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 2025-01-22.

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% 5% 5% 5% 4 5% 7% 28%	105217
	Night	14% 14% 5% 11% 6% 5% 4 4 5% 6% 27%	104157
Transparency	Day	53% 7% 12% 19% 9% 1	108818
	Night	55% 7% 12% 17% 8% 1	108644
Seeing	Day	25% 19% 29% 17% 8% 3	14796
	Night	24% 19% 26% 21% 9% 1	24260

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% 24% 13% 14% 5% 3 4 2 4 4 16%	2597
	11th to 20th	Day	10% 20% 9% 20% 7% 5% 4 3 4 4 14%	2765
	21st to end	Day	18% 20% 10% 12% 9% 5% 5% 3 4 4 10%	3133
February	1st to 10th	Day	14% 18% 6% 16% 9% 5% 5% 3 4 4 17%	1710
	11th to 20th	Day	11% 18% 8% 17% 7% 4 5% 3 4 4 19%	1663
	21st to end	Day	17% 16% 5% 14% 7% 5% 4 2 4 6% 21%	1531
March	1st to 10th	Day	17% 15% 6% 11% 4 4 3 2 4 5% 29%	3361
	11th to 20th	Day	9% 20% 7% 10% 5% 4 4 3 5% 6% 28%	3480
	21st to end	Day	13% 15% 4 12% 5% 5% 4 4 5% 6% 27%	3868
April	1st to 10th	Day	15% 19% 4 8% 4 3 4 3 4 7% 29%	2280
	11th to 20th	Day	18% 13% 3 8% 4 5% 4 3 5% 5% 32%	2254
	21st to end	Day	19% 13% 4 8% 4 4 4 4 5% 5% 30%	2397
May	1st to 10th	Day	20% 10% 3 7% 5% 4 4 3 5% 6% 31%	4190
	11th to 20th	Day	17% 10% 3 9% 6% 6% 5% 4 6% 8% 26%	4398
	21st to end	Day	11% 7% 2 7% 5% 5% 5% 4 6% 8% 41%	4837
June	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 5% 6% 8% 33%	2660
	11th to 20th	Day	13% 7% 2 8% 5% 5% 5% 5% 7% 9% 34%	2660
	21st to end	Day	14% 6% 2 7% 5% 5% 6% 4 6% 9% 35%	2660
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 7% 5% 7% 11% 35%	4440
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 6% 9% 10% 34%	4440
	21st to end	Day	8% 6% 3 8% 6% 7% 7% 6% 7% 8% 34%	4606
August	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	4104
	11th to 20th	Day	14% 8% 3 9% 6% 6% 6% 5% 8% 10% 26%	4044
	21st to end	Day	11% 8% 3 8% 5% 5% 6% 4 6% 8% 36%	4185
September	1st to 10th	Day	17% 8% 2 7% 4 5% 5% 3 5% 7% 38%	2280
	11th to 20th	Day	14% 6% 2 6% 4 5% 4 3 6% 7% 43%	2280
	21st to end	Day	17% 9% 3 8% 6% 4 6% 3 6% 6% 31%	1981
October	1st to 10th	Day	18% 10% 3 7% 5% 3 4 4 5% 7% 33%	1900
	11th to 20th	Day	20% 12% 3 9% 5% 5% 5% 3 4 6% 29%	1900
	21st to end	Day	23% 16% 3 8% 5% 4 4 3 4 6% 24%	2090
November	1st to 10th	Day	12% 17% 4 7% 4 4 4 4 5% 5% 32%	1777
	11th to 20th	Day	13% 19% 5% 9% 5% 5% 4 5% 6% 6% 23%	1600
	21st to end	Day	15% 21% 7% 11% 6% 3 4 3 6% 5% 20%	1600
December	1st to 10th	Day	15% 21% 11% 17% 6% 4 4 3 4 4 11%	2600
	11th to 20th	Day	16% 17% 7% 16% 6% 4 4 4 4 4 17%	2986
	21st to end	Day	16% 20% 8% 15% 7% 5% 4 3 5% 6% 12%	3960
January	1st to 10th	Night	13% 20% 8% 18% 7% 5% 5% 3 4 4 13%	4569
	11th to 20th	Night	9% 21% 9% 18% 9% 6% 4 3 5% 4 12%	4412
	21st to end	Night	17% 19% 8% 17% 9% 6% 4 3 3 3 12%	4546
February	1st to 10th	Night	13% 18% 5% 14% 10% 6% 5% 3 4 4 17%	2470
	11th to 20th	Night	10% 16% 6% 16% 9% 5% 6% 4 4 6% 17%	2403
	21st to end	Night	14% 15% 6% 17% 6% 4 4 3 5% 5% 21%	1922
March	1st to 10th	Night	17% 13% 5% 10% 6% 4 3 3 5% 5% 29%	3477
	11th to 20th	Night	14% 22% 6% 11% 5% 4 4 3 4 5% 23%	3479
	21st to end	Night	13% 12% 4 13% 5% 4 4 3 5% 5% 32%	3763
April	1st to 10th	Night	15% 16% 5% 7% 5% 4 4 4 5% 6% 30%	1900
	11th to 20th	Night	18% 12% 3 6% 4 3 4 4 5% 5% 34%	1875
	21st to end	Night	15% 12% 4 8% 4 3 4 4 5% 8% 33%	1800
May	1st to 10th	Night	21% 11% 3 7% 5% 3 4 4 5% 6% 30%	2751
	11th to 20th	Night	17% 9% 3 7% 5% 4 5% 4 6% 7% 33%	2495
	21st to end	Night	13% 6% 2 6% 3 4 4 4 5% 9% 43%	2738
June	1st to 10th	Night	15% 6% 2 8% 5% 4 5% 4 7% 8% 36%	1520
	11th to 20th	Night	11% 7% 2 6% 4 4 4 5% 7% 9% 40%	1520
	21st to end	Night	10% 6% 3 8% 5% 5% 5% 5% 6% 8% 41%	1520
July	1st to 10th	Night	10% 7% 2 9% 6% 5% 4 3 6% 8% 40%	2520
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 4 6% 8% 45%	2520
	21st to end	Night	11% 6% 2 6% 4 6% 5% 4 7% 7% 44%	2944
August	1st to 10th	Night	14% 9% 3 6% 4 5% 5% 4 7% 7% 36%	2825
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 4 5% 9% 37%	2785
	21st to end	Night	10% 6% 2 7% 5% 4 5% 4 6% 8% 43%	3438
September	1st to 10th	Night	15% 6% 2 5% 4 4 4 3 5% 7% 46%	1900
	11th to 20th	Night	14% 6% 2 6% 3 3 4 3 5% 8% 47%	1900
	21st to end	Night	17% 9% 3 6% 4 4 5% 4 5% 7% 36%	2194
October	1st to 10th	Night	16% 9% 4 7% 5% 4 4 4 5% 7% 36%	2280
	11th to 20th	Night	18% 10% 3 8% 4 4 4 4 6% 7% 32%	2280
	21st to end	Night	24% 13% 3 8% 4 4 4 3 5% 6% 27%	2508
November	1st to 10th	Night	13% 14% 3 7% 4 4 4 3 5% 7% 36%	2520
	11th to 20th	Night	15% 17% 4 8% 5% 5% 5% 4 5% 6% 26%	2799
	21st to end	Night	14% 18% 6% 12% 6% 5% 5% 4 5% 5% 20%	2800
December	1st to 10th	Night	10% 20% 8% 16% 8% 6% 7% 4 5% 5% 10%	4580
	11th to 20th	Night	16% 20% 8% 14% 7% 5% 4 3 4 4 17%	5208
	21st to end	Night	17% 19% 8% 13% 6% 6% 4 3 5% 5% 13%	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	72% 4 8% 11% 5%	2598
	11th to 20th	Day	73% 5% 8% 10% 4	2765
	21st to end	Day	76% 5% 10% 6% 2	3133
February	1st to 10th	Day	72% 6% 9% 10% 3	1710
	11th to 20th	Day	69% 4 6% 14% 6%	1672
	21st to end	Day	63% 5% 10% 15% 6% 1	1531
March	1st to 10th	Day	58% 4 9% 22% 6%	3364
	11th to 20th	Day	56% 6% 15% 15% 8% 1	3480
	21st to end	Day	56% 6% 7% 22% 7% 1	3868
April	1st to 10th	Day	56% 5% 10% 17% 10% 2	2280
	11th to 20th	Day	53% 6% 10% 17% 13% 1	2254
	21st to end	Day	54% 6% 9% 16% 13% 2	2397
May	1st to 10th	Day	52% 6% 9% 20% 11% 2	4202
	11th to 20th	Day	53% 10% 12% 17% 7% 1	4402
	21st to end	Day	40% 8% 12% 25% 13% 3	4837
June	1st to 10th	Day	47% 7% 12% 19% 14% 1	2660
	11th to 20th	Day	45% 7% 15% 21% 11% 1	2660
	21st to end	Day	43% 9% 13% 21% 13%	2660
July	1st to 10th	Day	41% 8% 15% 25% 11% 1	4370
	11th to 20th	Day	38% 10% 16% 22% 13%	4365
	21st to end	Day	43% 9% 15% 20% 12% 1	4459
August	1st to 10th	Day	47% 8% 14% 20% 12%	4091
	11th to 20th	Day	49% 9% 15% 21% 5%	4083
	21st to end	Day	43% 7% 11% 30% 8% 1	4185
September	1st to 10th	Day	44% 7% 12% 24% 11% 2	2280
	11th to 20th	Day	40% 7% 12% 27% 15% 1	2280
	21st to end	Day	51% 7% 11% 20% 11% 1	1981
October	1st to 10th	Day	43% 6% 13% 23% 15% 1	3160
	11th to 20th	Day	48% 5% 14% 16% 15% 2	3140
	21st to end	Day	47% 5% 14% 25% 8% 1	3428
November	1st to 10th	Day	52% 7% 12% 17% 11% 2	1777
	11th to 20th	Day	59% 6% 11% 15% 5% 2	1600
	21st to end	Day	66% 7% 11% 13% 4	1600
December	1st to 10th	Day	77% 6% 7% 9% 2	2600
	11th to 20th	Day	69% 6% 10% 10% 5%	2986
	21st to end	Day	73% 5% 10% 8% 3	3960
January	1st to 10th	Night	71% 7% 10% 9% 3	4569
	11th to 20th	Night	73% 7% 8% 9% 3	4412
	21st to end	Night	78% 5% 6% 8% 3	4546
February	1st to 10th	Night	68% 7% 9% 10% 5%	2470
	11th to 20th	Night	67% 6% 10% 12% 5%	2405
	21st to end	Night	64% 6% 10% 12% 8% 1	1922
March	1st to 10th	Night	58% 4 13% 18% 6% 1	3487
	11th to 20th	Night	61% 8% 10% 14% 6%	3480
	21st to end	Night	51% 6% 10% 20% 12% 2	3763
April	1st to 10th	Night	55% 6% 11% 18% 9%	1900
	11th to 20th	Night	51% 7% 11% 18% 12% 1	1875
	21st to end	Night	48% 7% 12% 20% 12% 2	1800
May	1st to 10th	Night	53% 8% 10% 19% 9% 2	2758
	11th to 20th	Night	47% 10% 17% 18% 8% 1	2496
	21st to end	Night	36% 10% 14% 20% 17% 3	2739
June	1st to 10th	Night	42% 8% 12% 22% 14% 1	1520
	11th to 20th	Night	37% 9% 15% 24% 13% 2	1520
	21st to end	Night	38% 10% 15% 23% 13% 1	1520
July	1st to 10th	Night	42% 7% 13% 22% 13% 2	2480
	11th to 20th	Night	34% 8% 16% 29% 13% 1	2472
	21st to end	Night	37% 8% 14% 24% 15% 2	2850
August	1st to 10th	Night	44% 7% 15% 24% 8% 1	2814
	11th to 20th	Night	43% 8% 16% 26% 7%	2812
	21st to end	Night	36% 8% 14% 32% 8% 1	3438
September	1st to 10th	Night	37% 7% 15% 26% 12% 2	1900
	11th to 20th	Night	35% 6% 13% 28% 16% 1	1900
	21st to end	Night	46% 8% 12% 21% 12% 1	2194
October	1st to 10th	Night	45% 7% 11% 23% 13%	3800
	11th to 20th	Night	44% 6% 13% 19% 14% 3	3779
	21st to end	Night	44% 5% 15% 26% 9% 1	4120
November	1st to 10th	Night	47% 7% 11% 22% 10% 2	2520
	11th to 20th	Night	57% 7% 12% 15% 7% 2	2799
	21st to end	Night	63% 7% 14% 12% 3	2800
December	1st to 10th	Night	73% 8% 11% 7% 1	4580
	11th to 20th	Night	69% 5% 9% 11% 6%	5208
	21st to end	Night	72% 6% 8% 9% 4	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	36% 28% 24% 10% 3 1	200
	11th to 20th	Day	31% 38% 24% 6% 1	250
	21st to end	Day	26% 34% 28% 9% 3	300
February	1st to 10th	Day	38% 31% 23% 7%	270
	11th to 20th	Day	38% 31% 27% 3	270
	21st to end	Day	40% 25% 27% 8% 1	343
March	1st to 10th	Day	32% 26% 28% 11% 3 1	460
	11th to 20th	Day	33% 26% 29% 8% 4	460
	21st to end	Day	36% 20% 29% 10% 3 1	506
April	1st to 10th	Day	34% 22% 27% 10% 5% 1	460
	11th to 20th	Day	35% 22% 24% 12% 6% 2	452
	21st to end	Day	30% 21% 29% 13% 5% 1	460
May	1st to 10th	Day	30% 19% 27% 14% 7% 3	463
	11th to 20th	Day	22% 16% 31% 19% 9% 2	487
	21st to end	Day	18% 16% 30% 22% 11% 4	529
June	1st to 10th	Day	21% 19% 31% 19% 8% 2	490
	11th to 20th	Day	17% 14% 34% 21% 12% 3	490
	21st to end	Day	17% 15% 38% 21% 6% 3	490
July	1st to 10th	Day	13% 11% 29% 30% 14% 4	485
	11th to 20th	Day	12% 10% 32% 28% 13% 5%	484
	21st to end	Day	12% 11% 38% 25% 10% 4	532
August	1st to 10th	Day	13% 12% 28% 26% 16% 6%	486
	11th to 20th	Day	15% 10% 32% 22% 15% 7%	479
	21st to end	Day	13% 14% 29% 22% 14% 6%	539
September	1st to 10th	Day	19% 12% 26% 23% 13% 6%	490
	11th to 20th	Day	14% 10% 24% 22% 20% 10%	490
	21st to end	Day	20% 17% 29% 19% 12% 3	490
October	1st to 10th	Day	23% 15% 27% 20% 12% 4	490
	11th to 20th	Day	24% 25% 28% 15% 6% 2	490
	21st to end	Day	36% 21% 26% 15% 3	539
November	1st to 10th	Day	28% 21% 26% 16% 7% 2	402
	11th to 20th	Day	36% 25% 27% 11% 1 1	200
	21st to end	Day	38% 25% 31% 7%	200
December	1st to 10th	Day	41% 19% 31% 7% 3 1	200
	11th to 20th	Day	44% 22% 26% 8% 1	200
	21st to end	Day	30% 31% 29% 8% 2	220
January	1st to 10th	Night	29% 28% 29% 12% 3	800
	11th to 20th	Night	28% 29% 28% 13% 3	800
	21st to end	Night	24% 30% 30% 15% 2	841
February	1st to 10th	Night	29% 29% 28% 14%	760
	11th to 20th	Night	30% 29% 28% 11% 2	743
	21st to end	Night	32% 26% 27% 13% 2	628
March	1st to 10th	Night	27% 26% 30% 15% 3	760
	11th to 20th	Night	32% 23% 23% 18% 4	760
	21st to end	Night	25% 23% 29% 17% 5% 1	836
April	1st to 10th	Night	28% 20% 29% 21% 2	760
	11th to 20th	Night	28% 18% 28% 19% 6% 1	751
	21st to end	Night	23% 15% 29% 24% 9%	660
May	1st to 10th	Night	21% 14% 21% 31% 11% 1	527
	11th to 20th	Night	16% 11% 31% 33% 9% 1	374
	21st to end	Night	15% 10% 25% 32% 18%	411
June	1st to 10th	Night	18% 8% 28% 31% 15% 1	380
	11th to 20th	Night	14% 11% 23% 38% 14%	380
	21st to end	Night	11% 8% 29% 40% 11%	380
July	1st to 10th	Night	10% 5% 23% 42% 18% 3	370
	11th to 20th	Night	9% 5% 26% 36% 23% 1	368
	21st to end	Night	11% 4 25% 37% 22% 2	533
August	1st to 10th	Night	10% 6% 24% 34% 23% 2	567
	11th to 20th	Night	13% 6% 20% 29% 27% 4	564
	21st to end	Night	13% 7% 23% 34% 21% 2	811
September	1st to 10th	Night	18% 9% 23% 28% 19% 3	760
	11th to 20th	Night	17% 8% 19% 26% 27% 4	760
	21st to end	Night	20% 15% 22% 24% 16% 3	760
October	1st to 10th	Night	20% 13% 27% 21% 17% 2	760
	11th to 20th	Night	26% 18% 26% 22% 6% 1	760
	21st to end	Night	34% 16% 29% 18% 3	836
November	1st to 10th	Night	25% 18% 28% 21% 7% 1	780
	11th to 20th	Night	28% 24% 26% 15% 5% 2	800
	21st to end	Night	28% 25% 27% 15% 5%	800
December	1st to 10th	Night	32% 28% 25% 10% 4	800
	11th to 20th	Night	30% 28% 26% 13% 3	800
	21st to end	Night	29% 28% 30% 11% 2	880

Page generated at 2025-01-19 03:06:49. Version ac07

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