Bon Accord Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bon Accord Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	9% 8% 3 8% 5% 5% 5% 4 7% 9% 36%	155172
	Night	8% 9% 4 10% 6% 5% 5% 4 6% 8% 34%	144615
Transparency	Day	43% 8% 18% 23% 7%	154871
	Night	46% 8% 17% 21% 9% 1	144501
Seeing	Day	12% 7% 25% 31% 19% 6%	18400
	Night	13% 6% 28% 36% 16% 2	26760

By Month

Percentage Cloud Cover NW (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	8% 15% 5% 11% 7% 6% 6% 5% 7% 8% 23%	4125
	11th to 20th	Day	3 7% 3 10% 7% 6% 6% 6% 7% 10% 35%	4119
	21st to end	Day	6% 11% 4 10% 5% 5% 6% 4 6% 9% 34%	4811
February	1st to 10th	Day	7% 15% 4 11% 7% 6% 6% 5% 7% 8% 23%	1680
	11th to 20th	Day	6% 11% 6% 11% 7% 5% 7% 5% 8% 7% 27%	1780
	21st to end	Day	4 13% 5% 12% 7% 7% 6% 5% 7% 8% 28%	1604
March	1st to 10th	Day	5% 9% 4 12% 7% 6% 6% 5% 7% 10% 29%	4618
	11th to 20th	Day	6% 12% 4 11% 5% 5% 5% 4 6% 9% 33%	5063
	21st to end	Day	9% 14% 5% 15% 6% 5% 4 4 6% 8% 25%	5686
April	1st to 10th	Day	8% 12% 3 9% 7% 5% 6% 5% 6% 8% 32%	2632
	11th to 20th	Day	9% 11% 4 8% 5% 5% 6% 5% 6% 8% 34%	2700
	21st to end	Day	10% 10% 3 9% 6% 5% 5% 5% 6% 7% 33%	2730
May	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 6% 8% 42%	8665
	11th to 20th	Day	14% 11% 4 9% 6% 5% 6% 4 7% 9% 26%	8755
	21st to end	Day	11% 6% 3 7% 6% 6% 6% 4 7% 9% 36%	9649
June	1st to 10th	Day	14% 8% 3 8% 6% 5% 5% 5% 6% 8% 31%	3150
	11th to 20th	Day	18% 8% 3 9% 7% 7% 5% 5% 7% 8% 23%	3288
	21st to end	Day	11% 6% 2 6% 6% 5% 6% 5% 7% 9% 37%	3360
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 42%	9144
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 4 6% 8% 43%	8820
	21st to end	Day	9% 5% 1 5% 4 4 5% 4 7% 9% 46%	9569
August	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 9% 39%	6659
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 40%	6122
	21st to end	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 46%	6570
September	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 8% 43%	2679
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 5% 9% 42%	2234
	21st to end	Day	8% 8% 2 6% 5% 5% 4 4 6% 8% 44%	2200
October	1st to 10th	Day	10% 8% 3 9% 5% 5% 5% 4 6% 8% 38%	3384
	11th to 20th	Day	12% 9% 4 7% 5% 4 4 4 6% 8% 37%	2974
	21st to end	Day	7% 11% 5% 9% 6% 4 5% 4 5% 7% 37%	3136
November	1st to 10th	Day	11% 13% 4 10% 7% 7% 5% 3 6% 7% 27%	1648
	11th to 20th	Day	6% 12% 5% 11% 7% 6% 4 4 6% 8% 30%	1501
	21st to end	Day	5% 13% 6% 13% 6% 5% 6% 4 7% 8% 28%	1470
December	1st to 10th	Day	9% 15% 6% 11% 6% 7% 6% 4 7% 8% 22%	2360
	11th to 20th	Day	9% 13% 4 10% 8% 7% 7% 5% 7% 8% 21%	2712
	21st to end	Day	4 6% 3 9% 7% 6% 6% 4 7% 9% 39%	3575
January	1st to 10th	Night	7% 13% 5% 14% 9% 7% 7% 4 6% 8% 22%	8768
	11th to 20th	Night	3 8% 3 10% 7% 6% 5% 4 6% 8% 41%	8779
	21st to end	Night	7% 11% 4 10% 7% 5% 5% 5% 7% 8% 31%	9413
February	1st to 10th	Night	6% 10% 5% 12% 7% 7% 7% 5% 7% 7% 26%	2940
	11th to 20th	Night	6% 12% 4 10% 7% 6% 7% 5% 6% 7% 29%	2726
	21st to end	Night	4 10% 5% 11% 7% 5% 7% 5% 7% 8% 30%	2202
March	1st to 10th	Night	3 8% 5% 11% 7% 5% 5% 5% 8% 10% 32%	5496
	11th to 20th	Night	5% 9% 4 13% 7% 6% 6% 4 7% 9% 30%	5156
	21st to end	Night	10% 14% 7% 14% 6% 5% 5% 3 5% 6% 24%	5527
April	1st to 10th	Night	8% 11% 3 9% 7% 7% 6% 5% 7% 7% 32%	1987
	11th to 20th	Night	9% 12% 4 10% 7% 6% 5% 4 6% 7% 30%	1871
	21st to end	Night	13% 9% 3 8% 5% 4 5% 4 6% 9% 36%	1890
May	1st to 10th	Night	13% 7% 2 6% 4 4 4 4 5% 8% 42%	4252
	11th to 20th	Night	13% 9% 4 10% 6% 7% 6% 5% 6% 7% 26%	4105
	21st to end	Night	11% 7% 2 7% 5% 5% 5% 4 7% 9% 37%	4522
June	1st to 10th	Night	16% 10% 3 7% 5% 5% 6% 5% 6% 7% 31%	1470
	11th to 20th	Night	17% 9% 3 8% 7% 7% 7% 7% 7% 8% 21%	1327
	21st to end	Night	10% 6% 3 7% 5% 5% 5% 5% 8% 8% 39%	1260
July	1st to 10th	Night	7% 4 2 6% 4 5% 5% 5% 7% 9% 46%	3798
	11th to 20th	Night	11% 6% 2 7% 5% 5% 5% 4 7% 9% 39%	4140
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 7% 8% 46%	4499
August	1st to 10th	Night	14% 8% 3 7% 4 4 5% 4 7% 8% 36%	3488
	11th to 20th	Night	11% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	3933
	21st to end	Night	9% 4 2 5% 4 4 4 4 6% 9% 48%	4578
September	1st to 10th	Night	11% 6% 3 7% 4 4 4 3 6% 8% 45%	1939
	11th to 20th	Night	12% 7% 3 5% 3 4 4 3 6% 7% 47%	2233
	21st to end	Night	10% 6% 3 6% 5% 3 6% 3 6% 6% 46%	2200
October	1st to 10th	Night	8% 7% 2 8% 6% 5% 5% 4 7% 8% 40%	3809
	11th to 20th	Night	9% 7% 3 7% 5% 4 5% 4 6% 9% 40%	4173
	21st to end	Night	7% 9% 3 8% 5% 5% 6% 5% 7% 9% 36%	4632
November	1st to 10th	Night	12% 15% 6% 9% 6% 6% 5% 4 5% 6% 27%	2870
	11th to 20th	Night	8% 13% 5% 11% 7% 6% 5% 4 7% 7% 27%	3119
	21st to end	Night	6% 12% 5% 12% 8% 5% 6% 4 6% 7% 28%	3150
December	1st to 10th	Night	10% 17% 5% 12% 7% 6% 5% 4 5% 6% 23%	5040
	11th to 20th	Night	8% 15% 5% 13% 6% 7% 7% 5% 7% 7% 19%	5700
	21st to end	Night	3 7% 4 10% 6% 5% 6% 4 6% 8% 41%	7623

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	57% 6% 10% 15% 11%	4125
	11th to 20th	Day	42% 7% 17% 16% 17% 1	4140
	21st to end	Day	46% 7% 22% 20% 5%	4812
February	1st to 10th	Day	56% 6% 15% 14% 9%	1680
	11th to 20th	Day	50% 10% 16% 15% 9%	1789
	21st to end	Day	51% 9% 14% 19% 7%	1604
March	1st to 10th	Day	48% 6% 18% 19% 8%	4618
	11th to 20th	Day	46% 8% 17% 21% 8%	5064
	21st to end	Day	55% 8% 12% 20% 6%	5686
April	1st to 10th	Day	46% 10% 18% 18% 7% 1	2632
	11th to 20th	Day	45% 9% 16% 22% 8% 1	2700
	21st to end	Day	46% 10% 15% 22% 6% 1	2730
May	1st to 10th	Day	38% 6% 14% 29% 12% 1	8665
	11th to 20th	Day	50% 8% 12% 25% 5%	8759
	21st to end	Day	40% 10% 22% 21% 7% 1	9650
June	1st to 10th	Day	47% 9% 15% 19% 9% 1	3150
	11th to 20th	Day	56% 10% 15% 15% 5%	3288
	21st to end	Day	40% 11% 21% 21% 6% 1	3360
July	1st to 10th	Day	34% 9% 22% 27% 8%	9069
	11th to 20th	Day	37% 7% 17% 28% 11% 1	8739
	21st to end	Day	33% 8% 23% 29% 7%	9362
August	1st to 10th	Day	38% 9% 23% 27% 2	6641
	11th to 20th	Day	36% 9% 24% 26% 6%	6164
	21st to end	Day	32% 7% 25% 32% 4	6569
September	1st to 10th	Day	38% 9% 19% 26% 6% 2	2679
	11th to 20th	Day	38% 8% 20% 26% 7%	2234
	21st to end	Day	37% 9% 19% 26% 8% 2	2200
October	1st to 10th	Day	43% 7% 23% 22% 5%	3384
	11th to 20th	Day	45% 7% 15% 25% 9%	2974
	21st to end	Day	45% 6% 18% 26% 3	3136
November	1st to 10th	Day	55% 7% 15% 16% 6% 1	1648
	11th to 20th	Day	51% 7% 18% 18% 6%	1501
	21st to end	Day	52% 9% 15% 17% 5% 1	1470
December	1st to 10th	Day	59% 7% 18% 11% 4 1	2360
	11th to 20th	Day	57% 8% 17% 13% 4	2714
	21st to end	Day	40% 7% 14% 28% 10% 1	3575
January	1st to 10th	Night	59% 6% 11% 12% 11% 1	8769
	11th to 20th	Night	41% 6% 17% 19% 16% 2	8820
	21st to end	Night	49% 8% 20% 17% 6%	9413
February	1st to 10th	Night	54% 8% 14% 15% 9% 1	2940
	11th to 20th	Night	50% 9% 15% 15% 11% 1	2727
	21st to end	Night	49% 8% 13% 20% 9% 1	2202
March	1st to 10th	Night	44% 7% 20% 21% 8%	5496
	11th to 20th	Night	46% 8% 18% 19% 9% 1	5156
	21st to end	Night	57% 9% 10% 15% 9%	5527
April	1st to 10th	Night	45% 10% 16% 17% 11%	1987
	11th to 20th	Night	49% 7% 13% 18% 11% 2	1871
	21st to end	Night	43% 8% 12% 23% 13% 1	1890
May	1st to 10th	Night	39% 6% 11% 28% 14% 2	4251
	11th to 20th	Night	52% 10% 12% 19% 7%	4105
	21st to end	Night	40% 10% 19% 25% 6%	4526
June	1st to 10th	Night	49% 9% 13% 18% 10% 1	1470
	11th to 20th	Night	55% 10% 14% 16% 4	1327
	21st to end	Night	38% 12% 22% 23% 5%	1260
July	1st to 10th	Night	32% 9% 23% 25% 12%	3764
	11th to 20th	Night	40% 8% 14% 29% 8% 1	4098
	21st to end	Night	34% 7% 23% 30% 6%	4401
August	1st to 10th	Night	44% 8% 21% 26% 1	3481
	11th to 20th	Night	42% 8% 20% 24% 6%	3957
	21st to end	Night	32% 6% 22% 33% 7%	4578
September	1st to 10th	Night	36% 8% 18% 28% 9% 2	1939
	11th to 20th	Night	35% 7% 17% 29% 11% 1	2233
	21st to end	Night	36% 9% 16% 25% 13% 2	2200
October	1st to 10th	Night	40% 6% 22% 24% 7%	3809
	11th to 20th	Night	40% 6% 14% 32% 7%	4173
	21st to end	Night	41% 8% 23% 23% 5%	4632
November	1st to 10th	Night	57% 6% 11% 19% 6%	2870
	11th to 20th	Night	54% 9% 14% 17% 7%	3119
	21st to end	Night	53% 10% 15% 17% 5% 1	3150
December	1st to 10th	Night	60% 7% 14% 12% 5% 1	5040
	11th to 20th	Night	61% 8% 16% 11% 5%	5697
	21st to end	Night	39% 8% 16% 24% 13% 1	7623

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	13% 5% 26% 32% 18% 5%	220
	11th to 20th	Day	9% 11% 27% 33% 15% 5%	220
	21st to end	Day	11% 12% 30% 30% 13% 4	242
February	1st to 10th	Day	10% 10% 28% 31% 15% 6%	220
	11th to 20th	Day	10% 7% 30% 29% 18% 5%	296
	21st to end	Day	7% 7% 34% 29% 16% 7%	320
March	1st to 10th	Day	12% 6% 27% 35% 17% 4	540
	11th to 20th	Day	11% 4 24% 37% 19% 5%	540
	21st to end	Day	14% 5% 26% 31% 21% 4	594
April	1st to 10th	Day	9% 8% 28% 33% 20% 3	540
	11th to 20th	Day	11% 7% 23% 34% 19% 5%	532
	21st to end	Day	13% 9% 25% 30% 18% 6%	540
May	1st to 10th	Day	11% 11% 25% 33% 17% 4	725
	11th to 20th	Day	12% 7% 24% 32% 18% 7%	775
	21st to end	Day	15% 5% 22% 30% 19% 9%	844
June	1st to 10th	Day	17% 10% 26% 28% 16% 3	780
	11th to 20th	Day	20% 6% 22% 25% 20% 7%	918
	21st to end	Day	12% 5% 18% 30% 24% 11%	990
July	1st to 10th	Day	11% 4 19% 34% 24% 8%	886
	11th to 20th	Day	11% 5% 24% 29% 24% 8%	768
	21st to end	Day	9% 5% 20% 30% 28% 8%	849
August	1st to 10th	Day	11% 6% 19% 28% 24% 13%	668
	11th to 20th	Day	9% 6% 24% 33% 23% 6%	557
	21st to end	Day	10% 6% 22% 29% 23% 9%	627
September	1st to 10th	Day	14% 9% 25% 28% 17% 8%	570
	11th to 20th	Day	15% 8% 23% 30% 18% 5%	543
	21st to end	Day	11% 14% 32% 26% 13% 4	530
October	1st to 10th	Day	14% 10% 26% 29% 17% 4	405
	11th to 20th	Day	14% 8% 31% 31% 14% 2	330
	21st to end	Day	14% 13% 36% 23% 10% 3	363
November	1st to 10th	Day	17% 7% 28% 34% 12% 2	330
	11th to 20th	Day	11% 6% 33% 35% 11% 4	236
	21st to end	Day	6% 8% 35% 30% 13% 8%	220
December	1st to 10th	Day	12% 8% 31% 31% 13% 4	220
	11th to 20th	Day	12% 10% 26% 36% 13% 3	220
	21st to end	Day	10% 5% 22% 33% 22% 7%	242
January	1st to 10th	Night	11% 6% 31% 36% 14% 2	1050
	11th to 20th	Night	9% 9% 33% 35% 13% 1	1050
	21st to end	Night	14% 11% 34% 29% 10% 2	1073
February	1st to 10th	Night	10% 8% 26% 37% 16% 3	840
	11th to 20th	Night	11% 4 32% 36% 16% 1	824
	21st to end	Night	7% 7% 29% 36% 17% 5%	692
March	1st to 10th	Night	9% 5% 28% 40% 15% 3	840
	11th to 20th	Night	11% 5% 31% 35% 15% 3	840
	21st to end	Night	14% 5% 27% 36% 16% 2	924
April	1st to 10th	Night	12% 4 26% 39% 17% 2	727
	11th to 20th	Night	14% 4 29% 38% 14% 1	623
	21st to end	Night	15% 7% 25% 35% 17%	630
May	1st to 10th	Night	14% 6% 26% 35% 17% 2	441
	11th to 20th	Night	11% 6% 22% 40% 18% 2	414
	21st to end	Night	20% 5% 19% 35% 19% 1	455
June	1st to 10th	Night	20% 8% 20% 38% 12% 2	420
	11th to 20th	Night	20% 4 22% 30% 21% 2	420
	21st to end	Night	11% 4 18% 38% 26% 4	420
July	1st to 10th	Night	12% 3 18% 41% 25%	410
	11th to 20th	Night	11% 4 21% 39% 22% 2	408
	21st to end	Night	10% 4 16% 42% 25% 4	460
August	1st to 10th	Night	10% 5% 17% 33% 31% 5%	418
	11th to 20th	Night	11% 4 19% 40% 26% 2	529
	21st to end	Night	10% 4 21% 40% 24% 1	693
September	1st to 10th	Night	15% 4 21% 38% 21% 2	667
	11th to 20th	Night	14% 4 20% 41% 19% 1	812
	21st to end	Night	13% 5% 30% 37% 13% 2	800
October	1st to 10th	Night	15% 5% 28% 38% 14% 1	800
	11th to 20th	Night	12% 7% 30% 36% 13% 2	800
	21st to end	Night	13% 10% 32% 33% 12%	900
November	1st to 10th	Night	20% 7% 31% 32% 10% 1	1025
	11th to 20th	Night	13% 7% 35% 32% 11% 3	1050
	21st to end	Night	10% 5% 35% 36% 13% 1	1050
December	1st to 10th	Night	14% 8% 32% 30% 13% 3	1050
	11th to 20th	Night	11% 7% 33% 34% 13% 2	1050
	21st to end	Night	10% 5% 27% 37% 17% 3	1155

Page generated at 2026-09-11 02:11:56. Version ac07

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