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 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	11% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	114228
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	114836
Transparency	Day	41% 7% 11% 17% 17% 7%	117880
	Night	36% 7% 12% 18% 18% 9%	119327
Seeing	Day	14% 5% 17% 24% 24% 16%	15461
	Night	14% 5% 18% 33% 24% 6%	25285

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	16% 8% 4 7% 4 4 3 3 4 6% 40%	4077
	11th to 20th	Day	13% 8% 3 7% 4 3 3 3 3 5% 49%	4080
	21st to end	Day	21% 9% 4 7% 4 4 4 4 5% 5% 34%	4488
February	1st to 10th	Day	16% 9% 3 8% 4 4 3 4 5% 6% 38%	1994
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 6% 6% 39%	1948
	21st to end	Day	17% 9% 3 8% 5% 4 4 3 5% 8% 35%	1700
March	1st to 10th	Day	20% 9% 3 8% 4 4 4 3 5% 7% 33%	4936
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	4980
	21st to end	Day	13% 7% 2 7% 5% 4 4 4 5% 7% 40%	5539
April	1st to 10th	Day	10% 8% 3 6% 5% 5% 6% 4 5% 7% 41%	2400
	11th to 20th	Day	13% 7% 3 6% 4 5% 4 4 7% 7% 39%	2374
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	2400
May	1st to 10th	Day	11% 7% 3 9% 7% 6% 6% 4 6% 7% 34%	3885
	11th to 20th	Day	11% 6% 3 8% 5% 5% 5% 4 5% 7% 43%	3764
	21st to end	Day	5% 4 2 7% 6% 7% 6% 6% 8% 9% 38%	4129
June	1st to 10th	Day	7% 5% 2 8% 7% 7% 7% 5% 8% 9% 33%	2456
	11th to 20th	Day	5% 6% 3 9% 8% 7% 7% 7% 8% 10% 32%	2470
	21st to end	Day	6% 6% 3 10% 9% 8% 8% 8% 9% 10% 24%	2470
July	1st to 10th	Day	6% 7% 3 10% 9% 8% 8% 6% 9% 10% 22%	4120
	11th to 20th	Day	6% 7% 3 10% 9% 9% 10% 7% 9% 11% 21%	4120
	21st to end	Day	12% 10% 4 10% 7% 7% 7% 5% 7% 9% 21%	4372
August	1st to 10th	Day	3 3 1 5% 4 5% 5% 5% 7% 9% 54%	3785
	11th to 20th	Day	6% 4 2 6% 5% 7% 7% 6% 9% 11% 37%	3729
	21st to end	Day	11% 7% 3 9% 7% 7% 6% 4 6% 7% 33%	4180
September	1st to 10th	Day	10% 5% 3 8% 6% 7% 6% 5% 7% 9% 34%	2280
	11th to 20th	Day	12% 6% 3 6% 5% 6% 5% 4 6% 8% 38%	2280
	21st to end	Day	9% 5% 2 6% 5% 4 4 4 6% 7% 48%	2048
October	1st to 10th	Day	6% 4 2 6% 3 3 4 4 5% 6% 56%	1900
	11th to 20th	Day	8% 4 2 5% 3 4 3 3 5% 7% 55%	1900
	21st to end	Day	11% 4 2 5% 4 3 4 3 5% 6% 54%	2090
November	1st to 10th	Day	8% 6% 2 5% 4 4 4 4 6% 8% 49%	1958
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 5% 5% 53%	2000
	21st to end	Day	12% 7% 2 7% 5% 4 4 4 5% 5% 45%	2000
December	1st to 10th	Day	17% 8% 3 6% 5% 4 4 3 4 5% 40%	3260
	11th to 20th	Day	10% 7% 3 7% 5% 5% 4 4 5% 6% 43%	3628
	21st to end	Day	16% 7% 3 6% 4 3 4 3 5% 6% 44%	4488
January	1st to 10th	Night	19% 6% 2 4 3 3 3 2 4 5% 48%	5888
	11th to 20th	Night	18% 7% 2 5% 3 3 2 3 4 5% 48%	5900
	21st to end	Night	24% 8% 2 6% 4 3 3 3 4 6% 35%	6490
February	1st to 10th	Night	15% 8% 3 6% 4 4 4 3 5% 6% 42%	2401
	11th to 20th	Night	20% 7% 3 6% 3 4 3 3 5% 5% 42%	2338
	21st to end	Night	20% 9% 2 6% 3 3 3 3 5% 6% 39%	1946
March	1st to 10th	Night	19% 10% 3 7% 4 3 4 3 4 6% 37%	4919
	11th to 20th	Night	14% 8% 3 5% 4 4 3 3 4 6% 46%	4980
	21st to end	Night	16% 8% 3 6% 4 3 4 3 5% 6% 42%	5387
April	1st to 10th	Night	13% 7% 3 5% 4 4 4 4 6% 8% 44%	2000
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 3 5% 7% 47%	1975
	21st to end	Night	7% 5% 3 5% 4 3 4 4 7% 7% 52%	2000
May	1st to 10th	Night	14% 7% 3 6% 4 4 4 4 6% 8% 38%	3230
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 3 6% 7% 41%	3135
	21st to end	Night	8% 5% 2 6% 4 5% 4 5% 8% 12% 41%	3426
June	1st to 10th	Night	5% 4 2 5% 3 4 5% 5% 9% 11% 45%	1718
	11th to 20th	Night	3 4 2 5% 4 4 7% 5% 9% 12% 45%	1710
	21st to end	Night	7% 4 2 7% 6% 6% 7% 5% 8% 10% 39%	1710
July	1st to 10th	Night	3 5% 3 6% 6% 6% 7% 6% 10% 14% 33%	2850
	11th to 20th	Night	4 5% 2 5% 5% 5% 6% 6% 10% 13% 38%	2850
	21st to end	Night	5% 6% 2 8% 6% 6% 7% 5% 8% 13% 31%	3184
August	1st to 10th	Night	2 2 1 4 3 3 4 4 6% 11% 61%	3144
	11th to 20th	Night	3 2 1 5% 4 5% 5% 5% 9% 15% 46%	3100
	21st to end	Night	6% 7% 3 7% 5% 6% 6% 4 7% 9% 41%	3476
September	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 4 7% 10% 42%	1900
	11th to 20th	Night	10% 7% 2 7% 4 5% 6% 4 7% 8% 40%	1900
	21st to end	Night	7% 4 2 6% 4 4 5% 4 6% 8% 50%	2107
October	1st to 10th	Night	5% 5% 2 4 3 3 3 4 4 7% 59%	2280
	11th to 20th	Night	7% 4 2 5% 3 3 3 3 5% 6% 59%	2280
	21st to end	Night	10% 5% 2 5% 3 4 4 3 4 5% 57%	2508
November	1st to 10th	Night	9% 5% 2 6% 3 3 3 3 4 6% 56%	2340
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 5% 55%	2400
	21st to end	Night	14% 6% 2 6% 3 4 3 3 5% 6% 48%	2400
December	1st to 10th	Night	18% 7% 3 7% 4 3 3 3 4 5% 41%	3920
	11th to 20th	Night	10% 9% 2 7% 4 4 4 3 5% 6% 45%	4554
	21st to end	Night	17% 9% 2 6% 4 3 3 2 3 5% 44%	6490

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	44% 4 7% 17% 22% 6%	4078
	11th to 20th	Day	39% 3 5% 16% 28% 9%	4080
	21st to end	Day	52% 4 7% 12% 16% 10%	4488
February	1st to 10th	Day	46% 6% 8% 13% 18% 9%	1994
	11th to 20th	Day	45% 5% 7% 15% 18% 9%	1958
	21st to end	Day	47% 5% 8% 16% 16% 8%	1700
March	1st to 10th	Day	50% 6% 8% 12% 13% 11%	4936
	11th to 20th	Day	37% 7% 9% 13% 18% 16%	4980
	21st to end	Day	42% 6% 9% 17% 19% 7%	5541
April	1st to 10th	Day	40% 7% 10% 16% 18% 9%	2400
	11th to 20th	Day	41% 6% 10% 18% 16% 9%	2374
	21st to end	Day	35% 8% 11% 20% 22% 4	2400
May	1st to 10th	Day	43% 10% 12% 20% 11% 4	3920
	11th to 20th	Day	39% 8% 9% 18% 23% 3	3768
	21st to end	Day	34% 11% 15% 29% 10% 1	4141
June	1st to 10th	Day	43% 10% 13% 22% 11% 1	2456
	11th to 20th	Day	42% 11% 15% 19% 12% 1	2470
	21st to end	Day	48% 13% 18% 16% 6%	2470
July	1st to 10th	Day	53% 9% 20% 15% 2	4056
	11th to 20th	Day	51% 13% 25% 9% 2	4051
	21st to end	Day	57% 10% 15% 14% 3	4234
August	1st to 10th	Day	25% 8% 13% 31% 22%	3773
	11th to 20th	Day	36% 9% 23% 26% 6% 1	3765
	21st to end	Day	49% 8% 14% 13% 15% 2	4179
September	1st to 10th	Day	44% 11% 15% 15% 11% 4	2280
	11th to 20th	Day	41% 8% 15% 15% 15% 6%	2280
	21st to end	Day	34% 7% 11% 22% 17% 9%	2048
October	1st to 10th	Day	25% 4 6% 12% 23% 30%	3160
	11th to 20th	Day	18% 3 5% 19% 27% 27%	3140
	21st to end	Day	25% 3 5% 11% 41% 16%	3428
November	1st to 10th	Day	33% 4 10% 19% 25% 9%	1958
	11th to 20th	Day	33% 4 7% 16% 26% 13%	2000
	21st to end	Day	39% 6% 7% 16% 23% 9%	2000
December	1st to 10th	Day	46% 5% 7% 15% 21% 6%	3260
	11th to 20th	Day	40% 4 10% 15% 22% 9%	3626
	21st to end	Day	40% 5% 8% 17% 25% 5%	4488
January	1st to 10th	Night	39% 4 7% 19% 25% 6%	5888
	11th to 20th	Night	39% 4 5% 15% 27% 10%	5900
	21st to end	Night	51% 3 6% 13% 16% 11%	6490
February	1st to 10th	Night	43% 5% 7% 15% 20% 10%	2401
	11th to 20th	Night	44% 5% 6% 16% 18% 12%	2339
	21st to end	Night	44% 6% 10% 14% 15% 11%	1946
March	1st to 10th	Night	48% 5% 8% 12% 16% 11%	4919
	11th to 20th	Night	38% 7% 8% 13% 17% 16%	4980
	21st to end	Night	41% 6% 9% 18% 18% 7%	5387
April	1st to 10th	Night	36% 7% 11% 17% 17% 11%	2000
	11th to 20th	Night	36% 6% 10% 19% 18% 11%	1975
	21st to end	Night	29% 8% 12% 22% 24% 7%	2000
May	1st to 10th	Night	38% 11% 14% 20% 13% 4	3260
	11th to 20th	Night	39% 8% 12% 20% 20% 2	3136
	21st to end	Night	30% 10% 20% 27% 11% 2	3435
June	1st to 10th	Night	29% 12% 20% 24% 15% 1	1718
	11th to 20th	Night	28% 11% 20% 27% 12% 2	1710
	21st to end	Night	36% 13% 20% 23% 8% 1	1710
July	1st to 10th	Night	38% 13% 31% 14% 3	2805
	11th to 20th	Night	32% 15% 38% 12% 3	2796
	21st to end	Night	40% 18% 21% 16% 4	3083
August	1st to 10th	Night	16% 9% 20% 31% 23% 1	3132
	11th to 20th	Night	26% 10% 30% 29% 5%	3130
	21st to end	Night	38% 10% 17% 18% 14% 2	3476
September	1st to 10th	Night	34% 13% 19% 20% 10% 5%	1900
	11th to 20th	Night	38% 10% 16% 17% 14% 5%	1900
	21st to end	Night	31% 7% 14% 22% 15% 11%	2107
October	1st to 10th	Night	24% 4 8% 14% 21% 29%	3800
	11th to 20th	Night	16% 3 6% 19% 28% 27%	3779
	21st to end	Night	20% 4 5% 12% 36% 23%	4120
November	1st to 10th	Night	30% 5% 10% 19% 24% 12%	2340
	11th to 20th	Night	32% 4 7% 15% 26% 16%	2400
	21st to end	Night	37% 5% 8% 18% 21% 11%	2400
December	1st to 10th	Night	46% 3 5% 18% 22% 5%	3920
	11th to 20th	Night	39% 5% 10% 16% 20% 10%	4555
	21st to end	Night	43% 4 8% 14% 23% 7%	6490

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	22% 14% 35% 18% 8% 2	300
	11th to 20th	Day	16% 14% 38% 24% 7% 1	300
	21st to end	Day	26% 10% 36% 24% 4	330
February	1st to 10th	Day	22% 10% 29% 29% 9% 1	300
	11th to 20th	Day	25% 14% 31% 21% 8%	300
	21st to end	Day	23% 16% 37% 19% 6%	249
March	1st to 10th	Day	26% 10% 32% 24% 7%	300
	11th to 20th	Day	24% 14% 34% 21% 7% 1	300
	21st to end	Day	18% 8% 38% 23% 12% 1	356
April	1st to 10th	Day	18% 10% 29% 29% 12% 2	500
	11th to 20th	Day	21% 8% 30% 24% 15% 2	492
	21st to end	Day	11% 5% 29% 30% 20% 5%	500
May	1st to 10th	Day	14% 5% 25% 32% 21% 4	503
	11th to 20th	Day	14% 2 18% 29% 27% 11%	487
	21st to end	Day	6% 2 13% 32% 32% 16%	529
June	1st to 10th	Day	10% 2 9% 29% 33% 18%	666
	11th to 20th	Day	8% 2 11% 26% 36% 17%	680
	21st to end	Day	9% 2 8% 23% 31% 27%	680
July	1st to 10th	Day	9% 2 4 19% 34% 31%	671
	11th to 20th	Day	7% 2 2 17% 38% 34%	668
	21st to end	Day	11% 1 3 20% 35% 30%	682
August	1st to 10th	Day	5% 2 1 14% 34% 44%	486
	11th to 20th	Day	8% 1 1 10% 31% 49%	479
	21st to end	Day	13% 3 3 16% 34% 32%	539
September	1st to 10th	Day	12% 3 2 17% 37% 28%	490
	11th to 20th	Day	13% 1 5% 21% 36% 24%	490
	21st to end	Day	10% 3 8% 21% 36% 23%	424
October	1st to 10th	Day	7% 2 16% 37% 26% 12%	300
	11th to 20th	Day	8% 3 17% 37% 22% 13%	300
	21st to end	Day	15% 6% 21% 28% 21% 8%	330
November	1st to 10th	Day	14% 2 22% 32% 25% 6%	300
	11th to 20th	Day	16% 6% 26% 32% 16% 3	300
	21st to end	Day	17% 9% 31% 26% 14% 3	300
December	1st to 10th	Day	20% 6% 27% 28% 12% 6%	300
	11th to 20th	Day	21% 8% 31% 31% 9%	300
	21st to end	Day	26% 6% 31% 24% 11% 2	330
January	1st to 10th	Night	22% 7% 31% 32% 8% 1	1000
	11th to 20th	Night	19% 8% 29% 34% 9% 1	1000
	21st to end	Night	23% 8% 30% 33% 6%	1100
February	1st to 10th	Night	20% 8% 30% 35% 6%	801
	11th to 20th	Night	23% 10% 26% 34% 7%	783
	21st to end	Night	25% 9% 34% 28% 4	626
March	1st to 10th	Night	18% 9% 28% 37% 8%	600
	11th to 20th	Night	17% 6% 30% 38% 10%	600
	21st to end	Night	15% 5% 32% 36% 12%	660
April	1st to 10th	Night	16% 8% 29% 36% 10% 1	600
	11th to 20th	Night	15% 8% 26% 37% 13% 1	593
	21st to end	Night	9% 4 27% 42% 16% 1	600
May	1st to 10th	Night	12% 2 24% 45% 17% 1	600
	11th to 20th	Night	12% 2 16% 43% 25% 2	563
	21st to end	Night	9% 1 9% 39% 35% 7%	617
June	1st to 10th	Night	7% 1 5% 39% 41% 7%	570
	11th to 20th	Night	5% 1 7% 33% 45% 9%	570
	21st to end	Night	9% 1 6% 25% 43% 15%	570
July	1st to 10th	Night	5% 1 1 21% 51% 20%	555
	11th to 20th	Night	4 2 1 16% 52% 26%	552
	21st to end	Night	6% 1 2 18% 54% 19%	624
August	1st to 10th	Night	4 1 1 18% 52% 24%	567
	11th to 20th	Night	5% 1 1 15% 52% 26%	564
	21st to end	Night	8% 1 3 18% 49% 21%	627
September	1st to 10th	Night	11% 2 4 18% 48% 18%	570
	11th to 20th	Night	15% 2 4 20% 48% 11%	570
	21st to end	Night	9% 2 5% 29% 46% 9%	717
October	1st to 10th	Night	8% 1 12% 36% 37% 6%	760
	11th to 20th	Night	9% 3 13% 42% 30% 3	760
	21st to end	Night	13% 4 17% 39% 25% 2	836
November	1st to 10th	Night	12% 4 18% 40% 23% 3	780
	11th to 20th	Night	15% 5% 22% 38% 19% 1	800
	21st to end	Night	19% 4 24% 37% 15% 1	800
December	1st to 10th	Night	20% 8% 21% 35% 16% 1	800
	11th to 20th	Night	21% 5% 25% 37% 12%	850
	21st to end	Night	25% 7% 26% 33% 10%	1100

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