Webb Schools Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Webb Schools Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	4 3 1 3 2 3 3 2 4 6% 69%	140866
	Night	5% 4 1 3 2 3 3 2 4 6% 67%	138785
Transparency	Day	19% 2 4 16% 28% 30%	135954
	Night	23% 3 5% 17% 26% 27%	134672
Seeing	Day	6% 10% 29% 35% 15% 5%	17620
	Night	6% 4 18% 37% 30% 5%	27532

By Month

Percentage Cloud Cover SW (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	10% 3 2 3 2 3 3 2 4 5% 63%	4762
	11th to 20th	Day	5% 3 1 2 1 2 2 2 3 7% 73%	5554
	21st to end	Day	8% 5% 2 5% 4 4 4 3 5% 8% 52%	6500
February	1st to 10th	Day	10% 5% 1 4 2 2 3 3 5% 5% 59%	2100
	11th to 20th	Day	12% 5% 1 3 3 3 3 3 5% 6% 57%	2050
	21st to end	Day	9% 7% 2 5% 4 4 3 3 4 6% 53%	1730
March	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 6% 7% 46%	4618
	11th to 20th	Day	9% 6% 2 4 4 4 4 3 5% 7% 53%	4784
	21st to end	Day	8% 4 1 4 4 5% 5% 4 5% 8% 51%	6073
April	1st to 10th	Day	6% 5% 1 4 4 4 3 3 5% 9% 57%	2520
	11th to 20th	Day	5% 4 2 4 3 5% 3 3 4 6% 61%	2442
	21st to end	Day	5% 5% 1 3 4 3 3 3 4 7% 64%	2400
May	1st to 10th	Day	4 6% 2 3 2 3 3 3 4 6% 66%	5660
	11th to 20th	Day	2 3 1 3 3 5% 5% 4 5% 7% 63%	5841
	21st to end	Day	2 2 1 2 2 3 5% 3 4 6% 68%	6423
June	1st to 10th	Day	3 2 1 3 2 3 3 2 4 8% 68%	2600
	11th to 20th	Day	2 2 1 1 2 2 3 2 3 6% 78%	2736
	21st to end	Day	1 2 1 2 2 2 2 1 3 4 81%	2800
July	1st to 10th	Day	1 1 1 1 1 2 3 90%	6319
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 84%	6100
	21st to end	Day	1 1 1 1 1 2 2 3 4 84%	6083
August	1st to 10th	Day	1 1 1 1 1 1 1 2 4 87%	5346
	11th to 20th	Day	1 1 1 1 1 1 3 92%	5364
	21st to end	Day	1 1 1 1 1 2 2 2 3 84%	5984
September	1st to 10th	Day	2 1 1 1 1 2 2 2 3 5% 81%	2400
	11th to 20th	Day	1 2 1 1 1 2 1 3 5% 82%	2230
	21st to end	Day	2 2 1 2 1 2 2 2 3 4 79%	2200
October	1st to 10th	Day	3 2 2 1 1 2 2 2 4 81%	3600
	11th to 20th	Day	2 2 1 1 1 2 3 2 5% 7% 75%	3431
	21st to end	Day	3 2 1 3 2 2 3 3 5% 6% 69%	3434
November	1st to 10th	Day	4 3 1 1 2 2 3 3 4 7% 70%	1854
	11th to 20th	Day	6% 3 1 3 2 3 3 2 4 5% 67%	1890
	21st to end	Day	5% 3 1 4 4 4 4 3 4 7% 60%	1890
December	1st to 10th	Day	4 2 1 4 2 2 3 3 5% 7% 67%	3040
	11th to 20th	Day	9% 4 1 4 3 3 5% 4 6% 8% 51%	3499
	21st to end	Day	10% 5% 2 5% 4 5% 5% 3 5% 7% 49%	4609
January	1st to 10th	Night	11% 4 1 4 3 3 3 2 4 4 61%	8081
	11th to 20th	Night	3 3 1 3 2 2 3 2 4 7% 71%	7380
	21st to end	Night	9% 6% 2 5% 3 4 4 3 5% 7% 51%	7747
February	1st to 10th	Night	10% 5% 2 4 3 3 3 3 4 5% 60%	2520
	11th to 20th	Night	9% 5% 2 4 3 3 4 3 4 6% 59%	2458
	21st to end	Night	9% 6% 2 4 3 4 4 4 4 5% 55%	2076
March	1st to 10th	Night	10% 10% 2 5% 3 4 5% 3 6% 6% 45%	5496
	11th to 20th	Night	7% 4 2 5% 3 4 5% 3 6% 7% 53%	5434
	21st to end	Night	7% 6% 2 5% 4 5% 5% 3 5% 7% 51%	5127
April	1st to 10th	Night	6% 5% 2 4 3 3 4 3 5% 7% 58%	2100
	11th to 20th	Night	4 5% 2 2 3 4 4 2 4 7% 62%	2039
	21st to end	Night	4 6% 1 3 2 3 4 3 4 7% 63%	2000
May	1st to 10th	Night	5% 7% 2 3 2 3 5% 3 4 6% 61%	4316
	11th to 20th	Night	2 4 1 2 2 3 5% 4 5% 9% 63%	4074
	21st to end	Night	1 3 1 2 4 5% 3 4 9% 66%	4474
June	1st to 10th	Night	1 3 1 2 2 2 4 3 5% 9% 68%	1800
	11th to 20th	Night	1 1 1 1 3 2 2 4 7% 77%	1655
	21st to end	Night	1 1 1 1 1 1 2 2 3 5% 82%	1600
July	1st to 10th	Night	1 1 1 4 92%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 2 6% 84%	3865
	21st to end	Night	1 1 1 1 1 2 2 4 5% 82%	4787
August	1st to 10th	Night	1 1 1 1 1 1 1 2 4 86%	4486
	11th to 20th	Night	1 1 1 3 6% 88%	4506
	21st to end	Night	1 1 1 2 1 1 2 1 3 5% 83%	5015
September	1st to 10th	Night	2 2 1 2 1 2 2 2 2 5% 80%	2000
	11th to 20th	Night	1 1 1 1 1 2 3 1 2 4 83%	2170
	21st to end	Night	1 2 1 1 1 2 1 2 3 4 82%	2200
October	1st to 10th	Night	2 2 1 2 1 1 2 1 2 4 83%	3600
	11th to 20th	Night	2 2 1 2 1 2 5% 2 7% 8% 66%	3722
	21st to end	Night	2 3 1 3 3 2 2 1 4 5% 74%	4370
November	1st to 10th	Night	3 3 1 3 2 2 3 2 3 5% 72%	2665
	11th to 20th	Night	5% 4 1 4 2 3 2 3 3 6% 67%	2730
	21st to end	Night	6% 4 2 4 3 4 4 3 5% 7% 59%	2730
December	1st to 10th	Night	5% 3 1 3 2 3 3 2 4 5% 68%	4370
	11th to 20th	Night	9% 4 1 4 3 4 5% 4 6% 8% 52%	4941
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 6% 11% 46%	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	26% 2 5% 17% 26% 24%	4763
	11th to 20th	Day	15% 2 7% 15% 19% 42%	5554
	21st to end	Day	31% 2 2 15% 23% 27%	6500
February	1st to 10th	Day	26% 2 4 13% 26% 28%	2100
	11th to 20th	Day	27% 2 4 19% 25% 24%	2060
	21st to end	Day	30% 2 5% 17% 25% 21%	1730
March	1st to 10th	Day	35% 3 5% 19% 23% 16%	4618
	11th to 20th	Day	31% 2 7% 26% 21% 12%	4784
	21st to end	Day	29% 2 10% 30% 18% 10%	6073
April	1st to 10th	Day	24% 4 9% 25% 23% 15%	2520
	11th to 20th	Day	24% 3 4 24% 27% 18%	2445
	21st to end	Day	22% 2 5% 26% 32% 13%	2400
May	1st to 10th	Day	23% 2 6% 19% 32% 17%	5660
	11th to 20th	Day	25% 3 5% 23% 34% 10%	5844
	21st to end	Day	22% 3 3 24% 38% 11%	6424
June	1st to 10th	Day	22% 4 6% 22% 32% 15%	2600
	11th to 20th	Day	16% 2 3 13% 32% 34%	2736
	21st to end	Day	13% 2 2 7% 29% 47%	2800
July	1st to 10th	Day	5% 1 1 4 19% 69%	6251
	11th to 20th	Day	8% 4 3 9% 38% 38%	6026
	21st to end	Day	7% 3 3 14% 35% 38%	5909
August	1st to 10th	Day	8% 1 1 11% 43% 37%	3890
	11th to 20th	Day	5% 2 1 11% 30% 52%	3880
	21st to end	Day	7% 2 2 7% 27% 56%	4311
September	1st to 10th	Day	11% 2 3 8% 33% 44%	2400
	11th to 20th	Day	12% 2 1 11% 35% 40%	2230
	21st to end	Day	11% 1 1 16% 32% 39%	2200
October	1st to 10th	Day	11% 1 2 10% 18% 57%	3600
	11th to 20th	Day	13% 4 2 13% 38% 31%	3431
	21st to end	Day	17% 3 2 9% 28% 42%	3434
November	1st to 10th	Day	15% 2 4 14% 29% 37%	1854
	11th to 20th	Day	19% 3 4 14% 22% 39%	1890
	21st to end	Day	24% 2 6% 12% 22% 33%	1890
December	1st to 10th	Day	18% 2 4 16% 24% 36%	3040
	11th to 20th	Day	29% 3 6% 14% 26% 21%	3498
	21st to end	Day	35% 2 6% 24% 23% 10%	4609
January	1st to 10th	Night	29% 3 3 14% 27% 23%	8081
	11th to 20th	Night	18% 2 7% 15% 16% 42%	7380
	21st to end	Night	36% 3 3 11% 22% 25%	7747
February	1st to 10th	Night	28% 2 5% 15% 23% 27%	2520
	11th to 20th	Night	28% 2 6% 19% 22% 23%	2459
	21st to end	Night	30% 2 5% 18% 22% 23%	2076
March	1st to 10th	Night	38% 2 7% 21% 20% 12%	5496
	11th to 20th	Night	32% 2 9% 26% 18% 13%	5434
	21st to end	Night	35% 4 11% 24% 20% 7%	5127
April	1st to 10th	Night	27% 3 10% 24% 21% 15%	2100
	11th to 20th	Night	25% 3 6% 25% 25% 16%	2039
	21st to end	Night	26% 4 6% 26% 28% 10%	2000
May	1st to 10th	Night	30% 4 5% 22% 30% 8%	4313
	11th to 20th	Night	32% 3 5% 28% 28% 5%	4075
	21st to end	Night	24% 4 4 32% 27% 9%	4474
June	1st to 10th	Night	26% 4 7% 21% 29% 13%	1800
	11th to 20th	Night	17% 4 3 15% 33% 28%	1655
	21st to end	Night	15% 3 2 7% 30% 43%	1600
July	1st to 10th	Night	5% 2 2 8% 17% 65%	3600
	11th to 20th	Night	9% 7% 4 12% 38% 30%	3817
	21st to end	Night	9% 5% 3 18% 36% 30%	4650
August	1st to 10th	Night	9% 1 2 14% 43% 31%	3256
	11th to 20th	Night	6% 2 1 15% 34% 41%	3256
	21st to end	Night	8% 4 1 6% 36% 45%	3608
September	1st to 10th	Night	17% 2 2 7% 32% 40%	2000
	11th to 20th	Night	13% 2 2 11% 35% 37%	2170
	21st to end	Night	13% 2 2 14% 35% 34%	2200
October	1st to 10th	Night	11% 2 2 11% 19% 55%	3600
	11th to 20th	Night	17% 9% 2 11% 32% 29%	3722
	21st to end	Night	17% 2 1 10% 26% 43%	4370
November	1st to 10th	Night	17% 2 4 15% 26% 35%	2665
	11th to 20th	Night	20% 3 4 15% 21% 38%	2730
	21st to end	Night	25% 2 6% 17% 18% 32%	2730
December	1st to 10th	Night	21% 2 3 14% 24% 36%	4370
	11th to 20th	Night	31% 3 7% 14% 24% 21%	4941
	21st to end	Night	35% 2 10% 24% 19% 9%	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	11% 14% 36% 30% 9% 1	228
	11th to 20th	Day	11% 10% 31% 34% 14% 1	429
	21st to end	Day	10% 16% 35% 29% 9% 1	594
February	1st to 10th	Day	14% 14% 32% 29% 10% 2	540
	11th to 20th	Day	14% 17% 35% 26% 8% 1	535
	21st to end	Day	11% 16% 36% 29% 8%	446
March	1st to 10th	Day	15% 21% 32% 24% 9%	540
	11th to 20th	Day	14% 15% 33% 27% 9% 1	540
	21st to end	Day	9% 20% 38% 24% 7% 2	594
April	1st to 10th	Day	10% 16% 30% 29% 12% 3	540
	11th to 20th	Day	7% 19% 38% 26% 8% 3	517
	21st to end	Day	5% 18% 32% 28% 12% 5%	500
May	1st to 10th	Day	9% 16% 32% 30% 10% 3	503
	11th to 20th	Day	6% 13% 34% 30% 13% 3	528
	21st to end	Day	6% 11% 35% 31% 13% 3	573
June	1st to 10th	Day	3 7% 28% 35% 22% 4	530
	11th to 20th	Day	1 7% 31% 40% 16% 5%	530
	21st to end	Day	2 5% 30% 42% 16% 7%	530
July	1st to 10th	Day	1 2 20% 47% 21% 10%	526
	11th to 20th	Day	1 1 16% 45% 23% 14%	524
	21st to end	Day	1 2 18% 47% 18% 14%	575
August	1st to 10th	Day	2 1 25% 44% 18% 10%	527
	11th to 20th	Day	1 2 20% 44% 21% 12%	517
	21st to end	Day	2 2 21% 45% 20% 10%	583
September	1st to 10th	Day	1 3 28% 40% 18% 9%	530
	11th to 20th	Day	2 7% 30% 40% 16% 5%	530
	21st to end	Day	2 8% 32% 32% 19% 7%	530
October	1st to 10th	Day	4 10% 32% 32% 19% 2	530
	11th to 20th	Day	3 11% 31% 36% 16% 3	530
	21st to end	Day	5% 9% 29% 32% 20% 5%	508
November	1st to 10th	Day	4 8% 21% 41% 24% 2	330
	11th to 20th	Day	8% 8% 27% 38% 16% 3	330
	21st to end	Day	5% 10% 30% 35% 18% 2	330
December	1st to 10th	Day	8% 7% 25% 39% 18% 3	330
	11th to 20th	Day	5% 8% 27% 36% 22% 2	330
	21st to end	Day	17% 13% 29% 32% 9% 1	363
January	1st to 10th	Night	12% 6% 25% 38% 15% 3	840
	11th to 20th	Night	7% 4 26% 43% 18% 2	840
	21st to end	Night	13% 6% 27% 37% 15% 2	924
February	1st to 10th	Night	10% 7% 24% 38% 19% 3	840
	11th to 20th	Night	11% 6% 28% 36% 16% 3	824
	21st to end	Night	11% 7% 25% 40% 15% 2	692
March	1st to 10th	Night	9% 6% 28% 39% 18% 1	840
	11th to 20th	Night	9% 6% 23% 37% 21% 5%	840
	21st to end	Night	9% 7% 25% 39% 19% 2	924
April	1st to 10th	Night	8% 5% 22% 39% 22% 3	840
	11th to 20th	Night	6% 7% 26% 36% 23% 2	815
	21st to end	Night	5% 7% 20% 37% 26% 5%	800
May	1st to 10th	Night	7% 6% 22% 37% 24% 3	699
	11th to 20th	Night	4 6% 20% 36% 31% 4	591
	21st to end	Night	3 7% 21% 37% 28% 4	650
June	1st to 10th	Night	2 3 19% 40% 33% 3	600
	11th to 20th	Night	3 9% 41% 43% 4	455
	21st to end	Night	1 1 3 38% 51% 6%	400
July	1st to 10th	Night	1 3 17% 69% 10%	390
	11th to 20th	Night	2 3 25% 59% 10%	483
	21st to end	Night	2 1 4 30% 53% 11%	793
August	1st to 10th	Night	2 1 6% 31% 48% 13%	796
	11th to 20th	Night	1 1 3 27% 54% 15%	792
	21st to end	Night	2 5% 27% 53% 13%	880
September	1st to 10th	Night	2 1 9% 27% 51% 11%	800
	11th to 20th	Night	1 2 8% 35% 44% 10%	800
	21st to end	Night	2 2 13% 34% 41% 9%	800
October	1st to 10th	Night	4 2 14% 44% 35% 3	800
	11th to 20th	Night	4 4 17% 39% 32% 5%	800
	21st to end	Night	3 4 19% 40% 26% 8%	880
November	1st to 10th	Night	4 3 17% 41% 30% 5%	820
	11th to 20th	Night	6% 3 18% 44% 22% 7%	840
	21st to end	Night	6% 5% 23% 40% 22% 4	840
December	1st to 10th	Night	9% 4 23% 42% 20% 3	840
	11th to 20th	Night	11% 4 25% 39% 19% 3	840
	21st to end	Night	10% 5% 31% 37% 14% 3	924

Page generated at 2026-04-13 23:44:27. Version ac07

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