Black Diamond
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Black Diamond 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	7% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	124026
	Night	5% 9% 3 8% 5% 5% 5% 4 6% 8% 41%	123811
Transparency	Day	39% 7% 15% 24% 13% 2	123773
	Night	39% 7% 15% 23% 13% 2	123707
Seeing	Day	13% 11% 28% 30% 14% 4	16251
	Night	10% 9% 31% 34% 13% 2	25303

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	1 7% 6% 12% 8% 9% 5% 4 7% 8% 33%	3179
	11th to 20th	Day	2 8% 4 11% 6% 7% 5% 4 6% 7% 41%	3311
	21st to end	Day	1 7% 2 8% 5% 5% 4 4 6% 11% 47%	4004
February	1st to 10th	Day	3 9% 4 13% 7% 4 6% 5% 7% 9% 32%	1600
	11th to 20th	Day	2 10% 5% 11% 7% 6% 6% 6% 7% 9% 31%	1744
	21st to end	Day	2 10% 5% 12% 6% 6% 5% 4 6% 8% 35%	1625
March	1st to 10th	Day	2 8% 4 10% 5% 6% 5% 4 6% 11% 40%	4518
	11th to 20th	Day	3 10% 4 9% 4 4 5% 4 6% 8% 42%	4928
	21st to end	Day	6% 19% 8% 14% 5% 4 4 4 6% 7% 23%	5590
April	1st to 10th	Day	8% 13% 5% 10% 6% 5% 5% 4 6% 9% 31%	2440
	11th to 20th	Day	9% 14% 4 8% 5% 4 5% 4 6% 8% 31%	2570
	21st to end	Day	11% 12% 3 7% 4 5% 5% 5% 6% 9% 34%	2600
May	1st to 10th	Day	13% 13% 3 6% 4 4 4 3 5% 6% 39%	6130
	11th to 20th	Day	12% 9% 4 9% 7% 6% 6% 5% 7% 8% 27%	6734
	21st to end	Day	12% 6% 3 7% 6% 6% 6% 4 5% 7% 37%	7405
June	1st to 10th	Day	16% 8% 3 7% 6% 5% 6% 5% 7% 8% 30%	3000
	11th to 20th	Day	20% 8% 4 8% 5% 5% 6% 5% 6% 7% 27%	3000
	21st to end	Day	9% 4 2 6% 4 5% 5% 5% 7% 10% 41%	3000
July	1st to 10th	Day	7% 5% 2 5% 4 4 4 4 7% 8% 49%	4890
	11th to 20th	Day	7% 3 1 4 3 3 5% 5% 7% 10% 52%	4890
	21st to end	Day	3 3 1 3 3 3 4 4 6% 9% 59%	4961
August	1st to 10th	Day	9% 6% 2 6% 3 4 5% 5% 8% 9% 43%	4401
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 5% 8% 10% 45%	4066
	21st to end	Day	9% 4 1 4 4 3 4 3 6% 8% 53%	4521
September	1st to 10th	Day	11% 6% 2 6% 4 3 4 3 5% 7% 51%	2342
	11th to 20th	Day	12% 8% 2 5% 4 4 4 4 6% 7% 45%	2090
	21st to end	Day	8% 7% 2 7% 4 4 5% 3 7% 7% 46%	2090
October	1st to 10th	Day	6% 7% 3 7% 4 5% 4 3 6% 9% 46%	3369
	11th to 20th	Day	4 6% 3 7% 5% 4 4 4 8% 11% 44%	2906
	21st to end	Day	6% 8% 3 6% 4 4 5% 4 7% 7% 47%	3101
November	1st to 10th	Day	6% 14% 4 9% 5% 5% 5% 4 6% 7% 35%	1662
	11th to 20th	Day	4 10% 4 7% 5% 4 4 5% 6% 8% 42%	1529
	21st to end	Day	3 11% 4 10% 6% 5% 5% 4 6% 10% 37%	1400
December	1st to 10th	Day	2 8% 4 10% 6% 6% 6% 4 8% 9% 38%	2290
	11th to 20th	Day	2 8% 4 11% 5% 6% 6% 5% 8% 9% 37%	2642
	21st to end	Day	3 9% 5% 9% 5% 4 4 4 5% 8% 43%	3498
January	1st to 10th	Night	2 11% 5% 11% 9% 8% 6% 5% 6% 7% 28%	6766
	11th to 20th	Night	2 7% 4 12% 6% 6% 7% 5% 7% 8% 36%	6629
	21st to end	Night	2 8% 4 10% 6% 5% 5% 4 6% 8% 43%	6952
February	1st to 10th	Night	3 10% 4 11% 6% 6% 7% 3 8% 8% 34%	2800
	11th to 20th	Night	2 13% 6% 10% 6% 6% 5% 4 6% 8% 33%	2542
	21st to end	Night	3 8% 4 12% 8% 7% 5% 4 6% 6% 37%	2002
March	1st to 10th	Night	2 8% 4 11% 7% 6% 6% 4 6% 10% 36%	5376
	11th to 20th	Night	2 9% 3 9% 6% 5% 5% 4 7% 8% 41%	5071
	21st to end	Night	8% 19% 6% 11% 6% 4 4 4 7% 7% 25%	5372
April	1st to 10th	Night	7% 15% 4 9% 4 4 5% 5% 6% 7% 34%	1959
	11th to 20th	Night	9% 12% 5% 10% 6% 4 4 4 5% 5% 34%	1781
	21st to end	Night	9% 12% 4 8% 5% 4 4 4 6% 8% 36%	1800
May	1st to 10th	Night	12% 12% 3 7% 4 4 4 3 5% 6% 40%	3835
	11th to 20th	Night	13% 9% 4 9% 5% 5% 5% 4 6% 9% 30%	3159
	21st to end	Night	10% 6% 3 7% 5% 5% 6% 4 5% 7% 42%	3470
June	1st to 10th	Night	13% 9% 3 6% 5% 4 4 3 7% 8% 37%	1400
	11th to 20th	Night	18% 9% 2 7% 4 4 4 5% 5% 9% 33%	1400
	21st to end	Night	9% 4 1 5% 4 4 5% 4 7% 11% 47%	1400
July	1st to 10th	Night	7% 5% 2 6% 5% 4 3 4 6% 8% 50%	2290
	11th to 20th	Night	6% 2 2 4 4 4 4 4 7% 8% 55%	2290
	21st to end	Night	5% 4 1 4 3 3 3 3 4 7% 62%	2589
August	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 7% 9% 44%	2529
	11th to 20th	Night	6% 5% 2 5% 5% 4 5% 4 7% 9% 47%	2772
	21st to end	Night	8% 3 2 3 3 3 4 3 6% 9% 55%	3146
September	1st to 10th	Night	10% 5% 2 5% 3 3 4 3 5% 7% 53%	1834
	11th to 20th	Night	13% 8% 2 4 2 3 3 2 5% 7% 51%	2090
	21st to end	Night	10% 7% 2 5% 4 3 4 3 6% 8% 49%	2090
October	1st to 10th	Night	7% 6% 2 5% 3 3 4 4 6% 8% 53%	3606
	11th to 20th	Night	5% 7% 2 5% 3 3 4 4 6% 8% 52%	4019
	21st to end	Night	4 9% 3 6% 4 4 5% 4 6% 7% 48%	4457
November	1st to 10th	Night	6% 13% 4 8% 4 4 6% 5% 5% 7% 37%	2617
	11th to 20th	Night	4 9% 3 8% 6% 4 5% 4 7% 8% 43%	2870
	21st to end	Night	3 10% 4 9% 5% 5% 5% 4 7% 9% 40%	3000
December	1st to 10th	Night	2 8% 3 8% 5% 5% 6% 5% 8% 9% 40%	4890
	11th to 20th	Night	2 9% 4 12% 7% 6% 6% 5% 7% 9% 34%	5550
	21st to end	Night	3 11% 4 10% 4 5% 5% 4 6% 8% 41%	7458

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	46% 7% 12% 21% 11% 3	3179
	11th to 20th	Day	40% 5% 13% 20% 18% 3	3332
	21st to end	Day	29% 9% 14% 32% 15% 1	4004
February	1st to 10th	Day	46% 10% 14% 17% 10% 3	1600
	11th to 20th	Day	47% 8% 13% 20% 9% 2	1753
	21st to end	Day	46% 7% 14% 17% 14% 1	1625
March	1st to 10th	Day	39% 5% 16% 22% 17% 1	4518
	11th to 20th	Day	38% 6% 14% 24% 17% 1	4929
	21st to end	Day	58% 7% 14% 14% 6%	5590
April	1st to 10th	Day	50% 7% 16% 18% 9% 1	2440
	11th to 20th	Day	49% 7% 14% 19% 10% 1	2570
	21st to end	Day	46% 8% 15% 20% 9% 1	2600
May	1st to 10th	Day	44% 6% 9% 22% 17% 3	6130
	11th to 20th	Day	49% 9% 13% 21% 7%	6738
	21st to end	Day	42% 7% 18% 21% 12% 1	7406
June	1st to 10th	Day	47% 10% 13% 18% 10% 1	3000
	11th to 20th	Day	52% 9% 13% 16% 8% 2	3000
	21st to end	Day	35% 11% 17% 24% 11% 2	3000
July	1st to 10th	Day	30% 8% 13% 27% 18% 5%	4816
	11th to 20th	Day	25% 7% 18% 32% 16% 2	4809
	21st to end	Day	19% 6% 11% 28% 24% 12%	4804
August	1st to 10th	Day	33% 9% 14% 36% 8% 1	4384
	11th to 20th	Day	30% 9% 24% 28% 8% 2	4105
	21st to end	Day	27% 6% 16% 35% 14% 2	4520
September	1st to 10th	Day	32% 7% 14% 24% 17% 5%	2342
	11th to 20th	Day	37% 8% 14% 26% 13% 2	2090
	21st to end	Day	35% 7% 14% 28% 13% 3	2090
October	1st to 10th	Day	34% 7% 15% 35% 9% 1	3369
	11th to 20th	Day	31% 6% 21% 26% 13% 3	2906
	21st to end	Day	35% 6% 12% 31% 13% 2	3101
November	1st to 10th	Day	47% 8% 13% 19% 11% 1	1662
	11th to 20th	Day	39% 7% 18% 23% 11% 2	1529
	21st to end	Day	40% 9% 18% 20% 9% 3	1400
December	1st to 10th	Day	41% 8% 14% 26% 9% 2	2290
	11th to 20th	Day	41% 9% 21% 20% 8% 1	2644
	21st to end	Day	39% 5% 10% 23% 18% 4	3498
January	1st to 10th	Night	51% 9% 10% 21% 6% 2	6766
	11th to 20th	Night	44% 6% 14% 18% 13% 5%	6670
	21st to end	Night	36% 7% 14% 25% 17% 1	6952
February	1st to 10th	Night	46% 8% 13% 19% 12% 3	2800
	11th to 20th	Night	47% 8% 15% 17% 12% 2	2543
	21st to end	Night	46% 6% 12% 21% 12% 3	2002
March	1st to 10th	Night	41% 7% 17% 20% 13% 2	5376
	11th to 20th	Night	38% 7% 17% 22% 16% 1	5071
	21st to end	Night	56% 8% 13% 13% 9% 1	5372
April	1st to 10th	Night	48% 6% 13% 21% 12% 1	1959
	11th to 20th	Night	49% 7% 12% 21% 9% 1	1781
	21st to end	Night	44% 10% 13% 23% 9% 2	1800
May	1st to 10th	Night	45% 6% 9% 23% 14% 2	3832
	11th to 20th	Night	47% 7% 13% 26% 6% 1	3159
	21st to end	Night	38% 8% 17% 27% 9% 1	3470
June	1st to 10th	Night	43% 7% 14% 24% 11% 1	1400
	11th to 20th	Night	47% 8% 13% 20% 10% 2	1400
	21st to end	Night	30% 9% 20% 27% 13% 2	1400
July	1st to 10th	Night	32% 7% 15% 24% 18% 4	2255
	11th to 20th	Night	24% 7% 21% 33% 11% 4	2248
	21st to end	Night	21% 5% 10% 31% 23% 10%	2507
August	1st to 10th	Night	35% 6% 15% 28% 14% 2	2521
	11th to 20th	Night	31% 8% 22% 27% 10% 1	2799
	21st to end	Night	25% 5% 19% 34% 15% 2	3146
September	1st to 10th	Night	30% 6% 16% 26% 17% 5%	1834
	11th to 20th	Night	33% 6% 14% 28% 14% 4	2090
	21st to end	Night	32% 6% 17% 26% 14% 4	2090
October	1st to 10th	Night	28% 7% 13% 29% 21% 2	3606
	11th to 20th	Night	30% 6% 20% 24% 19% 2	4019
	21st to end	Night	35% 6% 12% 31% 16% 1	4457
November	1st to 10th	Night	45% 8% 13% 20% 12% 2	2617
	11th to 20th	Night	37% 9% 15% 21% 16% 2	2870
	21st to end	Night	40% 8% 16% 21% 12% 4	3000
December	1st to 10th	Night	36% 10% 23% 20% 9% 2	4890
	11th to 20th	Night	44% 10% 21% 17% 8%	5547
	21st to end	Night	39% 5% 11% 24% 16% 4	7458

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	2 7% 30% 38% 16% 9%	200
	11th to 20th	Day	7% 8% 30% 38% 14% 5%	200
	21st to end	Day	7% 4 30% 45% 10% 3	220
February	1st to 10th	Day	8% 12% 39% 29% 11% 3	200
	11th to 20th	Day	5% 9% 40% 34% 11% 2	295
	21st to end	Day	5% 12% 36% 32% 15% 1	389
March	1st to 10th	Day	7% 14% 38% 27% 11% 3	500
	11th to 20th	Day	8% 12% 32% 30% 14% 3	500
	21st to end	Day	11% 13% 35% 26% 10% 4	550
April	1st to 10th	Day	13% 13% 31% 31% 10% 2	500
	11th to 20th	Day	16% 12% 29% 27% 13% 3	492
	21st to end	Day	17% 15% 29% 24% 10% 5%	500
May	1st to 10th	Day	19% 12% 24% 26% 15% 4	517
	11th to 20th	Day	18% 10% 22% 26% 19% 5%	725
	21st to end	Day	19% 10% 19% 29% 18% 4	789
June	1st to 10th	Day	16% 12% 28% 29% 11% 3	730
	11th to 20th	Day	22% 12% 26% 27% 11% 3	730
	21st to end	Day	11% 10% 25% 31% 19% 4	730
July	1st to 10th	Day	10% 8% 23% 35% 20% 5%	721
	11th to 20th	Day	12% 7% 25% 33% 19% 4	718
	21st to end	Day	5% 8% 30% 33% 19% 5%	637
August	1st to 10th	Day	12% 9% 26% 35% 15% 4	487
	11th to 20th	Day	8% 9% 29% 31% 16% 6%	477
	21st to end	Day	11% 11% 30% 28% 16% 5%	539
September	1st to 10th	Day	14% 13% 29% 26% 14% 4	490
	11th to 20th	Day	19% 9% 25% 30% 13% 3	490
	21st to end	Day	13% 19% 30% 26% 9% 3	490
October	1st to 10th	Day	14% 12% 28% 28% 14% 4	421
	11th to 20th	Day	12% 12% 29% 30% 12% 5%	300
	21st to end	Day	15% 15% 32% 25% 10% 3	330
November	1st to 10th	Day	17% 12% 33% 24% 11% 3	300
	11th to 20th	Day	12% 13% 33% 30% 9% 2	264
	21st to end	Day	9% 13% 25% 33% 13% 8%	200
December	1st to 10th	Day	7% 8% 32% 39% 12% 3	200
	11th to 20th	Day	7% 14% 28% 37% 11% 4	200
	21st to end	Day	7% 7% 26% 40% 15% 5%	220
January	1st to 10th	Night	8% 14% 37% 30% 10% 1	1000
	11th to 20th	Night	7% 9% 45% 29% 9% 1	955
	21st to end	Night	6% 15% 37% 33% 8% 2	880
February	1st to 10th	Night	6% 11% 36% 30% 12% 5%	800
	11th to 20th	Night	7% 8% 35% 39% 11% 1	784
	21st to end	Night	6% 6% 36% 32% 16% 4	660
March	1st to 10th	Night	7% 7% 38% 34% 11% 3	800
	11th to 20th	Night	7% 9% 37% 35% 11% 2	800
	21st to end	Night	11% 8% 28% 39% 11% 3	880
April	1st to 10th	Night	13% 7% 33% 30% 14% 2	759
	11th to 20th	Night	15% 7% 33% 34% 10%	593
	21st to end	Night	16% 8% 28% 36% 11% 2	600
May	1st to 10th	Night	17% 8% 23% 35% 16% 2	506
	11th to 20th	Night	14% 6% 24% 34% 21% 1	394
	21st to end	Night	21% 6% 18% 33% 19% 2	433
June	1st to 10th	Night	18% 7% 23% 36% 14% 2	400
	11th to 20th	Night	22% 5% 25% 35% 13% 1	400
	21st to end	Night	9% 4 21% 40% 25% 1	400
July	1st to 10th	Night	12% 4 14% 43% 26% 1	390
	11th to 20th	Night	9% 3 17% 49% 18% 3	388
	21st to end	Night	8% 4 16% 45% 24% 3	416
August	1st to 10th	Night	13% 1 11% 43% 29% 3	378
	11th to 20th	Night	10% 2 17% 43% 24% 3	537
	21st to end	Night	10% 6% 20% 42% 20% 2	627
September	1st to 10th	Night	13% 4 20% 38% 19% 5%	685
	11th to 20th	Night	18% 5% 18% 35% 21% 4	760
	21st to end	Night	13% 7% 29% 36% 11% 5%	760
October	1st to 10th	Night	14% 6% 27% 35% 15% 3	760
	11th to 20th	Night	11% 8% 31% 33% 14% 3	760
	21st to end	Night	10% 10% 33% 36% 10% 1	836
November	1st to 10th	Night	13% 13% 36% 29% 8% 1	862
	11th to 20th	Night	9% 18% 39% 28% 5% 1	1000
	21st to end	Night	7% 12% 40% 31% 8% 2	1000
December	1st to 10th	Night	7% 9% 39% 32% 10% 3	1000
	11th to 20th	Night	5% 11% 40% 34% 8% 2	1000
	21st to end	Night	8% 11% 38% 29% 11% 2	1100

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