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 2024-01-16.

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	5% 3 1 3 3 2 3 3 4 6% 67%	75205
	Night	5% 4 1 3 3 2 3 3 4 6% 66%	74608
Transparency	Day	21% 2 5% 17% 27% 28%	74929
	Night	24% 2 5% 17% 25% 26%	74407
Seeing	Day	6% 10% 29% 35% 15% 5%	14515
	Night	6% 4 18% 36% 31% 6%	24474

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 4 2 4 3 5% 6% 58%	1533
	11th to 20th	Day	9% 3 2 3 3 2 4 3 3 7% 60%	1728
	21st to end	Day	11% 5% 2 4 4 2 3 3 3 4 58%	1980
February	1st to 10th	Day	10% 4 2 4 2 1 3 3 4 6% 62%	1800
	11th to 20th	Day	11% 4 1 3 3 2 3 3 4 6% 61%	1750
	21st to end	Day	11% 7% 2 4 4 3 4 4 3 6% 53%	1480
March	1st to 10th	Day	10% 5% 2 5% 5% 4 4 4 6% 7% 47%	1800
	11th to 20th	Day	8% 5% 2 3 4 3 4 3 6% 9% 54%	1844
	21st to end	Day	6% 4 1 4 5% 4 5% 5% 5% 7% 55%	2362
April	1st to 10th	Day	6% 4 2 4 4 3 4 3 4 9% 57%	2160
	11th to 20th	Day	5% 3 2 4 4 3 4 4 3 6% 63%	2129
	21st to end	Day	6% 4 1 2 3 3 3 3 3 7% 65%	2160
May	1st to 10th	Day	6% 5% 2 3 3 3 4 4 4 7% 59%	2250
	11th to 20th	Day	4 4 2 3 3 2 3 3 5% 7% 64%	2301
	21st to end	Day	6% 3 2 3 3 2 4 4 4 6% 63%	2530
June	1st to 10th	Day	4 3 1 3 2 2 3 2 4 8% 68%	2340
	11th to 20th	Day	2 2 1 2 2 2 3 2 3 5% 78%	2462
	21st to end	Day	1 2 1 2 2 1 3 2 2 4 81%	2520
July	1st to 10th	Day	1 1 1 1 2 2 2 3 4 5% 79%	2520
	11th to 20th	Day	2 1 1 2 2 1 2 3 3 5% 79%	2430
	21st to end	Day	2 1 1 2 2 1 3 4 4 7% 72%	2419
August	1st to 10th	Day	1 1 1 2 2 1 2 2 2 4 83%	2149
	11th to 20th	Day	1 1 1 2 1 2 2 3 5% 83%	2122
	21st to end	Day	1 1 2 2 2 2 3 3 4 80%	2376
September	1st to 10th	Day	2 1 1 1 2 1 2 2 3 5% 80%	2160
	11th to 20th	Day	1 1 1 1 1 1 2 2 5% 84%	2008
	21st to end	Day	2 2 1 2 2 2 2 2 3 4 79%	1980
October	1st to 10th	Day	5% 3 1 3 2 1 3 3 3 6% 70%	1980
	11th to 20th	Day	4 2 1 2 2 2 2 3 3 4 77%	1900
	21st to end	Day	4 2 1 2 2 2 2 3 3 4 74%	1913
November	1st to 10th	Day	4 3 2 2 2 2 3 3 4 7% 70%	1674
	11th to 20th	Day	5% 3 1 2 2 2 4 2 4 6% 68%	1710
	21st to end	Day	6% 3 1 4 4 3 4 4 3 6% 62%	1710
December	1st to 10th	Day	7% 4 2 4 3 2 3 3 5% 7% 59%	1710
	11th to 20th	Day	14% 6% 2 5% 5% 3 3 4 6% 9% 43%	2169
	21st to end	Day	13% 6% 2 5% 6% 4 5% 4 4 5% 45%	3146
January	1st to 10th	Night	10% 4 1 3 3 3 3 3 5% 7% 57%	2616
	11th to 20th	Night	7% 4 1 3 2 2 3 4 3 8% 62%	2343
	21st to end	Night	10% 5% 2 3 3 2 3 3 3 6% 60%	2376
February	1st to 10th	Night	9% 4 2 4 3 2 3 3 3 5% 64%	2160
	11th to 20th	Night	8% 4 2 4 3 1 4 3 3 5% 63%	2098
	21st to end	Night	10% 6% 2 3 3 3 5% 4 4 5% 54%	1776
March	1st to 10th	Night	8% 6% 3 4 4 3 5% 4 6% 8% 50%	2160
	11th to 20th	Night	8% 5% 2 3 3 2 4 4 4 7% 58%	2115
	21st to end	Night	7% 5% 3 4 4 3 4 3 5% 7% 56%	1990
April	1st to 10th	Night	6% 4 3 4 4 2 4 4 4 7% 57%	1800
	11th to 20th	Night	4 5% 2 2 3 3 4 3 4 7% 64%	1779
	21st to end	Night	5% 4 1 3 3 2 3 3 3 7% 65%	1800
May	1st to 10th	Night	4 5% 2 3 4 3 5% 4 5% 6% 61%	1710
	11th to 20th	Night	2 3 3 3 3 3 3 3 4 7% 65%	1594
	21st to end	Night	2 3 1 2 3 2 4 3 4 9% 67%	1746
June	1st to 10th	Night	1 3 1 2 1 2 4 3 4 7% 71%	1620
	11th to 20th	Night	1 1 1 1 1 2 3 3 3 6% 78%	1490
	21st to end	Night	1 1 1 1 1 1 2 2 3 5% 82%	1440
July	1st to 10th	Night	1 1 1 1 2 2 2 7% 83%	1440
	11th to 20th	Night	2 1 1 2 2 2 2 3 3 6% 78%	1525
	21st to end	Night	2 2 1 2 2 1 3 3 4 7% 73%	1895
August	1st to 10th	Night	2 1 1 1 1 1 2 2 3 6% 80%	1790
	11th to 20th	Night	1 1 1 1 1 1 2 3 5% 84%	1780
	21st to end	Night	1 1 1 1 1 2 2 3 4 83%	1980
September	1st to 10th	Night	2 2 1 2 1 1 2 2 3 5% 80%	1800
	11th to 20th	Night	1 1 1 1 1 1 2 2 2 4 84%	1952
	21st to end	Night	1 2 1 1 2 1 2 1 3 4 82%	1980
October	1st to 10th	Night	3 3 1 3 3 2 3 3 3 6% 71%	1980
	11th to 20th	Night	3 2 1 2 2 2 2 2 2 5% 77%	2056
	21st to end	Night	3 3 1 2 3 2 3 2 3 4 73%	2440
November	1st to 10th	Night	4 3 1 2 3 2 3 2 3 5% 72%	2405
	11th to 20th	Night	4 3 1 3 3 2 3 3 3 6% 69%	2470
	21st to end	Night	6% 4 2 3 3 3 5% 3 5% 6% 59%	2470
December	1st to 10th	Night	8% 4 2 4 4 2 3 3 4 7% 58%	2470
	11th to 20th	Night	15% 6% 2 4 4 3 4 4 5% 8% 45%	3041
	21st to end	Night	9% 7% 2 5% 6% 4 4 5% 4 8% 46%	4521

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	27% 3 5% 18% 24% 22%	1533
	11th to 20th	Day	25% 3 6% 15% 23% 28%	1728
	21st to end	Day	29% 2 5% 17% 19% 28%	1980
February	1st to 10th	Day	24% 2 4 13% 26% 30%	1800
	11th to 20th	Day	24% 2 3 17% 27% 27%	1760
	21st to end	Day	31% 2 5% 16% 25% 21%	1480
March	1st to 10th	Day	32% 3 6% 19% 18% 22%	1800
	11th to 20th	Day	26% 3 9% 20% 24% 18%	1844
	21st to end	Day	25% 3 10% 24% 23% 15%	2362
April	1st to 10th	Day	25% 4 9% 27% 21% 14%	2160
	11th to 20th	Day	23% 3 4 23% 28% 19%	2132
	21st to end	Day	21% 2 6% 25% 32% 14%	2160
May	1st to 10th	Day	26% 3 7% 20% 28% 15%	2250
	11th to 20th	Day	24% 3 5% 23% 31% 14%	2304
	21st to end	Day	25% 3 6% 28% 28% 9%	2530
June	1st to 10th	Day	21% 3 6% 20% 32% 16%	2340
	11th to 20th	Day	17% 2 3 13% 33% 33%	2462
	21st to end	Day	14% 2 2 7% 30% 45%	2520
July	1st to 10th	Day	12% 3 2 10% 24% 50%	2451
	11th to 20th	Day	13% 3 4 17% 30% 33%	2356
	21st to end	Day	13% 2 7% 23% 35% 20%	2285
August	1st to 10th	Day	11% 1 1 12% 33% 42%	2135
	11th to 20th	Day	9% 2 12% 36% 40%	2122
	21st to end	Day	11% 3 2 10% 36% 39%	2376
September	1st to 10th	Day	12% 2 3 8% 33% 42%	2160
	11th to 20th	Day	11% 1 11% 34% 41%	2008
	21st to end	Day	11% 1 1 16% 34% 37%	1980
October	1st to 10th	Day	18% 2 4 18% 24% 34%	1980
	11th to 20th	Day	15% 2 3 13% 29% 38%	1900
	21st to end	Day	16% 3 4 10% 22% 46%	1913
November	1st to 10th	Day	16% 2 4 14% 28% 36%	1674
	11th to 20th	Day	17% 2 4 15% 21% 41%	1710
	21st to end	Day	23% 2 5% 12% 23% 34%	1710
December	1st to 10th	Day	25% 2 7% 17% 20% 28%	1710
	11th to 20th	Day	37% 3 8% 14% 19% 20%	2168
	21st to end	Day	41% 1 5% 22% 17% 13%	3146
January	1st to 10th	Night	29% 3 6% 15% 25% 23%	2616
	11th to 20th	Night	25% 3 6% 16% 21% 29%	2343
	21st to end	Night	28% 2 7% 15% 17% 30%	2376
February	1st to 10th	Night	25% 2 5% 15% 24% 29%	2160
	11th to 20th	Night	25% 2 5% 18% 24% 26%	2099
	21st to end	Night	32% 2 6% 16% 22% 23%	1776
March	1st to 10th	Night	32% 3 6% 21% 19% 19%	2160
	11th to 20th	Night	27% 2 9% 19% 25% 18%	2115
	21st to end	Night	29% 2 7% 24% 22% 14%	1990
April	1st to 10th	Night	29% 3 11% 24% 19% 15%	1800
	11th to 20th	Night	25% 2 7% 25% 24% 18%	1779
	21st to end	Night	25% 3 6% 25% 29% 11%	1800
May	1st to 10th	Night	28% 3 8% 23% 23% 15%	1710
	11th to 20th	Night	28% 3 7% 26% 25% 11%	1595
	21st to end	Night	24% 2 8% 33% 25% 8%	1746
June	1st to 10th	Night	24% 4 7% 21% 30% 15%	1620
	11th to 20th	Night	18% 2 4 16% 33% 27%	1490
	21st to end	Night	16% 3 2 7% 30% 42%	1440
July	1st to 10th	Night	13% 3 2 11% 28% 44%	1400
	11th to 20th	Night	15% 3 5% 20% 28% 29%	1477
	21st to end	Night	17% 3 6% 24% 32% 18%	1790
August	1st to 10th	Night	14% 2 3 12% 36% 34%	1780
	11th to 20th	Night	11% 3 1 12% 39% 34%	1780
	21st to end	Night	13% 4 1 10% 35% 37%	1980
September	1st to 10th	Night	17% 2 2 7% 31% 40%	1800
	11th to 20th	Night	12% 1 2 11% 33% 40%	1952
	21st to end	Night	13% 2 1 14% 36% 34%	1980
October	1st to 10th	Night	20% 3 4 18% 25% 31%	1980
	11th to 20th	Night	17% 3 3 14% 30% 33%	2056
	21st to end	Night	20% 2 2 10% 20% 45%	2440
November	1st to 10th	Night	18% 2 4 15% 25% 35%	2405
	11th to 20th	Night	18% 3 4 15% 20% 40%	2470
	21st to end	Night	25% 2 5% 17% 18% 33%	2470
December	1st to 10th	Night	30% 2 6% 15% 19% 30%	2470
	11th to 20th	Night	39% 2 7% 14% 22% 16%	3041
	21st to end	Night	42% 1 7% 20% 17% 13%	4521

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% 12% 35% 32% 10% 1	187
	11th to 20th	Day	13% 10% 32% 32% 13% 1	344
	21st to end	Day	10% 18% 36% 27% 8% 2	462
February	1st to 10th	Day	12% 14% 31% 29% 11% 3	420
	11th to 20th	Day	11% 17% 35% 28% 7% 1	415
	21st to end	Day	13% 18% 34% 28% 7%	346
March	1st to 10th	Day	15% 19% 32% 24% 9%	420
	11th to 20th	Day	13% 14% 35% 28% 9% 1	420
	21st to end	Day	10% 19% 40% 24% 6% 1	462
April	1st to 10th	Day	11% 14% 32% 29% 12% 2	420
	11th to 20th	Day	5% 20% 40% 26% 6% 3	412
	21st to end	Day	4 19% 35% 27% 12% 4	420
May	1st to 10th	Day	9% 15% 33% 30% 9% 3	423
	11th to 20th	Day	6% 12% 34% 31% 13% 3	448
	21st to end	Day	8% 11% 35% 30% 13% 3	485
June	1st to 10th	Day	3 8% 30% 34% 21% 4	450
	11th to 20th	Day	1 6% 29% 40% 17% 6%	450
	21st to end	Day	2 4 28% 43% 15% 7%	450
July	1st to 10th	Day	1 2 19% 48% 21% 10%	446
	11th to 20th	Day	1 1 16% 45% 23% 14%	444
	21st to end	Day	1 1 16% 49% 18% 14%	487
August	1st to 10th	Day	2 1 24% 45% 18% 10%	447
	11th to 20th	Day	1 2 18% 46% 21% 12%	437
	21st to end	Day	1 1 19% 48% 21% 10%	495
September	1st to 10th	Day	2 3 28% 41% 19% 7%	450
	11th to 20th	Day	2 6% 29% 41% 17% 5%	450
	21st to end	Day	2 8% 29% 34% 20% 7%	450
October	1st to 10th	Day	4 11% 33% 32% 19% 2	450
	11th to 20th	Day	3 10% 32% 37% 17% 2	450
	21st to end	Day	5% 9% 28% 33% 20% 4	428
November	1st to 10th	Day	5% 7% 23% 41% 22% 2	270
	11th to 20th	Day	6% 8% 25% 40% 18% 3	270
	21st to end	Day	6% 9% 30% 33% 20% 2	270
December	1st to 10th	Day	10% 9% 24% 39% 16% 1	270
	11th to 20th	Day	6% 9% 29% 36% 18% 2	270
	21st to end	Day	16% 12% 30% 31% 9% 2	297
January	1st to 10th	Night	11% 6% 24% 39% 16% 4	760
	11th to 20th	Night	8% 4 27% 41% 18% 2	744
	21st to end	Night	13% 6% 27% 37% 15% 2	792
February	1st to 10th	Night	9% 6% 23% 39% 21% 3	720
	11th to 20th	Night	10% 5% 27% 37% 18% 4	704
	21st to end	Night	12% 8% 24% 37% 16% 3	592
March	1st to 10th	Night	9% 4 28% 40% 19% 1	720
	11th to 20th	Night	9% 5% 22% 36% 23% 5%	720
	21st to end	Night	9% 6% 26% 38% 19% 2	792
April	1st to 10th	Night	8% 6% 20% 41% 22% 3	720
	11th to 20th	Night	6% 7% 26% 35% 24% 2	711
	21st to end	Night	5% 7% 20% 38% 25% 6%	720
May	1st to 10th	Night	6% 7% 23% 37% 24% 3	630
	11th to 20th	Night	4 5% 19% 36% 32% 4	531
	21st to end	Night	3 7% 21% 37% 27% 4	584
June	1st to 10th	Night	3 3 18% 40% 34% 3	540
	11th to 20th	Night	3 8% 40% 44% 5%	410
	21st to end	Night	1 1 2 38% 52% 6%	360
July	1st to 10th	Night	1 3 15% 72% 10%	350
	11th to 20th	Night	2 3 25% 60% 10%	433
	21st to end	Night	2 1 3 28% 54% 12%	713
August	1st to 10th	Night	2 1 7% 31% 48% 13%	716
	11th to 20th	Night	1 1 3 25% 55% 16%	712
	21st to end	Night	2 4 25% 55% 14%	792
September	1st to 10th	Night	2 1 9% 25% 51% 12%	720
	11th to 20th	Night	1 2 6% 33% 47% 11%	720
	21st to end	Night	2 2 12% 32% 43% 10%	720
October	1st to 10th	Night	4 2 14% 43% 35% 3	720
	11th to 20th	Night	3 3 16% 38% 34% 6%	720
	21st to end	Night	4 4 19% 39% 27% 8%	792
November	1st to 10th	Night	5% 3 17% 40% 30% 5%	740
	11th to 20th	Night	5% 2 17% 45% 24% 8%	760
	21st to end	Night	7% 4 24% 39% 23% 4	760
December	1st to 10th	Night	9% 4 22% 41% 20% 3	760
	11th to 20th	Night	13% 4 25% 37% 18% 3	760
	21st to end	Night	9% 5% 30% 38% 15% 3	836

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