Thomas Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thomas Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	1 1 1 2 1 2 2 2 4 5% 79%	135941
	Night	1 1 1 2 2 2 2 2 4 6% 78%	133549
Transparency	Day	10% 2 4 13% 26% 44%	131017
	Night	10% 2 5% 16% 29% 37%	129446
Seeing	Day	5% 11% 23% 31% 22% 7%	16976
	Night	3 8% 26% 37% 23% 4	26790

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	2 1 2 3 2 2 2 4 5% 76%	3719
	11th to 20th	Day	2 1 1 3 2 2 3 3 5% 7% 71%	3843
	21st to end	Day	4 3 1 4 3 3 3 3 5% 8% 62%	5000
February	1st to 10th	Day	4 2 1 3 3 3 2 2 4 7% 70%	2001
	11th to 20th	Day	2 1 1 2 2 2 2 4 6% 9% 70%	1950
	21st to end	Day	3 2 1 3 2 2 3 3 5% 6% 70%	1650
March	1st to 10th	Day	2 3 1 3 2 2 4 3 6% 8% 65%	4518
	11th to 20th	Day	2 3 1 3 2 3 3 4 6% 8% 65%	4665
	21st to end	Day	2 1 2 2 2 3 3 5% 10% 72%	5964
April	1st to 10th	Day	2 2 1 2 2 2 2 3 4 8% 74%	2400
	11th to 20th	Day	1 1 1 1 2 2 2 2 3 6% 78%	2369
	21st to end	Day	1 1 1 2 3 2 4 7% 80%	2400
May	1st to 10th	Day	1 1 2 1 2 2 2 3 4 81%	5765
	11th to 20th	Day	1 1 1 1 1 1 3 5% 85%	5841
	21st to end	Day	1 1 1 1 1 1 1 2 4 88%	6423
June	1st to 10th	Day	1 1 1 1 1 2 2 4 5% 81%	2715
	11th to 20th	Day	1 1 1 2 1 3 4 87%	2800
	21st to end	Day	1 1 1 1 1 2 1 2 2 89%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 92%	6319
	11th to 20th	Day	1 1 2 1 1 1 2 2 3 85%	6320
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	6348
August	1st to 10th	Day	1 1 1 1 1 2 2 2 3 4 83%	5346
	11th to 20th	Day	1 1 1 1 2 94%	5364
	21st to end	Day	1 1 1 1 1 1 2 2 3 3 85%	5984
September	1st to 10th	Day	1 1 2 1 2 2 2 3 4 83%	2400
	11th to 20th	Day	1 1 1 1 1 2 2 89%	2245
	21st to end	Day	1 1 1 1 2 1 2 3 89%	2200
October	1st to 10th	Day	1 1 1 1 1 1 1 4 88%	3600
	11th to 20th	Day	1 1 1 1 1 1 2 4 4 85%	3367
	21st to end	Day	1 1 2 2 2 2 2 4 5% 79%	3378
November	1st to 10th	Day	1 1 1 1 2 2 2 3 5% 81%	1854
	11th to 20th	Day	2 1 1 2 2 2 3 2 4 6% 74%	1890
	21st to end	Day	2 2 2 2 2 3 3 5% 7% 72%	1890
December	1st to 10th	Day	1 1 2 2 3 4 3 6% 7% 70%	3040
	11th to 20th	Day	2 3 1 3 2 3 4 3 6% 9% 64%	3481
	21st to end	Day	5% 5% 2 4 2 3 4 4 7% 9% 57%	4092
January	1st to 10th	Night	2 2 1 2 2 1 2 2 4 6% 75%	6446
	11th to 20th	Night	1 2 1 2 2 2 2 2 5% 8% 72%	6325
	21st to end	Night	4 4 1 3 2 2 3 3 5% 7% 65%	6232
February	1st to 10th	Night	3 2 1 3 3 3 3 3 4 7% 68%	2400
	11th to 20th	Night	2 2 1 3 2 3 2 3 3 6% 72%	2338
	21st to end	Night	3 2 1 2 2 2 3 2 4 7% 72%	1980
March	1st to 10th	Night	1 3 1 3 3 3 3 3 5% 7% 68%	5376
	11th to 20th	Night	2 2 1 3 2 3 4 4 6% 7% 66%	5335
	21st to end	Night	1 1 2 2 2 3 3 5% 8% 73%	5006
April	1st to 10th	Night	1 2 1 2 2 2 2 3 3 7% 76%	2000
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 5% 80%	1979
	21st to end	Night	1 1 2 1 2 2 4 6% 81%	2000
May	1st to 10th	Night	1 1 1 1 2 2 2 2 3 4 82%	4204
	11th to 20th	Night	1 1 1 1 1 2 5% 88%	4074
	21st to end	Night	1 1 1 1 1 2 3 91%	4474
June	1st to 10th	Night	1 1 1 2 1 3 5% 85%	1679
	11th to 20th	Night	1 1 1 1 1 2 3 90%	1600
	21st to end	Night	1 1 1 1 1 1 2 3 90%	1600
July	1st to 10th	Night	1 1 1 2 94%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 4 86%	3640
	21st to end	Night	1 1 1 1 1 1 1 2 3 89%	4507
August	1st to 10th	Night	1 1 1 1 1 2 3 5% 86%	4486
	11th to 20th	Night	1 1 1 2 94%	4506
	21st to end	Night	1 1 1 1 1 2 2 3 3 85%	5015
September	1st to 10th	Night	1 1 2 1 2 1 1 2 3 85%	2000
	11th to 20th	Night	1 1 1 2 2 1 2 2 89%	2155
	21st to end	Night	1 1 1 1 1 2 3 90%	2200
October	1st to 10th	Night	1 1 1 1 1 2 3 90%	3600
	11th to 20th	Night	1 1 1 1 1 1 2 4 88%	3786
	21st to end	Night	1 1 1 1 1 1 2 3 4 84%	4424
November	1st to 10th	Night	1 1 2 1 1 2 2 4 5% 81%	2665
	11th to 20th	Night	2 1 3 2 2 2 2 4 6% 76%	2730
	21st to end	Night	2 2 1 2 2 2 3 2 4 7% 72%	2730
December	1st to 10th	Night	2 1 1 2 2 2 3 3 5% 7% 72%	4370
	11th to 20th	Night	2 2 1 2 2 2 3 3 7% 10% 66%	4941
	21st to end	Night	4 4 2 4 3 4 4 4 7% 10% 55%	7106

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	12% 3 7% 35% 30% 14%	3719
	11th to 20th	Day	13% 5% 12% 18% 25% 27%	3843
	21st to end	Day	21% 4 7% 23% 26% 20%	5000
February	1st to 10th	Day	16% 4 7% 20% 31% 22%	2001
	11th to 20th	Day	13% 3 7% 21% 33% 22%	1960
	21st to end	Day	16% 3 9% 23% 31% 18%	1650
March	1st to 10th	Day	16% 4 6% 20% 27% 27%	4518
	11th to 20th	Day	16% 4 7% 25% 32% 15%	4665
	21st to end	Day	9% 4 10% 23% 37% 18%	5964
April	1st to 10th	Day	12% 3 7% 20% 33% 25%	2400
	11th to 20th	Day	9% 2 6% 17% 38% 27%	2372
	21st to end	Day	6% 3 5% 20% 38% 27%	2400
May	1st to 10th	Day	10% 1 3 15% 32% 38%	5765
	11th to 20th	Day	6% 1 2 10% 35% 46%	5844
	21st to end	Day	5% 1 2 7% 31% 54%	6424
June	1st to 10th	Day	7% 2 4 12% 25% 50%	2715
	11th to 20th	Day	3 1 3 7% 21% 65%	2800
	21st to end	Day	6% 1 1 3 12% 76%	2800
July	1st to 10th	Day	3 1 1 2 7% 85%	6251
	11th to 20th	Day	8% 1 1 4 10% 76%	6245
	21st to end	Day	6% 1 2 5% 22% 64%	6164
August	1st to 10th	Day	9% 1 1 4 21% 64%	3890
	11th to 20th	Day	3 1 1 3 12% 79%	3880
	21st to end	Day	4 1 1 4 14% 76%	4311
September	1st to 10th	Day	8% 2 3 5% 15% 67%	2400
	11th to 20th	Day	4 1 2 6% 27% 59%	2245
	21st to end	Day	5% 1 2 8% 29% 56%	2200
October	1st to 10th	Day	5% 1 2 7% 21% 64%	3600
	11th to 20th	Day	6% 1 2 7% 28% 56%	3367
	21st to end	Day	10% 1 2 8% 33% 47%	3378
November	1st to 10th	Day	7% 2 5% 14% 42% 30%	1854
	11th to 20th	Day	13% 2 7% 20% 32% 27%	1890
	21st to end	Day	13% 3 8% 19% 31% 26%	1890
December	1st to 10th	Day	14% 3 6% 24% 31% 22%	3040
	11th to 20th	Day	18% 6% 7% 24% 35% 10%	3480
	21st to end	Day	24% 5% 11% 27% 25% 8%	4092
January	1st to 10th	Night	13% 2 6% 28% 37% 14%	6446
	11th to 20th	Night	12% 4 10% 22% 28% 24%	6325
	21st to end	Night	20% 4 7% 20% 28% 20%	6232
February	1st to 10th	Night	18% 3 8% 18% 29% 24%	2400
	11th to 20th	Night	14% 3 6% 21% 32% 24%	2339
	21st to end	Night	13% 4 7% 24% 32% 20%	1980
March	1st to 10th	Night	14% 4 7% 22% 28% 25%	5376
	11th to 20th	Night	15% 5% 9% 30% 27% 14%	5335
	21st to end	Night	11% 3 8% 32% 34% 12%	5006
April	1st to 10th	Night	11% 4 6% 26% 33% 20%	2000
	11th to 20th	Night	10% 3 4 23% 36% 24%	1979
	21st to end	Night	7% 3 5% 22% 44% 19%	2000
May	1st to 10th	Night	9% 2 4 14% 41% 30%	4201
	11th to 20th	Night	4 1 2 14% 44% 35%	4075
	21st to end	Night	4 1 2 8% 42% 43%	4474
June	1st to 10th	Night	5% 1 3 15% 30% 45%	1679
	11th to 20th	Night	3 2 3 9% 20% 63%	1600
	21st to end	Night	4 1 2 4 15% 75%	1600
July	1st to 10th	Night	3 1 2 8% 87%	3600
	11th to 20th	Night	6% 1 2 4 12% 76%	3592
	21st to end	Night	5% 1 2 4 25% 63%	4380
August	1st to 10th	Night	7% 1 2 6% 22% 63%	3256
	11th to 20th	Night	3 1 2 3 12% 79%	3256
	21st to end	Night	4 1 2 17% 75%	3608
September	1st to 10th	Night	8% 1 2 5% 19% 65%	2000
	11th to 20th	Night	5% 1 2 7% 30% 56%	2155
	21st to end	Night	3 1 2 6% 29% 58%	2200
October	1st to 10th	Night	4 1 2 7% 16% 70%	3600
	11th to 20th	Night	4 1 2 6% 30% 56%	3786
	21st to end	Night	7% 1 3 6% 33% 50%	4424
November	1st to 10th	Night	8% 2 4 15% 39% 32%	2665
	11th to 20th	Night	11% 2 6% 17% 31% 32%	2730
	21st to end	Night	13% 2 7% 20% 31% 27%	2730
December	1st to 10th	Night	13% 2 4 21% 36% 23%	4370
	11th to 20th	Night	14% 5% 10% 27% 33% 12%	4941
	21st to end	Night	24% 5% 13% 29% 22% 8%	7106

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	10% 10% 25% 36% 17% 2	220
	11th to 20th	Day	9% 13% 22% 26% 23% 9%	278
	21st to end	Day	9% 12% 21% 34% 19% 5%	563
February	1st to 10th	Day	9% 18% 24% 30% 14% 5%	501
	11th to 20th	Day	9% 17% 30% 29% 12% 3	495
	21st to end	Day	9% 16% 31% 33% 8% 2	414
March	1st to 10th	Day	13% 23% 27% 24% 12% 1	500
	11th to 20th	Day	9% 21% 31% 26% 11% 2	500
	21st to end	Day	10% 24% 32% 25% 8% 2	550
April	1st to 10th	Day	10% 20% 27% 25% 14% 4	500
	11th to 20th	Day	7% 21% 32% 27% 12% 2	492
	21st to end	Day	6% 17% 30% 25% 16% 6%	500
May	1st to 10th	Day	6% 19% 32% 23% 15% 5%	503
	11th to 20th	Day	5% 17% 26% 30% 17% 5%	528
	21st to end	Day	5% 13% 23% 31% 23% 5%	573
June	1st to 10th	Day	3 9% 24% 31% 24% 8%	530
	11th to 20th	Day	3 11% 25% 30% 25% 6%	530
	21st to end	Day	2 8% 24% 32% 24% 10%	530
July	1st to 10th	Day	1 4 21% 35% 30% 9%	526
	11th to 20th	Day	1 1 15% 33% 32% 17%	524
	21st to end	Day	2 2 14% 39% 31% 13%	575
August	1st to 10th	Day	2 5% 23% 33% 27% 10%	527
	11th to 20th	Day	1 3 16% 35% 32% 13%	517
	21st to end	Day	1 3 19% 35% 32% 10%	583
September	1st to 10th	Day	1 4 21% 34% 30% 10%	530
	11th to 20th	Day	2 9% 20% 34% 28% 7%	530
	21st to end	Day	2 8% 18% 35% 31% 5%	530
October	1st to 10th	Day	4 8% 23% 30% 28% 7%	530
	11th to 20th	Day	4 9% 21% 33% 27% 7%	530
	21st to end	Day	3 8% 22% 31% 26% 9%	478
November	1st to 10th	Day	5% 11% 18% 32% 26% 8%	330
	11th to 20th	Day	4 9% 22% 35% 23% 7%	330
	21st to end	Day	4 11% 20% 33% 26% 5%	330
December	1st to 10th	Day	5% 7% 19% 38% 25% 5%	330
	11th to 20th	Day	3 9% 20% 37% 22% 9%	327
	21st to end	Day	13% 15% 20% 35% 13% 3	242
January	1st to 10th	Night	7% 8% 25% 33% 22% 6%	840
	11th to 20th	Night	3 6% 24% 41% 22% 3	840
	21st to end	Night	7% 8% 25% 38% 19% 4	924
February	1st to 10th	Night	4 11% 27% 35% 20% 4	800
	11th to 20th	Night	4 11% 32% 31% 18% 4	784
	21st to end	Night	6% 10% 32% 36% 13% 2	660
March	1st to 10th	Night	5% 16% 33% 30% 14% 2	800
	11th to 20th	Night	5% 16% 29% 29% 17% 4	800
	21st to end	Night	3 19% 36% 28% 13% 1	880
April	1st to 10th	Night	3 18% 35% 25% 17% 3	800
	11th to 20th	Night	3 16% 35% 31% 13% 1	791
	21st to end	Night	3 15% 35% 30% 15% 3	800
May	1st to 10th	Night	4 15% 39% 28% 12% 2	652
	11th to 20th	Night	3 14% 33% 34% 16% 1	591
	21st to end	Night	3 16% 33% 33% 15%	650
June	1st to 10th	Night	1 8% 31% 44% 15% 1	479
	11th to 20th	Night	7% 36% 40% 17% 1	400
	21st to end	Night	1 5% 26% 49% 19% 1	400
July	1st to 10th	Night	1 2 22% 44% 30% 2	390
	11th to 20th	Night	2 13% 53% 29% 2	388
	21st to end	Night	2 18% 48% 30% 2	669
August	1st to 10th	Night	2 2 24% 43% 27% 2	796
	11th to 20th	Night	1 1 17% 47% 30% 5%	792
	21st to end	Night	1 1 16% 40% 35% 6%	880
September	1st to 10th	Night	1 2 13% 38% 39% 6%	800
	11th to 20th	Night	1 4 22% 38% 31% 4	800
	21st to end	Night	1 6% 18% 37% 33% 7%	800
October	1st to 10th	Night	1 5% 22% 42% 26% 3	800
	11th to 20th	Night	1 6% 20% 39% 27% 8%	800
	21st to end	Night	2 7% 20% 31% 31% 8%	880
November	1st to 10th	Night	1 9% 20% 35% 28% 7%	820
	11th to 20th	Night	3 5% 19% 41% 26% 6%	840
	21st to end	Night	4 5% 23% 38% 25% 5%	840
December	1st to 10th	Night	5% 6% 22% 37% 25% 4	840
	11th to 20th	Night	4 8% 23% 40% 21% 4	840
	21st to end	Night	5% 9% 29% 33% 19% 4	924

Page generated at 2026-01-28 22:21:42. Version ac07

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