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 2025-02-21.

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	15% 11% 4 9% 5% 5% 4 4 5% 7% 31%	108587
	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 32%	109978
Transparency	Day	51% 6% 11% 19% 11% 1	112275
	Night	51% 6% 11% 19% 11% 2	114429
Seeing	Day	20% 12% 28% 21% 13% 5%	14961
	Night	20% 10% 27% 29% 13% 1	25051

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	15% 20% 6% 21% 5% 4 4 2 3 5% 15%	4056
	11th to 20th	Day	13% 22% 7% 16% 7% 6% 3 3 3 4 17%	4060
	21st to end	Day	21% 19% 8% 11% 7% 6% 4 3 4 3 13%	4466
February	1st to 10th	Day	19% 17% 4 13% 7% 4 4 3 4 4 22%	1800
	11th to 20th	Day	16% 14% 6% 13% 7% 5% 5% 3 5% 5% 23%	1828
	21st to end	Day	18% 16% 4 11% 4 4 3 4 4 4 28%	1570
March	1st to 10th	Day	22% 15% 5% 8% 4 4 4 3 4 6% 26%	3361
	11th to 20th	Day	13% 13% 4 10% 5% 5% 4 3 5% 5% 34%	3480
	21st to end	Day	21% 10% 4 8% 5% 4 4 3 4 6% 31%	3815
April	1st to 10th	Day	14% 13% 3 8% 6% 5% 4 3 5% 7% 31%	2116
	11th to 20th	Day	18% 11% 3 7% 5% 4 4 3 5% 5% 35%	2409
	21st to end	Day	19% 10% 4 8% 4 5% 4 4 4 6% 32%	2470
May	1st to 10th	Day	21% 10% 3 8% 5% 4 5% 4 5% 7% 29%	4082
	11th to 20th	Day	20% 8% 3 7% 4 4 4 3 6% 8% 33%	4081
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	4476
June	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 4 7% 9% 35%	2598
	11th to 20th	Day	13% 7% 3 8% 4 5% 5% 5% 6% 8% 36%	2660
	21st to end	Day	11% 6% 2 7% 5% 6% 5% 5% 7% 11% 37%	2543
July	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 4 6% 10% 39%	4120
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 10% 42%	4120
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 37%	4466
August	1st to 10th	Day	14% 7% 2 7% 4 5% 6% 4 7% 9% 34%	4104
	11th to 20th	Day	10% 7% 3 7% 5% 6% 6% 5% 7% 10% 35%	4044
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 6% 10% 41%	4449
September	1st to 10th	Day	13% 6% 2 5% 5% 4 4 4 5% 7% 46%	2162
	11th to 20th	Day	10% 6% 2 5% 5% 4 4 4 7% 9% 44%	2090
	21st to end	Day	20% 8% 3 7% 4 4 4 3 4 6% 37%	2090
October	1st to 10th	Day	14% 6% 3 7% 5% 4 4 4 6% 7% 41%	1971
	11th to 20th	Day	16% 10% 2 8% 4 5% 4 3 5% 5% 37%	1900
	21st to end	Day	20% 11% 3 8% 5% 4 4 3 6% 6% 32%	2090
November	1st to 10th	Day	11% 10% 3 7% 4 5% 4 3 6% 6% 42%	1767
	11th to 20th	Day	17% 13% 4 7% 5% 4 4 4 4 5% 31%	1800
	21st to end	Day	16% 16% 6% 8% 5% 6% 4 4 5% 5% 25%	1628
December	1st to 10th	Day	19% 14% 6% 16% 5% 4 4 2 3 4 23%	2600
	11th to 20th	Day	22% 18% 6% 12% 4 3 3 2 4 5% 21%	2990
	21st to end	Day	23% 22% 5% 12% 6% 5% 4 3 4 4 12%	4325
January	1st to 10th	Night	15% 18% 7% 21% 8% 4 4 3 3 3 14%	5889
	11th to 20th	Night	12% 20% 9% 15% 8% 6% 5% 3 4 4 13%	5900
	21st to end	Night	22% 17% 6% 11% 6% 4 4 3 3 4 20%	6490
February	1st to 10th	Night	20% 14% 5% 12% 6% 5% 4 3 4 4 23%	2600
	11th to 20th	Night	18% 15% 5% 10% 6% 5% 4 3 5% 6% 23%	2454
	21st to end	Night	19% 13% 5% 11% 7% 5% 4 4 4 5% 24%	1885
March	1st to 10th	Night	23% 13% 3 7% 3 3 3 3 4 4 33%	3492
	11th to 20th	Night	16% 11% 3 6% 4 4 4 4 5% 7% 36%	3480
	21st to end	Night	17% 11% 3 7% 4 3 4 3 4 6% 37%	3817
April	1st to 10th	Night	16% 12% 3 7% 4 5% 5% 3 6% 6% 34%	2062
	11th to 20th	Night	16% 10% 3 8% 4 4 4 3 5% 7% 36%	1737
	21st to end	Night	15% 9% 3 8% 5% 4 4 3 5% 7% 38%	1710
May	1st to 10th	Night	25% 10% 3 8% 5% 4 4 4 6% 6% 27%	2813
	11th to 20th	Night	18% 9% 2 5% 5% 4 4 3 5% 6% 39%	2812
	21st to end	Night	10% 5% 2 5% 4 4 4 4 7% 11% 44%	3079
June	1st to 10th	Night	8% 7% 3 8% 4 4 4 4 6% 8% 44%	1577
	11th to 20th	Night	10% 5% 2 5% 5% 5% 4 4 7% 8% 43%	1520
	21st to end	Night	10% 5% 2 5% 5% 5% 5% 4 6% 9% 44%	1626
July	1st to 10th	Night	10% 6% 2 7% 4 4 4 4 5% 8% 46%	2840
	11th to 20th	Night	9% 4 2 5% 5% 4 4 4 6% 9% 47%	2840
	21st to end	Night	8% 6% 2 7% 5% 4 5% 4 6% 8% 44%	3087
August	1st to 10th	Night	11% 8% 3 8% 5% 4 4 4 6% 10% 37%	2825
	11th to 20th	Night	11% 7% 4 7% 4 4 4 4 4 7% 45%	2785
	21st to end	Night	7% 3 2 6% 5% 5% 6% 5% 6% 9% 45%	3174
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 3 4 7% 51%	2014
	11th to 20th	Night	10% 4 1 5% 4 3 4 4 5% 7% 54%	2090
	21st to end	Night	15% 8% 3 6% 4 4 3 3 5% 6% 43%	2090
October	1st to 10th	Night	13% 5% 3 6% 4 3 5% 3 6% 7% 45%	2209
	11th to 20th	Night	17% 8% 3 8% 5% 5% 4 4 4 7% 37%	2280
	21st to end	Night	20% 10% 3 7% 3 4 4 3 5% 6% 34%	2508
November	1st to 10th	Night	14% 8% 2 6% 4 4 4 4 5% 6% 44%	2529
	11th to 20th	Night	16% 13% 4 8% 4 4 4 4 5% 6% 32%	2600
	21st to end	Night	19% 14% 5% 7% 5% 5% 4 3 5% 7% 27%	2768
December	1st to 10th	Night	16% 16% 6% 12% 6% 4 4 3 5% 6% 23%	4580
	11th to 20th	Night	19% 19% 7% 11% 5% 3 3 3 5% 5% 20%	5205
	21st to end	Night	22% 20% 6% 11% 5% 5% 4 3 4 5% 15%	6611

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	73% 5% 9% 9% 3	4057
	11th to 20th	Day	73% 4 9% 8% 6%	4060
	21st to end	Day	77% 5% 5% 9% 4	4466
February	1st to 10th	Day	67% 4 8% 14% 6% 1	1800
	11th to 20th	Day	64% 6% 8% 15% 7%	1837
	21st to end	Day	58% 5% 12% 15% 8% 2	1570
March	1st to 10th	Day	59% 6% 10% 20% 5%	3361
	11th to 20th	Day	49% 8% 11% 23% 8% 1	3480
	21st to end	Day	53% 7% 11% 17% 12% 1	3815
April	1st to 10th	Day	51% 7% 12% 18% 11% 1	2116
	11th to 20th	Day	50% 7% 11% 17% 14% 2	2409
	21st to end	Day	51% 6% 11% 20% 9% 2	2470
May	1st to 10th	Day	52% 10% 10% 20% 7% 1	4120
	11th to 20th	Day	48% 8% 13% 21% 9% 1	4085
	21st to end	Day	37% 11% 15% 20% 12% 4	4489
June	1st to 10th	Day	43% 9% 14% 22% 12% 1	2598
	11th to 20th	Day	44% 8% 13% 21% 13% 1	2660
	21st to end	Day	39% 9% 14% 22% 14% 2	2543
July	1st to 10th	Day	42% 6% 12% 23% 15% 2	4055
	11th to 20th	Day	37% 6% 13% 26% 18% 1	4051
	21st to end	Day	42% 7% 15% 17% 17% 3	4324
August	1st to 10th	Day	46% 7% 13% 23% 11%	4091
	11th to 20th	Day	41% 7% 10% 30% 11% 1	4083
	21st to end	Day	37% 6% 13% 32% 9% 2	4449
September	1st to 10th	Day	36% 8% 13% 20% 18% 4	2162
	11th to 20th	Day	35% 8% 15% 24% 16% 3	2090
	21st to end	Day	47% 6% 11% 20% 15% 1	2090
October	1st to 10th	Day	32% 4 11% 31% 20% 1	3270
	11th to 20th	Day	45% 4 11% 21% 15% 4	3140
	21st to end	Day	40% 4 13% 26% 16% 1	3428
November	1st to 10th	Day	43% 5% 12% 24% 15% 2	1767
	11th to 20th	Day	54% 5% 10% 19% 9% 3	1800
	21st to end	Day	61% 6% 11% 15% 6% 1	1628
December	1st to 10th	Day	68% 3 12% 14% 3	2600
	11th to 20th	Day	67% 4 11% 13% 5%	2986
	21st to end	Day	77% 5% 7% 8% 3	4325
January	1st to 10th	Night	74% 5% 7% 10% 4	5889
	11th to 20th	Night	75% 5% 9% 8% 4	5900
	21st to end	Night	70% 3 9% 11% 5% 1	6490
February	1st to 10th	Night	64% 5% 11% 15% 5%	2600
	11th to 20th	Night	62% 5% 10% 15% 7% 1	2456
	21st to end	Night	63% 6% 8% 13% 8% 2	1885
March	1st to 10th	Night	55% 5% 10% 21% 9%	3492
	11th to 20th	Night	44% 10% 15% 23% 7% 1	3480
	21st to end	Night	47% 7% 12% 24% 10% 1	3817
April	1st to 10th	Night	49% 6% 12% 18% 13% 2	2062
	11th to 20th	Night	47% 6% 12% 16% 16% 3	1737
	21st to end	Night	45% 8% 12% 18% 15% 2	1710
May	1st to 10th	Night	55% 9% 9% 18% 8% 1	2840
	11th to 20th	Night	44% 8% 11% 23% 12% 2	2813
	21st to end	Night	31% 10% 17% 24% 14% 4	3087
June	1st to 10th	Night	36% 10% 12% 23% 17% 3	1577
	11th to 20th	Night	36% 8% 15% 22% 16% 3	1520
	21st to end	Night	35% 9% 13% 24% 18% 1	1626
July	1st to 10th	Night	37% 6% 12% 23% 17% 5%	2795
	11th to 20th	Night	33% 5% 14% 30% 17% 1	2786
	21st to end	Night	37% 7% 14% 19% 22% 2	2988
August	1st to 10th	Night	42% 6% 14% 23% 13% 1	2814
	11th to 20th	Night	40% 5% 11% 25% 18% 1	2812
	21st to end	Night	35% 5% 14% 30% 13% 2	3174
September	1st to 10th	Night	33% 7% 13% 24% 19% 5%	2014
	11th to 20th	Night	29% 7% 12% 25% 24% 2	2090
	21st to end	Night	42% 6% 12% 21% 19% 1	2090
October	1st to 10th	Night	31% 4 9% 28% 26% 1	3690
	11th to 20th	Night	39% 4 12% 22% 16% 5%	3779
	21st to end	Night	37% 4 10% 29% 19% 2	4120
November	1st to 10th	Night	40% 6% 12% 23% 16% 3	2529
	11th to 20th	Night	53% 5% 11% 18% 11% 3	2600
	21st to end	Night	58% 6% 12% 18% 6% 1	2768
December	1st to 10th	Night	64% 4 11% 18% 4	4580
	11th to 20th	Night	66% 4 14% 13% 3	5208
	21st to end	Night	71% 6% 9% 12% 2	6611

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	29% 23% 33% 9% 5% 1	300
	11th to 20th	Day	28% 27% 35% 7% 2	300
	21st to end	Day	26% 26% 33% 13% 2	330
February	1st to 10th	Day	26% 23% 37% 12% 1 1	300
	11th to 20th	Day	32% 26% 32% 9% 1	300
	21st to end	Day	28% 29% 32% 10%	225
March	1st to 10th	Day	27% 19% 31% 17% 6%	408
	11th to 20th	Day	29% 14% 35% 16% 6%	460
	21st to end	Day	33% 14% 28% 19% 5% 1	506
April	1st to 10th	Day	23% 18% 33% 16% 8% 2	460
	11th to 20th	Day	24% 19% 26% 17% 9% 5%	452
	21st to end	Day	24% 14% 33% 20% 8% 2	460
May	1st to 10th	Day	27% 16% 26% 19% 10% 3	463
	11th to 20th	Day	20% 10% 32% 20% 12% 6%	487
	21st to end	Day	13% 6% 26% 27% 21% 7%	529
June	1st to 10th	Day	14% 7% 27% 32% 15% 5%	490
	11th to 20th	Day	18% 7% 22% 29% 19% 5%	490
	21st to end	Day	12% 8% 29% 26% 18% 6%	490
July	1st to 10th	Day	13% 3 19% 27% 23% 15%	485
	11th to 20th	Day	12% 5% 23% 25% 24% 11%	484
	21st to end	Day	12% 3 24% 26% 25% 10%	532
August	1st to 10th	Day	15% 5% 17% 28% 24% 12%	486
	11th to 20th	Day	15% 5% 20% 28% 22% 10%	479
	21st to end	Day	9% 6% 17% 26% 27% 15%	539
September	1st to 10th	Day	13% 6% 21% 25% 22% 12%	490
	11th to 20th	Day	10% 3 16% 26% 31% 13%	490
	21st to end	Day	24% 9% 22% 22% 14% 8%	490
October	1st to 10th	Day	12% 9% 29% 24% 20% 6%	490
	11th to 20th	Day	21% 11% 33% 23% 10% 3	490
	21st to end	Day	27% 15% 30% 20% 7% 1	539
November	1st to 10th	Day	16% 18% 36% 17% 10% 2	305
	11th to 20th	Day	28% 20% 30% 15% 7% 1	300
	21st to end	Day	27% 19% 38% 15% 1	213
December	1st to 10th	Day	29% 21% 33% 13% 5% 2	200
	11th to 20th	Day	27% 18% 39% 15% 2	200
	21st to end	Day	32% 21% 31% 13% 2	299
January	1st to 10th	Night	26% 17% 35% 18% 4	800
	11th to 20th	Night	27% 18% 34% 17% 4	800
	21st to end	Night	29% 15% 31% 22% 3	880
February	1st to 10th	Night	28% 14% 33% 22% 3	800
	11th to 20th	Night	26% 15% 36% 21% 2	783
	21st to end	Night	26% 15% 37% 19% 3	629
March	1st to 10th	Night	24% 14% 31% 27% 5%	760
	11th to 20th	Night	22% 14% 33% 25% 6% 1	760
	21st to end	Night	24% 11% 33% 25% 6% 1	836
April	1st to 10th	Night	22% 13% 34% 25% 5% 1	760
	11th to 20th	Night	20% 13% 34% 22% 10% 1	613
	21st to end	Night	20% 8% 34% 30% 7% 1	570
May	1st to 10th	Night	20% 8% 28% 34% 9% 1	570
	11th to 20th	Night	22% 5% 22% 36% 13% 2	561
	21st to end	Night	10% 2 20% 41% 24% 3	617
June	1st to 10th	Night	11% 3 24% 43% 19%	437
	11th to 20th	Night	10% 4 23% 46% 15% 1	380
	21st to end	Night	10% 2 24% 41% 22%	486
July	1st to 10th	Night	10% 3 13% 41% 28% 6%	555
	11th to 20th	Night	9% 3 17% 40% 28% 4	552
	21st to end	Night	9% 3 14% 42% 30% 2	624
August	1st to 10th	Night	9% 3 13% 42% 31% 2	567
	11th to 20th	Night	12% 3 12% 41% 27% 4	564
	21st to end	Night	8% 2 17% 34% 33% 6%	651
September	1st to 10th	Night	14% 4 18% 32% 28% 5%	760
	11th to 20th	Night	11% 2 13% 36% 33% 5%	760
	21st to end	Night	18% 6% 22% 30% 19% 4	760
October	1st to 10th	Night	16% 6% 22% 35% 18% 4	760
	11th to 20th	Night	21% 9% 30% 27% 11% 2	760
	21st to end	Night	26% 11% 25% 30% 8%	836
November	1st to 10th	Night	18% 9% 30% 30% 13% 1	780
	11th to 20th	Night	23% 12% 33% 24% 7% 1	800
	21st to end	Night	27% 13% 32% 22% 6%	800
December	1st to 10th	Night	29% 14% 32% 21% 4	800
	11th to 20th	Night	26% 13% 34% 23% 4	800
	21st to end	Night	28% 13% 33% 23% 2	880

Page generated at 2025-02-18 03:26:31. Version ac07

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