2800
July	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 10% 42%	6360
	11th to 20th	Day	8% 5% 2 6% 5% 6% 5% 5% 9% 11% 38%	6360
	21st to end	Day	10% 8% 3 9% 6% 7% 6% 6% 8% 8% 29%	6424
August	1st to 10th	Day	7% 5% 2 5% 3 3 4 4 8% 10% 48%	4234
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 9% 9% 40%	3888
	21st to end	Day	5% 4 1 4 4 5% 4 3 5% 9% 56%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 4 3 5% 8% 44%	2346
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 3 5% 9% 46%	2200
	21st to end	Day	13% 6% 2 5% 5% 4 4 3 5% 6% 48%	2200
October	1st to 10th	Day	10% 5% 2 4 4 3 3 3 4 6% 55%	2200
	11th to 20th	Day	12% 5% 2 5% 3 4 4 3 5% 5% 51%	2095
	21st to end	Day	18% 7% 3 5% 3 4 4 2 5% 6% 43%	1881
November	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 4 5% 52%	1762
	11th to 20th	Day	16% 9% 2 5% 4 3 4 3 5% 6% 43%	1800
	21st to end	Day	17% 10% 3 6% 4 3 3 4 4 6% 40%	1800
December	1st to 10th	Day	16% 11% 3 7% 3 3 3 3 4 5% 42%	2930
	11th to 20th	Day	15% 11% 6% 10% 6% 5% 4 3 5% 5% 29%	3373
	21st to end	Day	20% 18% 4 9% 4 5% 4 3 4 6% 23%	4009
January	1st to 10th	Night	15% 15% 5% 11% 5% 4 4 3 4 6% 28%	6348
	11th to 20th	Night	16% 13% 5% 11% 5% 5% 5% 2 4 4 30%	6333
	21st to end	Night	18% 12% 4 6% 4 3 3 3 4 5% 38%	6334
February	1st to 10th	Night	20% 12% 5% 7% 5% 4 5% 4 5% 5% 29%	2400
	11th to 20th	Night	20% 10% 4 9% 5% 4 3 3 3 5% 34%	2338
	21st to end	Night	19% 11% 4 8% 5% 4 4 3 5% 5% 33%	1980
March	1st to 10th	Night	19% 13% 3 7% 4 4 4 3 4 4 35%	5373
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 5% 47%	5123
	21st to end	Night	14% 8% 3 8% 6% 5% 4 4 6% 6% 36%	5312
April	1st to 10th	Night	13% 7% 3 5% 4 5% 4 4 6% 7% 42%	2000
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 5% 7% 42%	1975
	21st to end	Night	15% 8% 3 6% 6% 4 5% 3 5% 8% 38%	1865
May	1st to 10th	Night	18% 10% 4 9% 5% 5% 5% 4 5% 8% 28%	4033
	11th to 20th	Night	14% 8% 3 8% 6% 5% 5% 4 6% 8% 32%	4032
	21st to end	Night	11% 6% 3 8% 5% 5% 5% 5% 7% 10% 36%	4421
June	1st to 10th	Night	9% 5% 2 6% 4 3 4 3 7% 9% 47%	1706
	11th to 20th	Night	6% 4 1 5% 5% 4 5% 5% 6% 10% 49%	1600
	21st to end	Night	4 3 1 5% 5% 4 4 6% 7% 12% 49%	1600
July	1st to 10th	Night	7% 6% 2 6% 4 5% 5% 4 5% 8% 48%	3618
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 7% 11% 46%	3617
	21st to end	Night	8% 6% 3 9% 6% 6% 6% 5% 8% 10% 33%	4423
August	1st to 10th	Night	5% 4 2 5% 3 4 3 3 5% 9% 56%	2915
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 4 7% 10% 48%	3157
	21st to end	Night	3 2 1 3 3 2 3 3 5% 9% 65%	3586
September	1st to 10th	Night	11% 5% 2 6% 4 3 4 3 5% 6% 51%	2032
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 51%	2200
	21st to end	Night	12% 5% 2 5% 4 3 4 3 5% 6% 53%	2200
October	1st to 10th	Night	9% 4 2 4 3 3 2 2 5% 6% 60%	2200
	11th to 20th	Night	13% 5% 2 6% 4 4 4 2 4 6% 52%	2256
	21st to end	Night	16% 7% 2 6% 3 3 3 3 4 5% 47%	2717
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 4 5% 51%	2536
	11th to 20th	Night	14% 7% 2 7% 4 4 4 3 4 6% 45%	2600
	21st to end	Night	16% 10% 4 5% 4 4 4 3 5% 5% 41%	2600
December	1st to 10th	Night	18% 11% 4 7% 4 3 3 3 4 4 40%	4250
	11th to 20th	Night	18% 11% 3 9% 4 4 3 3 4 6% 35%	4822
	21st to end	Night	22% 16% 5% 10% 6% 5% 4 3 3 4 22%	6932

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	43% 27% 7% 16% 6% 1	3618
	11th to 20th	Day	40% 26% 3 15% 12% 2	3641
	21st to end	Day	41% 28% 6% 13% 9% 3	4604
February	1st to 10th	Day	20% 61% 4 9% 6% 1	2000
	11th to 20th	Day	21% 61% 3 10% 5%	1958
	21st to end	Day	18% 61% 2 10% 6% 2	1650
March	1st to 10th	Day	41% 28% 2 12% 13% 5%	4480
	11th to 20th	Day	26% 29% 6% 22% 16% 1	4837
	21st to end	Day	32% 29% 8% 19% 11%	5647
April	1st to 10th	Day	20% 62% 5% 8% 5%	2400
	11th to 20th	Day	18% 62% 4 11% 4 1	2374
	21st to end	Day	17% 62% 4 11% 4 2	2535
May	1st to 10th	Day	40% 31% 9% 12% 7%	5897
	11th to 20th	Day	37% 30% 9% 17% 8%	5865
	21st to end	Day	34% 30% 9% 14% 12% 1	6447
June	1st to 10th	Day	15% 63% 3 10% 8% 1	2688
	11th to 20th	Day	16% 63% 5% 8% 8%	2800
	21st to end	Day	14% 61% 6% 10% 7%	2800
July	1st to 10th	Day	30% 26% 9% 20% 16%	6291
	11th to 20th	Day	30% 26% 13% 24% 7%	6285
	21st to end	Day	41% 26% 12% 14% 6% 1	6240
August	1st to 10th	Day	19% 36% 9% 28% 8%	4221
	11th to 20th	Day	25% 36% 8% 21% 10%	3925
	21st to end	Day	15% 36% 7% 32% 9% 1	4311
September	1st to 10th	Day	14% 57% 5% 13% 10% 2	2346
	11th to 20th	Day	13% 57% 6% 12% 10% 1	2200
	21st to end	Day	17% 57% 3 12% 10%	2200
October	1st to 10th	Day	11% 35% 2 20% 31% 1	3590
	11th to 20th	Day	13% 36% 6% 19% 24% 2	3389
	21st to end	Day	17% 36% 4 20% 17% 6%	3081
November	1st to 10th	Day	19% 56% 4 12% 7% 1	1762
	11th to 20th	Day	18% 56% 2 11% 9% 3	1800
	21st to end	Day	22% 57% 4 11% 6% 1	1800
December	1st to 10th	Day	28% 35% 5% 18% 14% 1	2930
	11th to 20th	Day	40% 31% 6% 14% 8% 1	3370
	21st to end	Day	49% 27% 8% 11% 4	4009
January	1st to 10th	Night	46% 27% 5% 15% 6% 1	6348
	11th to 20th	Night	45% 26% 2 14% 10% 2	6333
	21st to end	Night	36% 28% 4 16% 13% 2	6334
February	1st to 10th	Night	23% 61% 4 7% 5% 1	2400
	11th to 20th	Night	22% 60% 3 8% 6%	2339
	21st to end	Night	20% 61% 3 8% 7% 1	1980
March	1st to 10th	Night	42% 29% 3 15% 8% 3	5373
	11th to 20th	Night	25% 29% 5% 19% 18% 3	5123
	21st to end	Night	35% 29% 5% 19% 12%	5312
April	1st to 10th	Night	18% 62% 4 9% 6% 1	2000
	11th to 20th	Night	18% 62% 4 9% 6% 1	1975
	21st to end	Night	18% 62% 4 10% 5% 2	1865
May	1st to 10th	Night	40% 32% 8% 15% 5%	4060
	11th to 20th	Night	38% 31% 8% 18% 5%	4033
	21st to end	Night	32% 30% 12% 16% 9% 1	4429
June	1st to 10th	Night	15% 62% 4 11% 7% 2	1706
	11th to 20th	Night	13% 63% 4 9% 9% 2	1600
	21st to end	Night	11% 61% 7% 12% 9%	1600
July	1st to 10th	Night	27% 27% 8% 22% 15% 1	3580
	11th to 20th	Night	27% 27% 14% 23% 8%	3572
	21st to end	Night	37% 27% 12% 15% 9%	4294
August	1st to 10th	Night	16% 36% 9% 30% 9%	2904
	11th to 20th	Night	22% 36% 9% 21% 11%	3186
	21st to end	Night	9% 35% 9% 29% 16% 1	3586
September	1st to 10th	Night	11% 57% 5% 13% 12% 2	2032
	11th to 20th	Night	12% 57% 4 12% 13% 2	2200
	21st to end	Night	14% 57% 4 13% 10% 1	2200
October	1st to 10th	Night	10% 35% 3 12% 38% 2	3590
	11th to 20th	Night	12% 35% 3 15% 31% 4	3676
	21st to end	Night	17% 36% 4 14% 24% 5%	4467
November	1st to 10th	Night	22% 56% 3 11% 8% 1	2536
	11th to 20th	Night	18% 56% 2 10% 10% 3	2600
	21st to end	Night	22% 57% 4 11% 6%	2600
December	1st to 10th	Night	33% 35% 5% 15% 11% 1	4250
	11th to 20th	Night	35% 31% 8% 15% 10% 1	4824
	21st to end	Night	53% 27% 5% 11% 4	6932

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	27% 16% 36% 15% 4 1	300
	11th to 20th	Day	30% 27% 28% 14% 2	300
	21st to end	Day	26% 27% 34% 11% 2	330
February	1st to 10th	Day	27% 24% 36% 11% 2	300
	11th to 20th	Day	30% 28% 33% 7% 1 1	300
	21st to end	Day	31% 21% 37% 8% 2	249
March	1st to 10th	Day	29% 20% 35% 13% 3	300
	11th to 20th	Day	27% 16% 38% 11% 7%	435
	21st to end	Day	28% 22% 31% 14% 4 1	550
April	1st to 10th	Day	24% 18% 28% 21% 8% 1	500
	11th to 20th	Day	24% 17% 28% 19% 8% 3	492
	21st to end	Day	21% 16% 33% 19% 8% 2	500
May	1st to 10th	Day	23% 12% 31% 22% 7% 4	503
	11th to 20th	Day	20% 10% 29% 24% 13% 4	527
	21st to end	Day	13% 9% 28% 25% 14% 11%	573
June	1st to 10th	Day	13% 6% 25% 28% 19% 9%	618
	11th to 20th	Day	8% 5% 23% 34% 21% 9%	730
	21st to end	Day	11% 5% 25% 28% 20% 11%	730
July	1st to 10th	Day	12% 4 15% 28% 25% 16%	721
	11th to 20th	Day	8% 3 18% 35% 22% 13%	718
	21st to end	Day	10% 2 15% 34% 28% 12%	576
August	1st to 10th	Day	9% 4 15% 30% 27% 16%	526
	11th to 20th	Day	12% 4 17% 31% 26% 11%	519
	21st to end	Day	9% 3 10% 29% 30% 18%	583
September	1st to 10th	Day	13% 2 16% 28% 32% 9%	530
	11th to 20th	Day	10% 3 14% 27% 31% 15%	530
	21st to end	Day	18% 7% 18% 26% 21% 10%	530
October	1st to 10th	Day	12% 6% 21% 35% 21% 6%	530
	11th to 20th	Day	15% 13% 30% 26% 11% 5%	498
	21st to end	Day	25% 13% 37% 18% 7%	330
November	1st to 10th	Day	15% 13% 35% 26% 7% 3	300
	11th to 20th	Day	24% 17% 31% 23% 4 1	300
	21st to end	Day	27% 12% 36% 17% 7% 1	300
December	1st to 10th	Day	22% 26% 30% 15% 6% 1	300
	11th to 20th	Day	26% 16% 35% 17% 5%	300
	21st to end	Day	33% 19% 32% 13% 3	330
January	1st to 10th	Night	22% 18% 31% 25% 4	1000
	11th to 20th	Night	27% 19% 32% 20% 3	986
	21st to end	Night	23% 16% 38% 21% 2	880
February	1st to 10th	Night	28% 14% 35% 21% 2	800
	11th to 20th	Night	27% 16% 36% 19% 2	783
	21st to end	Night	27% 15% 37% 19% 2	660
March	1st to 10th	Night	23% 16% 31% 25% 5%	800
	11th to 20th	Night	24% 13% 34% 23% 5% 1	800
	21st to end	Night	24% 12% 29% 29% 6%	815
April	1st to 10th	Night	17% 12% 35% 28% 8%	600
	11th to 20th	Night	19% 12% 33% 25% 12%	593
	21st to end	Night	20% 9% 33% 29% 9% 1	600
May	1st to 10th	Night	20% 7% 31% 27% 15% 1	600
	11th to 20th	Night	16% 6% 22% 37% 16% 3	591
	21st to end	Night	9% 2 21% 37% 26% 5%	650
June	1st to 10th	Night	11% 3 20% 36% 27% 3	600
	11th to 20th	Night	9% 2 14% 44% 28% 3	600
	21st to end	Night	5% 2 20% 38% 31% 4	600
July	1st to 10th	Night	11% 2 11% 27% 41% 9%	585
	11th to 20th	Night	8% 2 9% 35% 40% 6%	582
	21st to end	Night	10% 2 12% 35% 35% 7%	657
August	1st to 10th	Night	8% 2 9% 33% 42% 6%	597
	11th to 20th	Night	7% 2 10% 38% 37% 6%	594
	21st to end	Night	7% 1 8% 35% 42% 7%	660
September	1st to 10th	Night	12% 2 12% 33% 36% 5%	632
	11th to 20th	Night	10% 2 13% 34% 35% 6%	800
	21st to end	Night	13% 3 20% 31% 29% 4	800
October	1st to 10th	Night	11% 4 17% 34% 29% 5%	800
	11th to 20th	Night	15% 8% 26% 30% 18% 2	793
	21st to end	Night	20% 9% 33% 25% 12% 1	836
November	1st to 10th	Night	17% 9% 28% 29% 16% 1	780
	11th to 20th	Night	19% 15% 28% 28% 10%	800
	21st to end	Night	23% 14% 25% 28% 11%	800
December	1st to 10th	Night	27% 11% 31% 26% 5% 1	800
	11th to 20th	Night	25% 13% 30% 25% 6% 1	800
	21st to end	Night	30% 14% 30% 22% 4	1075

Page generated at 2025-10-16 01:50:26. Version ac07

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