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 2024-01-31.

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	16% 10% 4 7% 6% 3 5% 5% 5% 7% 32%	74740
	Night	16% 10% 4 7% 6% 3 4 4 5% 6% 34%	75383
Transparency	Day	51% 8% 12% 18% 10% 1	74461
	Night	50% 7% 12% 18% 11% 2	75181
Seeing	Day	20% 12% 27% 21% 14% 6%	13589
	Night	20% 9% 27% 29% 13% 2	23665

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	18% 15% 8% 10% 8% 4 4 4 3 5% 20%	1706
	11th to 20th	Day	18% 19% 8% 11% 8% 4 3 4 4 5% 17%	1710
	21st to end	Day	17% 19% 8% 13% 8% 3 5% 4 4 4 15%	1872
February	1st to 10th	Day	20% 16% 5% 12% 8% 3 4 3 4 4 21%	1620
	11th to 20th	Day	16% 14% 6% 12% 9% 3 5% 4 5% 5% 22%	1641
	21st to end	Day	18% 16% 4 10% 6% 2 4 4 4 5% 28%	1480
March	1st to 10th	Day	18% 12% 5% 8% 6% 3 4 3 4 6% 30%	1930
	11th to 20th	Day	17% 13% 4 9% 6% 3 5% 4 5% 6% 29%	1980
	21st to end	Day	23% 10% 4 7% 6% 3 4 5% 4 5% 28%	2178
April	1st to 10th	Day	14% 12% 4 7% 7% 3 5% 3 5% 8% 32%	2004
	11th to 20th	Day	18% 10% 4 6% 6% 3 4 4 5% 6% 34%	2280
	21st to end	Day	20% 9% 4 7% 5% 3 4 4 4 6% 32%	2340
May	1st to 10th	Day	18% 9% 4 7% 6% 3 5% 6% 5% 7% 30%	2340
	11th to 20th	Day	20% 8% 3 7% 6% 4 5% 4 7% 7% 31%	2301
	21st to end	Day	10% 5% 3 5% 6% 4 6% 6% 8% 9% 39%	2531
June	1st to 10th	Day	12% 7% 3 6% 6% 3 6% 5% 7% 9% 35%	2461
	11th to 20th	Day	13% 7% 3 7% 5% 4 6% 6% 6% 8% 35%	2520
	21st to end	Day	11% 5% 2 5% 6% 4 6% 6% 7% 11% 37%	2410
July	1st to 10th	Day	11% 6% 3 5% 6% 4 5% 5% 5% 8% 41%	2340
	11th to 20th	Day	11% 5% 2 5% 6% 3 6% 5% 7% 9% 40%	2340
	21st to end	Day	11% 5% 2 6% 6% 4 6% 6% 7% 10% 36%	2547
August	1st to 10th	Day	10% 6% 2 6% 5% 4 6% 7% 7% 10% 36%	2327
	11th to 20th	Day	10% 7% 3 6% 6% 4 7% 7% 7% 11% 34%	2303
	21st to end	Day	10% 5% 2 6% 5% 4 6% 7% 6% 9% 39%	2533
September	1st to 10th	Day	14% 6% 2 5% 5% 3 4 5% 5% 7% 44%	2049
	11th to 20th	Day	11% 6% 2 5% 6% 3 4 5% 7% 9% 43%	1980
	21st to end	Day	18% 7% 3 6% 5% 3 4 4 4 6% 39%	1980
October	1st to 10th	Day	15% 6% 3 7% 5% 3 4 5% 5% 7% 39%	1868
	11th to 20th	Day	17% 10% 3 7% 5% 4 4 4 5% 5% 36%	1800
	21st to end	Day	21% 11% 3 7% 5% 3 4 4 6% 6% 30%	1980
November	1st to 10th	Day	11% 10% 3 7% 5% 3 5% 4 5% 6% 42%	1677
	11th to 20th	Day	18% 12% 5% 6% 5% 3 4 5% 4 5% 32%	1710
	21st to end	Day	17% 14% 6% 7% 6% 4 5% 5% 6% 5% 27%	1547
December	1st to 10th	Day	20% 17% 9% 11% 5% 3 5% 3 3 5% 20%	1520
	11th to 20th	Day	18% 16% 6% 14% 6% 3 4 4 5% 6% 20%	1910
	21st to end	Day	24% 23% 6% 10% 6% 3 4 3 4 5% 12%	3025
January	1st to 10th	Night	18% 16% 6% 10% 7% 3 5% 5% 4 4 21%	2459
	11th to 20th	Night	18% 17% 7% 12% 7% 3 4 5% 5% 4 19%	2470
	21st to end	Night	20% 16% 6% 9% 9% 4 4 4 4 4 20%	2711
February	1st to 10th	Night	22% 12% 6% 10% 7% 4 4 4 4 4 22%	2340
	11th to 20th	Night	18% 14% 6% 9% 7% 4 5% 4 5% 6% 23%	2202
	21st to end	Night	20% 12% 6% 9% 7% 4 5% 4 4 4 25%	1776
March	1st to 10th	Night	17% 11% 4 8% 6% 3 4 4 5% 5% 33%	2030
	11th to 20th	Night	17% 9% 4 6% 5% 3 4 4 5% 7% 35%	1980
	21st to end	Night	19% 11% 5% 7% 6% 2 4 4 4 5% 33%	2178
April	1st to 10th	Night	15% 10% 4 6% 5% 4 5% 4 6% 6% 34%	1954
	11th to 20th	Night	16% 9% 4 7% 6% 3 4 4 5% 7% 36%	1645
	21st to end	Night	16% 9% 3 6% 6% 3 4 4 5% 7% 38%	1620
May	1st to 10th	Night	18% 8% 3 7% 7% 3 5% 7% 7% 6% 30%	1620
	11th to 20th	Night	17% 7% 3 5% 5% 3 5% 4 6% 7% 38%	1592
	21st to end	Night	10% 3 3 5% 5% 3 5% 5% 7% 10% 42%	1744
June	1st to 10th	Night	8% 6% 3 7% 5% 3 4 5% 6% 8% 45%	1494
	11th to 20th	Night	11% 5% 2 5% 6% 4 5% 5% 7% 9% 43%	1440
	21st to end	Night	10% 5% 2 4 5% 4 5% 5% 6% 10% 45%	1540
July	1st to 10th	Night	8% 4 2 6% 6% 3 5% 5% 5% 7% 48%	1620
	11th to 20th	Night	7% 3 3 5% 6% 3 4 6% 6% 11% 47%	1620
	21st to end	Night	9% 4 3 5% 7% 3 5% 6% 7% 8% 43%	1771
August	1st to 10th	Night	7% 5% 2 5% 5% 3 4 5% 7% 12% 46%	1611
	11th to 20th	Night	8% 6% 3 5% 5% 3 4 5% 6% 10% 46%	1598
	21st to end	Night	8% 4 2 5% 4 3 6% 6% 6% 8% 48%	1804
September	1st to 10th	Night	14% 5% 2 4 4 2 4 4 4 7% 50%	1908
	11th to 20th	Night	10% 3 2 4 4 2 4 5% 5% 7% 53%	1980
	21st to end	Night	14% 7% 3 5% 4 3 4 4 5% 7% 45%	1980
October	1st to 10th	Night	13% 5% 3 5% 4 3 5% 4 6% 7% 45%	2092
	11th to 20th	Night	18% 8% 3 7% 6% 4 4 5% 4 7% 36%	2160
	21st to end	Night	21% 10% 3 7% 5% 3 5% 4 5% 5% 33%	2376
November	1st to 10th	Night	14% 7% 3 5% 5% 3 4 4 5% 6% 43%	2399
	11th to 20th	Night	17% 12% 5% 8% 5% 3 4 4 5% 6% 33%	2470
	21st to end	Night	19% 13% 5% 7% 5% 4 5% 4 4 7% 28%	2629
December	1st to 10th	Night	20% 17% 6% 9% 6% 3 4 4 4 5% 22%	2660
	11th to 20th	Night	17% 17% 6% 9% 6% 3 3 4 5% 5% 24%	3285
	21st to end	Night	24% 20% 7% 9% 6% 3 4 4 4 5% 14%	4625

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	68% 5% 10% 12% 5%	1707
	11th to 20th	Day	72% 5% 12% 8% 3	1710
	21st to end	Day	73% 7% 7% 10% 4 1	1872
February	1st to 10th	Day	69% 4 8% 12% 5% 1	1620
	11th to 20th	Day	65% 6% 8% 13% 7%	1650
	21st to end	Day	59% 5% 12% 15% 8% 1	1480
March	1st to 10th	Day	56% 5% 12% 21% 7%	1930
	11th to 20th	Day	56% 6% 10% 16% 10% 2	1980
	21st to end	Day	58% 6% 8% 17% 10% 1	2178
April	1st to 10th	Day	50% 7% 13% 18% 11% 1	2004
	11th to 20th	Day	51% 7% 11% 16% 14% 2	2280
	21st to end	Day	53% 6% 11% 19% 9% 2	2340
May	1st to 10th	Day	52% 10% 11% 17% 9% 1	2340
	11th to 20th	Day	51% 8% 11% 16% 12% 1	2305
	21st to end	Day	38% 11% 15% 23% 11% 1	2531
June	1st to 10th	Day	43% 9% 14% 21% 12% 1	2461
	11th to 20th	Day	45% 9% 13% 20% 12% 1	2520
	21st to end	Day	38% 10% 14% 22% 14% 2	2410
July	1st to 10th	Day	41% 9% 12% 22% 13% 2	2275
	11th to 20th	Day	39% 9% 14% 23% 14% 2	2271
	21st to end	Day	38% 11% 15% 21% 12% 1	2405
August	1st to 10th	Day	40% 11% 14% 23% 11%	2314
	11th to 20th	Day	41% 11% 14% 22% 10% 1	2303
	21st to end	Day	38% 10% 13% 22% 13% 4	2533
September	1st to 10th	Day	38% 9% 14% 20% 17% 3	2049
	11th to 20th	Day	36% 9% 15% 24% 14% 2	1980
	21st to end	Day	45% 7% 11% 21% 16% 1	1980
October	1st to 10th	Day	43% 8% 11% 21% 16% 1	1868
	11th to 20th	Day	48% 7% 13% 20% 11% 1	1800
	21st to end	Day	54% 6% 13% 16% 9% 1	1980
November	1st to 10th	Day	43% 5% 12% 23% 15% 2	1677
	11th to 20th	Day	54% 5% 11% 19% 9% 3	1710
	21st to end	Day	59% 6% 11% 16% 7% 1	1547
December	1st to 10th	Day	69% 6% 9% 12% 3	1520
	11th to 20th	Day	65% 6% 10% 15% 4	1906
	21st to end	Day	76% 7% 9% 6% 2	3025
January	1st to 10th	Night	65% 6% 9% 13% 6%	2459
	11th to 20th	Night	68% 7% 11% 11% 4	2470
	21st to end	Night	68% 5% 10% 13% 4 1	2711
February	1st to 10th	Night	65% 5% 11% 14% 4	2340
	11th to 20th	Night	62% 5% 10% 14% 8% 1	2204
	21st to end	Night	63% 6% 8% 13% 8% 2	1776
March	1st to 10th	Night	54% 5% 10% 20% 10% 1	2030
	11th to 20th	Night	47% 7% 13% 21% 10% 2	1980
	21st to end	Night	53% 6% 8% 19% 11% 2	2178
April	1st to 10th	Night	48% 7% 12% 18% 14% 2	1954
	11th to 20th	Night	47% 6% 12% 16% 17% 3	1645
	21st to end	Night	46% 7% 12% 18% 15% 2	1620
May	1st to 10th	Night	50% 9% 11% 16% 11% 2	1620
	11th to 20th	Night	46% 7% 12% 17% 16% 3	1593
	21st to end	Night	35% 9% 16% 24% 14% 1	1745
June	1st to 10th	Night	36% 10% 12% 23% 17% 3	1494
	11th to 20th	Night	37% 8% 15% 22% 16% 2	1440
	21st to end	Night	34% 9% 14% 24% 18% 1	1540
July	1st to 10th	Night	35% 9% 12% 22% 18% 4	1575
	11th to 20th	Night	30% 9% 16% 25% 18% 2	1566
	21st to end	Night	35% 10% 17% 22% 14% 3	1672
August	1st to 10th	Night	30% 9% 17% 27% 17%	1600
	11th to 20th	Night	33% 9% 17% 25% 14% 2	1598
	21st to end	Night	32% 9% 15% 24% 17% 4	1804
September	1st to 10th	Night	34% 7% 14% 24% 17% 5%	1908
	11th to 20th	Night	29% 8% 13% 25% 24% 2	1980
	21st to end	Night	40% 6% 12% 22% 19% 1	1980
October	1st to 10th	Night	37% 7% 14% 21% 19% 1	2092
	11th to 20th	Night	47% 7% 11% 22% 13% 1	2160
	21st to end	Night	52% 7% 11% 19% 9% 2	2376
November	1st to 10th	Night	41% 6% 13% 22% 15% 3	2399
	11th to 20th	Night	52% 5% 11% 18% 11% 2	2470
	21st to end	Night	57% 6% 12% 18% 6% 1	2629
December	1st to 10th	Night	65% 6% 11% 12% 5%	2660
	11th to 20th	Night	61% 6% 13% 16% 3	3288
	21st to end	Night	72% 7% 10% 9% 2	4625

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	28% 22% 33% 10% 6% 1	270
	11th to 20th	Day	27% 25% 36% 8% 3	270
	21st to end	Day	27% 24% 34% 13% 2	294
February	1st to 10th	Day	31% 19% 37% 12% 1	240
	11th to 20th	Day	32% 28% 31% 9% 1	240
	21st to end	Day	28% 28% 34% 9% 1	198
March	1st to 10th	Day	26% 20% 32% 18% 5%	370
	11th to 20th	Day	29% 13% 35% 16% 7%	420
	21st to end	Day	34% 13% 28% 19% 5% 1	462
April	1st to 10th	Day	22% 18% 32% 17% 9% 2	420
	11th to 20th	Day	23% 18% 27% 17% 9% 6%	412
	21st to end	Day	26% 13% 32% 19% 8% 2	420
May	1st to 10th	Day	27% 14% 26% 20% 9% 4	423
	11th to 20th	Day	20% 10% 33% 20% 12% 5%	447
	21st to end	Day	14% 5% 25% 28% 21% 7%	485
June	1st to 10th	Day	14% 5% 27% 32% 16% 5%	450
	11th to 20th	Day	18% 8% 22% 27% 20% 5%	450
	21st to end	Day	13% 7% 28% 27% 19% 6%	450
July	1st to 10th	Day	14% 3 17% 27% 24% 17%	445
	11th to 20th	Day	12% 5% 22% 25% 23% 12%	444
	21st to end	Day	12% 3 24% 25% 25% 10%	488
August	1st to 10th	Day	15% 4 16% 28% 25% 13%	446
	11th to 20th	Day	15% 5% 19% 27% 23% 11%	439
	21st to end	Day	10% 6% 17% 25% 26% 15%	495
September	1st to 10th	Day	14% 6% 21% 25% 23% 12%	450
	11th to 20th	Day	11% 3 17% 25% 31% 13%	450
	21st to end	Day	21% 10% 22% 24% 15% 8%	450
October	1st to 10th	Day	12% 9% 29% 24% 20% 6%	450
	11th to 20th	Day	21% 12% 34% 23% 8% 1	450
	21st to end	Day	29% 15% 29% 19% 7%	495
November	1st to 10th	Day	16% 18% 36% 17% 11% 2	275
	11th to 20th	Day	26% 21% 29% 16% 7% 1	270
	21st to end	Day	26% 18% 40% 15% 2	192
December	1st to 10th	Day	28% 19% 33% 13% 4 2	180
	11th to 20th	Day	24% 19% 42% 13% 2	180
	21st to end	Day	32% 21% 32% 13% 2	269
January	1st to 10th	Night	26% 17% 35% 18% 4	760
	11th to 20th	Night	28% 16% 34% 18% 4	760
	21st to end	Night	29% 15% 30% 22% 3	835
February	1st to 10th	Night	29% 13% 34% 22% 3	720
	11th to 20th	Night	26% 14% 37% 21% 2	703
	21st to end	Night	27% 14% 38% 18% 3	592
March	1st to 10th	Night	23% 15% 32% 26% 5%	720
	11th to 20th	Night	22% 13% 33% 25% 7% 1	720
	21st to end	Night	24% 11% 33% 26% 6% 1	792
April	1st to 10th	Night	21% 13% 34% 26% 5% 1	720
	11th to 20th	Night	19% 13% 34% 22% 10% 1	581
	21st to end	Night	20% 8% 34% 29% 7% 1	540
May	1st to 10th	Night	20% 8% 29% 34% 9% 1	540
	11th to 20th	Night	22% 5% 23% 36% 13% 2	531
	21st to end	Night	10% 1 20% 40% 25% 4	584
June	1st to 10th	Night	10% 2 23% 44% 19%	414
	11th to 20th	Night	11% 5% 24% 44% 15% 1	360
	21st to end	Night	10% 2 23% 41% 23%	460
July	1st to 10th	Night	10% 3 12% 40% 29% 6%	525
	11th to 20th	Night	9% 3 16% 40% 28% 4	522
	21st to end	Night	9% 3 15% 42% 29% 2	591
August	1st to 10th	Night	9% 2 13% 42% 32% 3	537
	11th to 20th	Night	11% 3 11% 42% 29% 4	534
	21st to end	Night	9% 2 18% 33% 33% 6%	616
September	1st to 10th	Night	14% 4 18% 31% 29% 5%	720
	11th to 20th	Night	11% 2 13% 36% 32% 5%	720
	21st to end	Night	16% 6% 22% 31% 21% 4	720
October	1st to 10th	Night	16% 6% 21% 35% 18% 4	720
	11th to 20th	Night	21% 9% 31% 27% 11% 1	720
	21st to end	Night	27% 12% 25% 29% 7%	792
November	1st to 10th	Night	18% 9% 29% 29% 13% 1	740
	11th to 20th	Night	23% 12% 33% 24% 7% 1	760
	21st to end	Night	26% 13% 32% 22% 6%	760
December	1st to 10th	Night	29% 15% 31% 21% 4	760
	11th to 20th	Night	25% 12% 36% 23% 4	760
	21st to end	Night	28% 13% 34% 23% 2	836

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