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 2024-12-24.

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 2 3 3 2 4 6% 69%	105767
	Night	5% 4 1 3 2 3 3 2 4 6% 67%	103084
Transparency	Day	20% 2 4 17% 27% 30%	105528
	Night	23% 2 5% 18% 26% 27%	102914
Seeing	Day	6% 10% 29% 35% 15% 5%	15827
	Night	6% 4 18% 36% 30% 6%	25730

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	7% 3 1 3 3 3 3 2 4 7% 65%	2566
	11th to 20th	Day	9% 5% 2 3 2 3 3 2 4 8% 59%	2962
	21st to end	Day	13% 6% 1 4 4 5% 4 3 5% 7% 48%	3498
February	1st to 10th	Day	11% 4 1 4 2 2 3 3 4 5% 60%	1900
	11th to 20th	Day	11% 4 1 3 3 3 3 2 4 6% 60%	1850
	21st to end	Day	10% 8% 2 5% 4 4 3 3 4 6% 51%	1570
March	1st to 10th	Day	12% 7% 2 5% 4 5% 5% 4 8% 8% 39%	3138
	11th to 20th	Day	5% 5% 2 4 3 4 4 3 6% 8% 57%	3263
	21st to end	Day	10% 5% 1 4 4 5% 5% 4 6% 8% 48%	4151
April	1st to 10th	Day	6% 5% 1 4 4 4 3 3 5% 8% 57%	2280
	11th to 20th	Day	5% 4 2 4 3 4 3 3 3 6% 63%	2249
	21st to end	Day	5% 5% 1 2 4 3 3 2 4 7% 65%	2280
May	1st to 10th	Day	3 3 1 2 2 3 2 2 3 5% 72%	3955
	11th to 20th	Day	2 3 1 3 2 5% 4 3 4 7% 66%	4071
	21st to end	Day	3 3 1 3 3 5% 4 3 4 6% 66%	4477
June	1st to 10th	Day	4 2 1 3 2 3 3 2 4 8% 68%	2470
	11th to 20th	Day	2 2 1 1 2 2 2 2 3 5% 78%	2599
	21st to end	Day	1 2 1 2 2 2 2 1 3 4 81%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 88%	4420
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 81%	4265
	21st to end	Day	1 1 1 1 1 2 2 3 4 83%	4227
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 4 85%	3783
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	3724
	21st to end	Day	1 1 1 1 1 1 1 2 3 88%	4180
September	1st to 10th	Day	2 1 1 1 1 2 2 2 3 5% 80%	2280
	11th to 20th	Day	1 2 1 1 1 2 1 3 5% 83%	2119
	21st to end	Day	2 2 1 2 1 2 2 2 3 4 80%	2090
October	1st to 10th	Day	3 2 2 1 1 2 2 2 4 81%	3490
	11th to 20th	Day	2 2 1 1 1 2 3 2 5% 7% 75%	3326
	21st to end	Day	3 2 1 3 3 2 3 3 5% 6% 68%	3328
November	1st to 10th	Day	4 3 1 2 2 3 3 3 4 7% 70%	1764
	11th to 20th	Day	5% 3 1 3 2 3 3 2 4 5% 69%	1800
	21st to end	Day	5% 3 1 4 4 4 4 3 4 7% 60%	1800
December	1st to 10th	Day	7% 4 2 5% 2 3 3 3 5% 7% 60%	1800
	11th to 20th	Day	14% 6% 2 5% 4 3 3 3 6% 9% 43%	2259
	21st to end	Day	13% 7% 2 6% 5% 6% 5% 3 4 5% 45%	3173
January	1st to 10th	Night	7% 4 1 4 2 4 3 2 4 5% 64%	4352
	11th to 20th	Night	6% 5% 2 5% 3 3 4 3 4 9% 57%	3983
	21st to end	Night	12% 6% 2 4 3 5% 4 3 4 7% 49%	4180
February	1st to 10th	Night	10% 5% 2 4 3 2 2 2 4 5% 61%	2280
	11th to 20th	Night	8% 4 2 4 3 3 3 2 4 5% 63%	2218
	21st to end	Night	10% 7% 2 4 3 4 4 4 4 5% 52%	1884
March	1st to 10th	Night	10% 9% 2 5% 4 5% 5% 3 6% 7% 43%	3736
	11th to 20th	Night	5% 4 1 3 3 5% 4 3 5% 8% 60%	3716
	21st to end	Night	9% 7% 2 5% 4 5% 4 3 4 7% 50%	3498
April	1st to 10th	Night	6% 5% 2 4 4 3 4 4 4 7% 58%	1900
	11th to 20th	Night	4 5% 2 2 3 4 3 2 4 7% 64%	1879
	21st to end	Night	4 5% 1 3 2 3 3 3 3 7% 65%	1900
May	1st to 10th	Night	4 4 1 2 2 3 3 2 3 4 71%	3006
	11th to 20th	Night	1 2 1 2 2 3 5% 3 4 10% 67%	2834
	21st to end	Night	1 5% 1 2 3 5% 2 2 3 8% 69%	3110
June	1st to 10th	Night	1 3 1 2 2 2 4 3 4 9% 69%	1710
	11th to 20th	Night	1 1 1 1 1 3 2 2 3 7% 78%	1572
	21st to end	Night	1 1 1 1 1 1 2 2 3 4 83%	1520
July	1st to 10th	Night	1 1 1 1 1 4 90%	2540
	11th to 20th	Night	1 1 1 1 1 1 2 2 4 84%	2695
	21st to end	Night	1 1 1 1 1 1 1 2 4 84%	3326
August	1st to 10th	Night	1 1 1 1 1 1 1 3 4 85%	3166
	11th to 20th	Night	1 1 1 1 1 2 4 89%	3126
	21st to end	Night	1 1 1 1 1 1 2 5% 87%	3498
September	1st to 10th	Night	2 2 2 1 2 2 2 2 5% 80%	1900
	11th to 20th	Night	1 1 1 1 1 2 3 1 2 4 83%	2061
	21st to end	Night	1 2 1 1 1 2 1 1 3 4 83%	2090
October	1st to 10th	Night	2 2 1 2 1 1 2 1 2 4 83%	3490
	11th to 20th	Night	2 2 1 2 1 2 5% 2 7% 8% 66%	3608
	21st to end	Night	2 3 1 3 3 2 2 1 4 5% 74%	4234
November	1st to 10th	Night	3 3 1 3 2 2 3 2 3 5% 72%	2535
	11th to 20th	Night	4 3 1 4 2 3 3 3 3 6% 69%	2600
	21st to end	Night	5% 4 2 4 3 4 4 3 6% 7% 58%	2600
December	1st to 10th	Night	7% 4 2 5% 3 3 3 2 5% 7% 59%	2600
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 8% 46%	3171
	21st to end	Night	9% 7% 2 6% 5% 5% 4 4 5% 8% 46%	4566

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	22% 2 3 23% 31% 19%	2566
	11th to 20th	Day	26% 2 13% 25% 18% 16%	2962
	21st to end	Day	37% 1 4 17% 23% 18%	3498
February	1st to 10th	Day	26% 2 4 13% 26% 29%	1900
	11th to 20th	Day	25% 2 4 17% 26% 25%	1860
	21st to end	Day	33% 2 5% 17% 24% 19%	1570
March	1st to 10th	Day	37% 2 6% 23% 16% 16%	3138
	11th to 20th	Day	26% 2 9% 32% 17% 15%	3263
	21st to end	Day	32% 2 10% 32% 16% 8%	4151
April	1st to 10th	Day	25% 4 9% 26% 21% 15%	2280
	11th to 20th	Day	24% 2 4 23% 28% 19%	2252
	21st to end	Day	21% 2 6% 25% 33% 14%	2280
May	1st to 10th	Day	19% 2 6% 18% 38% 17%	3955
	11th to 20th	Day	28% 1 3 21% 37% 10%	4074
	21st to end	Day	25% 2 3 25% 39% 7%	4477
June	1st to 10th	Day	22% 3 6% 22% 32% 16%	2470
	11th to 20th	Day	16% 2 3 13% 33% 33%	2599
	21st to end	Day	14% 2 2 7% 30% 46%	2660
July	1st to 10th	Day	7% 2 1 6% 13% 72%	4351
	11th to 20th	Day	10% 3 3 10% 29% 46%	4191
	21st to end	Day	7% 1 4 19% 31% 38%	4094
August	1st to 10th	Day	8% 1 11% 43% 37%	3770
	11th to 20th	Day	5% 2 11% 30% 52%	3760
	21st to end	Day	6% 2 2 7% 27% 56%	4179
September	1st to 10th	Day	12% 2 3 8% 32% 44%	2280
	11th to 20th	Day	12% 2 1 11% 35% 40%	2119
	21st to end	Day	11% 1 1 15% 33% 39%	2090
October	1st to 10th	Day	11% 1 2 10% 17% 58%	3490
	11th to 20th	Day	12% 4 2 13% 38% 31%	3326
	21st to end	Day	17% 3 2 9% 27% 42%	3328
November	1st to 10th	Day	15% 2 4 14% 28% 37%	1764
	11th to 20th	Day	16% 2 4 15% 22% 41%	1800
	21st to end	Day	24% 2 6% 11% 23% 34%	1800
December	1st to 10th	Day	25% 2 7% 17% 20% 30%	1800
	11th to 20th	Day	36% 3 8% 14% 20% 20%	2258
	21st to end	Day	41% 1 5% 22% 18% 13%	3173
January	1st to 10th	Night	25% 2 5% 17% 31% 21%	4352
	11th to 20th	Night	29% 2 12% 25% 15% 17%	3983
	21st to end	Night	41% 2 5% 13% 22% 17%	4180
February	1st to 10th	Night	27% 2 5% 15% 23% 28%	2280
	11th to 20th	Night	26% 2 5% 18% 23% 25%	2219
	21st to end	Night	33% 2 6% 17% 21% 21%	1884
March	1st to 10th	Night	39% 2 6% 25% 16% 12%	3736
	11th to 20th	Night	27% 1 11% 30% 16% 15%	3716
	21st to end	Night	37% 1 10% 28% 16% 8%	3498
April	1st to 10th	Night	28% 2 11% 24% 20% 15%	1900
	11th to 20th	Night	25% 2 6% 25% 25% 17%	1879
	21st to end	Night	25% 3 6% 26% 29% 11%	1900
May	1st to 10th	Night	24% 2 5% 23% 36% 11%	3006
	11th to 20th	Night	33% 2 4 26% 28% 7%	2835
	21st to end	Night	27% 1 4 34% 29% 5%	3110
June	1st to 10th	Night	25% 3 7% 21% 29% 14%	1710
	11th to 20th	Night	18% 3 3 16% 34% 27%	1572
	21st to end	Night	15% 3 2 8% 30% 43%	1520
July	1st to 10th	Night	7% 2 1 6% 16% 68%	2500
	11th to 20th	Night	10% 3 3 12% 34% 38%	2647
	21st to end	Night	10% 2 4 22% 28% 35%	3222
August	1st to 10th	Night	9% 1 2 14% 43% 30%	3156
	11th to 20th	Night	6% 2 1 15% 34% 42%	3156
	21st to end	Night	8% 4 1 6% 36% 45%	3498
September	1st to 10th	Night	17% 2 2 7% 32% 41%	1900
	11th to 20th	Night	12% 2 2 11% 34% 39%	2061
	21st to end	Night	12% 2 1 14% 35% 35%	2090
October	1st to 10th	Night	11% 2 2 10% 19% 56%	3490
	11th to 20th	Night	17% 9% 2 11% 32% 29%	3608
	21st to end	Night	17% 2 1 10% 26% 43%	4234
November	1st to 10th	Night	18% 2 4 15% 25% 35%	2535
	11th to 20th	Night	18% 3 4 15% 21% 39%	2600
	21st to end	Night	25% 2 6% 17% 18% 33%	2600
December	1st to 10th	Night	28% 2 5% 14% 19% 31%	2600
	11th to 20th	Night	38% 2 7% 14% 22% 17%	3171
	21st to end	Night	42% 1 7% 20% 17% 13%	4566

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% 31% 32% 13% 1	361
	21st to end	Day	11% 17% 36% 27% 8% 2	506
February	1st to 10th	Day	14% 15% 31% 27% 10% 3	460
	11th to 20th	Day	12% 16% 34% 28% 9% 1	455
	21st to end	Day	12% 19% 34% 28% 7%	382
March	1st to 10th	Day	15% 19% 32% 25% 10%	460
	11th to 20th	Day	13% 16% 34% 28% 9% 1	460
	21st to end	Day	10% 20% 39% 24% 6% 1	506
April	1st to 10th	Day	10% 15% 32% 28% 12% 2	460
	11th to 20th	Day	6% 19% 40% 26% 7% 3	452
	21st to end	Day	4 18% 33% 28% 13% 5%	460
May	1st to 10th	Day	8% 16% 33% 30% 10% 3	463
	11th to 20th	Day	6% 12% 34% 31% 13% 4	488
	21st to end	Day	7% 11% 34% 31% 14% 3	529
June	1st to 10th	Day	3 7% 28% 35% 23% 5%	490
	11th to 20th	Day	1 7% 30% 40% 16% 6%	490
	21st to end	Day	2 4 29% 43% 16% 7%	490
July	1st to 10th	Day	1 2 19% 48% 21% 10%	486
	11th to 20th	Day	1 1 17% 45% 23% 14%	484
	21st to end	Day	1 2 17% 47% 19% 14%	531
August	1st to 10th	Day	2 1 24% 45% 18% 10%	487
	11th to 20th	Day	1 2 20% 44% 21% 12%	477
	21st to end	Day	1 2 20% 47% 20% 10%	539
September	1st to 10th	Day	1 3 28% 41% 19% 8%	490
	11th to 20th	Day	2 6% 30% 41% 17% 4	490
	21st to end	Day	2 8% 30% 34% 19% 7%	490
October	1st to 10th	Day	4 10% 31% 33% 20% 2	490
	11th to 20th	Day	2 11% 30% 37% 17% 3	490
	21st to end	Day	5% 9% 30% 32% 19% 5%	468
November	1st to 10th	Day	4 9% 23% 41% 22% 2	300
	11th to 20th	Day	5% 8% 27% 39% 17% 3	300
	21st to end	Day	5% 10% 31% 33% 19% 2	300
December	1st to 10th	Day	9% 8% 25% 40% 16% 3	300
	11th to 20th	Day	6% 9% 30% 34% 20% 2	300
	21st to end	Day	16% 12% 29% 32% 9% 2	306
January	1st to 10th	Night	11% 6% 24% 39% 16% 4	760
	11th to 20th	Night	8% 4 26% 41% 19% 2	760
	21st to end	Night	14% 6% 26% 37% 15% 2	836
February	1st to 10th	Night	10% 7% 23% 37% 20% 3	760
	11th to 20th	Night	10% 5% 27% 38% 18% 3	744
	21st to end	Night	12% 8% 25% 37% 15% 3	628
March	1st to 10th	Night	9% 5% 28% 40% 18% 1	760
	11th to 20th	Night	9% 6% 22% 37% 22% 5%	760
	21st to end	Night	9% 7% 26% 38% 18% 2	836
April	1st to 10th	Night	8% 6% 21% 39% 22% 3	760
	11th to 20th	Night	6% 7% 26% 36% 24% 2	751
	21st to end	Night	4 7% 20% 37% 25% 6%	760
May	1st to 10th	Night	6% 7% 23% 37% 24% 3	664
	11th to 20th	Night	4 5% 19% 36% 32% 4	561
	21st to end	Night	3 7% 21% 37% 28% 4	617
June	1st to 10th	Night	2 3 19% 39% 34% 3	570
	11th to 20th	Night	3 9% 41% 42% 5%	432
	21st to end	Night	1 1 2 37% 52% 6%	380
July	1st to 10th	Night	1 3 15% 71% 11%	370
	11th to 20th	Night	2 3 26% 60% 9%	458
	21st to end	Night	2 1 3 29% 54% 11%	753
August	1st to 10th	Night	2 1 6% 30% 48% 13%	756
	11th to 20th	Night	1 1 3 26% 54% 15%	752
	21st to end	Night	2 5% 26% 54% 14%	836
September	1st to 10th	Night	2 1 9% 25% 51% 12%	760
	11th to 20th	Night	1 2 7% 34% 45% 11%	760
	21st to end	Night	2 2 12% 34% 42% 9%	760
October	1st to 10th	Night	4 2 13% 43% 35% 3	760
	11th to 20th	Night	3 4 16% 38% 33% 6%	760
	21st to end	Night	3 4 19% 39% 26% 8%	836
November	1st to 10th	Night	4 3 18% 40% 30% 5%	780
	11th to 20th	Night	5% 3 18% 45% 23% 7%	800
	21st to end	Night	7% 5% 24% 40% 22% 4	800
December	1st to 10th	Night	9% 4 22% 41% 20% 3	800
	11th to 20th	Night	12% 4 25% 38% 18% 3	800
	21st to end	Night	9% 5% 30% 38% 15% 3	850

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