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 2024-11-02.

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 6% 7% 34%	101570
	Night	13% 11% 4 9% 5% 4 4 3 5% 6% 35%	97548
Transparency	Day	47% 8% 13% 21% 11% 1	101329
	Night	47% 8% 13% 21% 10% 1	97407
Seeing	Day	21% 17% 33% 19% 8% 3	16039
	Night	19% 14% 33% 25% 8% 1	25899

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	12% 14% 6% 14% 5% 6% 5% 4 6% 6% 21%	2527
	11th to 20th	Day	11% 15% 4 13% 7% 6% 7% 5% 6% 6% 20%	2530
	21st to end	Day	13% 19% 8% 15% 5% 4 4 3 4 5% 20%	2783
February	1st to 10th	Day	9% 13% 4 10% 6% 5% 5% 4 6% 7% 30%	1647
	11th to 20th	Day	10% 12% 3 11% 6% 5% 4 5% 5% 6% 31%	1771
	21st to end	Day	12% 14% 4 10% 6% 5% 4 3 5% 8% 28%	1570
March	1st to 10th	Day	11% 13% 3 8% 4 3 4 3 5% 5% 42%	3118
	11th to 20th	Day	7% 13% 5% 11% 9% 5% 5% 4 6% 6% 30%	3272
	21st to end	Day	19% 14% 6% 14% 5% 5% 4 3 4 5% 23%	4165
April	1st to 10th	Day	16% 15% 5% 9% 6% 4 3 3 6% 7% 27%	2280
	11th to 20th	Day	20% 12% 4 8% 5% 4 4 4 4 5% 30%	2254
	21st to end	Day	18% 12% 4 8% 4 5% 5% 4 5% 6% 31%	2451
May	1st to 10th	Day	16% 8% 3 8% 5% 4 5% 3 5% 6% 36%	4322
	11th to 20th	Day	14% 7% 3 7% 5% 4 4 3 6% 7% 40%	4399
	21st to end	Day	14% 8% 3 9% 6% 5% 5% 3 6% 7% 35%	4837
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	14% 11% 4 8% 5% 4 5% 4 5% 7% 32%	1681
	11th to 20th	Day	15% 15% 6% 11% 6% 5% 5% 4 5% 5% 24%	1642
	21st to end	Day	15% 13% 5% 10% 6% 6% 4 4 7% 7% 23%	1520
December	1st to 10th	Day	12% 14% 6% 12% 7% 4 5% 5% 6% 5% 25%	1520
	11th to 20th	Day	15% 15% 5% 11% 7% 6% 5% 3 5% 6% 22%	1914
	21st to end	Day	21% 23% 7% 11% 4 4 4 3 3 5% 16%	2783
January	1st to 10th	Night	13% 15% 6% 13% 4 4 4 3 5% 6% 27%	4428
	11th to 20th	Night	10% 15% 6% 13% 8% 7% 5% 4 6% 6% 20%	4440
	21st to end	Night	10% 16% 8% 15% 6% 5% 4 3 5% 6% 21%	4884
February	1st to 10th	Night	10% 12% 5% 11% 6% 4 6% 4 6% 7% 30%	2532
	11th to 20th	Night	9% 12% 3 10% 8% 7% 5% 3 5% 7% 31%	2294
	21st to end	Night	13% 16% 4 9% 6% 4 3 3 5% 7% 30%	1884
March	1st to 10th	Night	10% 9% 3 8% 4 4 4 3 5% 8% 41%	3722
	11th to 20th	Night	9% 10% 4 12% 5% 5% 4 3 5% 8% 34%	3716
	21st to end	Night	17% 13% 5% 10% 6% 4 4 3 5% 5% 27%	3466
April	1st to 10th	Night	19% 12% 4 8% 4 4 4 3 4 7% 31%	1900
	11th to 20th	Night	21% 10% 3 6% 4 4 3 4 4 5% 35%	1875
	21st to end	Night	17% 8% 4 8% 5% 4 5% 3 6% 5% 35%	1727
May	1st to 10th	Night	17% 9% 3 7% 5% 3 4 3 5% 6% 38%	2624
	11th to 20th	Night	13% 8% 2 7% 4 4 4 4 6% 6% 43%	2506
	21st to end	Night	16% 7% 3 7% 5% 5% 5% 4 5% 7% 37%	2750
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% 5% 5% 4 4 5% 7% 35%	2420
	11th to 20th	Night	15% 12% 4 9% 5% 6% 4 4 5% 6% 29%	2536
	21st to end	Night	14% 13% 4 10% 6% 5% 5% 4 5% 6% 26%	2660
December	1st to 10th	Night	13% 14% 6% 9% 6% 5% 5% 4 5% 7% 28%	2660
	11th to 20th	Night	13% 13% 7% 11% 7% 5% 4 4 5% 5% 26%	3283
	21st to end	Night	22% 21% 8% 10% 4 3 3 2 3 4 19%	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	59% 9% 11% 16% 5%	2528
	11th to 20th	Day	60% 9% 13% 13% 5%	2530
	21st to end	Day	64% 8% 12% 12% 4 1	2783
February	1st to 10th	Day	50% 7% 13% 19% 12% 1	1647
	11th to 20th	Day	49% 8% 12% 21% 8% 1	1780
	21st to end	Day	53% 6% 11% 17% 11% 1	1570
March	1st to 10th	Day	42% 6% 13% 20% 18% 1	3120
	11th to 20th	Day	49% 10% 13% 18% 9% 1	3272
	21st to end	Day	62% 7% 9% 15% 7% 1	4165
April	1st to 10th	Day	55% 7% 12% 17% 9% 1	2280
	11th to 20th	Day	55% 6% 8% 20% 10% 1	2254
	21st to end	Day	52% 7% 9% 18% 13% 2	2451
May	1st to 10th	Day	43% 9% 11% 21% 15% 1	4334
	11th to 20th	Day	40% 8% 14% 25% 12% 1	4403
	21st to end	Day	45% 9% 11% 18% 16% 1	4837
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	45% 8% 13% 24% 10%	1976
	11th to 20th	Day	56% 7% 12% 17% 7% 1	1841
	21st to end	Day	58% 7% 12% 17% 5%	1881
November	1st to 10th	Day	48% 7% 13% 22% 10% 1	1681
	11th to 20th	Day	59% 8% 12% 13% 7%	1642
	21st to end	Day	57% 10% 14% 15% 3 1	1520
December	1st to 10th	Day	56% 7% 13% 18% 5%	1520
	11th to 20th	Day	60% 8% 12% 13% 6% 1	1915
	21st to end	Day	69% 7% 7% 11% 5%	2783
January	1st to 10th	Night	55% 7% 13% 18% 6% 1	4428
	11th to 20th	Night	59% 9% 14% 14% 4	4440
	21st to end	Night	62% 8% 10% 15% 4	4884
February	1st to 10th	Night	50% 7% 11% 19% 12% 2	2532
	11th to 20th	Night	48% 7% 13% 20% 9% 2	2295
	21st to end	Night	54% 5% 13% 18% 9% 1	1884
March	1st to 10th	Night	38% 7% 21% 23% 10% 1	3733
	11th to 20th	Night	45% 8% 13% 24% 9% 1	3717
	21st to end	Night	55% 9% 10% 15% 10% 1	3466
April	1st to 10th	Night	53% 7% 11% 15% 14% 1	1900
	11th to 20th	Night	49% 7% 9% 21% 11% 3	1875
	21st to end	Night	49% 7% 11% 18% 13% 1	1727
May	1st to 10th	Night	44% 7% 13% 22% 13% 1	2631
	11th to 20th	Night	37% 9% 14% 24% 14% 2	2507
	21st to end	Night	45% 9% 14% 16% 14% 3	2751
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	40% 7% 13% 28% 11% 1	2200
	11th to 20th	Night	50% 7% 13% 21% 8% 1	2338
	21st to end	Night	54% 7% 13% 18% 8%	2717
November	1st to 10th	Night	47% 6% 13% 22% 11% 1	2420
	11th to 20th	Night	53% 8% 13% 16% 9%	2536
	21st to end	Night	54% 9% 15% 18% 4 1	2660
December	1st to 10th	Night	53% 8% 16% 18% 5%	2660
	11th to 20th	Night	56% 9% 11% 17% 6%	3281
	21st to end	Night	68% 5% 7% 14% 5%	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	22% 17% 34% 20% 6% 1	270
	11th to 20th	Day	29% 19% 32% 16% 4	270
	21st to end	Day	24% 16% 42% 15% 2	297
February	1st to 10th	Day	17% 30% 37% 11% 3 1	270
	11th to 20th	Day	23% 31% 30% 13% 3 1	270
	21st to end	Day	26% 27% 39% 7% 1	225
March	1st to 10th	Day	21% 26% 41% 11% 1	270
	11th to 20th	Day	25% 22% 35% 13% 5%	279
	21st to end	Day	26% 21% 30% 16% 7% 1	506
April	1st to 10th	Day	29% 19% 31% 17% 4	460
	11th to 20th	Day	31% 20% 27% 15% 5% 2	452
	21st to end	Day	30% 17% 28% 17% 7% 2	460
May	1st to 10th	Day	26% 19% 32% 13% 7% 1	586
	11th to 20th	Day	21% 19% 34% 17% 5% 3	675
	21st to end	Day	20% 14% 35% 19% 8% 4	734
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% 26% 33% 9% 2	330
November	1st to 10th	Day	21% 21% 37% 15% 4 1	273
	11th to 20th	Day	24% 27% 35% 9% 3 2	270
	21st to end	Day	27% 22% 36% 11% 4	270
December	1st to 10th	Day	22% 18% 40% 15% 4 1	270
	11th to 20th	Day	28% 24% 33% 12% 3	270
	21st to end	Day	32% 22% 33% 11% 2	297
January	1st to 10th	Night	17% 20% 41% 19% 3 1	950
	11th to 20th	Night	20% 17% 39% 17% 6% 1	950
	21st to end	Night	18% 14% 40% 21% 5%	1045
February	1st to 10th	Night	19% 18% 39% 18% 6%	950
	11th to 20th	Night	20% 22% 33% 19% 6%	819
	21st to end	Night	21% 21% 40% 17% 2	628
March	1st to 10th	Night	19% 17% 34% 24% 7%	760
	11th to 20th	Night	18% 19% 37% 20% 5% 1	726
	21st to end	Night	22% 10% 32% 27% 7% 1	627
April	1st to 10th	Night	26% 14% 33% 23% 5%	570
	11th to 20th	Night	28% 14% 27% 25% 5% 1	563
	21st to end	Night	23% 12% 32% 25% 6% 1	570
May	1st to 10th	Night	23% 12% 32% 26% 7% 1	570
	11th to 20th	Night	22% 10% 36% 25% 6% 1	561
	21st to end	Night	16% 9% 30% 34% 11% 1	617
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% 39% 21% 5%	745
	11th to 20th	Night	24% 20% 34% 17% 5% 1	826
	21st to end	Night	23% 19% 32% 22% 4	950
December	1st to 10th	Night	22% 20% 36% 16% 5% 1	950
	11th to 20th	Night	20% 19% 34% 19% 5% 1	950
	21st to end	Night	29% 17% 35% 16% 4	1045

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