Blue Mountain Vista Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Mountain Vista Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	17% 7% 2 7% 4 5% 5% 4 6% 7% 37%	131221
	Night	17% 7% 2 6% 4 4 4 3 5% 7% 41%	129925
Transparency	Day	43% 6% 12% 22% 14% 2	134766
	Night	42% 5% 12% 23% 16% 3	134466
Seeing	Day	23% 16% 28% 20% 10% 3	16494
	Night	21% 13% 28% 27% 10% 1	25915

By Month

Percentage Cloud Cover NE (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% 8% 3 8% 5% 4 5% 4 5% 6% 40%	4661
	11th to 20th	Day	12% 10% 5% 13% 6% 5% 5% 3 4 5% 33%	4847
	21st to end	Day	15% 8% 3 9% 5% 5% 5% 3 4 6% 37%	5780
February	1st to 10th	Day	15% 8% 3 9% 6% 5% 4 4 5% 6% 34%	1890
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 5% 6% 35%	1942
	21st to end	Day	18% 11% 3 8% 4 4 4 4 4 5% 35%	1730
March	1st to 10th	Day	21% 7% 2 5% 4 4 4 3 4 6% 39%	4606
	11th to 20th	Day	12% 7% 3 7% 5% 4 5% 3 6% 6% 42%	4980
	21st to end	Day	18% 7% 2 5% 3 4 4 4 5% 6% 42%	5728
April	1st to 10th	Day	17% 9% 2 7% 5% 5% 5% 4 5% 5% 35%	2400
	11th to 20th	Day	17% 8% 2 6% 5% 4 4 4 5% 5% 39%	2374
	21st to end	Day	17% 6% 2 6% 4 4 5% 4 6% 7% 39%	2400
May	1st to 10th	Day	28% 7% 2 6% 4 4 4 4 6% 7% 29%	5947
	11th to 20th	Day	25% 9% 2 7% 4 5% 4 3 5% 7% 29%	6318
	21st to end	Day	20% 5% 2 5% 4 4 4 4 7% 9% 34%	6949
June	1st to 10th	Day	13% 6% 2 7% 5% 6% 5% 4 7% 9% 36%	2800
	11th to 20th	Day	17% 6% 2 5% 5% 6% 5% 4 6% 10% 35%	2800
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 40%	2800
July	1st to 10th	Day	8% 6% 2 8% 5% 5% 6% 5% 7% 9% 38%	6360
	11th to 20th	Day	12% 7% 3 8% 5% 6% 6% 5% 7% 11% 31%	6360
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 39%	6751
August	1st to 10th	Day	22% 5% 2 6% 5% 5% 5% 4 7% 8% 31%	4234
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 7% 8% 39%	3949
	21st to end	Day	16% 5% 2 4 2 4 4 4 6% 8% 45%	4312
September	1st to 10th	Day	16% 5% 2 7% 4 4 4 3 5% 6% 44%	2400
	11th to 20th	Day	13% 5% 1 4 3 4 3 4 6% 6% 51%	2400
	21st to end	Day	22% 8% 2 5% 4 4 4 4 4 6% 36%	2144
October	1st to 10th	Day	19% 7% 2 4 3 3 4 3 6% 7% 42%	2000
	11th to 20th	Day	18% 5% 2 6% 4 4 4 3 5% 7% 43%	2000
	21st to end	Day	20% 6% 2 5% 4 4 4 4 5% 7% 40%	2155
November	1st to 10th	Day	14% 7% 2 4 3 2 3 2 4 7% 51%	1851
	11th to 20th	Day	16% 8% 2 5% 4 4 4 4 7% 6% 41%	1879
	21st to end	Day	18% 7% 3 5% 4 4 4 4 6% 6% 39%	1680
December	1st to 10th	Day	17% 9% 3 6% 5% 4 5% 4 5% 6% 37%	2680
	11th to 20th	Day	23% 10% 2 6% 3 3 3 3 5% 6% 37%	3066
	21st to end	Day	19% 8% 2 8% 5% 5% 4 4 6% 7% 32%	4048
January	1st to 10th	Night	12% 8% 3 8% 5% 4 4 3 5% 7% 39%	8234
	11th to 20th	Night	13% 8% 3 8% 5% 5% 5% 4 6% 7% 37%	8023
	21st to end	Night	15% 6% 2 6% 4 4 4 4 5% 6% 43%	8401
February	1st to 10th	Night	16% 8% 3 6% 4 4 4 4 5% 7% 39%	2730
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 4 6% 6% 36%	2563
	21st to end	Night	17% 9% 2 6% 5% 4 4 3 6% 6% 38%	2076
March	1st to 10th	Night	18% 10% 3 6% 4 3 4 3 4 5% 40%	5262
	11th to 20th	Night	12% 6% 3 6% 3 3 3 3 4 6% 50%	4979
	21st to end	Night	16% 9% 2 6% 4 4 4 3 5% 6% 42%	5202
April	1st to 10th	Night	19% 8% 2 6% 5% 5% 4 4 5% 6% 36%	2000
	11th to 20th	Night	15% 7% 2 6% 5% 4 4 4 5% 7% 39%	1975
	21st to end	Night	19% 6% 2 5% 3 3 4 3 5% 6% 44%	2000
May	1st to 10th	Night	30% 8% 2 5% 4 4 4 4 5% 7% 29%	4021
	11th to 20th	Night	24% 10% 2 6% 4 4 4 3 4 6% 33%	3575
	21st to end	Night	23% 6% 2 6% 4 3 4 3 5% 7% 37%	3926
June	1st to 10th	Night	17% 5% 2 6% 4 4 4 3 5% 7% 42%	1600
	11th to 20th	Night	15% 6% 2 5% 4 4 5% 4 6% 8% 41%	1600
	21st to end	Night	12% 5% 2 5% 3 4 5% 4 6% 10% 44%	1600
July	1st to 10th	Night	11% 6% 3 7% 5% 5% 6% 4 5% 10% 39%	3600
	11th to 20th	Night	16% 6% 3 8% 5% 5% 5% 5% 6% 9% 33%	3600
	21st to end	Night	13% 6% 2 5% 4 3 5% 4 5% 7% 47%	4097
August	1st to 10th	Night	22% 6% 2 7% 4 4 4 3 6% 7% 35%	2915
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 4 6% 42%	3067
	21st to end	Night	14% 4 2 4 3 3 4 3 6% 7% 49%	3586
September	1st to 10th	Night	16% 6% 2 6% 3 3 3 2 4 6% 48%	2000
	11th to 20th	Night	12% 6% 2 6% 4 3 4 3 4 7% 51%	2000
	21st to end	Night	20% 7% 3 5% 3 4 4 3 4 6% 41%	2251
October	1st to 10th	Night	22% 5% 1 4 3 3 3 3 5% 6% 44%	2400
	11th to 20th	Night	17% 6% 2 4 3 3 3 3 4 7% 47%	2400
	21st to end	Night	20% 5% 2 4 4 3 4 3 4 7% 45%	2682
November	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 5% 6% 50%	2667
	11th to 20th	Night	16% 8% 2 6% 3 3 3 4 5% 7% 42%	2735
	21st to end	Night	15% 7% 2 6% 4 5% 4 3 5% 8% 40%	2940
December	1st to 10th	Night	13% 8% 3 8% 4 4 4 3 5% 6% 41%	4720
	11th to 20th	Night	23% 8% 2 5% 3 4 3 3 5% 5% 38%	5348
	21st to end	Night	19% 10% 3 7% 4 4 4 3 5% 6% 36%	7150

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	42% 4 12% 25% 16% 1	4662
	11th to 20th	Day	51% 4 10% 18% 13% 5%	4878
	21st to end	Day	48% 4 12% 21% 13%	5780
February	1st to 10th	Day	47% 6% 13% 20% 12% 1	1890
	11th to 20th	Day	48% 5% 11% 23% 12% 2	1951
	21st to end	Day	50% 4 10% 23% 10% 2	1730
March	1st to 10th	Day	45% 4 11% 24% 11% 6%	4609
	11th to 20th	Day	41% 4 14% 27% 10% 5%	4980
	21st to end	Day	43% 5% 11% 15% 21% 5%	5728
April	1st to 10th	Day	48% 5% 9% 20% 14% 5%	2400
	11th to 20th	Day	45% 5% 11% 21% 14% 4	2374
	21st to end	Day	42% 6% 10% 22% 15% 5%	2400
May	1st to 10th	Day	53% 7% 13% 17% 10% 2	5956
	11th to 20th	Day	54% 6% 9% 20% 11% 1	6322
	21st to end	Day	44% 6% 12% 23% 13% 2	6949
June	1st to 10th	Day	41% 8% 13% 22% 14% 2	2800
	11th to 20th	Day	44% 8% 14% 23% 11% 2	2800
	21st to end	Day	38% 8% 14% 25% 14% 1	2800
July	1st to 10th	Day	39% 10% 12% 23% 14% 1	6290
	11th to 20th	Day	44% 9% 16% 21% 10% 1	6285
	21st to end	Day	41% 6% 11% 20% 21% 1	6556
August	1st to 10th	Day	48% 8% 13% 20% 10%	4221
	11th to 20th	Day	41% 7% 12% 28% 11% 1	3988
	21st to end	Day	35% 5% 12% 30% 16% 2	4312
September	1st to 10th	Day	40% 6% 11% 22% 19% 2	2400
	11th to 20th	Day	32% 5% 10% 25% 24% 3	2400
	21st to end	Day	49% 4 9% 17% 19% 2	2144
October	1st to 10th	Day	39% 4 9% 21% 22% 5%	3260
	11th to 20th	Day	31% 4 11% 28% 21% 5%	3240
	21st to end	Day	28% 4 15% 25% 25% 3	3457
November	1st to 10th	Day	35% 3 14% 25% 19% 3	1851
	11th to 20th	Day	41% 4 11% 24% 17% 3	1879
	21st to end	Day	44% 5% 13% 28% 9% 1	1680
December	1st to 10th	Day	46% 5% 15% 24% 9% 1	2680
	11th to 20th	Day	48% 4 15% 23% 9% 1	3066
	21st to end	Day	48% 6% 14% 19% 12% 1	4048
January	1st to 10th	Night	43% 5% 15% 22% 14% 2	8234
	11th to 20th	Night	43% 5% 13% 19% 15% 5%	8067
	21st to end	Night	40% 4 10% 24% 20% 2	8401
February	1st to 10th	Night	44% 4 13% 25% 11% 2	2730
	11th to 20th	Night	47% 4 11% 20% 15% 3	2565
	21st to end	Night	45% 3 10% 22% 16% 3	2076
March	1st to 10th	Night	45% 5% 9% 22% 12% 6%	5272
	11th to 20th	Night	35% 4 17% 26% 13% 5%	4980
	21st to end	Night	44% 4 8% 17% 23% 5%	5202
April	1st to 10th	Night	47% 5% 10% 21% 14% 3	2000
	11th to 20th	Night	43% 6% 11% 19% 20% 1	1975
	21st to end	Night	40% 6% 10% 22% 18% 4	2000
May	1st to 10th	Night	55% 6% 10% 18% 10% 2	4028
	11th to 20th	Night	52% 8% 11% 16% 12% 1	3576
	21st to end	Night	47% 6% 10% 23% 12% 2	3927
June	1st to 10th	Night	41% 6% 10% 24% 19% 1	1600
	11th to 20th	Night	39% 8% 14% 25% 13% 1	1600
	21st to end	Night	34% 9% 14% 28% 14% 1	1600
July	1st to 10th	Night	42% 7% 14% 19% 16% 3	3560
	11th to 20th	Night	47% 7% 15% 20% 11% 1	3552
	21st to end	Night	35% 6% 13% 23% 21% 1	3977
August	1st to 10th	Night	48% 7% 16% 19% 10% 1	2904
	11th to 20th	Night	43% 6% 11% 24% 16% 1	3094
	21st to end	Night	34% 5% 11% 29% 18% 1	3586
September	1st to 10th	Night	38% 5% 9% 25% 20% 3	2000
	11th to 20th	Night	34% 5% 8% 26% 24% 3	2000
	21st to end	Night	44% 4 10% 20% 20% 2	2251
October	1st to 10th	Night	41% 4 10% 22% 21% 2	3920
	11th to 20th	Night	29% 4 9% 30% 23% 5%	3899
	21st to end	Night	28% 3 13% 31% 23% 2	4330
November	1st to 10th	Night	34% 5% 13% 24% 20% 4	2667
	11th to 20th	Night	42% 4 10% 24% 17% 4	2735
	21st to end	Night	41% 5% 12% 27% 13% 2	2940
December	1st to 10th	Night	43% 4 13% 29% 9% 1	4720
	11th to 20th	Night	47% 3 13% 23% 12% 2	5348
	21st to end	Night	49% 4 11% 20% 15% 1	7150

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	31% 30% 26% 10% 3 1	220
	11th to 20th	Day	39% 26% 25% 8% 2	256
	21st to end	Day	27% 30% 28% 11% 3 1	363
February	1st to 10th	Day	26% 28% 34% 10% 2	330
	11th to 20th	Day	33% 24% 34% 8% 1	429
	21st to end	Day	31% 21% 29% 18% 1	446
March	1st to 10th	Day	30% 21% 29% 17% 3	508
	11th to 20th	Day	26% 26% 31% 11% 4 1	500
	21st to end	Day	30% 24% 28% 13% 5% 1	550
April	1st to 10th	Day	29% 23% 27% 16% 4 1	500
	11th to 20th	Day	25% 25% 28% 13% 7% 2	492
	21st to end	Day	27% 19% 30% 15% 6% 2	500
May	1st to 10th	Day	31% 19% 26% 15% 7% 3	503
	11th to 20th	Day	25% 17% 31% 19% 7% 2	527
	21st to end	Day	22% 10% 29% 21% 15% 3	573
June	1st to 10th	Day	16% 16% 29% 23% 13% 3	530
	11th to 20th	Day	22% 12% 28% 22% 12% 2	530
	21st to end	Day	17% 10% 32% 26% 12% 3	530
July	1st to 10th	Day	14% 7% 25% 30% 19% 6%	525
	11th to 20th	Day	10% 8% 25% 31% 21% 4	524
	21st to end	Day	14% 8% 31% 31% 13% 4	576
August	1st to 10th	Day	13% 7% 25% 29% 17% 9%	526
	11th to 20th	Day	15% 5% 21% 35% 17% 7%	519
	21st to end	Day	15% 7% 26% 28% 14% 9%	583
September	1st to 10th	Day	18% 11% 20% 25% 20% 6%	530
	11th to 20th	Day	13% 5% 24% 31% 18% 9%	530
	21st to end	Day	28% 11% 23% 20% 13% 4	530
October	1st to 10th	Day	23% 10% 23% 22% 15% 6%	530
	11th to 20th	Day	22% 16% 28% 21% 12% 2	530
	21st to end	Day	23% 16% 31% 19% 10% 1	558
November	1st to 10th	Day	19% 18% 27% 24% 9% 3	425
	11th to 20th	Day	24% 21% 29% 19% 6%	419
	21st to end	Day	30% 26% 30% 10% 4	220
December	1st to 10th	Day	31% 25% 28% 11% 5%	220
	11th to 20th	Day	35% 25% 27% 11% 3	220
	21st to end	Day	29% 23% 31% 14% 2 1	242
January	1st to 10th	Night	20% 20% 38% 19% 3	840
	11th to 20th	Night	27% 18% 29% 22% 3	840
	21st to end	Night	19% 21% 36% 22% 2	924
February	1st to 10th	Night	23% 17% 35% 24% 2	840
	11th to 20th	Night	27% 20% 35% 17% 1	823
	21st to end	Night	26% 20% 33% 20% 3	692
March	1st to 10th	Night	24% 19% 31% 22% 4	809
	11th to 20th	Night	24% 18% 32% 20% 5% 1	800
	21st to end	Night	23% 17% 36% 20% 4	880
April	1st to 10th	Night	25% 16% 34% 20% 5%	800
	11th to 20th	Night	20% 14% 36% 23% 6%	791
	21st to end	Night	24% 12% 31% 25% 8%	800
May	1st to 10th	Night	26% 10% 28% 26% 9% 1	594
	11th to 20th	Night	20% 11% 30% 28% 10% 1	394
	21st to end	Night	20% 8% 19% 39% 12% 2	433
June	1st to 10th	Night	18% 7% 24% 34% 17%	400
	11th to 20th	Night	19% 4 26% 38% 12% 1	400
	21st to end	Night	12% 5% 26% 40% 16% 1	400
July	1st to 10th	Night	12% 4 20% 38% 25% 2	390
	11th to 20th	Night	13% 4 17% 40% 23% 2	388
	21st to end	Night	11% 4 18% 44% 21% 1	508
August	1st to 10th	Night	15% 2 15% 40% 26% 2	597
	11th to 20th	Night	16% 6% 13% 37% 27% 2	708
	21st to end	Night	17% 4 17% 35% 26% 2	880
September	1st to 10th	Night	18% 5% 21% 35% 20% 2	800
	11th to 20th	Night	14% 5% 17% 36% 23% 5%	800
	21st to end	Night	24% 5% 20% 30% 20% 2	800
October	1st to 10th	Night	24% 7% 21% 26% 18% 3	800
	11th to 20th	Night	21% 11% 25% 33% 11% 1	800
	21st to end	Night	23% 12% 28% 28% 9%	880
November	1st to 10th	Night	17% 13% 31% 28% 11% 1	820
	11th to 20th	Night	21% 18% 27% 26% 6% 1	840
	21st to end	Night	20% 18% 30% 27% 6%	840
December	1st to 10th	Night	25% 18% 31% 21% 4 1	840
	11th to 20th	Night	24% 16% 30% 26% 4	840
	21st to end	Night	26% 14% 31% 26% 3	924

Page generated at 2026-02-28 01:40:22. Version ac07

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