Forestville
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Forestville clear sky chart from 2005-11-06 to 2026-03-18.

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	16% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	131070
	Night	15% 11% 4 9% 5% 5% 4 3 5% 6% 33%	130591
Transparency	Day	49% 8% 13% 20% 9% 1	134499
	Night	51% 7% 11% 20% 10% 1	135250
Seeing	Day	21% 8% 29% 28% 12% 2	17479
	Night	22% 7% 28% 30% 11% 1	25652

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% 18% 6% 11% 5% 4 4 3 4 5% 24%	4093
	11th to 20th	Day	10% 14% 6% 14% 7% 7% 5% 4 5% 6% 23%	4536
	21st to end	Day	4 9% 5% 12% 7% 6% 6% 5% 6% 7% 33%	5124
February	1st to 10th	Day	7% 12% 5% 10% 8% 6% 5% 4 6% 7% 32%	1735
	11th to 20th	Day	11% 13% 6% 13% 7% 6% 3 3 5% 5% 27%	1873
	21st to end	Day	10% 12% 4 9% 6% 5% 5% 3 6% 7% 32%	1730
March	1st to 10th	Day	16% 15% 4 8% 5% 5% 5% 4 6% 8% 26%	4576
	11th to 20th	Day	17% 17% 3 8% 5% 5% 6% 3 5% 6% 23%	5007
	21st to end	Day	18% 17% 4 8% 5% 4 3 3 4 6% 29%	5553
April	1st to 10th	Day	19% 11% 3 7% 6% 5% 5% 4 5% 6% 29%	2419
	11th to 20th	Day	16% 10% 3 7% 5% 5% 4 3 5% 7% 36%	2572
	21st to end	Day	21% 10% 3 7% 5% 3 5% 3 5% 5% 33%	2600
May	1st to 10th	Day	18% 9% 3 8% 5% 5% 4 3 5% 6% 34%	5926
	11th to 20th	Day	26% 8% 2 6% 4 4 4 3 4 6% 33%	6609
	21st to end	Day	18% 10% 3 8% 4 4 4 3 5% 8% 33%	7451
June	1st to 10th	Day	21% 9% 3 7% 5% 5% 4 4 6% 7% 31%	3000
	11th to 20th	Day	17% 6% 3 7% 5% 5% 5% 4 6% 8% 34%	3000
	21st to end	Day	22% 8% 2 8% 5% 5% 5% 4 5% 6% 30%	3000
July	1st to 10th	Day	15% 7% 3 7% 5% 5% 6% 5% 7% 10% 32%	6820
	11th to 20th	Day	15% 6% 2 7% 5% 5% 6% 5% 7% 9% 32%	6820
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 7% 9% 42%	6928
August	1st to 10th	Day	12% 5% 2 6% 6% 6% 7% 7% 8% 9% 31%	4468
	11th to 20th	Day	17% 7% 3 8% 5% 5% 6% 4 7% 9% 30%	4175
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 5% 8% 38%	4675
September	1st to 10th	Day	15% 6% 2 7% 4 5% 5% 4 6% 7% 39%	2403
	11th to 20th	Day	17% 6% 2 6% 4 4 3 3 5% 6% 44%	2200
	21st to end	Day	18% 7% 2 6% 4 4 4 4 5% 6% 39%	2200
October	1st to 10th	Day	16% 6% 2 8% 5% 4 5% 4 6% 8% 36%	2120
	11th to 20th	Day	23% 7% 3 7% 5% 5% 5% 3 6% 7% 31%	1885
	21st to end	Day	25% 10% 3 8% 5% 4 4 3 4 5% 30%	1980
November	1st to 10th	Day	17% 10% 3 9% 5% 6% 5% 4 5% 6% 29%	1819
	11th to 20th	Day	20% 11% 4 7% 5% 5% 5% 4 5% 5% 29%	1680
	21st to end	Day	24% 14% 4 7% 5% 4 4 2 4 5% 26%	1502
December	1st to 10th	Day	16% 15% 5% 10% 4 3 3 3 4 4 33%	2350
	11th to 20th	Day	17% 12% 4 8% 5% 4 4 3 4 5% 34%	2688
	21st to end	Day	17% 14% 4 9% 3 3 2 2 3 5% 38%	3553
January	1st to 10th	Night	15% 18% 5% 11% 5% 4 4 3 4 4 26%	8831
	11th to 20th	Night	11% 17% 6% 14% 6% 5% 4 3 5% 6% 24%	8340
	21st to end	Night	3 8% 4 11% 6% 6% 6% 4 7% 9% 37%	9057
February	1st to 10th	Night	8% 12% 4 10% 6% 6% 5% 3 5% 6% 34%	2883
	11th to 20th	Night	11% 16% 5% 10% 6% 5% 5% 4 5% 5% 28%	2631
	21st to end	Night	12% 14% 4 8% 6% 5% 4 3 4 4 34%	2076
March	1st to 10th	Night	16% 14% 4 8% 6% 4 5% 4 5% 6% 29%	5484
	11th to 20th	Night	17% 17% 4 10% 5% 5% 4 3 4 5% 25%	5106
	21st to end	Night	18% 17% 4 7% 4 3 3 3 4 4 33%	5376
April	1st to 10th	Night	22% 13% 4 8% 6% 4 3 3 3 4 29%	1980
	11th to 20th	Night	21% 9% 2 5% 4 5% 5% 4 6% 5% 35%	1778
	21st to end	Night	24% 11% 3 6% 3 3 3 3 4 5% 35%	1800
May	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 5% 6% 37%	4054
	11th to 20th	Night	24% 8% 3 7% 4 3 3 3 6% 7% 32%	3300
	21st to end	Night	18% 8% 3 7% 5% 4 5% 4 6% 7% 33%	3446
June	1st to 10th	Night	20% 6% 2 8% 6% 5% 6% 4 6% 7% 31%	1400
	11th to 20th	Night	17% 7% 2 7% 4 5% 4 3 6% 7% 37%	1400
	21st to end	Night	24% 9% 3 7% 5% 4 4 4 4 5% 31%	1400
July	1st to 10th	Night	11% 6% 3 7% 5% 6% 6% 5% 8% 8% 35%	3160
	11th to 20th	Night	12% 7% 3 7% 5% 4 6% 5% 6% 9% 35%	3160
	21st to end	Night	11% 6% 2 6% 4 4 4 4 5% 8% 45%	3921
August	1st to 10th	Night	16% 5% 3 8% 4 5% 5% 4 6% 7% 36%	2649
	11th to 20th	Night	14% 7% 3 8% 6% 5% 6% 5% 7% 10% 29%	2872
	21st to end	Night	14% 7% 2 7% 5% 4 4 3 5% 6% 43%	3223
September	1st to 10th	Night	15% 6% 2 6% 5% 4 4 3 7% 8% 41%	1995
	11th to 20th	Night	18% 6% 2 6% 4 4 4 4 4 6% 42%	2200
	21st to end	Night	18% 6% 2 5% 5% 4 3 3 4 5% 45%	2200
October	1st to 10th	Night	16% 7% 2 7% 4 4 4 4 5% 5% 42%	2279
	11th to 20th	Night	24% 7% 3 7% 4 4 3 3 5% 6% 35%	2510
	21st to end	Night	24% 7% 3 7% 4 5% 5% 4 5% 6% 32%	2860
November	1st to 10th	Night	19% 11% 3 8% 5% 4 4 4 5% 5% 32%	2699
	11th to 20th	Night	20% 12% 3 8% 4 5% 4 4 4 5% 32%	2940
	21st to end	Night	20% 16% 4 9% 5% 5% 4 3 5% 5% 25%	3113
December	1st to 10th	Night	14% 13% 5% 10% 5% 5% 4 3 4 4 34%	5060
	11th to 20th	Night	19% 13% 4 7% 5% 4 3 3 5% 6% 32%	5741
	21st to end	Night	15% 13% 6% 9% 5% 4 3 3 4 5% 33%	7667

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	62% 6% 10% 18% 5%	4094
	11th to 20th	Day	61% 7% 9% 17% 6% 1	4566
	21st to end	Day	46% 8% 12% 20% 14%	5124
February	1st to 10th	Day	50% 8% 12% 17% 11% 2	1735
	11th to 20th	Day	60% 6% 10% 15% 10%	1881
	21st to end	Day	48% 8% 9% 21% 12% 2	1730
March	1st to 10th	Day	53% 8% 13% 12% 12% 2	4579
	11th to 20th	Day	61% 7% 10% 14% 8% 1	5007
	21st to end	Day	56% 5% 8% 18% 10% 3	5553
April	1st to 10th	Day	53% 6% 10% 15% 13% 2	2419
	11th to 20th	Day	46% 7% 12% 23% 11% 2	2572
	21st to end	Day	52% 6% 10% 18% 12% 2	2600
May	1st to 10th	Day	49% 9% 14% 18% 9% 1	5937
	11th to 20th	Day	51% 9% 13% 16% 8% 2	6613
	21st to end	Day	48% 6% 13% 23% 9% 1	7451
June	1st to 10th	Day	51% 8% 10% 18% 11% 2	3000
	11th to 20th	Day	46% 8% 14% 17% 13% 2	3000
	21st to end	Day	53% 8% 13% 15% 10% 1	3000
July	1st to 10th	Day	45% 9% 14% 25% 7%	6745
	11th to 20th	Day	45% 11% 18% 21% 5%	6739
	21st to end	Day	36% 9% 16% 29% 9%	6729
August	1st to 10th	Day	42% 15% 18% 19% 7%	4454
	11th to 20th	Day	48% 12% 18% 18% 4	4214
	21st to end	Day	43% 10% 16% 20% 10% 1	4675
September	1st to 10th	Day	41% 9% 17% 22% 11%	2403
	11th to 20th	Day	40% 7% 16% 26% 10% 1	2200
	21st to end	Day	43% 8% 16% 24% 9%	2200
October	1st to 10th	Day	51% 8% 15% 19% 7% 1	3458
	11th to 20th	Day	46% 8% 14% 28% 3	3049
	21st to end	Day	47% 7% 16% 20% 11%	3180
November	1st to 10th	Day	53% 8% 16% 16% 6% 1	1819
	11th to 20th	Day	55% 7% 12% 19% 6%	1680
	21st to end	Day	62% 6% 11% 14% 7%	1502
December	1st to 10th	Day	55% 6% 10% 19% 10%	2350
	11th to 20th	Day	53% 6% 11% 20% 9%	2688
	21st to end	Day	51% 5% 12% 22% 7% 3	3553
January	1st to 10th	Night	63% 5% 9% 17% 6%	8831
	11th to 20th	Night	61% 6% 9% 18% 6%	8385
	21st to end	Night	42% 8% 12% 23% 15%	9057
February	1st to 10th	Night	52% 6% 9% 20% 12% 2	2883
	11th to 20th	Night	57% 7% 9% 17% 9%	2634
	21st to end	Night	53% 5% 10% 22% 9% 1	2076
March	1st to 10th	Night	55% 7% 10% 15% 12% 1	5494
	11th to 20th	Night	61% 6% 10% 13% 8% 2	5107
	21st to end	Night	55% 4 7% 16% 17% 1	5376
April	1st to 10th	Night	59% 4 7% 13% 14% 3	1980
	11th to 20th	Night	49% 6% 9% 19% 15% 2	1778
	21st to end	Night	53% 4 8% 19% 14% 3	1800
May	1st to 10th	Night	47% 8% 11% 16% 16% 1	4060
	11th to 20th	Night	51% 8% 10% 21% 9% 1	3301
	21st to end	Night	48% 9% 10% 21% 12% 1	3447
June	1st to 10th	Night	50% 8% 11% 18% 11% 2	1400
	11th to 20th	Night	46% 6% 13% 18% 14% 3	1400
	21st to end	Night	55% 7% 8% 17% 12% 1	1400
July	1st to 10th	Night	42% 10% 15% 24% 9%	3125
	11th to 20th	Night	43% 9% 18% 26% 4	3118
	21st to end	Night	37% 8% 20% 23% 11%	3805
August	1st to 10th	Night	45% 9% 15% 23% 7%	2639
	11th to 20th	Night	48% 11% 19% 14% 7%	2899
	21st to end	Night	41% 8% 12% 27% 12%	3223
September	1st to 10th	Night	40% 9% 16% 24% 10% 1	1995
	11th to 20th	Night	43% 8% 16% 23% 10% 1	2200
	21st to end	Night	42% 7% 12% 26% 14%	2200
October	1st to 10th	Night	46% 7% 11% 27% 8% 1	3721
	11th to 20th	Night	47% 7% 12% 29% 5%	4085
	21st to end	Night	48% 10% 14% 20% 8%	4610
November	1st to 10th	Night	53% 7% 13% 18% 8% 1	2699
	11th to 20th	Night	54% 7% 13% 18% 7%	2940
	21st to end	Night	61% 7% 11% 12% 8%	3113
December	1st to 10th	Night	55% 6% 12% 20% 7%	5060
	11th to 20th	Night	54% 6% 9% 23% 8%	5742
	21st to end	Night	54% 6% 10% 20% 8% 2	7667

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	30% 9% 28% 25% 8%	220
	11th to 20th	Day	20% 15% 25% 29% 8% 2	220
	21st to end	Day	12% 10% 34% 25% 15% 3	242
February	1st to 10th	Day	13% 12% 31% 31% 9% 5%	275
	11th to 20th	Day	16% 11% 37% 27% 9% 1	443
	21st to end	Day	16% 11% 38% 26% 9% 1	446
March	1st to 10th	Day	24% 11% 30% 26% 8% 1	540
	11th to 20th	Day	24% 12% 31% 21% 10% 3	528
	21st to end	Day	25% 11% 33% 25% 6% 1	550
April	1st to 10th	Day	24% 15% 29% 22% 8% 1	500
	11th to 20th	Day	21% 10% 31% 26% 11% 1	492
	21st to end	Day	24% 9% 28% 29% 9% 1	500
May	1st to 10th	Day	25% 9% 28% 25% 10% 2	503
	11th to 20th	Day	23% 5% 29% 27% 13% 3	649
	21st to end	Day	23% 9% 31% 26% 10% 2	790
June	1st to 10th	Day	25% 4 30% 30% 10% 1	730
	11th to 20th	Day	20% 7% 28% 28% 15% 3	730
	21st to end	Day	23% 5% 27% 32% 11% 2	730
July	1st to 10th	Day	19% 4 24% 37% 15% 2	720
	11th to 20th	Day	16% 5% 24% 37% 17% 2	718
	21st to end	Day	12% 8% 24% 36% 17% 2	578
August	1st to 10th	Day	16% 4 21% 38% 19% 3	526
	11th to 20th	Day	18% 4 21% 34% 19% 3	519
	21st to end	Day	15% 9% 22% 32% 18% 3	583
September	1st to 10th	Day	16% 6% 30% 29% 16% 3	530
	11th to 20th	Day	18% 8% 26% 32% 12% 3	530
	21st to end	Day	19% 6% 29% 29% 13% 3	530
October	1st to 10th	Day	17% 7% 33% 26% 13% 3	486
	11th to 20th	Day	26% 10% 27% 24% 10% 3	420
	21st to end	Day	29% 12% 26% 24% 8% 2	462
November	1st to 10th	Day	22% 15% 35% 23% 4 1	425
	11th to 20th	Day	26% 10% 28% 24% 10% 1	430
	21st to end	Day	32% 11% 29% 23% 4 1	252
December	1st to 10th	Day	22% 11% 28% 24% 12% 3	220
	11th to 20th	Day	23% 16% 33% 19% 7% 1	220
	21st to end	Day	23% 12% 31% 23% 10% 1	242
January	1st to 10th	Night	24% 9% 31% 30% 6%	950
	11th to 20th	Night	23% 11% 33% 26% 7%	860
	21st to end	Night	13% 7% 36% 33% 10% 1	924
February	1st to 10th	Night	13% 8% 39% 30% 10% 1	840
	11th to 20th	Night	18% 12% 34% 27% 9%	823
	21st to end	Night	18% 8% 31% 32% 10% 1	692
March	1st to 10th	Night	21% 9% 33% 30% 7% 1	840
	11th to 20th	Night	22% 11% 31% 26% 9% 1	829
	21st to end	Night	22% 9% 31% 29% 7% 1	880
April	1st to 10th	Night	28% 7% 31% 25% 8%	780
	11th to 20th	Night	24% 7% 28% 27% 12% 2	594
	21st to end	Night	30% 7% 22% 31% 10% 1	600
May	1st to 10th	Night	28% 5% 22% 34% 10% 1	600
	11th to 20th	Night	23% 3 23% 33% 15% 2	397
	21st to end	Night	23% 3 30% 30% 12% 1	433
June	1st to 10th	Night	21% 3 22% 39% 15% 1	400
	11th to 20th	Night	19% 4 21% 32% 22% 3	400
	21st to end	Night	28% 4 19% 35% 14% 1	400
July	1st to 10th	Night	17% 5% 21% 40% 15% 2	390
	11th to 20th	Night	14% 4 18% 43% 20% 1	388
	21st to end	Night	17% 3 18% 38% 21% 2	438
August	1st to 10th	Night	16% 2 12% 44% 24% 2	435
	11th to 20th	Night	17% 3 17% 40% 21% 2	592
	21st to end	Night	13% 4 17% 42% 19% 4	660
September	1st to 10th	Night	18% 3 26% 36% 15% 1	745
	11th to 20th	Night	22% 5% 23% 31% 17% 3	800
	21st to end	Night	20% 5% 26% 32% 17% 2	800
October	1st to 10th	Night	19% 6% 29% 32% 13% 2	800
	11th to 20th	Night	26% 5% 26% 27% 14% 1	800
	21st to end	Night	27% 7% 30% 27% 9% 1	880
November	1st to 10th	Night	26% 11% 30% 27% 6% 1	837
	11th to 20th	Night	24% 11% 30% 25% 9% 1	950
	21st to end	Night	28% 9% 28% 28% 6%	950
December	1st to 10th	Night	24% 9% 31% 28% 8% 1	950
	11th to 20th	Night	25% 12% 33% 22% 7% 1	950
	21st to end	Night	24% 11% 31% 25% 7% 1	1045

Page generated at 2026-03-15 02:23:57. Version ac07

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