Blue Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Mountain clear sky chart from 2005-11-06 to 2025-07-21.

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	28% 13% 3 6% 4 3 3 3 4 5% 28%	124592
	Night	31% 14% 3 6% 3 3 3 2 4 4 28%	123195
Transparency	Day	59% 5% 11% 16% 8% 1	124350
	Night	61% 5% 10% 15% 8% 1	123080
Seeing	Day	35% 11% 22% 22% 8% 2	15866
	Night	40% 7% 18% 23% 12% 1	24215

By Month

Percentage Cloud Cover NW (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	42% 21% 3 6% 3 2 3 2 3 3 12%	3386
	11th to 20th	Day	21% 15% 5% 7% 4 4 4 3 4 6% 28%	3616
	21st to end	Day	39% 12% 2 4 2 2 3 2 3 3 28%	4025
February	1st to 10th	Day	35% 17% 3 9% 4 4 3 3 3 3 15%	1800
	11th to 20th	Day	34% 21% 4 6% 3 3 3 2 3 4 19%	1755
	21st to end	Day	30% 24% 4 7% 3 3 2 2 3 3 18%	1573
March	1st to 10th	Day	27% 20% 3 8% 5% 4 4 4 4 4 17%	4552
	11th to 20th	Day	25% 23% 4 7% 4 4 3 2 2 2 24%	5000
	21st to end	Day	35% 19% 3 8% 5% 4 4 3 4 4 11%	5722
April	1st to 10th	Day	32% 21% 3 6% 5% 4 4 3 4 4 13%	2400
	11th to 20th	Day	29% 18% 3 7% 4 3 3 3 4 4 20%	2372
	21st to end	Day	33% 17% 4 8% 4 4 4 3 3 4 15%	2683
May	1st to 10th	Day	21% 14% 2 7% 4 5% 5% 4 4 5% 30%	6320
	11th to 20th	Day	32% 13% 3 7% 4 4 4 3 4 6% 20%	6277
	21st to end	Day	33% 12% 3 6% 4 3 3 3 5% 7% 22%	6903
June	1st to 10th	Day	33% 10% 2 6% 4 4 3 3 4 5% 23%	2817
	11th to 20th	Day	38% 12% 3 8% 5% 4 4 4 4 4 14%	3000
	21st to end	Day	24% 9% 3 7% 5% 4 5% 4 6% 6% 30%	3000
July	1st to 10th	Day	13% 5% 1 4 3 3 3 3 4 4 57%	4890
	11th to 20th	Day	10% 5% 1 3 3 2 2 3 3 5% 63%	4627
	21st to end	Day	17% 6% 2 4 3 3 3 3 4 6% 50%	4791
August	1st to 10th	Day	10% 4 1 4 3 3 3 3 4 6% 58%	4401
	11th to 20th	Day	19% 9% 3 8% 5% 4 5% 3 5% 6% 33%	4030
	21st to end	Day	22% 8% 3 7% 3 3 3 3 4 6% 38%	4306
September	1st to 10th	Day	17% 7% 3 5% 4 3 4 3 4 6% 44%	2280
	11th to 20th	Day	22% 8% 3 7% 4 4 4 4 4 5% 35%	2280
	21st to end	Day	27% 8% 2 6% 2 3 4 2 3 5% 37%	2019
October	1st to 10th	Day	26% 7% 2 4 3 3 4 3 6% 8% 34%	3180
	11th to 20th	Day	41% 9% 2 4 3 2 2 2 3 4 25%	3161
	21st to end	Day	40% 12% 4 6% 4 3 3 3 4 5% 18%	3200
November	1st to 10th	Day	45% 12% 2 5% 5% 3 3 3 4 3 15%	1630
	11th to 20th	Day	44% 16% 3 6% 2 2 2 2 3 3 18%	1600
	21st to end	Day	39% 17% 3 6% 3 2 4 2 3 4 17%	1600
December	1st to 10th	Day	29% 12% 2 5% 4 2 3 3 4 5% 32%	2620
	11th to 20th	Day	44% 18% 3 6% 3 3 3 3 2 2 13%	3025
	21st to end	Day	39% 21% 3 7% 4 3 3 2 3 3 10%	3751
January	1st to 10th	Night	40% 20% 4 6% 4 3 3 2 3 3 13%	6522
	11th to 20th	Night	24% 14% 3 6% 3 3 3 3 5% 7% 30%	6282
	21st to end	Night	36% 11% 2 5% 3 2 3 2 3 4 30%	6930
February	1st to 10th	Night	34% 19% 3 6% 4 4 4 3 3 3 18%	2600
	11th to 20th	Night	32% 20% 3 6% 3 3 4 2 3 4 20%	2532
	21st to end	Night	32% 21% 3 6% 3 3 3 3 3 4 19%	2056
March	1st to 10th	Night	22% 22% 3 8% 4 4 4 3 4 5% 19%	5338
	11th to 20th	Night	30% 22% 3 5% 2 2 2 2 2 4 26%	4999
	21st to end	Night	41% 20% 3 6% 3 3 2 3 3 4 12%	5253
April	1st to 10th	Night	32% 19% 3 7% 4 4 4 4 5% 5% 15%	2000
	11th to 20th	Night	30% 16% 2 5% 4 3 3 3 4 5% 24%	1976
	21st to end	Night	35% 14% 2 6% 5% 4 3 3 5% 5% 19%	1716
May	1st to 10th	Night	23% 13% 2 6% 3 3 3 2 3 3 40%	3640
	11th to 20th	Night	34% 14% 2 4 3 2 3 3 3 4 27%	3616
	21st to end	Night	34% 9% 3 7% 4 3 4 4 5% 6% 21%	3972
June	1st to 10th	Night	31% 8% 2 6% 4 4 4 3 4 5% 28%	1578
	11th to 20th	Night	35% 10% 3 6% 3 5% 3 2 5% 5% 23%	1400
	21st to end	Night	22% 6% 2 7% 4 3 5% 3 5% 5% 37%	1400
July	1st to 10th	Night	12% 4 2 4 3 2 2 2 3 3 63%	2290
	11th to 20th	Night	8% 3 1 2 2 2 2 2 3 5% 70%	2549
	21st to end	Night	17% 5% 2 4 2 2 2 2 4 4 55%	2762
August	1st to 10th	Night	7% 2 1 3 2 3 3 2 5% 6% 66%	2529
	11th to 20th	Night	21% 7% 2 5% 2 3 3 2 5% 5% 44%	2805
	21st to end	Night	26% 6% 2 4 3 2 2 2 3 3 46%	3367
September	1st to 10th	Night	17% 6% 2 4 3 3 3 2 3 5% 52%	1900
	11th to 20th	Night	26% 9% 2 6% 4 3 3 2 4 5% 36%	1900
	21st to end	Night	27% 8% 2 5% 2 3 3 3 4 5% 37%	2157
October	1st to 10th	Night	24% 7% 2 5% 4 4 4 3 6% 7% 35%	3800
	11th to 20th	Night	40% 7% 2 4 2 2 3 3 4 5% 28%	3777
	21st to end	Night	42% 13% 2 5% 3 2 2 2 4 3 21%	4361
November	1st to 10th	Night	42% 12% 3 6% 4 3 3 2 4 4 16%	2666
	11th to 20th	Night	44% 13% 3 5% 3 3 3 2 2 3 20%	2800
	21st to end	Night	39% 15% 3 6% 4 3 3 2 3 5% 18%	2800
December	1st to 10th	Night	28% 12% 3 5% 4 3 3 3 3 4 32%	4560
	11th to 20th	Night	42% 18% 4 6% 3 3 3 2 3 4 13%	5180
	21st to end	Night	40% 23% 3 6% 4 3 3 2 3 2 9%	7182

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	80% 3 4 10% 2 1	3386
	11th to 20th	Day	58% 4 9% 19% 9% 1	3640
	21st to end	Day	63% 3 5% 15% 13%	4025
February	1st to 10th	Day	74% 5% 7% 8% 4 1	1800
	11th to 20th	Day	73% 4 6% 9% 7% 1	1764
	21st to end	Day	72% 5% 6% 12% 4	1573
March	1st to 10th	Day	68% 5% 9% 11% 6%	4552
	11th to 20th	Day	68% 3 8% 19% 2	5000
	21st to end	Day	76% 5% 10% 7% 1	5722
April	1st to 10th	Day	75% 7% 11% 7% 1	2400
	11th to 20th	Day	68% 6% 9% 12% 5% 1	2372
	21st to end	Day	74% 5% 8% 8% 4 1	2683
May	1st to 10th	Day	55% 6% 14% 20% 6%	6320
	11th to 20th	Day	66% 6% 16% 10% 1	6280
	21st to end	Day	63% 7% 18% 12% 1	6904
June	1st to 10th	Day	63% 6% 15% 11% 5%	2817
	11th to 20th	Day	73% 7% 9% 9% 2	3000
	21st to end	Day	53% 8% 14% 18% 6% 1	3000
July	1st to 10th	Day	30% 5% 9% 22% 23% 10%	4816
	11th to 20th	Day	25% 4 9% 27% 33% 2	4549
	21st to end	Day	34% 5% 14% 32% 12% 2	4640
August	1st to 10th	Day	27% 5% 11% 40% 16% 1	4384
	11th to 20th	Day	49% 8% 16% 19% 7% 1	4069
	21st to end	Day	48% 4 11% 25% 11% 1	4305
September	1st to 10th	Day	40% 7% 12% 23% 13% 5%	2280
	11th to 20th	Day	52% 6% 9% 17% 14% 3	2280
	21st to end	Day	51% 5% 15% 15% 10% 3	2019
October	1st to 10th	Day	48% 5% 12% 28% 6%	3180
	11th to 20th	Day	64% 3 9% 14% 9% 1	3161
	21st to end	Day	70% 4 9% 8% 8% 1	3200
November	1st to 10th	Day	74% 6% 8% 8% 5%	1630
	11th to 20th	Day	73% 4 6% 8% 7% 2	1600
	21st to end	Day	72% 5% 5% 8% 8% 3	1600
December	1st to 10th	Day	57% 4 6% 17% 14% 2	2620
	11th to 20th	Day	78% 4 6% 7% 3 1	3028
	21st to end	Day	78% 6% 7% 7% 2	3751
January	1st to 10th	Night	78% 3 4 9% 4 1	6522
	11th to 20th	Night	55% 4 9% 19% 11% 1	6320
	21st to end	Night	60% 3 9% 15% 12% 1	6930
February	1st to 10th	Night	72% 5% 7% 10% 5% 1	2600
	11th to 20th	Night	69% 6% 7% 9% 7% 2	2534
	21st to end	Night	71% 4 8% 12% 4 1	2056
March	1st to 10th	Night	66% 5% 9% 11% 8%	5338
	11th to 20th	Night	64% 4 10% 19% 2	5000
	21st to end	Night	77% 4 10% 8% 1	5253
April	1st to 10th	Night	71% 7% 12% 9% 1	2000
	11th to 20th	Night	64% 5% 11% 12% 7% 2	1976
	21st to end	Night	67% 7% 10% 11% 5% 1	1716
May	1st to 10th	Night	51% 3 16% 21% 8%	3637
	11th to 20th	Night	62% 5% 15% 14% 3	3616
	21st to end	Night	64% 8% 16% 11% 1	3972
June	1st to 10th	Night	59% 6% 16% 14% 5%	1578
	11th to 20th	Night	63% 8% 13% 13% 3	1400
	21st to end	Night	48% 8% 19% 18% 5% 2	1400
July	1st to 10th	Night	29% 4 10% 22% 19% 16%	2255
	11th to 20th	Night	21% 3 10% 26% 38% 2	2504
	21st to end	Night	33% 6% 11% 28% 19% 3	2674
August	1st to 10th	Night	20% 5% 15% 39% 20% 2	2521
	11th to 20th	Night	44% 6% 20% 20% 8% 2	2832
	21st to end	Night	45% 4 12% 27% 12% 1	3367
September	1st to 10th	Night	36% 5% 14% 24% 15% 5%	1900
	11th to 20th	Night	51% 6% 8% 17% 14% 4	1900
	21st to end	Night	51% 6% 12% 18% 11% 3	2157
October	1st to 10th	Night	48% 5% 12% 28% 6%	3800
	11th to 20th	Night	60% 4 9% 15% 11% 1	3777
	21st to end	Night	69% 3 7% 11% 9% 1	4361
November	1st to 10th	Night	73% 5% 8% 9% 4 1	2666
	11th to 20th	Night	73% 4 7% 8% 6% 2	2800
	21st to end	Night	72% 4 6% 9% 9% 2	2800
December	1st to 10th	Night	58% 4 7% 14% 15% 3	4560
	11th to 20th	Night	76% 3 6% 10% 3	5176
	21st to end	Night	81% 5% 6% 6% 2	7182

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	50% 14% 19% 10% 8% 1	200
	11th to 20th	Day	45% 11% 20% 18% 7%	200
	21st to end	Day	51% 8% 15% 18% 6%	228
February	1st to 10th	Day	47% 12% 21% 16% 4 1	400
	11th to 20th	Day	52% 11% 21% 14% 3	395
	21st to end	Day	44% 13% 25% 15% 3 1	377
March	1st to 10th	Day	40% 15% 25% 16% 4	500
	11th to 20th	Day	41% 11% 21% 18% 8% 1	500
	21st to end	Day	49% 13% 21% 14% 3	550
April	1st to 10th	Day	46% 13% 21% 15% 5% 1	500
	11th to 20th	Day	41% 17% 20% 16% 6% 1	492
	21st to end	Day	41% 14% 22% 16% 6% 1	500
May	1st to 10th	Day	32% 12% 27% 23% 5%	503
	11th to 20th	Day	41% 13% 19% 19% 7% 2	528
	21st to end	Day	40% 13% 18% 21% 7% 2	573
June	1st to 10th	Day	32% 14% 22% 22% 9% 2	547
	11th to 20th	Day	36% 13% 27% 18% 5% 1	730
	21st to end	Day	24% 11% 25% 30% 7% 2	730
July	1st to 10th	Day	20% 9% 28% 32% 10% 2	721
	11th to 20th	Day	16% 12% 27% 30% 12% 3	562
	21st to end	Day	17% 7% 25% 32% 15% 4	532
August	1st to 10th	Day	17% 6% 26% 31% 15% 5%	487
	11th to 20th	Day	14% 10% 22% 34% 15% 5%	477
	21st to end	Day	22% 11% 23% 30% 11% 3	539
September	1st to 10th	Day	22% 13% 25% 28% 11% 2	490
	11th to 20th	Day	25% 13% 22% 27% 11% 2	490
	21st to end	Day	28% 11% 25% 22% 11% 2	490
October	1st to 10th	Day	36% 7% 21% 24% 11% 2	490
	11th to 20th	Day	42% 8% 20% 18% 11% 2	490
	21st to end	Day	46% 9% 21% 15% 8% 2	392
November	1st to 10th	Day	51% 9% 19% 16% 5%	233
	11th to 20th	Day	54% 12% 13% 13% 8% 2	200
	21st to end	Day	49% 6% 21% 13% 8% 4	200
December	1st to 10th	Day	55% 6% 9% 17% 10% 5%	200
	11th to 20th	Day	54% 10% 21% 12% 4	200
	21st to end	Day	52% 6% 22% 15% 4	220
January	1st to 10th	Night	56% 9% 18% 14% 4	848
	11th to 20th	Night	45% 10% 24% 16% 5% 1	800
	21st to end	Night	46% 8% 23% 20% 3	880
February	1st to 10th	Night	45% 11% 25% 15% 5%	800
	11th to 20th	Night	46% 8% 23% 20% 3	784
	21st to end	Night	48% 8% 21% 18% 5%	660
March	1st to 10th	Night	42% 10% 20% 20% 8%	800
	11th to 20th	Night	47% 7% 22% 19% 6% 1	800
	21st to end	Night	51% 9% 18% 17% 5% 1	880
April	1st to 10th	Night	45% 9% 16% 22% 8% 2	800
	11th to 20th	Night	39% 12% 19% 23% 6% 1	788
	21st to end	Night	43% 8% 17% 24% 8% 1	516
May	1st to 10th	Night	31% 8% 17% 31% 15%	400
	11th to 20th	Night	35% 3 13% 30% 18% 1	394
	21st to end	Night	34% 8% 10% 25% 20% 2	433
June	1st to 10th	Night	35% 7% 11% 27% 18% 2	400
	11th to 20th	Night	37% 7% 13% 28% 14% 1	400
	21st to end	Night	23% 5% 13% 33% 25% 2	400
July	1st to 10th	Night	16% 4 9% 33% 34% 4	390
	11th to 20th	Night	14% 5% 7% 39% 32% 3	388
	21st to end	Night	15% 3 6% 33% 38% 5%	416
August	1st to 10th	Night	13% 2 4 33% 43% 6%	378
	11th to 20th	Night	15% 4 8% 30% 36% 7%	557
	21st to end	Night	22% 4 10% 33% 29% 1	757
September	1st to 10th	Night	19% 4 12% 36% 24% 4	760
	11th to 20th	Night	29% 5% 12% 26% 24% 4	760
	21st to end	Night	32% 4 13% 34% 16% 1	760
October	1st to 10th	Night	30% 6% 19% 29% 14% 1	760
	11th to 20th	Night	40% 5% 21% 24% 11% 1	760
	21st to end	Night	46% 5% 23% 21% 5% 1	836
November	1st to 10th	Night	51% 7% 24% 16% 2	780
	11th to 20th	Night	53% 10% 17% 16% 4	800
	21st to end	Night	48% 10% 22% 17% 3	800
December	1st to 10th	Night	41% 8% 24% 21% 6% 1	800
	11th to 20th	Night	53% 9% 24% 11% 3 1	800
	21st to end	Night	58% 8% 19% 12% 2	930

Page generated at 2025-07-17 22:10:05. Version ac07

Copyright © 2025 Attilla Danko.