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 2024-11-28.

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% 9% 44%	100923
	Night	9% 6% 2 6% 4 4 4 4 6% 9% 45%	98574
Transparency	Day	33% 9% 16% 19% 16% 7%	104863
	Night	34% 7% 15% 18% 18% 8%	102760
Seeing	Day	10% 4 17% 27% 27% 14%	14797
	Night	11% 5% 18% 28% 29% 10%	24748

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	13% 5% 2 5% 3 4 5% 4 6% 8% 45%	2836
	11th to 20th	Day	11% 5% 2 4 3 3 3 4 7% 9% 49%	3137
	21st to end	Day	19% 7% 2 6% 4 4 4 3 6% 7% 37%	3476
February	1st to 10th	Day	14% 8% 2 7% 4 4 4 3 5% 6% 45%	1900
	11th to 20th	Day	15% 6% 2 5% 3 3 3 2 5% 6% 47%	1848
	21st to end	Day	14% 7% 4 7% 5% 4 3 4 6% 6% 41%	1570
March	1st to 10th	Day	16% 9% 3 6% 4 4 4 4 5% 7% 39%	3120
	11th to 20th	Day	9% 5% 2 7% 4 5% 6% 5% 6% 8% 44%	3411
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 7% 47%	3845
April	1st to 10th	Day	11% 5% 2 6% 4 3 4 4 6% 7% 48%	2280
	11th to 20th	Day	9% 5% 2 6% 4 3 4 4 6% 7% 49%	2254
	21st to end	Day	6% 3 1 4 3 3 4 3 5% 8% 59%	2280
May	1st to 10th	Day	6% 4 2 6% 4 4 4 4 6% 9% 50%	3915
	11th to 20th	Day	13% 5% 2 6% 3 4 4 4 5% 8% 45%	4081
	21st to end	Day	4 3 1 4 3 4 5% 5% 7% 12% 52%	4476
June	1st to 10th	Day	8% 5% 2 6% 4 5% 6% 5% 9% 11% 38%	2470
	11th to 20th	Day	4 4 2 5% 5% 5% 6% 5% 10% 15% 39%	2470
	21st to end	Day	6% 5% 2 7% 5% 5% 6% 6% 10% 12% 35%	2470
July	1st to 10th	Day	5% 4 1 6% 5% 7% 8% 7% 12% 15% 30%	4120
	11th to 20th	Day	4 3 2 5% 5% 6% 7% 6% 10% 13% 39%	4120
	21st to end	Day	3 3 2 7% 6% 7% 8% 8% 12% 17% 28%	4153
August	1st to 10th	Day	5% 3 1 5% 4 5% 7% 6% 10% 14% 41%	3785
	11th to 20th	Day	5% 3 1 5% 3 5% 6% 5% 8% 12% 47%	3729
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 6% 9% 12% 39%	4180
September	1st to 10th	Day	9% 6% 3 5% 5% 5% 6% 6% 8% 11% 36%	2171
	11th to 20th	Day	11% 6% 3 6% 5% 4 5% 5% 8% 10% 37%	2090
	21st to end	Day	7% 5% 1 5% 4 4 4 4 6% 9% 51%	2090
October	1st to 10th	Day	8% 4 2 5% 4 4 4 4 6% 7% 53%	2090
	11th to 20th	Day	7% 3 1 3 4 3 3 3 5% 6% 63%	2090
	21st to end	Day	8% 4 2 5% 4 3 4 3 5% 5% 57%	2144
November	1st to 10th	Day	8% 5% 2 4 3 3 4 4 5% 6% 56%	1762
	11th to 20th	Day	9% 4 2 5% 4 3 2 3 5% 7% 56%	1800
	21st to end	Day	10% 5% 2 5% 4 4 4 3 5% 6% 51%	1773
December	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 6% 8% 47%	1710
	11th to 20th	Day	13% 9% 3 6% 4 3 4 3 5% 6% 44%	2153
	21st to end	Day	17% 9% 3 7% 4 3 3 3 4 6% 42%	3124
January	1st to 10th	Night	14% 6% 2 7% 5% 4 5% 3 5% 7% 42%	4109
	11th to 20th	Night	10% 7% 2 6% 5% 4 4 3 6% 7% 46%	3818
	21st to end	Night	18% 9% 3 6% 4 3 4 4 5% 6% 38%	4180
February	1st to 10th	Night	13% 8% 2 6% 5% 3 4 3 4 6% 46%	2280
	11th to 20th	Night	16% 7% 3 5% 4 3 3 3 4 6% 46%	2218
	21st to end	Night	15% 7% 3 6% 3 3 4 4 5% 6% 44%	1884
March	1st to 10th	Night	17% 11% 4 8% 4 4 4 3 4 7% 34%	3733
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 5% 7% 10% 45%	3547
	21st to end	Night	12% 6% 2 6% 4 4 3 4 5% 7% 46%	3800
April	1st to 10th	Night	9% 7% 2 5% 5% 5% 4 4 6% 8% 46%	1900
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 6% 7% 47%	1875
	21st to end	Night	6% 3 1 4 3 4 3 3 5% 8% 59%	1900
May	1st to 10th	Night	6% 5% 2 5% 4 4 5% 4 7% 10% 47%	2976
	11th to 20th	Night	8% 6% 2 7% 5% 5% 4 4 6% 8% 45%	2812
	21st to end	Night	4 3 1 4 3 4 4 3 7% 11% 54%	3079
June	1st to 10th	Night	6% 6% 2 7% 5% 6% 5% 4 8% 14% 39%	1710
	11th to 20th	Night	4 5% 1 5% 4 5% 5% 5% 9% 14% 45%	1710
	21st to end	Night	5% 5% 2 5% 4 5% 5% 5% 9% 13% 42%	1710
July	1st to 10th	Night	4 4 2 6% 5% 6% 7% 6% 11% 16% 32%	2840
	11th to 20th	Night	2 3 1 3 3 3 4 6% 8% 13% 52%	2840
	21st to end	Night	2 3 1 5% 6% 6% 7% 6% 12% 17% 35%	3394
August	1st to 10th	Night	4 3 2 5% 4 4 6% 6% 8% 13% 46%	3144
	11th to 20th	Night	4 4 2 6% 4 4 5% 5% 8% 12% 46%	3100
	21st to end	Night	5% 4 2 7% 5% 5% 7% 7% 9% 10% 38%	3476
September	1st to 10th	Night	8% 5% 2 8% 5% 6% 7% 7% 8% 12% 33%	1989
	11th to 20th	Night	9% 5% 2 7% 5% 5% 5% 6% 6% 10% 39%	2090
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 50%	2090
October	1st to 10th	Night	7% 5% 2 5% 4 4 4 4 6% 7% 51%	2090
	11th to 20th	Night	6% 4 2 4 3 3 2 3 4 5% 64%	2090
	21st to end	Night	8% 4 2 5% 4 4 5% 3 5% 5% 56%	2448
November	1st to 10th	Night	7% 4 1 5% 3 4 4 4 5% 8% 56%	2536
	11th to 20th	Night	10% 5% 2 5% 4 3 3 3 4 7% 53%	2600
	21st to end	Night	11% 5% 2 6% 4 3 3 4 5% 7% 50%	2562
December	1st to 10th	Night	12% 7% 3 5% 4 3 4 4 6% 8% 45%	2470
	11th to 20th	Night	14% 7% 2 6% 3 4 4 4 5% 6% 45%	3042
	21st to end	Night	16% 10% 3 7% 4 3 3 3 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	36% 6% 7% 18% 23% 11%	2837
	11th to 20th	Day	31% 5% 9% 18% 22% 15%	3137
	21st to end	Day	47% 5% 5% 14% 20% 9%	3476
February	1st to 10th	Day	41% 5% 8% 16% 20% 10%	1900
	11th to 20th	Day	38% 5% 6% 19% 21% 11%	1858
	21st to end	Day	41% 7% 9% 17% 17% 9%	1570
March	1st to 10th	Day	44% 7% 9% 20% 15% 6%	3120
	11th to 20th	Day	34% 9% 15% 16% 18% 8%	3411
	21st to end	Day	34% 7% 15% 19% 16% 9%	3845
April	1st to 10th	Day	34% 7% 13% 18% 18% 11%	2280
	11th to 20th	Day	33% 6% 10% 21% 21% 9%	2254
	21st to end	Day	23% 6% 13% 26% 23% 9%	2280
May	1st to 10th	Day	26% 11% 15% 25% 18% 5%	3950
	11th to 20th	Day	36% 9% 13% 24% 17% 2	4085
	21st to end	Day	21% 10% 20% 32% 16% 2	4489
June	1st to 10th	Day	35% 11% 19% 26% 8% 1	2470
	11th to 20th	Day	29% 12% 23% 26% 9% 1	2470
	21st to end	Day	37% 14% 22% 18% 8% 1	2470
July	1st to 10th	Day	34% 18% 36% 10% 1	4055
	11th to 20th	Day	32% 15% 41% 10% 2	4051
	21st to end	Day	37% 20% 29% 13% 1	4020
August	1st to 10th	Day	27% 12% 23% 25% 11% 2	3773
	11th to 20th	Day	27% 11% 25% 25% 12% 1	3765
	21st to end	Day	33% 16% 27% 14% 9% 1	4179
September	1st to 10th	Day	38% 12% 22% 15% 10% 3	2171
	11th to 20th	Day	39% 11% 17% 16% 13% 4	2090
	21st to end	Day	29% 8% 14% 20% 20% 8%	2090
October	1st to 10th	Day	35% 6% 7% 14% 17% 21%	3480
	11th to 20th	Day	15% 3 5% 21% 23% 33%	3458
	21st to end	Day	19% 4 4 14% 43% 16%	3510
November	1st to 10th	Day	28% 5% 10% 20% 22% 15%	1762
	11th to 20th	Day	29% 4 8% 18% 28% 13%	1800
	21st to end	Day	33% 6% 8% 21% 21% 11%	1773
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	42% 5% 6% 11% 23% 14%	4109
	11th to 20th	Night	38% 4 6% 15% 28% 9%	3818
	21st to end	Night	46% 5% 7% 14% 20% 7%	4180
February	1st to 10th	Night	40% 4 7% 17% 21% 10%	2280
	11th to 20th	Night	40% 4 8% 15% 20% 14%	2219
	21st to end	Night	40% 5% 9% 17% 18% 12%	1884
March	1st to 10th	Night	51% 5% 10% 15% 13% 6%	3733
	11th to 20th	Night	31% 7% 14% 20% 19% 9%	3547
	21st to end	Night	37% 5% 11% 20% 17% 10%	3800
April	1st to 10th	Night	35% 6% 13% 16% 19% 11%	1900
	11th to 20th	Night	34% 5% 8% 22% 22% 9%	1875
	21st to end	Night	23% 5% 11% 26% 27% 8%	1900
May	1st to 10th	Night	29% 8% 20% 22% 17% 6%	3006
	11th to 20th	Night	34% 8% 15% 23% 17% 2	2813
	21st to end	Night	21% 8% 18% 35% 16% 3	3087
June	1st to 10th	Night	34% 9% 23% 26% 8% 1	1710
	11th to 20th	Night	26% 11% 25% 26% 10% 2	1710
	21st to end	Night	31% 12% 28% 18% 10% 1	1710
July	1st to 10th	Night	35% 15% 36% 12% 2	2795
	11th to 20th	Night	20% 15% 52% 10% 2	2786
	21st to end	Night	30% 20% 34% 13% 2	3286
August	1st to 10th	Night	26% 10% 26% 25% 11% 1	3132
	11th to 20th	Night	28% 11% 27% 27% 7%	3130
	21st to end	Night	35% 14% 24% 16% 9% 1	3476
September	1st to 10th	Night	39% 12% 24% 13% 10% 3	1989
	11th to 20th	Night	38% 10% 16% 19% 13% 4	2090
	21st to end	Night	31% 8% 14% 21% 20% 7%	2090
October	1st to 10th	Night	35% 6% 8% 13% 19% 19%	3480
	11th to 20th	Night	15% 2 4 18% 30% 29%	3461
	21st to end	Night	20% 4 4 15% 42% 16%	4020
November	1st to 10th	Night	28% 4 8% 20% 23% 17%	2536
	11th to 20th	Night	32% 4 6% 18% 27% 13%	2600
	21st to end	Night	34% 5% 10% 18% 22% 12%	2562
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% 32% 34% 12% 2	270
	21st to end	Day	22% 13% 36% 23% 5%	297
February	1st to 10th	Day	18% 8% 38% 27% 9%	270
	11th to 20th	Day	19% 10% 30% 34% 6%	270
	21st to end	Day	10% 13% 43% 29% 5%	225
March	1st to 10th	Day	11% 9% 41% 31% 7% 1	270
	11th to 20th	Day	12% 7% 35% 32% 12% 2	417
	21st to end	Day	11% 6% 33% 33% 15% 2	506
April	1st to 10th	Day	12% 11% 26% 35% 14% 2	460
	11th to 20th	Day	10% 9% 26% 32% 21% 3	452
	21st to end	Day	7% 3 24% 38% 25% 4	460
May	1st to 10th	Day	6% 3 20% 35% 29% 7%	463
	11th to 20th	Day	8% 3 13% 34% 29% 13%	487
	21st to end	Day	8% 2 7% 30% 36% 16%	529
June	1st to 10th	Day	11% 1 8% 26% 39% 14%	490
	11th to 20th	Day	6% 1 9% 21% 41% 22%	490
	21st to end	Day	8% 1 6% 23% 35% 27%	490
July	1st to 10th	Day	6% 5% 13% 37% 38%	485
	11th to 20th	Day	7% 2 11% 37% 43%	484
	21st to end	Day	5% 4 13% 42% 36%	532
August	1st to 10th	Day	7% 1 4 13% 39% 36%	486
	11th to 20th	Day	9% 2 12% 39% 37%	479
	21st to end	Day	9% 6% 17% 41% 27%	539
September	1st to 10th	Day	12% 2 7% 16% 37% 27%	490
	11th to 20th	Day	15% 1 6% 21% 42% 14%	490
	21st to end	Day	9% 1 8% 26% 40% 17%	490
October	1st to 10th	Day	9% 2 17% 37% 27% 8%	490
	11th to 20th	Day	6% 3 15% 33% 31% 11%	490
	21st to end	Day	9% 6% 16% 36% 27% 7%	498
November	1st to 10th	Day	14% 3 14% 33% 31% 5%	300
	11th to 20th	Day	12% 5% 22% 36% 21% 4	300
	21st to end	Day	10% 8% 24% 37% 17% 5%	291
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% 28% 34% 14% 1	760
	21st to end	Night	20% 10% 32% 31% 7%	836
February	1st to 10th	Night	17% 10% 28% 33% 11% 1	760
	11th to 20th	Night	19% 8% 29% 31% 12% 1	743
	21st to end	Night	15% 9% 29% 39% 8%	628
March	1st to 10th	Night	14% 7% 28% 37% 13% 1	760
	11th to 20th	Night	10% 8% 26% 36% 19% 2	760
	21st to end	Night	11% 6% 25% 35% 22% 1	831
April	1st to 10th	Night	12% 6% 24% 33% 23% 2	570
	11th to 20th	Night	11% 4 23% 33% 27% 2	563
	21st to end	Night	7% 2 18% 35% 32% 6%	570
May	1st to 10th	Night	7% 3 14% 35% 33% 8%	570
	11th to 20th	Night	4 2 12% 34% 37% 10%	561
	21st to end	Night	6% 7% 25% 50% 12%	617
June	1st to 10th	Night	8% 1 5% 23% 49% 14%	570
	11th to 20th	Night	5% 2 5% 18% 51% 19%	570
	21st to end	Night	6% 1 6% 16% 48% 24%	570
July	1st to 10th	Night	6% 1 3 10% 46% 34%	555
	11th to 20th	Night	5% 2 7% 39% 46%	552
	21st to end	Night	4 1 2 11% 50% 33%	624
August	1st to 10th	Night	5% 4 8% 42% 41%	567
	11th to 20th	Night	7% 1 4 7% 47% 34%	564
	21st to end	Night	7% 2 7% 15% 43% 26%	627
September	1st to 10th	Night	9% 2 5% 18% 42% 23%	659
	11th to 20th	Night	11% 2 9% 23% 43% 12%	760
	21st to end	Night	9% 2 11% 26% 41% 12%	760
October	1st to 10th	Night	8% 2 16% 36% 32% 6%	760
	11th to 20th	Night	7% 3 15% 33% 33% 8%	760
	21st to end	Night	10% 4 17% 36% 30% 4	836
November	1st to 10th	Night	9% 4 19% 38% 27% 3	780
	11th to 20th	Night	11% 7% 25% 34% 22% 2	800
	21st to end	Night	12% 7% 25% 32% 21% 3	789
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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