Inlet Outlet Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Inlet Outlet Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	16% 7% 2 6% 4 4 4 4 5% 7% 41%	126844
	Night	16% 8% 2 6% 4 4 4 3 5% 6% 43%	123498
Transparency	Day	41% 6% 12% 23% 16% 2	130371
	Night	41% 4 11% 23% 17% 3	128045
Seeing	Day	18% 5% 23% 31% 18% 4	16412
	Night	19% 6% 28% 30% 14% 2	25567

By Month

Percentage Cloud Cover NE (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	13% 6% 2 4 3 2 4 3 5% 6% 52%	3597
	11th to 20th	Day	14% 8% 3 5% 4 4 4 3 4 5% 46%	3570
	21st to end	Day	18% 10% 4 8% 4 4 3 3 3 5% 39%	4374
February	1st to 10th	Day	15% 10% 3 6% 5% 3 3 3 5% 6% 41%	1830
	11th to 20th	Day	16% 8% 3 5% 4 4 4 4 5% 6% 42%	1948
	21st to end	Day	17% 8% 3 6% 4 3 4 4 5% 5% 40%	1650
March	1st to 10th	Day	21% 6% 1 4 2 3 3 3 4 6% 46%	4476
	11th to 20th	Day	12% 10% 2 6% 4 4 5% 3 5% 6% 43%	4786
	21st to end	Day	17% 7% 2 5% 4 3 4 3 5% 6% 43%	5922
April	1st to 10th	Day	18% 8% 2 5% 5% 4 4 3 5% 7% 39%	2400
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 3 6% 8% 41%	2374
	21st to end	Day	17% 6% 2 5% 4 4 5% 4 6% 7% 40%	2403
May	1st to 10th	Day	24% 9% 3 6% 4 4 5% 4 6% 7% 28%	5886
	11th to 20th	Day	25% 8% 2 5% 3 3 3 3 4 6% 36%	6197
	21st to end	Day	20% 7% 2 6% 4 4 4 4 6% 8% 36%	6949
June	1st to 10th	Day	15% 6% 2 5% 5% 4 5% 5% 7% 10% 38%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 5% 5% 9% 39%	2800
	21st to end	Day	10% 5% 2 5% 5% 5% 6% 5% 7% 10% 42%	2800
July	1st to 10th	Day	9% 5% 2 7% 5% 7% 7% 5% 7% 9% 36%	6360
	11th to 20th	Day	10% 8% 3 8% 6% 6% 6% 6% 8% 10% 30%	6360
	21st to end	Day	10% 7% 2 7% 4 4 5% 4 7% 9% 40%	6926
August	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 4 7% 9% 33%	4199
	11th to 20th	Day	12% 6% 3 6% 4 4 5% 4 6% 9% 41%	3850
	21st to end	Day	9% 4 1 5% 3 3 4 4 7% 10% 47%	4312
September	1st to 10th	Day	14% 5% 2 6% 4 3 3 3 5% 8% 47%	2400
	11th to 20th	Day	11% 5% 2 6% 3 3 4 3 5% 8% 50%	2310
	21st to end	Day	18% 7% 3 6% 4 4 4 4 5% 6% 39%	2200
October	1st to 10th	Day	20% 5% 2 4 3 3 3 4 5% 7% 45%	2095
	11th to 20th	Day	15% 6% 2 5% 4 3 3 2 5% 7% 49%	2000
	21st to end	Day	17% 6% 2 5% 3 2 3 3 5% 7% 46%	2055
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 5% 6% 51%	1851
	11th to 20th	Day	16% 5% 2 4 5% 3 4 3 5% 6% 47%	1863
	21st to end	Day	16% 6% 2 4 3 4 5% 3 5% 7% 46%	1755
December	1st to 10th	Day	13% 6% 2 6% 3 4 3 4 6% 7% 47%	2600
	11th to 20th	Day	22% 8% 2 5% 2 2 2 2 4 5% 46%	2986
	21st to end	Day	15% 7% 3 6% 4 4 4 4 5% 8% 40%	3960
January	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 4 6% 53%	6349
	11th to 20th	Night	15% 8% 2 5% 3 3 3 3 4 5% 49%	6315
	21st to end	Night	18% 9% 2 6% 4 3 4 3 5% 6% 40%	6564
February	1st to 10th	Night	17% 10% 2 6% 4 3 3 3 6% 6% 41%	2567
	11th to 20th	Night	18% 9% 3 4 4 3 4 3 4 5% 43%	2338
	21st to end	Night	19% 7% 2 6% 3 3 3 3 5% 6% 43%	1980
March	1st to 10th	Night	18% 9% 3 6% 3 3 2 2 3 5% 46%	5364
	11th to 20th	Night	13% 13% 3 5% 4 3 3 3 4 5% 45%	5171
	21st to end	Night	18% 8% 3 7% 4 4 3 3 4 5% 40%	5009
April	1st to 10th	Night	17% 9% 3 7% 5% 4 3 4 5% 5% 39%	2000
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 6% 42%	1975
	21st to end	Night	18% 6% 3 5% 3 3 4 4 4 7% 43%	1995
May	1st to 10th	Night	23% 10% 3 7% 3 5% 4 4 6% 7% 27%	4054
	11th to 20th	Night	24% 8% 4 6% 3 3 3 3 5% 7% 34%	3725
	21st to end	Night	21% 9% 2 6% 3 3 4 3 5% 8% 35%	3926
June	1st to 10th	Night	16% 8% 3 6% 5% 4 4 4 5% 8% 40%	1600
	11th to 20th	Night	16% 7% 2 6% 4 4 5% 4 6% 8% 40%	1600
	21st to end	Night	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	1600
July	1st to 10th	Night	8% 8% 3 8% 6% 6% 6% 5% 6% 9% 36%	3600
	11th to 20th	Night	11% 10% 3 8% 5% 5% 6% 4 7% 10% 32%	3600
	21st to end	Night	11% 7% 2 7% 5% 4 4 4 7% 8% 41%	3926
August	1st to 10th	Night	21% 8% 3 7% 5% 4 4 4 6% 8% 31%	2902
	11th to 20th	Night	11% 7% 3 7% 4 4 5% 4 6% 8% 42%	3197
	21st to end	Night	8% 5% 2 5% 4 3 4 3 5% 8% 53%	3586
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 7% 50%	2089
	21st to end	Night	20% 7% 3 5% 4 3 4 3 5% 7% 39%	2200
October	1st to 10th	Night	19% 6% 3 5% 4 4 3 3 4 6% 44%	2300
	11th to 20th	Night	16% 4 2 5% 3 3 3 3 4 6% 50%	2400
	21st to end	Night	18% 7% 3 5% 3 3 3 3 4 6% 45%	2783
November	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 5% 6% 52%	2667
	11th to 20th	Night	16% 7% 3 5% 4 3 3 3 4 5% 48%	2692
	21st to end	Night	14% 6% 2 4 4 3 5% 3 6% 7% 47%	2640
December	1st to 10th	Night	14% 7% 2 5% 3 3 3 3 4 6% 49%	4580
	11th to 20th	Night	23% 7% 2 5% 3 2 2 3 4 6% 43%	5208
	21st to end	Night	17% 9% 3 6% 3 4 4 3 5% 6% 41%	6996

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	33% 3 8% 31% 23% 2	3598
	11th to 20th	Day	41% 4 8% 24% 17% 6%	3600
	21st to end	Day	49% 4 9% 23% 14% 2	4374
February	1st to 10th	Day	44% 5% 13% 23% 14% 2	1830
	11th to 20th	Day	43% 4 9% 24% 17% 3	1957
	21st to end	Day	45% 5% 10% 22% 14% 4	1650
March	1st to 10th	Day	40% 4 10% 22% 16% 8%	4479
	11th to 20th	Day	41% 5% 9% 27% 14% 4	4786
	21st to end	Day	41% 5% 11% 15% 23% 5%	5922
April	1st to 10th	Day	44% 6% 11% 19% 17% 5%	2400
	11th to 20th	Day	40% 6% 12% 22% 17% 3	2374
	21st to end	Day	41% 6% 11% 19% 19% 4	2403
May	1st to 10th	Day	54% 7% 14% 14% 9% 1	5897
	11th to 20th	Day	50% 5% 9% 23% 13% 1	6201
	21st to end	Day	45% 7% 9% 23% 15% 1	6949
June	1st to 10th	Day	39% 9% 13% 21% 16% 2	2800
	11th to 20th	Day	39% 10% 13% 24% 12% 2	2800
	21st to end	Day	35% 9% 15% 23% 16% 1	2800
July	1st to 10th	Day	43% 10% 12% 20% 14% 1	6290
	11th to 20th	Day	46% 8% 19% 18% 9% 1	6285
	21st to end	Day	38% 6% 12% 22% 20% 1	6728
August	1st to 10th	Day	45% 7% 16% 22% 9% 1	4185
	11th to 20th	Day	38% 6% 14% 30% 11% 1	3886
	21st to end	Day	29% 6% 15% 30% 19% 2	4312
September	1st to 10th	Day	36% 6% 14% 22% 20% 2	2400
	11th to 20th	Day	32% 7% 10% 26% 23% 2	2310
	21st to end	Day	45% 6% 11% 18% 19% 2	2200
October	1st to 10th	Day	33% 5% 12% 22% 23% 5%	3407
	11th to 20th	Day	25% 3 10% 24% 32% 5%	3240
	21st to end	Day	27% 4 14% 27% 25% 3	3293
November	1st to 10th	Day	35% 3 13% 26% 20% 2	1851
	11th to 20th	Day	38% 3 10% 23% 24% 3	1863
	21st to end	Day	39% 4 13% 29% 12% 2	1755
December	1st to 10th	Day	38% 3 15% 27% 14% 2	2600
	11th to 20th	Day	43% 2 10% 29% 12% 3	2986
	21st to end	Day	42% 6% 11% 22% 17% 2	3960
January	1st to 10th	Night	34% 3 9% 26% 24% 5%	6349
	11th to 20th	Night	39% 3 10% 24% 17% 7%	6360
	21st to end	Night	46% 3 9% 21% 18% 3	6564
February	1st to 10th	Night	44% 4 14% 23% 13% 2	2567
	11th to 20th	Night	45% 4 9% 21% 19% 4	2340
	21st to end	Night	43% 3 8% 24% 17% 5%	1980
March	1st to 10th	Night	43% 3 8% 24% 13% 9%	5374
	11th to 20th	Night	42% 4 11% 25% 14% 5%	5172
	21st to end	Night	46% 4 6% 15% 24% 5%	5009
April	1st to 10th	Night	47% 4 9% 19% 17% 4	2000
	11th to 20th	Night	43% 5% 9% 19% 21% 3	1975
	21st to end	Night	41% 5% 8% 20% 20% 5%	1995
May	1st to 10th	Night	55% 6% 12% 16% 9% 2	4060
	11th to 20th	Night	52% 4 11% 18% 14% 1	3726
	21st to end	Night	47% 6% 12% 21% 13% 2	3927
June	1st to 10th	Night	43% 7% 9% 20% 20% 1	1600
	11th to 20th	Night	40% 8% 13% 26% 12% 1	1600
	21st to end	Night	38% 7% 15% 23% 15% 2	1600
July	1st to 10th	Night	46% 7% 12% 22% 12% 1	3560
	11th to 20th	Night	47% 6% 17% 22% 9% 1	3552
	21st to end	Night	38% 6% 15% 23% 17% 1	3809
August	1st to 10th	Night	50% 7% 18% 16% 9% 1	2892
	11th to 20th	Night	40% 6% 13% 30% 11% 1	3227
	21st to end	Night	30% 5% 11% 32% 21% 2	3586
September	1st to 10th	Night	40% 6% 10% 23% 18% 2	2000
	11th to 20th	Night	34% 4 10% 24% 24% 3	2089
	21st to end	Night	46% 5% 7% 22% 19% 2	2200
October	1st to 10th	Night	38% 4 10% 23% 22% 3	3755
	11th to 20th	Night	25% 3 10% 27% 30% 5%	3899
	21st to end	Night	29% 3 11% 30% 24% 4	4495
November	1st to 10th	Night	34% 4 12% 25% 21% 4	2667
	11th to 20th	Night	40% 4 8% 23% 20% 5%	2692
	21st to end	Night	36% 5% 13% 27% 17% 1	2640
December	1st to 10th	Night	38% 3 12% 29% 15% 3	4580
	11th to 20th	Night	45% 3 9% 25% 14% 4	5208
	21st to end	Night	44% 4 11% 21% 17% 3	6996

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	16% 10% 46% 22% 6% 1	200
	11th to 20th	Day	19% 19% 36% 23% 3 1	200
	21st to end	Day	21% 8% 38% 25% 7% 1	310
February	1st to 10th	Day	22% 6% 35% 31% 5% 1	330
	11th to 20th	Day	22% 13% 37% 26% 1	496
	21st to end	Day	20% 13% 35% 26% 5%	414
March	1st to 10th	Day	18% 9% 39% 28% 6%	500
	11th to 20th	Day	17% 9% 38% 29% 6% 2	500
	21st to end	Day	23% 8% 31% 29% 8% 1	550
April	1st to 10th	Day	20% 8% 35% 29% 8% 1	500
	11th to 20th	Day	18% 6% 29% 34% 12% 1	492
	21st to end	Day	20% 7% 26% 33% 12% 1	500
May	1st to 10th	Day	25% 5% 25% 30% 12% 3	503
	11th to 20th	Day	24% 2 20% 38% 14% 2	527
	21st to end	Day	22% 2 12% 32% 25% 6%	573
June	1st to 10th	Day	18% 3 17% 33% 26% 5%	530
	11th to 20th	Day	15% 2 18% 37% 23% 5%	530
	21st to end	Day	12% 3 13% 39% 28% 5%	530
July	1st to 10th	Day	11% 3 9% 33% 34% 9%	525
	11th to 20th	Day	10% 2 8% 34% 35% 12%	524
	21st to end	Day	12% 2 13% 36% 32% 5%	576
August	1st to 10th	Day	12% 1 8% 39% 32% 8%	526
	11th to 20th	Day	12% 1 8% 33% 37% 9%	519
	21st to end	Day	11% 2 10% 36% 30% 10%	583
September	1st to 10th	Day	18% 2 16% 30% 23% 11%	530
	11th to 20th	Day	14% 1 11% 35% 29% 10%	530
	21st to end	Day	24% 4 17% 32% 20% 4	530
October	1st to 10th	Day	23% 4 15% 26% 24% 8%	530
	11th to 20th	Day	16% 5% 26% 28% 21% 4	530
	21st to end	Day	19% 8% 29% 26% 15% 3	503
November	1st to 10th	Day	14% 6% 27% 35% 16% 3	425
	11th to 20th	Day	15% 11% 35% 27% 12%	421
	21st to end	Day	18% 8% 33% 29% 10% 1	355
December	1st to 10th	Day	21% 10% 39% 21% 9% 1	200
	11th to 20th	Day	27% 12% 31% 22% 9%	200
	21st to end	Day	20% 12% 29% 27% 12%	220
January	1st to 10th	Night	20% 11% 41% 23% 5%	800
	11th to 20th	Night	23% 12% 35% 25% 5%	800
	21st to end	Night	20% 11% 44% 23% 3	880
February	1st to 10th	Night	22% 10% 40% 26% 3	800
	11th to 20th	Night	23% 13% 43% 20% 2	783
	21st to end	Night	23% 12% 35% 26% 4	660
March	1st to 10th	Night	18% 10% 41% 25% 6% 1	800
	11th to 20th	Night	20% 8% 37% 28% 6% 1	800
	21st to end	Night	21% 6% 35% 30% 7% 1	880
April	1st to 10th	Night	20% 7% 34% 31% 7% 1	800
	11th to 20th	Night	20% 5% 28% 37% 9%	791
	21st to end	Night	20% 4 29% 36% 10% 1	795
May	1st to 10th	Night	25% 4 23% 33% 13% 1	600
	11th to 20th	Night	25% 2 24% 36% 11% 1	466
	21st to end	Night	21% 2 11% 35% 27% 4	433
June	1st to 10th	Night	19% 2 18% 36% 23% 2	400
	11th to 20th	Night	18% 1 13% 43% 24% 1	400
	21st to end	Night	10% 2 12% 47% 28% 2	400
July	1st to 10th	Night	12% 3 12% 38% 31% 4	390
	11th to 20th	Night	12% 2 9% 37% 36% 4	388
	21st to end	Night	11% 1 12% 41% 33% 1	438
August	1st to 10th	Night	13% 2 9% 40% 33% 4	584
	11th to 20th	Night	13% 2 11% 34% 35% 6%	790
	21st to end	Night	13% 2 13% 36% 30% 7%	880
September	1st to 10th	Night	16% 2 17% 34% 25% 6%	800
	11th to 20th	Night	16% 2 14% 31% 30% 7%	800
	21st to end	Night	23% 3 18% 32% 22% 3	800
October	1st to 10th	Night	22% 3 23% 25% 21% 6%	800
	11th to 20th	Night	18% 7% 26% 34% 14% 2	800
	21st to end	Night	22% 5% 33% 27% 11% 1	880
November	1st to 10th	Night	15% 7% 32% 32% 13% 2	820
	11th to 20th	Night	20% 9% 33% 27% 10% 1	829
	21st to end	Night	15% 12% 38% 26% 9% 1	800
December	1st to 10th	Night	24% 12% 35% 22% 7% 1	800
	11th to 20th	Night	22% 9% 37% 27% 4 1	800
	21st to end	Night	21% 8% 41% 27% 4	880

Page generated at 2025-11-15 00:55:59. Version ac07

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