De Forest
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the De Forest clear sky chart from 2005-11-06 to 2025-11-18.

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	12% 8% 3 8% 5% 4 4 4 6% 8% 38%	127175
	Night	12% 9% 3 8% 5% 4 4 4 5% 6% 39%	123465
Transparency	Day	42% 7% 13% 24% 13% 1	130657
	Night	44% 6% 12% 23% 14% 2	128056
Seeing	Day	16% 10% 29% 26% 14% 5%	17346
	Night	17% 10% 31% 30% 11% 1	27171

By Month

Percentage Cloud Cover SE (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% 9% 5% 17% 8% 7% 5% 3 5% 6% 27%	3617
	11th to 20th	Day	7% 10% 7% 10% 5% 4 5% 7% 7% 9% 28%	3620
	21st to end	Day	12% 12% 6% 11% 3 4 3 3 4 5% 35%	4148
February	1st to 10th	Day	12% 11% 5% 10% 6% 5% 4 4 5% 7% 31%	1800
	11th to 20th	Day	12% 11% 3 8% 6% 5% 5% 4 4 6% 36%	1879
	21st to end	Day	11% 12% 3 11% 6% 5% 5% 4 5% 6% 32%	1650
March	1st to 10th	Day	14% 7% 3 5% 3 3 4 4 5% 7% 46%	4480
	11th to 20th	Day	11% 5% 2 7% 4 5% 5% 3 6% 7% 44%	4794
	21st to end	Day	23% 11% 3 10% 5% 4 4 3 4 5% 29%	5911
April	1st to 10th	Day	17% 13% 4 7% 5% 4 4 3 6% 6% 30%	2400
	11th to 20th	Day	17% 9% 3 7% 6% 4 5% 4 6% 8% 32%	2374
	21st to end	Day	18% 9% 3 8% 5% 4 4 4 6% 7% 33%	2475
May	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 6% 7% 41%	6082
	11th to 20th	Day	13% 6% 2 7% 5% 4 4 4 5% 7% 42%	6319
	21st to end	Day	16% 7% 2 5% 4 4 5% 4 6% 8% 39%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 5% 6% 8% 45%	2800
	11th to 20th	Day	11% 6% 3 7% 4 4 4 4 6% 9% 42%	2800
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 8% 10% 40%	2800
July	1st to 10th	Day	9% 6% 3 7% 4 4 5% 4 8% 10% 40%	6360
	11th to 20th	Day	6% 6% 2 6% 4 4 4 4 6% 9% 48%	6360
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 10% 40%	6924
August	1st to 10th	Day	9% 6% 2 6% 4 5% 5% 4 7% 9% 42%	4512
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 48%	3944
	21st to end	Day	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	4312
September	1st to 10th	Day	12% 5% 2 6% 5% 4 5% 4 5% 7% 44%	2400
	11th to 20th	Day	12% 5% 2 5% 3 3 4 3 5% 8% 49%	2333
	21st to end	Day	11% 7% 3 7% 4 4 4 4 5% 7% 45%	2200
October	1st to 10th	Day	13% 6% 3 6% 4 4 3 3 5% 6% 48%	2044
	11th to 20th	Day	19% 9% 2 7% 5% 4 4 4 5% 7% 34%	2000
	21st to end	Day	16% 10% 3 7% 4 4 3 3 4 5% 39%	2006
November	1st to 10th	Day	11% 9% 3 6% 4 4 4 3 5% 6% 44%	1852
	11th to 20th	Day	12% 12% 5% 9% 7% 4 5% 4 5% 7% 31%	1844
	21st to end	Day	15% 13% 4 9% 5% 4 4 2 4 6% 35%	1600
December	1st to 10th	Day	8% 9% 4 9% 6% 5% 5% 5% 7% 8% 36%	2610
	11th to 20th	Day	12% 14% 4 11% 7% 4 5% 4 6% 7% 25%	3008
	21st to end	Day	18% 21% 8% 13% 6% 5% 4 3 4 5% 14%	3982
January	1st to 10th	Night	8% 10% 6% 13% 7% 5% 5% 3 5% 5% 33%	6348
	11th to 20th	Night	10% 11% 6% 14% 6% 5% 5% 4 6% 7% 25%	6360
	21st to end	Night	11% 15% 7% 10% 5% 4 4 4 5% 6% 31%	6815
February	1st to 10th	Night	11% 13% 4 9% 6% 5% 4 4 5% 5% 33%	2600
	11th to 20th	Night	11% 10% 3 9% 5% 4 4 4 5% 5% 40%	2408
	21st to end	Night	15% 12% 4 8% 5% 3 4 3 4 7% 36%	1980
March	1st to 10th	Night	15% 8% 2 7% 4 4 4 4 5% 5% 43%	5373
	11th to 20th	Night	10% 7% 2 5% 4 4 4 4 6% 9% 46%	5210
	21st to end	Night	21% 11% 3 9% 5% 4 4 3 4 5% 30%	5018
April	1st to 10th	Night	17% 11% 2 6% 5% 4 4 4 4 7% 37%	2000
	11th to 20th	Night	19% 8% 2 7% 5% 4 3 3 5% 6% 37%	1975
	21st to end	Night	17% 8% 2 6% 4 4 4 5% 6% 7% 38%	1925
May	1st to 10th	Night	16% 7% 3 7% 4 4 4 3 6% 6% 41%	3819
	11th to 20th	Night	10% 5% 2 5% 4 4 5% 4 6% 7% 47%	3596
	21st to end	Night	15% 8% 3 7% 5% 5% 5% 4 5% 7% 37%	3942
June	1st to 10th	Night	7% 5% 3 6% 5% 5% 4 4 6% 9% 47%	1600
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 9% 46%	1600
	21st to end	Night	9% 7% 2 5% 4 4 3 4 5% 8% 49%	1600
July	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 6% 8% 44%	3618
	11th to 20th	Night	6% 5% 2 5% 4 5% 4 4 5% 7% 54%	3617
	21st to end	Night	6% 5% 2 6% 4 5% 4 4 6% 7% 51%	3947
August	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 50%	2644
	11th to 20th	Night	9% 6% 2 6% 4 4 4 3 5% 7% 49%	3106
	21st to end	Night	9% 5% 1 5% 4 4 4 3 5% 9% 50%	3586
September	1st to 10th	Night	9% 4 2 5% 3 4 3 3 5% 7% 55%	2000
	11th to 20th	Night	11% 4 2 5% 3 4 4 4 5% 6% 52%	2045
	21st to end	Night	10% 6% 2 5% 5% 3 4 3 5% 7% 49%	2200
October	1st to 10th	Night	11% 6% 2 6% 3 3 3 3 5% 6% 51%	2353
	11th to 20th	Night	17% 7% 2 6% 5% 4 4 2 5% 7% 42%	2400
	21st to end	Night	18% 8% 3 7% 4 3 4 4 6% 7% 36%	2832
November	1st to 10th	Night	12% 8% 3 6% 5% 4 3 3 5% 6% 44%	2666
	11th to 20th	Night	14% 11% 3 9% 5% 4 5% 4 5% 6% 35%	2707
	21st to end	Night	16% 11% 3 7% 4 4 5% 4 5% 5% 36%	2800
December	1st to 10th	Night	8% 9% 3 7% 5% 4 4 4 6% 7% 43%	4580
	11th to 20th	Night	15% 16% 6% 10% 5% 4 5% 3 4 4 26%	5199
	21st to end	Night	19% 19% 6% 12% 5% 4 4 3 4 5% 18%	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	58% 6% 12% 18% 6%	3618
	11th to 20th	Day	50% 6% 12% 13% 17% 1	3620
	21st to end	Day	51% 5% 12% 19% 12%	4148
February	1st to 10th	Day	52% 5% 14% 18% 9% 1	1800
	11th to 20th	Day	47% 6% 13% 22% 9% 2	1888
	21st to end	Day	52% 7% 10% 18% 10% 2	1650
March	1st to 10th	Day	38% 5% 13% 28% 12% 4	4480
	11th to 20th	Day	38% 6% 14% 28% 13% 1	4794
	21st to end	Day	57% 6% 11% 15% 10% 2	5913
April	1st to 10th	Day	53% 7% 10% 20% 10%	2400
	11th to 20th	Day	48% 7% 11% 20% 11% 2	2374
	21st to end	Day	50% 7% 12% 19% 11% 1	2475
May	1st to 10th	Day	40% 7% 12% 24% 16% 2	6117
	11th to 20th	Day	40% 8% 11% 26% 15% 1	6323
	21st to end	Day	39% 8% 11% 26% 13% 2	6949
June	1st to 10th	Day	35% 8% 15% 25% 15% 1	2800
	11th to 20th	Day	38% 7% 13% 24% 17% 1	2800
	21st to end	Day	38% 8% 16% 23% 14% 2	2800
July	1st to 10th	Day	37% 8% 15% 31% 9% 1	6291
	11th to 20th	Day	32% 7% 14% 32% 14% 1	6285
	21st to end	Day	38% 8% 16% 21% 16% 1	6728
August	1st to 10th	Day	36% 7% 16% 28% 11% 1	4497
	11th to 20th	Day	33% 6% 15% 33% 12% 1	3982
	21st to end	Day	35% 6% 14% 28% 16% 1	4311
September	1st to 10th	Day	36% 8% 12% 25% 17% 1	2400
	11th to 20th	Day	32% 6% 14% 31% 16% 1	2333
	21st to end	Day	38% 5% 12% 30% 15% 1	2200
October	1st to 10th	Day	23% 4 6% 40% 25% 1	3330
	11th to 20th	Day	36% 4 10% 28% 20% 2	3240
	21st to end	Day	43% 4 10% 29% 13% 2	3218
November	1st to 10th	Day	41% 5% 15% 24% 14% 2	1852
	11th to 20th	Day	51% 6% 12% 21% 9% 1	1844
	21st to end	Day	51% 5% 16% 21% 7%	1600
December	1st to 10th	Day	44% 6% 22% 22% 6% 1	2610
	11th to 20th	Day	57% 5% 11% 18% 7% 2	3005
	21st to end	Day	72% 6% 7% 9% 6%	3982
January	1st to 10th	Night	51% 6% 10% 23% 11%	6348
	11th to 20th	Night	57% 6% 9% 11% 17% 1	6360
	21st to end	Night	54% 5% 12% 16% 12%	6815
February	1st to 10th	Night	52% 5% 12% 18% 12% 2	2600
	11th to 20th	Night	46% 6% 11% 23% 14% 1	2409
	21st to end	Night	49% 5% 12% 20% 11% 3	1980
March	1st to 10th	Night	43% 5% 13% 23% 10% 7%	5373
	11th to 20th	Night	34% 5% 13% 31% 13% 4	5210
	21st to end	Night	57% 5% 9% 16% 12% 1	5018
April	1st to 10th	Night	46% 8% 10% 20% 15% 1	2000
	11th to 20th	Night	47% 5% 8% 21% 16% 3	1975
	21st to end	Night	43% 8% 12% 23% 12% 2	1925
May	1st to 10th	Night	41% 7% 12% 18% 20% 2	3846
	11th to 20th	Night	33% 10% 16% 21% 18% 2	3597
	21st to end	Night	42% 9% 10% 23% 13% 3	3950
June	1st to 10th	Night	32% 7% 13% 28% 18% 1	1600
	11th to 20th	Night	35% 7% 12% 24% 21% 2	1600
	21st to end	Night	34% 6% 16% 23% 18% 3	1600
July	1st to 10th	Night	37% 6% 14% 29% 12% 2	3580
	11th to 20th	Night	29% 6% 13% 32% 18% 1	3572
	21st to end	Night	31% 7% 15% 24% 21% 2	3832
August	1st to 10th	Night	32% 7% 15% 25% 20% 2	2635
	11th to 20th	Night	34% 6% 14% 31% 15% 1	3134
	21st to end	Night	31% 7% 14% 27% 19% 3	3586
September	1st to 10th	Night	28% 8% 12% 27% 23% 2	2000
	11th to 20th	Night	31% 6% 12% 31% 18% 1	2045
	21st to end	Night	33% 6% 10% 29% 20% 1	2200
October	1st to 10th	Night	25% 4 6% 37% 26% 1	3847
	11th to 20th	Night	33% 4 11% 28% 21% 3	3899
	21st to end	Night	43% 5% 9% 27% 15% 1	4570
November	1st to 10th	Night	40% 6% 11% 26% 16% 2	2666
	11th to 20th	Night	48% 7% 12% 20% 10% 2	2707
	21st to end	Night	49% 6% 13% 22% 9% 1	2800
December	1st to 10th	Night	40% 5% 18% 27% 8% 2	4580
	11th to 20th	Night	61% 5% 10% 15% 9% 1	5201
	21st to end	Night	69% 5% 8% 13% 5%	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	15% 13% 36% 24% 8% 3	300
	11th to 20th	Day	14% 16% 40% 21% 8% 1	300
	21st to end	Day	20% 13% 35% 25% 6% 1	330
February	1st to 10th	Day	15% 15% 42% 24% 3 2	300
	11th to 20th	Day	19% 18% 34% 22% 7%	300
	21st to end	Day	19% 16% 41% 21% 2	249
March	1st to 10th	Day	16% 13% 44% 22% 5%	300
	11th to 20th	Day	19% 18% 33% 20% 5% 4	300
	21st to end	Day	28% 12% 28% 23% 7% 1	520
April	1st to 10th	Day	24% 16% 35% 22% 4	500
	11th to 20th	Day	24% 19% 25% 20% 10% 2	492
	21st to end	Day	23% 14% 29% 23% 10% 2	500
May	1st to 10th	Day	19% 11% 31% 28% 9% 2	593
	11th to 20th	Day	20% 11% 29% 24% 12% 4	725
	21st to end	Day	17% 9% 29% 24% 15% 6%	789
June	1st to 10th	Day	8% 9% 30% 32% 16% 5%	730
	11th to 20th	Day	13% 7% 31% 28% 16% 5%	730
	21st to end	Day	14% 8% 29% 28% 16% 4	730
July	1st to 10th	Day	9% 4 18% 30% 25% 12%	721
	11th to 20th	Day	8% 6% 26% 30% 20% 9%	718
	21st to end	Day	8% 3 25% 35% 23% 6%	794
August	1st to 10th	Day	8% 3 21% 32% 24% 11%	726
	11th to 20th	Day	14% 5% 18% 28% 23% 12%	578
	21st to end	Day	9% 7% 23% 30% 21% 10%	583
September	1st to 10th	Day	14% 8% 22% 29% 18% 9%	530
	11th to 20th	Day	12% 4 19% 30% 26% 9%	530
	21st to end	Day	14% 10% 28% 24% 16% 7%	530
October	1st to 10th	Day	17% 10% 26% 27% 16% 3	374
	11th to 20th	Day	23% 16% 34% 18% 8%	330
	21st to end	Day	23% 17% 34% 19% 6% 1	363
November	1st to 10th	Day	14% 19% 37% 22% 5% 2	330
	11th to 20th	Day	19% 18% 34% 21% 8%	321
	21st to end	Day	23% 12% 43% 17% 5%	300
December	1st to 10th	Day	16% 17% 43% 18% 6% 1	300
	11th to 20th	Day	18% 11% 39% 21% 10% 1	300
	21st to end	Day	25% 17% 38% 16% 3 1	330
January	1st to 10th	Night	15% 16% 37% 24% 8% 1	1000
	11th to 20th	Night	19% 13% 38% 25% 5% 1	1000
	21st to end	Night	19% 12% 36% 26% 6% 1	1100
February	1st to 10th	Night	17% 12% 41% 25% 5%	1000
	11th to 20th	Night	16% 16% 37% 27% 5%	853
	21st to end	Night	23% 13% 38% 22% 4 1	660
March	1st to 10th	Night	21% 10% 36% 24% 8% 1	800
	11th to 20th	Night	17% 11% 36% 28% 7%	700
	21st to end	Night	23% 10% 30% 28% 7% 1	660
April	1st to 10th	Night	21% 14% 36% 25% 4	600
	11th to 20th	Night	22% 14% 28% 30% 6%	593
	21st to end	Night	20% 10% 32% 29% 7% 1	600
May	1st to 10th	Night	20% 10% 32% 30% 8% 1	600
	11th to 20th	Night	18% 7% 33% 32% 10%	591
	21st to end	Night	15% 5% 26% 38% 14% 1	650
June	1st to 10th	Night	10% 4 30% 42% 14%	600
	11th to 20th	Night	16% 4 30% 38% 12%	600
	21st to end	Night	11% 5% 27% 42% 15% 1	600
July	1st to 10th	Night	9% 3 14% 41% 30% 3	585
	11th to 20th	Night	8% 4 22% 41% 23% 2	582
	21st to end	Night	5% 2 19% 49% 22% 2	657
August	1st to 10th	Night	6% 3 17% 47% 26% 2	597
	11th to 20th	Night	11% 3 14% 42% 24% 5%	594
	21st to end	Night	10% 2 19% 43% 24% 2	660
September	1st to 10th	Night	11% 4 23% 35% 22% 6%	600
	11th to 20th	Night	12% 3 23% 35% 24% 4	645
	21st to end	Night	13% 7% 25% 35% 19% 1	800
October	1st to 10th	Night	15% 7% 29% 33% 17% 1	800
	11th to 20th	Night	20% 12% 30% 29% 8%	800
	21st to end	Night	22% 13% 30% 28% 7% 1	880
November	1st to 10th	Night	16% 11% 37% 25% 9% 1	820
	11th to 20th	Night	20% 15% 38% 22% 5%	844
	21st to end	Night	21% 16% 33% 23% 7%	1000
December	1st to 10th	Night	20% 15% 38% 21% 6%	1000
	11th to 20th	Night	22% 13% 35% 23% 6% 1	1000
	21st to end	Night	28% 13% 34% 21% 3	1100

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