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 2024-02-20.

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	18% 10% 4 7% 6% 3 5% 5% 5% 6% 32%	78186
	Night	18% 11% 4 7% 6% 3 4 4 5% 6% 32%	81005
Transparency	Day	52% 4 12% 20% 10% 1	77891
	Night	54% 4 11% 19% 10% 1	80834
Seeing	Day	22% 8% 27% 25% 13% 5%	13484
	Night	23% 7% 26% 32% 12% 1	22701

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	18% 17% 7% 12% 9% 3 4 4 3 4 20%	2517
	11th to 20th	Day	15% 13% 7% 12% 7% 3 5% 4 5% 6% 22%	2520
	21st to end	Day	15% 17% 7% 9% 8% 5% 6% 5% 4 5% 19%	2807
February	1st to 10th	Day	14% 15% 6% 10% 8% 3 5% 4 5% 5% 25%	1710
	11th to 20th	Day	14% 16% 6% 11% 5% 4 4 4 4 5% 27%	1691
	21st to end	Day	19% 13% 6% 8% 7% 5% 5% 3 4 5% 25%	1480
March	1st to 10th	Day	16% 12% 5% 8% 8% 3 4 5% 5% 5% 29%	1797
	11th to 20th	Day	17% 12% 3 7% 7% 4 5% 4 5% 5% 30%	1932
	21st to end	Day	20% 9% 4 7% 5% 3 3 4 4 6% 36%	2326
April	1st to 10th	Day	22% 12% 4 6% 5% 2 4 3 4 5% 32%	2160
	11th to 20th	Day	21% 8% 3 6% 5% 3 4 5% 4 6% 36%	2134
	21st to end	Day	23% 10% 4 6% 5% 3 5% 4 4 5% 31%	2288
May	1st to 10th	Day	21% 8% 3 6% 6% 3 4 5% 5% 6% 34%	2520
	11th to 20th	Day	18% 6% 3 5% 5% 3 4 4 5% 7% 38%	2478
	21st to end	Day	18% 7% 3 5% 6% 4 4 6% 5% 8% 36%	2725
June	1st to 10th	Day	23% 7% 3 6% 5% 4 5% 5% 6% 7% 30%	2520
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 6% 6% 8% 37%	2520
	21st to end	Day	20% 6% 3 6% 5% 3 6% 5% 7% 7% 30%	2520
July	1st to 10th	Day	13% 5% 2 5% 5% 4 6% 6% 6% 8% 40%	2520
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 6% 7% 10% 35%	2520
	21st to end	Day	12% 6% 2 6% 7% 3 5% 7% 7% 9% 37%	2744
August	1st to 10th	Day	12% 6% 3 5% 6% 4 6% 6% 5% 9% 38%	2506
	11th to 20th	Day	13% 6% 3 6% 5% 3 6% 5% 6% 9% 39%	2268
	21st to end	Day	13% 6% 2 5% 6% 3 4 5% 5% 8% 43%	2376
September	1st to 10th	Day	16% 5% 2 5% 5% 3 5% 5% 5% 7% 42%	2160
	11th to 20th	Day	14% 5% 2 4 5% 3 5% 4 4 8% 48%	2133
	21st to end	Day	19% 6% 2 4 4 3 4 5% 5% 7% 41%	1966
October	1st to 10th	Day	22% 8% 2 6% 5% 3 4 4 5% 6% 36%	1800
	11th to 20th	Day	24% 9% 4 7% 6% 4 4 4 5% 6% 27%	1800
	21st to end	Day	26% 10% 4 6% 6% 4 5% 4 5% 5% 27%	1796
November	1st to 10th	Day	17% 9% 3 6% 6% 3 5% 4 6% 7% 35%	1671
	11th to 20th	Day	22% 11% 4 8% 6% 4 5% 4 5% 5% 27%	1563
	21st to end	Day	23% 13% 4 8% 7% 4 4 4 5% 6% 23%	1520
December	1st to 10th	Day	23% 15% 6% 10% 6% 3 6% 4 4 5% 18%	1520
	11th to 20th	Day	22% 15% 4 8% 7% 3 5% 4 3 5% 24%	1906
	21st to end	Day	27% 19% 5% 8% 6% 2 3 3 3 3 20%	2772
January	1st to 10th	Night	18% 17% 6% 9% 6% 4 4 4 4 4 23%	4429
	11th to 20th	Night	14% 15% 6% 10% 8% 4 5% 5% 5% 5% 23%	4440
	21st to end	Night	16% 20% 6% 9% 7% 4 4 3 4 5% 21%	4848
February	1st to 10th	Night	14% 13% 5% 10% 8% 4 5% 4 5% 6% 25%	2470
	11th to 20th	Night	14% 11% 6% 10% 9% 4 4 5% 4 5% 28%	2360
	21st to end	Night	17% 13% 5% 8% 7% 3 4 4 5% 5% 30%	1776
March	1st to 10th	Night	17% 12% 4 9% 8% 3 5% 4 4 5% 29%	2150
	11th to 20th	Night	19% 13% 4 7% 6% 3 4 4 3 5% 33%	2025
	21st to end	Night	17% 9% 3 7% 6% 2 4 4 4 4 40%	2030
April	1st to 10th	Night	23% 10% 3 5% 6% 3 4 4 4 6% 32%	1800
	11th to 20th	Night	21% 8% 3 6% 6% 2 4 4 5% 6% 37%	1775
	21st to end	Night	21% 9% 3 7% 5% 2 4 4 4 6% 34%	1688
May	1st to 10th	Night	22% 7% 3 6% 5% 2 4 4 4 7% 36%	1440
	11th to 20th	Night	20% 6% 2 5% 5% 2 3 4 4 6% 43%	1415
	21st to end	Night	15% 5% 3 5% 5% 3 5% 5% 5% 7% 42%	1550
June	1st to 10th	Night	18% 6% 3 6% 6% 3 4 6% 8% 9% 32%	1440
	11th to 20th	Night	15% 6% 3 6% 4 3 5% 5% 5% 6% 41%	1440
	21st to end	Night	18% 7% 2 6% 5% 3 5% 6% 5% 6% 37%	1440
July	1st to 10th	Night	10% 5% 3 5% 4 3 4 5% 6% 7% 46%	1440
	11th to 20th	Night	11% 6% 3 5% 4 3 5% 4 6% 9% 43%	1440
	21st to end	Night	11% 5% 3 5% 5% 4 5% 5% 6% 8% 43%	1574
August	1st to 10th	Night	11% 6% 3 5% 5% 3 4 6% 7% 8% 43%	1432
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 7% 7% 46%	1611
	21st to end	Night	12% 5% 2 5% 5% 4 4 5% 5% 7% 45%	1980
September	1st to 10th	Night	13% 5% 2 5% 6% 4 5% 5% 5% 7% 45%	1800
	11th to 20th	Night	15% 6% 2 3 4 2 3 4 4 7% 49%	1826
	21st to end	Night	18% 5% 2 5% 5% 3 4 3 4 8% 44%	1991
October	1st to 10th	Night	19% 7% 3 6% 4 3 5% 4 6% 6% 38%	2160
	11th to 20th	Night	22% 8% 3 6% 6% 4 5% 5% 4 5% 33%	2160
	21st to end	Night	25% 9% 3 7% 6% 3 4 4 4 5% 29%	2560
November	1st to 10th	Night	21% 9% 3 6% 6% 3 4 4 5% 6% 33%	2407
	11th to 20th	Night	19% 10% 4 8% 7% 3 4 4 5% 5% 31%	2616
	21st to end	Night	22% 12% 5% 8% 7% 3 6% 4 4 5% 24%	2660
December	1st to 10th	Night	22% 16% 6% 10% 6% 3 5% 5% 4 5% 18%	2660
	11th to 20th	Night	23% 14% 5% 8% 6% 3 4 3 4 4 26%	3288
	21st to end	Night	30% 19% 4 8% 5% 2 3 3 3 4 20%	4884

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	68% 4 11% 11% 6% 1	2518
	11th to 20th	Day	62% 4 11% 16% 6% 1	2520
	21st to end	Day	67% 5% 12% 10% 5%	2807
February	1st to 10th	Day	61% 3 10% 18% 6% 2	1710
	11th to 20th	Day	58% 3 12% 16% 9% 1	1700
	21st to end	Day	62% 3 9% 14% 10% 1	1480
March	1st to 10th	Day	57% 3 10% 17% 11% 2	1800
	11th to 20th	Day	55% 3 12% 16% 12% 3	1932
	21st to end	Day	50% 4 8% 18% 15% 5%	2326
April	1st to 10th	Day	55% 3 7% 19% 12% 3	2160
	11th to 20th	Day	50% 3 7% 23% 15% 2	2134
	21st to end	Day	56% 3 9% 14% 13% 4	2288
May	1st to 10th	Day	51% 5% 6% 20% 15% 3	2520
	11th to 20th	Day	46% 4 9% 22% 16% 3	2482
	21st to end	Day	46% 6% 12% 20% 14% 2	2725
June	1st to 10th	Day	53% 4 11% 19% 11% 1	2520
	11th to 20th	Day	43% 6% 12% 23% 14% 2	2520
	21st to end	Day	50% 7% 13% 19% 10% 1	2520
July	1st to 10th	Day	40% 5% 14% 26% 13% 1	2450
	11th to 20th	Day	41% 6% 15% 27% 11%	2445
	21st to end	Day	41% 6% 17% 26% 11%	2591
August	1st to 10th	Day	42% 6% 18% 25% 9%	2492
	11th to 20th	Day	40% 5% 18% 27% 9% 1	2268
	21st to end	Day	39% 5% 16% 27% 13% 1	2376
September	1st to 10th	Day	39% 6% 14% 27% 14%	2160
	11th to 20th	Day	36% 5% 14% 31% 14% 1	2133
	21st to end	Day	42% 6% 15% 28% 9%	1966
October	1st to 10th	Day	49% 4 11% 24% 11% 1	1800
	11th to 20th	Day	57% 5% 14% 18% 7% 1	1800
	21st to end	Day	59% 3 13% 17% 7% 1	1796
November	1st to 10th	Day	50% 4 14% 23% 7% 1	1671
	11th to 20th	Day	58% 3 14% 16% 9%	1563
	21st to end	Day	63% 4 13% 13% 5% 1	1520
December	1st to 10th	Day	69% 5% 12% 11% 3	1520
	11th to 20th	Day	63% 4 13% 15% 4	1906
	21st to end	Day	71% 3 7% 14% 4	2772
January	1st to 10th	Night	66% 4 15% 11% 4	4429
	11th to 20th	Night	63% 5% 14% 13% 5% 1	4440
	21st to end	Night	67% 4 12% 11% 6% 1	4848
February	1st to 10th	Night	61% 4 10% 17% 7% 1	2470
	11th to 20th	Night	58% 4 9% 20% 8%	2362
	21st to end	Night	56% 4 8% 19% 12% 1	1776
March	1st to 10th	Night	58% 3 9% 17% 10% 2	2160
	11th to 20th	Night	55% 3 10% 18% 12% 3	2026
	21st to end	Night	48% 4 7% 20% 16% 4	2030
April	1st to 10th	Night	54% 4 8% 17% 15% 2	1800
	11th to 20th	Night	50% 4 6% 21% 17% 2	1775
	21st to end	Night	51% 4 8% 19% 15% 4	1688
May	1st to 10th	Night	49% 3 7% 19% 16% 5%	1440
	11th to 20th	Night	44% 4 8% 21% 19% 4	1416
	21st to end	Night	41% 5% 14% 20% 18% 3	1551
June	1st to 10th	Night	46% 7% 13% 23% 11%	1440
	11th to 20th	Night	43% 5% 11% 23% 16% 2	1440
	21st to end	Night	45% 7% 13% 19% 15%	1440
July	1st to 10th	Night	35% 7% 13% 27% 17% 1	1400
	11th to 20th	Night	37% 5% 14% 31% 13%	1392
	21st to end	Night	38% 7% 17% 24% 14%	1486
August	1st to 10th	Night	39% 6% 18% 26% 11% 1	1422
	11th to 20th	Night	37% 6% 18% 29% 11%	1611
	21st to end	Night	38% 6% 17% 29% 11% 1	1980
September	1st to 10th	Night	39% 6% 11% 27% 15% 1	1800
	11th to 20th	Night	37% 4 12% 35% 10% 1	1826
	21st to end	Night	41% 5% 13% 31% 11%	1991
October	1st to 10th	Night	46% 5% 11% 24% 12% 1	2160
	11th to 20th	Night	53% 5% 12% 22% 9%	2160
	21st to end	Night	58% 4 13% 17% 8% 1	2560
November	1st to 10th	Night	51% 4 15% 21% 9%	2407
	11th to 20th	Night	56% 4 12% 18% 9% 1	2616
	21st to end	Night	62% 5% 13% 16% 5%	2660
December	1st to 10th	Night	68% 5% 10% 14% 4	2660
	11th to 20th	Night	62% 4 8% 18% 7% 1	3288
	21st to end	Night	71% 4 7% 14% 4	4884

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% 9% 27% 29% 5% 2	180
	11th to 20th	Day	24% 11% 35% 22% 6% 2	180
	21st to end	Day	21% 7% 31% 30% 10% 1	209
February	1st to 10th	Day	29% 12% 39% 15% 5%	270
	11th to 20th	Day	25% 14% 38% 21% 2	304
	21st to end	Day	29% 11% 31% 26% 3	346
March	1st to 10th	Day	23% 9% 32% 26% 9%	420
	11th to 20th	Day	23% 14% 33% 23% 7% 1	420
	21st to end	Day	28% 11% 28% 20% 11% 3	462
April	1st to 10th	Day	26% 15% 28% 18% 11% 2	420
	11th to 20th	Day	28% 13% 23% 19% 13% 3	412
	21st to end	Day	28% 10% 30% 21% 10% 2	420
May	1st to 10th	Day	24% 8% 30% 27% 7% 3	423
	11th to 20th	Day	19% 11% 29% 25% 12% 4	447
	21st to end	Day	19% 8% 32% 25% 12% 4	485
June	1st to 10th	Day	26% 6% 26% 27% 11% 5%	450
	11th to 20th	Day	15% 9% 25% 28% 17% 6%	450
	21st to end	Day	20% 7% 25% 28% 15% 5%	450
July	1st to 10th	Day	14% 5% 20% 34% 19% 8%	445
	11th to 20th	Day	14% 4 26% 31% 21% 5%	444
	21st to end	Day	14% 4 24% 32% 21% 6%	488
August	1st to 10th	Day	14% 4 21% 29% 24% 7%	446
	11th to 20th	Day	16% 3 15% 29% 26% 10%	439
	21st to end	Day	15% 5% 21% 28% 21% 10%	495
September	1st to 10th	Day	18% 4 24% 22% 20% 13%	450
	11th to 20th	Day	16% 5% 21% 22% 22% 14%	450
	21st to end	Day	20% 7% 25% 21% 18% 10%	450
October	1st to 10th	Day	25% 9% 24% 20% 16% 6%	450
	11th to 20th	Day	28% 10% 28% 21% 12% 2	450
	21st to end	Day	31% 9% 24% 25% 11%	403
November	1st to 10th	Day	26% 11% 22% 27% 13% 2	365
	11th to 20th	Day	28% 10% 32% 20% 8% 2	223
	21st to end	Day	26% 15% 23% 26% 7% 2	180
December	1st to 10th	Day	31% 8% 29% 22% 9%	180
	11th to 20th	Day	26% 14% 37% 21% 2 1	180
	21st to end	Day	34% 13% 33% 15% 5% 1	198
January	1st to 10th	Night	28% 9% 29% 28% 6%	760
	11th to 20th	Night	28% 7% 28% 29% 7% 1	760
	21st to end	Night	20% 7% 28% 36% 9% 1	836
February	1st to 10th	Night	21% 9% 33% 30% 6%	760
	11th to 20th	Night	20% 9% 36% 29% 6%	742
	21st to end	Night	24% 8% 29% 34% 5%	592
March	1st to 10th	Night	23% 9% 29% 33% 6%	720
	11th to 20th	Night	24% 8% 32% 28% 7% 1	720
	21st to end	Night	22% 7% 30% 31% 10%	792
April	1st to 10th	Night	28% 8% 27% 29% 7%	720
	11th to 20th	Night	24% 10% 28% 28% 9% 1	711
	21st to end	Night	26% 7% 27% 31% 8% 1	608
May	1st to 10th	Night	25% 5% 25% 34% 9% 2	360
	11th to 20th	Night	25% 4 27% 32% 11% 1	354
	21st to end	Night	16% 5% 30% 35% 13%	389
June	1st to 10th	Night	17% 4 24% 43% 11% 1	360
	11th to 20th	Night	17% 4 23% 40% 15% 1	360
	21st to end	Night	18% 3 23% 36% 19% 2	360
July	1st to 10th	Night	11% 3 19% 45% 21%	350
	11th to 20th	Night	13% 2 18% 46% 20% 1	348
	21st to end	Night	14% 3 16% 47% 19% 1	394
August	1st to 10th	Night	13% 3 12% 48% 23% 1	358
	11th to 20th	Night	14% 3 13% 39% 29% 3	547
	21st to end	Night	15% 3 19% 39% 22% 3	792
September	1st to 10th	Night	14% 3 20% 41% 20% 2	720
	11th to 20th	Night	19% 4 15% 35% 24% 3	720
	21st to end	Night	19% 4 21% 32% 22% 2	720
October	1st to 10th	Night	23% 5% 23% 32% 16% 1	720
	11th to 20th	Night	26% 6% 25% 33% 10% 1	720
	21st to end	Night	27% 7% 27% 33% 7%	792
November	1st to 10th	Night	26% 7% 23% 31% 12% 1	740
	11th to 20th	Night	24% 9% 30% 26% 11%	760
	21st to end	Night	29% 7% 28% 27% 9%	760
December	1st to 10th	Night	29% 8% 29% 27% 6% 1	760
	11th to 20th	Night	28% 11% 28% 25% 6% 1	760
	21st to end	Night	35% 9% 29% 22% 5%	836

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