T.O.M.S. Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the T.O.M.S. Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	13% 7% 2 6% 4 4 4 4 6% 7% 42%	130683
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	130239
Transparency	Day	40% 5% 11% 23% 18% 4	134395
	Night	38% 4 10% 22% 19% 6%	134603
Seeing	Day	16% 9% 22% 24% 19% 9%	16852
	Night	16% 8% 24% 29% 19% 3	26678

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	17% 11% 3 9% 3 3 3 2 4 5% 39%	4691
	11th to 20th	Day	14% 7% 2 8% 3 2 3 2 4 4 52%	5130
	21st to end	Day	21% 8% 3 7% 4 4 4 3 4 5% 38%	6225
February	1st to 10th	Day	19% 11% 3 6% 3 3 3 2 4 5% 40%	2100
	11th to 20th	Day	21% 10% 4 6% 4 4 3 3 4 5% 38%	1998
	21st to end	Day	21% 8% 3 6% 5% 3 3 3 5% 5% 38%	1650
March	1st to 10th	Day	16% 13% 3 9% 5% 4 4 3 4 4 35%	4480
	11th to 20th	Day	12% 7% 3 7% 4 4 3 3 4 6% 47%	4698
	21st to end	Day	13% 7% 3 7% 4 4 4 3 5% 7% 43%	5967
April	1st to 10th	Day	14% 7% 2 5% 4 4 4 4 5% 6% 44%	2400
	11th to 20th	Day	14% 7% 3 7% 4 3 4 3 5% 6% 46%	2374
	21st to end	Day	12% 7% 2 6% 4 4 5% 4 6% 8% 43%	2400
May	1st to 10th	Day	17% 8% 2 7% 5% 5% 5% 4 7% 8% 31%	5607
	11th to 20th	Day	14% 9% 3 8% 6% 5% 5% 4 7% 8% 31%	5861
	21st to end	Day	17% 8% 3 7% 4 4 4 4 6% 8% 35%	6433
June	1st to 10th	Day	9% 6% 3 6% 5% 5% 5% 5% 7% 10% 40%	2718
	11th to 20th	Day	5% 4 2 6% 4 6% 6% 4 8% 9% 46%	2800
	21st to end	Day	5% 4 1 5% 5% 6% 6% 5% 8% 10% 44%	2800
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 5% 8% 11% 38%	6360
	11th to 20th	Day	7% 5% 2 6% 5% 6% 7% 6% 9% 12% 34%	6360
	21st to end	Day	11% 7% 3 7% 5% 6% 5% 5% 8% 12% 31%	6190
August	1st to 10th	Day	3 2 1 4 4 5% 5% 4 7% 10% 55%	3905
	11th to 20th	Day	7% 4 2 6% 4 4 5% 5% 6% 10% 46%	3849
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 8% 53%	4312
September	1st to 10th	Day	9% 5% 2 6% 5% 5% 4 4 7% 9% 44%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 8% 45%	2246
	21st to end	Day	10% 4 2 6% 4 4 3 4 6% 6% 52%	2200
October	1st to 10th	Day	10% 5% 2 5% 3 3 3 3 4 5% 58%	2200
	11th to 20th	Day	10% 4 2 5% 3 3 3 3 4 6% 58%	2042
	21st to end	Day	16% 5% 2 5% 3 3 3 3 4 4 51%	2125
November	1st to 10th	Day	13% 5% 2 5% 3 2 3 4 4 5% 54%	1852
	11th to 20th	Day	13% 7% 2 5% 3 3 3 3 4 6% 52%	1890
	21st to end	Day	15% 9% 2 5% 4 3 3 4 5% 5% 45%	1890
December	1st to 10th	Day	16% 9% 3 6% 4 3 4 3 4 4 46%	3020
	11th to 20th	Day	21% 10% 3 6% 4 3 3 3 4 4 40%	3440
	21st to end	Day	16% 10% 3 8% 4 4 4 3 5% 7% 35%	4070
January	1st to 10th	Night	18% 11% 3 8% 3 3 3 3 4 5% 39%	8232
	11th to 20th	Night	14% 8% 3 7% 3 3 3 2 3 5% 50%	7843
	21st to end	Night	21% 10% 3 7% 4 3 3 2 4 4 39%	7949
February	1st to 10th	Night	20% 10% 3 6% 4 3 3 3 4 5% 39%	2520
	11th to 20th	Night	21% 10% 3 7% 3 3 4 3 3 4 39%	2399
	21st to end	Night	21% 8% 2 7% 4 3 4 3 5% 5% 38%	1980
March	1st to 10th	Night	20% 10% 3 7% 4 4 3 3 4 6% 36%	5373
	11th to 20th	Night	12% 7% 2 5% 3 4 3 3 4 7% 50%	5303
	21st to end	Night	16% 7% 3 6% 5% 4 4 3 4 5% 43%	4962
April	1st to 10th	Night	14% 6% 2 6% 4 3 4 4 5% 7% 46%	2000
	11th to 20th	Night	14% 6% 2 5% 3 3 3 4 4 7% 50%	1975
	21st to end	Night	11% 7% 3 6% 4 4 4 4 4 7% 46%	2000
May	1st to 10th	Night	17% 9% 3 7% 5% 4 4 4 6% 7% 35%	4272
	11th to 20th	Night	16% 10% 4 8% 5% 4 5% 4 5% 7% 33%	4032
	21st to end	Night	13% 7% 3 7% 4 4 5% 4 6% 8% 38%	4421
June	1st to 10th	Night	7% 6% 2 6% 4 4 5% 5% 6% 10% 45%	1675
	11th to 20th	Night	3 3 2 4 4 4 4 5% 7% 11% 53%	1600
	21st to end	Night	3 3 2 4 3 4 5% 4 7% 10% 55%	1600
July	1st to 10th	Night	6% 4 2 7% 6% 5% 6% 5% 8% 12% 41%	3618
	11th to 20th	Night	5% 5% 2 5% 4 4 5% 5% 8% 15% 43%	3617
	21st to end	Night	8% 6% 3 8% 5% 5% 5% 5% 9% 13% 34%	4631
August	1st to 10th	Night	2 2 1 4 3 4 4 4 6% 9% 62%	3244
	11th to 20th	Night	4 3 2 4 3 3 4 4 8% 11% 54%	3200
	21st to end	Night	5% 4 1 5% 4 4 4 4 6% 9% 54%	3586
September	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 7% 7% 50%	2000
	11th to 20th	Night	11% 4 1 6% 4 4 4 4 5% 7% 50%	2132
	21st to end	Night	10% 4 2 5% 4 3 3 4 5% 6% 55%	2200
October	1st to 10th	Night	9% 4 2 3 3 3 2 2 4 5% 64%	2200
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 3 6% 56%	2354
	21st to end	Night	16% 8% 2 4 2 3 3 2 4 5% 51%	2713
November	1st to 10th	Night	12% 5% 2 4 3 3 4 3 4 7% 53%	2666
	11th to 20th	Night	11% 5% 2 6% 3 3 4 3 5% 7% 52%	2730
	21st to end	Night	17% 7% 2 5% 3 3 3 3 4 5% 48%	2730
December	1st to 10th	Night	20% 8% 2 5% 3 3 3 2 4 5% 44%	4380
	11th to 20th	Night	22% 9% 2 5% 3 3 2 2 3 5% 44%	4952
	21st to end	Night	19% 9% 3 6% 4 4 5% 4 6% 6% 34%	7150

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	49% 2 9% 25% 13% 1	4692
	11th to 20th	Day	36% 3 5% 23% 31% 3	5130
	21st to end	Day	49% 2 6% 20% 18% 5%	6225
February	1st to 10th	Day	48% 3 7% 20% 18% 3	2100
	11th to 20th	Day	50% 3 6% 20% 15% 5%	2008
	21st to end	Day	48% 3 8% 15% 19% 7%	1650
March	1st to 10th	Day	53% 4 5% 14% 15% 9%	4480
	11th to 20th	Day	39% 4 7% 20% 19% 11%	4698
	21st to end	Day	42% 4 9% 24% 20% 1	5969
April	1st to 10th	Day	39% 6% 10% 22% 19% 5%	2400
	11th to 20th	Day	40% 4 8% 20% 22% 6%	2374
	21st to end	Day	38% 5% 10% 26% 18% 2	2400
May	1st to 10th	Day	48% 6% 12% 22% 11% 1	5642
	11th to 20th	Day	50% 6% 16% 18% 9% 1	5865
	21st to end	Day	46% 5% 10% 21% 15% 2	6447
June	1st to 10th	Day	37% 9% 12% 22% 19% 2	2718
	11th to 20th	Day	32% 7% 14% 27% 18% 3	2800
	21st to end	Day	31% 10% 15% 23% 17% 3	2800
July	1st to 10th	Day	38% 6% 19% 25% 11% 1	6291
	11th to 20th	Day	39% 5% 22% 28% 4 1	6285
	21st to end	Day	45% 6% 23% 22% 4	6014
August	1st to 10th	Day	22% 7% 15% 35% 20% 1	3893
	11th to 20th	Day	32% 6% 16% 27% 16% 3	3885
	21st to end	Day	30% 5% 12% 25% 24% 4	4311
September	1st to 10th	Day	35% 8% 11% 25% 16% 5%	2400
	11th to 20th	Day	38% 6% 10% 20% 22% 4	2246
	21st to end	Day	31% 6% 11% 23% 23% 7%	2200
October	1st to 10th	Day	21% 3 5% 15% 38% 19%	3590
	11th to 20th	Day	19% 3 5% 21% 36% 17%	3308
	21st to end	Day	28% 2 4 25% 33% 9%	3415
November	1st to 10th	Day	33% 4 8% 21% 28% 7%	1852
	11th to 20th	Day	34% 3 9% 20% 27% 7%	1890
	21st to end	Day	41% 4 7% 22% 23% 4	1890
December	1st to 10th	Day	44% 3 4 25% 21% 3	3020
	11th to 20th	Day	49% 3 6% 20% 16% 5%	3437
	21st to end	Day	48% 4 10% 25% 12% 2	4070
January	1st to 10th	Night	50% 2 10% 21% 15% 2	8233
	11th to 20th	Night	38% 3 5% 19% 29% 6%	7843
	21st to end	Night	50% 2 5% 19% 17% 7%	7949
February	1st to 10th	Night	48% 3 8% 19% 18% 3	2520
	11th to 20th	Night	50% 3 6% 18% 18% 5%	2400
	21st to end	Night	48% 3 7% 17% 19% 7%	1980
March	1st to 10th	Night	50% 3 7% 16% 17% 7%	5373
	11th to 20th	Night	35% 4 7% 19% 19% 16%	5303
	21st to end	Night	44% 4 7% 21% 21% 4	4962
April	1st to 10th	Night	38% 5% 11% 23% 19% 5%	2000
	11th to 20th	Night	35% 4 8% 21% 24% 8%	1975
	21st to end	Night	38% 5% 12% 24% 19% 3	2000
May	1st to 10th	Night	47% 6% 13% 20% 13% 2	4302
	11th to 20th	Night	50% 6% 15% 20% 9% 1	4033
	21st to end	Night	41% 7% 12% 24% 14% 2	4429
June	1st to 10th	Night	32% 8% 15% 23% 19% 3	1675
	11th to 20th	Night	24% 7% 16% 29% 22% 3	1600
	21st to end	Night	24% 8% 16% 30% 19% 4	1600
July	1st to 10th	Night	34% 8% 21% 27% 10% 1	3580
	11th to 20th	Night	30% 5% 27% 32% 5% 2	3572
	21st to end	Night	40% 6% 26% 21% 6%	4494
August	1st to 10th	Night	17% 7% 12% 38% 24% 1	3232
	11th to 20th	Night	22% 7% 24% 27% 17% 2	3230
	21st to end	Night	25% 5% 15% 23% 27% 4	3586
September	1st to 10th	Night	31% 7% 13% 23% 20% 6%	2000
	11th to 20th	Night	34% 6% 14% 18% 25% 3	2132
	21st to end	Night	29% 5% 10% 24% 24% 8%	2200
October	1st to 10th	Night	18% 4 5% 17% 36% 21%	3590
	11th to 20th	Night	20% 3 5% 15% 35% 22%	3827
	21st to end	Night	27% 2 5% 26% 25% 15%	4373
November	1st to 10th	Night	33% 4 7% 22% 25% 9%	2666
	11th to 20th	Night	33% 4 7% 20% 27% 9%	2730
	21st to end	Night	39% 3 7% 21% 24% 6%	2730
December	1st to 10th	Night	43% 3 6% 24% 22% 2	4380
	11th to 20th	Night	46% 2 5% 20% 23% 3	4954
	21st to end	Night	48% 5% 9% 25% 11% 3	7150

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	25% 22% 32% 16% 4	330
	11th to 20th	Day	28% 21% 34% 13% 2 2	330
	21st to end	Day	23% 21% 41% 12% 3	363
February	1st to 10th	Day	23% 24% 34% 14% 4 1	330
	11th to 20th	Day	38% 16% 33% 9% 3 1	315
	21st to end	Day	33% 20% 34% 10% 3	249
March	1st to 10th	Day	26% 19% 32% 18% 5%	300
	11th to 20th	Day	22% 19% 36% 19% 3	318
	21st to end	Day	24% 16% 31% 21% 7%	550
April	1st to 10th	Day	23% 17% 27% 24% 8% 1	500
	11th to 20th	Day	19% 17% 32% 21% 10% 1	492
	21st to end	Day	17% 15% 28% 25% 13% 3	500
May	1st to 10th	Day	17% 14% 26% 27% 11% 5%	503
	11th to 20th	Day	16% 6% 26% 26% 17% 10%	527
	21st to end	Day	14% 5% 21% 28% 20% 13%	573
June	1st to 10th	Day	10% 5% 19% 29% 22% 15%	648
	11th to 20th	Day	6% 3 16% 30% 31% 15%	730
	21st to end	Day	7% 2 18% 31% 29% 14%	730
July	1st to 10th	Day	7% 3 11% 29% 33% 17%	721
	11th to 20th	Day	10% 1 11% 26% 32% 21%	718
	21st to end	Day	8% 2 10% 25% 33% 21%	629
August	1st to 10th	Day	7% 2 10% 28% 36% 18%	526
	11th to 20th	Day	12% 2 10% 28% 33% 14%	519
	21st to end	Day	12% 2 9% 22% 36% 19%	583
September	1st to 10th	Day	9% 2 13% 25% 36% 14%	530
	11th to 20th	Day	13% 2 10% 25% 32% 18%	530
	21st to end	Day	13% 4 14% 30% 28% 11%	530
October	1st to 10th	Day	12% 3 17% 35% 26% 9%	530
	11th to 20th	Day	9% 10% 31% 30% 17% 3	372
	21st to end	Day	20% 12% 30% 25% 11% 2	363
November	1st to 10th	Day	22% 8% 24% 28% 15% 3	330
	11th to 20th	Day	14% 15% 36% 28% 5% 1	330
	21st to end	Day	21% 16% 29% 19% 12% 2	330
December	1st to 10th	Day	25% 21% 32% 17% 5%	330
	11th to 20th	Day	24% 15% 33% 20% 7%	330
	21st to end	Day	26% 19% 34% 17% 4	363
January	1st to 10th	Night	24% 16% 34% 23% 3	1050
	11th to 20th	Night	24% 18% 33% 22% 4	1050
	21st to end	Night	22% 12% 39% 24% 2	998
February	1st to 10th	Night	24% 14% 35% 24% 3	840
	11th to 20th	Night	27% 19% 30% 23% 2	804
	21st to end	Night	25% 17% 35% 20% 2	660
March	1st to 10th	Night	21% 15% 31% 30% 3	800
	11th to 20th	Night	21% 12% 35% 27% 5% 1	760
	21st to end	Night	18% 11% 34% 25% 11%	660
April	1st to 10th	Night	16% 11% 35% 27% 10% 1	600
	11th to 20th	Night	16% 11% 32% 28% 12% 1	593
	21st to end	Night	13% 10% 32% 30% 13% 2	600
May	1st to 10th	Night	16% 5% 27% 34% 16% 2	600
	11th to 20th	Night	13% 5% 24% 33% 20% 4	591
	21st to end	Night	10% 2 16% 39% 26% 7%	650
June	1st to 10th	Night	10% 2 14% 36% 32% 7%	600
	11th to 20th	Night	4 1 13% 36% 40% 7%	600
	21st to end	Night	4 1 11% 36% 39% 9%	600
July	1st to 10th	Night	8% 2 7% 27% 45% 12%	585
	11th to 20th	Night	8% 1 9% 26% 40% 16%	582
	21st to end	Night	7% 1 7% 31% 44% 12%	657
August	1st to 10th	Night	5% 1 6% 30% 49% 11%	597
	11th to 20th	Night	7% 2 8% 35% 41% 8%	594
	21st to end	Night	8% 1 8% 30% 44% 10%	660
September	1st to 10th	Night	9% 1 11% 35% 38% 6%	600
	11th to 20th	Night	13% 1 14% 28% 38% 7%	732
	21st to end	Night	11% 3 15% 33% 34% 4	800
October	1st to 10th	Night	11% 3 17% 33% 31% 5%	800
	11th to 20th	Night	15% 6% 22% 37% 20% 2	800
	21st to end	Night	21% 8% 24% 29% 18% 1	880
November	1st to 10th	Night	14% 7% 24% 36% 18% 2	820
	11th to 20th	Night	13% 10% 33% 32% 12% 1	840
	21st to end	Night	22% 11% 29% 26% 12% 1	840
December	1st to 10th	Night	26% 11% 30% 27% 7%	840
	11th to 20th	Night	22% 12% 32% 27% 7%	840
	21st to end	Night	26% 14% 30% 26% 4	1155

Page generated at 2026-02-13 00:42:17. Version ac07

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