Treadwell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Treadwell Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	17% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	144292
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	137597
Transparency	Day	50% 5% 12% 21% 10% 1	147735
	Night	54% 5% 11% 19% 10% 1	142243
Seeing	Day	22% 10% 28% 23% 13% 4	16714
	Night	23% 7% 26% 32% 11% 1	25671

By Month

Percentage Cloud Cover NE (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	14% 18% 7% 15% 6% 5% 4 2 4 5% 20%	4661
	11th to 20th	Day	13% 20% 6% 15% 6% 4 4 3 5% 5% 20%	4650
	21st to end	Day	9% 17% 7% 13% 8% 7% 6% 4 4 4 21%	5201
February	1st to 10th	Day	13% 15% 5% 12% 7% 5% 4 3 5% 5% 25%	1890
	11th to 20th	Day	13% 17% 5% 11% 6% 5% 4 4 4 6% 26%	1885
	21st to end	Day	19% 15% 5% 9% 7% 5% 4 3 5% 4 24%	1730
March	1st to 10th	Day	19% 14% 5% 11% 6% 4 4 3 5% 5% 25%	4576
	11th to 20th	Day	13% 13% 4 9% 6% 5% 4 4 6% 7% 30%	4983
	21st to end	Day	19% 10% 3 8% 6% 4 4 3 4 5% 34%	5979
April	1st to 10th	Day	22% 13% 3 7% 5% 3 4 2 4 5% 32%	2520
	11th to 20th	Day	22% 10% 3 7% 4 4 4 3 4 5% 35%	2494
	21st to end	Day	22% 10% 3 7% 5% 4 5% 3 4 4 32%	2670
May	1st to 10th	Day	23% 9% 3 8% 5% 5% 5% 4 6% 7% 25%	8237
	11th to 20th	Day	19% 8% 3 8% 5% 4 5% 4 5% 7% 32%	8221
	21st to end	Day	22% 9% 2 6% 4 4 5% 4 5% 6% 34%	9060
June	1st to 10th	Day	22% 8% 3 6% 5% 5% 4 5% 6% 7% 30%	2940
	11th to 20th	Day	15% 6% 2 6% 5% 5% 6% 5% 6% 8% 36%	2940
	21st to end	Day	19% 7% 3 7% 4 5% 5% 5% 7% 7% 31%	2940
July	1st to 10th	Day	13% 5% 2 7% 5% 5% 5% 5% 6% 9% 37%	8280
	11th to 20th	Day	12% 4 2 5% 4 4 4 4 6% 10% 44%	8280
	21st to end	Day	12% 5% 2 6% 5% 4 5% 4 6% 10% 42%	8993
August	1st to 10th	Day	14% 6% 3 8% 5% 6% 6% 4 6% 8% 36%	4703
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 9% 39%	4147
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 5% 9% 40%	4312
September	1st to 10th	Day	15% 5% 2 6% 4 4 5% 3 5% 7% 44%	2400
	11th to 20th	Day	13% 5% 2 4 4 4 4 3 4 7% 50%	2369
	21st to end	Day	19% 6% 2 5% 3 4 4 4 5% 6% 42%	2184
October	1st to 10th	Day	21% 7% 2 6% 4 4 4 3 5% 7% 37%	2000
	11th to 20th	Day	22% 9% 4 7% 6% 4 4 3 6% 6% 29%	2000
	21st to end	Day	24% 10% 3 8% 5% 5% 5% 3 5% 5% 28%	1995
November	1st to 10th	Day	17% 11% 3 7% 6% 5% 4 3 5% 7% 34%	1851
	11th to 20th	Day	20% 12% 4 9% 5% 5% 5% 3 5% 5% 28%	1727
	21st to end	Day	23% 15% 4 9% 6% 5% 4 3 5% 5% 21%	1680
December	1st to 10th	Day	21% 21% 5% 10% 4 5% 4 4 5% 6% 15%	2680
	11th to 20th	Day	19% 18% 5% 9% 5% 4 4 3 3 4 25%	3066
	21st to end	Day	23% 18% 4 8% 5% 4 4 3 4 4 23%	4048
January	1st to 10th	Night	14% 19% 7% 15% 5% 4 3 2 4 4 21%	8234
	11th to 20th	Night	13% 20% 8% 16% 7% 5% 4 3 4 4 18%	8235
	21st to end	Night	11% 17% 6% 12% 7% 6% 5% 3 5% 6% 22%	8979
February	1st to 10th	Night	13% 14% 5% 12% 7% 5% 4 3 5% 6% 25%	2730
	11th to 20th	Night	13% 12% 5% 12% 7% 5% 4 4 4 5% 28%	2621
	21st to end	Night	15% 14% 5% 10% 6% 5% 4 3 5% 5% 29%	2076
March	1st to 10th	Night	22% 14% 4 9% 5% 4 3 2 4 4 28%	5484
	11th to 20th	Night	14% 13% 3 7% 5% 5% 5% 4 6% 7% 31%	5194
	21st to end	Night	18% 11% 4 9% 5% 4 4 4 4 5% 33%	5195
April	1st to 10th	Night	22% 11% 2 7% 5% 4 4 3 4 6% 31%	2100
	11th to 20th	Night	20% 9% 3 7% 5% 4 4 3 4 6% 35%	2075
	21st to end	Night	22% 9% 2 7% 4 3 4 3 5% 6% 35%	1967
May	1st to 10th	Night	21% 9% 3 8% 6% 5% 5% 5% 6% 8% 25%	4660
	11th to 20th	Night	20% 7% 3 6% 4 5% 5% 4 5% 7% 34%	4639
	21st to end	Night	21% 9% 3 7% 4 5% 5% 4 5% 6% 32%	5114
June	1st to 10th	Night	17% 7% 2 7% 5% 4 5% 4 8% 9% 33%	1680
	11th to 20th	Night	15% 7% 3 6% 4 4 4 4 5% 6% 40%	1680
	21st to end	Night	19% 7% 2 7% 4 5% 5% 5% 5% 6% 36%	1680
July	1st to 10th	Night	14% 5% 2 7% 5% 5% 6% 5% 7% 9% 36%	4680
	11th to 20th	Night	12% 5% 2 6% 4 4 4 3 5% 8% 46%	4680
	21st to end	Night	14% 5% 2 6% 5% 4 5% 4 5% 7% 43%	5084
August	1st to 10th	Night	14% 6% 3 7% 5% 5% 5% 4 6% 7% 39%	2666
	11th to 20th	Night	17% 7% 2 5% 3 4 4 3 6% 7% 43%	2953
	21st to end	Night	16% 5% 2 7% 4 4 4 4 5% 6% 43%	3586
September	1st to 10th	Night	12% 5% 2 6% 5% 4 5% 4 5% 7% 46%	2000
	11th to 20th	Night	14% 6% 2 4 3 2 3 3 5% 6% 51%	2030
	21st to end	Night	18% 5% 2 5% 4 4 4 3 4 7% 44%	2213
October	1st to 10th	Night	18% 7% 3 6% 4 4 5% 3 6% 6% 40%	2400
	11th to 20th	Night	20% 8% 3 7% 5% 5% 4 4 4 5% 34%	2400
	21st to end	Night	24% 9% 3 8% 5% 4 4 3 4 5% 31%	2845
November	1st to 10th	Night	22% 9% 3 7% 5% 4 3 3 5% 6% 32%	2667
	11th to 20th	Night	18% 11% 4 9% 5% 4 4 3 5% 5% 31%	2892
	21st to end	Night	21% 14% 5% 9% 6% 5% 5% 3 4 5% 23%	2940
December	1st to 10th	Night	19% 18% 5% 12% 5% 5% 5% 4 4 5% 18%	4720
	11th to 20th	Night	25% 14% 5% 9% 5% 4 3 2 3 4 26%	5348
	21st to end	Night	26% 18% 4 9% 4 3 3 3 3 5% 23%	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	66% 5% 9% 15% 4 1	4662
	11th to 20th	Day	66% 4 8% 13% 9% 1	4680
	21st to end	Day	65% 5% 11% 10% 9% 1	5201
February	1st to 10th	Day	60% 3 10% 19% 6% 2	1890
	11th to 20th	Day	59% 3 12% 17% 9% 1	1894
	21st to end	Day	62% 3 9% 13% 10% 1	1730
March	1st to 10th	Day	61% 3 9% 12% 11% 3	4579
	11th to 20th	Day	53% 5% 16% 15% 10% 1	4983
	21st to end	Day	52% 4 6% 18% 18% 3	5979
April	1st to 10th	Day	55% 4 8% 19% 12% 3	2520
	11th to 20th	Day	51% 4 6% 22% 15% 2	2494
	21st to end	Day	55% 3 9% 16% 13% 4	2670
May	1st to 10th	Day	55% 6% 8% 20% 10% 1	8249
	11th to 20th	Day	51% 6% 13% 21% 9% 1	8225
	21st to end	Day	50% 5% 11% 22% 12% 1	9061
June	1st to 10th	Day	52% 4 10% 20% 12% 1	2940
	11th to 20th	Day	44% 6% 12% 24% 13% 2	2940
	21st to end	Day	48% 7% 13% 21% 10% 1	2940
July	1st to 10th	Day	41% 7% 14% 25% 11%	8210
	11th to 20th	Day	34% 6% 15% 32% 13%	8205
	21st to end	Day	36% 7% 17% 31% 9%	8795
August	1st to 10th	Day	44% 6% 18% 23% 9%	4689
	11th to 20th	Day	40% 6% 17% 30% 7% 1	4187
	21st to end	Day	41% 7% 17% 23% 12% 1	4312
September	1st to 10th	Day	38% 6% 14% 29% 13%	2400
	11th to 20th	Day	34% 5% 14% 33% 14% 1	2369
	21st to end	Day	42% 6% 14% 27% 11% 1	2184
October	1st to 10th	Day	46% 6% 11% 29% 8%	3260
	11th to 20th	Day	47% 5% 15% 26% 7% 1	3240
	21st to end	Day	48% 4 19% 22% 6%	3195
November	1st to 10th	Day	51% 4 14% 23% 7% 1	1851
	11th to 20th	Day	57% 3 13% 16% 10% 1	1727
	21st to end	Day	64% 5% 13% 13% 5%	1680
December	1st to 10th	Day	68% 6% 12% 12% 2	2680
	11th to 20th	Day	63% 4 10% 15% 7%	3066
	21st to end	Day	65% 4 9% 17% 6%	4048
January	1st to 10th	Night	68% 3 11% 12% 5%	8234
	11th to 20th	Night	70% 4 9% 11% 6%	8280
	21st to end	Night	62% 4 10% 13% 11% 1	8979
February	1st to 10th	Night	60% 4 10% 18% 7% 1	2730
	11th to 20th	Night	58% 4 9% 21% 8%	2623
	21st to end	Night	56% 5% 8% 19% 11% 1	2076
March	1st to 10th	Night	59% 3 8% 14% 12% 3	5494
	11th to 20th	Night	52% 5% 14% 16% 9% 3	5195
	21st to end	Night	52% 5% 6% 23% 12% 2	5195
April	1st to 10th	Night	55% 4 8% 18% 14% 2	2100
	11th to 20th	Night	52% 4 6% 21% 15% 1	2075
	21st to end	Night	50% 4 8% 19% 16% 3	1967
May	1st to 10th	Night	56% 6% 12% 18% 7% 2	4666
	11th to 20th	Night	48% 6% 11% 21% 12% 1	4640
	21st to end	Night	52% 5% 10% 18% 15% 1	5115
June	1st to 10th	Night	46% 6% 11% 23% 12% 1	1680
	11th to 20th	Night	43% 6% 11% 22% 16% 2	1680
	21st to end	Night	46% 7% 13% 19% 14% 1	1680
July	1st to 10th	Night	42% 8% 18% 20% 12% 1	4640
	11th to 20th	Night	35% 6% 15% 32% 12%	4632
	21st to end	Night	41% 6% 13% 27% 12%	4967
August	1st to 10th	Night	44% 6% 15% 26% 8% 1	2656
	11th to 20th	Night	41% 5% 17% 30% 7%	2979
	21st to end	Night	40% 7% 16% 23% 12% 2	3586
September	1st to 10th	Night	37% 6% 12% 29% 15% 1	2000
	11th to 20th	Night	35% 5% 13% 35% 11% 1	2030
	21st to end	Night	40% 4 13% 29% 13%	2213
October	1st to 10th	Night	42% 6% 15% 25% 11% 1	3920
	11th to 20th	Night	45% 5% 15% 26% 9%	3899
	21st to end	Night	47% 5% 17% 20% 10%	4595
November	1st to 10th	Night	52% 4 14% 21% 8% 1	2667
	11th to 20th	Night	56% 4 12% 18% 10% 1	2892
	21st to end	Night	62% 5% 12% 15% 5%	2940
December	1st to 10th	Night	68% 5% 12% 12% 3	4720
	11th to 20th	Night	63% 4 7% 16% 10% 1	5348
	21st to end	Night	66% 3 7% 14% 10%	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% 11% 31% 26% 5% 1	220
	11th to 20th	Day	26% 13% 35% 19% 5% 1	220
	21st to end	Day	19% 12% 31% 28% 9% 1	256
February	1st to 10th	Day	27% 13% 38% 15% 5%	330
	11th to 20th	Day	25% 16% 37% 21% 2	372
	21st to end	Day	27% 11% 33% 26% 3	446
March	1st to 10th	Day	25% 11% 31% 24% 8%	540
	11th to 20th	Day	26% 14% 33% 21% 6% 1	540
	21st to end	Day	27% 13% 31% 17% 9% 3	594
April	1st to 10th	Day	29% 17% 27% 16% 10% 1	540
	11th to 20th	Day	30% 14% 24% 18% 11% 3	532
	21st to end	Day	27% 13% 31% 19% 9% 2	540
May	1st to 10th	Day	25% 10% 30% 26% 6% 3	543
	11th to 20th	Day	20% 12% 29% 23% 13% 4	567
	21st to end	Day	21% 10% 30% 24% 12% 3	617
June	1st to 10th	Day	25% 7% 26% 25% 12% 5%	570
	11th to 20th	Day	15% 11% 28% 26% 15% 5%	570
	21st to end	Day	20% 9% 28% 25% 13% 4	570
July	1st to 10th	Day	13% 6% 24% 33% 16% 7%	565
	11th to 20th	Day	13% 7% 28% 29% 18% 4	564
	21st to end	Day	13% 6% 25% 31% 20% 5%	620
August	1st to 10th	Day	14% 5% 22% 30% 22% 6%	566
	11th to 20th	Day	15% 5% 19% 28% 24% 9%	535
	21st to end	Day	15% 5% 22% 27% 20% 9%	583
September	1st to 10th	Day	17% 5% 25% 22% 19% 12%	530
	11th to 20th	Day	13% 4 19% 22% 25% 16%	530
	21st to end	Day	20% 8% 25% 21% 17% 9%	530
October	1st to 10th	Day	23% 9% 25% 21% 16% 6%	530
	11th to 20th	Day	25% 12% 29% 22% 11% 1	530
	21st to end	Day	29% 9% 25% 24% 11%	470
November	1st to 10th	Day	28% 12% 23% 24% 11% 2	425
	11th to 20th	Day	28% 10% 33% 18% 8% 2	267
	21st to end	Day	31% 15% 23% 23% 6% 2	220
December	1st to 10th	Day	31% 8% 30% 22% 9%	220
	11th to 20th	Day	26% 15% 39% 18% 3	220
	21st to end	Day	31% 14% 34% 15% 5% 1	242
January	1st to 10th	Night	27% 9% 30% 28% 5%	840
	11th to 20th	Night	28% 9% 27% 28% 7%	840
	21st to end	Night	19% 8% 28% 35% 9% 1	924
February	1st to 10th	Night	20% 9% 33% 30% 7%	840
	11th to 20th	Night	19% 9% 37% 28% 6%	823
	21st to end	Night	24% 9% 28% 34% 5%	692
March	1st to 10th	Night	24% 10% 30% 31% 6%	840
	11th to 20th	Night	25% 9% 32% 27% 6%	840
	21st to end	Night	23% 8% 30% 29% 8%	924
April	1st to 10th	Night	28% 9% 27% 28% 7%	840
	11th to 20th	Night	24% 10% 29% 27% 8%	831
	21st to end	Night	26% 7% 28% 30% 7% 1	707
May	1st to 10th	Night	25% 6% 26% 33% 8% 2	420
	11th to 20th	Night	24% 5% 27% 31% 12%	414
	21st to end	Night	17% 6% 30% 34% 13%	455
June	1st to 10th	Night	16% 4 25% 42% 12% 1	420
	11th to 20th	Night	18% 5% 24% 39% 13% 1	420
	21st to end	Night	19% 3 25% 36% 16% 1	420
July	1st to 10th	Night	11% 3 19% 47% 20%	410
	11th to 20th	Night	12% 4 19% 46% 18% 1	408
	21st to end	Night	15% 3 17% 46% 19% 1	460
August	1st to 10th	Night	13% 4 12% 48% 23% 1	418
	11th to 20th	Night	15% 3 14% 37% 28% 3	621
	21st to end	Night	15% 3 19% 39% 22% 2	880
September	1st to 10th	Night	13% 4 20% 42% 20% 1	800
	11th to 20th	Night	18% 4 14% 35% 27% 3	800
	21st to end	Night	19% 5% 22% 33% 21% 2	800
October	1st to 10th	Night	22% 5% 23% 33% 17% 1	800
	11th to 20th	Night	25% 6% 26% 33% 10% 1	800
	21st to end	Night	26% 7% 26% 33% 8%	880
November	1st to 10th	Night	27% 8% 23% 31% 11% 1	820
	11th to 20th	Night	24% 9% 30% 26% 11%	840
	21st to end	Night	28% 9% 28% 26% 8%	840
December	1st to 10th	Night	28% 9% 29% 27% 6% 1	840
	11th to 20th	Night	29% 12% 28% 25% 6% 1	840
	21st to end	Night	35% 9% 28% 22% 5% 1	924

Page generated at 2026-08-12 03:37:14. Version ac07

Copyright © 2026 Attilla Danko.