Deep Woods Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deep Woods Observatory clear sky chart from 2005-11-06 to 2025-05-26.

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% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	114104
	Night	13% 11% 4 9% 5% 5% 4 4 5% 7% 34%	115384
Transparency	Day	47% 7% 12% 21% 12% 1	117596
	Night	47% 7% 13% 21% 11% 1	120054
Seeing	Day	21% 17% 33% 19% 8% 3	16744
	Night	20% 14% 33% 25% 8% 1	26742

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	8% 10% 5% 15% 9% 7% 5% 4 6% 6% 23%	3617
	11th to 20th	Day	10% 14% 5% 11% 9% 6% 7% 5% 6% 6% 21%	3589
	21st to end	Day	10% 16% 7% 15% 6% 4 4 3 5% 6% 25%	3982
February	1st to 10th	Day	9% 13% 4 10% 6% 5% 5% 4 6% 7% 31%	1734
	11th to 20th	Day	10% 12% 3 11% 6% 6% 4 5% 5% 7% 31%	1867
	21st to end	Day	12% 14% 5% 10% 6% 5% 4 3 5% 8% 27%	1650
March	1st to 10th	Day	12% 10% 2 6% 4 4 4 3 6% 6% 42%	4478
	11th to 20th	Day	10% 11% 4 10% 8% 5% 5% 4 6% 7% 30%	4688
	21st to end	Day	22% 13% 5% 12% 5% 4 3 3 3 4 25%	5969
April	1st to 10th	Day	15% 16% 5% 9% 6% 4 4 3 6% 7% 26%	2400
	11th to 20th	Day	19% 12% 5% 9% 5% 4 4 4 4 5% 30%	2374
	21st to end	Day	18% 12% 4 8% 4 5% 5% 4 5% 6% 31%	2580
May	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 3 5% 6% 37%	4459
	11th to 20th	Day	14% 7% 3 7% 5% 4 4 3 6% 6% 40%	4539
	21st to end	Day	14% 8% 3 8% 6% 5% 5% 3 6% 7% 35%	4908
June	1st to 10th	Day	12% 7% 3 6% 6% 4 5% 4 6% 8% 40%	2660
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 7% 8% 36%	2660
	21st to end	Day	13% 6% 3 8% 5% 5% 5% 4 7% 9% 35%	2660
July	1st to 10th	Day	14% 5% 2 6% 6% 6% 5% 4 7% 9% 37%	4440
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 5% 7% 9% 48%	4440
	21st to end	Day	8% 6% 2 6% 4 4 6% 4 7% 8% 45%	4814
August	1st to 10th	Day	10% 6% 2 7% 5% 6% 5% 4 7% 8% 39%	4423
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 6% 7% 46%	4051
	21st to end	Day	15% 7% 3 6% 5% 4 5% 3 4 7% 41%	4180
September	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 3 5% 6% 43%	2280
	11th to 20th	Day	13% 7% 2 7% 4 5% 5% 4 6% 8% 40%	2188
	21st to end	Day	14% 9% 4 8% 5% 5% 5% 3 5% 8% 35%	2090
October	1st to 10th	Day	18% 8% 3 7% 4 4 4 4 5% 7% 36%	1976
	11th to 20th	Day	20% 13% 3 8% 5% 5% 5% 3 6% 6% 25%	1841
	21st to end	Day	21% 15% 4 9% 5% 3 4 3 5% 5% 27%	1881
November	1st to 10th	Day	15% 11% 4 7% 5% 4 5% 4 5% 7% 32%	1762
	11th to 20th	Day	15% 16% 6% 11% 6% 5% 5% 4 5% 5% 23%	1728
	21st to end	Day	15% 14% 5% 10% 6% 6% 4 4 6% 7% 23%	1600
December	1st to 10th	Day	9% 14% 6% 20% 9% 6% 5% 4 5% 4 18%	2610
	11th to 20th	Day	11% 20% 6% 13% 6% 5% 4 3 4 4 24%	3004
	21st to end	Day	21% 22% 7% 10% 3 4 3 2 3 4 20%	3982
January	1st to 10th	Night	9% 12% 5% 13% 8% 5% 5% 4 5% 6% 27%	6348
	11th to 20th	Night	8% 14% 5% 13% 8% 7% 6% 4 6% 7% 22%	6316
	21st to end	Night	8% 13% 7% 15% 7% 5% 5% 3 5% 6% 26%	6996
February	1st to 10th	Night	11% 12% 5% 10% 6% 4 6% 4 6% 7% 30%	2665
	11th to 20th	Night	8% 12% 3 10% 8% 7% 5% 4 5% 7% 31%	2418
	21st to end	Night	13% 15% 4 9% 6% 5% 4 3 5% 7% 30%	1980
March	1st to 10th	Night	11% 8% 3 8% 4 4 4 3 5% 8% 41%	5362
	11th to 20th	Night	11% 11% 4 11% 4 5% 4 4 5% 9% 31%	5314
	21st to end	Night	20% 12% 5% 9% 5% 4 4 3 5% 6% 27%	4962
April	1st to 10th	Night	19% 13% 4 8% 4 4 4 3 4 7% 31%	2000
	11th to 20th	Night	21% 10% 3 6% 4 4 3 4 4 5% 35%	1975
	21st to end	Night	17% 8% 4 8% 5% 4 4 3 6% 5% 36%	1818
May	1st to 10th	Night	16% 9% 3 7% 5% 3 4 3 5% 6% 39%	2707
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 6% 6% 43%	2586
	21st to end	Night	16% 7% 3 7% 5% 5% 5% 4 5% 7% 37%	2795
June	1st to 10th	Night	11% 6% 2 5% 5% 5% 4 4 5% 8% 45%	1520
	11th to 20th	Night	13% 8% 3 6% 5% 4 5% 4 6% 6% 40%	1520
	21st to end	Night	13% 6% 2 5% 4 5% 5% 4 7% 8% 41%	1520
July	1st to 10th	Night	14% 5% 2 5% 3 3 4 3 5% 9% 45%	2530
	11th to 20th	Night	5% 4 1 5% 3 3 4 4 6% 7% 59%	2530
	21st to end	Night	8% 6% 2 5% 4 3 4 3 4 7% 53%	2750
August	1st to 10th	Night	10% 6% 3 7% 4 4 4 4 5% 9% 43%	2517
	11th to 20th	Night	12% 5% 2 5% 3 4 4 4 5% 6% 49%	2777
	21st to end	Night	14% 7% 3 6% 4 4 4 3 5% 6% 47%	3476
September	1st to 10th	Night	10% 6% 2 6% 4 3 3 3 5% 6% 52%	1900
	11th to 20th	Night	12% 5% 3 5% 3 4 4 3 5% 7% 49%	1972
	21st to end	Night	15% 8% 2 6% 4 4 4 4 4 6% 42%	2090
October	1st to 10th	Night	15% 9% 2 5% 4 4 4 3 4 6% 43%	2200
	11th to 20th	Night	17% 12% 4 7% 4 5% 4 3 6% 6% 33%	2338
	21st to end	Night	19% 14% 3 8% 5% 4 4 3 6% 7% 27%	2717
November	1st to 10th	Night	15% 10% 3 7% 4 5% 4 4 5% 7% 35%	2536
	11th to 20th	Night	15% 13% 4 9% 5% 6% 4 4 5% 6% 29%	2670
	21st to end	Night	14% 13% 5% 11% 6% 5% 5% 4 5% 6% 25%	2800
December	1st to 10th	Night	13% 11% 4 10% 7% 6% 5% 5% 7% 8% 23%	4580
	11th to 20th	Night	11% 16% 8% 13% 7% 5% 4 3 4 4 24%	5203
	21st to end	Night	20% 19% 7% 10% 5% 4 3 3 4 5% 21%	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	56% 8% 10% 17% 9% 1	3618
	11th to 20th	Day	58% 8% 11% 14% 7% 2	3620
	21st to end	Day	58% 7% 11% 13% 10% 1	3982
February	1st to 10th	Day	50% 7% 13% 18% 12% 1	1734
	11th to 20th	Day	49% 8% 12% 21% 9% 1	1876
	21st to end	Day	53% 6% 11% 17% 11% 1	1650
March	1st to 10th	Day	41% 5% 15% 18% 18% 4	4480
	11th to 20th	Day	50% 8% 13% 18% 10% 1	4688
	21st to end	Day	62% 6% 8% 13% 11%	5969
April	1st to 10th	Day	55% 7% 11% 17% 9% 1	2400
	11th to 20th	Day	55% 6% 9% 20% 10% 1	2374
	21st to end	Day	52% 7% 9% 17% 13% 2	2580
May	1st to 10th	Day	43% 9% 11% 21% 15% 1	4471
	11th to 20th	Day	40% 8% 14% 25% 12% 1	4543
	21st to end	Day	45% 8% 11% 18% 16% 1	4908
June	1st to 10th	Day	39% 8% 13% 25% 13% 1	2660
	11th to 20th	Day	43% 9% 14% 19% 14% 1	2660
	21st to end	Day	43% 9% 14% 22% 10% 1	2660
July	1st to 10th	Day	41% 8% 15% 24% 12% 1	4371
	11th to 20th	Day	28% 8% 14% 33% 14% 3	4365
	21st to end	Day	34% 7% 12% 30% 17% 1	4662
August	1st to 10th	Day	39% 9% 15% 24% 13% 1	4408
	11th to 20th	Day	37% 7% 16% 29% 11%	4093
	21st to end	Day	43% 6% 15% 22% 13% 1	4179
September	1st to 10th	Day	40% 6% 12% 28% 13%	2280
	11th to 20th	Day	40% 8% 18% 26% 7%	2188
	21st to end	Day	46% 7% 12% 25% 9%	2090
October	1st to 10th	Day	33% 5% 13% 28% 20%	3288
	11th to 20th	Day	52% 5% 11% 19% 12%	3031
	21st to end	Day	54% 6% 16% 20% 4	3081
November	1st to 10th	Day	48% 7% 13% 22% 10% 1	1762
	11th to 20th	Day	59% 8% 12% 13% 7% 1	1728
	21st to end	Day	57% 10% 14% 15% 4 1	1600
December	1st to 10th	Day	62% 9% 11% 14% 4	2610
	11th to 20th	Day	62% 6% 9% 12% 11%	3005
	21st to end	Day	67% 5% 6% 15% 7%	3982
January	1st to 10th	Night	53% 8% 11% 22% 7%	6348
	11th to 20th	Night	57% 8% 12% 13% 10% 1	6360
	21st to end	Night	55% 7% 11% 19% 7%	6996
February	1st to 10th	Night	51% 6% 11% 19% 12% 2	2665
	11th to 20th	Night	49% 7% 12% 20% 10% 2	2419
	21st to end	Night	53% 5% 13% 19% 9% 1	1980
March	1st to 10th	Night	39% 5% 19% 20% 14% 1	5373
	11th to 20th	Night	47% 7% 16% 20% 9% 1	5315
	21st to end	Night	56% 7% 11% 14% 11% 1	4962
April	1st to 10th	Night	53% 6% 11% 15% 14% 1	2000
	11th to 20th	Night	50% 7% 9% 21% 11% 3	1975
	21st to end	Night	48% 7% 11% 19% 14% 1	1818
May	1st to 10th	Night	44% 7% 13% 22% 13% 1	2714
	11th to 20th	Night	37% 9% 14% 23% 14% 2	2587
	21st to end	Night	45% 8% 14% 16% 14% 3	2796
June	1st to 10th	Night	35% 8% 14% 26% 14% 1	1520
	11th to 20th	Night	43% 8% 12% 22% 15% 1	1520
	21st to end	Night	36% 10% 15% 27% 10% 2	1520
July	1st to 10th	Night	37% 7% 19% 23% 12% 3	2490
	11th to 20th	Night	22% 8% 17% 37% 14% 2	2482
	21st to end	Night	31% 7% 13% 27% 21% 1	2662
August	1st to 10th	Night	36% 9% 15% 20% 18% 2	2508
	11th to 20th	Night	34% 8% 15% 28% 13% 2	2801
	21st to end	Night	39% 7% 13% 24% 15% 1	3476
September	1st to 10th	Night	31% 8% 14% 32% 14% 1	1900
	11th to 20th	Night	33% 9% 16% 33% 10%	1972
	21st to end	Night	41% 8% 15% 26% 9% 1	2090
October	1st to 10th	Night	32% 5% 9% 36% 16% 2	3668
	11th to 20th	Night	41% 6% 11% 26% 14%	3887
	21st to end	Night	53% 5% 12% 22% 8%	4467
November	1st to 10th	Night	48% 6% 13% 22% 11% 1	2536
	11th to 20th	Night	53% 8% 13% 16% 9% 1	2670
	21st to end	Night	55% 9% 14% 17% 4 1	2800
December	1st to 10th	Night	51% 9% 20% 16% 3	4580
	11th to 20th	Night	61% 6% 9% 15% 8%	5201
	21st to end	Night	64% 5% 6% 16% 9%	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	21% 15% 37% 20% 6% 1	300
	11th to 20th	Day	28% 19% 33% 16% 3	300
	21st to end	Day	24% 18% 42% 14% 2	330
February	1st to 10th	Day	18% 32% 36% 11% 3 1	300
	11th to 20th	Day	24% 30% 32% 12% 2 1	300
	21st to end	Day	27% 27% 37% 8% 1	249
March	1st to 10th	Day	23% 26% 40% 10% 1	300
	11th to 20th	Day	27% 22% 34% 12% 4	310
	21st to end	Day	27% 21% 30% 15% 6% 1	550
April	1st to 10th	Day	31% 19% 30% 16% 3	500
	11th to 20th	Day	31% 20% 27% 15% 5% 1	492
	21st to end	Day	30% 17% 29% 16% 6% 1	500
May	1st to 10th	Day	26% 18% 32% 15% 7% 1	633
	11th to 20th	Day	21% 20% 34% 17% 5% 3	725
	21st to end	Day	20% 15% 35% 19% 7% 4	760
June	1st to 10th	Day	17% 18% 32% 21% 9% 3	680
	11th to 20th	Day	20% 13% 37% 20% 9% 2	680
	21st to end	Day	18% 15% 32% 23% 9% 4	680
July	1st to 10th	Day	9% 10% 36% 27% 13% 5%	671
	11th to 20th	Day	12% 12% 34% 26% 10% 6%	668
	21st to end	Day	8% 11% 35% 26% 14% 6%	739
August	1st to 10th	Day	13% 11% 31% 26% 15% 5%	676
	11th to 20th	Day	17% 7% 25% 27% 18% 7%	556
	21st to end	Day	20% 11% 26% 23% 16% 5%	539
September	1st to 10th	Day	19% 11% 26% 26% 12% 6%	490
	11th to 20th	Day	15% 10% 33% 28% 10% 3	490
	21st to end	Day	20% 14% 28% 24% 11% 3	490
October	1st to 10th	Day	23% 19% 30% 17% 9% 2	376
	11th to 20th	Day	28% 26% 32% 12% 2	300
	21st to end	Day	31% 25% 33% 9% 2	330
November	1st to 10th	Day	23% 21% 37% 14% 4 1	300
	11th to 20th	Day	27% 26% 34% 9% 2 2	300
	21st to end	Day	28% 22% 35% 11% 4	300
December	1st to 10th	Day	22% 18% 40% 14% 4 1	300
	11th to 20th	Day	26% 26% 33% 12% 3	300
	21st to end	Day	32% 21% 33% 12% 2 1	330
January	1st to 10th	Night	17% 19% 41% 19% 3 1	1000
	11th to 20th	Night	20% 17% 40% 17% 6% 1	1000
	21st to end	Night	19% 15% 41% 21% 5%	1100
February	1st to 10th	Night	20% 18% 38% 18% 6%	1000
	11th to 20th	Night	19% 23% 33% 19% 6%	863
	21st to end	Night	20% 20% 40% 17% 2	660
March	1st to 10th	Night	19% 17% 34% 23% 6%	800
	11th to 20th	Night	18% 20% 37% 20% 5% 1	764
	21st to end	Night	23% 11% 32% 26% 7% 1	660
April	1st to 10th	Night	26% 14% 33% 23% 4	600
	11th to 20th	Night	28% 14% 27% 25% 5% 1	593
	21st to end	Night	23% 13% 33% 25% 6% 1	600
May	1st to 10th	Night	23% 12% 32% 26% 7% 1	600
	11th to 20th	Night	21% 10% 36% 25% 6% 1	591
	21st to end	Night	16% 9% 30% 34% 11% 1	634
June	1st to 10th	Night	14% 11% 29% 34% 12%	570
	11th to 20th	Night	18% 8% 30% 34% 8% 1	570
	21st to end	Night	15% 6% 31% 35% 13% 1	570
July	1st to 10th	Night	10% 5% 29% 40% 16% 1	555
	11th to 20th	Night	10% 5% 34% 32% 18% 1	552
	21st to end	Night	8% 4 30% 40% 16% 2	624
August	1st to 10th	Night	12% 4 27% 38% 17% 1	567
	11th to 20th	Night	13% 4 23% 35% 22% 3	564
	21st to end	Night	15% 6% 26% 34% 18% 1	627
September	1st to 10th	Night	13% 8% 26% 32% 18% 3	570
	11th to 20th	Night	15% 7% 30% 31% 15% 2	642
	21st to end	Night	19% 10% 30% 30% 10% 1	760
October	1st to 10th	Night	20% 14% 30% 23% 13% 1	760
	11th to 20th	Night	23% 18% 29% 23% 7%	760
	21st to end	Night	28% 12% 32% 22% 6%	836
November	1st to 10th	Night	21% 15% 38% 21% 5%	780
	11th to 20th	Night	24% 20% 34% 17% 5% 1	870
	21st to end	Night	23% 19% 33% 21% 4	1000
December	1st to 10th	Night	22% 20% 36% 17% 5% 1	1000
	11th to 20th	Night	21% 19% 35% 19% 5% 1	1000
	21st to end	Night	29% 16% 35% 16% 4	1100

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