Foresthill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Foresthill 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	10% 4 1 3 2 2 2 2 3 5% 65%	139958
	Night	12% 5% 1 3 2 2 3 2 4 5% 60%	138967
Transparency	Day	24% 4 7% 18% 25% 24%	134906
	Night	28% 4 7% 17% 23% 21%	135003
Seeing	Day	12% 10% 26% 33% 15% 4	16827
	Night	14% 3 15% 36% 26% 5%	26258

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	24% 8% 2 3 2 2 3 3 4 5% 43%	4722
	11th to 20th	Day	13% 5% 2 4 3 3 3 2 3 4 59%	5100
	21st to end	Day	16% 7% 2 5% 3 3 4 3 5% 7% 45%	5843
February	1st to 10th	Day	22% 8% 2 3 2 2 3 2 4 5% 46%	1890
	11th to 20th	Day	17% 11% 2 4 3 4 3 3 5% 5% 43%	1993
	21st to end	Day	22% 10% 2 3 3 3 2 2 4 5% 44%	1730
March	1st to 10th	Day	19% 16% 3 7% 4 4 3 2 5% 6% 32%	4703
	11th to 20th	Day	18% 9% 2 5% 3 2 2 2 3 5% 48%	5078
	21st to end	Day	23% 11% 3 5% 3 3 4 3 4 6% 34%	5739
April	1st to 10th	Day	16% 8% 3 4 3 3 3 4 5% 7% 44%	2400
	11th to 20th	Day	14% 5% 1 4 3 3 3 4 5% 6% 52%	2369
	21st to end	Day	12% 4 1 4 3 3 2 2 5% 7% 57%	2400
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 3 4 73%	5600
	11th to 20th	Day	5% 2 1 2 1 2 2 2 3 5% 74%	6277
	21st to end	Day	4 2 1 2 1 1 2 2 3 4 78%	6902
June	1st to 10th	Day	6% 4 1 3 3 3 3 2 5% 7% 63%	2800
	11th to 20th	Day	2 2 1 2 1 2 2 2 3 5% 79%	2800
	21st to end	Day	2 2 1 1 1 1 1 1 3 3 84%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 94%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 2 89%	6320
	21st to end	Day	1 1 2 2 2 2 2 3 3 83%	6515
August	1st to 10th	Day	1 1 1 2 1 1 1 2 2 3 84%	5777
	11th to 20th	Day	1 1 1 1 2 2 91%	5413
	21st to end	Day	2 1 2 1 2 2 1 2 3 83%	5984
September	1st to 10th	Day	2 1 1 1 1 1 2 2 3 86%	2400
	11th to 20th	Day	5% 3 1 3 2 1 2 2 3 4 75%	2400
	21st to end	Day	4 2 1 2 2 2 2 2 2 5% 77%	2168
October	1st to 10th	Day	4 2 1 3 2 2 3 3 4 5% 73%	3280
	11th to 20th	Day	7% 2 1 2 2 2 2 2 3 5% 72%	3261
	21st to end	Day	6% 3 1 4 3 3 4 3 4 6% 63%	3558
November	1st to 10th	Day	14% 5% 1 3 4 3 2 2 4 7% 54%	1868
	11th to 20th	Day	16% 4 1 3 4 3 4 3 5% 7% 49%	1890
	21st to end	Day	14% 5% 1 3 3 3 3 3 5% 5% 55%	1759
December	1st to 10th	Day	10% 3 1 3 3 3 3 3 5% 7% 59%	2700
	11th to 20th	Day	28% 8% 2 4 3 3 3 3 5% 6% 34%	3108
	21st to end	Day	27% 6% 2 6% 3 3 4 4 6% 8% 31%	4092
January	1st to 10th	Night	22% 9% 2 4 3 3 3 3 5% 7% 39%	8172
	11th to 20th	Night	13% 4 1 3 2 3 3 3 4 5% 59%	7861
	21st to end	Night	16% 6% 2 6% 4 3 4 3 4 6% 46%	8371
February	1st to 10th	Night	21% 7% 1 4 3 3 3 2 4 5% 48%	2730
	11th to 20th	Night	18% 8% 1 5% 3 3 4 3 5% 6% 43%	2512
	21st to end	Night	19% 8% 2 3 2 3 3 3 4 6% 47%	2076
March	1st to 10th	Night	22% 16% 2 4 3 3 3 2 4 6% 36%	5406
	11th to 20th	Night	23% 11% 1 3 2 2 2 2 3 5% 48%	5077
	21st to end	Night	24% 11% 2 5% 3 3 4 3 6% 6% 34%	5236
April	1st to 10th	Night	15% 9% 2 5% 3 3 3 2 5% 6% 47%	2000
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 4 6% 52%	1979
	21st to end	Night	10% 5% 2 3 3 3 2 3 4 6% 61%	2000
May	1st to 10th	Night	4 4 1 2 2 2 2 2 3 6% 71%	4386
	11th to 20th	Night	4 2 1 2 2 2 3 2 3 5% 74%	3616
	21st to end	Night	4 1 2 2 2 2 2 2 4 80%	3972
June	1st to 10th	Night	6% 4 1 3 2 3 3 3 4 5% 66%	1600
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 80%	1600
	21st to end	Night	2 2 2 2 1 2 2 2 3 82%	1600
July	1st to 10th	Night	1 1 1 1 1 2 94%	3640
	11th to 20th	Night	1 1 2 1 1 1 1 2 3 86%	3640
	21st to end	Night	1 1 1 2 2 2 3 2 4 5% 77%	4335
August	1st to 10th	Night	2 2 1 2 1 1 1 2 3 3 82%	4035
	11th to 20th	Night	1 1 1 1 2 2 91%	4454
	21st to end	Night	3 1 1 2 1 2 2 2 2 3 82%	5015
September	1st to 10th	Night	2 1 1 1 1 1 1 1 3 3 87%	2000
	11th to 20th	Night	4 2 1 2 1 1 2 2 3 4 77%	2000
	21st to end	Night	3 2 1 2 2 2 2 2 3 3 78%	2226
October	1st to 10th	Night	4 2 1 3 2 2 2 2 3 3 77%	3920
	11th to 20th	Night	5% 2 2 1 2 2 2 2 4 78%	3897
	21st to end	Night	10% 3 1 3 2 3 3 2 4 6% 63%	4253
November	1st to 10th	Night	16% 5% 1 3 2 2 3 3 4 7% 56%	2650
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 7% 53%	2730
	21st to end	Night	13% 5% 2 4 3 3 3 2 4 6% 55%	2856
December	1st to 10th	Night	12% 3 1 2 2 2 3 3 4 6% 62%	4700
	11th to 20th	Night	28% 9% 2 4 3 3 4 4 6% 8% 31%	5316
	21st to end	Night	29% 8% 3 6% 4 4 4 3 4 5% 30%	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	43% 3 13% 19% 15% 7%	4723
	11th to 20th	Day	30% 4 5% 12% 24% 26%	5100
	21st to end	Day	39% 4 6% 17% 19% 15%	5843
February	1st to 10th	Day	42% 3 9% 18% 18% 11%	1890
	11th to 20th	Day	43% 4 9% 18% 20% 6%	2003
	21st to end	Day	44% 3 9% 21% 15% 8%	1730
March	1st to 10th	Day	51% 7% 8% 15% 16% 4	4703
	11th to 20th	Day	40% 4 10% 28% 12% 5%	5078
	21st to end	Day	48% 8% 13% 19% 11% 1	5739
April	1st to 10th	Day	38% 7% 12% 22% 16% 3	2400
	11th to 20th	Day	32% 7% 13% 24% 19% 6%	2372
	21st to end	Day	26% 5% 13% 29% 22% 4	2400
May	1st to 10th	Day	16% 4 9% 27% 29% 15%	5600
	11th to 20th	Day	14% 3 8% 30% 36% 8%	6280
	21st to end	Day	11% 3 8% 28% 43% 7%	6904
June	1st to 10th	Day	21% 6% 9% 24% 31% 10%	2800
	11th to 20th	Day	10% 3 7% 20% 37% 23%	2800
	21st to end	Day	9% 2 3 13% 38% 36%	2800
July	1st to 10th	Day	2 1 1 6% 20% 70%	6251
	11th to 20th	Day	5% 1 2 5% 12% 75%	6245
	21st to end	Day	8% 2 3 14% 28% 46%	6327
August	1st to 10th	Day	10% 2 2 8% 22% 56%	4206
	11th to 20th	Day	4 1 2 6% 39% 47%	3917
	21st to end	Day	6% 1 2 8% 42% 40%	4311
September	1st to 10th	Day	6% 2 3 10% 28% 52%	2400
	11th to 20th	Day	15% 3 5% 20% 37% 21%	2400
	21st to end	Day	13% 2 5% 18% 33% 29%	2168
October	1st to 10th	Day	15% 3 4 12% 26% 41%	3280
	11th to 20th	Day	16% 3 5% 17% 39% 19%	3261
	21st to end	Day	22% 3 7% 20% 32% 15%	3558
November	1st to 10th	Day	30% 4 10% 19% 23% 14%	1868
	11th to 20th	Day	35% 5% 10% 19% 22% 10%	1890
	21st to end	Day	32% 4 8% 18% 25% 13%	1759
December	1st to 10th	Day	26% 4 6% 23% 28% 14%	2700
	11th to 20th	Day	52% 3 7% 16% 14% 8%	3108
	21st to end	Day	49% 6% 12% 18% 11% 4	4092
January	1st to 10th	Night	44% 5% 14% 17% 14% 7%	8172
	11th to 20th	Night	28% 3 6% 12% 23% 28%	7861
	21st to end	Night	41% 4 5% 14% 19% 17%	8371
February	1st to 10th	Night	40% 3 6% 19% 19% 14%	2730
	11th to 20th	Night	42% 5% 7% 20% 19% 8%	2513
	21st to end	Night	39% 4 9% 22% 18% 7%	2076
March	1st to 10th	Night	50% 5% 7% 18% 13% 6%	5406
	11th to 20th	Night	42% 3 9% 22% 19% 5%	5077
	21st to end	Night	48% 8% 13% 19% 10% 2	5236
April	1st to 10th	Night	39% 5% 12% 20% 18% 5%	2000
	11th to 20th	Night	32% 6% 12% 24% 16% 8%	1979
	21st to end	Night	26% 6% 11% 33% 20% 5%	2000
May	1st to 10th	Night	16% 5% 11% 27% 27% 14%	4383
	11th to 20th	Night	15% 2 8% 31% 33% 12%	3616
	21st to end	Night	11% 3 9% 32% 40% 5%	3972
June	1st to 10th	Night	20% 7% 10% 27% 25% 11%	1600
	11th to 20th	Night	10% 3 6% 21% 34% 26%	1600
	21st to end	Night	10% 3 4 12% 34% 38%	1600
July	1st to 10th	Night	2 1 1 5% 25% 66%	3600
	11th to 20th	Night	7% 2 2 5% 12% 72%	3592
	21st to end	Night	10% 3 3 13% 23% 49%	4211
August	1st to 10th	Night	13% 2 2 9% 24% 51%	2929
	11th to 20th	Night	4 1 2 8% 36% 48%	3217
	21st to end	Night	8% 1 2 12% 34% 43%	3608
September	1st to 10th	Night	6% 1 3 10% 32% 49%	2000
	11th to 20th	Night	12% 3 6% 19% 39% 21%	2000
	21st to end	Night	13% 2 5% 19% 32% 30%	2226
October	1st to 10th	Night	15% 2 4 11% 23% 45%	3920
	11th to 20th	Night	13% 2 5% 24% 36% 20%	3897
	21st to end	Night	24% 3 4 19% 33% 18%	4253
November	1st to 10th	Night	29% 5% 10% 21% 20% 16%	2650
	11th to 20th	Night	31% 4 10% 20% 24% 10%	2730
	21st to end	Night	31% 3 7% 19% 24% 15%	2856
December	1st to 10th	Night	24% 3 7% 22% 25% 19%	4700
	11th to 20th	Night	51% 4 7% 14% 18% 7%	5316
	21st to end	Night	55% 6% 10% 14% 10% 5%	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	39% 7% 26% 18% 9% 2	220
	11th to 20th	Day	22% 7% 21% 34% 12% 4	346
	21st to end	Day	23% 9% 23% 29% 13% 4	473
February	1st to 10th	Day	28% 11% 24% 25% 10% 2	430
	11th to 20th	Day	26% 17% 28% 20% 8% 1	505
	21st to end	Day	23% 15% 28% 26% 7% 1	446
March	1st to 10th	Day	29% 18% 28% 18% 6% 1	540
	11th to 20th	Day	26% 18% 23% 19% 11% 3	529
	21st to end	Day	26% 16% 30% 20% 7% 1	550
April	1st to 10th	Day	20% 16% 30% 23% 9% 1	500
	11th to 20th	Day	16% 17% 29% 26% 10% 1	492
	21st to end	Day	11% 13% 30% 29% 14% 3	500
May	1st to 10th	Day	10% 16% 33% 27% 11% 4	503
	11th to 20th	Day	11% 14% 25% 30% 15% 5%	528
	21st to end	Day	9% 10% 26% 32% 16% 6%	573
June	1st to 10th	Day	6% 9% 26% 37% 13% 8%	530
	11th to 20th	Day	5% 12% 29% 35% 15% 4	530
	21st to end	Day	3 4 29% 39% 16% 8%	530
July	1st to 10th	Day	1 4 26% 45% 18% 6%	526
	11th to 20th	Day	1 25% 47% 19% 7%	524
	21st to end	Day	1 1 27% 46% 18% 8%	575
August	1st to 10th	Day	3 4 24% 45% 19% 6%	527
	11th to 20th	Day	2 3 28% 41% 18% 7%	517
	21st to end	Day	2 3 19% 51% 22% 4	583
September	1st to 10th	Day	2 4 28% 44% 18% 4	530
	11th to 20th	Day	7% 5% 29% 37% 19% 3	530
	21st to end	Day	4 8% 27% 41% 17% 2	530
October	1st to 10th	Day	5% 11% 27% 32% 20% 5%	530
	11th to 20th	Day	8% 9% 24% 35% 18% 6%	530
	21st to end	Day	11% 10% 24% 32% 18% 5%	583
November	1st to 10th	Day	17% 5% 21% 29% 23% 6%	344
	11th to 20th	Day	15% 9% 21% 29% 20% 5%	330
	21st to end	Day	14% 10% 18% 33% 18% 6%	261
December	1st to 10th	Day	19% 9% 20% 27% 17% 9%	220
	11th to 20th	Day	22% 10% 20% 25% 16% 7%	220
	21st to end	Day	30% 8% 21% 26% 12% 3	242
January	1st to 10th	Night	28% 4 19% 28% 17% 5%	840
	11th to 20th	Night	18% 3 21% 40% 15% 3	840
	21st to end	Night	22% 4 21% 30% 19% 4	924
February	1st to 10th	Night	24% 5% 20% 33% 17% 2	840
	11th to 20th	Night	22% 7% 19% 31% 18% 3	824
	21st to end	Night	22% 6% 23% 35% 12% 2	692
March	1st to 10th	Night	27% 7% 20% 29% 15% 2	840
	11th to 20th	Night	24% 6% 17% 29% 22% 3	828
	21st to end	Night	24% 6% 18% 33% 17% 2	880
April	1st to 10th	Night	17% 5% 19% 38% 19% 2	800
	11th to 20th	Night	15% 5% 19% 35% 22% 3	791
	21st to end	Night	11% 5% 17% 39% 24% 5%	800
May	1st to 10th	Night	9% 5% 22% 38% 22% 4	730
	11th to 20th	Night	9% 2 14% 37% 34% 4	394
	21st to end	Night	8% 2 13% 37% 36% 4	433
June	1st to 10th	Night	7% 2 14% 37% 36% 5%	400
	11th to 20th	Night	3 2 14% 45% 33% 4	400
	21st to end	Night	2 2 9% 42% 42% 5%	400
July	1st to 10th	Night	1 6% 41% 49% 4	390
	11th to 20th	Night	1 1 4 43% 46% 6%	388
	21st to end	Night	1 5% 32% 52% 10%	598
August	1st to 10th	Night	3 1 5% 40% 44% 7%	597
	11th to 20th	Night	1 1 7% 44% 39% 8%	765
	21st to end	Night	2 1 6% 38% 50% 4	880
September	1st to 10th	Night	2 1 8% 40% 44% 6%	800
	11th to 20th	Night	4 1 10% 38% 38% 10%	800
	21st to end	Night	3 2 13% 41% 31% 9%	800
October	1st to 10th	Night	7% 3 13% 42% 29% 6%	800
	11th to 20th	Night	9% 3 13% 38% 32% 5%	800
	21st to end	Night	10% 4 16% 34% 27% 9%	880
November	1st to 10th	Night	17% 2 14% 35% 25% 7%	820
	11th to 20th	Night	15% 3 20% 36% 21% 5%	840
	21st to end	Night	15% 5% 18% 38% 18% 5%	840
December	1st to 10th	Night	19% 4 18% 36% 17% 5%	840
	11th to 20th	Night	29% 3 17% 31% 17% 3	840
	21st to end	Night	30% 4 21% 30% 14% 2	924

Page generated at 2026-03-14 23:23:51. Version ac07

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