Thomas Conway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Thomas Conway 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	13% 9% 3 8% 5% 4 5% 4 6% 7% 36%	127311
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 37%	124939
Transparency	Day	45% 6% 12% 22% 13% 2	130849
	Night	46% 5% 11% 21% 15% 2	129462
Seeing	Day	16% 10% 29% 24% 14% 5%	17398
	Night	18% 10% 30% 30% 12% 1	27343

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	13% 12% 8% 15% 6% 5% 5% 4 5% 6% 22%	3697
	11th to 20th	Day	11% 13% 7% 14% 7% 6% 5% 5% 5% 3 22%	3700
	21st to end	Day	17% 18% 5% 10% 4 3 4 3 4 6% 26%	4137
February	1st to 10th	Day	16% 14% 5% 10% 7% 5% 4 3 5% 6% 26%	1912
	11th to 20th	Day	14% 12% 4 8% 7% 5% 4 3 5% 6% 32%	1948
	21st to end	Day	16% 13% 5% 10% 5% 3 4 4 6% 6% 29%	1650
March	1st to 10th	Day	19% 13% 3 7% 4 3 3 3 4 5% 37%	4480
	11th to 20th	Day	8% 9% 4 7% 5% 4 5% 4 6% 7% 41%	4658
	21st to end	Day	18% 13% 6% 11% 6% 5% 5% 3 5% 6% 21%	5936
April	1st to 10th	Day	18% 12% 3 8% 4 4 4 4 5% 7% 32%	2400
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 4 5% 7% 37%	2374
	21st to end	Day	19% 8% 3 7% 4 4 5% 4 5% 7% 33%	2535
May	1st to 10th	Day	19% 8% 2 7% 5% 5% 5% 4 6% 8% 31%	5862
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 5% 8% 40%	5970
	21st to end	Day	13% 7% 2 7% 4 4 5% 4 5% 9% 40%	6935
June	1st to 10th	Day	11% 6% 2 5% 4 4 5% 5% 6% 9% 43%	2800
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 6% 8% 45%	2800
	21st to end	Day	10% 5% 2 7% 5% 6% 5% 5% 7% 9% 40%	2800
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 10% 36%	6360
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 11% 40%	6360
	21st to end	Day	10% 6% 3 7% 4 5% 5% 4 7% 9% 40%	6924
August	1st to 10th	Day	7% 5% 2 7% 4 4 5% 4 6% 8% 48%	4347
	11th to 20th	Day	9% 4 2 6% 5% 5% 5% 4 6% 8% 46%	3918
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	4312
September	1st to 10th	Day	10% 5% 2 5% 4 5% 4 4 6% 7% 49%	2400
	11th to 20th	Day	11% 5% 2 5% 3 4 5% 3 6% 7% 50%	2235
	21st to end	Day	10% 6% 3 6% 4 4 4 4 7% 8% 44%	2200
October	1st to 10th	Day	11% 6% 2 6% 4 4 4 3 6% 7% 46%	2200
	11th to 20th	Day	19% 7% 2 6% 4 5% 5% 3 5% 6% 38%	1968
	21st to end	Day	19% 9% 3 7% 5% 5% 4 4 6% 7% 32%	1980
November	1st to 10th	Day	13% 7% 2 6% 3 4 4 3 6% 8% 45%	1852
	11th to 20th	Day	18% 10% 3 8% 4 4 3 3 4 7% 35%	1890
	21st to end	Day	17% 12% 4 9% 4 4 3 3 4 4 35%	1890
December	1st to 10th	Day	10% 12% 2 8% 6% 4 4 3 5% 6% 40%	2723
	11th to 20th	Day	14% 15% 5% 12% 6% 5% 4 4 6% 6% 23%	3088
	21st to end	Day	27% 21% 7% 11% 5% 4 4 3 4 4 10%	4070
January	1st to 10th	Night	14% 15% 8% 12% 6% 4 4 3 4 5% 25%	6488
	11th to 20th	Night	13% 17% 7% 13% 7% 5% 5% 4 5% 4 20%	6500
	21st to end	Night	20% 14% 6% 8% 4 4 4 2 4 4 30%	7079
February	1st to 10th	Night	16% 13% 5% 8% 6% 5% 4 3 4 6% 30%	2487
	11th to 20th	Night	15% 11% 4 8% 5% 4 4 4 4 7% 34%	2338
	21st to end	Night	19% 13% 3 8% 4 4 3 3 5% 5% 33%	1980
March	1st to 10th	Night	16% 11% 4 7% 4 4 3 3 4 5% 39%	5373
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 7% 45%	5302
	21st to end	Night	18% 10% 5% 8% 5% 5% 6% 4 5% 6% 29%	5036
April	1st to 10th	Night	14% 8% 4 7% 5% 4 4 4 5% 7% 38%	2000
	11th to 20th	Night	18% 9% 3 7% 4 3 4 3 5% 6% 39%	1975
	21st to end	Night	17% 8% 2 8% 5% 5% 5% 4 6% 6% 36%	1865
May	1st to 10th	Night	19% 9% 3 7% 5% 4 4 3 5% 7% 33%	4033
	11th to 20th	Night	14% 8% 2 5% 4 4 4 4 6% 8% 42%	3913
	21st to end	Night	14% 8% 3 7% 5% 5% 4 3 5% 7% 39%	3942
June	1st to 10th	Night	9% 5% 1 6% 4 4 5% 4 5% 8% 49%	1600
	11th to 20th	Night	8% 6% 3 5% 4 4 4 4 7% 9% 46%	1600
	21st to end	Night	9% 5% 3 7% 6% 4 6% 4 6% 7% 44%	1600
July	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 3 7% 10% 43%	3618
	11th to 20th	Night	8% 5% 2 6% 5% 4 4 4 6% 9% 48%	3617
	21st to end	Night	8% 6% 3 7% 5% 4 5% 4 6% 8% 44%	3947
August	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 4 7% 10% 47%	2802
	11th to 20th	Night	8% 4 2 5% 3 4 4 4 6% 8% 51%	3128
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 8% 53%	3586
September	1st to 10th	Night	10% 4 2 5% 4 4 3 3 5% 6% 54%	2000
	11th to 20th	Night	8% 3 1 4 3 3 4 4 6% 8% 55%	2145
	21st to end	Night	10% 6% 2 6% 4 5% 4 4 5% 8% 47%	2200
October	1st to 10th	Night	11% 5% 2 6% 4 4 3 3 5% 6% 53%	2200
	11th to 20th	Night	16% 7% 2 5% 4 3 3 3 4 6% 46%	2429
	21st to end	Night	19% 9% 2 7% 4 4 3 3 4 6% 39%	2860
November	1st to 10th	Night	13% 8% 3 6% 4 3 3 3 6% 7% 45%	2666
	11th to 20th	Night	14% 9% 3 7% 4 4 5% 5% 5% 6% 38%	2730
	21st to end	Night	18% 11% 4 8% 5% 3 4 3 5% 5% 35%	2730
December	1st to 10th	Night	13% 10% 2 8% 4 4 4 3 4 5% 43%	4681
	11th to 20th	Night	17% 13% 4 8% 6% 5% 4 3 4 5% 30%	5339
	21st to end	Night	25% 19% 6% 11% 6% 4 4 3 4 4 15%	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	61% 6% 11% 14% 7% 1	3698
	11th to 20th	Day	63% 5% 8% 9% 11% 4	3700
	21st to end	Day	61% 3 12% 17% 7%	4137
February	1st to 10th	Day	60% 4 11% 16% 7% 1	1912
	11th to 20th	Day	52% 5% 10% 22% 9% 1	1958
	21st to end	Day	55% 4 12% 20% 7% 2	1650
March	1st to 10th	Day	50% 5% 7% 23% 12% 3	4480
	11th to 20th	Day	41% 5% 11% 32% 10% 1	4658
	21st to end	Day	63% 5% 13% 13% 6% 1	5938
April	1st to 10th	Day	52% 5% 10% 20% 12%	2400
	11th to 20th	Day	47% 5% 11% 23% 11% 3	2374
	21st to end	Day	50% 6% 12% 19% 11% 2	2535
May	1st to 10th	Day	49% 7% 11% 21% 11% 1	5897
	11th to 20th	Day	42% 6% 12% 26% 13% 1	5974
	21st to end	Day	40% 6% 11% 24% 16% 2	6949
June	1st to 10th	Day	36% 7% 11% 25% 17% 3	2800
	11th to 20th	Day	35% 7% 11% 24% 20% 3	2800
	21st to end	Day	38% 7% 16% 20% 16% 3	2800
July	1st to 10th	Day	40% 6% 13% 24% 15% 2	6291
	11th to 20th	Day	35% 7% 16% 26% 15% 1	6285
	21st to end	Day	39% 7% 16% 22% 14% 2	6728
August	1st to 10th	Day	33% 7% 15% 29% 16% 1	4332
	11th to 20th	Day	34% 7% 12% 31% 16% 1	3956
	21st to end	Day	30% 6% 14% 27% 19% 4	4311
September	1st to 10th	Day	33% 7% 12% 27% 17% 4	2400
	11th to 20th	Day	31% 6% 12% 26% 22% 4	2235
	21st to end	Day	36% 6% 10% 26% 20% 2	2200
October	1st to 10th	Day	26% 4 7% 32% 29% 2	3590
	11th to 20th	Day	36% 5% 11% 23% 24% 1	3171
	21st to end	Day	44% 4 10% 26% 15% 2	3180
November	1st to 10th	Day	38% 4 15% 25% 16% 2	1852
	11th to 20th	Day	50% 3 12% 22% 12% 1	1890
	21st to end	Day	52% 5% 11% 22% 10%	1890
December	1st to 10th	Day	46% 3 15% 26% 10%	2723
	11th to 20th	Day	60% 5% 12% 17% 5%	3085
	21st to end	Day	78% 4 8% 8% 3	4070
January	1st to 10th	Night	61% 5% 10% 16% 9%	6488
	11th to 20th	Night	66% 4 10% 8% 10% 2	6500
	21st to end	Night	59% 3 10% 17% 10% 1	7079
February	1st to 10th	Night	57% 4 11% 17% 10% 2	2487
	11th to 20th	Night	50% 5% 11% 20% 12% 2	2339
	21st to end	Night	53% 4 11% 19% 11% 3	1980
March	1st to 10th	Night	48% 4 9% 26% 11% 2	5373
	11th to 20th	Night	38% 5% 13% 27% 16% 2	5302
	21st to end	Night	55% 5% 10% 21% 8% 1	5036
April	1st to 10th	Night	44% 5% 12% 23% 13% 2	2000
	11th to 20th	Night	46% 4 9% 21% 16% 4	1975
	21st to end	Night	46% 6% 10% 21% 13% 3	1865
May	1st to 10th	Night	51% 5% 10% 16% 14% 4	4060
	11th to 20th	Night	39% 6% 14% 25% 13% 2	3914
	21st to end	Night	44% 6% 10% 19% 18% 4	3950
June	1st to 10th	Night	32% 6% 12% 26% 21% 3	1600
	11th to 20th	Night	34% 5% 12% 23% 23% 3	1600
	21st to end	Night	37% 8% 14% 23% 15% 4	1600
July	1st to 10th	Night	36% 6% 12% 27% 14% 4	3580
	11th to 20th	Night	33% 6% 15% 23% 21% 2	3572
	21st to end	Night	37% 6% 16% 23% 15% 4	3832
August	1st to 10th	Night	31% 7% 17% 28% 16% 1	2793
	11th to 20th	Night	29% 6% 15% 29% 20% 1	3156
	21st to end	Night	31% 5% 11% 31% 19% 3	3586
September	1st to 10th	Night	31% 5% 11% 26% 21% 5%	2000
	11th to 20th	Night	26% 6% 10% 27% 27% 4	2145
	21st to end	Night	36% 5% 9% 27% 20% 3	2200
October	1st to 10th	Night	22% 3 8% 32% 32% 2	3590
	11th to 20th	Night	34% 4 9% 23% 28% 2	3952
	21st to end	Night	39% 3 9% 24% 21% 3	4610
November	1st to 10th	Night	39% 5% 10% 26% 18% 3	2666
	11th to 20th	Night	46% 5% 12% 22% 13% 2	2730
	21st to end	Night	52% 4 11% 23% 11%	2730
December	1st to 10th	Night	44% 4 14% 26% 11% 1	4681
	11th to 20th	Night	57% 4 12% 16% 10% 1	5341
	21st to end	Night	72% 4 9% 9% 5%	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	22% 14% 35% 22% 7% 1	330
	11th to 20th	Day	21% 18% 42% 14% 4 1	330
	21st to end	Day	20% 21% 33% 20% 5%	363
February	1st to 10th	Day	23% 19% 38% 16% 4	300
	11th to 20th	Day	26% 15% 38% 18% 2 1	300
	21st to end	Day	27% 17% 41% 12% 4	249
March	1st to 10th	Day	22% 21% 35% 19% 2	300
	11th to 20th	Day	21% 17% 37% 18% 7%	360
	21st to end	Day	27% 17% 30% 19% 5% 1	550
April	1st to 10th	Day	26% 13% 33% 21% 7% 1	500
	11th to 20th	Day	21% 18% 30% 17% 10% 3	492
	21st to end	Day	23% 14% 33% 20% 8% 2	500
May	1st to 10th	Day	23% 12% 31% 21% 11% 2	503
	11th to 20th	Day	20% 8% 33% 22% 15% 4	571
	21st to end	Day	13% 10% 30% 24% 16% 7%	789
June	1st to 10th	Day	12% 8% 29% 28% 17% 5%	730
	11th to 20th	Day	11% 5% 31% 31% 18% 4	730
	21st to end	Day	13% 7% 31% 30% 13% 6%	730
July	1st to 10th	Day	9% 4 20% 30% 21% 15%	721
	11th to 20th	Day	10% 3 22% 33% 21% 11%	718
	21st to end	Day	9% 3 24% 32% 24% 7%	794
August	1st to 10th	Day	7% 3 18% 30% 29% 13%	601
	11th to 20th	Day	10% 5% 19% 29% 26% 12%	519
	21st to end	Day	9% 3 17% 34% 27% 12%	583
September	1st to 10th	Day	10% 6% 20% 30% 25% 9%	530
	11th to 20th	Day	10% 4 15% 27% 30% 15%	530
	21st to end	Day	12% 8% 25% 25% 20% 10%	530
October	1st to 10th	Day	13% 5% 29% 29% 18% 5%	530
	11th to 20th	Day	19% 20% 26% 25% 9% 1	339
	21st to end	Day	28% 14% 35% 19% 4 1	363
November	1st to 10th	Day	15% 14% 38% 22% 8% 3	330
	11th to 20th	Day	22% 15% 37% 16% 8% 1	330
	21st to end	Day	25% 14% 39% 18% 4	330
December	1st to 10th	Day	18% 18% 39% 18% 6%	330
	11th to 20th	Day	21% 18% 38% 18% 5%	330
	21st to end	Day	31% 17% 36% 14% 3	363
January	1st to 10th	Night	21% 14% 38% 22% 4	1050
	11th to 20th	Night	21% 16% 38% 20% 4	1050
	21st to end	Night	22% 14% 36% 24% 4	1155
February	1st to 10th	Night	24% 13% 39% 22% 3	887
	11th to 20th	Night	20% 15% 36% 26% 4	783
	21st to end	Night	24% 13% 39% 21% 2	660
March	1st to 10th	Night	21% 13% 38% 24% 5%	800
	11th to 20th	Night	18% 12% 41% 24% 5% 1	800
	21st to end	Night	23% 13% 36% 22% 7%	695
April	1st to 10th	Night	18% 14% 38% 24% 6% 1	600
	11th to 20th	Night	20% 10% 36% 25% 7% 1	593
	21st to end	Night	21% 10% 33% 29% 8% 1	600
May	1st to 10th	Night	20% 7% 33% 30% 9% 1	600
	11th to 20th	Night	18% 5% 28% 33% 16% 1	591
	21st to end	Night	12% 4 26% 41% 16% 1	650
June	1st to 10th	Night	10% 3 25% 46% 15% 1	600
	11th to 20th	Night	11% 5% 30% 39% 16% 1	600
	21st to end	Night	10% 6% 25% 38% 21% 1	600
July	1st to 10th	Night	10% 2 16% 41% 28% 4	585
	11th to 20th	Night	8% 3 19% 38% 29% 3	582
	21st to end	Night	5% 2 14% 47% 31% 1	657
August	1st to 10th	Night	7% 1 11% 47% 30% 4	597
	11th to 20th	Night	7% 2 13% 42% 31% 4	594
	21st to end	Night	8% 2 15% 40% 32% 3	660
September	1st to 10th	Night	11% 3 16% 38% 28% 4	600
	11th to 20th	Night	8% 4 16% 39% 29% 4	745
	21st to end	Night	11% 6% 24% 32% 24% 3	800
October	1st to 10th	Night	13% 5% 24% 38% 19% 1	800
	11th to 20th	Night	19% 13% 29% 29% 10%	800
	21st to end	Night	24% 13% 31% 25% 7%	880
November	1st to 10th	Night	17% 11% 36% 25% 9% 1	820
	11th to 20th	Night	19% 16% 35% 24% 6%	840
	21st to end	Night	23% 14% 34% 23% 6%	840
December	1st to 10th	Night	23% 13% 36% 24% 4	1024
	11th to 20th	Night	25% 13% 33% 25% 4	1050
	21st to end	Night	29% 14% 32% 22% 2	1155

Page generated at 2026-01-29 00:21:43. Version ac07

Copyright © 2026 Attilla Danko.