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 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	9% 5% 2 5% 5% 3 5% 5% 7% 9% 45%	74802
	Night	9% 5% 2 5% 5% 3 4 5% 6% 9% 46%	74962
Transparency	Day	34% 9% 15% 19% 16% 6%	74535
	Night	34% 7% 14% 19% 18% 8%	74748
Seeing	Day	10% 4 17% 27% 28% 14%	13610
	Night	11% 5% 18% 28% 29% 10%	23634

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	12% 6% 3 5% 4 2 6% 5% 6% 8% 43%	1706
	11th to 20th	Day	11% 5% 2 5% 5% 2 4 5% 5% 8% 49%	1820
	21st to end	Day	16% 7% 3 5% 5% 3 4 4 5% 5% 43%	1980
February	1st to 10th	Day	14% 7% 3 5% 5% 3 4 3 5% 6% 45%	1800
	11th to 20th	Day	14% 5% 3 5% 3 2 4 3 5% 7% 48%	1748
	21st to end	Day	14% 6% 4 6% 5% 4 4 5% 6% 6% 40%	1480
March	1st to 10th	Day	8% 5% 2 4 4 3 4 5% 6% 8% 52%	1800
	11th to 20th	Day	11% 4 3 6% 4 3 5% 5% 5% 7% 48%	1939
	21st to end	Day	9% 4 2 4 5% 3 5% 5% 5% 7% 52%	2195
April	1st to 10th	Day	11% 4 2 5% 4 3 4 6% 6% 7% 49%	2160
	11th to 20th	Day	10% 5% 2 5% 5% 3 4 5% 7% 7% 49%	2134
	21st to end	Day	6% 2 2 4 3 2 4 4 5% 8% 60%	2160
May	1st to 10th	Day	5% 3 2 3 3 2 3 4 6% 8% 60%	2240
	11th to 20th	Day	5% 4 2 5% 4 3 6% 6% 6% 11% 48%	2301
	21st to end	Day	5% 4 1 5% 4 3 5% 6% 7% 11% 48%	2531
June	1st to 10th	Day	8% 5% 3 5% 5% 4 6% 6% 9% 11% 39%	2340
	11th to 20th	Day	4 4 2 4 5% 3 7% 7% 10% 15% 40%	2340
	21st to end	Day	7% 5% 3 6% 6% 4 7% 7% 9% 12% 35%	2340
July	1st to 10th	Day	8% 5% 2 6% 6% 5% 7% 8% 10% 13% 30%	2340
	11th to 20th	Day	7% 4 3 5% 7% 4 8% 9% 10% 14% 29%	2340
	21st to end	Day	5% 3 3 6% 6% 4 7% 8% 12% 15% 32%	2371
August	1st to 10th	Day	7% 4 2 5% 6% 4 7% 9% 12% 15% 29%	2148
	11th to 20th	Day	9% 4 2 6% 6% 4 7% 8% 10% 14% 30%	2125
	21st to end	Day	10% 7% 2 5% 6% 3 5% 6% 8% 10% 36%	2376
September	1st to 10th	Day	8% 5% 3 4 6% 4 6% 8% 9% 11% 37%	2057
	11th to 20th	Day	10% 5% 3 5% 6% 3 6% 6% 8% 10% 38%	1980
	21st to end	Day	7% 5% 2 4 5% 4 5% 5% 6% 9% 50%	1980
October	1st to 10th	Day	7% 4 2 5% 5% 3 4 5% 6% 7% 53%	1980
	11th to 20th	Day	7% 2 1 3 4 2 3 3 5% 6% 62%	1980
	21st to end	Day	8% 4 2 4 5% 3 4 4 5% 5% 56%	2032
November	1st to 10th	Day	8% 5% 2 3 3 2 4 4 5% 6% 58%	1672
	11th to 20th	Day	9% 4 2 5% 4 3 3 3 5% 7% 57%	1710
	21st to end	Day	10% 5% 2 4 5% 3 4 4 5% 6% 51%	1710
December	1st to 10th	Day	12% 6% 2 4 4 3 4 4 5% 8% 48%	1710
	11th to 20th	Day	14% 8% 3 5% 5% 3 4 4 5% 6% 44%	2153
	21st to end	Day	17% 9% 3 6% 4 3 3 3 4 6% 42%	3124
January	1st to 10th	Night	13% 6% 2 6% 6% 3 4 4 5% 6% 45%	2459
	11th to 20th	Night	11% 6% 3 5% 4 2 4 4 6% 8% 48%	2213
	21st to end	Night	17% 7% 4 5% 5% 3 4 4 5% 6% 41%	2376
February	1st to 10th	Night	14% 7% 3 5% 5% 2 4 3 4 6% 46%	2160
	11th to 20th	Night	15% 6% 3 4 4 2 3 4 4 7% 47%	2098
	21st to end	Night	15% 6% 4 6% 4 3 4 5% 5% 6% 43%	1776
March	1st to 10th	Night	11% 5% 2 4 4 3 4 5% 5% 9% 49%	2160
	11th to 20th	Night	9% 5% 3 5% 5% 3 5% 5% 6% 8% 47%	2019
	21st to end	Night	8% 4 2 4 4 3 4 4 6% 9% 52%	2174
April	1st to 10th	Night	9% 6% 2 5% 5% 3 5% 5% 5% 8% 47%	1800
	11th to 20th	Night	10% 5% 3 5% 5% 3 4 5% 6% 7% 47%	1775
	21st to end	Night	7% 3 1 3 3 3 3 4 5% 9% 60%	1800
May	1st to 10th	Night	6% 3 2 3 3 2 4 4 6% 10% 56%	1716
	11th to 20th	Night	4 4 2 4 5% 4 5% 5% 7% 9% 51%	1592
	21st to end	Night	5% 4 1 4 4 3 4 5% 7% 12% 49%	1744
June	1st to 10th	Night	6% 5% 3 6% 5% 5% 5% 5% 7% 13% 39%	1620
	11th to 20th	Night	4 4 2 3 5% 3 5% 6% 9% 14% 45%	1620
	21st to end	Night	6% 4 2 4 5% 3 6% 6% 9% 13% 42%	1620
July	1st to 10th	Night	6% 4 2 6% 5% 4 6% 8% 7% 11% 39%	1620
	11th to 20th	Night	4 4 2 4 5% 3 6% 9% 10% 14% 39%	1620
	21st to end	Night	3 3 3 4 6% 4 6% 7% 10% 15% 40%	1941
August	1st to 10th	Night	5% 3 2 4 5% 4 7% 7% 9% 13% 41%	1790
	11th to 20th	Night	7% 5% 3 6% 6% 4 6% 6% 9% 14% 35%	1776
	21st to end	Night	8% 5% 3 7% 7% 4 6% 7% 9% 9% 37%	1980
September	1st to 10th	Night	6% 5% 2 6% 7% 4 6% 9% 8% 13% 34%	1884
	11th to 20th	Night	9% 5% 2 6% 7% 4 5% 7% 6% 10% 40%	1980
	21st to end	Night	7% 5% 2 4 5% 3 4 5% 6% 8% 50%	1980
October	1st to 10th	Night	7% 5% 2 4 4 3 4 6% 6% 8% 51%	1980
	11th to 20th	Night	6% 4 2 4 4 3 2 4 4 6% 62%	1980
	21st to end	Night	9% 4 2 4 5% 3 5% 4 5% 6% 54%	2319
November	1st to 10th	Night	7% 4 1 3 4 3 4 4 4 8% 58%	2406
	11th to 20th	Night	10% 5% 2 4 4 2 4 4 4 7% 54%	2470
	21st to end	Night	12% 5% 2 4 5% 3 4 4 5% 8% 49%	2470
December	1st to 10th	Night	12% 6% 3 4 4 2 4 4 5% 8% 46%	2470
	11th to 20th	Night	14% 7% 2 5% 4 3 4 4 5% 7% 45%	3042
	21st to end	Night	17% 9% 3 6% 5% 3 3 4 4 5% 41%	4532

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	37% 7% 9% 16% 22% 9%	1707
	11th to 20th	Day	34% 5% 11% 17% 23% 10%	1820
	21st to end	Day	44% 5% 7% 15% 20% 10%	1980
February	1st to 10th	Day	41% 5% 9% 16% 20% 9%	1800
	11th to 20th	Day	37% 6% 7% 20% 22% 10%	1758
	21st to end	Day	42% 8% 9% 16% 16% 9%	1480
March	1st to 10th	Day	30% 6% 9% 21% 24% 10%	1800
	11th to 20th	Day	35% 6% 16% 16% 17% 10%	1939
	21st to end	Day	31% 7% 15% 21% 18% 8%	2195
April	1st to 10th	Day	33% 7% 13% 18% 19% 10%	2160
	11th to 20th	Day	33% 6% 10% 21% 21% 9%	2134
	21st to end	Day	23% 6% 13% 26% 23% 8%	2160
May	1st to 10th	Day	22% 6% 15% 25% 23% 9%	2240
	11th to 20th	Day	28% 11% 18% 21% 19% 3	2305
	21st to end	Day	26% 10% 18% 25% 19% 3	2531
June	1st to 10th	Day	35% 11% 19% 27% 7% 1	2340
	11th to 20th	Day	28% 12% 24% 26% 9% 1	2340
	21st to end	Day	37% 14% 23% 18% 8% 1	2340
July	1st to 10th	Day	40% 17% 26% 15% 2	2275
	11th to 20th	Day	40% 16% 28% 13% 3	2271
	21st to end	Day	33% 17% 29% 19% 3	2238
August	1st to 10th	Day	36% 16% 26% 19% 4	2136
	11th to 20th	Day	39% 16% 27% 14% 4	2125
	21st to end	Day	39% 14% 22% 17% 7% 2	2376
September	1st to 10th	Day	36% 12% 22% 15% 11% 3	2057
	11th to 20th	Day	38% 12% 17% 16% 12% 5%	1980
	21st to end	Day	30% 8% 15% 19% 21% 8%	1980
October	1st to 10th	Day	30% 8% 11% 20% 20% 11%	1980
	11th to 20th	Day	24% 5% 8% 24% 27% 13%	1980
	21st to end	Day	30% 6% 5% 16% 27% 16%	2032
November	1st to 10th	Day	27% 5% 9% 21% 22% 16%	1672
	11th to 20th	Day	28% 4 8% 18% 28% 13%	1710
	21st to end	Day	33% 6% 8% 21% 20% 11%	1710
December	1st to 10th	Day	35% 6% 13% 18% 20% 7%	1710
	11th to 20th	Day	42% 5% 6% 17% 20% 9%	2150
	21st to end	Day	45% 4 6% 21% 20% 5%	3124
January	1st to 10th	Night	41% 4 8% 13% 24% 10%	2459
	11th to 20th	Night	35% 5% 8% 18% 24% 10%	2213
	21st to end	Night	44% 5% 7% 12% 24% 9%	2376
February	1st to 10th	Night	40% 4 7% 17% 21% 10%	2160
	11th to 20th	Night	39% 4 8% 15% 20% 14%	2099
	21st to end	Night	41% 5% 8% 17% 17% 12%	1776
March	1st to 10th	Night	32% 5% 11% 19% 23% 10%	2160
	11th to 20th	Night	34% 7% 12% 22% 17% 9%	2019
	21st to end	Night	29% 6% 13% 23% 20% 10%	2174
April	1st to 10th	Night	34% 6% 13% 16% 19% 11%	1800
	11th to 20th	Night	34% 6% 8% 22% 22% 8%	1775
	21st to end	Night	23% 5% 11% 26% 28% 8%	1800
May	1st to 10th	Night	24% 5% 14% 22% 25% 10%	1716
	11th to 20th	Night	28% 9% 19% 20% 21% 3	1593
	21st to end	Night	26% 8% 19% 24% 18% 4	1745
June	1st to 10th	Night	34% 9% 23% 26% 8%	1620
	11th to 20th	Night	26% 11% 26% 27% 9% 1	1620
	21st to end	Night	30% 13% 28% 18% 9% 1	1620
July	1st to 10th	Night	35% 14% 28% 20% 4	1575
	11th to 20th	Night	29% 17% 35% 15% 3	1566
	21st to end	Night	28% 12% 34% 22% 3	1833
August	1st to 10th	Night	29% 13% 33% 21% 3	1778
	11th to 20th	Night	36% 14% 30% 16% 3	1776
	21st to end	Night	40% 13% 21% 17% 8% 2	1980
September	1st to 10th	Night	37% 13% 24% 14% 10% 3	1884
	11th to 20th	Night	38% 10% 16% 19% 12% 4	1980
	21st to end	Night	31% 8% 15% 20% 20% 7%	1980
October	1st to 10th	Night	30% 7% 10% 20% 21% 12%	1980
	11th to 20th	Night	25% 4 7% 23% 30% 11%	1980
	21st to end	Night	31% 6% 5% 16% 25% 16%	2319
November	1st to 10th	Night	26% 4 7% 20% 24% 18%	2406
	11th to 20th	Night	31% 4 6% 19% 27% 13%	2470
	21st to end	Night	34% 5% 10% 18% 21% 12%	2470
December	1st to 10th	Night	36% 6% 11% 20% 21% 6%	2470
	11th to 20th	Night	40% 5% 7% 14% 22% 12%	3044
	21st to end	Night	46% 5% 6% 20% 21% 3	4532

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	15% 12% 37% 29% 8%	270
	11th to 20th	Day	9% 10% 30% 35% 13% 2	249
	21st to end	Day	23% 13% 35% 23% 5%	264
February	1st to 10th	Day	19% 9% 36% 29% 8%	240
	11th to 20th	Day	19% 10% 30% 35% 6%	240
	21st to end	Day	11% 10% 42% 31% 6%	198
March	1st to 10th	Day	9% 8% 41% 33% 8% 1	240
	11th to 20th	Day	13% 7% 34% 31% 13% 2	379
	21st to end	Day	10% 6% 32% 34% 16% 2	462
April	1st to 10th	Day	12% 10% 24% 36% 15% 3	420
	11th to 20th	Day	10% 9% 25% 31% 21% 3	412
	21st to end	Day	8% 2 23% 38% 25% 4	420
May	1st to 10th	Day	6% 3 20% 35% 29% 7%	423
	11th to 20th	Day	7% 2 12% 35% 30% 14%	447
	21st to end	Day	9% 1 7% 27% 38% 18%	485
June	1st to 10th	Day	12% 1 7% 25% 40% 15%	450
	11th to 20th	Day	6% 1 9% 20% 41% 23%	450
	21st to end	Day	8% 1 7% 23% 34% 28%	450
July	1st to 10th	Day	7% 5% 13% 38% 37%	445
	11th to 20th	Day	7% 2 12% 37% 41%	444
	21st to end	Day	5% 4 14% 42% 34%	488
August	1st to 10th	Day	8% 1 4 13% 39% 37%	446
	11th to 20th	Day	10% 2 11% 40% 36%	439
	21st to end	Day	10% 6% 17% 40% 26%	495
September	1st to 10th	Day	9% 1 7% 16% 38% 29%	450
	11th to 20th	Day	14% 1 6% 20% 44% 15%	450
	21st to end	Day	9% 1 7% 26% 40% 17%	450
October	1st to 10th	Day	8% 2 17% 37% 28% 8%	450
	11th to 20th	Day	6% 4 14% 31% 34% 12%	450
	21st to end	Day	9% 6% 18% 35% 26% 6%	457
November	1st to 10th	Day	14% 3 14% 34% 30% 4	270
	11th to 20th	Day	11% 5% 23% 37% 21% 3	270
	21st to end	Day	10% 8% 23% 38% 16% 4	270
December	1st to 10th	Day	17% 7% 27% 30% 16% 3	270
	11th to 20th	Day	19% 9% 27% 36% 9%	270
	21st to end	Day	21% 5% 26% 31% 14% 3	297
January	1st to 10th	Night	16% 10% 34% 28% 10% 1	760
	11th to 20th	Night	12% 11% 27% 34% 14% 1	736
	21st to end	Night	20% 9% 31% 32% 7%	792
February	1st to 10th	Night	17% 10% 28% 33% 11% 1	720
	11th to 20th	Night	19% 8% 29% 30% 13% 1	703
	21st to end	Night	16% 8% 28% 40% 8%	592
March	1st to 10th	Night	13% 7% 28% 38% 14% 1	720
	11th to 20th	Night	10% 7% 25% 35% 20% 2	720
	21st to end	Night	11% 5% 25% 36% 23% 2	788
April	1st to 10th	Night	12% 5% 23% 33% 24% 2	540
	11th to 20th	Night	11% 4 23% 32% 28% 2	533
	21st to end	Night	7% 3 18% 34% 31% 7%	540
May	1st to 10th	Night	8% 3 14% 35% 33% 8%	540
	11th to 20th	Night	4 2 11% 33% 39% 11%	531
	21st to end	Night	7% 1 6% 23% 51% 12%	584
June	1st to 10th	Night	8% 1 6% 22% 49% 15%	540
	11th to 20th	Night	5% 2 4 17% 52% 20%	540
	21st to end	Night	6% 1 6% 16% 47% 24%	540
July	1st to 10th	Night	7% 1 3 10% 46% 33%	525
	11th to 20th	Night	5% 2 7% 40% 45%	522
	21st to end	Night	4 2 11% 52% 31%	591
August	1st to 10th	Night	6% 3 7% 43% 42%	537
	11th to 20th	Night	8% 1 4 7% 48% 33%	534
	21st to end	Night	7% 2 7% 14% 43% 26%	594
September	1st to 10th	Night	7% 1 6% 18% 44% 24%	624
	11th to 20th	Night	10% 1 9% 22% 44% 13%	720
	21st to end	Night	8% 1 11% 26% 41% 12%	720
October	1st to 10th	Night	8% 2 16% 36% 33% 6%	720
	11th to 20th	Night	8% 3 14% 32% 35% 8%	720
	21st to end	Night	11% 4 18% 34% 29% 4	792
November	1st to 10th	Night	9% 4 20% 38% 26% 3	740
	11th to 20th	Night	10% 7% 25% 34% 21% 2	760
	21st to end	Night	13% 6% 25% 33% 21% 3	760
December	1st to 10th	Night	15% 7% 25% 31% 19% 3	760
	11th to 20th	Night	14% 7% 32% 30% 17% 1	760
	21st to end	Night	20% 9% 26% 31% 14% 1	836

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