Broch nam Broc Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Broch nam Broc Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	21% 9% 3 7% 4 4 4 3 5% 6% 35%	131539
	Night	26% 9% 3 7% 4 4 4 3 4 5% 31%	129181
Transparency	Day	49% 9% 16% 17% 8% 1	131248
	Night	54% 8% 14% 16% 7% 1	129049
Seeing	Day	26% 9% 20% 25% 16% 5%	16765
	Night	33% 7% 15% 24% 19% 3	25150

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	41% 12% 4 9% 5% 4 5% 3 3 4 11%	3249
	11th to 20th	Day	20% 12% 3 6% 5% 4 4 3 4 6% 33%	3319
	21st to end	Day	40% 11% 3 6% 3 2 2 2 3 3 25%	4120
February	1st to 10th	Day	32% 12% 4 9% 5% 5% 4 3 4 5% 17%	1801
	11th to 20th	Day	29% 12% 4 8% 5% 4 4 3 4 5% 22%	1755
	21st to end	Day	27% 16% 4 7% 5% 3 4 3 6% 5% 19%	1521
March	1st to 10th	Day	25% 14% 4 9% 6% 5% 4 3 5% 6% 19%	4638
	11th to 20th	Day	22% 12% 4 8% 5% 5% 5% 3 5% 6% 25%	5000
	21st to end	Day	27% 12% 4 9% 6% 5% 5% 4 5% 6% 17%	5656
April	1st to 10th	Day	27% 12% 3 8% 6% 5% 4 4 6% 6% 19%	2400
	11th to 20th	Day	21% 10% 3 8% 7% 5% 5% 4 6% 6% 27%	2443
	21st to end	Day	23% 9% 4 8% 5% 4 4 4 5% 6% 28%	2670
May	1st to 10th	Day	15% 8% 3 7% 4 3 4 3 5% 5% 43%	6320
	11th to 20th	Day	20% 9% 3 7% 4 4 4 4 6% 7% 31%	6277
	21st to end	Day	22% 7% 3 6% 4 4 5% 4 7% 8% 30%	6921
June	1st to 10th	Day	21% 7% 3 7% 5% 5% 4 4 5% 7% 32%	3000
	11th to 20th	Day	21% 9% 3 8% 6% 4 5% 4 5% 7% 29%	3000
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 5% 7% 38%	3000
July	1st to 10th	Day	9% 3 1 4 3 3 3 3 4 6% 62%	6780
	11th to 20th	Day	5% 2 1 3 2 2 2 2 4 5% 72%	6780
	21st to end	Day	10% 3 1 3 2 3 3 3 4 6% 62%	6884
August	1st to 10th	Day	6% 3 1 4 3 2 2 3 4 6% 67%	4520
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 7% 48%	4156
	21st to end	Day	17% 7% 3 6% 4 4 4 3 5% 8% 41%	4519
September	1st to 10th	Day	11% 7% 2 7% 4 4 4 3 4 6% 49%	2400
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 3 5% 6% 40%	2400
	21st to end	Day	23% 7% 3 6% 4 3 3 3 6% 7% 35%	2110
October	1st to 10th	Day	21% 6% 2 7% 5% 5% 5% 4 6% 7% 32%	3280
	11th to 20th	Day	35% 8% 2 6% 3 3 3 4 5% 6% 24%	3261
	21st to end	Day	31% 12% 3 8% 4 5% 4 4 6% 6% 16%	3309
November	1st to 10th	Day	38% 10% 3 6% 5% 4 4 3 4 6% 17%	1648
	11th to 20th	Day	37% 11% 2 6% 5% 4 3 3 4 4 21%	1680
	21st to end	Day	36% 12% 4 8% 5% 4 3 3 5% 5% 17%	1680
December	1st to 10th	Day	26% 10% 3 8% 5% 5% 4 3 5% 5% 26%	2700
	11th to 20th	Day	40% 14% 3 8% 4 3 3 4 4 4 13%	2767
	21st to end	Day	37% 14% 5% 9% 5% 5% 3 3 3 5% 11%	3575
January	1st to 10th	Night	39% 13% 4 9% 5% 4 4 2 3 3 14%	6916
	11th to 20th	Night	23% 9% 2 5% 3 4 4 4 6% 8% 33%	6796
	21st to end	Night	36% 11% 3 6% 3 3 2 2 3 3 28%	7078
February	1st to 10th	Night	34% 13% 3 7% 5% 3 3 3 3 4 21%	2600
	11th to 20th	Night	29% 14% 3 7% 5% 4 3 3 3 4 25%	2532
	21st to end	Night	30% 12% 4 6% 4 4 3 3 4 5% 24%	2108
March	1st to 10th	Night	22% 15% 4 7% 4 5% 5% 3 5% 6% 23%	5254
	11th to 20th	Night	27% 11% 3 7% 4 4 4 3 4 6% 27%	4999
	21st to end	Night	36% 13% 4 8% 5% 4 3 3 4 4 17%	5318
April	1st to 10th	Night	28% 11% 3 6% 6% 5% 5% 3 6% 6% 21%	2000
	11th to 20th	Night	22% 11% 2 8% 5% 5% 4 3 5% 5% 29%	1905
	21st to end	Night	26% 10% 3 8% 5% 4 4 4 5% 6% 26%	1730
May	1st to 10th	Night	15% 10% 3 7% 4 3 3 2 3 4 46%	3640
	11th to 20th	Night	18% 8% 3 7% 4 5% 4 4 4 6% 36%	3616
	21st to end	Night	16% 10% 4 10% 5% 5% 6% 4 6% 6% 28%	3938
June	1st to 10th	Night	20% 7% 3 8% 4 5% 5% 4 5% 7% 33%	1400
	11th to 20th	Night	19% 10% 3 8% 6% 4 5% 4 6% 7% 28%	1400
	21st to end	Night	15% 6% 2 6% 4 6% 4 3 5% 8% 42%	1400
July	1st to 10th	Night	8% 3 1 4 4 3 3 2 3 5% 63%	3180
	11th to 20th	Night	4 3 1 2 2 2 2 2 3 4 77%	3180
	21st to end	Night	8% 3 1 4 3 3 3 2 3 6% 64%	3957
August	1st to 10th	Night	5% 2 1 3 2 2 2 3 4 6% 69%	2627
	11th to 20th	Night	12% 4 2 5% 3 3 3 3 5% 8% 51%	2902
	21st to end	Night	14% 5% 2 6% 5% 3 4 4 6% 8% 42%	3394
September	1st to 10th	Night	9% 3 1 5% 4 3 4 3 5% 6% 56%	2000
	11th to 20th	Night	19% 5% 2 7% 3 4 4 3 5% 6% 43%	2000
	21st to end	Night	20% 6% 3 7% 4 4 4 3 4 6% 40%	2286
October	1st to 10th	Night	21% 7% 3 7% 5% 5% 5% 4 5% 6% 33%	3920
	11th to 20th	Night	32% 6% 2 5% 4 4 4 3 5% 6% 29%	3897
	21st to end	Night	32% 9% 2 7% 5% 4 4 3 4 5% 25%	4493
November	1st to 10th	Night	38% 9% 3 7% 5% 4 4 3 4 4 18%	2870
	11th to 20th	Night	37% 10% 2 7% 5% 4 3 3 3 4 22%	2940
	21st to end	Night	36% 12% 3 8% 4 4 4 3 3 4 19%	2940
December	1st to 10th	Night	27% 9% 3 7% 5% 6% 5% 4 5% 7% 22%	4700
	11th to 20th	Night	42% 13% 4 8% 4 4 3 3 3 4 13%	5642
	21st to end	Night	37% 14% 4 9% 5% 4 4 2 3 4 14%	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	78% 7% 7% 7% 1	3249
	11th to 20th	Day	52% 8% 11% 19% 8% 2	3340
	21st to end	Day	66% 5% 7% 15% 6%	4120
February	1st to 10th	Day	70% 7% 8% 7% 6% 2	1801
	11th to 20th	Day	65% 8% 9% 9% 7% 1	1764
	21st to end	Day	66% 9% 9% 10% 5% 1	1521
March	1st to 10th	Day	62% 9% 12% 13% 4	4638
	11th to 20th	Day	59% 8% 12% 18% 2	5000
	21st to end	Day	65% 11% 13% 9% 3	5656
April	1st to 10th	Day	64% 11% 15% 9% 1	2400
	11th to 20th	Day	57% 9% 12% 13% 7% 2	2443
	21st to end	Day	57% 10% 14% 13% 6% 1	2670
May	1st to 10th	Day	43% 8% 16% 23% 10%	6320
	11th to 20th	Day	49% 11% 21% 15% 3	6280
	21st to end	Day	50% 11% 21% 16% 2	6922
June	1st to 10th	Day	52% 10% 17% 15% 6% 1	3000
	11th to 20th	Day	55% 12% 16% 13% 4	3000
	21st to end	Day	45% 10% 18% 19% 7% 1	3000
July	1st to 10th	Day	24% 7% 21% 23% 18% 8%	6706
	11th to 20th	Day	17% 5% 20% 34% 22% 3	6699
	21st to end	Day	23% 7% 29% 29% 10% 2	6690
August	1st to 10th	Day	19% 7% 16% 40% 17% 1	4503
	11th to 20th	Day	35% 11% 25% 20% 7% 2	4195
	21st to end	Day	42% 8% 18% 22% 10% 1	4518
September	1st to 10th	Day	36% 9% 17% 21% 12% 4	2400
	11th to 20th	Day	44% 10% 13% 18% 14% 2	2400
	21st to end	Day	48% 9% 17% 16% 9% 1	2110
October	1st to 10th	Day	48% 9% 18% 20% 5%	3280
	11th to 20th	Day	59% 7% 14% 12% 7% 1	3261
	21st to end	Day	64% 11% 10% 10% 4	3309
November	1st to 10th	Day	69% 7% 10% 9% 3 1	1648
	11th to 20th	Day	69% 7% 8% 10% 5% 2	1680
	21st to end	Day	69% 6% 9% 9% 6% 1	1680
December	1st to 10th	Day	58% 9% 11% 15% 8%	2700
	11th to 20th	Day	72% 9% 9% 7% 2 1	2770
	21st to end	Day	75% 9% 9% 5% 1	3575
January	1st to 10th	Night	76% 7% 6% 8% 2	6916
	11th to 20th	Night	47% 9% 17% 17% 9% 1	6837
	21st to end	Night	62% 5% 10% 17% 6%	7078
February	1st to 10th	Night	67% 7% 9% 9% 5% 3	2600
	11th to 20th	Night	64% 8% 9% 11% 6% 2	2534
	21st to end	Night	63% 7% 10% 13% 6% 1	2108
March	1st to 10th	Night	58% 9% 14% 13% 6%	5254
	11th to 20th	Night	56% 9% 13% 19% 3	5000
	21st to end	Night	70% 9% 12% 8% 1	5318
April	1st to 10th	Night	62% 11% 12% 13% 2	2000
	11th to 20th	Night	56% 8% 12% 14% 7% 3	1905
	21st to end	Night	58% 8% 13% 12% 7% 1	1730
May	1st to 10th	Night	42% 6% 12% 30% 9%	3637
	11th to 20th	Night	47% 11% 23% 16% 4	3616
	21st to end	Night	53% 12% 17% 16% 2	3938
June	1st to 10th	Night	50% 9% 17% 15% 7% 2	1400
	11th to 20th	Night	52% 12% 17% 16% 3	1400
	21st to end	Night	41% 8% 20% 21% 7% 3	1400
July	1st to 10th	Night	24% 6% 17% 27% 17% 8%	3145
	11th to 20th	Night	13% 4 20% 34% 26% 3	3138
	21st to end	Night	22% 5% 21% 31% 17% 2	3846
August	1st to 10th	Night	15% 5% 17% 37% 24% 2	2619
	11th to 20th	Night	31% 8% 28% 22% 8% 3	2929
	21st to end	Night	38% 9% 20% 21% 10% 1	3394
September	1st to 10th	Night	28% 9% 19% 25% 14% 4	2000
	11th to 20th	Night	42% 8% 14% 18% 15% 3	2000
	21st to end	Night	46% 9% 15% 20% 9% 2	2286
October	1st to 10th	Night	49% 10% 19% 17% 6%	3920
	11th to 20th	Night	56% 6% 14% 16% 7% 1	3897
	21st to end	Night	60% 10% 13% 12% 5%	4493
November	1st to 10th	Night	70% 7% 10% 9% 4 1	2870
	11th to 20th	Night	68% 8% 9% 8% 5% 2	2940
	21st to end	Night	69% 7% 10% 8% 6% 1	2940
December	1st to 10th	Night	57% 11% 14% 12% 5%	4700
	11th to 20th	Night	74% 7% 8% 8% 2 1	5638
	21st to end	Night	73% 9% 9% 8% 2	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	42% 12% 20% 14% 7% 5%	220
	11th to 20th	Day	40% 11% 15% 18% 13% 4	220
	21st to end	Day	43% 13% 13% 12% 15% 4	257
February	1st to 10th	Day	43% 11% 19% 17% 8% 3	401
	11th to 20th	Day	37% 11% 21% 20% 10% 2	395
	21st to end	Day	36% 13% 25% 15% 9% 1	350
March	1st to 10th	Day	35% 11% 22% 22% 10% 2	500
	11th to 20th	Day	35% 10% 18% 21% 13% 3	500
	21st to end	Day	38% 12% 19% 20% 8% 2	550
April	1st to 10th	Day	33% 11% 22% 19% 12% 2	500
	11th to 20th	Day	25% 11% 28% 22% 12% 3	492
	21st to end	Day	26% 11% 24% 21% 12% 5%	500
May	1st to 10th	Day	18% 10% 25% 30% 15% 3	503
	11th to 20th	Day	24% 8% 18% 28% 17% 5%	528
	21st to end	Day	21% 12% 24% 23% 15% 4	578
June	1st to 10th	Day	21% 8% 25% 30% 13% 3	730
	11th to 20th	Day	22% 10% 24% 30% 11% 3	730
	21st to end	Day	17% 8% 21% 31% 19% 3	730
July	1st to 10th	Day	15% 5% 21% 36% 19% 5%	721
	11th to 20th	Day	9% 6% 21% 39% 19% 6%	718
	21st to end	Day	9% 4 15% 38% 24% 10%	585
August	1st to 10th	Day	12% 5% 16% 31% 26% 12%	527
	11th to 20th	Day	11% 3 14% 34% 28% 9%	517
	21st to end	Day	13% 8% 20% 28% 23% 8%	583
September	1st to 10th	Day	17% 5% 18% 32% 19% 9%	530
	11th to 20th	Day	20% 8% 17% 28% 22% 6%	530
	21st to end	Day	25% 8% 22% 23% 18% 5%	530
October	1st to 10th	Day	29% 6% 19% 22% 18% 5%	530
	11th to 20th	Day	37% 7% 15% 20% 16% 5%	530
	21st to end	Day	40% 12% 17% 17% 11% 4	438
November	1st to 10th	Day	40% 17% 16% 16% 10% 1	220
	11th to 20th	Day	43% 10% 17% 13% 11% 6%	220
	21st to end	Day	46% 7% 19% 11% 11% 7%	220
December	1st to 10th	Day	45% 10% 13% 10% 12% 10%	220
	11th to 20th	Day	51% 14% 13% 14% 7% 1	220
	21st to end	Day	48% 10% 18% 16% 7% 1	242
January	1st to 10th	Night	48% 12% 15% 15% 8% 1	950
	11th to 20th	Night	41% 9% 13% 23% 13% 1	928
	21st to end	Night	41% 8% 14% 22% 13% 1	924
February	1st to 10th	Night	42% 12% 19% 19% 7% 1	800
	11th to 20th	Night	38% 10% 17% 25% 9% 1	784
	21st to end	Night	37% 10% 22% 20% 10% 1	660
March	1st to 10th	Night	33% 9% 19% 23% 15% 2	800
	11th to 20th	Night	39% 8% 18% 21% 13% 1	800
	21st to end	Night	40% 9% 16% 22% 11% 2	880
April	1st to 10th	Night	34% 8% 16% 26% 15% 2	800
	11th to 20th	Night	26% 8% 21% 27% 15% 3	717
	21st to end	Night	32% 6% 16% 27% 17% 3	530
May	1st to 10th	Night	22% 6% 13% 32% 26% 3	400
	11th to 20th	Night	21% 4 9% 27% 35% 5%	394
	21st to end	Night	20% 5% 11% 32% 27% 5%	433
June	1st to 10th	Night	23% 5% 13% 29% 26% 4	400
	11th to 20th	Night	20% 8% 14% 32% 22% 4	400
	21st to end	Night	17% 4 10% 30% 35% 5%	400
July	1st to 10th	Night	11% 3 8% 31% 42% 5%	390
	11th to 20th	Night	11% 3 9% 31% 38% 8%	388
	21st to end	Night	5% 2 7% 26% 50% 9%	438
August	1st to 10th	Night	12% 1 5% 22% 45% 14%	403
	11th to 20th	Night	10% 2 4 26% 45% 13%	594
	21st to end	Night	12% 4 9% 26% 40% 8%	744
September	1st to 10th	Night	13% 2 8% 35% 33% 9%	800
	11th to 20th	Night	21% 4 11% 23% 32% 9%	800
	21st to end	Night	26% 5% 13% 27% 26% 3	800
October	1st to 10th	Night	25% 5% 12% 28% 25% 5%	800
	11th to 20th	Night	35% 6% 14% 23% 17% 4	800
	21st to end	Night	38% 8% 17% 24% 11% 2	880
November	1st to 10th	Night	45% 7% 18% 22% 6% 1	820
	11th to 20th	Night	47% 8% 18% 17% 9% 2	840
	21st to end	Night	43% 10% 15% 18% 12% 3	840
December	1st to 10th	Night	38% 7% 17% 22% 14% 1	840
	11th to 20th	Night	51% 10% 18% 16% 4 1	928
	21st to end	Night	48% 11% 19% 17% 4	1045

Page generated at 2026-01-28 22:21:46. Version ac07

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