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 2025-03-12.

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% 9% 3 9% 6% 5% 5% 4 6% 8% 35%	111554
	Night	7% 9% 4 10% 7% 6% 6% 4 7% 8% 33%	116023
Transparency	Day	45% 9% 17% 22% 7%	111297
	Night	46% 8% 16% 21% 8% 1	115927
Seeing	Day	13% 8% 26% 31% 17% 5%	16050
	Night	12% 6% 29% 37% 15% 2	25476

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	5% 11% 6% 16% 10% 8% 6% 4 6% 6% 22%	3179
	11th to 20th	Day	4 8% 3 12% 8% 9% 10% 6% 7% 11% 23%	3159
	21st to end	Day	7% 14% 6% 16% 7% 5% 5% 4 5% 8% 24%	3498
February	1st to 10th	Day	7% 13% 5% 14% 9% 7% 7% 6% 6% 7% 21%	1708
	11th to 20th	Day	7% 11% 4 11% 9% 7% 7% 5% 7% 8% 25%	1755
	21st to end	Day	5% 9% 5% 13% 9% 7% 8% 4 6% 8% 24%	1485
March	1st to 10th	Day	4 12% 5% 11% 7% 7% 7% 7% 7% 8% 27%	3246
	11th to 20th	Day	5% 11% 4 11% 7% 5% 5% 3 7% 8% 34%	3501
	21st to end	Day	6% 9% 5% 12% 7% 6% 5% 4 5% 6% 35%	3948
April	1st to 10th	Day	6% 11% 3 9% 7% 6% 4 4 6% 8% 36%	2280
	11th to 20th	Day	11% 10% 4 9% 6% 6% 5% 4 7% 8% 31%	2421
	21st to end	Day	13% 10% 3 7% 5% 5% 5% 4 7% 8% 33%	2617
May	1st to 10th	Day	15% 11% 4 8% 5% 5% 6% 4 6% 6% 31%	4430
	11th to 20th	Day	11% 9% 3 9% 6% 5% 6% 4 7% 7% 33%	4508
	21st to end	Day	14% 8% 3 10% 6% 6% 5% 5% 7% 8% 28%	5225
June	1st to 10th	Day	14% 8% 3 8% 6% 5% 6% 4 6% 7% 32%	3040
	11th to 20th	Day	21% 10% 3 8% 5% 4 4 4 6% 8% 27%	3040
	21st to end	Day	12% 6% 3 8% 5% 5% 5% 5% 7% 8% 36%	3040
July	1st to 10th	Day	8% 4 2 4 4 4 5% 4 7% 10% 47%	4886
	11th to 20th	Day	7% 4 2 6% 5% 5% 5% 6% 8% 10% 42%	4740
	21st to end	Day	6% 4 2 5% 4 4 5% 5% 7% 9% 49%	5138
August	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 5% 8% 47%	4457
	11th to 20th	Day	7% 5% 2 5% 4 5% 5% 4 7% 11% 45%	4197
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 46%	4505
September	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 5% 9% 45%	2280
	11th to 20th	Day	15% 7% 3 7% 4 5% 4 3 6% 7% 39%	2280
	21st to end	Day	9% 7% 2 5% 4 4 5% 4 5% 8% 47%	2000
October	1st to 10th	Day	8% 10% 3 7% 4 5% 4 3 6% 7% 43%	3170
	11th to 20th	Day	6% 8% 3 8% 5% 4 4 4 8% 11% 39%	3137
	21st to end	Day	6% 8% 5% 11% 5% 6% 6% 5% 6% 7% 35%	2877
November	1st to 10th	Day	12% 14% 6% 12% 6% 6% 4 5% 4 7% 25%	1568
	11th to 20th	Day	10% 16% 5% 15% 7% 5% 4 4 6% 7% 22%	1600
	21st to end	Day	7% 12% 5% 15% 8% 7% 5% 5% 6% 8% 23%	1400
December	1st to 10th	Day	6% 13% 5% 12% 8% 7% 7% 6% 6% 6% 23%	1960
	11th to 20th	Day	6% 12% 8% 17% 10% 10% 6% 4 5% 4 18%	2259
	21st to end	Day	3 7% 4 14% 9% 7% 5% 4 6% 7% 35%	3020
January	1st to 10th	Night	4 11% 5% 15% 9% 8% 7% 5% 8% 8% 19%	6766
	11th to 20th	Night	3 10% 4 13% 8% 9% 11% 6% 7% 9% 21%	6739
	21st to end	Night	6% 11% 4 14% 9% 5% 5% 4 7% 8% 27%	7458
February	1st to 10th	Night	6% 13% 4 12% 9% 6% 6% 5% 7% 8% 22%	2708
	11th to 20th	Night	5% 10% 4 10% 8% 7% 6% 6% 8% 7% 30%	2532
	21st to end	Night	2 9% 4 11% 8% 6% 6% 5% 8% 10% 30%	2145
March	1st to 10th	Night	5% 12% 4 11% 7% 6% 7% 6% 9% 10% 24%	3839
	11th to 20th	Night	6% 10% 4 10% 6% 6% 6% 5% 8% 11% 29%	3506
	21st to end	Night	7% 7% 3 10% 7% 6% 6% 6% 7% 9% 33%	3701
April	1st to 10th	Night	5% 10% 2 8% 6% 6% 6% 4 7% 7% 39%	1900
	11th to 20th	Night	8% 11% 3 8% 7% 5% 5% 4 7% 8% 33%	1709
	21st to end	Night	13% 9% 3 8% 7% 6% 4 3 5% 7% 35%	1563
May	1st to 10th	Night	15% 10% 4 9% 6% 4 4 4 5% 8% 31%	2540
	11th to 20th	Night	12% 8% 2 6% 5% 5% 5% 5% 6% 7% 38%	2376
	21st to end	Night	16% 9% 3 8% 6% 4 5% 4 6% 7% 31%	2350
June	1st to 10th	Night	12% 7% 3 8% 6% 5% 5% 4 6% 8% 35%	1140
	11th to 20th	Night	16% 10% 2 8% 5% 5% 5% 5% 6% 8% 30%	1140
	21st to end	Night	10% 4 2 6% 5% 6% 5% 3 6% 9% 43%	1140
July	1st to 10th	Night	7% 4 2 6% 4 4 5% 4 8% 10% 47%	2057
	11th to 20th	Night	5% 4 2 5% 5% 4 4 3 5% 9% 54%	2220
	21st to end	Night	5% 3 1 4 3 3 4 3 6% 8% 58%	2414
August	1st to 10th	Night	8% 4 2 7% 4 4 5% 4 6% 8% 49%	2481
	11th to 20th	Night	7% 4 2 6% 5% 4 6% 4 8% 10% 44%	2639
	21st to end	Night	10% 4 2 5% 3 4 3 4 6% 8% 51%	3162
September	1st to 10th	Night	10% 6% 2 5% 4 3 5% 3 7% 8% 48%	1900
	11th to 20th	Night	12% 6% 2 6% 3 4 4 3 6% 7% 45%	1900
	21st to end	Night	7% 7% 2 5% 4 4 5% 4 6% 8% 49%	2176
October	1st to 10th	Night	9% 9% 3 6% 4 4 4 3 5% 8% 46%	3800
	11th to 20th	Night	5% 7% 2 7% 4 5% 5% 4 6% 8% 48%	3778
	21st to end	Night	7% 10% 4 9% 5% 5% 5% 3 6% 8% 38%	4648
November	1st to 10th	Night	15% 15% 5% 9% 6% 5% 4 4 6% 6% 25%	2730
	11th to 20th	Night	8% 12% 4 16% 7% 7% 6% 4 6% 6% 24%	2800
	21st to end	Night	6% 13% 5% 14% 8% 5% 5% 3 6% 9% 25%	2995
December	1st to 10th	Night	7% 13% 5% 14% 8% 8% 7% 5% 6% 6% 21%	5220
	11th to 20th	Night	5% 13% 6% 16% 8% 7% 7% 5% 7% 8% 17%	5933
	21st to end	Night	2 7% 4 13% 10% 6% 5% 4 5% 6% 37%	7918

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	60% 7% 9% 19% 5%	3179
	11th to 20th	Day	53% 7% 12% 15% 13% 1	3180
	21st to end	Day	57% 7% 18% 13% 4	3498
February	1st to 10th	Day	58% 7% 12% 14% 9%	1708
	11th to 20th	Day	52% 7% 15% 16% 9% 1	1764
	21st to end	Day	54% 8% 12% 18% 8%	1485
March	1st to 10th	Day	51% 8% 19% 15% 7%	3246
	11th to 20th	Day	45% 11% 20% 19% 5%	3502
	21st to end	Day	48% 8% 12% 19% 13%	3948
April	1st to 10th	Day	44% 9% 16% 19% 11%	2280
	11th to 20th	Day	49% 7% 14% 20% 9% 1	2421
	21st to end	Day	45% 8% 16% 22% 8% 1	2617
May	1st to 10th	Day	50% 8% 13% 20% 8% 1	4430
	11th to 20th	Day	46% 9% 13% 22% 10%	4512
	21st to end	Day	49% 12% 14% 16% 8%	5226
June	1st to 10th	Day	47% 10% 15% 18% 9% 1	3040
	11th to 20th	Day	55% 8% 14% 16% 7%	3040
	21st to end	Day	43% 10% 18% 22% 6%	3040
July	1st to 10th	Day	29% 10% 27% 27% 7%	4812
	11th to 20th	Day	32% 11% 19% 33% 5%	4659
	21st to end	Day	29% 9% 22% 34% 6%	4976
August	1st to 10th	Day	35% 8% 21% 32% 4	4439
	11th to 20th	Day	30% 10% 24% 30% 6%	4239
	21st to end	Day	35% 7% 23% 30% 4	4504
September	1st to 10th	Day	35% 8% 18% 24% 12% 2	2280
	11th to 20th	Day	44% 7% 16% 26% 6%	2280
	21st to end	Day	34% 8% 19% 28% 11% 1	2000
October	1st to 10th	Day	39% 7% 22% 24% 8%	3170
	11th to 20th	Day	37% 7% 17% 34% 6%	3137
	21st to end	Day	46% 8% 19% 23% 4	2877
November	1st to 10th	Day	58% 8% 14% 14% 6%	1568
	11th to 20th	Day	61% 10% 13% 13% 4	1600
	21st to end	Day	58% 8% 15% 16% 3	1400
December	1st to 10th	Day	57% 11% 13% 13% 5% 1	1960
	11th to 20th	Day	67% 6% 12% 11% 4	2260
	21st to end	Day	46% 7% 12% 20% 14% 1	3020
January	1st to 10th	Night	57% 8% 13% 15% 6%	6766
	11th to 20th	Night	56% 7% 13% 14% 10%	6780
	21st to end	Night	52% 8% 20% 16% 3	7458
February	1st to 10th	Night	55% 6% 13% 17% 9%	2708
	11th to 20th	Night	48% 8% 14% 19% 10%	2533
	21st to end	Night	44% 7% 16% 22% 10%	2145
March	1st to 10th	Night	49% 9% 17% 19% 6%	3839
	11th to 20th	Night	44% 11% 19% 20% 7%	3506
	21st to end	Night	41% 8% 15% 22% 14%	3701
April	1st to 10th	Night	40% 8% 14% 22% 15% 1	1900
	11th to 20th	Night	46% 6% 13% 22% 12% 1	1709
	21st to end	Night	48% 7% 12% 21% 11% 2	1563
May	1st to 10th	Night	49% 8% 11% 21% 10%	2540
	11th to 20th	Night	39% 10% 11% 24% 14% 2	2376
	21st to end	Night	47% 11% 13% 22% 6% 1	2350
June	1st to 10th	Night	45% 7% 16% 22% 8% 2	1140
	11th to 20th	Night	48% 10% 14% 18% 9% 1	1140
	21st to end	Night	35% 9% 23% 23% 9% 1	1140
July	1st to 10th	Night	29% 10% 24% 28% 8% 1	2023
	11th to 20th	Night	26% 7% 17% 42% 8%	2178
	21st to end	Night	23% 8% 25% 34% 11% 1	2337
August	1st to 10th	Night	32% 8% 23% 31% 5%	2474
	11th to 20th	Night	31% 8% 24% 28% 8% 1	2663
	21st to end	Night	30% 6% 21% 35% 7%	3162
September	1st to 10th	Night	32% 8% 17% 28% 11% 4	1900
	11th to 20th	Night	37% 8% 15% 30% 9% 1	1900
	21st to end	Night	32% 7% 17% 29% 13% 2	2176
October	1st to 10th	Night	37% 5% 22% 30% 6%	3800
	11th to 20th	Night	34% 5% 14% 35% 12%	3778
	21st to end	Night	43% 7% 21% 24% 5%	4648
November	1st to 10th	Night	58% 7% 13% 16% 6% 1	2730
	11th to 20th	Night	60% 8% 14% 14% 4	2800
	21st to end	Night	54% 9% 17% 16% 4	2995
December	1st to 10th	Night	61% 9% 13% 12% 4 1	5220
	11th to 20th	Night	62% 8% 15% 12% 3	5931
	21st to end	Night	45% 7% 11% 23% 13% 1	7918

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	11% 9% 24% 35% 16% 6%	200
	11th to 20th	Day	12% 4 30% 30% 21% 5%	200
	21st to end	Day	11% 12% 26% 29% 17% 5%	220
February	1st to 10th	Day	10% 9% 33% 32% 13% 3	300
	11th to 20th	Day	8% 4 30% 34% 19% 4	300
	21st to end	Day	4 8% 26% 37% 19% 5%	249
March	1st to 10th	Day	8% 8% 27% 37% 16% 2	330
	11th to 20th	Day	10% 8% 36% 28% 12% 6%	465
	21st to end	Day	12% 6% 30% 34% 15% 3	506
April	1st to 10th	Day	9% 12% 32% 31% 11% 5%	460
	11th to 20th	Day	14% 11% 30% 31% 11% 3	452
	21st to end	Day	15% 10% 31% 28% 13% 4	607
May	1st to 10th	Day	13% 11% 25% 29% 17% 5%	653
	11th to 20th	Day	10% 9% 23% 32% 20% 6%	675
	21st to end	Day	14% 7% 24% 28% 19% 8%	734
June	1st to 10th	Day	16% 9% 27% 33% 12% 3	680
	11th to 20th	Day	24% 11% 24% 22% 15% 4	680
	21st to end	Day	14% 8% 20% 30% 21% 8%	680
July	1st to 10th	Day	12% 4 20% 35% 23% 7%	671
	11th to 20th	Day	10% 7% 22% 32% 22% 7%	668
	21st to end	Day	8% 5% 22% 32% 25% 8%	739
August	1st to 10th	Day	12% 5% 21% 30% 23% 9%	676
	11th to 20th	Day	9% 8% 20% 34% 25% 4	584
	21st to end	Day	14% 5% 27% 29% 20% 5%	539
September	1st to 10th	Day	14% 9% 29% 29% 14% 4	490
	11th to 20th	Day	21% 7% 26% 30% 14% 1	490
	21st to end	Day	15% 11% 25% 31% 16% 3	395
October	1st to 10th	Day	15% 5% 28% 35% 11% 6%	300
	11th to 20th	Day	12% 10% 30% 34% 11% 3	300
	21st to end	Day	13% 10% 36% 27% 12% 3	330
November	1st to 10th	Day	17% 7% 29% 32% 12% 2	300
	11th to 20th	Day	17% 11% 33% 29% 9% 1	300
	21st to end	Day	7% 12% 31% 36% 12% 3	257
December	1st to 10th	Day	8% 7% 23% 36% 20% 7%	200
	11th to 20th	Day	9% 9% 35% 36% 11% 1	200
	21st to end	Day	8% 5% 26% 35% 17% 10%	220
January	1st to 10th	Night	10% 7% 32% 35% 15% 1	1000
	11th to 20th	Night	10% 6% 33% 39% 11% 1	1000
	21st to end	Night	13% 8% 35% 33% 9% 1	1100
February	1st to 10th	Night	11% 6% 27% 37% 17% 2	907
	11th to 20th	Night	9% 5% 29% 38% 17% 3	784
	21st to end	Night	5% 5% 22% 40% 25% 4	660
March	1st to 10th	Night	10% 6% 30% 39% 14% 2	800
	11th to 20th	Night	9% 7% 35% 35% 13% 1	766
	21st to end	Night	11% 6% 28% 37% 15% 3	760
April	1st to 10th	Night	9% 7% 30% 37% 14% 2	570
	11th to 20th	Night	13% 8% 33% 30% 15% 1	563
	21st to end	Night	16% 7% 28% 36% 12% 1	570
May	1st to 10th	Night	14% 7% 27% 35% 15% 2	570
	11th to 20th	Night	11% 6% 28% 34% 19% 2	486
	21st to end	Night	17% 8% 21% 34% 19% 2	411
June	1st to 10th	Night	16% 6% 28% 33% 15% 2	380
	11th to 20th	Night	21% 4 24% 30% 18% 3	380
	21st to end	Night	9% 5% 21% 41% 22% 2	380
July	1st to 10th	Night	9% 4 18% 45% 23% 2	370
	11th to 20th	Night	10% 4 22% 43% 19% 1	368
	21st to end	Night	7% 4 22% 41% 23% 3	416
August	1st to 10th	Night	7% 4 20% 36% 29% 3	532
	11th to 20th	Night	8% 4 20% 39% 27% 2	564
	21st to end	Night	9% 4 26% 39% 21% 1	627
September	1st to 10th	Night	12% 3 28% 39% 18% 2	570
	11th to 20th	Night	15% 4 25% 40% 15% 1	570
	21st to end	Night	10% 6% 31% 39% 13% 1	751
October	1st to 10th	Night	18% 7% 28% 35% 13% 1	760
	11th to 20th	Night	12% 5% 27% 39% 13% 3	760
	21st to end	Night	16% 9% 34% 32% 9% 1	1026
November	1st to 10th	Night	21% 6% 29% 34% 10% 1	975
	11th to 20th	Night	13% 6% 32% 34% 12% 2	1000
	21st to end	Night	10% 8% 35% 36% 10% 1	1000
December	1st to 10th	Night	10% 7% 29% 37% 14% 3	1000
	11th to 20th	Night	12% 8% 29% 39% 11% 1	1000
	21st to end	Night	9% 4 27% 40% 19% 2	1100

Page generated at 2025-03-09 07:08:41. Version ac07

Copyright © 2025 Attilla Danko.