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 2024-03-28.

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	31% 13% 3 5% 4 2 3 4 4 5% 26%	78845
	Night	33% 13% 3 5% 4 2 3 3 4 4 25%	81071
Transparency	Day	63% 5% 10% 14% 7% 1	78564
	Night	63% 5% 10% 13% 7% 1	80932
Seeing	Day	36% 11% 22% 22% 8% 2	14091
	Night	40% 7% 17% 23% 12% 1	22696

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% 24% 4 5% 4 2 2 2 2 2 10%	2356
	11th to 20th	Day	30% 13% 6% 6% 4 3 3 3 4 6% 22%	2516
	21st to end	Day	53% 15% 2 4 2 1 2 3 2 3 12%	2801
February	1st to 10th	Day	36% 15% 4 8% 5% 3 4 4 3 4 14%	1710
	11th to 20th	Day	35% 20% 4 5% 4 2 2 3 2 4 19%	1665
	21st to end	Day	30% 23% 4 6% 5% 3 3 2 3 3 18%	1496
March	1st to 10th	Day	32% 18% 3 7% 5% 3 3 3 4 4 17%	1909
	11th to 20th	Day	35% 18% 3 6% 4 3 3 3 3 4 19%	2090
	21st to end	Day	36% 21% 5% 6% 5% 3 4 3 4 4 10%	2352
April	1st to 10th	Day	32% 20% 4 6% 5% 3 5% 4 4 4 13%	2160
	11th to 20th	Day	32% 17% 4 6% 5% 3 4 4 3 4 17%	2132
	21st to end	Day	34% 16% 5% 7% 5% 3 4 4 3 4 14%	2414
May	1st to 10th	Day	28% 13% 3 6% 5% 3 4 4 4 5% 22%	2520
	11th to 20th	Day	30% 11% 4 6% 5% 3 4 4 4 6% 23%	2477
	21st to end	Day	38% 12% 3 7% 5% 2 3 3 3 4 19%	2723
June	1st to 10th	Day	36% 9% 3 5% 5% 4 3 4 4 5% 21%	2535
	11th to 20th	Day	40% 11% 3 7% 6% 3 4 4 4 4 14%	2700
	21st to end	Day	24% 8% 3 6% 5% 3 5% 5% 5% 6% 31%	2700
July	1st to 10th	Day	19% 7% 2 5% 4 3 5% 4 4 5% 41%	2700
	11th to 20th	Day	17% 7% 2 4 4 2 4 5% 4 7% 43%	2557
	21st to end	Day	16% 4 2 3 4 2 3 3 3 6% 54%	2741
August	1st to 10th	Day	17% 6% 2 5% 4 2 3 4 4 5% 48%	2507
	11th to 20th	Day	14% 5% 2 4 4 2 4 4 5% 8% 49%	2302
	21st to end	Day	20% 8% 2 6% 4 2 4 4 5% 7% 39%	2450
September	1st to 10th	Day	18% 6% 3 5% 4 3 3 4 4 6% 44%	2160
	11th to 20th	Day	23% 7% 3 5% 5% 3 3 5% 4 5% 36%	2160
	21st to end	Day	28% 9% 2 5% 3 2 3 3 3 5% 38%	1914
October	1st to 10th	Day	29% 10% 3 4 5% 3 4 4 4 6% 30%	1800
	11th to 20th	Day	35% 9% 2 4 4 2 3 3 4 6% 28%	1800
	21st to end	Day	41% 9% 3 5% 3 2 3 2 3 4 24%	1841
November	1st to 10th	Day	45% 12% 3 5% 6% 2 2 4 3 3 15%	1547
	11th to 20th	Day	44% 14% 3 5% 3 1 2 2 3 3 19%	1520
	21st to end	Day	41% 15% 3 5% 4 1 3 2 3 4 18%	1520
December	1st to 10th	Day	37% 15% 3 5% 4 1 2 2 3 3 24%	1520
	11th to 20th	Day	37% 17% 4 5% 4 2 3 4 2 3 19%	1925
	21st to end	Day	32% 20% 4 8% 6% 2 3 3 4 4 13%	2625
January	1st to 10th	Night	42% 21% 4 5% 4 2 3 3 3 3 11%	4566
	11th to 20th	Night	34% 14% 4 5% 4 2 3 3 4 6% 22%	4382
	21st to end	Night	47% 13% 3 5% 4 2 3 3 3 3 13%	4853
February	1st to 10th	Night	35% 17% 3 5% 5% 3 4 3 3 4 18%	2470
	11th to 20th	Night	32% 20% 3 5% 4 2 4 3 3 4 20%	2402
	21st to end	Night	32% 20% 3 6% 4 2 4 3 3 4 19%	1957
March	1st to 10th	Night	32% 17% 3 6% 4 2 3 4 4 6% 20%	2267
	11th to 20th	Night	35% 16% 3 5% 4 1 3 3 3 6% 22%	2089
	21st to end	Night	37% 20% 3 6% 4 2 3 4 4 4 14%	2159
April	1st to 10th	Night	32% 18% 3 7% 5% 3 4 4 4 6% 15%	1800
	11th to 20th	Night	32% 15% 3 5% 5% 3 4 3 4 5% 22%	1777
	21st to end	Night	36% 12% 3 6% 5% 3 3 4 4 5% 18%	1545
May	1st to 10th	Night	26% 11% 2 6% 5% 2 4 3 4 5% 33%	1440
	11th to 20th	Night	29% 8% 3 6% 4 2 4 5% 5% 5% 28%	1416
	21st to end	Night	32% 8% 3 8% 5% 3 4 5% 5% 6% 22%	1552
June	1st to 10th	Night	33% 8% 3 6% 4 3 4 4 4 5% 27%	1421
	11th to 20th	Night	36% 9% 3 5% 4 4 4 3 5% 6% 22%	1260
	21st to end	Night	21% 6% 3 5% 5% 2 4 4 5% 6% 39%	1260
July	1st to 10th	Night	16% 6% 2 5% 5% 2 4 4 5% 4 48%	1260
	11th to 20th	Night	15% 4 2 4 3 2 3 4 3 6% 53%	1399
	21st to end	Night	13% 5% 2 3 2 2 3 2 4 5% 60%	1576
August	1st to 10th	Night	13% 3 1 3 3 2 3 3 4 6% 59%	1432
	11th to 20th	Night	12% 4 1 3 3 2 3 3 4 6% 59%	1595
	21st to end	Night	17% 7% 3 4 4 2 3 3 4 4 50%	1905
September	1st to 10th	Night	17% 5% 2 3 4 2 3 3 3 5% 52%	1800
	11th to 20th	Night	25% 8% 2 5% 5% 2 3 3 3 5% 39%	1800
	21st to end	Night	27% 8% 3 5% 3 2 3 3 4 6% 37%	2042
October	1st to 10th	Night	28% 7% 2 4 4 3 4 3 4 5% 35%	2160
	11th to 20th	Night	35% 8% 2 5% 3 2 4 4 4 5% 29%	2160
	21st to end	Night	37% 10% 2 4 3 2 3 3 4 4 27%	2515
November	1st to 10th	Night	43% 12% 3 5% 5% 2 3 3 4 4 16%	2529
	11th to 20th	Night	44% 11% 3 4 3 2 3 3 2 3 21%	2660
	21st to end	Night	41% 13% 3 5% 4 2 3 3 3 4 19%	2660
December	1st to 10th	Night	36% 13% 3 4 5% 2 4 3 4 4 23%	2660
	11th to 20th	Night	39% 17% 4 5% 4 2 3 3 3 4 16%	3280
	21st to end	Night	35% 20% 4 6% 6% 3 4 3 3 3 12%	5022

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	83% 2 3 8% 2 2	2356
	11th to 20th	Day	65% 4 7% 14% 8% 2	2540
	21st to end	Day	80% 4 5% 6% 5%	2801
February	1st to 10th	Day	75% 5% 7% 8% 3 1	1710
	11th to 20th	Day	73% 5% 6% 9% 6% 2	1674
	21st to end	Day	72% 5% 7% 11% 4	1496
March	1st to 10th	Day	72% 4 9% 11% 4	1909
	11th to 20th	Day	72% 4 7% 12% 3 1	2090
	21st to end	Day	81% 4 8% 6% 1	2352
April	1st to 10th	Day	76% 6% 10% 8% 1	2160
	11th to 20th	Day	72% 5% 7% 11% 4 1	2132
	21st to end	Day	75% 5% 7% 7% 4 1	2414
May	1st to 10th	Day	64% 7% 12% 13% 4	2520
	11th to 20th	Day	63% 6% 16% 12% 3	2480
	21st to end	Day	70% 6% 12% 12% 1	2724
June	1st to 10th	Day	66% 6% 14% 9% 5%	2535
	11th to 20th	Day	74% 7% 9% 8% 2	2700
	21st to end	Day	53% 9% 14% 18% 6% 1	2700
July	1st to 10th	Day	45% 7% 11% 25% 11% 1	2626
	11th to 20th	Day	41% 6% 12% 27% 14%	2479
	21st to end	Day	31% 5% 11% 34% 15% 4	2589
August	1st to 10th	Day	39% 6% 14% 27% 13% 2	2490
	11th to 20th	Day	33% 7% 17% 28% 12% 2	2302
	21st to end	Day	46% 5% 14% 26% 8% 1	2450
September	1st to 10th	Day	40% 7% 12% 23% 13% 5%	2160
	11th to 20th	Day	51% 6% 9% 17% 15% 3	2160
	21st to end	Day	51% 5% 14% 15% 10% 3	1914
October	1st to 10th	Day	56% 5% 11% 20% 7% 1	1800
	11th to 20th	Day	58% 4 12% 17% 8% 1	1800
	21st to end	Day	66% 4 7% 12% 11% 1	1841
November	1st to 10th	Day	74% 5% 8% 7% 5%	1547
	11th to 20th	Day	73% 4 6% 8% 7% 2	1520
	21st to end	Day	73% 4 4 8% 9% 3	1520
December	1st to 10th	Day	68% 5% 6% 12% 9% 1	1520
	11th to 20th	Day	71% 5% 9% 9% 5% 1	1928
	21st to end	Day	73% 7% 7% 9% 4	2625
January	1st to 10th	Night	81% 4 4 7% 2 2	4566
	11th to 20th	Night	66% 3 5% 14% 10% 2	4420
	21st to end	Night	77% 5% 6% 6% 5% 1	4853
February	1st to 10th	Night	72% 5% 7% 10% 4 1	2470
	11th to 20th	Night	69% 7% 7% 9% 7% 2	2404
	21st to end	Night	71% 4 8% 12% 4 1	1957
March	1st to 10th	Night	67% 6% 9% 12% 6%	2267
	11th to 20th	Night	67% 5% 9% 14% 4 1	2090
	21st to end	Night	75% 5% 10% 8% 1	2159
April	1st to 10th	Night	71% 6% 12% 10% 1	1800
	11th to 20th	Night	67% 5% 10% 11% 6% 1	1777
	21st to end	Night	68% 7% 9% 10% 5% 1	1545
May	1st to 10th	Night	56% 5% 18% 15% 5%	1440
	11th to 20th	Night	58% 6% 18% 12% 5% 1	1416
	21st to end	Night	64% 8% 14% 13% 1	1552
June	1st to 10th	Night	61% 5% 15% 13% 5%	1421
	11th to 20th	Night	65% 8% 13% 11% 3	1260
	21st to end	Night	46% 8% 19% 18% 6% 3	1260
July	1st to 10th	Night	42% 6% 14% 23% 12% 3	1225
	11th to 20th	Night	36% 5% 15% 28% 14% 1	1354
	21st to end	Night	26% 6% 12% 33% 18% 5%	1488
August	1st to 10th	Night	28% 6% 17% 29% 16% 3	1424
	11th to 20th	Night	28% 5% 19% 29% 15% 4	1595
	21st to end	Night	39% 5% 14% 29% 11% 2	1905
September	1st to 10th	Night	36% 5% 14% 25% 15% 5%	1800
	11th to 20th	Night	50% 5% 8% 18% 15% 5%	1800
	21st to end	Night	51% 6% 11% 18% 11% 4	2042
October	1st to 10th	Night	52% 5% 12% 20% 9% 1	2160
	11th to 20th	Night	59% 5% 9% 17% 9% 1	2160
	21st to end	Night	62% 4 8% 13% 12% 1	2515
November	1st to 10th	Night	73% 5% 8% 9% 4 1	2529
	11th to 20th	Night	72% 4 7% 9% 7% 2	2660
	21st to end	Night	71% 3 5% 9% 9% 2	2660
December	1st to 10th	Night	67% 4 8% 10% 8% 2	2660
	11th to 20th	Night	73% 4 8% 10% 4 1	3276
	21st to end	Night	76% 6% 7% 8% 3	5022

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% 16% 10% 8% 1	180
	11th to 20th	Day	48% 11% 19% 15% 7%	180
	21st to end	Day	56% 8% 14% 17% 6%	205
February	1st to 10th	Day	49% 11% 19% 16% 3 1	370
	11th to 20th	Day	53% 11% 19% 15% 3	365
	21st to end	Day	44% 12% 25% 16% 3	348
March	1st to 10th	Day	40% 15% 25% 16% 5%	460
	11th to 20th	Day	39% 12% 21% 19% 8% 1	460
	21st to end	Day	50% 13% 20% 14% 3	491
April	1st to 10th	Day	47% 13% 19% 15% 5% 1	420
	11th to 20th	Day	44% 16% 18% 16% 6% 1	412
	21st to end	Day	43% 15% 20% 17% 4	420
May	1st to 10th	Day	32% 12% 26% 24% 5%	423
	11th to 20th	Day	39% 13% 19% 20% 7% 3	448
	21st to end	Day	40% 14% 17% 21% 7% 2	485
June	1st to 10th	Day	34% 14% 21% 21% 9% 2	465
	11th to 20th	Day	37% 12% 27% 18% 5% 1	630
	21st to end	Day	23% 11% 25% 31% 8% 2	630
July	1st to 10th	Day	21% 10% 26% 32% 10% 2	621
	11th to 20th	Day	19% 13% 27% 28% 11% 3	478
	21st to end	Day	16% 6% 25% 33% 15% 4	487
August	1st to 10th	Day	19% 6% 25% 31% 15% 5%	447
	11th to 20th	Day	13% 9% 22% 36% 16% 5%	437
	21st to end	Day	22% 9% 24% 31% 11% 3	495
September	1st to 10th	Day	23% 13% 26% 28% 10% 1	450
	11th to 20th	Day	26% 10% 22% 28% 12% 2	450
	21st to end	Day	30% 10% 24% 22% 11% 2	450
October	1st to 10th	Day	37% 6% 20% 24% 11% 2	450
	11th to 20th	Day	41% 8% 19% 19% 11% 2	450
	21st to end	Day	47% 8% 20% 15% 8% 2	356
November	1st to 10th	Day	51% 9% 20% 17% 4	210
	11th to 20th	Day	52% 11% 13% 14% 8% 2	180
	21st to end	Day	52% 6% 19% 11% 8% 4	180
December	1st to 10th	Day	59% 7% 9% 14% 7% 4	180
	11th to 20th	Day	51% 10% 21% 13% 4	180
	21st to end	Day	49% 7% 24% 16% 4 1	198
January	1st to 10th	Night	57% 8% 17% 14% 4	804
	11th to 20th	Night	46% 10% 22% 16% 5% 1	760
	21st to end	Night	48% 8% 22% 19% 3	836
February	1st to 10th	Night	47% 9% 24% 16% 5%	760
	11th to 20th	Night	46% 7% 23% 21% 4	744
	21st to end	Night	47% 8% 21% 19% 5%	628
March	1st to 10th	Night	43% 10% 19% 21% 8%	760
	11th to 20th	Night	47% 6% 21% 20% 6% 1	760
	21st to end	Night	51% 8% 18% 17% 5% 1	820
April	1st to 10th	Night	44% 8% 16% 22% 8% 2	720
	11th to 20th	Night	40% 12% 17% 23% 7% 1	709
	21st to end	Night	44% 7% 17% 24% 8% 1	465
May	1st to 10th	Night	31% 7% 14% 31% 16%	360
	11th to 20th	Night	33% 3 12% 30% 19% 1	354
	21st to end	Night	34% 8% 11% 25% 21% 2	389
June	1st to 10th	Night	36% 7% 10% 25% 19% 3	360
	11th to 20th	Night	39% 7% 13% 26% 14% 1	360
	21st to end	Night	22% 4 14% 32% 26% 2	360
July	1st to 10th	Night	17% 4 6% 33% 35% 5%	350
	11th to 20th	Night	15% 5% 7% 38% 31% 3	348
	21st to end	Night	15% 3 6% 34% 38% 5%	394
August	1st to 10th	Night	13% 2 4 32% 43% 6%	358
	11th to 20th	Night	14% 4 9% 31% 36% 7%	527
	21st to end	Night	21% 4 10% 34% 29% 2	717
September	1st to 10th	Night	20% 4 13% 37% 23% 4	720
	11th to 20th	Night	29% 5% 12% 26% 25% 4	720
	21st to end	Night	32% 3 13% 34% 17% 1	720
October	1st to 10th	Night	31% 6% 18% 30% 15% 1	720
	11th to 20th	Night	39% 5% 20% 24% 11% 1	720
	21st to end	Night	45% 5% 23% 22% 5% 1	792
November	1st to 10th	Night	51% 7% 24% 16% 2	740
	11th to 20th	Night	52% 10% 17% 16% 4	760
	21st to end	Night	49% 9% 21% 17% 3	760
December	1st to 10th	Night	42% 9% 23% 19% 6% 1	760
	11th to 20th	Night	53% 8% 24% 11% 3 1	760
	21st to end	Night	57% 8% 20% 13% 2	881

Page generated at 2024-03-24 20:37:02. Version ac07

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