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 2025-04-11.

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% 6% 32%	113462
	Night	18% 12% 4 9% 5% 4 4 3 5% 6% 31%	114821
Transparency	Day	51% 5% 12% 21% 10% 1	116951
	Night	54% 5% 12% 19% 10% 1	119496
Seeing	Day	22% 9% 27% 24% 13% 4	14949
	Night	23% 7% 26% 32% 11% 1	24145

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	15% 20% 7% 16% 6% 5% 4 2 3 4 19%	3597
	11th to 20th	Day	13% 16% 6% 14% 7% 4 4 3 5% 5% 22%	3570
	21st to end	Day	12% 19% 7% 10% 6% 6% 6% 4 4 4 22%	4023
February	1st to 10th	Day	14% 16% 5% 12% 7% 5% 4 3 5% 5% 25%	1800
	11th to 20th	Day	14% 17% 5% 11% 6% 4 4 4 4 5% 26%	1793
	21st to end	Day	19% 14% 5% 9% 6% 5% 4 3 5% 5% 25%	1650
March	1st to 10th	Day	19% 14% 5% 11% 6% 4 4 3 5% 4 25%	4476
	11th to 20th	Day	13% 13% 4 9% 6% 5% 4 4 6% 7% 30%	4876
	21st to end	Day	20% 10% 3 7% 5% 4 4 3 4 5% 35%	5850
April	1st to 10th	Day	21% 13% 3 7% 5% 3 4 2 4 5% 32%	2400
	11th to 20th	Day	22% 9% 3 6% 4 4 4 3 4 5% 35%	2254
	21st to end	Day	22% 10% 3 7% 5% 4 5% 3 4 4 32%	2416
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 6% 6% 29%	4426
	11th to 20th	Day	19% 7% 3 8% 5% 5% 4 4 5% 7% 32%	4398
	21st to end	Day	16% 7% 3 7% 5% 4 5% 4 6% 8% 37%	4837
June	1st to 10th	Day	22% 8% 3 6% 5% 5% 4 5% 5% 7% 29%	2660
	11th to 20th	Day	14% 6% 2 7% 4 5% 6% 5% 6% 8% 36%	2660
	21st to end	Day	20% 7% 3 6% 5% 5% 5% 5% 7% 7% 30%	2660
July	1st to 10th	Day	13% 6% 3 8% 5% 6% 5% 5% 6% 8% 35%	4440
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	4440
	21st to end	Day	12% 5% 2 6% 5% 4 5% 4 5% 8% 45%	4814
August	1st to 10th	Day	14% 6% 3 8% 5% 6% 6% 4 6% 8% 34%	4423
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 6% 9% 38%	3966
	21st to end	Day	16% 5% 2 6% 5% 4 5% 4 5% 8% 40%	4180
September	1st to 10th	Day	15% 5% 2 6% 4 4 5% 3 5% 7% 43%	2280
	11th to 20th	Day	13% 5% 2 4 4 4 4 3 4 7% 49%	2251
	21st to end	Day	19% 6% 2 5% 3 4 4 4 5% 6% 42%	2075
October	1st to 10th	Day	22% 8% 2 6% 4 4 4 3 5% 7% 35%	1900
	11th to 20th	Day	23% 10% 4 7% 6% 4 4 3 5% 6% 28%	1900
	21st to end	Day	24% 11% 3 7% 5% 5% 4 3 5% 6% 28%	1895
November	1st to 10th	Day	17% 10% 3 6% 5% 5% 4 3 5% 7% 35%	1761
	11th to 20th	Day	20% 12% 4 9% 5% 5% 5% 3 5% 5% 29%	1645
	21st to end	Day	23% 14% 4 9% 6% 5% 4 3 5% 5% 21%	1600
December	1st to 10th	Day	22% 21% 5% 10% 4 4 4 4 5% 6% 15%	2600
	11th to 20th	Day	19% 18% 5% 9% 5% 4 4 3 3 4 26%	2986
	21st to end	Day	23% 18% 4 8% 5% 3 4 3 4 4 23%	3960
January	1st to 10th	Night	15% 21% 7% 14% 5% 4 3 2 4 4 21%	6349
	11th to 20th	Night	12% 17% 7% 14% 7% 6% 5% 3 4 4 20%	6315
	21st to end	Night	13% 19% 6% 12% 6% 5% 5% 3 5% 5% 22%	6932
February	1st to 10th	Night	14% 14% 5% 12% 7% 5% 4 3 5% 6% 25%	2600
	11th to 20th	Night	14% 12% 5% 12% 7% 5% 4 4 4 5% 28%	2493
	21st to end	Night	15% 15% 5% 10% 6% 5% 3 3 5% 5% 29%	1980
March	1st to 10th	Night	23% 14% 4 8% 5% 4 3 2 4 4 28%	5364
	11th to 20th	Night	13% 13% 3 7% 5% 5% 5% 4 6% 8% 31%	5081
	21st to end	Night	18% 11% 3 9% 5% 4 4 4 4 5% 33%	5082
April	1st to 10th	Night	22% 11% 3 6% 5% 4 4 3 4 6% 31%	2000
	11th to 20th	Night	20% 9% 2 7% 5% 4 4 3 5% 6% 35%	1882
	21st to end	Night	21% 10% 2 7% 4 4 4 3 5% 6% 34%	1780
May	1st to 10th	Night	20% 9% 3 8% 6% 4 4 4 5% 8% 30%	2514
	11th to 20th	Night	21% 8% 2 6% 4 4 4 3 4 7% 37%	2495
	21st to end	Night	17% 7% 2 7% 5% 4 5% 4 6% 7% 37%	2738
June	1st to 10th	Night	17% 7% 2 7% 5% 4 5% 4 8% 9% 32%	1520
	11th to 20th	Night	15% 7% 3 6% 4 4 4 5% 5% 6% 41%	1520
	21st to end	Night	19% 8% 2 7% 4 5% 5% 5% 5% 6% 36%	1520
July	1st to 10th	Night	15% 6% 3 8% 5% 6% 5% 5% 7% 7% 35%	2520
	11th to 20th	Night	15% 6% 3 6% 4 4 4 3 6% 8% 42%	2520
	21st to end	Night	12% 6% 2 5% 4 4 5% 4 5% 7% 45%	2739
August	1st to 10th	Night	14% 6% 3 7% 5% 4 5% 4 6% 7% 38%	2506
	11th to 20th	Night	18% 7% 2 5% 3 4 4 3 6% 7% 42%	2827
	21st to end	Night	16% 5% 2 7% 4 4 4 4 5% 6% 43%	3476
September	1st to 10th	Night	12% 5% 2 6% 5% 4 5% 4 5% 7% 45%	1900
	11th to 20th	Night	14% 6% 2 4 4 2 3 3 5% 6% 50%	1928
	21st to end	Night	18% 5% 2 5% 4 4 4 3 4 7% 44%	2102
October	1st to 10th	Night	19% 7% 3 7% 4 4 5% 3 6% 6% 38%	2280
	11th to 20th	Night	21% 9% 3 7% 5% 5% 4 4 4 5% 34%	2280
	21st to end	Night	24% 9% 3 8% 5% 4 4 3 4 5% 30%	2703
November	1st to 10th	Night	21% 9% 3 7% 5% 4 3 3 5% 6% 32%	2537
	11th to 20th	Night	19% 11% 4 9% 6% 4 4 3 5% 5% 31%	2754
	21st to end	Night	21% 14% 4 9% 6% 5% 5% 3 5% 5% 23%	2800
December	1st to 10th	Night	20% 18% 5% 12% 5% 5% 5% 4 4 6% 18%	4580
	11th to 20th	Night	25% 14% 5% 9% 5% 3 3 2 3 4 26%	5208
	21st to end	Night	26% 18% 3 9% 4 3 3 3 3 5% 23%	6996

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	69% 4 8% 14% 4 1	3598
	11th to 20th	Day	63% 4 9% 15% 7% 1	3600
	21st to end	Day	64% 5% 11% 11% 9% 1	4023
February	1st to 10th	Day	61% 3 10% 18% 7% 2	1800
	11th to 20th	Day	59% 3 12% 16% 9% 1	1802
	21st to end	Day	61% 3 10% 14% 11% 1	1650
March	1st to 10th	Day	62% 3 9% 12% 11% 3	4479
	11th to 20th	Day	52% 5% 16% 15% 10% 1	4876
	21st to end	Day	52% 4 6% 18% 18% 3	5850
April	1st to 10th	Day	54% 4 7% 19% 13% 3	2400
	11th to 20th	Day	51% 4 6% 22% 15% 2	2254
	21st to end	Day	56% 3 9% 15% 13% 4	2416
May	1st to 10th	Day	52% 6% 8% 22% 10% 2	4440
	11th to 20th	Day	50% 5% 15% 19% 9% 2	4402
	21st to end	Day	45% 5% 12% 22% 14% 1	4837
June	1st to 10th	Day	53% 4 11% 20% 11% 1	2660
	11th to 20th	Day	43% 6% 12% 23% 13% 2	2660
	21st to end	Day	50% 6% 13% 20% 10% 1	2660
July	1st to 10th	Day	44% 6% 14% 24% 11%	4370
	11th to 20th	Day	41% 7% 16% 29% 8%	4365
	21st to end	Day	36% 6% 15% 32% 11%	4661
August	1st to 10th	Day	45% 6% 17% 22% 9%	4409
	11th to 20th	Day	41% 6% 17% 29% 6%	4006
	21st to end	Day	41% 7% 17% 22% 12% 1	4180
September	1st to 10th	Day	38% 6% 14% 28% 13%	2280
	11th to 20th	Day	35% 5% 15% 31% 14% 1	2251
	21st to end	Day	42% 6% 14% 27% 11%	2075
October	1st to 10th	Day	47% 6% 11% 28% 8%	3160
	11th to 20th	Day	47% 5% 15% 26% 7%	3140
	21st to end	Day	48% 4 20% 21% 6%	3095
November	1st to 10th	Day	50% 4 14% 24% 7% 1	1761
	11th to 20th	Day	57% 3 13% 17% 10% 1	1645
	21st to end	Day	64% 5% 13% 13% 5% 1	1600
December	1st to 10th	Day	68% 6% 12% 12% 2	2600
	11th to 20th	Day	63% 4 10% 16% 7%	2986
	21st to end	Day	64% 4 9% 17% 6%	3960
January	1st to 10th	Night	70% 3 12% 12% 4	6349
	11th to 20th	Night	67% 5% 11% 13% 5%	6360
	21st to end	Night	64% 4 10% 10% 11% 1	6932
February	1st to 10th	Night	60% 4 10% 17% 7% 1	2600
	11th to 20th	Night	58% 4 10% 20% 8%	2495
	21st to end	Night	56% 4 8% 19% 11% 1	1980
March	1st to 10th	Night	60% 3 8% 14% 12% 4	5374
	11th to 20th	Night	51% 5% 15% 16% 9% 3	5082
	21st to end	Night	52% 5% 6% 23% 12% 2	5082
April	1st to 10th	Night	55% 4 8% 17% 14% 2	2000
	11th to 20th	Night	51% 4 6% 21% 16% 2	1882
	21st to end	Night	51% 4 8% 19% 16% 4	1780
May	1st to 10th	Night	53% 5% 11% 20% 9% 3	2520
	11th to 20th	Night	47% 5% 12% 20% 13% 2	2496
	21st to end	Night	46% 5% 12% 19% 17% 2	2739
June	1st to 10th	Night	47% 7% 12% 23% 11%	1520
	11th to 20th	Night	42% 5% 12% 23% 16% 2	1520
	21st to end	Night	47% 7% 13% 19% 15% 1	1520
July	1st to 10th	Night	45% 7% 15% 20% 12% 1	2480
	11th to 20th	Night	39% 6% 16% 31% 8%	2472
	21st to end	Night	37% 7% 17% 25% 14%	2651
August	1st to 10th	Night	44% 6% 15% 26% 8%	2496
	11th to 20th	Night	41% 5% 17% 29% 7%	2853
	21st to end	Night	40% 7% 16% 23% 12% 2	3476
September	1st to 10th	Night	38% 6% 11% 28% 16% 1	1900
	11th to 20th	Night	36% 5% 13% 35% 11% 1	1928
	21st to end	Night	40% 4 13% 30% 12%	2102
October	1st to 10th	Night	43% 6% 15% 25% 10%	3800
	11th to 20th	Night	45% 5% 15% 26% 9%	3779
	21st to end	Night	47% 5% 17% 20% 10%	4453
November	1st to 10th	Night	52% 4 15% 21% 9% 1	2537
	11th to 20th	Night	55% 4 12% 18% 10% 1	2754
	21st to end	Night	62% 5% 12% 16% 5%	2800
December	1st to 10th	Night	68% 5% 12% 12% 3	4580
	11th to 20th	Night	63% 4 7% 16% 10% 1	5208
	21st to end	Night	66% 3 7% 14% 10%	6996

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	27% 11% 30% 27% 5% 2	200
	11th to 20th	Day	24% 12% 36% 21% 6% 2	200
	21st to end	Day	20% 10% 31% 29% 9% 1	233
February	1st to 10th	Day	29% 15% 37% 14% 5%	300
	11th to 20th	Day	26% 15% 36% 21% 1	340
	21st to end	Day	28% 11% 31% 27% 3	414
March	1st to 10th	Day	25% 10% 31% 25% 9%	500
	11th to 20th	Day	23% 15% 34% 22% 6% 1	500
	21st to end	Day	27% 12% 31% 18% 9% 3	550
April	1st to 10th	Day	27% 16% 28% 17% 10% 1	500
	11th to 20th	Day	29% 13% 24% 18% 12% 3	452
	21st to end	Day	27% 10% 31% 20% 10% 2	460
May	1st to 10th	Day	24% 8% 29% 29% 7% 3	463
	11th to 20th	Day	19% 10% 27% 25% 14% 5%	487
	21st to end	Day	18% 9% 31% 25% 13% 4	529
June	1st to 10th	Day	26% 6% 25% 27% 12% 5%	490
	11th to 20th	Day	14% 10% 26% 28% 16% 6%	490
	21st to end	Day	22% 9% 25% 26% 14% 4	490
July	1st to 10th	Day	14% 5% 22% 33% 18% 8%	485
	11th to 20th	Day	14% 5% 26% 30% 20% 5%	484
	21st to end	Day	14% 4 23% 33% 20% 6%	532
August	1st to 10th	Day	14% 5% 21% 30% 23% 7%	486
	11th to 20th	Day	16% 4 17% 28% 25% 10%	479
	21st to end	Day	16% 5% 21% 29% 20% 10%	539
September	1st to 10th	Day	18% 4 25% 22% 19% 12%	490
	11th to 20th	Day	14% 4 19% 22% 24% 17%	490
	21st to end	Day	20% 7% 25% 21% 18% 9%	490
October	1st to 10th	Day	25% 9% 24% 20% 16% 6%	490
	11th to 20th	Day	26% 11% 28% 21% 12% 1	490
	21st to end	Day	30% 9% 25% 25% 11%	436
November	1st to 10th	Day	27% 11% 23% 25% 12% 2	395
	11th to 20th	Day	27% 10% 34% 20% 8% 2	245
	21st to end	Day	30% 14% 24% 24% 7% 2	200
December	1st to 10th	Day	32% 8% 29% 22% 10%	200
	11th to 20th	Day	27% 15% 36% 20% 3 1	200
	21st to end	Day	30% 14% 34% 15% 5% 1	220
January	1st to 10th	Night	28% 9% 30% 27% 6%	800
	11th to 20th	Night	28% 9% 28% 29% 7% 1	800
	21st to end	Night	20% 7% 28% 36% 9% 1	880
February	1st to 10th	Night	21% 10% 33% 30% 6%	800
	11th to 20th	Night	20% 10% 36% 28% 6%	783
	21st to end	Night	24% 8% 28% 34% 5%	660
March	1st to 10th	Night	24% 9% 29% 32% 6%	800
	11th to 20th	Night	24% 8% 33% 28% 6% 1	800
	21st to end	Night	23% 8% 30% 30% 9%	880
April	1st to 10th	Night	28% 9% 27% 29% 7%	800
	11th to 20th	Night	25% 10% 29% 27% 9% 1	754
	21st to end	Night	27% 7% 27% 31% 8% 1	640
May	1st to 10th	Night	25% 5% 26% 34% 8% 2	380
	11th to 20th	Night	25% 5% 26% 32% 12% 1	374
	21st to end	Night	16% 5% 30% 35% 13%	411
June	1st to 10th	Night	17% 4 24% 43% 12% 1	380
	11th to 20th	Night	17% 4 23% 41% 14% 1	380
	21st to end	Night	20% 3 23% 35% 18% 2	380
July	1st to 10th	Night	12% 3 19% 45% 20%	370
	11th to 20th	Night	13% 3 18% 46% 19% 1	368
	21st to end	Night	15% 3 16% 46% 19% 1	416
August	1st to 10th	Night	13% 3 12% 47% 23% 1	378
	11th to 20th	Night	15% 3 13% 37% 28% 3	579
	21st to end	Night	15% 3 18% 39% 22% 2	836
September	1st to 10th	Night	14% 4 21% 42% 19% 1	760
	11th to 20th	Night	18% 4 14% 34% 27% 3	760
	21st to end	Night	19% 4 21% 33% 21% 2	760
October	1st to 10th	Night	23% 5% 23% 32% 17% 1	760
	11th to 20th	Night	26% 6% 25% 33% 11% 1	760
	21st to end	Night	26% 8% 27% 32% 7%	836
November	1st to 10th	Night	26% 8% 23% 31% 11% 1	780
	11th to 20th	Night	24% 9% 31% 26% 11%	800
	21st to end	Night	29% 9% 28% 26% 9%	800
December	1st to 10th	Night	29% 9% 29% 27% 6% 1	800
	11th to 20th	Night	29% 11% 28% 25% 6% 1	800
	21st to end	Night	35% 8% 28% 23% 5% 1	880

Page generated at 2025-04-08 09:17:24. Version ac07

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