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 2024-04-10.

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	8% 8% 4 7% 6% 3 5% 5% 6% 8% 38%	82570
	Night	6% 9% 4 7% 6% 4 5% 5% 6% 8% 40%	86227
Transparency	Day	41% 9% 15% 22% 11% 2	82278
	Night	40% 8% 15% 22% 12% 2	86099
Seeing	Day	13% 11% 28% 30% 14% 4	14458
	Night	11% 9% 31% 34% 13% 2	23789

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	2 7% 6% 11% 7% 4 5% 5% 8% 10% 35%	2219
	11th to 20th	Day	2 7% 3 9% 7% 5% 7% 6% 7% 8% 39%	2295
	21st to end	Day	2 8% 3 7% 7% 4 5% 6% 6% 9% 42%	2794
February	1st to 10th	Day	3 8% 5% 11% 8% 3 6% 6% 8% 10% 33%	1520
	11th to 20th	Day	3 9% 5% 9% 8% 4 7% 7% 7% 9% 31%	1654
	21st to end	Day	2 10% 6% 11% 7% 4 6% 6% 5% 8% 34%	1546
March	1st to 10th	Day	2 8% 4 11% 8% 6% 6% 6% 6% 8% 35%	3138
	11th to 20th	Day	5% 9% 4 6% 4 3 4 5% 5% 9% 47%	3444
	21st to end	Day	6% 19% 8% 13% 6% 3 5% 5% 5% 5% 25%	3903
April	1st to 10th	Day	9% 11% 6% 8% 8% 4 5% 5% 6% 9% 30%	2307
	11th to 20th	Day	10% 13% 5% 7% 6% 3 5% 5% 6% 8% 31%	2310
	21st to end	Day	12% 11% 3 6% 5% 3 5% 6% 7% 9% 33%	2340
May	1st to 10th	Day	13% 11% 4 7% 6% 4 6% 6% 7% 8% 28%	2432
	11th to 20th	Day	13% 7% 3 6% 6% 3 5% 6% 6% 8% 35%	2654
	21st to end	Day	21% 7% 3 6% 6% 4 6% 5% 5% 9% 28%	2917
June	1st to 10th	Day	17% 8% 3 6% 6% 4 6% 7% 7% 8% 28%	2700
	11th to 20th	Day	21% 8% 4 7% 5% 4 6% 6% 6% 7% 26%	2700
	21st to end	Day	10% 4 2 5% 5% 4 6% 7% 7% 10% 42%	2700
July	1st to 10th	Day	9% 5% 3 5% 6% 4 5% 6% 8% 8% 42%	2700
	11th to 20th	Day	10% 4 2 4 4 2 5% 6% 7% 9% 46%	2700
	21st to end	Day	5% 3 2 3 4 2 5% 6% 7% 8% 55%	2841
August	1st to 10th	Day	9% 5% 2 4 5% 3 5% 6% 6% 9% 46%	2507
	11th to 20th	Day	9% 4 2 4 4 3 4 6% 6% 8% 50%	2325
	21st to end	Day	10% 4 2 4 5% 3 4 5% 5% 9% 49%	2574
September	1st to 10th	Day	11% 6% 2 5% 5% 3 3 4 5% 7% 50%	2220
	11th to 20th	Day	12% 8% 2 4 4 3 4 5% 6% 7% 46%	1980
	21st to end	Day	9% 7% 3 6% 4 3 4 4 6% 8% 46%	1980
October	1st to 10th	Day	10% 11% 3 6% 5% 4 4 4 5% 7% 41%	1915
	11th to 20th	Day	7% 8% 3 7% 6% 3 4 5% 6% 10% 40%	1658
	21st to end	Day	6% 11% 4 6% 5% 2 5% 5% 9% 8% 37%	1782
November	1st to 10th	Day	6% 13% 5% 9% 5% 4 5% 5% 5% 8% 35%	1578
	11th to 20th	Day	5% 10% 5% 5% 5% 3 5% 5% 6% 8% 43%	1453
	21st to end	Day	3 9% 6% 8% 6% 4 5% 5% 5% 10% 39%	1330
December	1st to 10th	Day	3 9% 5% 9% 7% 4 6% 5% 5% 8% 37%	1330
	11th to 20th	Day	2 10% 5% 11% 7% 4 6% 6% 8% 8% 33%	1682
	21st to end	Day	3 8% 4 7% 6% 3 4 5% 6% 9% 46%	2442
January	1st to 10th	Night	2 10% 4 10% 10% 5% 7% 7% 7% 9% 30%	4726
	11th to 20th	Night	2 6% 5% 11% 8% 4 7% 8% 6% 8% 36%	4631
	21st to end	Night	3 10% 4 9% 8% 4 5% 5% 6% 8% 39%	4862
February	1st to 10th	Night	3 9% 5% 10% 7% 4 7% 5% 7% 9% 35%	2660
	11th to 20th	Night	3 12% 7% 8% 6% 5% 6% 5% 7% 8% 34%	2412
	21st to end	Night	3 6% 5% 11% 10% 5% 6% 5% 6% 6% 38%	1905
March	1st to 10th	Night	1 7% 4 11% 11% 5% 8% 6% 6% 7% 33%	3736
	11th to 20th	Night	3 8% 3 6% 5% 4 5% 4 6% 8% 48%	3535
	21st to end	Night	9% 21% 7% 10% 7% 3 4 4 6% 6% 23%	3750
April	1st to 10th	Night	7% 14% 6% 8% 5% 3 5% 6% 6% 7% 34%	1861
	11th to 20th	Night	10% 12% 5% 9% 6% 3 4 5% 5% 5% 33%	1601
	21st to end	Night	9% 11% 4 6% 6% 3 5% 4 6% 9% 36%	1620
May	1st to 10th	Night	11% 11% 4 7% 6% 3 6% 5% 6% 8% 34%	1523
	11th to 20th	Night	12% 6% 3 5% 5% 4 5% 6% 4 9% 42%	1239
	21st to end	Night	20% 6% 3 5% 4 4 6% 5% 5% 8% 34%	1358
June	1st to 10th	Night	14% 8% 3 6% 5% 3 5% 4 6% 9% 36%	1260
	11th to 20th	Night	19% 8% 3 6% 5% 3 4 6% 5% 9% 32%	1260
	21st to end	Night	8% 4 1 4 5% 3 5% 5% 6% 11% 47%	1260
July	1st to 10th	Night	8% 5% 2 4 5% 3 3 4 7% 10% 49%	1260
	11th to 20th	Night	9% 3 3 5% 5% 3 4 6% 6% 8% 49%	1260
	21st to end	Night	7% 4 1 3 3 2 4 5% 5% 9% 58%	1473
August	1st to 10th	Night	11% 4 2 4 4 3 5% 4 6% 9% 49%	1432
	11th to 20th	Night	9% 4 1 2 4 2 4 4 6% 10% 54%	1576
	21st to end	Night	9% 4 1 3 4 2 3 4 6% 9% 55%	1782
September	1st to 10th	Night	11% 5% 2 4 4 2 3 5% 5% 7% 51%	1736
	11th to 20th	Night	13% 8% 2 4 3 2 3 3 5% 7% 51%	1980
	21st to end	Night	10% 7% 2 4 5% 3 3 4 5% 9% 48%	1980
October	1st to 10th	Night	11% 8% 3 6% 4 2 3 5% 5% 7% 44%	2044
	11th to 20th	Night	8% 9% 3 5% 5% 3 4 5% 7% 8% 44%	2299
	21st to end	Night	6% 9% 4 6% 6% 4 5% 6% 6% 8% 41%	2574
November	1st to 10th	Night	6% 12% 4 7% 5% 3 6% 6% 5% 7% 38%	2482
	11th to 20th	Night	4 8% 3 6% 7% 3 5% 5% 6% 8% 44%	2726
	21st to end	Night	3 8% 5% 8% 6% 3 5% 5% 7% 9% 41%	2850
December	1st to 10th	Night	3 9% 5% 8% 7% 4 7% 6% 6% 9% 37%	2850
	11th to 20th	Night	1 7% 4 10% 9% 4 6% 6% 6% 10% 35%	3510
	21st to end	Night	2 8% 4 8% 6% 4 5% 5% 6% 7% 44%	5214

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	40% 9% 16% 23% 10% 3	2219
	11th to 20th	Day	41% 7% 11% 18% 18% 4	2316
	21st to end	Day	34% 11% 16% 29% 9% 1	2794
February	1st to 10th	Day	46% 10% 14% 18% 9% 3	1520
	11th to 20th	Day	47% 8% 14% 19% 10% 2	1663
	21st to end	Day	47% 8% 14% 17% 13% 1	1546
March	1st to 10th	Day	45% 7% 16% 17% 14% 1	3138
	11th to 20th	Day	33% 7% 18% 27% 15% 1	3445
	21st to end	Day	58% 8% 13% 16% 5%	3903
April	1st to 10th	Day	50% 7% 17% 17% 8% 1	2307
	11th to 20th	Day	50% 7% 14% 18% 10% 1	2310
	21st to end	Day	47% 8% 15% 20% 8% 1	2340
May	1st to 10th	Day	51% 10% 14% 19% 7% 1	2432
	11th to 20th	Day	45% 9% 13% 22% 10% 1	2658
	21st to end	Day	52% 9% 15% 17% 6% 1	2918
June	1st to 10th	Day	50% 10% 13% 17% 8% 1	2700
	11th to 20th	Day	53% 10% 13% 14% 7% 2	2700
	21st to end	Day	35% 11% 17% 23% 11% 3	2700
July	1st to 10th	Day	35% 10% 16% 23% 13% 2	2626
	11th to 20th	Day	33% 10% 15% 25% 16% 2	2619
	21st to end	Day	24% 10% 14% 31% 18% 4	2683
August	1st to 10th	Day	32% 11% 15% 28% 12% 2	2490
	11th to 20th	Day	29% 10% 20% 27% 11% 3	2325
	21st to end	Day	30% 9% 15% 27% 16% 3	2574
September	1st to 10th	Day	33% 7% 14% 25% 17% 4	2220
	11th to 20th	Day	36% 8% 13% 26% 14% 2	1980
	21st to end	Day	36% 7% 14% 28% 13% 3	1980
October	1st to 10th	Day	42% 9% 13% 24% 11% 2	1915
	11th to 20th	Day	40% 7% 16% 23% 11% 3	1658
	21st to end	Day	42% 9% 14% 22% 11% 3	1782
November	1st to 10th	Day	48% 8% 13% 19% 12% 1	1578
	11th to 20th	Day	38% 7% 18% 23% 11% 2	1453
	21st to end	Day	39% 9% 19% 21% 10% 3	1330
December	1st to 10th	Day	43% 8% 13% 18% 13% 4	1330
	11th to 20th	Day	45% 10% 16% 19% 9% 1	1684
	21st to end	Day	33% 7% 11% 24% 18% 6%	2442
January	1st to 10th	Night	46% 11% 13% 21% 8% 2	4726
	11th to 20th	Night	44% 7% 13% 16% 14% 6%	4672
	21st to end	Night	39% 10% 14% 23% 12% 1	4862
February	1st to 10th	Night	45% 8% 13% 20% 11% 3	2660
	11th to 20th	Night	46% 8% 15% 18% 12% 2	2413
	21st to end	Night	47% 6% 11% 21% 12% 3	1905
March	1st to 10th	Night	46% 9% 13% 20% 11% 1	3736
	11th to 20th	Night	31% 10% 21% 22% 16% 1	3535
	21st to end	Night	59% 8% 12% 15% 5% 1	3750
April	1st to 10th	Night	48% 6% 14% 20% 11% 1	1861
	11th to 20th	Night	50% 7% 13% 19% 9% 1	1601
	21st to end	Night	44% 10% 13% 22% 8% 2	1620
May	1st to 10th	Night	47% 8% 11% 23% 10%	1523
	11th to 20th	Night	38% 9% 12% 30% 9% 1	1239
	21st to end	Night	49% 7% 15% 21% 7% 1	1358
June	1st to 10th	Night	44% 7% 15% 23% 11%	1260
	11th to 20th	Night	47% 8% 14% 20% 9% 2	1260
	21st to end	Night	30% 9% 20% 26% 13% 2	1260
July	1st to 10th	Night	31% 8% 18% 27% 15% 1	1225
	11th to 20th	Night	31% 9% 18% 26% 13% 3	1218
	21st to end	Night	23% 7% 15% 31% 20% 4	1391
August	1st to 10th	Night	32% 8% 16% 29% 13% 3	1424
	11th to 20th	Night	26% 9% 19% 31% 14% 2	1576
	21st to end	Night	26% 6% 14% 31% 19% 4	1782
September	1st to 10th	Night	31% 6% 16% 26% 16% 4	1736
	11th to 20th	Night	33% 6% 13% 29% 15% 5%	1980
	21st to end	Night	33% 6% 17% 25% 14% 4	1980
October	1st to 10th	Night	39% 8% 12% 24% 16% 1	2044
	11th to 20th	Night	36% 7% 16% 24% 13% 3	2299
	21st to end	Night	40% 8% 15% 24% 12% 1	2574
November	1st to 10th	Night	44% 9% 13% 20% 13% 2	2482
	11th to 20th	Night	37% 9% 15% 21% 17% 2	2726
	21st to end	Night	38% 8% 16% 21% 12% 4	2850
December	1st to 10th	Night	42% 8% 15% 20% 11% 4	2850
	11th to 20th	Night	41% 12% 19% 21% 8% 1	3507
	21st to end	Night	36% 7% 12% 25% 14% 6%	5214

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% 29% 36% 16% 10%	180
	11th to 20th	Day	7% 7% 29% 37% 15% 5%	180
	21st to end	Day	8% 5% 28% 45% 11% 4	198
February	1st to 10th	Day	8% 13% 37% 28% 11% 3	180
	11th to 20th	Day	5% 10% 39% 32% 11% 2	265
	21st to end	Day	5% 12% 35% 32% 15% 1	358
March	1st to 10th	Day	6% 14% 39% 28% 10% 3	460
	11th to 20th	Day	9% 13% 31% 29% 15% 3	460
	21st to end	Day	11% 13% 36% 25% 11% 4	506
April	1st to 10th	Day	14% 14% 30% 32% 10% 1	457
	11th to 20th	Day	18% 11% 29% 26% 14% 3	412
	21st to end	Day	18% 16% 29% 22% 10% 4	420
May	1st to 10th	Day	19% 12% 24% 26% 14% 5%	435
	11th to 20th	Day	19% 10% 21% 26% 19% 5%	625
	21st to end	Day	21% 9% 18% 29% 18% 4	679
June	1st to 10th	Day	18% 11% 27% 30% 11% 4	630
	11th to 20th	Day	23% 12% 25% 26% 11% 2	630
	21st to end	Day	12% 10% 25% 30% 20% 4	630
July	1st to 10th	Day	11% 8% 21% 34% 21% 5%	621
	11th to 20th	Day	13% 8% 26% 32% 19% 3	618
	21st to end	Day	5% 8% 31% 33% 18% 5%	587
August	1st to 10th	Day	11% 9% 26% 34% 15% 4	447
	11th to 20th	Day	8% 10% 29% 31% 15% 6%	437
	21st to end	Day	10% 11% 30% 28% 16% 5%	495
September	1st to 10th	Day	15% 14% 30% 26% 12% 3	450
	11th to 20th	Day	19% 10% 26% 29% 13% 4	450
	21st to end	Day	15% 19% 29% 26% 9% 3	450
October	1st to 10th	Day	14% 12% 26% 28% 15% 4	385
	11th to 20th	Day	13% 13% 28% 30% 12% 4	270
	21st to end	Day	15% 16% 31% 25% 10% 3	297
November	1st to 10th	Day	18% 11% 33% 23% 11% 3	270
	11th to 20th	Day	13% 12% 33% 31% 10% 2	238
	21st to end	Day	8% 13% 24% 34% 12% 9%	180
December	1st to 10th	Day	7% 8% 30% 39% 12% 3	180
	11th to 20th	Day	6% 15% 29% 37% 9% 4	180
	21st to end	Day	7% 8% 25% 39% 16% 6%	198
January	1st to 10th	Night	8% 14% 37% 31% 9% 1	950
	11th to 20th	Night	7% 9% 43% 30% 9% 1	908
	21st to end	Night	6% 16% 35% 33% 8% 2	836
February	1st to 10th	Night	6% 12% 36% 28% 12% 5%	760
	11th to 20th	Night	7% 8% 35% 37% 11% 1	744
	21st to end	Night	6% 6% 35% 32% 17% 4	628
March	1st to 10th	Night	6% 7% 39% 33% 11% 4	760
	11th to 20th	Night	8% 9% 37% 33% 11% 2	760
	21st to end	Night	11% 8% 27% 39% 11% 3	836
April	1st to 10th	Night	14% 7% 33% 31% 14% 2	721
	11th to 20th	Night	16% 7% 31% 34% 11%	533
	21st to end	Night	16% 7% 28% 35% 11% 2	540
May	1st to 10th	Night	17% 8% 23% 34% 17% 2	456
	11th to 20th	Night	15% 5% 22% 35% 21% 1	354
	21st to end	Night	22% 6% 18% 33% 18% 2	389
June	1st to 10th	Night	18% 6% 21% 37% 15% 3	360
	11th to 20th	Night	24% 5% 23% 34% 13% 1	360
	21st to end	Night	9% 4 21% 38% 27% 1	360
July	1st to 10th	Night	11% 4 14% 43% 27% 1	350
	11th to 20th	Night	9% 3 18% 49% 17% 4	348
	21st to end	Night	8% 4 16% 45% 24% 3	394
August	1st to 10th	Night	13% 1 12% 42% 29% 3	358
	11th to 20th	Night	11% 2 17% 43% 24% 3	508
	21st to end	Night	10% 5% 19% 43% 20% 3	594
September	1st to 10th	Night	14% 4 21% 38% 19% 4	648
	11th to 20th	Night	18% 4 19% 34% 21% 4	720
	21st to end	Night	13% 7% 28% 36% 11% 5%	720
October	1st to 10th	Night	14% 6% 26% 35% 15% 3	720
	11th to 20th	Night	12% 8% 31% 33% 14% 3	720
	21st to end	Night	10% 10% 33% 36% 10% 2	792
November	1st to 10th	Night	13% 13% 35% 29% 8% 1	817
	11th to 20th	Night	9% 18% 38% 28% 5% 1	950
	21st to end	Night	7% 12% 41% 29% 8% 2	950
December	1st to 10th	Night	7% 9% 39% 31% 11% 3	950
	11th to 20th	Night	5% 12% 40% 33% 8% 2	950
	21st to end	Night	8% 11% 37% 30% 11% 2	1045

Page generated at 2024-04-07 07:28:15. Version ac07

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