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 2025-05-11.

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% 9% 3 9% 6% 5% 5% 4 6% 8% 34%	117763
	Night	8% 10% 4 10% 6% 5% 5% 4 6% 8% 33%	119867
Transparency	Day	44% 9% 18% 21% 7%	117506
	Night	47% 8% 16% 20% 8% 1	119770
Seeing	Day	12% 7% 24% 31% 19% 6%	16447
	Night	13% 6% 27% 36% 16% 2	25187

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	5% 12% 5% 12% 8% 6% 6% 5% 8% 8% 23%	3179
	11th to 20th	Day	3 9% 3 11% 7% 6% 5% 6% 7% 11% 30%	3159
	21st to end	Day	8% 12% 4 9% 4 4 5% 4 6% 9% 34%	3710
February	1st to 10th	Day	7% 16% 4 11% 7% 6% 6% 5% 7% 8% 22%	1600
	11th to 20th	Day	6% 11% 6% 11% 7% 5% 7% 5% 8% 7% 27%	1693
	21st to end	Day	4 13% 4 11% 7% 6% 6% 5% 7% 8% 29%	1530
March	1st to 10th	Day	5% 9% 4 13% 7% 6% 6% 5% 7% 10% 29%	4518
	11th to 20th	Day	6% 12% 4 11% 5% 5% 5% 4 6% 9% 34%	4954
	21st to end	Day	9% 14% 5% 15% 5% 4 4 4 6% 8% 25%	5563
April	1st to 10th	Day	7% 12% 3 9% 6% 5% 5% 5% 6% 7% 33%	2507
	11th to 20th	Day	9% 12% 4 8% 5% 5% 6% 5% 6% 8% 34%	2570
	21st to end	Day	10% 10% 3 8% 6% 6% 5% 5% 6% 7% 33%	2600
May	1st to 10th	Day	15% 9% 3 8% 6% 5% 5% 4 5% 7% 33%	4814
	11th to 20th	Day	12% 10% 3 8% 5% 5% 5% 5% 7% 9% 31%	4697
	21st to end	Day	12% 8% 4 8% 6% 6% 6% 5% 7% 9% 30%	5161
June	1st to 10th	Day	14% 8% 3 9% 6% 6% 5% 5% 6% 8% 31%	2850
	11th to 20th	Day	18% 8% 3 9% 6% 7% 5% 5% 7% 8% 24%	2974
	21st to end	Day	10% 5% 2 6% 6% 5% 6% 5% 8% 9% 38%	3040
July	1st to 10th	Day	9% 5% 2 5% 5% 5% 5% 4 7% 8% 45%	4910
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 6% 8% 47%	4740
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 9% 43%	5138
August	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 8% 41%	4544
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 43%	4156
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 6% 9% 43%	4521
September	1st to 10th	Day	12% 6% 3 6% 5% 4 5% 4 6% 8% 43%	2425
	11th to 20th	Day	13% 7% 2 6% 5% 5% 4 4 6% 9% 39%	2090
	21st to end	Day	9% 8% 2 6% 5% 5% 5% 4 6% 8% 43%	2090
October	1st to 10th	Day	10% 8% 3 9% 5% 5% 5% 4 6% 8% 38%	3280
	11th to 20th	Day	13% 9% 4 7% 5% 4 4 4 6% 8% 36%	2883
	21st to end	Day	7% 11% 5% 9% 6% 5% 5% 4 5% 7% 36%	3039
November	1st to 10th	Day	10% 13% 4 10% 8% 7% 5% 3 6% 7% 27%	1568
	11th to 20th	Day	6% 13% 5% 11% 7% 6% 4 4 6% 8% 29%	1430
	21st to end	Day	5% 13% 6% 13% 6% 6% 6% 4 6% 8% 28%	1400
December	1st to 10th	Day	9% 14% 6% 11% 6% 7% 6% 4 7% 8% 22%	2290
	11th to 20th	Day	9% 13% 4 10% 7% 7% 7% 5% 7% 8% 21%	2642
	21st to end	Day	3 6% 3 9% 7% 6% 6% 4 7% 9% 39%	3498
January	1st to 10th	Night	4 10% 5% 15% 10% 7% 7% 4 6% 8% 24%	6766
	11th to 20th	Night	3 9% 3 11% 8% 7% 5% 4 6% 7% 37%	6739
	21st to end	Night	8% 13% 4 9% 6% 5% 5% 4 6% 8% 32%	7276
February	1st to 10th	Night	6% 11% 5% 13% 7% 8% 7% 5% 6% 7% 26%	2800
	11th to 20th	Night	6% 12% 4 10% 7% 6% 7% 6% 7% 7% 29%	2593
	21st to end	Night	5% 10% 5% 11% 7% 5% 7% 6% 7% 8% 31%	2100
March	1st to 10th	Night	3 8% 5% 11% 7% 5% 5% 5% 8% 10% 32%	5376
	11th to 20th	Night	5% 9% 4 13% 7% 6% 6% 4 7% 9% 31%	5045
	21st to end	Night	10% 14% 7% 15% 6% 5% 5% 3 5% 6% 24%	5408
April	1st to 10th	Night	8% 11% 3 9% 7% 7% 6% 5% 7% 6% 33%	1892
	11th to 20th	Night	9% 12% 4 10% 7% 6% 5% 4 6% 7% 31%	1781
	21st to end	Night	13% 9% 3 8% 5% 4 5% 4 6% 9% 35%	1800
May	1st to 10th	Night	22% 10% 3 8% 5% 5% 4 4 5% 5% 30%	2362
	11th to 20th	Night	13% 9% 3 9% 6% 6% 5% 4 6% 7% 30%	2202
	21st to end	Night	13% 9% 3 8% 6% 6% 6% 5% 7% 9% 29%	2414
June	1st to 10th	Night	15% 9% 3 7% 5% 5% 6% 5% 7% 7% 30%	1330
	11th to 20th	Night	16% 9% 3 8% 7% 7% 7% 7% 7% 8% 21%	1201
	21st to end	Night	8% 6% 3 7% 5% 5% 6% 5% 8% 8% 39%	1140
July	1st to 10th	Night	9% 5% 2 7% 4 4 4 5% 7% 8% 45%	2046
	11th to 20th	Night	9% 4 2 6% 5% 5% 5% 4 7% 10% 41%	2220
	21st to end	Night	13% 6% 2 6% 4 4 5% 3 8% 9% 41%	2414
August	1st to 10th	Night	12% 6% 2 7% 4 4 5% 3 7% 8% 42%	2383
	11th to 20th	Night	12% 8% 3 6% 5% 5% 4 4 7% 8% 38%	2677
	21st to end	Night	12% 5% 2 5% 4 4 4 4 6% 9% 46%	3146
September	1st to 10th	Night	11% 6% 3 7% 4 4 4 3 5% 7% 45%	1753
	11th to 20th	Night	12% 7% 3 5% 3 4 4 3 6% 8% 45%	2090
	21st to end	Night	10% 6% 3 6% 5% 4 5% 4 6% 6% 46%	2090
October	1st to 10th	Night	8% 7% 2 8% 6% 5% 5% 4 7% 8% 39%	3693
	11th to 20th	Night	10% 7% 3 7% 5% 4 5% 4 6% 9% 40%	4044
	21st to end	Night	7% 9% 3 8% 5% 5% 6% 5% 7% 9% 36%	4488
November	1st to 10th	Night	12% 15% 6% 9% 6% 6% 5% 4 5% 6% 26%	2730
	11th to 20th	Night	8% 14% 5% 11% 7% 6% 5% 4 7% 7% 26%	2970
	21st to end	Night	6% 13% 5% 13% 8% 5% 6% 4 6% 7% 27%	3000
December	1st to 10th	Night	10% 17% 5% 12% 7% 6% 5% 4 5% 6% 24%	4890
	11th to 20th	Night	8% 15% 5% 13% 6% 7% 7% 5% 7% 7% 19%	5550
	21st to end	Night	3 7% 3 10% 6% 5% 6% 4 6% 8% 41%	7458

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% 8% 12% 18% 9%	3179
	11th to 20th	Day	46% 8% 13% 13% 18% 1	3180
	21st to end	Day	44% 8% 21% 20% 6%	3710
February	1st to 10th	Day	57% 7% 14% 13% 8%	1600
	11th to 20th	Day	50% 10% 16% 15% 8%	1702
	21st to end	Day	49% 10% 14% 20% 7%	1530
March	1st to 10th	Day	48% 7% 18% 19% 8%	4518
	11th to 20th	Day	46% 8% 17% 21% 8%	4955
	21st to end	Day	55% 8% 12% 20% 6%	5563
April	1st to 10th	Day	45% 11% 17% 18% 8% 1	2507
	11th to 20th	Day	45% 9% 15% 22% 9% 1	2570
	21st to end	Day	45% 10% 16% 21% 6% 2	2600
May	1st to 10th	Day	48% 8% 12% 24% 7%	4814
	11th to 20th	Day	43% 11% 14% 24% 8%	4701
	21st to end	Day	44% 13% 19% 16% 8%	5162
June	1st to 10th	Day	48% 9% 16% 19% 8%	2850
	11th to 20th	Day	55% 10% 16% 14% 5%	2974
	21st to end	Day	39% 12% 21% 21% 6% 1	3040
July	1st to 10th	Day	34% 9% 17% 28% 12% 1	4835
	11th to 20th	Day	32% 9% 18% 27% 13%	4659
	21st to end	Day	37% 9% 22% 24% 9%	4976
August	1st to 10th	Day	37% 10% 21% 30% 2	4526
	11th to 20th	Day	34% 9% 24% 24% 8%	4198
	21st to end	Day	36% 9% 27% 25% 4	4520
September	1st to 10th	Day	37% 10% 19% 25% 7% 2	2425
	11th to 20th	Day	40% 8% 20% 25% 6%	2090
	21st to end	Day	37% 9% 18% 26% 8% 2	2090
October	1st to 10th	Day	43% 7% 23% 22% 5%	3280
	11th to 20th	Day	45% 7% 15% 25% 8%	2883
	21st to end	Day	46% 7% 18% 26% 3	3039
November	1st to 10th	Day	56% 7% 15% 15% 6% 1	1568
	11th to 20th	Day	52% 7% 17% 18% 5%	1430
	21st to end	Day	52% 10% 16% 17% 5% 1	1400
December	1st to 10th	Day	58% 7% 18% 11% 4 1	2290
	11th to 20th	Day	57% 9% 18% 13% 4	2644
	21st to end	Day	39% 7% 14% 28% 10% 1	3498
January	1st to 10th	Night	55% 8% 13% 13% 9% 1	6766
	11th to 20th	Night	45% 6% 14% 17% 17% 1	6780
	21st to end	Night	48% 8% 20% 18% 5%	7276
February	1st to 10th	Night	54% 8% 13% 15% 9% 1	2800
	11th to 20th	Night	50% 9% 15% 15% 10% 1	2594
	21st to end	Night	47% 9% 14% 20% 9% 1	2100
March	1st to 10th	Night	44% 7% 20% 21% 8%	5376
	11th to 20th	Night	46% 8% 18% 19% 9% 1	5045
	21st to end	Night	57% 9% 10% 15% 9%	5408
April	1st to 10th	Night	45% 9% 15% 17% 12%	1892
	11th to 20th	Night	49% 7% 12% 18% 11% 2	1781
	21st to end	Night	44% 8% 12% 22% 13% 1	1800
May	1st to 10th	Night	52% 7% 10% 21% 9% 1	2362
	11th to 20th	Night	45% 13% 12% 20% 10% 1	2202
	21st to end	Night	45% 14% 16% 16% 8% 1	2414
June	1st to 10th	Night	48% 10% 14% 18% 9% 1	1330
	11th to 20th	Night	55% 10% 15% 15% 5%	1201
	21st to end	Night	37% 13% 22% 22% 5% 1	1140
July	1st to 10th	Night	33% 10% 17% 23% 16% 1	2012
	11th to 20th	Night	35% 10% 16% 29% 10%	2178
	21st to end	Night	36% 9% 20% 28% 7%	2337
August	1st to 10th	Night	39% 9% 21% 30% 1	2376
	11th to 20th	Night	41% 8% 19% 22% 9%	2701
	21st to end	Night	35% 6% 25% 28% 5%	3146
September	1st to 10th	Night	36% 8% 18% 26% 9% 2	1753
	11th to 20th	Night	37% 7% 18% 28% 9% 1	2090
	21st to end	Night	36% 9% 15% 25% 13% 2	2090
October	1st to 10th	Night	40% 6% 22% 24% 6%	3693
	11th to 20th	Night	40% 6% 15% 33% 7%	4044
	21st to end	Night	41% 8% 23% 23% 5%	4488
November	1st to 10th	Night	57% 6% 12% 19% 5%	2730
	11th to 20th	Night	55% 9% 13% 16% 6%	2970
	21st to end	Night	53% 10% 15% 17% 4 1	3000
December	1st to 10th	Night	60% 7% 15% 13% 5% 1	4890
	11th to 20th	Night	61% 8% 16% 10% 4	5547
	21st to end	Night	39% 8% 15% 24% 13% 1	7458

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	11% 5% 27% 33% 19% 6%	200
	11th to 20th	Day	9% 11% 25% 34% 16% 6%	200
	21st to end	Day	11% 13% 31% 29% 13% 4	220
February	1st to 10th	Day	11% 10% 28% 30% 16% 7%	200
	11th to 20th	Day	10% 6% 30% 29% 19% 6%	269
	21st to end	Day	6% 7% 34% 29% 17% 7%	294
March	1st to 10th	Day	11% 5% 26% 36% 18% 4	500
	11th to 20th	Day	10% 4 24% 38% 19% 5%	500
	21st to end	Day	14% 4 26% 31% 21% 4	550
April	1st to 10th	Day	8% 8% 28% 34% 19% 3	500
	11th to 20th	Day	11% 7% 24% 33% 18% 6%	492
	21st to end	Day	13% 9% 25% 31% 17% 5%	500
May	1st to 10th	Day	11% 10% 24% 33% 18% 4	676
	11th to 20th	Day	11% 7% 24% 33% 18% 7%	676
	21st to end	Day	15% 5% 21% 31% 18% 10%	734
June	1st to 10th	Day	16% 10% 25% 29% 16% 3	680
	11th to 20th	Day	20% 6% 22% 26% 20% 7%	804
	21st to end	Day	11% 5% 17% 30% 26% 10%	870
July	1st to 10th	Day	12% 3 19% 32% 25% 9%	775
	11th to 20th	Day	11% 5% 22% 28% 25% 8%	668
	21st to end	Day	9% 5% 20% 30% 27% 9%	739
August	1st to 10th	Day	11% 6% 19% 28% 23% 13%	578
	11th to 20th	Day	7% 6% 23% 34% 23% 6%	477
	21st to end	Day	12% 7% 22% 29% 22% 9%	539
September	1st to 10th	Day	14% 9% 26% 27% 17% 7%	490
	11th to 20th	Day	17% 8% 25% 30% 16% 4	490
	21st to end	Day	11% 14% 30% 26% 14% 4	490
October	1st to 10th	Day	15% 9% 26% 29% 17% 4	371
	11th to 20th	Day	15% 7% 30% 31% 15% 2	300
	21st to end	Day	14% 13% 36% 23% 11% 3	330
November	1st to 10th	Day	17% 7% 24% 37% 14% 2	300
	11th to 20th	Day	12% 7% 31% 35% 12% 4	215
	21st to end	Day	6% 9% 35% 32% 12% 8%	200
December	1st to 10th	Day	10% 9% 32% 32% 14% 5%	200
	11th to 20th	Day	12% 9% 24% 39% 14% 3	200
	21st to end	Day	7% 4 22% 36% 23% 7%	220
January	1st to 10th	Night	10% 6% 31% 36% 15% 2	1000
	11th to 20th	Night	10% 8% 32% 36% 13% 1	1000
	21st to end	Night	15% 11% 34% 28% 11% 2	1022
February	1st to 10th	Night	10% 8% 25% 37% 17% 4	800
	11th to 20th	Night	10% 4 31% 36% 17% 1	784
	21st to end	Night	7% 6% 29% 36% 18% 5%	660
March	1st to 10th	Night	9% 5% 27% 40% 16% 3	800
	11th to 20th	Night	11% 6% 30% 35% 16% 3	800
	21st to end	Night	14% 5% 25% 36% 17% 2	880
April	1st to 10th	Night	12% 4 25% 38% 18% 2	692
	11th to 20th	Night	14% 4 29% 38% 14% 1	593
	21st to end	Night	15% 7% 26% 36% 17%	600
May	1st to 10th	Night	15% 6% 27% 33% 18% 2	420
	11th to 20th	Night	12% 6% 21% 41% 18% 2	375
	21st to end	Night	20% 5% 19% 34% 19% 1	411
June	1st to 10th	Night	20% 7% 19% 39% 13% 2	380
	11th to 20th	Night	21% 4 22% 30% 22% 2	380
	21st to end	Night	10% 4 19% 37% 26% 4	380
July	1st to 10th	Night	13% 3 17% 41% 26%	370
	11th to 20th	Night	11% 4 22% 39% 22% 3	368
	21st to end	Night	11% 4 15% 41% 25% 4	416
August	1st to 10th	Night	8% 5% 17% 32% 31% 6%	378
	11th to 20th	Night	11% 4 18% 39% 26% 2	478
	21st to end	Night	11% 4 21% 40% 23% 1	627
September	1st to 10th	Night	15% 3 21% 38% 21% 2	603
	11th to 20th	Night	15% 4 21% 41% 18% 1	760
	21st to end	Night	13% 5% 29% 38% 13% 2	760
October	1st to 10th	Night	15% 4 28% 38% 14% 1	760
	11th to 20th	Night	12% 6% 29% 38% 13% 2	760
	21st to end	Night	13% 9% 31% 33% 13%	855
November	1st to 10th	Night	19% 6% 30% 32% 10% 1	975
	11th to 20th	Night	13% 7% 34% 32% 11% 3	1000
	21st to end	Night	10% 5% 36% 36% 12% 2	1000
December	1st to 10th	Night	14% 8% 31% 31% 14% 3	1000
	11th to 20th	Night	11% 7% 32% 35% 14% 2	1000
	21st to end	Night	10% 5% 27% 37% 18% 4	1100

Page generated at 2025-05-08 07:34:00. Version ac07

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