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 2024-01-15.

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	16% 7% 3 6% 5% 3 5% 5% 6% 7% 37%	74736
	Night	17% 6% 2 5% 5% 3 4 4 5% 7% 41%	75031
Transparency	Day	44% 6% 12% 22% 14% 2	74445
	Night	42% 6% 12% 23% 15% 2	74856
Seeing	Day	22% 16% 28% 20% 10% 3	13754
	Night	21% 12% 28% 28% 11% 1	23175

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% 4 7% 6% 3 5% 6% 6% 6% 37%	1517
	11th to 20th	Day	17% 9% 4 8% 7% 4 6% 4 4 5% 31%	1530
	21st to end	Day	14% 9% 3 7% 6% 4 5% 5% 5% 7% 34%	1782
February	1st to 10th	Day	18% 7% 3 8% 7% 4 5% 5% 5% 6% 32%	1620
	11th to 20th	Day	15% 8% 3 8% 5% 4 6% 4 5% 7% 36%	1658
	21st to end	Day	20% 10% 4 8% 5% 3 4 5% 4 5% 34%	1480
March	1st to 10th	Day	14% 8% 3 5% 6% 3 5% 5% 4 5% 42%	1837
	11th to 20th	Day	15% 8% 3 7% 7% 3 5% 4 6% 6% 36%	1980
	21st to end	Day	22% 7% 3 6% 4 3 5% 4 4 5% 37%	2274
April	1st to 10th	Day	16% 8% 3 6% 5% 4 5% 5% 6% 5% 37%	2160
	11th to 20th	Day	18% 6% 2 6% 5% 3 4 5% 5% 6% 40%	2134
	21st to end	Day	18% 5% 2 6% 5% 4 5% 5% 5% 8% 38%	2160
May	1st to 10th	Day	20% 7% 3 5% 5% 3 6% 4 6% 6% 34%	2357
	11th to 20th	Day	20% 7% 2 7% 6% 4 5% 4 5% 7% 33%	2478
	21st to end	Day	17% 6% 2 5% 5% 2 5% 6% 7% 8% 38%	2725
June	1st to 10th	Day	13% 5% 3 6% 5% 3 6% 5% 7% 9% 36%	2520
	11th to 20th	Day	16% 6% 2 5% 5% 4 6% 5% 6% 10% 34%	2520
	21st to end	Day	12% 5% 2 5% 6% 4 5% 5% 7% 9% 40%	2520
July	1st to 10th	Day	11% 6% 3 6% 6% 4 6% 6% 7% 10% 35%	2520
	11th to 20th	Day	9% 5% 2 5% 5% 4 6% 6% 6% 11% 41%	2520
	21st to end	Day	12% 6% 3 7% 6% 4 6% 6% 6% 8% 37%	2678
August	1st to 10th	Day	13% 5% 3 6% 6% 4 6% 6% 8% 9% 35%	2327
	11th to 20th	Day	12% 7% 3 6% 6% 4 5% 5% 6% 8% 38%	2183
	21st to end	Day	16% 6% 2 4 4 3 5% 6% 7% 8% 38%	2376
September	1st to 10th	Day	18% 5% 2 6% 5% 3 4 4 5% 6% 42%	2160
	11th to 20th	Day	14% 5% 1 3 4 2 4 4 6% 6% 50%	2160
	21st to end	Day	20% 7% 2 5% 4 3 4 5% 5% 6% 39%	1930
October	1st to 10th	Day	19% 7% 2 4 4 3 3 5% 6% 8% 40%	1800
	11th to 20th	Day	19% 6% 2 5% 5% 3 4 4 4 7% 42%	1800
	21st to end	Day	22% 6% 2 5% 5% 3 4 5% 5% 6% 37%	1940
November	1st to 10th	Day	15% 7% 2 3 4 1 3 3 4 7% 50%	1671
	11th to 20th	Day	17% 6% 2 4 5% 2 4 5% 6% 6% 42%	1701
	21st to end	Day	18% 5% 3 4 4 3 4 5% 6% 7% 40%	1520
December	1st to 10th	Day	21% 9% 4 6% 5% 3 5% 4 4 6% 33%	1520
	11th to 20th	Day	22% 8% 3 6% 4 3 4 4 5% 6% 37%	1906
	21st to end	Day	20% 9% 3 8% 7% 4 5% 5% 5% 7% 27%	2772
January	1st to 10th	Night	14% 7% 3 5% 5% 3 4 4 6% 8% 40%	2649
	11th to 20th	Night	18% 8% 3 5% 6% 3 5% 5% 6% 7% 34%	2524
	21st to end	Night	12% 8% 3 6% 6% 3 5% 4 6% 7% 40%	2574
February	1st to 10th	Night	18% 8% 3 6% 5% 2 5% 4 5% 8% 38%	2340
	11th to 20th	Night	16% 7% 3 6% 6% 3 5% 5% 6% 7% 37%	2188
	21st to end	Night	19% 8% 2 6% 5% 3 4 4 5% 6% 39%	1776
March	1st to 10th	Night	15% 6% 2 4 4 3 4 5% 6% 7% 43%	2107
	11th to 20th	Night	18% 8% 3 5% 4 2 3 4 4 5% 45%	1979
	21st to end	Night	17% 8% 2 5% 5% 3 5% 4 5% 7% 39%	2080
April	1st to 10th	Night	18% 7% 3 5% 5% 3 4 5% 5% 7% 38%	1800
	11th to 20th	Night	14% 6% 2 5% 6% 3 5% 5% 5% 8% 41%	1775
	21st to end	Night	20% 5% 2 4 4 3 4 4 5% 5% 43%	1800
May	1st to 10th	Night	20% 8% 3 5% 5% 3 4 4 4 7% 37%	1616
	11th to 20th	Night	19% 6% 2 5% 5% 4 5% 5% 5% 6% 40%	1415
	21st to end	Night	16% 6% 2 5% 6% 3 4 5% 5% 8% 39%	1550
June	1st to 10th	Night	17% 5% 2 5% 5% 3 4 4 5% 7% 43%	1440
	11th to 20th	Night	14% 5% 2 5% 5% 3 6% 5% 6% 9% 41%	1440
	21st to end	Night	12% 4 3 4 3 3 5% 6% 6% 10% 45%	1440
July	1st to 10th	Night	11% 4 3 6% 6% 4 5% 6% 5% 9% 40%	1440
	11th to 20th	Night	11% 4 2 5% 5% 3 4 5% 6% 10% 45%	1440
	21st to end	Night	10% 6% 3 5% 6% 3 5% 5% 6% 10% 42%	1636
August	1st to 10th	Night	13% 5% 2 5% 5% 3 5% 5% 7% 9% 42%	1611
	11th to 20th	Night	13% 6% 3 6% 6% 3 6% 5% 5% 7% 40%	1700
	21st to end	Night	14% 5% 2 4 4 3 5% 5% 7% 7% 43%	1980
September	1st to 10th	Night	17% 5% 2 5% 5% 3 3 3 4 7% 46%	1800
	11th to 20th	Night	12% 5% 2 5% 4 2 4 4 4 7% 51%	1800
	21st to end	Night	18% 6% 2 4 4 3 4 5% 3 6% 45%	2025
October	1st to 10th	Night	23% 5% 2 4 4 3 4 3 5% 6% 43%	2160
	11th to 20th	Night	18% 6% 2 4 4 2 3 4 4 7% 46%	2160
	21st to end	Night	22% 5% 2 4 5% 2 4 4 4 6% 43%	2413
November	1st to 10th	Night	13% 5% 2 4 4 3 4 4 4 6% 50%	2407
	11th to 20th	Night	17% 7% 3 5% 4 2 3 4 5% 7% 43%	2474
	21st to end	Night	16% 6% 2 5% 5% 3 5% 4 5% 9% 41%	2660
December	1st to 10th	Night	20% 6% 3 7% 6% 3 5% 5% 5% 7% 34%	2660
	11th to 20th	Night	19% 8% 2 5% 4 3 5% 5% 6% 6% 38%	3288
	21st to end	Night	19% 9% 4 6% 6% 3 4 4 5% 7% 34%	4884

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	45% 5% 12% 24% 11% 2	1518
	11th to 20th	Day	54% 5% 12% 17% 10% 2	1530
	21st to end	Day	49% 4 11% 22% 12% 1	1782
February	1st to 10th	Day	50% 6% 14% 18% 11% 1	1620
	11th to 20th	Day	46% 5% 11% 24% 12% 2	1667
	21st to end	Day	52% 4 9% 22% 10% 3	1480
March	1st to 10th	Day	43% 5% 12% 26% 13% 2	1840
	11th to 20th	Day	47% 5% 10% 20% 14% 3	1980
	21st to end	Day	49% 4 9% 18% 16% 3	2274
April	1st to 10th	Day	46% 5% 10% 21% 14% 5%	2160
	11th to 20th	Day	44% 5% 10% 22% 15% 4	2134
	21st to end	Day	43% 6% 10% 21% 15% 5%	2160
May	1st to 10th	Day	48% 6% 9% 19% 15% 3	2357
	11th to 20th	Day	50% 6% 10% 19% 13% 2	2482
	21st to end	Day	42% 8% 13% 21% 13% 2	2725
June	1st to 10th	Day	42% 8% 14% 22% 13% 2	2520
	11th to 20th	Day	44% 8% 13% 22% 10% 2	2520
	21st to end	Day	39% 9% 13% 25% 13% 1	2520
July	1st to 10th	Day	41% 8% 13% 22% 15% 1	2450
	11th to 20th	Day	35% 8% 14% 25% 15% 2	2445
	21st to end	Day	43% 7% 11% 21% 15% 3	2528
August	1st to 10th	Day	40% 9% 14% 21% 15% 1	2314
	11th to 20th	Day	41% 8% 14% 23% 13% 1	2183
	21st to end	Day	40% 7% 11% 22% 18% 3	2376
September	1st to 10th	Day	42% 6% 11% 21% 17% 3	2160
	11th to 20th	Day	33% 6% 10% 24% 24% 3	2160
	21st to end	Day	45% 4 9% 19% 21% 2	1930
October	1st to 10th	Day	43% 5% 13% 23% 15% 2	1800
	11th to 20th	Day	41% 5% 12% 24% 17% 1	1800
	21st to end	Day	46% 5% 10% 22% 15% 2	1940
November	1st to 10th	Day	36% 3 14% 25% 18% 3	1671
	11th to 20th	Day	41% 4 11% 24% 18% 2	1701
	21st to end	Day	42% 5% 13% 29% 9% 2	1520
December	1st to 10th	Day	52% 5% 14% 18% 10% 1	1520
	11th to 20th	Day	47% 5% 17% 23% 8% 1	1906
	21st to end	Day	54% 6% 13% 19% 7%	2772
January	1st to 10th	Night	42% 5% 12% 24% 14% 3	2649
	11th to 20th	Night	48% 5% 14% 21% 9% 2	2524
	21st to end	Night	42% 5% 11% 25% 15% 2	2574
February	1st to 10th	Night	46% 4 13% 24% 10% 2	2340
	11th to 20th	Night	46% 5% 11% 20% 16% 3	2190
	21st to end	Night	46% 3 9% 22% 16% 3	1776
March	1st to 10th	Night	39% 5% 12% 26% 15% 3	2117
	11th to 20th	Night	43% 4 10% 23% 17% 3	1980
	21st to end	Night	45% 5% 10% 18% 18% 4	2080
April	1st to 10th	Night	45% 5% 11% 22% 14% 3	1800
	11th to 20th	Night	41% 6% 11% 20% 20% 2	1775
	21st to end	Night	41% 6% 10% 21% 17% 5%	1800
May	1st to 10th	Night	47% 6% 10% 20% 15% 3	1616
	11th to 20th	Night	44% 7% 11% 18% 18% 2	1416
	21st to end	Night	43% 7% 15% 19% 13% 3	1551
June	1st to 10th	Night	41% 6% 10% 23% 18% 1	1440
	11th to 20th	Night	39% 8% 14% 24% 14% 1	1440
	21st to end	Night	34% 9% 14% 29% 13% 1	1440
July	1st to 10th	Night	41% 7% 15% 21% 15% 3	1400
	11th to 20th	Night	34% 7% 16% 25% 15% 2	1392
	21st to end	Night	35% 9% 17% 22% 15% 2	1545
August	1st to 10th	Night	37% 8% 16% 22% 16% 1	1600
	11th to 20th	Night	43% 8% 13% 18% 16% 1	1700
	21st to end	Night	39% 7% 13% 20% 19% 2	1980
September	1st to 10th	Night	40% 6% 9% 24% 18% 3	1800
	11th to 20th	Night	35% 6% 8% 25% 24% 3	1800
	21st to end	Night	41% 4 9% 21% 22% 2	2025
October	1st to 10th	Night	42% 6% 12% 24% 15% 2	2160
	11th to 20th	Night	38% 5% 11% 29% 14% 2	2160
	21st to end	Night	41% 5% 10% 25% 16% 2	2413
November	1st to 10th	Night	35% 5% 13% 24% 19% 4	2407
	11th to 20th	Night	41% 3 10% 25% 16% 4	2474
	21st to end	Night	40% 5% 13% 28% 13% 2	2660
December	1st to 10th	Night	49% 5% 12% 21% 12% 1	2660
	11th to 20th	Night	45% 4 15% 20% 13% 2	3288
	21st to end	Night	49% 5% 14% 23% 8%	4884

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	29% 28% 28% 11% 3 1	180
	11th to 20th	Day	36% 25% 28% 9% 3	194
	21st to end	Day	23% 30% 31% 12% 3 1	264
February	1st to 10th	Day	28% 32% 29% 10% 2	240
	11th to 20th	Day	29% 26% 36% 9% 1	325
	21st to end	Day	32% 21% 28% 18% 1	346
March	1st to 10th	Day	27% 21% 30% 19% 3	420
	11th to 20th	Day	25% 28% 32% 11% 4 1	420
	21st to end	Day	31% 23% 26% 14% 6% 1	462
April	1st to 10th	Day	25% 24% 27% 17% 5% 1	420
	11th to 20th	Day	22% 24% 29% 16% 8% 2	412
	21st to end	Day	29% 20% 30% 13% 6% 2	420
May	1st to 10th	Day	30% 20% 25% 14% 7% 3	423
	11th to 20th	Day	24% 17% 31% 19% 8% 1	447
	21st to end	Day	22% 9% 27% 22% 16% 4	485
June	1st to 10th	Day	16% 15% 32% 22% 13% 3	450
	11th to 20th	Day	22% 13% 28% 22% 12% 3	450
	21st to end	Day	16% 10% 34% 24% 13% 3	450
July	1st to 10th	Day	14% 7% 23% 28% 20% 7%	445
	11th to 20th	Day	9% 7% 24% 33% 22% 4	444
	21st to end	Day	14% 8% 31% 30% 13% 5%	488
August	1st to 10th	Day	12% 7% 25% 29% 17% 9%	446
	11th to 20th	Day	15% 3 23% 34% 17% 8%	439
	21st to end	Day	16% 6% 26% 28% 14% 10%	495
September	1st to 10th	Day	20% 10% 21% 23% 20% 6%	450
	11th to 20th	Day	14% 5% 26% 28% 17% 10%	450
	21st to end	Day	24% 13% 24% 20% 15% 5%	450
October	1st to 10th	Day	25% 9% 23% 21% 16% 6%	450
	11th to 20th	Day	21% 15% 28% 22% 12% 2	450
	21st to end	Day	24% 16% 32% 17% 9% 1	475
November	1st to 10th	Day	20% 16% 26% 24% 11% 3	365
	11th to 20th	Day	25% 20% 29% 20% 6%	361
	21st to end	Day	26% 24% 34% 11% 5%	180
December	1st to 10th	Day	30% 26% 26% 12% 6% 1	180
	11th to 20th	Day	33% 24% 27% 12% 4	180
	21st to end	Day	28% 24% 30% 15% 2 2	198
January	1st to 10th	Night	19% 19% 39% 19% 3	760
	11th to 20th	Night	27% 16% 30% 23% 4	739
	21st to end	Night	17% 20% 37% 24% 2	792
February	1st to 10th	Night	24% 17% 33% 23% 2	720
	11th to 20th	Night	24% 21% 36% 17% 1	703
	21st to end	Night	27% 21% 32% 18% 2	592
March	1st to 10th	Night	23% 20% 32% 22% 4	720
	11th to 20th	Night	25% 17% 32% 20% 5% 1	720
	21st to end	Night	22% 16% 36% 22% 5%	792
April	1st to 10th	Night	24% 17% 34% 20% 6%	720
	11th to 20th	Night	18% 14% 37% 24% 7%	711
	21st to end	Night	25% 12% 31% 24% 7%	720
May	1st to 10th	Night	25% 11% 28% 26% 10% 1	536
	11th to 20th	Night	18% 10% 33% 28% 10% 1	354
	21st to end	Night	19% 7% 18% 41% 13% 2	389
June	1st to 10th	Night	18% 6% 25% 33% 18%	360
	11th to 20th	Night	18% 4 26% 39% 12% 1	360
	21st to end	Night	11% 4 26% 42% 16% 1	360
July	1st to 10th	Night	12% 3 21% 36% 25% 2	350
	11th to 20th	Night	13% 3 17% 42% 24% 2	348
	21st to end	Night	11% 4 18% 43% 21% 2	456
August	1st to 10th	Night	15% 1 16% 40% 26% 2	537
	11th to 20th	Night	17% 5% 13% 37% 27% 2	636
	21st to end	Night	17% 3 17% 35% 26% 2	792
September	1st to 10th	Night	18% 4 21% 34% 21% 2	720
	11th to 20th	Night	15% 6% 18% 33% 23% 5%	720
	21st to end	Night	21% 5% 22% 30% 20% 2	720
October	1st to 10th	Night	25% 7% 21% 27% 17% 3	720
	11th to 20th	Night	21% 10% 24% 33% 11% 1	720
	21st to end	Night	25% 12% 28% 27% 9%	792
November	1st to 10th	Night	16% 12% 32% 27% 12% 1	740
	11th to 20th	Night	20% 18% 27% 27% 7% 1	760
	21st to end	Night	18% 17% 31% 27% 6%	760
December	1st to 10th	Night	26% 17% 30% 21% 5% 1	760
	11th to 20th	Night	22% 16% 32% 26% 5%	760
	21st to end	Night	25% 14% 32% 26% 3	836

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