WCAC
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WCAC clear sky chart from 2005-11-06 to 2026-06-16.

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	15% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	137047
	Night	15% 11% 4 9% 5% 4 4 3 5% 6% 32%	134940
Transparency	Day	51% 5% 11% 20% 11% 1	140691
	Night	52% 5% 11% 19% 11% 1	139368
Seeing	Day	21% 13% 28% 21% 13% 5%	16715
	Night	20% 10% 27% 29% 12% 1	26734

By Month

Percentage Cloud Cover SE (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	16% 19% 6% 20% 5% 4 4 3 3 5% 15%	5258
	11th to 20th	Day	11% 20% 7% 17% 8% 6% 4 3 3 4 17%	5280
	21st to end	Day	19% 17% 7% 12% 9% 7% 4 3 4 4 14%	5780
February	1st to 10th	Day	18% 16% 4 13% 7% 4 4 3 4 4 23%	1890
	11th to 20th	Day	16% 14% 6% 13% 7% 5% 4 3 5% 5% 23%	1922
	21st to end	Day	16% 15% 4 11% 4 4 3 4 5% 5% 28%	1730
March	1st to 10th	Day	17% 13% 6% 11% 4 4 5% 3 4 5% 28%	4926
	11th to 20th	Day	11% 11% 3 10% 5% 5% 4 3 5% 6% 37%	5090
	21st to end	Day	19% 11% 4 8% 5% 4 4 3 5% 6% 29%	5586
April	1st to 10th	Day	14% 13% 3 8% 6% 4 4 3 6% 7% 31%	2339
	11th to 20th	Day	16% 11% 3 7% 5% 5% 4 3 5% 6% 35%	2665
	21st to end	Day	18% 10% 3 7% 4 5% 4 4 5% 6% 33%	2730
May	1st to 10th	Day	20% 9% 3 8% 5% 5% 5% 4 5% 6% 28%	7616
	11th to 20th	Day	17% 8% 3 7% 5% 4 5% 4 6% 8% 33%	7626
	21st to end	Day	20% 7% 2 6% 4 4 4 4 6% 7% 36%	8391
June	1st to 10th	Day	12% 7% 3 7% 5% 4 6% 4 7% 8% 36%	2872
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 5% 6% 8% 36%	2870
	21st to end	Day	11% 5% 2 7% 5% 6% 5% 5% 7% 10% 38%	2677
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	5900
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 10% 36%	5900
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	6424
August	1st to 10th	Day	14% 7% 2 7% 4 5% 6% 4 6% 9% 35%	4234
	11th to 20th	Day	10% 7% 3 7% 5% 6% 6% 5% 7% 10% 36%	4174
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 6% 10% 41%	4589
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 7% 47%	2276
	11th to 20th	Day	9% 6% 2 5% 5% 4 4 4 7% 9% 45%	2200
	21st to end	Day	19% 8% 3 7% 4 4 4 3 5% 6% 37%	2200
October	1st to 10th	Day	14% 6% 2 7% 5% 4 4 4 6% 7% 42%	2075
	11th to 20th	Day	16% 10% 2 7% 4 5% 4 3 5% 5% 38%	2000
	21st to end	Day	20% 11% 3 8% 5% 4 4 3 6% 6% 32%	2200
November	1st to 10th	Day	11% 10% 3 7% 4 5% 4 3 6% 6% 41%	1857
	11th to 20th	Day	17% 13% 4 7% 5% 4 4 4 4 5% 32%	1890
	21st to end	Day	17% 16% 6% 8% 5% 6% 4 4 5% 5% 25%	1709
December	1st to 10th	Day	19% 14% 6% 16% 5% 4 4 2 3 4 23%	2680
	11th to 20th	Day	22% 18% 6% 12% 4 3 3 2 4 5% 21%	3070
	21st to end	Day	23% 22% 5% 12% 6% 5% 4 3 4 4 12%	4421
January	1st to 10th	Night	14% 18% 6% 20% 8% 5% 4 3 4 3 13%	7635
	11th to 20th	Night	11% 19% 9% 16% 8% 6% 6% 3 4 4 15%	7680
	21st to end	Night	18% 16% 5% 11% 6% 7% 4 3 4 5% 22%	8401
February	1st to 10th	Night	19% 13% 5% 12% 6% 5% 4 3 4 4 24%	2730
	11th to 20th	Night	17% 14% 6% 11% 6% 5% 4 4 5% 6% 23%	2580
	21st to end	Night	18% 13% 5% 12% 7% 5% 4 4 4 5% 25%	2076
March	1st to 10th	Night	22% 12% 4 9% 4 3 3 2 3 4 33%	5133
	11th to 20th	Night	13% 9% 3 6% 4 4 5% 4 6% 8% 39%	5090
	21st to end	Night	16% 11% 3 8% 5% 4 5% 3 4 5% 35%	5588
April	1st to 10th	Night	17% 12% 3 7% 4 5% 5% 3 5% 6% 33%	2279
	11th to 20th	Night	15% 9% 3 9% 4 4 4 3 6% 7% 35%	1922
	21st to end	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 38%	1890
May	1st to 10th	Night	23% 9% 3 8% 5% 4 5% 4 6% 6% 28%	5237
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 5% 8% 39%	5234
	21st to end	Night	19% 8% 2 5% 4 4 4 3 6% 8% 37%	5761
June	1st to 10th	Night	8% 7% 2 8% 4 4 4 4 6% 7% 45%	1743
	11th to 20th	Night	10% 5% 2 5% 6% 5% 4 5% 7% 8% 42%	1640
	21st to end	Night	9% 5% 2 5% 4 5% 5% 4 6% 9% 44%	1712
July	1st to 10th	Night	10% 6% 2 7% 4 4 5% 4 6% 9% 44%	4058
	11th to 20th	Night	9% 6% 2 6% 6% 5% 5% 5% 7% 9% 41%	4057
	21st to end	Night	8% 6% 2 7% 5% 4 5% 4 5% 8% 45%	4427
August	1st to 10th	Night	10% 7% 3 8% 5% 4 4 4 6% 10% 38%	2915
	11th to 20th	Night	11% 7% 3 7% 4 4 4 3 4 7% 45%	2875
	21st to end	Night	7% 4 2 6% 5% 5% 6% 5% 6% 9% 45%	3274
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 3 4 7% 52%	2120
	11th to 20th	Night	9% 4 1 5% 4 3 4 4 5% 7% 54%	2200
	21st to end	Night	16% 8% 3 6% 4 4 3 3 5% 6% 43%	2200
October	1st to 10th	Night	13% 5% 3 6% 4 3 5% 3 6% 7% 47%	2325
	11th to 20th	Night	16% 8% 3 7% 4 5% 4 4 4 7% 38%	2400
	21st to end	Night	19% 10% 3 7% 3 4 4 3 5% 6% 35%	2640
November	1st to 10th	Night	13% 8% 2 6% 4 4 4 3 5% 6% 44%	2659
	11th to 20th	Night	16% 13% 4 9% 4 4 4 4 5% 6% 32%	2730
	21st to end	Night	19% 14% 5% 8% 5% 5% 4 3 5% 6% 27%	2907
December	1st to 10th	Night	16% 16% 6% 12% 6% 4 4 3 5% 6% 23%	4720
	11th to 20th	Night	19% 19% 7% 11% 5% 4 3 3 5% 5% 21%	5345
	21st to end	Night	22% 20% 6% 11% 5% 5% 4 3 4 5% 15%	6757

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	72% 4 10% 10% 3	5259
	11th to 20th	Day	72% 3 8% 8% 8%	5280
	21st to end	Day	75% 4 5% 12% 4	5780
February	1st to 10th	Day	66% 4 8% 14% 7% 1	1890
	11th to 20th	Day	64% 6% 8% 15% 7%	1931
	21st to end	Day	58% 5% 12% 16% 7% 1	1730
March	1st to 10th	Day	58% 5% 8% 23% 7%	4926
	11th to 20th	Day	47% 6% 11% 26% 9% 2	5090
	21st to end	Day	55% 5% 8% 19% 12% 1	5586
April	1st to 10th	Day	52% 6% 12% 17% 11% 1	2339
	11th to 20th	Day	50% 6% 12% 17% 13% 2	2665
	21st to end	Day	51% 6% 11% 21% 10% 2	2730
May	1st to 10th	Day	55% 6% 10% 19% 8% 2	7651
	11th to 20th	Day	48% 5% 13% 22% 11% 1	7630
	21st to end	Day	46% 6% 10% 19% 15% 4	8405
June	1st to 10th	Day	42% 8% 14% 22% 13% 1	2872
	11th to 20th	Day	45% 8% 13% 20% 13% 1	2870
	21st to end	Day	38% 9% 15% 22% 14% 2	2677
July	1st to 10th	Day	40% 4 11% 29% 14% 1	5835
	11th to 20th	Day	43% 4 14% 24% 14% 1	5831
	21st to end	Day	41% 5% 13% 21% 18% 2	6240
August	1st to 10th	Day	45% 7% 13% 23% 11%	4221
	11th to 20th	Day	41% 7% 10% 30% 11% 1	4213
	21st to end	Day	37% 6% 13% 32% 9% 2	4589
September	1st to 10th	Day	36% 8% 13% 21% 19% 3	2276
	11th to 20th	Day	34% 8% 15% 25% 16% 3	2200
	21st to end	Day	47% 6% 11% 19% 16% 1	2200
October	1st to 10th	Day	31% 4 11% 31% 21% 1	3374
	11th to 20th	Day	44% 4 11% 22% 15% 4	3240
	21st to end	Day	40% 4 12% 26% 17% 1	3538
November	1st to 10th	Day	43% 5% 12% 24% 15% 2	1857
	11th to 20th	Day	54% 5% 10% 19% 9% 3	1890
	21st to end	Day	62% 6% 11% 15% 6% 1	1709
December	1st to 10th	Day	68% 3 12% 14% 3	2680
	11th to 20th	Day	67% 4 11% 13% 5%	3066
	21st to end	Day	77% 5% 7% 8% 3	4421
January	1st to 10th	Night	75% 4 8% 10% 3	7636
	11th to 20th	Night	73% 5% 8% 9% 5%	7680
	21st to end	Night	67% 3 9% 14% 6% 1	8401
February	1st to 10th	Night	64% 5% 10% 15% 6%	2730
	11th to 20th	Night	63% 5% 10% 15% 7% 1	2582
	21st to end	Night	62% 6% 8% 13% 8% 1	2076
March	1st to 10th	Night	56% 4 10% 20% 10%	5133
	11th to 20th	Night	41% 7% 14% 27% 9% 2	5090
	21st to end	Night	50% 5% 10% 20% 13% 1	5588
April	1st to 10th	Night	51% 6% 11% 18% 12% 2	2279
	11th to 20th	Night	47% 6% 12% 17% 15% 2	1922
	21st to end	Night	46% 7% 11% 19% 15% 2	1890
May	1st to 10th	Night	56% 6% 9% 17% 9% 3	5264
	11th to 20th	Night	45% 5% 10% 24% 15% 1	5235
	21st to end	Night	45% 6% 12% 21% 14% 2	5771
June	1st to 10th	Night	36% 9% 11% 23% 18% 3	1743
	11th to 20th	Night	38% 7% 15% 22% 16% 3	1640
	21st to end	Night	35% 9% 13% 24% 18% 1	1712
July	1st to 10th	Night	38% 5% 11% 25% 18% 3	4015
	11th to 20th	Night	39% 5% 16% 27% 12% 1	4006
	21st to end	Night	36% 5% 13% 20% 23% 2	4297
August	1st to 10th	Night	41% 6% 14% 24% 13% 1	2904
	11th to 20th	Night	39% 5% 11% 26% 17% 1	2902
	21st to end	Night	35% 5% 14% 31% 13% 2	3274
September	1st to 10th	Night	32% 6% 13% 25% 19% 5%	2120
	11th to 20th	Night	28% 7% 13% 26% 24% 2	2200
	21st to end	Night	43% 6% 11% 20% 19% 1	2200
October	1st to 10th	Night	31% 4 9% 28% 27% 1	3806
	11th to 20th	Night	39% 4 12% 22% 17% 5%	3899
	21st to end	Night	37% 4 10% 29% 19% 2	4252
November	1st to 10th	Night	40% 6% 13% 23% 16% 3	2659
	11th to 20th	Night	53% 5% 11% 19% 10% 3	2730
	21st to end	Night	58% 6% 12% 18% 6% 1	2907
December	1st to 10th	Night	64% 4 11% 18% 4	4720
	11th to 20th	Night	66% 4 13% 13% 3	5348
	21st to end	Night	71% 5% 9% 12% 2	6757

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	31% 22% 33% 8% 5% 1	330
	11th to 20th	Day	32% 27% 32% 7% 2	330
	21st to end	Day	28% 25% 32% 13% 1	363
February	1st to 10th	Day	25% 22% 37% 13% 2 1	330
	11th to 20th	Day	32% 26% 31% 9% 1	330
	21st to end	Day	27% 29% 35% 9%	273
March	1st to 10th	Day	27% 19% 34% 15% 5%	482
	11th to 20th	Day	30% 14% 34% 15% 6% 1	540
	21st to end	Day	31% 17% 28% 19% 4 1	594
April	1st to 10th	Day	25% 19% 32% 15% 7% 2	540
	11th to 20th	Day	24% 21% 25% 17% 8% 4	532
	21st to end	Day	24% 14% 32% 20% 8% 2	540
May	1st to 10th	Day	27% 17% 27% 18% 8% 3	543
	11th to 20th	Day	21% 10% 34% 20% 11% 5%	567
	21st to end	Day	18% 6% 26% 25% 19% 6%	617
June	1st to 10th	Day	15% 6% 28% 31% 15% 5%	570
	11th to 20th	Day	18% 8% 24% 29% 17% 5%	550
	21st to end	Day	12% 8% 28% 27% 19% 6%	530
July	1st to 10th	Day	13% 3 18% 27% 24% 15%	525
	11th to 20th	Day	12% 6% 23% 26% 23% 11%	524
	21st to end	Day	13% 3 23% 26% 25% 10%	576
August	1st to 10th	Day	14% 4 17% 29% 24% 12%	526
	11th to 20th	Day	14% 5% 20% 28% 22% 11%	519
	21st to end	Day	9% 6% 18% 26% 26% 14%	583
September	1st to 10th	Day	13% 6% 21% 24% 23% 12%	530
	11th to 20th	Day	9% 3 15% 26% 32% 15%	530
	21st to end	Day	23% 8% 22% 24% 14% 8%	530
October	1st to 10th	Day	12% 9% 28% 25% 21% 6%	530
	11th to 20th	Day	20% 11% 34% 23% 9% 2	530
	21st to end	Day	27% 16% 31% 20% 7% 1	583
November	1st to 10th	Day	17% 19% 37% 17% 9% 1	335
	11th to 20th	Day	29% 20% 28% 15% 7% 1	330
	21st to end	Day	30% 18% 36% 15% 1	234
December	1st to 10th	Day	28% 20% 34% 12% 4 1	220
	11th to 20th	Day	27% 20% 37% 14% 2	220
	21st to end	Day	34% 20% 31% 13% 2	329
January	1st to 10th	Night	27% 17% 35% 18% 3	840
	11th to 20th	Night	27% 19% 33% 17% 4	840
	21st to end	Night	28% 16% 31% 21% 3	924
February	1st to 10th	Night	27% 13% 34% 23% 3	840
	11th to 20th	Night	26% 16% 36% 21% 2	823
	21st to end	Night	26% 15% 37% 19% 3	692
March	1st to 10th	Night	25% 15% 32% 25% 4	840
	11th to 20th	Night	23% 14% 33% 24% 6% 1	840
	21st to end	Night	23% 13% 34% 24% 6% 1	924
April	1st to 10th	Night	24% 13% 34% 24% 5%	840
	11th to 20th	Night	20% 14% 34% 22% 9% 1	678
	21st to end	Night	21% 8% 33% 30% 7%	630
May	1st to 10th	Night	20% 8% 29% 33% 9% 1	630
	11th to 20th	Night	21% 6% 23% 36% 13% 1	621
	21st to end	Night	13% 2 20% 41% 21% 3	683
June	1st to 10th	Night	11% 3 24% 43% 19% 1	483
	11th to 20th	Night	10% 5% 24% 46% 14% 1	410
	21st to end	Night	10% 2 24% 40% 23% 1	512
July	1st to 10th	Night	10% 3 13% 41% 28% 5%	585
	11th to 20th	Night	9% 2 17% 39% 29% 4	582
	21st to end	Night	9% 2 14% 42% 30% 2	657
August	1st to 10th	Night	9% 3 13% 42% 31% 2	597
	11th to 20th	Night	12% 3 12% 41% 29% 4	594
	21st to end	Night	9% 2 17% 35% 33% 6%	685
September	1st to 10th	Night	13% 4 18% 33% 28% 5%	800
	11th to 20th	Night	11% 2 13% 37% 34% 5%	800
	21st to end	Night	19% 6% 21% 31% 20% 4	800
October	1st to 10th	Night	15% 6% 21% 35% 20% 4	800
	11th to 20th	Night	20% 9% 30% 28% 11% 2	800
	21st to end	Night	25% 11% 27% 29% 8%	880
November	1st to 10th	Night	18% 10% 30% 30% 12%	820
	11th to 20th	Night	24% 13% 32% 23% 7% 1	840
	21st to end	Night	28% 13% 32% 21% 6%	840
December	1st to 10th	Night	29% 15% 31% 22% 4	840
	11th to 20th	Night	25% 14% 34% 23% 4	840
	21st to end	Night	29% 14% 33% 23% 2	924

Page generated at 2026-06-13 02:31:38. Version ac07

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