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 2024-01-12.

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	14% 7% 3 6% 5% 3 5% 5% 6% 7% 39%	74956
	Night	14% 8% 3 6% 5% 3 4 4 5% 7% 41%	74736
Transparency	Day	28% 38% 7% 14% 10% 2	74669
	Night	29% 37% 7% 14% 11% 2	74544
Seeing	Day	17% 11% 25% 24% 15% 7%	14192
	Night	18% 9% 25% 28% 17% 2	24056

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	17% 8% 4 9% 5% 3 5% 3 5% 5% 36%	1517
	11th to 20th	Day	20% 10% 5% 10% 6% 3 4 4 4 5% 30%	1456
	21st to end	Day	15% 11% 4 7% 6% 3 4 4 5% 6% 35%	1836
February	1st to 10th	Day	19% 11% 5% 8% 5% 3 3 3 3 6% 33%	1800
	11th to 20th	Day	18% 8% 5% 7% 6% 3 4 4 5% 7% 34%	1748
	21st to end	Day	19% 10% 4 9% 6% 3 3 5% 6% 5% 31%	1480
March	1st to 10th	Day	18% 8% 3 5% 5% 2 5% 4 4 6% 40%	1800
	11th to 20th	Day	19% 9% 3 7% 6% 3 4 4 5% 6% 33%	1921
	21st to end	Day	18% 9% 4 7% 6% 4 4 4 6% 7% 31%	2250
April	1st to 10th	Day	12% 7% 3 6% 6% 3 4 5% 6% 8% 40%	2160
	11th to 20th	Day	16% 9% 3 6% 5% 3 5% 4 4 7% 40%	2134
	21st to end	Day	19% 6% 3 5% 6% 4 4 4 5% 6% 38%	2281
May	1st to 10th	Day	18% 7% 3 6% 6% 3 4 5% 6% 8% 35%	2340
	11th to 20th	Day	16% 7% 4 6% 5% 4 5% 5% 5% 7% 36%	2301
	21st to end	Day	7% 6% 2 6% 7% 4 7% 6% 8% 10% 35%	2531
June	1st to 10th	Day	11% 4 3 5% 6% 3 6% 5% 5% 7% 46%	2418
	11th to 20th	Day	5% 4 2 6% 6% 4 5% 6% 6% 10% 45%	2520
	21st to end	Day	7% 5% 2 4 6% 4 5% 6% 7% 9% 44%	2520
July	1st to 10th	Day	13% 6% 3 5% 6% 3 6% 5% 8% 10% 35%	2520
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 6% 8% 9% 42%	2520
	21st to end	Day	10% 5% 2 5% 5% 4 5% 6% 8% 10% 40%	2547
August	1st to 10th	Day	8% 5% 2 6% 5% 3 5% 6% 8% 10% 41%	2327
	11th to 20th	Day	9% 5% 2 6% 6% 4 6% 6% 7% 9% 41%	2150
	21st to end	Day	8% 5% 2 5% 5% 4 5% 5% 5% 9% 46%	2376
September	1st to 10th	Day	13% 6% 2 5% 6% 4 5% 3 5% 8% 43%	2112
	11th to 20th	Day	11% 6% 2 5% 5% 3 5% 5% 5% 9% 45%	1980
	21st to end	Day	11% 5% 3 4 5% 3 4 4 5% 6% 51%	1980
October	1st to 10th	Day	11% 5% 2 4 4 2 4 4 4 7% 54%	1980
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 5% 6% 49%	1904
	21st to end	Day	19% 6% 3 5% 4 3 3 4 4 6% 42%	1782
November	1st to 10th	Day	11% 4 2 5% 4 3 4 4 4 5% 54%	1672
	11th to 20th	Day	16% 8% 3 4 4 2 4 4 5% 6% 44%	1710
	21st to end	Day	17% 8% 3 6% 4 2 4 4 4 5% 42%	1710
December	1st to 10th	Day	19% 12% 4 7% 5% 3 3 4 4 6% 33%	1710
	11th to 20th	Day	15% 12% 6% 8% 6% 3 5% 4 5% 5% 33%	2153
	21st to end	Day	20% 16% 4 7% 5% 3 5% 4 5% 7% 25%	2810
January	1st to 10th	Night	16% 11% 4 8% 7% 3 4 4 4 6% 33%	2648
	11th to 20th	Night	20% 11% 5% 8% 6% 3 4 4 5% 5% 28%	2522
	21st to end	Night	16% 10% 5% 7% 5% 3 4 5% 5% 6% 33%	2516
February	1st to 10th	Night	21% 11% 6% 6% 5% 3 4 4 5% 5% 28%	2160
	11th to 20th	Night	19% 9% 4 8% 7% 3 4 4 3 6% 35%	2098
	21st to end	Night	21% 11% 4 8% 5% 3 4 4 5% 6% 31%	1776
March	1st to 10th	Night	17% 8% 4 6% 5% 3 4 4 4 5% 41%	2160
	11th to 20th	Night	19% 10% 3 7% 6% 3 4 4 5% 5% 35%	2039
	21st to end	Night	18% 9% 3 7% 6% 3 4 4 5% 7% 34%	2120
April	1st to 10th	Night	12% 6% 3 4 5% 4 5% 5% 6% 8% 45%	1800
	11th to 20th	Night	16% 7% 3 5% 5% 3 4 3 5% 7% 42%	1775
	21st to end	Night	15% 7% 4 5% 6% 3 5% 4 5% 8% 38%	1679
May	1st to 10th	Night	16% 7% 3 7% 5% 3 4 4 5% 7% 37%	1620
	11th to 20th	Night	11% 6% 3 6% 7% 4 5% 6% 5% 8% 39%	1592
	21st to end	Night	5% 4 2 6% 6% 5% 6% 7% 9% 10% 40%	1744
June	1st to 10th	Night	9% 4 2 5% 5% 2 4 4 7% 9% 48%	1536
	11th to 20th	Night	5% 4 1 5% 5% 3 5% 6% 5% 11% 50%	1440
	21st to end	Night	4 3 2 4 5% 3 5% 6% 6% 13% 49%	1440
July	1st to 10th	Night	8% 6% 2 5% 5% 4 5% 6% 5% 9% 47%	1440
	11th to 20th	Night	7% 3 2 4 4 2 4 5% 7% 11% 50%	1440
	21st to end	Night	9% 3 2 5% 6% 3 4 6% 7% 10% 46%	1767
August	1st to 10th	Night	7% 4 2 4 5% 3 5% 4 6% 9% 51%	1611
	11th to 20th	Night	5% 5% 2 5% 5% 3 5% 6% 7% 11% 45%	1747
	21st to end	Night	6% 4 2 4 4 2 3 4 6% 11% 55%	1980
September	1st to 10th	Night	12% 5% 2 5% 4 3 3 4 5% 7% 51%	1828
	11th to 20th	Night	9% 5% 2 4 5% 3 3 5% 5% 8% 51%	1980
	21st to end	Night	10% 5% 2 4 4 2 4 4 5% 6% 55%	1980
October	1st to 10th	Night	9% 4 2 4 4 3 3 3 5% 6% 59%	1980
	11th to 20th	Night	15% 5% 2 5% 5% 3 4 4 4 6% 48%	2052
	21st to end	Night	17% 7% 2 6% 4 2 4 4 4 5% 46%	2574
November	1st to 10th	Night	12% 6% 2 4 4 3 3 4 4 5% 53%	2406
	11th to 20th	Night	15% 6% 2 5% 5% 3 4 4 4 6% 46%	2470
	21st to end	Night	17% 9% 4 4 4 3 4 4 5% 5% 42%	2470
December	1st to 10th	Night	20% 11% 4 5% 5% 3 3 4 4 6% 34%	2470
	11th to 20th	Night	18% 10% 4 9% 5% 3 4 4 4 5% 34%	3042
	21st to end	Night	18% 15% 5% 9% 7% 4 5% 4 4 5% 23%	4834

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	32% 41% 7% 16% 5%	1518
	11th to 20th	Day	36% 41% 6% 11% 6%	1456
	21st to end	Day	33% 38% 7% 17% 4 1	1836
February	1st to 10th	Day	37% 36% 5% 16% 6% 1	1800
	11th to 20th	Day	33% 37% 7% 12% 8% 2	1758
	21st to end	Day	36% 38% 7% 10% 7% 2	1480
March	1st to 10th	Day	29% 36% 7% 15% 12% 1	1800
	11th to 20th	Day	36% 37% 7% 11% 8% 1	1921
	21st to end	Day	36% 39% 8% 13% 3 1	2250
April	1st to 10th	Day	25% 38% 7% 16% 13% 1	2160
	11th to 20th	Day	29% 36% 5% 14% 14% 2	2134
	21st to end	Day	32% 37% 7% 16% 7% 1	2281
May	1st to 10th	Day	33% 38% 8% 14% 7% 1	2340
	11th to 20th	Day	32% 39% 7% 10% 10% 1	2305
	21st to end	Day	24% 40% 11% 12% 11% 1	2531
June	1st to 10th	Day	24% 38% 8% 17% 11% 2	2418
	11th to 20th	Day	20% 38% 11% 17% 12% 2	2520
	21st to end	Day	21% 40% 8% 14% 13% 4	2520
July	1st to 10th	Day	25% 41% 8% 14% 9% 2	2451
	11th to 20th	Day	24% 40% 10% 12% 10% 3	2445
	21st to end	Day	25% 41% 10% 17% 7% 1	2405
August	1st to 10th	Day	22% 40% 10% 16% 11% 1	2314
	11th to 20th	Day	23% 38% 8% 19% 10% 2	2150
	21st to end	Day	22% 38% 7% 16% 13% 5%	2376
September	1st to 10th	Day	29% 38% 6% 13% 11% 3	2112
	11th to 20th	Day	24% 38% 7% 15% 14% 2	1980
	21st to end	Day	24% 37% 5% 15% 16% 3	1980
October	1st to 10th	Day	19% 36% 6% 14% 22% 3	1980
	11th to 20th	Day	25% 37% 5% 17% 13% 2	1904
	21st to end	Day	29% 38% 5% 15% 11% 2	1782
November	1st to 10th	Day	17% 40% 5% 20% 15% 4	1672
	11th to 20th	Day	26% 39% 6% 14% 11% 2	1710
	21st to end	Day	27% 39% 5% 14% 13% 1	1710
December	1st to 10th	Day	34% 40% 6% 14% 6%	1710
	11th to 20th	Day	38% 32% 6% 13% 9% 1	2150
	21st to end	Day	47% 27% 9% 11% 5%	2810
January	1st to 10th	Night	33% 40% 7% 13% 7%	2648
	11th to 20th	Night	35% 43% 7% 10% 5%	2522
	21st to end	Night	34% 37% 6% 16% 6% 1	2516
February	1st to 10th	Night	38% 37% 7% 13% 4 1	2160
	11th to 20th	Night	35% 37% 5% 14% 9% 1	2099
	21st to end	Night	38% 36% 8% 10% 6% 2	1776
March	1st to 10th	Night	28% 36% 5% 15% 13% 2	2160
	11th to 20th	Night	34% 38% 7% 13% 6% 2	2039
	21st to end	Night	34% 38% 9% 14% 4 1	2120
April	1st to 10th	Night	24% 38% 8% 16% 13% 3	1800
	11th to 20th	Night	27% 35% 6% 13% 14% 4	1775
	21st to end	Night	31% 37% 6% 15% 10% 1	1679
May	1st to 10th	Night	33% 38% 7% 13% 7% 2	1620
	11th to 20th	Night	30% 39% 7% 10% 12% 1	1593
	21st to end	Night	21% 42% 9% 14% 12% 2	1745
June	1st to 10th	Night	19% 38% 10% 16% 14% 1	1536
	11th to 20th	Night	17% 39% 10% 15% 15% 3	1440
	21st to end	Night	18% 39% 11% 15% 12% 6%	1440
July	1st to 10th	Night	22% 40% 9% 15% 11% 2	1400
	11th to 20th	Night	19% 40% 10% 16% 11% 4	1392
	21st to end	Night	21% 42% 9% 19% 8% 1	1669
August	1st to 10th	Night	19% 38% 9% 17% 14% 2	1600
	11th to 20th	Night	18% 38% 10% 19% 14% 2	1747
	21st to end	Night	16% 38% 11% 16% 16% 4	1980
September	1st to 10th	Night	25% 38% 7% 12% 14% 4	1828
	11th to 20th	Night	22% 37% 5% 18% 14% 3	1980
	21st to end	Night	22% 37% 5% 15% 17% 4	1980
October	1st to 10th	Night	19% 35% 5% 14% 24% 4	1980
	11th to 20th	Night	26% 37% 5% 16% 14% 3	2052
	21st to end	Night	27% 37% 5% 14% 14% 3	2574
November	1st to 10th	Night	17% 40% 6% 17% 17% 4	2406
	11th to 20th	Night	26% 39% 5% 13% 13% 4	2470
	21st to end	Night	28% 39% 5% 14% 12% 2	2470
December	1st to 10th	Night	32% 39% 8% 14% 7%	2470
	11th to 20th	Night	37% 33% 6% 13% 10% 1	3044
	21st to end	Night	50% 27% 7% 10% 5%	4834

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	25% 16% 37% 16% 5% 1	270
	11th to 20th	Day	29% 24% 28% 16% 2	243
	21st to end	Day	24% 29% 34% 11% 2	264
February	1st to 10th	Day	28% 24% 35% 10% 2	240
	11th to 20th	Day	31% 26% 35% 7% 1	240
	21st to end	Day	31% 20% 38% 8% 3	198
March	1st to 10th	Day	26% 21% 40% 11% 2	240
	11th to 20th	Day	27% 17% 36% 12% 8% 1	361
	21st to end	Day	26% 22% 31% 15% 5% 1	462
April	1st to 10th	Day	20% 17% 29% 24% 9% 1	420
	11th to 20th	Day	24% 16% 27% 20% 10% 3	412
	21st to end	Day	22% 15% 32% 19% 9% 2	420
May	1st to 10th	Day	22% 12% 32% 22% 7% 5%	423
	11th to 20th	Day	19% 9% 30% 26% 12% 4	447
	21st to end	Day	12% 6% 26% 27% 16% 12%	485
June	1st to 10th	Day	13% 6% 21% 29% 20% 10%	528
	11th to 20th	Day	6% 5% 24% 33% 22% 9%	630
	21st to end	Day	11% 6% 25% 27% 20% 11%	630
July	1st to 10th	Day	12% 4 14% 27% 24% 18%	621
	11th to 20th	Day	9% 3 17% 33% 24% 15%	618
	21st to end	Day	9% 2 15% 34% 28% 12%	488
August	1st to 10th	Day	10% 4 13% 30% 27% 16%	446
	11th to 20th	Day	13% 5% 15% 31% 26% 10%	439
	21st to end	Day	11% 3 10% 27% 31% 18%	495
September	1st to 10th	Day	14% 2 14% 28% 33% 9%	450
	11th to 20th	Day	11% 3 15% 26% 31% 14%	450
	21st to end	Day	15% 7% 18% 26% 22% 11%	450
October	1st to 10th	Day	12% 6% 22% 35% 18% 7%	450
	11th to 20th	Day	17% 14% 32% 25% 9% 4	428
	21st to end	Day	27% 13% 35% 18% 7%	297
November	1st to 10th	Day	13% 13% 36% 27% 8% 4	270
	11th to 20th	Day	23% 17% 30% 24% 5% 1	270
	21st to end	Day	26% 11% 37% 18% 8% 1	270
December	1st to 10th	Day	21% 25% 30% 16% 7% 1	270
	11th to 20th	Day	24% 17% 35% 18% 6%	270
	21st to end	Day	31% 21% 31% 14% 3	297
January	1st to 10th	Night	22% 18% 31% 25% 4	950
	11th to 20th	Night	26% 18% 31% 21% 4	893
	21st to end	Night	23% 17% 38% 21% 2	792
February	1st to 10th	Night	29% 14% 34% 21% 2	720
	11th to 20th	Night	26% 17% 36% 20% 1	703
	21st to end	Night	28% 16% 37% 18% 2	592
March	1st to 10th	Night	22% 16% 33% 25% 5%	720
	11th to 20th	Night	24% 13% 34% 23% 6% 1	720
	21st to end	Night	24% 12% 26% 31% 7%	734
April	1st to 10th	Night	15% 11% 36% 30% 9%	540
	11th to 20th	Night	19% 11% 32% 25% 13%	533
	21st to end	Night	20% 9% 33% 28% 9% 1	540
May	1st to 10th	Night	18% 7% 33% 26% 16% 1	540
	11th to 20th	Night	15% 6% 23% 36% 16% 4	531
	21st to end	Night	8% 2 20% 36% 28% 5%	584
June	1st to 10th	Night	11% 3 19% 36% 28% 4	540
	11th to 20th	Night	8% 2 14% 45% 27% 3	540
	21st to end	Night	5% 2 20% 39% 31% 4	540
July	1st to 10th	Night	11% 2 10% 26% 42% 10%	525
	11th to 20th	Night	6% 2 9% 34% 42% 7%	522
	21st to end	Night	10% 2 12% 36% 33% 7%	591
August	1st to 10th	Night	9% 1 8% 34% 42% 6%	537
	11th to 20th	Night	7% 2 10% 38% 37% 7%	534
	21st to end	Night	8% 1 8% 34% 43% 6%	594
September	1st to 10th	Night	13% 1 11% 31% 37% 6%	568
	11th to 20th	Night	11% 2 14% 33% 34% 6%	720
	21st to end	Night	11% 3 21% 30% 30% 4	720
October	1st to 10th	Night	11% 5% 18% 34% 27% 5%	720
	11th to 20th	Night	16% 9% 28% 30% 15% 2	720
	21st to end	Night	21% 9% 32% 26% 11% 1	792
November	1st to 10th	Night	16% 8% 28% 29% 17% 1	740
	11th to 20th	Night	18% 15% 29% 28% 10%	760
	21st to end	Night	23% 13% 25% 28% 11%	760
December	1st to 10th	Night	26% 12% 30% 26% 5% 1	760
	11th to 20th	Night	25% 12% 30% 25% 7% 1	760
	21st to end	Night	29% 15% 30% 22% 4	1021

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