Ossian Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ossian Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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% 12% 5% 10% 6% 5% 5% 4 5% 6% 28%	132224
	Night	13% 13% 5% 12% 6% 5% 5% 4 5% 6% 26%	130570
Transparency	Day	54% 6% 11% 18% 9% 1	135812
	Night	57% 6% 11% 16% 8% 1	135068
Seeing	Day	26% 19% 28% 16% 8% 3	16505
	Night	24% 19% 26% 21% 9% 1	25953

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	11% 20% 10% 22% 9% 3 4 2 4 3 12%	4661
	11th to 20th	Day	8% 19% 9% 22% 9% 8% 4 3 4 4 11%	4948
	21st to end	Day	13% 13% 7% 15% 10% 9% 7% 4 4 5% 13%	5780
February	1st to 10th	Day	13% 18% 6% 15% 9% 5% 5% 3 4 4 17%	1890
	11th to 20th	Day	11% 18% 8% 17% 7% 5% 5% 3 4 5% 18%	1843
	21st to end	Day	16% 17% 5% 13% 7% 5% 4 3 5% 6% 21%	1687
March	1st to 10th	Day	15% 15% 7% 11% 5% 4 3 2 3 5% 30%	4940
	11th to 20th	Day	10% 17% 7% 9% 4 4 4 3 5% 6% 32%	5090
	21st to end	Day	13% 15% 5% 13% 6% 5% 5% 4 4 5% 26%	5669
April	1st to 10th	Day	14% 19% 4 9% 5% 4 4 3 4 7% 27%	2520
	11th to 20th	Day	17% 14% 3 8% 4 5% 5% 3 5% 5% 31%	2494
	21st to end	Day	19% 12% 4 8% 4 4 4 4 5% 5% 31%	2649
May	1st to 10th	Day	25% 10% 3 7% 5% 5% 5% 3 5% 5% 28%	6130
	11th to 20th	Day	17% 12% 3 9% 6% 5% 5% 3 6% 7% 28%	6402
	21st to end	Day	18% 9% 3 8% 5% 5% 5% 4 6% 6% 31%	6949
June	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 5% 6% 8% 34%	2800
	11th to 20th	Day	14% 7% 3 8% 5% 5% 5% 5% 7% 9% 33%	2800
	21st to end	Day	13% 6% 2 7% 5% 5% 6% 4 6% 9% 36%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 7% 5% 7% 10% 38%	6360
	11th to 20th	Day	8% 5% 2 7% 6% 7% 6% 6% 9% 11% 34%	6360
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 5% 6% 7% 41%	6620
August	1st to 10th	Day	16% 6% 3 8% 5% 5% 5% 4 6% 8% 34%	4234
	11th to 20th	Day	14% 8% 3 9% 6% 6% 6% 5% 8% 10% 26%	4174
	21st to end	Day	11% 7% 3 8% 5% 5% 6% 4 6% 8% 36%	4317
September	1st to 10th	Day	16% 8% 2 6% 4 5% 4 3 6% 7% 39%	2400
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 3 5% 7% 44%	2400
	21st to end	Day	17% 9% 3 8% 5% 4 6% 3 6% 7% 32%	2085
October	1st to 10th	Day	17% 10% 2 7% 5% 3 4 4 5% 7% 35%	2000
	11th to 20th	Day	19% 12% 3 9% 5% 5% 5% 3 4 6% 28%	2000
	21st to end	Day	23% 15% 3 8% 4 4 4 3 4 6% 24%	2200
November	1st to 10th	Day	13% 17% 4 8% 4 4 4 4 5% 5% 31%	1868
	11th to 20th	Day	13% 19% 6% 9% 6% 5% 4 4 5% 6% 23%	1680
	21st to end	Day	15% 22% 7% 12% 5% 3 4 3 5% 5% 19%	1680
December	1st to 10th	Day	15% 21% 11% 17% 6% 4 4 3 4 4 11%	2680
	11th to 20th	Day	16% 17% 7% 16% 6% 4 4 4 4 4 17%	3066
	21st to end	Day	16% 20% 8% 16% 7% 5% 4 3 5% 6% 12%	4048
January	1st to 10th	Night	11% 18% 8% 26% 10% 4 4 2 3 3 11%	8234
	11th to 20th	Night	7% 19% 9% 23% 10% 7% 5% 3 5% 4 9%	7922
	21st to end	Night	11% 13% 7% 15% 10% 10% 8% 4 4 4 14%	8401
February	1st to 10th	Night	12% 18% 5% 15% 10% 7% 5% 3 4 4 17%	2730
	11th to 20th	Night	10% 16% 6% 17% 8% 5% 6% 4 4 5% 17%	2663
	21st to end	Night	13% 15% 6% 17% 7% 4 4 3 5% 5% 20%	2118
March	1st to 10th	Night	14% 12% 6% 11% 9% 5% 4 3 5% 5% 27%	5117
	11th to 20th	Night	13% 18% 5% 9% 4 3 3 3 4 5% 32%	5089
	21st to end	Night	13% 13% 5% 13% 6% 4 4 3 5% 5% 28%	5504
April	1st to 10th	Night	16% 16% 5% 8% 5% 4 4 4 5% 5% 28%	2100
	11th to 20th	Night	18% 13% 3 7% 4 4 4 4 5% 5% 33%	2075
	21st to end	Night	15% 12% 4 8% 4 3 4 4 4 8% 34%	1989
May	1st to 10th	Night	26% 12% 3 8% 5% 3 4 4 5% 5% 26%	4031
	11th to 20th	Night	16% 9% 4 8% 5% 5% 5% 3 5% 6% 34%	3623
	21st to end	Night	18% 10% 3 7% 4 4 4 3 5% 8% 34%	3926
June	1st to 10th	Night	15% 6% 2 8% 5% 4 5% 4 7% 8% 36%	1600
	11th to 20th	Night	12% 7% 2 7% 4 4 4 5% 7% 9% 39%	1600
	21st to end	Night	9% 5% 2 7% 5% 5% 5% 5% 6% 8% 42%	1600
July	1st to 10th	Night	10% 7% 3 8% 6% 5% 4 4 6% 10% 39%	3600
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 5% 7% 9% 42%	3600
	21st to end	Night	10% 6% 2 6% 4 5% 4 3 6% 7% 47%	4215
August	1st to 10th	Night	13% 9% 3 6% 4 5% 5% 4 7% 7% 37%	2915
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 4 5% 9% 38%	2875
	21st to end	Night	10% 6% 2 6% 5% 4 5% 4 6% 8% 44%	3547
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 5% 7% 47%	2000
	11th to 20th	Night	13% 6% 2 6% 3 3 4 3 5% 8% 48%	2000
	21st to end	Night	16% 9% 3 6% 4 4 5% 4 5% 7% 36%	2310
October	1st to 10th	Night	15% 8% 3 7% 5% 4 4 4 5% 7% 38%	2400
	11th to 20th	Night	18% 10% 3 8% 4 4 4 4 5% 7% 32%	2400
	21st to end	Night	23% 13% 3 8% 4 4 4 3 5% 6% 28%	2640
November	1st to 10th	Night	14% 14% 3 7% 4 4 4 3 5% 7% 35%	2649
	11th to 20th	Night	14% 17% 4 8% 5% 5% 5% 4 5% 6% 25%	2939
	21st to end	Night	13% 18% 6% 12% 6% 5% 5% 4 5% 5% 20%	2940
December	1st to 10th	Night	10% 20% 9% 17% 8% 6% 7% 4 5% 5% 10%	4720
	11th to 20th	Night	15% 20% 7% 14% 7% 5% 4 3 4 4 17%	5348
	21st to end	Night	17% 19% 8% 13% 7% 6% 4 3 5% 5% 13%	7150

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	77% 4 7% 9% 3	4662
	11th to 20th	Day	76% 4 7% 7% 5% 1	4980
	21st to end	Day	72% 5% 10% 10% 3	5780
February	1st to 10th	Day	71% 6% 8% 11% 4	1890
	11th to 20th	Day	69% 4 6% 13% 6%	1852
	21st to end	Day	63% 5% 10% 15% 6% 1	1687
March	1st to 10th	Day	58% 4 11% 18% 9%	4943
	11th to 20th	Day	53% 5% 17% 16% 8% 2	5090
	21st to end	Day	59% 5% 6% 19% 9% 2	5669
April	1st to 10th	Day	57% 5% 10% 16% 10% 1	2520
	11th to 20th	Day	54% 6% 10% 17% 13% 1	2494
	21st to end	Day	53% 6% 9% 17% 13% 2	2649
May	1st to 10th	Day	57% 6% 8% 17% 10% 2	6139
	11th to 20th	Day	54% 8% 9% 18% 11%	6406
	21st to end	Day	51% 8% 11% 19% 10% 2	6949
June	1st to 10th	Day	47% 7% 12% 19% 14% 1	2800
	11th to 20th	Day	46% 8% 14% 21% 11% 1	2800
	21st to end	Day	43% 9% 14% 21% 13% 1	2800
July	1st to 10th	Day	40% 7% 13% 26% 13% 1	6290
	11th to 20th	Day	40% 9% 16% 21% 13%	6285
	21st to end	Day	39% 8% 13% 25% 15%	6431
August	1st to 10th	Day	46% 7% 14% 20% 12%	4221
	11th to 20th	Day	49% 9% 15% 21% 6%	4213
	21st to end	Day	43% 7% 12% 30% 8% 1	4317
September	1st to 10th	Day	43% 7% 12% 26% 11% 1	2400
	11th to 20th	Day	39% 7% 11% 28% 14% 1	2400
	21st to end	Day	50% 7% 11% 19% 11% 1	2085
October	1st to 10th	Day	42% 6% 12% 23% 16% 1	3260
	11th to 20th	Day	49% 5% 14% 16% 15% 2	3240
	21st to end	Day	47% 5% 14% 24% 9% 1	3538
November	1st to 10th	Day	53% 7% 12% 17% 10% 2	1868
	11th to 20th	Day	60% 6% 11% 16% 5% 2	1680
	21st to end	Day	66% 6% 10% 13% 4	1680
December	1st to 10th	Day	76% 6% 7% 9% 2	2680
	11th to 20th	Day	69% 6% 10% 10% 5%	3066
	21st to end	Day	73% 5% 10% 8% 3	4048
January	1st to 10th	Night	77% 6% 7% 7% 3	8234
	11th to 20th	Night	75% 6% 8% 7% 4	7964
	21st to end	Night	72% 4 9% 9% 6% 1	8401
February	1st to 10th	Night	69% 7% 9% 10% 5%	2730
	11th to 20th	Night	68% 6% 10% 11% 4	2665
	21st to end	Night	65% 6% 10% 11% 8% 1	2118
March	1st to 10th	Night	60% 4 14% 14% 7%	5127
	11th to 20th	Night	53% 6% 15% 16% 6% 3	5090
	21st to end	Night	56% 6% 9% 16% 12% 2	5504
April	1st to 10th	Night	57% 6% 11% 17% 9% 1	2100
	11th to 20th	Night	52% 7% 11% 18% 12% 1	2075
	21st to end	Night	48% 7% 12% 21% 11% 2	1989
May	1st to 10th	Night	60% 6% 8% 16% 8% 2	4038
	11th to 20th	Night	49% 8% 15% 19% 9% 1	3624
	21st to end	Night	48% 8% 12% 18% 12% 2	3927
June	1st to 10th	Night	43% 8% 12% 22% 15% 1	1600
	11th to 20th	Night	38% 9% 15% 23% 13% 2	1600
	21st to end	Night	36% 10% 15% 23% 14% 2	1600
July	1st to 10th	Night	41% 7% 14% 25% 13% 1	3560
	11th to 20th	Night	37% 9% 15% 26% 12% 1	3552
	21st to end	Night	34% 6% 13% 26% 19% 2	4090
August	1st to 10th	Night	44% 7% 15% 24% 9% 1	2904
	11th to 20th	Night	42% 8% 16% 26% 8%	2902
	21st to end	Night	36% 8% 14% 33% 8% 1	3547
September	1st to 10th	Night	36% 7% 15% 28% 13% 2	2000
	11th to 20th	Night	34% 6% 13% 30% 15% 1	2000
	21st to end	Night	46% 8% 12% 21% 12% 1	2310
October	1st to 10th	Night	44% 7% 11% 24% 14% 1	3920
	11th to 20th	Night	45% 6% 13% 19% 15% 3	3899
	21st to end	Night	44% 5% 15% 25% 9% 1	4252
November	1st to 10th	Night	48% 6% 11% 22% 10% 2	2649
	11th to 20th	Night	57% 7% 12% 15% 7% 2	2939
	21st to end	Night	64% 7% 14% 12% 3	2940
December	1st to 10th	Night	73% 8% 10% 7% 1	4720
	11th to 20th	Night	69% 5% 9% 11% 6%	5348
	21st to end	Night	73% 6% 8% 9% 4	7150

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	36% 30% 22% 9% 3	220
	11th to 20th	Day	33% 37% 23% 5% 1	275
	21st to end	Day	31% 32% 27% 7% 2	363
February	1st to 10th	Day	38% 30% 23% 7% 1	330
	11th to 20th	Day	40% 28% 30% 2	330
	21st to end	Day	38% 28% 27% 7%	403
March	1st to 10th	Day	32% 27% 27% 10% 2 1	540
	11th to 20th	Day	37% 24% 27% 8% 4	540
	21st to end	Day	37% 22% 28% 10% 3 1	594
April	1st to 10th	Day	35% 24% 26% 9% 5% 1	540
	11th to 20th	Day	35% 24% 23% 11% 5% 2	532
	21st to end	Day	30% 21% 30% 14% 5% 1	540
May	1st to 10th	Day	32% 20% 28% 12% 6% 3	543
	11th to 20th	Day	24% 16% 30% 19% 9% 2	551
	21st to end	Day	20% 16% 29% 21% 10% 3	573
June	1st to 10th	Day	21% 20% 31% 19% 8% 2	530
	11th to 20th	Day	18% 15% 33% 20% 11% 3	530
	21st to end	Day	16% 15% 37% 22% 7% 3	530
July	1st to 10th	Day	12% 12% 29% 30% 13% 3	525
	11th to 20th	Day	11% 10% 32% 29% 13% 5%	524
	21st to end	Day	13% 11% 36% 25% 11% 4	576
August	1st to 10th	Day	12% 11% 28% 27% 17% 6%	526
	11th to 20th	Day	15% 10% 31% 23% 15% 7%	519
	21st to end	Day	13% 14% 30% 22% 15% 6%	583
September	1st to 10th	Day	19% 12% 25% 23% 14% 7%	530
	11th to 20th	Day	13% 9% 25% 22% 22% 10%	530
	21st to end	Day	19% 16% 30% 19% 12% 3	530
October	1st to 10th	Day	22% 15% 26% 21% 13% 4	530
	11th to 20th	Day	25% 25% 28% 15% 5% 2	530
	21st to end	Day	36% 20% 26% 16% 3	583
November	1st to 10th	Day	30% 23% 24% 15% 7% 2	433
	11th to 20th	Day	36% 25% 26% 10% 1 1	220
	21st to end	Day	39% 25% 29% 6%	220
December	1st to 10th	Day	40% 20% 30% 6% 2 1	220
	11th to 20th	Day	45% 22% 25% 7% 1	220
	21st to end	Day	33% 31% 28% 7% 2	242
January	1st to 10th	Night	28% 28% 29% 12% 3	840
	11th to 20th	Night	29% 29% 27% 13% 3	840
	21st to end	Night	25% 29% 30% 14% 2	924
February	1st to 10th	Night	29% 28% 28% 14% 1	840
	11th to 20th	Night	30% 28% 28% 12% 1	823
	21st to end	Night	32% 27% 27% 12% 2	692
March	1st to 10th	Night	27% 26% 30% 15% 2	840
	11th to 20th	Night	33% 22% 24% 18% 3	840
	21st to end	Night	26% 24% 29% 16% 4 1	924
April	1st to 10th	Night	29% 21% 28% 20% 2	840
	11th to 20th	Night	28% 19% 28% 18% 6% 1	831
	21st to end	Night	23% 14% 29% 25% 8%	729
May	1st to 10th	Night	22% 14% 23% 30% 10% 1	583
	11th to 20th	Night	16% 12% 31% 32% 9%	406
	21st to end	Night	16% 10% 25% 32% 17%	433
June	1st to 10th	Night	18% 9% 27% 31% 14% 1	400
	11th to 20th	Night	14% 12% 23% 38% 13%	400
	21st to end	Night	11% 8% 30% 40% 11%	400
July	1st to 10th	Night	9% 5% 23% 42% 17% 3	390
	11th to 20th	Night	9% 5% 25% 38% 22% 1	388
	21st to end	Night	11% 4 24% 39% 21% 1	561
August	1st to 10th	Night	10% 6% 24% 35% 23% 2	597
	11th to 20th	Night	13% 6% 20% 30% 27% 4	594
	21st to end	Night	13% 7% 23% 34% 21% 2	854
September	1st to 10th	Night	18% 9% 23% 29% 20% 3	800
	11th to 20th	Night	16% 7% 18% 27% 28% 4	800
	21st to end	Night	19% 15% 22% 25% 16% 3	800
October	1st to 10th	Night	19% 13% 26% 22% 18% 2	800
	11th to 20th	Night	25% 18% 27% 22% 6% 1	800
	21st to end	Night	33% 16% 29% 18% 4	880
November	1st to 10th	Night	26% 18% 28% 20% 7%	820
	11th to 20th	Night	29% 23% 26% 15% 5% 2	840
	21st to end	Night	28% 25% 27% 15% 5%	840
December	1st to 10th	Night	32% 29% 25% 11% 4	840
	11th to 20th	Night	31% 28% 26% 13% 3	840
	21st to end	Night	30% 27% 29% 11% 2	924

Page generated at 2026-05-14 01:59:00. Version ac07

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