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 2024-11-01.

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	10% 9% 3 9% 6% 5% 5% 4 6% 8% 35%	102529
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 8% 33%	96481
Transparency	Day	44% 10% 18% 21% 7%	102272
	Night	46% 9% 16% 20% 8% 1	96384
Seeing	Day	12% 7% 24% 31% 19% 6%	15885
	Night	13% 6% 27% 36% 16% 2	24382

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% 13% 10% 8% 6% 5% 7% 8% 21%	2219
	11th to 20th	Day	5% 10% 4 10% 7% 6% 5% 6% 8% 9% 31%	2199
	21st to end	Day	7% 12% 5% 10% 5% 4 6% 4 6% 8% 33%	2584
February	1st to 10th	Day	8% 17% 4 11% 7% 6% 6% 4 7% 8% 22%	1520
	11th to 20th	Day	6% 12% 6% 11% 7% 5% 7% 5% 8% 6% 26%	1606
	21st to end	Day	4 13% 4 11% 7% 6% 6% 5% 7% 8% 28%	1456
March	1st to 10th	Day	4 12% 6% 16% 6% 5% 4 4 6% 11% 27%	3138
	11th to 20th	Day	5% 11% 3 10% 4 4 4 4 6% 10% 38%	3457
	21st to end	Day	8% 13% 5% 17% 6% 5% 5% 4 6% 7% 24%	3876
April	1st to 10th	Day	8% 12% 3 10% 6% 5% 6% 5% 6% 8% 32%	2381
	11th to 20th	Day	9% 12% 4 8% 5% 5% 5% 4 6% 7% 33%	2440
	21st to end	Day	11% 10% 3 8% 5% 5% 5% 5% 6% 7% 33%	2470
May	1st to 10th	Day	15% 10% 3 8% 6% 5% 5% 4 5% 7% 32%	4666
	11th to 20th	Day	12% 10% 3 8% 5% 5% 5% 5% 7% 9% 31%	4694
	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	11% 9% 3 9% 5% 5% 5% 4 4 6% 38%	1971
	11th to 20th	Day	11% 9% 4 7% 5% 4 5% 4 7% 8% 37%	1739
	21st to end	Day	9% 11% 4 10% 7% 5% 5% 4 6% 6% 32%	1843
November	1st to 10th	Day	10% 13% 4 10% 8% 7% 5% 3 6% 7% 26%	1488
	11th to 20th	Day	6% 13% 5% 11% 7% 5% 4 4 5% 8% 30%	1359
	21st to end	Day	5% 12% 6% 13% 6% 5% 6% 4 7% 8% 29%	1330
December	1st to 10th	Day	5% 13% 5% 10% 7% 8% 6% 4 7% 8% 26%	1330
	11th to 20th	Day	7% 10% 4 11% 7% 8% 7% 5% 7% 8% 25%	1682
	21st to end	Day	3 7% 3 7% 7% 6% 6% 4 7% 9% 41%	2442
January	1st to 10th	Night	4 11% 5% 16% 9% 7% 7% 5% 6% 7% 24%	4726
	11th to 20th	Night	4 8% 4 10% 9% 7% 6% 5% 7% 7% 34%	4699
	21st to end	Night	7% 13% 5% 10% 6% 5% 5% 5% 6% 8% 30%	5088
February	1st to 10th	Night	6% 11% 5% 13% 7% 7% 6% 4 7% 7% 26%	2660
	11th to 20th	Night	6% 12% 4 10% 7% 6% 7% 6% 7% 7% 29%	2460
	21st to end	Night	5% 10% 5% 11% 7% 5% 7% 5% 6% 8% 31%	1998
March	1st to 10th	Night	4 10% 6% 13% 8% 6% 5% 4 7% 9% 29%	3736
	11th to 20th	Night	5% 8% 3 11% 6% 5% 6% 4 7% 9% 36%	3522
	21st to end	Night	7% 12% 7% 16% 8% 6% 5% 3 6% 7% 23%	3773
April	1st to 10th	Night	8% 11% 3 10% 7% 7% 6% 6% 7% 6% 31%	1798
	11th to 20th	Night	9% 12% 4 10% 7% 6% 5% 5% 6% 7% 30%	1691
	21st to end	Night	13% 9% 3 8% 5% 3 5% 4 6% 9% 35%	1710
May	1st to 10th	Night	22% 10% 3 8% 5% 5% 4 4 5% 5% 30%	2290
	11th to 20th	Night	13% 9% 3 9% 6% 6% 5% 4 6% 8% 30%	2199
	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	11% 9% 3 9% 5% 5% 4 3 6% 7% 38%	2209
	11th to 20th	Night	9% 8% 4 8% 5% 4 5% 4 7% 8% 38%	2440
	21st to end	Night	8% 11% 4 10% 6% 5% 6% 5% 7% 7% 33%	2736
November	1st to 10th	Night	12% 15% 6% 9% 7% 6% 5% 4 5% 6% 26%	2597
	11th to 20th	Night	8% 14% 5% 11% 7% 6% 5% 4 7% 7% 26%	2821
	21st to end	Night	6% 12% 5% 12% 8% 5% 6% 4 6% 8% 28%	2850
December	1st to 10th	Night	8% 14% 3 12% 7% 6% 6% 3 6% 7% 29%	2850
	11th to 20th	Night	7% 13% 6% 13% 6% 6% 6% 5% 7% 7% 24%	3510
	21st to end	Night	3 8% 3 8% 6% 5% 6% 4 6% 9% 41%	5214

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% 10% 12% 17% 4 1	2219
	11th to 20th	Day	45% 9% 13% 15% 17% 1	2220
	21st to end	Day	46% 9% 20% 19% 5%	2584
February	1st to 10th	Day	57% 7% 15% 13% 7%	1520
	11th to 20th	Day	51% 11% 17% 14% 7%	1615
	21st to end	Day	50% 10% 14% 20% 7%	1456
March	1st to 10th	Day	51% 7% 16% 20% 6%	3138
	11th to 20th	Day	39% 11% 22% 20% 8%	3458
	21st to end	Day	55% 11% 14% 18% 2	3876
April	1st to 10th	Day	46% 11% 18% 17% 7% 1	2381
	11th to 20th	Day	45% 9% 15% 22% 8%	2440
	21st to end	Day	46% 10% 16% 20% 6% 2	2470
May	1st to 10th	Day	49% 8% 12% 24% 7%	4666
	11th to 20th	Day	43% 11% 14% 24% 8%	4698
	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	47% 7% 19% 20% 7%	1971
	11th to 20th	Day	43% 9% 18% 22% 8%	1739
	21st to end	Day	49% 9% 14% 22% 6%	1843
November	1st to 10th	Day	57% 8% 14% 15% 6% 1	1488
	11th to 20th	Day	52% 7% 17% 18% 5%	1359
	21st to end	Day	50% 10% 16% 17% 5% 1	1330
December	1st to 10th	Day	54% 9% 14% 16% 7% 1	1330
	11th to 20th	Day	51% 11% 17% 15% 6%	1684
	21st to end	Day	38% 8% 13% 26% 14% 1	2442
January	1st to 10th	Night	56% 10% 14% 14% 6% 1	4726
	11th to 20th	Night	46% 8% 14% 18% 14% 1	4740
	21st to end	Night	49% 9% 19% 15% 6%	5088
February	1st to 10th	Night	54% 8% 14% 16% 7% 1	2660
	11th to 20th	Night	50% 9% 16% 15% 9% 1	2461
	21st to end	Night	48% 9% 14% 20% 8% 1	1998
March	1st to 10th	Night	47% 8% 18% 20% 6%	3736
	11th to 20th	Night	40% 9% 22% 18% 10% 1	3522
	21st to end	Night	54% 11% 13% 15% 5%	3773
April	1st to 10th	Night	46% 10% 16% 17% 10%	1798
	11th to 20th	Night	50% 7% 12% 18% 11% 2	1691
	21st to end	Night	44% 8% 13% 21% 14% 1	1710
May	1st to 10th	Night	52% 8% 10% 20% 9% 1	2290
	11th to 20th	Night	45% 13% 12% 20% 10% 1	2199
	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	43% 7% 16% 24% 10%	2209
	11th to 20th	Night	42% 8% 16% 25% 8% 1	2440
	21st to end	Night	46% 10% 15% 22% 7%	2736
November	1st to 10th	Night	57% 7% 12% 19% 6%	2597
	11th to 20th	Night	55% 9% 14% 16% 6%	2821
	21st to end	Night	52% 10% 16% 17% 4 1	2850
December	1st to 10th	Night	53% 7% 12% 17% 9% 1	2850
	11th to 20th	Night	55% 9% 16% 14% 6%	3507
	21st to end	Night	37% 9% 17% 20% 15% 2	5214

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% 26% 33% 19% 6%	180
	11th to 20th	Day	8% 11% 24% 34% 17% 6%	180
	21st to end	Day	11% 12% 28% 31% 14% 4	198
February	1st to 10th	Day	12% 11% 28% 30% 14% 5%	180
	11th to 20th	Day	11% 7% 30% 29% 18% 6%	242
	21st to end	Day	7% 5% 33% 29% 18% 7%	268
March	1st to 10th	Day	11% 5% 27% 36% 17% 3	460
	11th to 20th	Day	9% 4 25% 38% 19% 5%	460
	21st to end	Day	14% 4 26% 30% 22% 4	506
April	1st to 10th	Day	8% 8% 29% 35% 17% 3	460
	11th to 20th	Day	12% 6% 23% 33% 19% 6%	452
	21st to end	Day	14% 9% 26% 29% 18% 5%	460
May	1st to 10th	Day	12% 9% 24% 33% 18% 4	627
	11th to 20th	Day	11% 7% 24% 33% 18% 7%	675
	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% 22% 12% 4	330
November	1st to 10th	Day	17% 7% 23% 37% 13% 3	270
	11th to 20th	Day	12% 7% 30% 36% 11% 4	194
	21st to end	Day	6% 8% 37% 32% 9% 8%	180
December	1st to 10th	Day	9% 8% 29% 33% 15% 5%	180
	11th to 20th	Day	11% 8% 24% 40% 13% 3	180
	21st to end	Day	7% 5% 22% 36% 24% 7%	198
January	1st to 10th	Night	11% 5% 31% 36% 15% 2	950
	11th to 20th	Night	9% 7% 32% 36% 14% 1	950
	21st to end	Night	15% 10% 33% 29% 11% 2	971
February	1st to 10th	Night	10% 8% 25% 37% 17% 4	760
	11th to 20th	Night	11% 5% 31% 35% 17% 1	744
	21st to end	Night	7% 5% 28% 37% 19% 5%	628
March	1st to 10th	Night	9% 6% 27% 39% 16% 3	760
	11th to 20th	Night	11% 6% 31% 34% 15% 3	760
	21st to end	Night	14% 5% 25% 37% 17% 2	836
April	1st to 10th	Night	12% 5% 25% 37% 18% 2	658
	11th to 20th	Night	14% 4 27% 39% 14% 1	563
	21st to end	Night	15% 6% 26% 35% 18%	570
May	1st to 10th	Night	15% 6% 26% 33% 18% 2	399
	11th to 20th	Night	12% 6% 21% 41% 18% 2	374
	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	20% 6% 30% 32% 11% 2	928
	11th to 20th	Night	13% 7% 34% 32% 11% 3	950
	21st to end	Night	10% 5% 36% 36% 12% 1	950
December	1st to 10th	Night	14% 8% 30% 31% 15% 3	950
	11th to 20th	Night	10% 7% 33% 35% 14% 2	950
	21st to end	Night	10% 5% 27% 36% 18% 4	1045

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