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 2026-01-17.

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	8% 4 2 4 3 3 3 3 4 6% 61%	136006
	Night	10% 6% 2 6% 3 3 4 3 5% 6% 51%	133102
Transparency	Day	26% 5% 8% 21% 23% 17%	130892
	Night	32% 6% 8% 18% 21% 14%	129206
Seeing	Day	11% 11% 24% 29% 18% 6%	16877
	Night	12% 6% 16% 31% 29% 6%	26212

By Month

Percentage Cloud Cover SW (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	13% 6% 2 6% 4 4 4 4 6% 8% 44%	4001
	11th to 20th	Day	16% 6% 2 6% 3 3 3 2 4 5% 49%	4155
	21st to end	Day	18% 6% 3 6% 4 3 3 2 5% 6% 44%	4510
February	1st to 10th	Day	19% 6% 2 5% 3 4 3 3 4 4 46%	1800
	11th to 20th	Day	13% 7% 3 7% 4 4 5% 3 6% 8% 42%	1780
	21st to end	Day	15% 6% 2 5% 3 3 4 3 4 6% 48%	1653
March	1st to 10th	Day	16% 10% 3 7% 4 4 4 4 5% 6% 37%	4952
	11th to 20th	Day	14% 4 1 5% 3 4 4 4 5% 8% 47%	5000
	21st to end	Day	18% 6% 2 5% 4 4 5% 4 6% 9% 35%	5486
April	1st to 10th	Day	13% 6% 2 6% 5% 4 4 3 5% 8% 43%	2400
	11th to 20th	Day	8% 6% 2 5% 4 4 4 4 5% 7% 52%	2369
	21st to end	Day	6% 5% 2 4 3 3 3 3 5% 9% 58%	2400
May	1st to 10th	Day	4 2 1 2 1 2 2 2 4 5% 75%	5830
	11th to 20th	Day	3 2 1 3 1 2 2 2 4 5% 75%	5917
	21st to end	Day	2 3 1 2 2 2 2 1 3 5% 77%	6902
June	1st to 10th	Day	5% 6% 1 5% 2 2 3 3 4 8% 61%	2800
	11th to 20th	Day	2 3 1 2 2 2 2 2 4 6% 74%	2800
	21st to end	Day	2 3 2 3 2 2 2 1 3 4 76%	2800
July	1st to 10th	Day	1 3 1 3 2 1 1 1 3 4 80%	6289
	11th to 20th	Day	1 4 2 4 2 2 2 2 3 5% 75%	5880
	21st to end	Day	1 4 2 4 2 2 2 2 3 4 74%	6402
August	1st to 10th	Day	1 3 1 5% 2 2 3 2 3 6% 73%	5777
	11th to 20th	Day	1 3 1 3 1 1 1 1 3 4 80%	5664
	21st to end	Day	1 3 2 5% 3 3 3 2 4 6% 68%	5984
September	1st to 10th	Day	1 3 1 4 2 2 2 2 4 6% 73%	2400
	11th to 20th	Day	4 4 2 5% 3 2 3 3 4 7% 63%	2375
	21st to end	Day	4 3 1 3 1 2 2 3 3 6% 72%	2115
October	1st to 10th	Day	4 2 1 3 2 3 2 2 4 6% 70%	3280
	11th to 20th	Day	9% 4 1 5% 2 2 2 2 4 5% 64%	3261
	21st to end	Day	7% 3 1 5% 3 4 3 3 5% 7% 57%	3558
November	1st to 10th	Day	10% 4 1 3 4 4 3 3 6% 7% 55%	2050
	11th to 20th	Day	13% 6% 3 6% 4 4 3 3 5% 7% 47%	1836
	21st to end	Day	11% 5% 1 4 3 4 4 3 5% 6% 54%	1680
December	1st to 10th	Day	10% 4 2 4 3 3 4 4 8% 11% 47%	2700
	11th to 20th	Day	27% 8% 2 6% 4 4 4 4 7% 7% 27%	3108
	21st to end	Day	26% 7% 2 6% 3 4 4 4 6% 7% 31%	4092
January	1st to 10th	Night	13% 6% 2 5% 5% 5% 6% 4 6% 9% 39%	6160
	11th to 20th	Night	16% 6% 2 6% 4 4 5% 4 6% 6% 40%	5958
	21st to end	Night	18% 7% 3 7% 3 4 4 4 4 5% 41%	6468
February	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 4 6% 48%	2600
	11th to 20th	Night	13% 8% 3 6% 4 3 5% 3 5% 8% 43%	2505
	21st to end	Night	13% 6% 2 4 2 3 3 3 4 6% 54%	1974
March	1st to 10th	Night	16% 9% 3 7% 4 4 4 4 5% 7% 36%	4942
	11th to 20th	Night	15% 6% 2 5% 3 4 3 4 5% 5% 47%	5000
	21st to end	Night	15% 9% 3 7% 4 4 4 3 5% 7% 39%	5476
April	1st to 10th	Night	10% 6% 2 7% 4 3 3 3 5% 6% 51%	2000
	11th to 20th	Night	8% 6% 2 6% 4 3 4 2 3 7% 54%	1979
	21st to end	Night	6% 5% 2 4 3 2 3 3 4 8% 60%	2000
May	1st to 10th	Night	4 5% 2 4 2 2 3 2 3 5% 70%	4148
	11th to 20th	Night	2 4 2 8% 3 3 3 3 4 5% 62%	3987
	21st to end	Night	2 2 1 3 2 2 2 2 3 5% 77%	3972
June	1st to 10th	Night	4 6% 2 4 4 3 3 3 5% 8% 58%	1600
	11th to 20th	Night	2 3 1 3 2 2 3 2 4 5% 73%	1600
	21st to end	Night	3 4 2 4 2 2 2 2 3 5% 71%	1600
July	1st to 10th	Night	1 4 2 6% 3 3 2 2 3 5% 71%	3640
	11th to 20th	Night	1 9% 5% 7% 3 3 3 3 5% 6% 56%	4100
	21st to end	Night	1 10% 4 7% 3 3 3 3 5% 7% 53%	4472
August	1st to 10th	Night	2 3 3 12% 4 3 3 3 5% 5% 57%	4035
	11th to 20th	Night	4 2 5% 2 2 2 2 4 5% 71%	4187
	21st to end	Night	1 4 3 11% 5% 4 3 3 4 5% 57%	5015
September	1st to 10th	Night	1 4 2 4 3 3 3 3 4 8% 67%	2000
	11th to 20th	Night	5% 6% 2 6% 2 3 4 3 5% 7% 58%	2020
	21st to end	Night	4 5% 3 5% 3 2 2 2 3 4 67%	2285
October	1st to 10th	Night	3 3 1 4 3 2 2 2 5% 5% 69%	3920
	11th to 20th	Night	6% 6% 3 7% 4 4 3 3 4 5% 56%	3897
	21st to end	Night	7% 4 2 5% 3 3 4 4 5% 6% 57%	4253
November	1st to 10th	Night	10% 5% 2 4 4 3 4 3 5% 8% 52%	2465
	11th to 20th	Night	13% 6% 3 5% 3 3 3 3 5% 8% 47%	2782
	21st to end	Night	10% 5% 2 5% 4 3 3 3 4 7% 53%	2940
December	1st to 10th	Night	12% 4 2 6% 5% 5% 6% 4 7% 7% 42%	4700
	11th to 20th	Night	26% 7% 2 7% 4 5% 5% 4 6% 8% 25%	5316
	21st to end	Night	32% 8% 2 5% 3 3 3 2 4 5% 32%	7106

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	38% 8% 15% 21% 13% 6%	4001
	11th to 20th	Day	38% 6% 7% 14% 19% 16%	4155
	21st to end	Day	42% 5% 7% 14% 13% 19%	4510
February	1st to 10th	Day	42% 5% 7% 19% 18% 8%	1800
	11th to 20th	Day	41% 6% 10% 20% 17% 6%	1789
	21st to end	Day	37% 6% 11% 24% 15% 7%	1653
March	1st to 10th	Day	46% 7% 7% 21% 17% 2	4952
	11th to 20th	Day	35% 5% 16% 26% 10% 8%	5000
	21st to end	Day	42% 10% 16% 22% 9% 1	5486
April	1st to 10th	Day	40% 6% 13% 22% 15% 4	2400
	11th to 20th	Day	32% 7% 14% 23% 19% 6%	2372
	21st to end	Day	24% 6% 15% 30% 21% 4	2400
May	1st to 10th	Day	12% 4 9% 28% 28% 18%	5830
	11th to 20th	Day	12% 3 8% 34% 33% 9%	5920
	21st to end	Day	11% 4 10% 35% 37% 3	6904
June	1st to 10th	Day	24% 5% 11% 29% 26% 6%	2800
	11th to 20th	Day	13% 4 10% 21% 37% 15%	2800
	21st to end	Day	14% 3 5% 16% 36% 26%	2800
July	1st to 10th	Day	12% 3 2 8% 25% 51%	6221
	11th to 20th	Day	14% 3 2 6% 21% 54%	5811
	21st to end	Day	15% 2 5% 17% 30% 31%	6217
August	1st to 10th	Day	19% 4 4 12% 27% 34%	4206
	11th to 20th	Day	10% 2 4 15% 41% 29%	4099
	21st to end	Day	18% 3 4 16% 34% 25%	4311
September	1st to 10th	Day	13% 4 5% 13% 30% 36%	2400
	11th to 20th	Day	22% 4 9% 25% 31% 10%	2375
	21st to end	Day	16% 3 6% 23% 29% 22%	2115
October	1st to 10th	Day	16% 3 4 13% 24% 40%	3280
	11th to 20th	Day	24% 4 6% 24% 24% 19%	3261
	21st to end	Day	27% 4 10% 24% 25% 11%	3558
November	1st to 10th	Day	29% 6% 9% 28% 18% 10%	2050
	11th to 20th	Day	38% 6% 12% 22% 15% 7%	1836
	21st to end	Day	31% 6% 11% 20% 20% 12%	1680
December	1st to 10th	Day	28% 7% 9% 27% 23% 7%	2700
	11th to 20th	Day	55% 5% 8% 17% 12% 3	3108
	21st to end	Day	53% 5% 12% 20% 7% 3	4092
January	1st to 10th	Night	39% 8% 19% 18% 12% 3	6160
	11th to 20th	Night	38% 11% 8% 17% 14% 12%	5958
	21st to end	Night	42% 8% 8% 9% 20% 13%	6468
February	1st to 10th	Night	39% 5% 9% 19% 17% 11%	2600
	11th to 20th	Night	40% 6% 10% 20% 17% 7%	2507
	21st to end	Night	32% 5% 12% 25% 20% 6%	1974
March	1st to 10th	Night	44% 11% 9% 19% 16% 2	4942
	11th to 20th	Night	37% 4 13% 20% 18% 8%	5000
	21st to end	Night	41% 11% 14% 22% 10% 2	5476
April	1st to 10th	Night	34% 8% 13% 22% 20% 4	2000
	11th to 20th	Night	31% 5% 13% 24% 17% 9%	1979
	21st to end	Night	24% 6% 11% 31% 24% 6%	2000
May	1st to 10th	Night	18% 4 5% 25% 32% 17%	4145
	11th to 20th	Night	20% 5% 6% 30% 26% 13%	3988
	21st to end	Night	12% 4 8% 33% 38% 5%	3972
June	1st to 10th	Night	25% 5% 10% 23% 30% 8%	1600
	11th to 20th	Night	15% 3 8% 20% 35% 20%	1600
	21st to end	Night	18% 4 4 15% 31% 29%	1600
July	1st to 10th	Night	18% 5% 2 8% 23% 45%	3600
	11th to 20th	Night	31% 3 3 6% 18% 39%	4046
	21st to end	Night	30% 5% 5% 10% 23% 26%	4345
August	1st to 10th	Night	34% 11% 3 10% 20% 23%	2929
	11th to 20th	Night	15% 2 3 12% 37% 31%	3025
	21st to end	Night	24% 6% 4 15% 29% 21%	3608
September	1st to 10th	Night	16% 5% 4 12% 31% 33%	2000
	11th to 20th	Night	27% 5% 6% 21% 29% 13%	2020
	21st to end	Night	21% 4 5% 20% 26% 24%	2285
October	1st to 10th	Night	17% 4 5% 11% 24% 39%	3920
	11th to 20th	Night	31% 4 6% 20% 20% 19%	3897
	21st to end	Night	28% 4 7% 21% 25% 15%	4253
November	1st to 10th	Night	32% 5% 10% 26% 18% 9%	2465
	11th to 20th	Night	35% 6% 14% 23% 15% 7%	2782
	21st to end	Night	32% 6% 9% 20% 21% 12%	2940
December	1st to 10th	Night	31% 11% 6% 24% 18% 9%	4700
	11th to 20th	Night	53% 9% 9% 14% 12% 3	5316
	21st to end	Night	56% 6% 10% 17% 10% 2	7106

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	28% 13% 25% 18% 13% 3	346
	11th to 20th	Day	20% 10% 24% 24% 19% 4	419
	21st to end	Day	15% 13% 24% 28% 16% 3	440
February	1st to 10th	Day	22% 13% 21% 23% 18% 4	400
	11th to 20th	Day	20% 18% 25% 24% 12% 1	408
	21st to end	Day	18% 15% 26% 26% 14% 1	414
March	1st to 10th	Day	25% 18% 27% 20% 7% 2	500
	11th to 20th	Day	23% 16% 27% 19% 10% 4	500
	21st to end	Day	19% 18% 33% 20% 7% 2	550
April	1st to 10th	Day	16% 20% 28% 22% 10% 3	500
	11th to 20th	Day	15% 19% 29% 23% 9% 5%	492
	21st to end	Day	8% 19% 32% 23% 12% 6%	500
May	1st to 10th	Day	11% 17% 31% 26% 12% 3	503
	11th to 20th	Day	11% 16% 30% 24% 15% 4	528
	21st to end	Day	10% 12% 27% 27% 18% 6%	573
June	1st to 10th	Day	8% 13% 29% 30% 14% 7%	530
	11th to 20th	Day	5% 13% 31% 33% 13% 4	530
	21st to end	Day	2 5% 28% 34% 24% 7%	530
July	1st to 10th	Day	4 5% 22% 41% 22% 5%	526
	11th to 20th	Day	1 1 20% 42% 28% 7%	524
	21st to end	Day	2 2 15% 47% 27% 7%	575
August	1st to 10th	Day	3 4 22% 42% 21% 8%	527
	11th to 20th	Day	4 3 23% 36% 25% 8%	517
	21st to end	Day	4 3 20% 39% 26% 8%	583
September	1st to 10th	Day	2 5% 21% 37% 26% 9%	530
	11th to 20th	Day	5% 9% 25% 31% 22% 9%	530
	21st to end	Day	6% 9% 22% 32% 24% 8%	530
October	1st to 10th	Day	7% 9% 25% 25% 22% 12%	530
	11th to 20th	Day	9% 10% 18% 26% 25% 11%	530
	21st to end	Day	13% 9% 18% 28% 24% 8%	583
November	1st to 10th	Day	11% 11% 19% 25% 26% 8%	526
	11th to 20th	Day	13% 11% 19% 24% 22% 12%	301
	21st to end	Day	11% 8% 21% 27% 21% 11%	220
December	1st to 10th	Day	16% 13% 13% 24% 24% 10%	220
	11th to 20th	Day	26% 10% 19% 20% 16% 8%	220
	21st to end	Day	27% 12% 20% 24% 14% 3	242
January	1st to 10th	Night	22% 8% 15% 30% 21% 4	840
	11th to 20th	Night	20% 6% 14% 33% 23% 4	824
	21st to end	Night	19% 6% 14% 32% 25% 4	880
February	1st to 10th	Night	21% 7% 15% 32% 23% 3	800
	11th to 20th	Night	17% 9% 18% 31% 20% 4	784
	21st to end	Night	15% 10% 21% 34% 18% 2	660
March	1st to 10th	Night	21% 12% 17% 27% 19% 3	800
	11th to 20th	Night	19% 9% 18% 31% 20% 3	800
	21st to end	Night	17% 9% 26% 28% 16% 3	880
April	1st to 10th	Night	12% 10% 29% 28% 19% 3	800
	11th to 20th	Night	11% 13% 24% 28% 20% 5%	791
	21st to end	Night	8% 12% 27% 25% 23% 5%	800
May	1st to 10th	Night	9% 7% 24% 28% 27% 4	622
	11th to 20th	Night	6% 3 23% 31% 33% 4	557
	21st to end	Night	6% 10% 22% 30% 27% 4	433
June	1st to 10th	Night	7% 6% 23% 36% 27% 2	400
	11th to 20th	Night	4 7% 21% 43% 24% 2	400
	21st to end	Night	3 2 16% 37% 39% 3	400
July	1st to 10th	Night	3 1 14% 36% 40% 6%	390
	11th to 20th	Night	3 1 6% 31% 49% 10%	582
	21st to end	Night	4 7% 33% 46% 11%	657
August	1st to 10th	Night	4 1 12% 32% 40% 10%	597
	11th to 20th	Night	2 10% 36% 43% 8%	651
	21st to end	Night	4 1 8% 34% 45% 8%	880
September	1st to 10th	Night	2 2 10% 31% 46% 9%	800
	11th to 20th	Night	7% 3 12% 34% 35% 10%	800
	21st to end	Night	6% 3 14% 30% 36% 12%	800
October	1st to 10th	Night	7% 4 14% 31% 34% 10%	800
	11th to 20th	Night	10% 4 12% 28% 39% 8%	800
	21st to end	Night	11% 4 12% 27% 35% 11%	880
November	1st to 10th	Night	12% 4 16% 26% 34% 8%	820
	11th to 20th	Night	15% 5% 13% 34% 28% 5%	840
	21st to end	Night	11% 7% 16% 35% 26% 5%	840
December	1st to 10th	Night	21% 5% 14% 31% 24% 6%	840
	11th to 20th	Night	26% 7% 14% 29% 21% 4	840
	21st to end	Night	27% 7% 19% 29% 16% 1	924

Page generated at 2026-01-13 22:14:30. Version ac07

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