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 2026-07-16.

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% 8% 3 9% 6% 5% 5% 4 6% 8% 35%	141643
	Night	7% 9% 4 10% 6% 6% 6% 4 7% 8% 33%	139687
Transparency	Day	44% 8% 16% 23% 8% 1	141341
	Night	46% 8% 15% 22% 9% 1	139575
Seeing	Day	13% 8% 27% 31% 17% 5%	17983
	Night	12% 6% 29% 37% 15% 2	27120

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	6% 12% 5% 16% 11% 8% 6% 4 5% 6% 21%	4125
	11th to 20th	Day	3 6% 2 11% 8% 9% 9% 5% 8% 12% 26%	4119
	21st to end	Day	6% 14% 6% 15% 7% 4 4 4 6% 9% 25%	4531
February	1st to 10th	Day	7% 13% 5% 14% 8% 7% 7% 5% 5% 7% 22%	1794
	11th to 20th	Day	7% 11% 4 11% 9% 7% 7% 5% 7% 8% 25%	1845
	21st to end	Day	4 9% 5% 13% 9% 7% 8% 4 6% 8% 24%	1557
March	1st to 10th	Day	4 11% 5% 13% 9% 8% 7% 6% 7% 9% 23%	4629
	11th to 20th	Day	6% 10% 4 14% 9% 6% 5% 3 6% 8% 29%	5109
	21st to end	Day	9% 13% 5% 12% 6% 5% 5% 4 5% 6% 31%	5786
April	1st to 10th	Day	6% 10% 3 8% 6% 6% 4 4 6% 8% 37%	2520
	11th to 20th	Day	10% 11% 4 9% 6% 6% 5% 4 6% 7% 31%	2679
	21st to end	Day	12% 10% 3 7% 4 5% 5% 4 7% 8% 34%	2893
May	1st to 10th	Day	8% 7% 3 6% 5% 5% 5% 4 6% 7% 43%	8224
	11th to 20th	Day	16% 10% 3 9% 5% 5% 6% 4 7% 8% 27%	8415
	21st to end	Day	10% 6% 3 8% 5% 5% 5% 5% 7% 9% 37%	9769
June	1st to 10th	Day	13% 8% 3 8% 6% 6% 6% 4 6% 7% 33%	3360
	11th to 20th	Day	20% 9% 3 9% 6% 4 4 5% 6% 8% 26%	3360
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 7% 8% 35%	3360
July	1st to 10th	Day	7% 5% 2 4 4 4 5% 4 7% 11% 48%	7151
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 9% 39%	6856
	21st to end	Day	7% 4 2 5% 4 5% 6% 5% 7% 9% 47%	7376
August	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 5% 8% 46%	4599
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 7% 11% 44%	4333
	21st to end	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 46%	4648
September	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 5% 9% 45%	2400
	11th to 20th	Day	15% 6% 3 7% 4 5% 4 3 6% 7% 40%	2400
	21st to end	Day	8% 7% 2 5% 4 4 4 4 5% 8% 48%	2105
October	1st to 10th	Day	8% 9% 3 7% 4 5% 5% 3 6% 7% 43%	3270
	11th to 20th	Day	6% 8% 3 8% 5% 4 4 4 8% 11% 39%	3237
	21st to end	Day	6% 9% 4 11% 5% 6% 6% 5% 6% 7% 35%	2969
November	1st to 10th	Day	12% 14% 6% 11% 6% 5% 4 5% 4 6% 26%	1648
	11th to 20th	Day	10% 15% 5% 15% 7% 5% 4 5% 5% 6% 23%	1680
	21st to end	Day	7% 12% 6% 14% 9% 7% 5% 5% 6% 8% 22%	1470
December	1st to 10th	Day	6% 14% 5% 12% 8% 7% 7% 6% 7% 6% 23%	2020
	11th to 20th	Day	6% 12% 8% 17% 10% 10% 6% 5% 5% 5% 18%	2319
	21st to end	Day	3 8% 4 14% 9% 7% 5% 4 6% 7% 34%	3087
January	1st to 10th	Night	6% 12% 5% 15% 9% 8% 7% 5% 7% 8% 19%	8768
	11th to 20th	Night	3 8% 4 13% 7% 8% 10% 6% 7% 9% 25%	8779
	21st to end	Night	5% 11% 5% 14% 8% 5% 5% 4 7% 9% 27%	9650
February	1st to 10th	Night	6% 12% 5% 12% 9% 7% 6% 5% 7% 8% 22%	2843
	11th to 20th	Night	6% 10% 4 9% 8% 7% 6% 6% 8% 7% 30%	2662
	21st to end	Night	2 9% 5% 11% 8% 6% 6% 5% 8% 10% 30%	2249
March	1st to 10th	Night	5% 10% 4 12% 7% 8% 8% 6% 9% 10% 23%	5467
	11th to 20th	Night	6% 11% 5% 13% 7% 6% 6% 5% 7% 9% 26%	5110
	21st to end	Night	9% 10% 4 11% 7% 5% 6% 5% 7% 8% 27%	5423
April	1st to 10th	Night	5% 10% 2 8% 6% 6% 6% 5% 7% 8% 38%	2100
	11th to 20th	Night	9% 11% 3 8% 7% 5% 5% 4 8% 8% 32%	1891
	21st to end	Night	13% 9% 3 9% 7% 5% 4 3 4 7% 35%	1727
May	1st to 10th	Night	9% 8% 3 7% 5% 3 3 4 5% 8% 45%	4724
	11th to 20th	Night	15% 11% 4 8% 5% 5% 6% 4 7% 8% 28%	4448
	21st to end	Night	10% 8% 2 8% 6% 4 5% 5% 6% 8% 37%	4402
June	1st to 10th	Night	11% 7% 3 8% 6% 6% 5% 5% 6% 8% 36%	1260
	11th to 20th	Night	15% 9% 2 8% 5% 5% 5% 5% 6% 9% 30%	1260
	21st to end	Night	12% 5% 2 6% 5% 6% 5% 3 6% 9% 42%	1260
July	1st to 10th	Night	7% 4 2 6% 3 4 5% 4 7% 11% 49%	3012
	11th to 20th	Night	7% 4 2 5% 5% 5% 5% 4 6% 9% 47%	3215
	21st to end	Night	5% 4 2 5% 4 5% 5% 4 6% 9% 51%	3467
August	1st to 10th	Night	8% 4 2 6% 4 4 5% 4 6% 7% 48%	2559
	11th to 20th	Night	7% 4 2 6% 5% 4 6% 4 8% 10% 44%	2723
	21st to end	Night	9% 4 2 5% 3 4 3 3 6% 8% 51%	3261
September	1st to 10th	Night	9% 6% 2 5% 4 3 5% 3 7% 8% 49%	2000
	11th to 20th	Night	12% 6% 2 5% 3 4 4 4 6% 7% 45%	2000
	21st to end	Night	7% 6% 2 5% 4 4 5% 4 6% 8% 50%	2291
October	1st to 10th	Night	9% 9% 3 6% 4 3 4 3 5% 8% 46%	3920
	11th to 20th	Night	6% 7% 2 6% 4 5% 5% 4 6% 8% 48%	3898
	21st to end	Night	7% 10% 4 9% 4 5% 5% 3 6% 8% 38%	4797
November	1st to 10th	Night	15% 14% 5% 9% 6% 5% 4 4 6% 6% 25%	2870
	11th to 20th	Night	8% 13% 4 16% 7% 6% 6% 4 6% 6% 25%	2940
	21st to end	Night	6% 13% 5% 14% 8% 5% 6% 3 7% 9% 25%	3145
December	1st to 10th	Night	7% 13% 5% 14% 8% 8% 7% 5% 6% 6% 21%	5380
	11th to 20th	Night	5% 13% 6% 16% 8% 8% 7% 5% 7% 8% 17%	6093
	21st to end	Night	3 7% 4 13% 10% 6% 5% 4 5% 6% 37%	8093

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	62% 6% 8% 17% 7%	4125
	11th to 20th	Day	49% 7% 14% 18% 12%	4140
	21st to end	Day	55% 6% 15% 15% 9%	4531
February	1st to 10th	Day	58% 7% 12% 14% 9%	1794
	11th to 20th	Day	52% 7% 14% 17% 9% 1	1854
	21st to end	Day	54% 8% 12% 19% 7%	1557
March	1st to 10th	Day	54% 7% 19% 14% 5%	4629
	11th to 20th	Day	50% 9% 16% 18% 7%	5110
	21st to end	Day	52% 7% 10% 19% 12%	5786
April	1st to 10th	Day	43% 8% 17% 20% 11%	2520
	11th to 20th	Day	49% 7% 14% 20% 9% 1	2679
	21st to end	Day	45% 8% 15% 22% 9% 1	2893
May	1st to 10th	Day	37% 7% 10% 28% 14% 4	8224
	11th to 20th	Day	51% 7% 11% 22% 8%	8419
	21st to end	Day	38% 10% 14% 25% 11% 1	9770
June	1st to 10th	Day	47% 10% 15% 18% 9% 1	3360
	11th to 20th	Day	54% 9% 14% 16% 7%	3360
	21st to end	Day	45% 9% 18% 22% 6%	3360
July	1st to 10th	Day	28% 9% 28% 30% 6%	7077
	11th to 20th	Day	37% 10% 17% 32% 4	6775
	21st to end	Day	30% 10% 22% 31% 6%	7169
August	1st to 10th	Day	36% 8% 21% 31% 4	4581
	11th to 20th	Day	31% 10% 24% 30% 6%	4375
	21st to end	Day	34% 7% 23% 30% 5%	4647
September	1st to 10th	Day	36% 8% 18% 25% 11% 2	2400
	11th to 20th	Day	43% 7% 17% 27% 6%	2400
	21st to end	Day	33% 7% 19% 28% 11% 1	2105
October	1st to 10th	Day	38% 7% 22% 24% 9%	3270
	11th to 20th	Day	37% 7% 16% 34% 6%	3237
	21st to end	Day	46% 8% 19% 24% 4	2969
November	1st to 10th	Day	58% 8% 13% 15% 6%	1648
	11th to 20th	Day	60% 9% 13% 13% 5%	1680
	21st to end	Day	59% 8% 14% 16% 3	1470
December	1st to 10th	Day	57% 11% 13% 13% 5% 1	2020
	11th to 20th	Day	67% 6% 12% 11% 4	2320
	21st to end	Day	47% 7% 12% 19% 13% 1	3087
January	1st to 10th	Night	58% 7% 12% 14% 8%	8769
	11th to 20th	Night	52% 7% 14% 16% 11%	8820
	21st to end	Night	51% 7% 18% 15% 8%	9650
February	1st to 10th	Night	55% 7% 12% 17% 9%	2843
	11th to 20th	Night	48% 8% 14% 20% 10%	2663
	21st to end	Night	45% 7% 16% 22% 10%	2249
March	1st to 10th	Night	50% 9% 17% 16% 7%	5467
	11th to 20th	Night	49% 9% 16% 19% 7%	5110
	21st to end	Night	49% 8% 12% 19% 12%	5423
April	1st to 10th	Night	41% 8% 15% 22% 14% 1	2100
	11th to 20th	Night	46% 6% 14% 22% 11% 1	1891
	21st to end	Night	48% 6% 11% 22% 11% 1	1727
May	1st to 10th	Night	35% 6% 9% 30% 18% 2	4724
	11th to 20th	Night	49% 8% 11% 22% 9% 1	4448
	21st to end	Night	40% 9% 12% 26% 12% 1	4406
June	1st to 10th	Night	45% 7% 15% 23% 9% 2	1260
	11th to 20th	Night	48% 10% 15% 19% 8%	1260
	21st to end	Night	37% 9% 22% 23% 8% 1	1260
July	1st to 10th	Night	28% 8% 26% 32% 6%	2978
	11th to 20th	Night	31% 8% 17% 37% 8%	3173
	21st to end	Night	29% 8% 22% 32% 8%	3369
August	1st to 10th	Night	33% 9% 23% 30% 6%	2552
	11th to 20th	Night	32% 8% 24% 28% 7% 1	2747
	21st to end	Night	30% 6% 21% 35% 8%	3261
September	1st to 10th	Night	31% 8% 16% 30% 11% 4	2000
	11th to 20th	Night	36% 8% 16% 30% 9% 1	2000
	21st to end	Night	31% 7% 17% 31% 13% 2	2291
October	1st to 10th	Night	37% 5% 22% 30% 6%	3920
	11th to 20th	Night	33% 5% 14% 35% 12%	3898
	21st to end	Night	43% 7% 21% 24% 5%	4797
November	1st to 10th	Night	58% 6% 13% 16% 6% 1	2870
	11th to 20th	Night	59% 8% 14% 14% 5%	2940
	21st to end	Night	54% 9% 17% 16% 4	3145
December	1st to 10th	Night	61% 9% 13% 12% 4 1	5380
	11th to 20th	Night	62% 8% 15% 12% 4	6091
	21st to end	Night	46% 7% 11% 23% 13% 1	8093

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	12% 9% 24% 35% 15% 6%	220
	11th to 20th	Day	11% 5% 31% 29% 19% 4	220
	21st to end	Day	11% 12% 27% 28% 17% 5%	242
February	1st to 10th	Day	10% 10% 33% 32% 12% 3	330
	11th to 20th	Day	8% 5% 29% 35% 19% 4	330
	21st to end	Day	5% 9% 27% 36% 19% 4	273
March	1st to 10th	Day	10% 10% 27% 36% 16% 2	361
	11th to 20th	Day	11% 8% 36% 28% 12% 5%	540
	21st to end	Day	13% 7% 32% 33% 14% 3	594
April	1st to 10th	Day	9% 10% 33% 32% 11% 4	540
	11th to 20th	Day	13% 12% 32% 29% 11% 3	532
	21st to end	Day	15% 10% 29% 28% 14% 3	703
May	1st to 10th	Day	12% 11% 25% 31% 17% 4	753
	11th to 20th	Day	12% 10% 23% 31% 18% 6%	775
	21st to end	Day	12% 8% 24% 29% 19% 8%	844
June	1st to 10th	Day	16% 11% 27% 31% 12% 3	780
	11th to 20th	Day	22% 10% 25% 23% 16% 4	780
	21st to end	Day	15% 8% 21% 28% 20% 7%	780
July	1st to 10th	Day	12% 5% 20% 35% 22% 6%	771
	11th to 20th	Day	10% 8% 23% 30% 22% 7%	744
	21st to end	Day	8% 5% 22% 32% 25% 7%	794
August	1st to 10th	Day	13% 5% 21% 29% 23% 9%	726
	11th to 20th	Day	9% 8% 21% 34% 24% 4	630
	21st to end	Day	13% 5% 26% 29% 21% 6%	583
September	1st to 10th	Day	14% 9% 28% 30% 15% 5%	530
	11th to 20th	Day	20% 7% 26% 30% 15% 3	530
	21st to end	Day	14% 11% 26% 30% 16% 2	430
October	1st to 10th	Day	14% 5% 28% 37% 10% 6%	330
	11th to 20th	Day	13% 10% 31% 33% 11% 3	330
	21st to end	Day	13% 12% 35% 27% 11% 2	363
November	1st to 10th	Day	17% 8% 30% 31% 11% 2	330
	11th to 20th	Day	16% 11% 34% 29% 9% 1	330
	21st to end	Day	6% 12% 31% 37% 11% 3	283
December	1st to 10th	Day	9% 7% 23% 37% 18% 6%	220
	11th to 20th	Day	9% 10% 37% 33% 10% 1	220
	21st to end	Day	8% 6% 26% 35% 16% 9%	242
January	1st to 10th	Night	10% 7% 32% 36% 15% 1	1050
	11th to 20th	Night	10% 7% 32% 39% 10% 1	1050
	21st to end	Night	13% 8% 35% 33% 10% 1	1155
February	1st to 10th	Night	10% 6% 28% 37% 17% 1	952
	11th to 20th	Night	9% 5% 28% 39% 16% 2	824
	21st to end	Night	5% 4 23% 40% 24% 3	692
March	1st to 10th	Night	10% 6% 30% 38% 14% 2	840
	11th to 20th	Night	9% 7% 35% 35% 13% 1	840
	21st to end	Night	12% 7% 28% 38% 14% 2	840
April	1st to 10th	Night	9% 8% 29% 38% 14% 2	630
	11th to 20th	Night	13% 8% 34% 30% 13% 1	623
	21st to end	Night	15% 8% 28% 36% 13% 1	630
May	1st to 10th	Night	13% 7% 27% 37% 14% 2	630
	11th to 20th	Night	13% 7% 28% 33% 17% 2	538
	21st to end	Night	16% 7% 22% 35% 18% 2	455
June	1st to 10th	Night	14% 7% 29% 33% 15% 2	420
	11th to 20th	Night	20% 4 24% 32% 18% 3	420
	21st to end	Night	11% 5% 21% 40% 21% 2	420
July	1st to 10th	Night	9% 4 18% 44% 23% 1	410
	11th to 20th	Night	11% 4 22% 43% 19% 1	398
	21st to end	Night	7% 3 22% 42% 24% 3	438
August	1st to 10th	Night	8% 5% 20% 36% 29% 3	560
	11th to 20th	Night	8% 5% 20% 39% 25% 2	594
	21st to end	Night	9% 4 24% 39% 22% 2	660
September	1st to 10th	Night	12% 3 27% 40% 17% 2	600
	11th to 20th	Night	14% 4 23% 41% 16% 1	600
	21st to end	Night	10% 6% 31% 39% 12% 1	791
October	1st to 10th	Night	17% 7% 29% 35% 13% 1	800
	11th to 20th	Night	13% 5% 28% 39% 13% 3	800
	21st to end	Night	16% 9% 34% 32% 9% 1	1080
November	1st to 10th	Night	21% 7% 29% 33% 9% 1	1025
	11th to 20th	Night	13% 7% 33% 34% 11% 2	1050
	21st to end	Night	9% 8% 35% 36% 11% 1	1050
December	1st to 10th	Night	10% 6% 30% 37% 14% 3	1050
	11th to 20th	Night	12% 8% 30% 38% 11% 1	1050
	21st to end	Night	9% 4 27% 39% 18% 2	1155

Page generated at 2026-07-13 01:14:53. Version ac07

Copyright © 2026 Attilla Danko.