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 2024-01-14.

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	23% 8% 3 6% 5% 3 4 4 5% 6% 32%	74771
	Night	27% 9% 3 6% 5% 3 4 4 4 5% 30%	75006
Transparency	Day	53% 9% 14% 16% 7% 1	74462
	Night	57% 8% 12% 15% 7% 1	74833
Seeing	Day	26% 8% 20% 25% 16% 4	14138
	Night	33% 7% 14% 24% 19% 3	22555

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	43% 9% 4 7% 5% 3 4 4 3 4 13%	1329
	11th to 20th	Day	34% 12% 4 6% 5% 4 4 4 4 5% 20%	1313
	21st to end	Day	37% 11% 3 6% 6% 3 3 4 4 5% 17%	1597
February	1st to 10th	Day	35% 11% 4 7% 6% 3 5% 3 4 5% 17%	1620
	11th to 20th	Day	30% 12% 4 7% 6% 3 5% 4 4 6% 21%	1575
	21st to end	Day	26% 15% 5% 6% 6% 3 3 4 6% 5% 21%	1364
March	1st to 10th	Day	30% 12% 4 7% 6% 4 4 3 4 5% 21%	1842
	11th to 20th	Day	31% 11% 4 8% 6% 3 4 4 4 5% 20%	1980
	21st to end	Day	28% 11% 4 8% 7% 3 5% 5% 4 5% 19%	2245
April	1st to 10th	Day	27% 12% 4 8% 6% 3 5% 5% 5% 6% 20%	2160
	11th to 20th	Day	23% 9% 3 8% 7% 4 5% 5% 6% 6% 25%	2195
	21st to end	Day	23% 9% 4 8% 6% 3 5% 5% 5% 6% 26%	2402
May	1st to 10th	Day	18% 7% 3 7% 5% 4 5% 5% 5% 7% 34%	2520
	11th to 20th	Day	17% 8% 3 6% 6% 4 5% 5% 5% 8% 33%	2477
	21st to end	Day	19% 9% 4 7% 6% 3 4 4 5% 6% 32%	2727
June	1st to 10th	Day	22% 6% 3 7% 6% 4 5% 4 5% 7% 31%	2700
	11th to 20th	Day	21% 8% 3 7% 6% 3 5% 5% 5% 7% 29%	2700
	21st to end	Day	13% 7% 3 6% 6% 4 5% 5% 5% 7% 40%	2700
July	1st to 10th	Day	12% 5% 1 5% 4 2 4 4 5% 6% 51%	2700
	11th to 20th	Day	10% 3 3 4 4 3 4 4 5% 7% 54%	2700
	21st to end	Day	7% 3 2 2 3 2 3 3 4 5% 66%	2751
August	1st to 10th	Day	9% 4 2 4 4 2 3 3 5% 6% 58%	2500
	11th to 20th	Day	8% 4 1 4 3 3 3 4 5% 7% 57%	2298
	21st to end	Day	13% 6% 2 5% 5% 3 4 4 4 8% 47%	2498
September	1st to 10th	Day	12% 7% 2 6% 5% 3 3 3 4 6% 49%	2160
	11th to 20th	Day	17% 6% 3 7% 6% 4 4 3 4 6% 41%	2160
	21st to end	Day	24% 6% 3 5% 5% 3 3 4 5% 6% 36%	1900
October	1st to 10th	Day	22% 8% 3 7% 5% 3 5% 4 5% 6% 33%	1800
	11th to 20th	Day	29% 9% 3 7% 5% 3 3 5% 6% 6% 23%	1800
	21st to end	Day	32% 10% 3 6% 4 3 3 4 6% 6% 23%	1847
November	1st to 10th	Day	39% 9% 3 5% 6% 3 4 4 4 6% 17%	1488
	11th to 20th	Day	36% 11% 3 5% 5% 3 3 3 3 4 23%	1520
	21st to end	Day	36% 11% 4 7% 6% 3 3 3 5% 5% 18%	1520
December	1st to 10th	Day	33% 12% 3 6% 4 3 4 3 3 4 25%	1520
	11th to 20th	Day	35% 14% 3 8% 5% 2 3 5% 4 6% 15%	1721
	21st to end	Day	34% 15% 5% 9% 7% 4 4 4 3 5% 12%	2442
January	1st to 10th	Night	42% 12% 4 7% 5% 2 3 3 3 3 16%	2836
	11th to 20th	Night	38% 10% 3 5% 4 2 4 4 4 5% 21%	2722
	21st to end	Night	35% 13% 3 6% 5% 2 3 2 3 5% 21%	2758
February	1st to 10th	Night	36% 11% 4 6% 6% 3 3 4 3 4 21%	2340
	11th to 20th	Night	30% 14% 3 6% 5% 3 3 3 3 5% 24%	2272
	21st to end	Night	29% 11% 5% 6% 5% 3 4 4 4 5% 26%	1891
March	1st to 10th	Night	30% 10% 4 5% 4 3 5% 4 5% 5% 25%	2116
	11th to 20th	Night	33% 10% 3 7% 5% 3 4 4 4 5% 22%	1979
	21st to end	Night	32% 11% 4 7% 6% 3 4 4 4 5% 19%	2110
April	1st to 10th	Night	28% 10% 4 6% 6% 3 5% 4 5% 7% 22%	1800
	11th to 20th	Night	24% 10% 3 7% 6% 4 5% 4 5% 5% 27%	1713
	21st to end	Night	26% 10% 3 7% 6% 3 4 5% 4 6% 25%	1558
May	1st to 10th	Night	17% 8% 3 6% 5% 3 4 5% 5% 6% 39%	1440
	11th to 20th	Night	14% 6% 3 6% 5% 3 6% 7% 7% 8% 35%	1416
	21st to end	Night	16% 7% 3 7% 5% 4 5% 5% 6% 6% 36%	1546
June	1st to 10th	Night	20% 7% 3 8% 6% 3 5% 5% 5% 7% 31%	1260
	11th to 20th	Night	19% 9% 4 7% 6% 3 5% 5% 6% 7% 27%	1260
	21st to end	Night	13% 5% 3 5% 5% 4 4 5% 5% 8% 44%	1260
July	1st to 10th	Night	10% 4 3 5% 6% 2 4 3 3 5% 54%	1260
	11th to 20th	Night	8% 5% 2 4 3 2 3 3 4 5% 60%	1260
	21st to end	Night	6% 2 1 2 3 2 4 3 3 6% 68%	1564
August	1st to 10th	Night	8% 3 1 4 3 2 3 5% 4 6% 61%	1436
	11th to 20th	Night	5% 3 2 3 4 1 3 3 5% 9% 62%	1602
	21st to end	Night	8% 5% 2 5% 5% 2 5% 4 7% 8% 47%	1857
September	1st to 10th	Night	9% 3 1 4 5% 3 4 4 4 6% 56%	1800
	11th to 20th	Night	19% 5% 2 6% 4 3 4 4 5% 6% 43%	1800
	21st to end	Night	19% 6% 3 6% 5% 3 4 3 4 6% 41%	2056
October	1st to 10th	Night	20% 6% 3 6% 5% 4 5% 4 4 5% 38%	2160
	11th to 20th	Night	29% 7% 3 6% 4 3 4 5% 4 6% 30%	2160
	21st to end	Night	31% 7% 2 6% 5% 2 4 4 4 5% 30%	2508
November	1st to 10th	Night	38% 9% 3 6% 6% 3 4 4 4 4 18%	2590
	11th to 20th	Night	36% 9% 3 6% 6% 3 3 3 3 4 23%	2660
	21st to end	Night	37% 10% 5% 7% 5% 3 4 4 3 4 19%	2660
December	1st to 10th	Night	32% 9% 3 5% 5% 3 4 4 4 6% 24%	2660
	11th to 20th	Night	40% 13% 4 8% 5% 3 4 3 2 4 14%	3482
	21st to end	Night	37% 13% 4 8% 6% 3 4 3 3 3 16%	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	77% 6% 7% 7% 3 1	1329
	11th to 20th	Day	68% 9% 8% 9% 4 1	1313
	21st to end	Day	70% 7% 9% 10% 3 1	1597
February	1st to 10th	Day	71% 7% 8% 7% 5% 2	1620
	11th to 20th	Day	65% 9% 9% 9% 7% 1	1584
	21st to end	Day	65% 9% 9% 10% 5% 1	1364
March	1st to 10th	Day	66% 8% 12% 9% 4 1	1842
	11th to 20th	Day	69% 6% 9% 11% 4 1	1980
	21st to end	Day	67% 10% 11% 10% 2	2245
April	1st to 10th	Day	65% 11% 14% 9% 1	2160
	11th to 20th	Day	61% 9% 11% 11% 7% 1	2195
	21st to end	Day	59% 10% 14% 11% 6% 1	2402
May	1st to 10th	Day	50% 11% 15% 16% 7% 1	2520
	11th to 20th	Day	48% 12% 18% 15% 6%	2480
	21st to end	Day	54% 11% 16% 17% 3	2728
June	1st to 10th	Day	54% 9% 16% 14% 5% 1	2700
	11th to 20th	Day	56% 13% 16% 11% 4	2700
	21st to end	Day	44% 11% 17% 19% 8% 1	2700
July	1st to 10th	Day	34% 10% 17% 25% 12% 3	2626
	11th to 20th	Day	30% 9% 17% 32% 11% 2	2619
	21st to end	Day	20% 8% 18% 36% 15% 4	2598
August	1st to 10th	Day	28% 9% 21% 29% 12% 2	2483
	11th to 20th	Day	26% 10% 20% 29% 13% 3	2298
	21st to end	Day	37% 9% 19% 27% 7% 1	2498
September	1st to 10th	Day	38% 8% 16% 22% 12% 4	2160
	11th to 20th	Day	44% 10% 12% 17% 15% 2	2160
	21st to end	Day	48% 9% 16% 16% 9% 2	1900
October	1st to 10th	Day	50% 9% 15% 16% 9%	1800
	11th to 20th	Day	59% 9% 14% 13% 4 1	1800
	21st to end	Day	62% 10% 10% 11% 8%	1847
November	1st to 10th	Day	70% 6% 10% 9% 3 1	1488
	11th to 20th	Day	68% 7% 8% 10% 5% 2	1520
	21st to end	Day	69% 6% 9% 9% 6% 1	1520
December	1st to 10th	Day	65% 9% 6% 11% 9% 1	1520
	11th to 20th	Day	69% 9% 9% 8% 3 1	1724
	21st to end	Day	74% 10% 9% 5% 2 1	2442
January	1st to 10th	Night	74% 6% 6% 10% 3 1	2836
	11th to 20th	Night	66% 8% 11% 10% 5% 1	2722
	21st to end	Night	66% 6% 9% 14% 4 1	2758
February	1st to 10th	Night	68% 7% 9% 9% 5% 3	2340
	11th to 20th	Night	65% 8% 8% 11% 6% 2	2274
	21st to end	Night	62% 6% 11% 13% 6% 1	1891
March	1st to 10th	Night	61% 8% 14% 12% 5%	2116
	11th to 20th	Night	65% 7% 10% 13% 5% 1	1980
	21st to end	Night	68% 9% 13% 10% 1	2110
April	1st to 10th	Night	62% 10% 12% 14% 2	1800
	11th to 20th	Night	58% 8% 11% 14% 6% 2	1713
	21st to end	Night	60% 8% 12% 11% 7% 1	1558
May	1st to 10th	Night	45% 11% 18% 17% 8% 1	1440
	11th to 20th	Night	44% 12% 19% 18% 7%	1416
	21st to end	Night	47% 11% 19% 18% 5%	1546
June	1st to 10th	Night	52% 9% 16% 15% 6% 2	1260
	11th to 20th	Night	53% 12% 18% 13% 3	1260
	21st to end	Night	39% 9% 22% 21% 7% 3	1260
July	1st to 10th	Night	34% 7% 14% 26% 14% 4	1225
	11th to 20th	Night	29% 7% 18% 30% 15% 1	1218
	21st to end	Night	17% 6% 15% 35% 22% 5%	1477
August	1st to 10th	Night	23% 7% 19% 29% 19% 3	1428
	11th to 20th	Night	20% 7% 23% 30% 15% 5%	1602
	21st to end	Night	33% 8% 18% 29% 11% 1	1857
September	1st to 10th	Night	30% 9% 17% 26% 14% 4	1800
	11th to 20th	Night	42% 9% 13% 17% 16% 4	1800
	21st to end	Night	45% 9% 14% 20% 10% 2	2056
October	1st to 10th	Night	47% 9% 14% 20% 10%	2160
	11th to 20th	Night	56% 7% 15% 15% 6% 1	2160
	21st to end	Night	57% 9% 11% 14% 9%	2508
November	1st to 10th	Night	70% 7% 10% 9% 4 1	2590
	11th to 20th	Night	67% 7% 9% 8% 6% 2	2660
	21st to end	Night	68% 7% 9% 8% 6% 1	2660
December	1st to 10th	Night	61% 9% 10% 12% 7% 1	2660
	11th to 20th	Night	74% 8% 8% 5% 4 1	3478
	21st to end	Night	73% 8% 8% 8% 2 1	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	41% 13% 21% 11% 8% 6%	180
	11th to 20th	Day	46% 13% 17% 11% 10% 3	166
	21st to end	Day	44% 11% 14% 14% 15% 2	189
February	1st to 10th	Day	46% 10% 18% 16% 6% 2	340
	11th to 20th	Day	38% 12% 20% 19% 10% 2	335
	21st to end	Day	35% 12% 25% 16% 10% 1	295
March	1st to 10th	Day	35% 11% 22% 20% 9% 2	420
	11th to 20th	Day	35% 10% 19% 21% 12% 3	420
	21st to end	Day	39% 11% 18% 21% 8% 2	462
April	1st to 10th	Day	34% 11% 23% 19% 12% 2	420
	11th to 20th	Day	27% 10% 26% 22% 12% 3	412
	21st to end	Day	26% 11% 25% 23% 12% 3	420
May	1st to 10th	Day	18% 9% 26% 31% 14% 2	423
	11th to 20th	Day	23% 6% 17% 29% 19% 5%	448
	21st to end	Day	20% 11% 25% 24% 16% 5%	489
June	1st to 10th	Day	23% 7% 26% 28% 13% 3	630
	11th to 20th	Day	23% 9% 25% 30% 11% 3	630
	21st to end	Day	16% 8% 21% 31% 20% 3	630
July	1st to 10th	Day	16% 4 20% 36% 20% 4	621
	11th to 20th	Day	10% 6% 21% 37% 20% 5%	618
	21st to end	Day	8% 3 16% 39% 25% 9%	497
August	1st to 10th	Day	12% 4 16% 30% 26% 12%	447
	11th to 20th	Day	10% 2 14% 36% 29% 10%	437
	21st to end	Day	14% 7% 21% 29% 22% 8%	495
September	1st to 10th	Day	18% 6% 19% 32% 18% 6%	450
	11th to 20th	Day	21% 7% 17% 29% 22% 5%	450
	21st to end	Day	25% 7% 21% 23% 19% 5%	450
October	1st to 10th	Day	30% 6% 18% 20% 20% 6%	450
	11th to 20th	Day	35% 6% 16% 19% 18% 6%	450
	21st to end	Day	40% 10% 17% 16% 12% 5%	366
November	1st to 10th	Day	40% 16% 17% 14% 11% 2	180
	11th to 20th	Day	42% 9% 16% 12% 13% 8%	180
	21st to end	Day	44% 6% 21% 11% 12% 7%	180
December	1st to 10th	Day	46% 11% 15% 11% 10% 8%	180
	11th to 20th	Day	49% 11% 15% 16% 9% 1	180
	21st to end	Day	47% 10% 19% 17% 7% 1	198
January	1st to 10th	Night	49% 12% 15% 14% 9% 1	850
	11th to 20th	Night	45% 9% 12% 21% 13% 1	803
	21st to end	Night	42% 9% 14% 22% 12% 1	792
February	1st to 10th	Night	43% 11% 18% 19% 7% 1	720
	11th to 20th	Night	38% 9% 17% 25% 10% 1	704
	21st to end	Night	36% 10% 22% 21% 11% 2	592
March	1st to 10th	Night	34% 9% 19% 22% 15% 2	720
	11th to 20th	Night	40% 7% 18% 21% 12% 1	720
	21st to end	Night	40% 7% 16% 22% 12% 3	792
April	1st to 10th	Night	35% 7% 16% 25% 15% 2	720
	11th to 20th	Night	28% 7% 20% 26% 16% 3	645
	21st to end	Night	34% 5% 15% 28% 16% 2	478
May	1st to 10th	Night	22% 5% 13% 32% 26% 3	360
	11th to 20th	Night	20% 4 7% 28% 36% 5%	354
	21st to end	Night	21% 3 10% 33% 28% 5%	389
June	1st to 10th	Night	24% 5% 13% 28% 26% 4	360
	11th to 20th	Night	20% 8% 14% 32% 21% 4	360
	21st to end	Night	14% 4 10% 31% 36% 5%	360
July	1st to 10th	Night	11% 3 8% 30% 43% 5%	350
	11th to 20th	Night	12% 3 8% 30% 39% 8%	348
	21st to end	Night	5% 2 6% 26% 52% 9%	394
August	1st to 10th	Night	12% 1 6% 22% 44% 15%	362
	11th to 20th	Night	8% 2 3 26% 48% 13%	534
	21st to end	Night	12% 4 9% 27% 41% 8%	669
September	1st to 10th	Night	13% 2 9% 37% 32% 7%	720
	11th to 20th	Night	21% 4 11% 22% 32% 10%	720
	21st to end	Night	25% 5% 13% 27% 27% 4	720
October	1st to 10th	Night	25% 4 11% 28% 26% 5%	720
	11th to 20th	Night	35% 5% 14% 23% 18% 5%	720
	21st to end	Night	38% 7% 17% 24% 12% 2	792
November	1st to 10th	Night	44% 7% 18% 23% 7% 1	740
	11th to 20th	Night	45% 8% 18% 17% 10% 2	760
	21st to end	Night	43% 10% 15% 17% 13% 3	760
December	1st to 10th	Night	38% 7% 18% 23% 14% 1	760
	11th to 20th	Night	50% 9% 18% 17% 4 1	832
	21st to end	Night	49% 11% 19% 17% 4	935

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