Collinsport Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Collinsport Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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% 9% 4 9% 5% 5% 5% 4 6% 7% 34%	125941
	Night	15% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	123062
Transparency	Day	31% 39% 8% 14% 8% 1	129552
	Night	33% 39% 7% 13% 8% 1	127520
Seeing	Day	16% 5% 18% 30% 25% 8%	16397
	Night	20% 7% 23% 29% 17% 4	26105

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% 17% 7% 19% 8% 5% 4 3 4 5% 16%	3617
	11th to 20th	Day	13% 19% 6% 15% 9% 6% 7% 3 4 4 13%	4014
	21st to end	Day	21% 16% 6% 13% 6% 5% 4 4 4 4 16%	4466
February	1st to 10th	Day	17% 14% 4 13% 7% 5% 4 4 4 6% 22%	1800
	11th to 20th	Day	14% 12% 5% 13% 8% 6% 4 4 5% 5% 25%	1872
	21st to end	Day	17% 10% 4 9% 7% 4 5% 5% 6% 6% 26%	1650
March	1st to 10th	Day	16% 10% 5% 10% 5% 5% 4 3 5% 5% 31%	4682
	11th to 20th	Day	9% 9% 3 9% 6% 5% 4 4 6% 7% 38%	4980
	21st to end	Day	16% 9% 4 9% 6% 5% 6% 4 6% 7% 29%	5465
April	1st to 10th	Day	13% 11% 3 7% 5% 4 5% 4 7% 7% 33%	2260
	11th to 20th	Day	15% 8% 3 7% 5% 5% 6% 3 5% 6% 36%	2558
	21st to end	Day	16% 8% 3 7% 5% 4 4 3 4 7% 37%	2600
May	1st to 10th	Day	19% 8% 3 8% 5% 4 5% 3 5% 6% 33%	5862
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	5861
	21st to end	Day	14% 7% 2 7% 4 4 4 4 6% 9% 40%	6433
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 8% 41%	2620
	11th to 20th	Day	9% 6% 3 6% 4 4 5% 4 7% 9% 44%	2958
	21st to end	Day	9% 4 2 6% 5% 4 5% 5% 7% 10% 43%	2800
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	6360
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 45%	6112
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 6% 9% 45%	6424
August	1st to 10th	Day	10% 6% 2 5% 4 4 4 4 6% 9% 47%	4234
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 4 8% 9% 42%	4174
	21st to end	Day	8% 5% 3 6% 4 4 5% 4 7% 10% 43%	4675
September	1st to 10th	Day	12% 6% 2 5% 3 3 4 4 5% 7% 49%	2224
	11th to 20th	Day	10% 6% 2 6% 3 4 4 4 6% 9% 48%	2178
	21st to end	Day	19% 7% 2 6% 3 4 4 4 5% 6% 40%	2090
October	1st to 10th	Day	14% 8% 3 7% 4 4 4 3 5% 6% 44%	2014
	11th to 20th	Day	19% 8% 3 7% 4 4 4 4 5% 6% 36%	1900
	21st to end	Day	19% 11% 3 7% 5% 5% 5% 4 5% 7% 29%	2005
November	1st to 10th	Day	10% 10% 4 8% 4 4 4 4 7% 8% 38%	1762
	11th to 20th	Day	17% 16% 5% 8% 5% 4 4 3 4 5% 29%	1800
	21st to end	Day	17% 15% 5% 11% 4 4 5% 5% 6% 6% 22%	1695
December	1st to 10th	Day	15% 15% 9% 14% 7% 5% 5% 3 4 6% 18%	2600
	11th to 20th	Day	19% 18% 7% 12% 6% 6% 5% 3 5% 5% 13%	2990
	21st to end	Day	22% 18% 7% 13% 7% 6% 4 3 3 4 14%	4206
January	1st to 10th	Night	15% 16% 8% 19% 6% 4 4 3 4 5% 15%	6348
	11th to 20th	Night	12% 19% 8% 15% 8% 6% 8% 4 3 4 13%	5926
	21st to end	Night	21% 16% 7% 12% 5% 4 4 2 4 5% 20%	6490
February	1st to 10th	Night	17% 15% 4 12% 7% 6% 4 4 5% 5% 22%	2600
	11th to 20th	Night	17% 13% 4 12% 7% 6% 4 4 5% 5% 24%	2410
	21st to end	Night	20% 11% 4 10% 6% 5% 4 3 5% 5% 27%	1980
March	1st to 10th	Night	18% 12% 4 7% 4 4 4 3 4 6% 34%	5157
	11th to 20th	Night	11% 7% 3 7% 5% 6% 5% 4 6% 8% 39%	4980
	21st to end	Night	15% 10% 4 8% 5% 4 5% 3 5% 6% 34%	5467
April	1st to 10th	Night	16% 13% 3 7% 6% 4 4 4 4 6% 32%	2157
	11th to 20th	Night	15% 9% 3 8% 5% 4 4 4 6% 7% 36%	1789
	21st to end	Night	16% 8% 2 7% 5% 5% 4 4 6% 6% 37%	1800
May	1st to 10th	Night	19% 10% 4 9% 6% 5% 4 3 5% 6% 27%	4033
	11th to 20th	Night	15% 8% 3 8% 4 4 4 4 6% 8% 36%	4032
	21st to end	Night	13% 6% 2 6% 4 5% 5% 4 6% 9% 39%	4421
June	1st to 10th	Night	12% 6% 3 6% 4 4 5% 5% 6% 7% 44%	1756
	11th to 20th	Night	9% 6% 2 7% 5% 5% 4 5% 7% 9% 42%	1422
	21st to end	Night	9% 4 2 6% 5% 5% 4 5% 7% 10% 44%	1600
July	1st to 10th	Night	8% 5% 2 6% 5% 4 6% 5% 8% 10% 43%	3618
	11th to 20th	Night	9% 5% 2 7% 5% 5% 4 4 8% 9% 42%	3837
	21st to end	Night	7% 5% 2 5% 4 4 4 4 7% 10% 46%	4427
August	1st to 10th	Night	13% 7% 2 7% 4 4 4 3 6% 9% 42%	2915
	11th to 20th	Night	9% 6% 2 7% 4 4 5% 4 6% 7% 46%	2875
	21st to end	Night	6% 5% 2 6% 6% 5% 6% 4 7% 7% 45%	3223
September	1st to 10th	Night	10% 7% 2 6% 5% 3 4 4 5% 8% 46%	2152
	11th to 20th	Night	9% 5% 2 6% 4 4 3 3 5% 8% 50%	2179
	21st to end	Night	15% 7% 2 8% 6% 5% 5% 3 5% 7% 39%	2090
October	1st to 10th	Night	14% 7% 2 6% 4 4 3 4 6% 7% 43%	2166
	11th to 20th	Night	21% 9% 3 7% 4 4 4 3 4 6% 35%	2280
	21st to end	Night	23% 10% 3 8% 5% 4 5% 3 5% 5% 29%	2591
November	1st to 10th	Night	14% 12% 3 8% 6% 4 4 3 5% 5% 36%	2536
	11th to 20th	Night	17% 16% 4 10% 5% 5% 4 3 5% 6% 26%	2600
	21st to end	Night	21% 15% 5% 10% 5% 4 5% 4 5% 6% 21%	2700
December	1st to 10th	Night	13% 16% 10% 13% 7% 5% 4 3 4 5% 20%	4580
	11th to 20th	Night	18% 17% 6% 12% 5% 5% 5% 3 5% 5% 19%	5205
	21st to end	Night	24% 19% 5% 12% 6% 4 4 3 4 5% 12%	6720

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	54% 29% 5% 8% 3	3618
	11th to 20th	Day	57% 27% 5% 4 6% 1	4014
	21st to end	Day	52% 30% 7% 7% 4	4466
February	1st to 10th	Day	25% 62% 5% 6% 2	1800
	11th to 20th	Day	24% 61% 4 9% 3	1881
	21st to end	Day	19% 63% 6% 7% 4 1	1650
March	1st to 10th	Day	40% 29% 7% 16% 7%	4682
	11th to 20th	Day	31% 30% 12% 20% 6% 1	4980
	21st to end	Day	37% 30% 10% 13% 10%	5465
April	1st to 10th	Day	22% 62% 5% 7% 4	2260
	11th to 20th	Day	18% 63% 5% 8% 5% 1	2558
	21st to end	Day	18% 62% 5% 9% 5% 1	2600
May	1st to 10th	Day	39% 30% 6% 13% 10% 1	5897
	11th to 20th	Day	35% 31% 11% 16% 6% 1	5865
	21st to end	Day	31% 30% 8% 17% 10% 5%	6447
June	1st to 10th	Day	17% 63% 4 11% 5% 1	2620
	11th to 20th	Day	17% 63% 5% 9% 5% 1	2958
	21st to end	Day	13% 61% 8% 12% 6% 1	2800
July	1st to 10th	Day	27% 28% 8% 23% 13% 1	6291
	11th to 20th	Day	24% 29% 12% 22% 13% 1	6038
	21st to end	Day	27% 28% 10% 17% 17% 1	6240
August	1st to 10th	Day	23% 38% 11% 18% 10%	4221
	11th to 20th	Day	24% 39% 8% 24% 5%	4213
	21st to end	Day	23% 38% 11% 21% 6%	4674
September	1st to 10th	Day	13% 57% 9% 14% 7% 1	2224
	11th to 20th	Day	13% 58% 4 15% 9%	2178
	21st to end	Day	17% 61% 4 10% 7%	2090
October	1st to 10th	Day	13% 37% 11% 23% 15%	3352
	11th to 20th	Day	24% 38% 7% 16% 15%	3140
	21st to end	Day	24% 38% 8% 17% 13%	3281
November	1st to 10th	Day	24% 57% 5% 9% 4	1762
	11th to 20th	Day	24% 57% 4 9% 5%	1800
	21st to end	Day	31% 57% 5% 5% 2	1695
December	1st to 10th	Day	46% 38% 8% 6% 2	2600
	11th to 20th	Day	52% 33% 5% 8% 1	2986
	21st to end	Day	57% 27% 5% 6% 4	4206
January	1st to 10th	Night	54% 29% 4 9% 4	6348
	11th to 20th	Night	57% 27% 4 6% 4 1	5926
	21st to end	Night	50% 28% 9% 6% 7%	6490
February	1st to 10th	Night	26% 62% 4 5% 2	2600
	11th to 20th	Night	24% 61% 3 8% 3	2412
	21st to end	Night	23% 62% 3 8% 3	1980
March	1st to 10th	Night	37% 30% 10% 15% 7% 1	5157
	11th to 20th	Night	29% 31% 10% 21% 9% 1	4980
	21st to end	Night	35% 30% 7% 14% 13% 1	5467
April	1st to 10th	Night	23% 62% 4 7% 3 1	2157
	11th to 20th	Night	20% 63% 3 7% 6% 1	1789
	21st to end	Night	17% 62% 5% 9% 6% 1	1800
May	1st to 10th	Night	42% 31% 5% 9% 11% 3	4060
	11th to 20th	Night	35% 30% 8% 20% 7% 1	4033
	21st to end	Night	31% 30% 8% 16% 12% 2	4429
June	1st to 10th	Night	16% 63% 3 11% 7% 1	1756
	11th to 20th	Night	17% 62% 4 10% 5% 1	1422
	21st to end	Night	14% 61% 5% 12% 7% 1	1600
July	1st to 10th	Night	25% 29% 11% 20% 11% 4	3580
	11th to 20th	Night	28% 29% 10% 21% 10% 2	3791
	21st to end	Night	24% 28% 10% 18% 18% 3	4297
August	1st to 10th	Night	28% 38% 8% 15% 10% 1	2904
	11th to 20th	Night	24% 38% 6% 24% 8%	2902
	21st to end	Night	23% 38% 8% 20% 9% 1	3223
September	1st to 10th	Night	15% 58% 7% 13% 6% 1	2152
	11th to 20th	Night	13% 57% 4 13% 11% 1	2179
	21st to end	Night	18% 60% 4 11% 8%	2090
October	1st to 10th	Night	14% 37% 5% 23% 20%	3608
	11th to 20th	Night	25% 38% 6% 18% 13%	3779
	21st to end	Night	22% 38% 5% 21% 12% 2	4265
November	1st to 10th	Night	27% 55% 5% 8% 4 1	2536
	11th to 20th	Night	25% 57% 3 8% 6% 1	2600
	21st to end	Night	29% 57% 6% 6% 1	2700
December	1st to 10th	Night	44% 37% 9% 8% 3	4580
	11th to 20th	Night	47% 33% 9% 9% 2	5208
	21st to end	Night	56% 27% 7% 7% 2	6720

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	20% 11% 27% 30% 9% 2	300
	11th to 20th	Day	21% 11% 34% 26% 7% 1	300
	21st to end	Day	25% 6% 27% 31% 10% 1	330
February	1st to 10th	Day	20% 6% 31% 34% 8% 1	300
	11th to 20th	Day	21% 9% 31% 31% 7% 1	300
	21st to end	Day	18% 7% 35% 32% 8%	249
March	1st to 10th	Day	19% 7% 25% 34% 15%	385
	11th to 20th	Day	16% 7% 28% 33% 13% 2	500
	21st to end	Day	23% 8% 23% 36% 10% 1	550
April	1st to 10th	Day	16% 6% 24% 39% 13% 2	500
	11th to 20th	Day	20% 6% 22% 34% 15% 4	492
	21st to end	Day	19% 4 23% 32% 20% 2	500
May	1st to 10th	Day	21% 4 19% 32% 21% 4	503
	11th to 20th	Day	16% 4 17% 30% 28% 6%	527
	21st to end	Day	11% 3 11% 31% 31% 13%	573
June	1st to 10th	Day	11% 3 14% 33% 30% 8%	541
	11th to 20th	Day	11% 2 11% 35% 33% 8%	730
	21st to end	Day	11% 2 11% 32% 34% 10%	730
July	1st to 10th	Day	9% 2 7% 27% 37% 18%	721
	11th to 20th	Day	10% 1 8% 29% 40% 13%	614
	21st to end	Day	9% 3 6% 29% 43% 11%	576
August	1st to 10th	Day	10% 2 8% 26% 39% 16%	526
	11th to 20th	Day	10% 1 10% 27% 39% 13%	519
	21st to end	Day	9% 2 8% 28% 40% 14%	583
September	1st to 10th	Day	13% 2 12% 22% 33% 18%	530
	11th to 20th	Day	10% 3 7% 22% 39% 19%	522
	21st to end	Day	21% 4 17% 21% 27% 10%	490
October	1st to 10th	Day	13% 4 17% 24% 30% 12%	490
	11th to 20th	Day	20% 8% 21% 29% 17% 5%	490
	21st to end	Day	24% 7% 25% 26% 13% 4	454
November	1st to 10th	Day	13% 8% 23% 27% 24% 6%	300
	11th to 20th	Day	25% 12% 24% 25% 12% 3	300
	21st to end	Day	23% 10% 30% 30% 7%	247
December	1st to 10th	Day	19% 10% 33% 30% 9%	200
	11th to 20th	Day	20% 14% 35% 24% 8%	200
	21st to end	Day	26% 9% 28% 26% 11%	325
January	1st to 10th	Night	25% 10% 32% 25% 7% 1	1000
	11th to 20th	Night	26% 10% 33% 24% 7%	813
	21st to end	Night	25% 9% 31% 29% 7% 1	880
February	1st to 10th	Night	23% 6% 33% 31% 6% 1	800
	11th to 20th	Night	22% 9% 31% 33% 5%	783
	21st to end	Night	24% 8% 35% 28% 6%	660
March	1st to 10th	Night	21% 5% 30% 33% 11% 1	800
	11th to 20th	Night	17% 7% 31% 32% 10% 3	800
	21st to end	Night	21% 7% 24% 34% 12% 3	880
April	1st to 10th	Night	20% 8% 27% 33% 10% 1	757
	11th to 20th	Night	19% 5% 25% 35% 14% 3	593
	21st to end	Night	18% 5% 24% 36% 15% 3	600
May	1st to 10th	Night	20% 4 19% 36% 18% 4	600
	11th to 20th	Night	21% 5% 17% 29% 22% 6%	591
	21st to end	Night	11% 3 14% 31% 31% 11%	650
June	1st to 10th	Night	14% 2 17% 34% 28% 4	573
	11th to 20th	Night	9% 15% 36% 34% 5%	422
	21st to end	Night	10% 1 16% 35% 32% 5%	600
July	1st to 10th	Night	11% 2 9% 32% 35% 11%	585
	11th to 20th	Night	9% 2 12% 34% 32% 10%	582
	21st to end	Night	8% 1 10% 36% 40% 6%	657
August	1st to 10th	Night	9% 2 10% 35% 35% 10%	597
	11th to 20th	Night	11% 1 10% 32% 36% 10%	594
	21st to end	Night	10% 3 11% 30% 36% 11%	660
September	1st to 10th	Night	12% 4 16% 25% 32% 11%	764
	11th to 20th	Night	11% 4 10% 26% 36% 13%	793
	21st to end	Night	17% 5% 18% 27% 25% 8%	760
October	1st to 10th	Night	18% 5% 18% 28% 23% 8%	760
	11th to 20th	Night	26% 10% 23% 25% 14% 2	760
	21st to end	Night	29% 10% 24% 26% 11% 1	836
November	1st to 10th	Night	19% 9% 26% 28% 14% 2	780
	11th to 20th	Night	25% 14% 29% 21% 9% 2	800
	21st to end	Night	30% 12% 28% 23% 7% 1	800
December	1st to 10th	Night	26% 15% 29% 22% 8%	800
	11th to 20th	Night	26% 12% 33% 22% 6% 1	800
	21st to end	Night	31% 9% 28% 27% 5%	975

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