BRAS Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BRAS Dark Sky Site 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	10% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	147501
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	143787
Transparency	Day	40% 7% 14% 18% 15% 6%	151112
	Night	37% 6% 14% 18% 17% 8%	148257
Seeing	Day	14% 5% 17% 25% 24% 15%	17355
	Night	14% 4 18% 33% 25% 6%	26958

By Month

Percentage Cloud Cover SE (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	15% 8% 4 8% 4 5% 3 3 4 6% 40%	5289
	11th to 20th	Day	11% 7% 2 6% 4 4 4 3 4 6% 49%	5310
	21st to end	Day	22% 8% 3 8% 5% 4 4 3 4 5% 35%	5812
February	1st to 10th	Day	15% 9% 3 7% 4 4 3 4 5% 6% 39%	2094
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 6% 6% 39%	2048
	21st to end	Day	16% 9% 3 8% 4 4 4 3 5% 8% 36%	1782
March	1st to 10th	Day	20% 9% 3 8% 4 4 4 3 6% 7% 33%	5046
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 5% 7% 44%	5090
	21st to end	Day	13% 7% 2 7% 5% 4 4 4 5% 7% 41%	5661
April	1st to 10th	Day	10% 8% 3 7% 5% 5% 6% 5% 5% 7% 40%	2520
	11th to 20th	Day	13% 7% 3 6% 4 5% 4 4 7% 7% 39%	2494
	21st to end	Day	9% 5% 3 6% 5% 5% 6% 5% 7% 9% 42%	2520
May	1st to 10th	Day	18% 7% 3 8% 6% 5% 5% 4 6% 7% 31%	7021
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 6% 8% 42%	7031
	21st to end	Day	7% 6% 3 8% 6% 7% 7% 6% 9% 11% 31%	7736
June	1st to 10th	Day	7% 5% 3 8% 7% 7% 7% 5% 8% 10% 33%	2715
	11th to 20th	Day	6% 6% 3 9% 8% 7% 7% 6% 8% 9% 32%	2730
	21st to end	Day	5% 6% 3 9% 9% 8% 8% 7% 8% 10% 27%	2730
July	1st to 10th	Day	4 6% 3 8% 7% 7% 8% 6% 9% 12% 29%	7680
	11th to 20th	Day	6% 6% 3 9% 7% 8% 8% 6% 8% 12% 28%	7680
	21st to end	Day	7% 6% 3 7% 6% 6% 7% 6% 8% 13% 32%	8160
August	1st to 10th	Day	2 2 1 4 3 4 5% 5% 8% 11% 55%	5545
	11th to 20th	Day	4 3 1 5% 4 5% 6% 6% 9% 12% 45%	5486
	21st to end	Day	9% 6% 3 7% 6% 6% 6% 5% 7% 9% 37%	6080
September	1st to 10th	Day	9% 5% 3 7% 6% 7% 6% 5% 7% 10% 35%	2520
	11th to 20th	Day	11% 6% 3 6% 5% 6% 5% 4 6% 9% 39%	2436
	21st to end	Day	9% 5% 2 6% 5% 4 4 4 6% 7% 48%	2155
October	1st to 10th	Day	6% 4 2 6% 3 3 4 4 5% 6% 56%	2000
	11th to 20th	Day	8% 4 2 5% 4 4 3 3 5% 6% 56%	2000
	21st to end	Day	10% 4 2 5% 4 3 4 3 5% 6% 54%	2200
November	1st to 10th	Day	8% 6% 2 5% 4 4 4 4 5% 8% 50%	2058
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 5% 4 54%	2100
	21st to end	Day	12% 6% 2 7% 5% 4 5% 4 6% 6% 45%	2100
December	1st to 10th	Day	18% 9% 3 6% 5% 4 4 3 4 5% 39%	3360
	11th to 20th	Day	10% 7% 3 7% 5% 4 4 4 5% 6% 44%	3725
	21st to end	Day	15% 7% 3 6% 4 3 4 3 5% 6% 44%	4587
January	1st to 10th	Night	20% 6% 2 4 3 3 3 2 4 6% 45%	7634
	11th to 20th	Night	15% 6% 2 6% 4 3 3 3 4 5% 48%	7680
	21st to end	Night	23% 8% 2 7% 4 4 3 3 4 6% 35%	8402
February	1st to 10th	Night	15% 8% 3 6% 4 4 4 3 4 6% 43%	2521
	11th to 20th	Night	20% 7% 3 6% 3 4 3 3 5% 5% 41%	2458
	21st to end	Night	20% 9% 2 6% 3 3 3 3 4 6% 41%	2040
March	1st to 10th	Night	19% 10% 3 7% 4 3 4 3 4 6% 37%	5029
	11th to 20th	Night	14% 8% 3 5% 4 4 3 3 4 6% 46%	5090
	21st to end	Night	16% 8% 2 6% 4 3 4 3 5% 6% 43%	5507
April	1st to 10th	Night	13% 7% 3 5% 4 4 4 4 6% 7% 44%	2100
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 3 5% 7% 47%	2075
	21st to end	Night	8% 5% 2 5% 4 3 4 4 7% 7% 50%	2100
May	1st to 10th	Night	21% 9% 3 6% 4 4 4 3 5% 7% 34%	5832
	11th to 20th	Night	14% 7% 2 6% 5% 4 4 3 6% 8% 40%	5829
	21st to end	Night	11% 7% 3 8% 5% 5% 5% 5% 7% 11% 34%	6415
June	1st to 10th	Night	6% 4 2 6% 3 4 6% 5% 8% 11% 44%	1898
	11th to 20th	Night	4 5% 2 5% 4 4 6% 5% 9% 12% 44%	1890
	21st to end	Night	7% 4 2 7% 6% 6% 6% 5% 8% 10% 40%	1890
July	1st to 10th	Night	3 4 2 6% 5% 5% 6% 6% 10% 14% 40%	5308
	11th to 20th	Night	4 4 2 6% 5% 5% 6% 5% 9% 13% 41%	5307
	21st to end	Night	4 5% 2 7% 5% 5% 7% 5% 9% 14% 39%	5934
August	1st to 10th	Night	2 2 1 3 3 3 4 4 7% 12% 58%	4604
	11th to 20th	Night	2 2 1 4 4 4 5% 4 8% 13% 54%	4560
	21st to end	Night	4 5% 2 6% 4 5% 6% 4 8% 11% 43%	5046
September	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 5% 8% 10% 42%	2100
	11th to 20th	Night	9% 7% 2 7% 4 5% 6% 4 7% 9% 41%	2030
	21st to end	Night	7% 4 2 6% 4 4 5% 4 6% 8% 50%	2218
October	1st to 10th	Night	5% 5% 2 4 3 3 4 4 4 6% 59%	2400
	11th to 20th	Night	7% 4 2 5% 3 3 3 3 5% 6% 60%	2400
	21st to end	Night	10% 5% 2 5% 2 3 4 3 4 5% 57%	2640
November	1st to 10th	Night	9% 5% 2 6% 4 3 3 3 4 6% 56%	2460
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 5% 56%	2520
	21st to end	Night	14% 6% 2 6% 3 4 3 3 5% 6% 48%	2520
December	1st to 10th	Night	19% 8% 3 7% 4 3 3 3 4 5% 41%	4040
	11th to 20th	Night	10% 9% 2 7% 4 4 4 3 5% 6% 45%	4677
	21st to end	Night	17% 9% 2 6% 4 3 3 2 3 5% 45%	6633

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	45% 4 8% 17% 21% 4	5290
	11th to 20th	Day	37% 3 6% 18% 27% 10%	5310
	21st to end	Day	52% 3 6% 11% 15% 12%	5812
February	1st to 10th	Day	45% 5% 8% 14% 18% 9%	2094
	11th to 20th	Day	45% 5% 7% 16% 18% 9%	2058
	21st to end	Day	46% 5% 8% 16% 17% 8%	1782
March	1st to 10th	Day	50% 6% 8% 13% 13% 10%	5046
	11th to 20th	Day	37% 7% 9% 14% 18% 16%	5090
	21st to end	Day	41% 6% 9% 18% 19% 7%	5663
April	1st to 10th	Day	41% 7% 11% 16% 17% 9%	2520
	11th to 20th	Day	41% 6% 10% 19% 16% 9%	2494
	21st to end	Day	36% 7% 12% 20% 21% 4	2520
May	1st to 10th	Day	50% 7% 11% 19% 11% 3	7056
	11th to 20th	Day	37% 6% 13% 23% 17% 3	7035
	21st to end	Day	42% 9% 17% 22% 8% 1	7749
June	1st to 10th	Day	43% 10% 13% 21% 12% 1	2715
	11th to 20th	Day	42% 11% 16% 18% 12% 1	2730
	21st to end	Day	46% 12% 18% 18% 5%	2730
July	1st to 10th	Day	44% 8% 30% 16% 2	7616
	11th to 20th	Day	45% 10% 32% 12% 2	7611
	21st to end	Day	42% 8% 21% 26% 2	7980
August	1st to 10th	Day	22% 7% 20% 28% 19% 3	5533
	11th to 20th	Day	28% 7% 26% 32% 6%	5522
	21st to end	Day	43% 7% 16% 19% 14% 2	6079
September	1st to 10th	Day	42% 11% 17% 16% 11% 3	2520
	11th to 20th	Day	40% 8% 15% 16% 15% 5%	2436
	21st to end	Day	34% 6% 11% 22% 17% 9%	2155
October	1st to 10th	Day	25% 4 6% 12% 23% 29%	3260
	11th to 20th	Day	18% 3 5% 19% 27% 27%	3240
	21st to end	Day	24% 3 5% 12% 41% 15%	3538
November	1st to 10th	Day	32% 4 9% 19% 25% 10%	2058
	11th to 20th	Day	32% 4 8% 17% 27% 13%	2100
	21st to end	Day	39% 5% 8% 17% 23% 9%	2100
December	1st to 10th	Day	47% 4 7% 15% 20% 6%	3360
	11th to 20th	Day	40% 4 10% 15% 22% 9%	3723
	21st to end	Day	40% 5% 8% 17% 25% 5%	4587
January	1st to 10th	Night	41% 3 9% 18% 22% 6%	7635
	11th to 20th	Night	38% 3 4 16% 25% 13%	7680
	21st to end	Night	52% 3 6% 13% 17% 9%	8402
February	1st to 10th	Night	42% 5% 6% 15% 21% 10%	2521
	11th to 20th	Night	45% 5% 6% 16% 18% 11%	2459
	21st to end	Night	44% 6% 10% 14% 16% 11%	2040
March	1st to 10th	Night	48% 5% 8% 12% 16% 11%	5029
	11th to 20th	Night	38% 7% 8% 14% 17% 16%	5090
	21st to end	Night	41% 6% 9% 18% 19% 7%	5507
April	1st to 10th	Night	37% 7% 12% 17% 16% 11%	2100
	11th to 20th	Night	36% 6% 10% 20% 18% 11%	2075
	21st to end	Night	30% 8% 12% 21% 22% 6%	2100
May	1st to 10th	Night	48% 7% 12% 17% 13% 3	5862
	11th to 20th	Night	40% 6% 15% 22% 15% 2	5830
	21st to end	Night	41% 9% 20% 22% 7% 1	6427
June	1st to 10th	Night	30% 12% 20% 23% 15% 1	1898
	11th to 20th	Night	30% 10% 20% 26% 12% 2	1890
	21st to end	Night	35% 12% 22% 24% 7% 1	1890
July	1st to 10th	Night	32% 10% 40% 16% 3	5265
	11th to 20th	Night	32% 13% 39% 14% 2	5256
	21st to end	Night	33% 12% 24% 28% 4	5803
August	1st to 10th	Night	17% 8% 26% 26% 19% 4	4592
	11th to 20th	Night	21% 7% 34% 32% 5%	4590
	21st to end	Night	34% 9% 20% 24% 12% 2	5046
September	1st to 10th	Night	33% 13% 21% 19% 10% 5%	2100
	11th to 20th	Night	37% 10% 16% 18% 14% 5%	2030
	21st to end	Night	30% 7% 14% 23% 16% 11%	2218
October	1st to 10th	Night	24% 4 8% 14% 21% 29%	3920
	11th to 20th	Night	16% 3 5% 19% 29% 27%	3899
	21st to end	Night	20% 4 5% 12% 36% 23%	4252
November	1st to 10th	Night	30% 5% 10% 19% 25% 13%	2460
	11th to 20th	Night	30% 4 7% 16% 26% 16%	2520
	21st to end	Night	36% 5% 9% 18% 21% 11%	2520
December	1st to 10th	Night	47% 3 5% 17% 22% 5%	4040
	11th to 20th	Night	39% 5% 10% 16% 20% 11%	4678
	21st to end	Night	43% 4 8% 15% 23% 7%	6633

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	21% 15% 35% 19% 8% 2	330
	11th to 20th	Day	16% 14% 38% 24% 7% 2	330
	21st to end	Day	29% 9% 34% 23% 4	363
February	1st to 10th	Day	21% 11% 30% 30% 8% 1	330
	11th to 20th	Day	24% 15% 32% 21% 8% 1	330
	21st to end	Day	22% 18% 35% 19% 5%	273
March	1st to 10th	Day	25% 11% 32% 25% 7%	330
	11th to 20th	Day	23% 14% 33% 21% 8% 1	330
	21st to end	Day	17% 7% 35% 25% 14% 1	390
April	1st to 10th	Day	18% 10% 29% 30% 12% 2	540
	11th to 20th	Day	21% 8% 30% 25% 14% 2	532
	21st to end	Day	12% 5% 31% 29% 19% 4	540
May	1st to 10th	Day	15% 5% 26% 31% 20% 3	543
	11th to 20th	Day	14% 3 20% 29% 24% 10%	567
	21st to end	Day	7% 1 14% 32% 30% 15%	617
June	1st to 10th	Day	10% 2 10% 30% 31% 16%	765
	11th to 20th	Day	9% 2 12% 27% 35% 16%	780
	21st to end	Day	9% 2 7% 25% 31% 26%	780
July	1st to 10th	Day	9% 2 4 20% 35% 30%	771
	11th to 20th	Day	8% 2 2 17% 38% 33%	768
	21st to end	Day	10% 1 4 20% 36% 29%	786
August	1st to 10th	Day	5% 2 2 17% 34% 41%	566
	11th to 20th	Day	7% 1 1 9% 32% 49%	559
	21st to end	Day	12% 2 3 18% 35% 30%	627
September	1st to 10th	Day	11% 3 3 20% 38% 26%	570
	11th to 20th	Day	12% 1 5% 21% 37% 25%	542
	21st to end	Day	10% 3 8% 23% 35% 21%	460
October	1st to 10th	Day	6% 2 16% 38% 25% 12%	330
	11th to 20th	Day	7% 3 17% 37% 22% 13%	330
	21st to end	Day	15% 6% 24% 28% 20% 8%	363
November	1st to 10th	Day	14% 3 21% 32% 24% 6%	330
	11th to 20th	Day	15% 6% 25% 35% 16% 4	330
	21st to end	Day	16% 10% 32% 25% 14% 3	330
December	1st to 10th	Day	23% 7% 27% 27% 11% 5%	330
	11th to 20th	Day	20% 7% 31% 32% 9%	330
	21st to end	Day	25% 7% 29% 24% 13% 2	363
January	1st to 10th	Night	22% 7% 30% 32% 7%	1050
	11th to 20th	Night	18% 9% 29% 34% 9%	1050
	21st to end	Night	23% 8% 30% 32% 6%	1155
February	1st to 10th	Night	20% 8% 31% 35% 6%	841
	11th to 20th	Night	24% 10% 26% 33% 7%	823
	21st to end	Night	25% 9% 35% 28% 4	656
March	1st to 10th	Night	18% 8% 28% 37% 8%	630
	11th to 20th	Night	17% 6% 29% 37% 11%	630
	21st to end	Night	15% 5% 31% 37% 12%	693
April	1st to 10th	Night	17% 8% 29% 36% 10%	630
	11th to 20th	Night	15% 7% 27% 38% 13% 1	623
	21st to end	Night	10% 4 27% 42% 15% 1	630
May	1st to 10th	Night	14% 2 24% 44% 16% 1	630
	11th to 20th	Night	13% 2 19% 41% 24% 2	621
	21st to end	Night	9% 1 10% 39% 34% 6%	683
June	1st to 10th	Night	7% 1 5% 40% 40% 7%	630
	11th to 20th	Night	7% 1 7% 32% 45% 8%	630
	21st to end	Night	9% 1 6% 25% 45% 15%	630
July	1st to 10th	Night	5% 1 1 20% 53% 20%	615
	11th to 20th	Night	5% 1 1 16% 52% 25%	612
	21st to end	Night	6% 1 2 20% 53% 18%	690
August	1st to 10th	Night	4 1 1 18% 53% 23%	627
	11th to 20th	Night	4 1 1 16% 51% 27%	624
	21st to end	Night	8% 1 3 19% 49% 20%	693
September	1st to 10th	Night	10% 2 4 20% 47% 16%	630
	11th to 20th	Night	14% 2 4 20% 49% 12%	609
	21st to end	Night	9% 2 6% 29% 46% 8%	755
October	1st to 10th	Night	9% 1 12% 36% 37% 6%	800
	11th to 20th	Night	9% 3 13% 41% 31% 3	800
	21st to end	Night	14% 4 17% 39% 24% 2	880
November	1st to 10th	Night	12% 4 17% 40% 24% 3	820
	11th to 20th	Night	15% 5% 22% 40% 19% 1	840
	21st to end	Night	18% 4 25% 37% 14% 1	840
December	1st to 10th	Night	22% 8% 21% 34% 15% 1	840
	11th to 20th	Night	20% 5% 25% 37% 13%	893
	21st to end	Night	25% 7% 25% 33% 10% 1	1155

Page generated at 2026-09-11 03:11:53. Version ac07

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