Blue Mountain Beach
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Mountain Beach 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	9% 5% 2 5% 4 4 5% 4 7% 10% 45%	127045
	Night	9% 5% 2 6% 4 4 4 4 6% 9% 46%	125005
Transparency	Day	32% 9% 16% 19% 16% 7%	130949
	Night	33% 7% 15% 19% 18% 8%	129162
Seeing	Day	10% 4 16% 27% 28% 14%	16318
	Night	11% 5% 18% 28% 29% 10%	26310

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	10% 4 2 4 3 4 4 3 5% 7% 53%	4146
	11th to 20th	Day	14% 6% 2 5% 3 3 3 4 6% 8% 47%	4597
	21st to end	Day	15% 6% 2 6% 4 5% 5% 4 6% 8% 39%	5082
February	1st to 10th	Day	13% 8% 2 7% 3 5% 4 3 5% 6% 44%	2000
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 5% 6% 47%	1948
	21st to end	Day	13% 7% 4 7% 5% 4 3 4 6% 6% 43%	1650
March	1st to 10th	Day	15% 7% 3 5% 4 4 4 3 5% 7% 43%	4480
	11th to 20th	Day	8% 5% 2 6% 3 4 5% 4 5% 7% 53%	4883
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 6% 46%	5507
April	1st to 10th	Day	11% 5% 2 6% 4 3 4 4 6% 7% 48%	2400
	11th to 20th	Day	9% 5% 2 5% 4 3 4 4 6% 7% 50%	2374
	21st to end	Day	6% 3 1 4 3 3 4 3 5% 8% 59%	2400
May	1st to 10th	Day	11% 6% 3 7% 4 4 4 4 6% 9% 43%	5625
	11th to 20th	Day	11% 5% 2 5% 3 3 4 4 7% 11% 45%	5861
	21st to end	Day	4 3 1 5% 3 4 5% 5% 7% 12% 50%	6433
June	1st to 10th	Day	7% 5% 2 6% 4 5% 6% 5% 9% 12% 39%	2600
	11th to 20th	Day	4 4 2 5% 5% 5% 7% 5% 11% 15% 39%	2600
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 6% 10% 12% 34%	2600
July	1st to 10th	Day	5% 4 1 6% 5% 6% 7% 7% 11% 15% 31%	5900
	11th to 20th	Day	3 3 2 5% 5% 5% 7% 6% 10% 14% 41%	5900
	21st to end	Day	3 3 2 6% 5% 6% 8% 7% 12% 17% 33%	5971
August	1st to 10th	Day	5% 3 1 5% 4 5% 7% 6% 10% 14% 40%	3905
	11th to 20th	Day	5% 3 1 5% 3 5% 5% 5% 8% 13% 47%	3849
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 6% 9% 12% 39%	4312
September	1st to 10th	Day	9% 5% 3 5% 5% 5% 6% 6% 8% 11% 38%	2285
	11th to 20th	Day	10% 5% 2 6% 5% 4 5% 5% 8% 10% 40%	2200
	21st to end	Day	7% 5% 1 5% 4 4 4 4 5% 9% 52%	2200
October	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 6% 7% 52%	2200
	11th to 20th	Day	7% 3 1 4 3 3 3 3 5% 6% 64%	2200
	21st to end	Day	8% 4 2 5% 4 3 4 3 5% 5% 57%	2257
November	1st to 10th	Day	8% 5% 2 3 3 3 4 4 5% 6% 57%	1852
	11th to 20th	Day	9% 4 2 5% 3 3 2 3 5% 7% 57%	1890
	21st to end	Day	10% 5% 2 5% 5% 4 4 3 5% 6% 50%	1890
December	1st to 10th	Day	14% 6% 2 4 3 3 3 2 5% 7% 50%	3020
	11th to 20th	Day	10% 6% 2 5% 3 3 3 3 4 6% 56%	3463
	21st to end	Day	15% 8% 3 6% 3 3 3 3 4 6% 46%	4565
January	1st to 10th	Night	11% 5% 2 6% 5% 4 4 3 5% 7% 47%	6019
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 6% 40%	5578
	21st to end	Night	16% 8% 3 6% 4 4 4 4 6% 8% 38%	6116
February	1st to 10th	Night	13% 8% 2 6% 4 3 4 3 4 6% 46%	2401
	11th to 20th	Night	16% 7% 3 5% 4 3 3 3 4 6% 45%	2338
	21st to end	Night	15% 7% 3 6% 3 3 3 4 5% 6% 45%	1980
March	1st to 10th	Night	18% 10% 3 7% 3 3 3 3 4 7% 38%	5373
	11th to 20th	Night	8% 5% 2 5% 3 3 4 4 6% 8% 52%	5075
	21st to end	Night	12% 7% 2 6% 4 4 3 3 5% 7% 48%	5450
April	1st to 10th	Night	9% 7% 2 5% 4 5% 4 4 6% 8% 46%	2000
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 7% 49%	1975
	21st to end	Night	6% 3 1 4 4 3 3 3 5% 9% 59%	2000
May	1st to 10th	Night	9% 6% 2 6% 4 5% 5% 4 7% 10% 44%	4266
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 7% 9% 47%	4032
	21st to end	Night	4 3 2 5% 3 5% 4 4 7% 12% 53%	4421
June	1st to 10th	Night	6% 5% 2 7% 5% 6% 5% 4 8% 14% 39%	1800
	11th to 20th	Night	4 4 1 4 4 5% 5% 5% 10% 14% 45%	1800
	21st to end	Night	5% 5% 2 5% 4 5% 5% 5% 9% 13% 41%	1800
July	1st to 10th	Night	4 3 2 7% 6% 6% 7% 7% 11% 16% 31%	4058
	11th to 20th	Night	2 2 1 3 3 4 5% 6% 10% 15% 48%	4057
	21st to end	Night	3 3 1 5% 5% 6% 7% 6% 12% 17% 35%	4874
August	1st to 10th	Night	4 3 2 5% 4 5% 6% 6% 8% 13% 45%	3244
	11th to 20th	Night	4 4 2 6% 4 4 5% 5% 8% 12% 46%	3200
	21st to end	Night	5% 4 2 7% 6% 5% 7% 6% 9% 10% 38%	3586
September	1st to 10th	Night	7% 5% 2 8% 5% 5% 6% 7% 8% 12% 34%	2094
	11th to 20th	Night	9% 5% 2 6% 5% 5% 5% 5% 6% 10% 41%	2200
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 51%	2200
October	1st to 10th	Night	7% 5% 2 5% 4 4 4 4 6% 7% 51%	2200
	11th to 20th	Night	5% 4 2 4 3 3 2 3 4 5% 65%	2200
	21st to end	Night	8% 4 2 5% 4 4 5% 3 5% 5% 55%	2577
November	1st to 10th	Night	7% 4 1 5% 3 4 4 3 5% 8% 56%	2666
	11th to 20th	Night	10% 5% 2 5% 4 3 3 3 4 7% 55%	2730
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 7% 49%	2730
December	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 7% 50%	4380
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 6% 53%	4952
	21st to end	Night	15% 8% 2 6% 4 3 3 3 4 5% 46%	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	30% 5% 7% 18% 30% 10%	4147
	11th to 20th	Day	35% 4 7% 18% 23% 13%	4597
	21st to end	Day	42% 5% 8% 15% 19% 10%	5082
February	1st to 10th	Day	41% 5% 8% 16% 20% 9%	2000
	11th to 20th	Day	38% 6% 6% 19% 21% 10%	1958
	21st to end	Day	41% 7% 9% 17% 17% 10%	1650
March	1st to 10th	Day	40% 6% 8% 19% 16% 10%	4480
	11th to 20th	Day	29% 7% 12% 14% 20% 19%	4883
	21st to end	Day	36% 6% 13% 20% 16% 9%	5509
April	1st to 10th	Day	33% 7% 13% 18% 18% 11%	2400
	11th to 20th	Day	32% 6% 10% 21% 22% 10%	2374
	21st to end	Day	23% 6% 14% 26% 22% 8%	2400
May	1st to 10th	Day	36% 9% 15% 23% 14% 4	5660
	11th to 20th	Day	30% 9% 19% 28% 13% 1	5865
	21st to end	Day	23% 9% 21% 29% 16% 2	6447
June	1st to 10th	Day	34% 11% 20% 26% 8% 1	2600
	11th to 20th	Day	29% 12% 24% 25% 9% 1	2600
	21st to end	Day	37% 14% 22% 19% 8% 1	2600
July	1st to 10th	Day	35% 17% 36% 11% 1	5835
	11th to 20th	Day	29% 15% 41% 14% 1	5831
	21st to end	Day	32% 19% 31% 17% 1	5799
August	1st to 10th	Day	29% 12% 23% 25% 10% 2	3893
	11th to 20th	Day	26% 11% 26% 24% 12% 1	3885
	21st to end	Day	33% 16% 27% 15% 9% 1	4311
September	1st to 10th	Day	37% 11% 22% 16% 11% 3	2285
	11th to 20th	Day	37% 11% 16% 18% 14% 4	2200
	21st to end	Day	29% 7% 14% 21% 20% 8%	2200
October	1st to 10th	Day	36% 6% 7% 14% 17% 21%	3590
	11th to 20th	Day	15% 3 5% 20% 23% 33%	3568
	21st to end	Day	19% 4 4 14% 43% 16%	3623
November	1st to 10th	Day	27% 5% 10% 20% 22% 16%	1852
	11th to 20th	Day	28% 4 8% 18% 29% 13%	1890
	21st to end	Day	33% 6% 9% 20% 20% 11%	1890
December	1st to 10th	Day	34% 4 10% 18% 26% 7%	3020
	11th to 20th	Day	31% 4 7% 24% 24% 10%	3460
	21st to end	Day	40% 4 7% 18% 28% 4	4565
January	1st to 10th	Night	36% 5% 6% 14% 27% 12%	6019
	11th to 20th	Night	44% 4 5% 14% 26% 7%	5578
	21st to end	Night	43% 5% 9% 18% 18% 8%	6116
February	1st to 10th	Night	40% 4 7% 17% 21% 10%	2401
	11th to 20th	Night	41% 4 8% 14% 20% 13%	2339
	21st to end	Night	39% 5% 8% 17% 18% 12%	1980
March	1st to 10th	Night	47% 4 8% 16% 19% 7%	5373
	11th to 20th	Night	29% 6% 11% 17% 19% 17%	5075
	21st to end	Night	36% 4 10% 22% 15% 11%	5450
April	1st to 10th	Night	34% 6% 13% 16% 18% 12%	2000
	11th to 20th	Night	33% 5% 7% 21% 23% 11%	1975
	21st to end	Night	23% 5% 12% 27% 26% 7%	2000
May	1st to 10th	Night	33% 7% 21% 22% 12% 4	4296
	11th to 20th	Night	32% 7% 18% 27% 15% 1	4033
	21st to end	Night	23% 7% 19% 35% 14% 2	4429
June	1st to 10th	Night	33% 10% 23% 25% 8% 1	1800
	11th to 20th	Night	26% 12% 26% 25% 9% 1	1800
	21st to end	Night	30% 13% 28% 19% 9% 1	1800
July	1st to 10th	Night	35% 16% 36% 12% 1	4015
	11th to 20th	Night	21% 16% 50% 12% 1	4006
	21st to end	Night	30% 18% 33% 18% 1	4732
August	1st to 10th	Night	27% 10% 26% 25% 11% 1	3232
	11th to 20th	Night	28% 11% 27% 27% 7%	3230
	21st to end	Night	35% 14% 24% 16% 9% 1	3586
September	1st to 10th	Night	38% 12% 24% 15% 10% 2	2094
	11th to 20th	Night	37% 10% 15% 20% 14% 4	2200
	21st to end	Night	30% 7% 14% 22% 20% 6%	2200
October	1st to 10th	Night	35% 6% 8% 13% 19% 18%	3590
	11th to 20th	Night	15% 2 4 18% 30% 30%	3571
	21st to end	Night	20% 4 4 15% 41% 16%	4149
November	1st to 10th	Night	27% 4 8% 20% 23% 18%	2666
	11th to 20th	Night	31% 4 6% 19% 27% 13%	2730
	21st to end	Night	33% 5% 10% 18% 21% 12%	2730
December	1st to 10th	Night	35% 4 8% 18% 22% 13%	4380
	11th to 20th	Night	32% 4 8% 22% 23% 11%	4954
	21st to end	Night	41% 4 6% 19% 26% 4	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	14% 12% 33% 31% 9%	330
	11th to 20th	Day	11% 9% 33% 34% 11% 2	330
	21st to end	Day	20% 14% 33% 26% 5%	363
February	1st to 10th	Day	16% 8% 34% 29% 12%	300
	11th to 20th	Day	19% 10% 32% 32% 6%	300
	21st to end	Day	10% 12% 41% 31% 7%	249
March	1st to 10th	Day	12% 10% 39% 32% 6% 1	300
	11th to 20th	Day	11% 8% 35% 31% 12% 2	455
	21st to end	Day	11% 6% 32% 34% 15% 2	550
April	1st to 10th	Day	12% 10% 25% 36% 14% 2	500
	11th to 20th	Day	9% 9% 25% 33% 21% 2	492
	21st to end	Day	7% 2 22% 37% 28% 4	500
May	1st to 10th	Day	7% 4 20% 35% 28% 6%	503
	11th to 20th	Day	8% 3 13% 36% 28% 12%	527
	21st to end	Day	8% 2 8% 30% 36% 16%	573
June	1st to 10th	Day	10% 1 8% 27% 40% 14%	530
	11th to 20th	Day	5% 1 8% 21% 42% 22%	530
	21st to end	Day	8% 1 6% 22% 36% 28%	530
July	1st to 10th	Day	6% 1 5% 13% 38% 38%	525
	11th to 20th	Day	6% 1 2 10% 38% 43%	524
	21st to end	Day	5% 4 13% 43% 36%	576
August	1st to 10th	Day	7% 1 4 13% 38% 37%	526
	11th to 20th	Day	9% 2 11% 39% 39%	519
	21st to end	Day	9% 5% 17% 41% 27%	583
September	1st to 10th	Day	11% 2 6% 17% 38% 26%	530
	11th to 20th	Day	14% 1 6% 20% 44% 15%	530
	21st to end	Day	8% 1 7% 25% 41% 17%	530
October	1st to 10th	Day	9% 3 16% 37% 28% 8%	530
	11th to 20th	Day	5% 4 14% 34% 32% 11%	530
	21st to end	Day	9% 6% 16% 35% 28% 6%	540
November	1st to 10th	Day	13% 3 14% 33% 32% 5%	330
	11th to 20th	Day	11% 4 20% 35% 25% 5%	330
	21st to end	Day	10% 8% 24% 38% 16% 5%	330
December	1st to 10th	Day	20% 6% 26% 31% 14% 3	330
	11th to 20th	Day	17% 8% 24% 36% 14% 1	330
	21st to end	Day	18% 4 25% 32% 16% 4	363
January	1st to 10th	Night	15% 10% 34% 30% 10% 1	840
	11th to 20th	Night	13% 11% 28% 34% 13% 1	840
	21st to end	Night	20% 9% 32% 32% 7%	924
February	1st to 10th	Night	16% 10% 27% 33% 12% 1	801
	11th to 20th	Night	19% 9% 29% 30% 12% 1	783
	21st to end	Night	15% 9% 29% 39% 8%	660
March	1st to 10th	Night	14% 8% 28% 37% 13% 1	800
	11th to 20th	Night	10% 8% 26% 36% 18% 2	800
	21st to end	Night	11% 6% 24% 36% 21% 1	875
April	1st to 10th	Night	12% 6% 24% 33% 23% 2	600
	11th to 20th	Night	10% 4 23% 33% 27% 2	593
	21st to end	Night	7% 2 17% 34% 33% 7%	600
May	1st to 10th	Night	8% 4 14% 35% 33% 7%	600
	11th to 20th	Night	5% 2 12% 34% 38% 9%	591
	21st to end	Night	6% 1 7% 25% 50% 11%	650
June	1st to 10th	Night	8% 1 5% 25% 48% 14%	600
	11th to 20th	Night	5% 2 5% 17% 51% 21%	600
	21st to end	Night	6% 1 6% 16% 48% 24%	600
July	1st to 10th	Night	6% 1 3 9% 47% 34%	585
	11th to 20th	Night	5% 2 7% 41% 46%	582
	21st to end	Night	4 1 2 10% 51% 33%	657
August	1st to 10th	Night	5% 4 8% 43% 40%	597
	11th to 20th	Night	7% 1 4 7% 47% 34%	594
	21st to end	Night	7% 2 7% 15% 43% 25%	660
September	1st to 10th	Night	9% 2 6% 20% 43% 22%	694
	11th to 20th	Night	10% 2 9% 23% 44% 12%	800
	21st to end	Night	8% 2 11% 25% 42% 12%	800
October	1st to 10th	Night	8% 3 16% 37% 32% 5%	800
	11th to 20th	Night	7% 3 15% 33% 34% 8%	800
	21st to end	Night	10% 4 17% 36% 29% 4	880
November	1st to 10th	Night	8% 5% 18% 38% 27% 3	820
	11th to 20th	Night	10% 7% 24% 35% 23% 2	840
	21st to end	Night	12% 7% 25% 32% 21% 3	840
December	1st to 10th	Night	16% 7% 27% 30% 18% 3	840
	11th to 20th	Night	13% 6% 31% 31% 18% 1	840
	21st to end	Night	19% 9% 25% 32% 14% 2	924

Page generated at 2026-01-29 00:21:44. Version ac07

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