Foothill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Foothill Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	4 3 1 4 3 3 3 3 5% 8% 62%	157720
	Night	5% 4 2 5% 4 4 4 3 6% 8% 57%	151910
Transparency	Day	21% 4 9% 20% 27% 19%	152654
	Night	26% 5% 10% 19% 24% 17%	147965
Seeing	Day	6% 10% 25% 32% 20% 7%	18070
	Night	8% 5% 16% 35% 30% 6%	27385

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	9% 6% 2 7% 4 4 5% 4 7% 9% 42%	5100
	11th to 20th	Day	5% 5% 2 6% 4 4 4 3 4 6% 57%	5340
	21st to end	Day	8% 8% 3 8% 5% 5% 5% 4 6% 9% 40%	5843
February	1st to 10th	Day	9% 8% 3 7% 4 5% 4 4 6% 6% 45%	1890
	11th to 20th	Day	9% 7% 3 9% 5% 4 5% 3 7% 7% 41%	1949
	21st to end	Day	9% 7% 2 8% 5% 5% 3 4 6% 6% 45%	1730
March	1st to 10th	Day	8% 7% 3 9% 6% 7% 7% 5% 8% 7% 33%	4976
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 4 7% 8% 47%	5110
	21st to end	Day	6% 6% 2 7% 6% 6% 6% 6% 9% 10% 36%	5690
April	1st to 10th	Day	9% 7% 3 6% 4 4 4 3 6% 8% 47%	2520
	11th to 20th	Day	7% 5% 2 4 4 3 4 3 6% 7% 54%	2489
	21st to end	Day	5% 5% 2 4 4 3 3 3 5% 9% 57%	2520
May	1st to 10th	Day	3 3 2 4 3 2 2 2 4 6% 68%	7228
	11th to 20th	Day	1 2 2 1 1 1 1 3 6% 81%	7707
	21st to end	Day	2 2 1 3 2 2 2 2 4 8% 72%	8992
June	1st to 10th	Day	3 4 1 3 3 2 3 3 5% 8% 65%	2940
	11th to 20th	Day	2 1 1 1 1 2 2 4 7% 78%	2940
	21st to end	Day	1 1 1 2 1 2 2 2 3 5% 79%	2940
July	1st to 10th	Day	1 1 1 2 1 4 6% 83%	8187
	11th to 20th	Day	2 2 1 3 2 2 3 2 5% 7% 73%	7650
	21st to end	Day	1 1 1 2 2 2 4 8% 77%	8304
August	1st to 10th	Day	1 1 1 1 1 2 2 4 7% 80%	7547
	11th to 20th	Day	1 2 2 3 4 3 6% 8% 71%	7171
	21st to end	Day	1 1 1 2 2 2 3 3 6% 9% 70%	7750
September	1st to 10th	Day	1 1 1 2 1 2 2 2 3 6% 78%	2520
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 8% 67%	2520
	21st to end	Day	2 3 1 3 1 2 2 2 4 7% 75%	2227
October	1st to 10th	Day	2 2 1 3 3 2 3 2 4 7% 72%	3380
	11th to 20th	Day	3 3 1 3 3 2 3 3 6% 7% 66%	3361
	21st to end	Day	2 3 1 3 3 4 3 3 6% 9% 61%	3639
November	1st to 10th	Day	5% 5% 2 5% 3 4 5% 3 5% 7% 56%	2032
	11th to 20th	Day	6% 6% 3 6% 5% 5% 5% 4 5% 7% 48%	1890
	21st to end	Day	7% 6% 2 5% 4 3 4 4 6% 7% 53%	1738
December	1st to 10th	Day	4 3 2 6% 5% 4 4 3 7% 10% 52%	2700
	11th to 20th	Day	9% 7% 3 9% 7% 6% 6% 4 8% 10% 32%	3108
	21st to end	Day	10% 9% 4 10% 6% 7% 6% 4 6% 8% 29%	4092
January	1st to 10th	Night	9% 7% 2 7% 5% 4 4 4 7% 9% 42%	7797
	11th to 20th	Night	6% 5% 3 7% 4 4 4 3 5% 7% 53%	7650
	21st to end	Night	8% 8% 3 9% 5% 5% 5% 4 5% 8% 40%	8371
February	1st to 10th	Night	8% 8% 3 6% 5% 4 4 3 5% 6% 48%	2730
	11th to 20th	Night	9% 7% 3 7% 5% 4 5% 4 6% 8% 42%	2558
	21st to end	Night	7% 6% 2 7% 5% 4 4 3 5% 8% 48%	2076
March	1st to 10th	Night	8% 7% 4 10% 6% 5% 5% 5% 6% 6% 37%	5123
	11th to 20th	Night	8% 6% 2 6% 5% 4 4 3 5% 7% 48%	5110
	21st to end	Night	8% 9% 3 8% 5% 5% 5% 4 5% 7% 41%	5526
April	1st to 10th	Night	7% 6% 2 6% 5% 4 4 4 7% 7% 48%	2100
	11th to 20th	Night	5% 6% 2 6% 4 3 5% 3 5% 8% 53%	2079
	21st to end	Night	4 3 1 3 3 3 4 4 5% 9% 59%	2100
May	1st to 10th	Night	2 4 1 4 3 3 3 2 5% 6% 66%	5723
	11th to 20th	Night	1 1 1 2 2 2 3 3 5% 6% 75%	5159
	21st to end	Night	1 2 1 3 2 2 3 3 5% 8% 71%	5181
June	1st to 10th	Night	3 3 1 4 3 2 3 3 6% 10% 63%	1680
	11th to 20th	Night	1 1 2 2 2 1 2 4 6% 79%	1680
	21st to end	Night	1 2 1 2 2 1 2 3 4 6% 77%	1680
July	1st to 10th	Night	1 1 1 2 2 3 5% 5% 81%	4755
	11th to 20th	Night	2 2 1 3 3 4 4 4 7% 6% 67%	5340
	21st to end	Night	2 2 2 3 3 7% 11% 68%	5806
August	1st to 10th	Night	1 1 2 2 2 3 3 6% 8% 73%	5275
	11th to 20th	Night	1 1 3 3 4 4 4 7% 9% 65%	5706
	21st to end	Night	1 2 1 2 2 3 4 4 8% 9% 65%	6499
September	1st to 10th	Night	1 1 1 2 2 2 3 3 6% 7% 73%	2100
	11th to 20th	Night	3 3 1 3 3 3 3 3 6% 7% 65%	2100
	21st to end	Night	1 2 1 3 2 1 3 2 5% 8% 72%	2386
October	1st to 10th	Night	2 2 2 2 2 3 2 4 7% 74%	4040
	11th to 20th	Night	3 4 1 4 3 3 3 3 4 7% 67%	4017
	21st to end	Night	3 3 1 4 3 3 4 3 6% 7% 63%	4349
November	1st to 10th	Night	5% 4 2 4 4 3 3 4 6% 9% 56%	2485
	11th to 20th	Night	7% 6% 3 5% 4 4 4 3 6% 8% 50%	2730
	21st to end	Night	6% 5% 2 4 4 3 4 3 5% 7% 55%	2877
December	1st to 10th	Night	5% 3 2 5% 4 4 5% 3 5% 7% 58%	4700
	11th to 20th	Night	10% 8% 3 8% 6% 6% 5% 5% 8% 11% 30%	5316
	21st to end	Night	10% 9% 3 10% 7% 6% 6% 5% 6% 7% 31%	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	36% 8% 17% 22% 15% 3	5101
	11th to 20th	Day	29% 4 7% 11% 26% 23%	5340
	21st to end	Day	40% 5% 8% 16% 19% 13%	5843
February	1st to 10th	Day	38% 5% 9% 20% 17% 11%	1890
	11th to 20th	Day	40% 5% 11% 18% 19% 6%	1958
	21st to end	Day	39% 6% 9% 21% 18% 6%	1730
March	1st to 10th	Day	44% 8% 10% 18% 18% 2	4976
	11th to 20th	Day	30% 7% 15% 27% 13% 8%	5110
	21st to end	Day	38% 8% 18% 23% 11% 2	5690
April	1st to 10th	Day	36% 6% 14% 22% 19% 4	2520
	11th to 20th	Day	29% 6% 16% 23% 20% 5%	2492
	21st to end	Day	25% 5% 15% 28% 22% 5%	2520
May	1st to 10th	Day	17% 4 7% 26% 34% 12%	7228
	11th to 20th	Day	8% 2 8% 30% 39% 14%	7710
	21st to end	Day	12% 4 12% 31% 37% 5%	8994
June	1st to 10th	Day	19% 5% 14% 25% 28% 9%	2940
	11th to 20th	Day	9% 3 10% 23% 37% 18%	2940
	21st to end	Day	10% 2 6% 17% 33% 32%	2940
July	1st to 10th	Day	4 1 3 11% 29% 52%	8119
	11th to 20th	Day	12% 1 4 7% 19% 56%	7581
	21st to end	Day	6% 3 4 16% 31% 40%	8119
August	1st to 10th	Day	8% 1 4 12% 39% 35%	5976
	11th to 20th	Day	10% 4 6% 21% 36% 23%	5653
	21st to end	Day	12% 2 6% 19% 39% 21%	6077
September	1st to 10th	Day	9% 3 5% 15% 31% 38%	2520
	11th to 20th	Day	17% 3 9% 24% 29% 16%	2520
	21st to end	Day	12% 2 7% 22% 35% 22%	2227
October	1st to 10th	Day	14% 3 6% 12% 27% 39%	3380
	11th to 20th	Day	16% 4 9% 25% 30% 16%	3361
	21st to end	Day	18% 5% 8% 25% 29% 15%	3639
November	1st to 10th	Day	27% 6% 10% 22% 23% 11%	2032
	11th to 20th	Day	34% 6% 11% 21% 21% 7%	1890
	21st to end	Day	30% 4 10% 17% 29% 9%	1738
December	1st to 10th	Day	26% 5% 7% 23% 31% 7%	2700
	11th to 20th	Day	45% 7% 11% 16% 14% 6%	3108
	21st to end	Day	51% 7% 13% 17% 10% 2	4092
January	1st to 10th	Night	37% 8% 18% 20% 15% 3	7797
	11th to 20th	Night	31% 4 8% 11% 28% 18%	7650
	21st to end	Night	41% 5% 8% 14% 20% 13%	8371
February	1st to 10th	Night	38% 4 9% 18% 19% 12%	2730
	11th to 20th	Night	39% 5% 11% 19% 20% 6%	2560
	21st to end	Night	35% 5% 13% 27% 16% 5%	2076
March	1st to 10th	Night	44% 7% 13% 17% 17% 3	5123
	11th to 20th	Night	34% 6% 12% 23% 17% 8%	5110
	21st to end	Night	43% 8% 13% 21% 12% 3	5526
April	1st to 10th	Night	33% 6% 16% 22% 18% 4	2100
	11th to 20th	Night	30% 7% 14% 24% 18% 7%	2079
	21st to end	Night	21% 6% 15% 32% 21% 6%	2100
May	1st to 10th	Night	20% 4 10% 22% 30% 14%	5721
	11th to 20th	Night	10% 3 9% 30% 30% 17%	5160
	21st to end	Night	13% 4 14% 24% 40% 5%	5182
June	1st to 10th	Night	17% 7% 13% 26% 28% 10%	1680
	11th to 20th	Night	9% 3 11% 21% 34% 23%	1680
	21st to end	Night	11% 1 6% 17% 32% 32%	1680
July	1st to 10th	Night	4 1 4 11% 29% 50%	4715
	11th to 20th	Night	12% 2 8% 10% 16% 52%	5286
	21st to end	Night	7% 3 7% 17% 29% 37%	5679
August	1st to 10th	Night	10% 2 7% 13% 31% 36%	4169
	11th to 20th	Night	12% 7% 7% 18% 31% 26%	4493
	21st to end	Night	12% 3 7% 18% 37% 23%	5092
September	1st to 10th	Night	10% 4 8% 16% 24% 39%	2100
	11th to 20th	Night	17% 4 8% 25% 30% 17%	2100
	21st to end	Night	12% 3 8% 20% 31% 26%	2386
October	1st to 10th	Night	13% 3 5% 12% 23% 44%	4040
	11th to 20th	Night	17% 4 8% 25% 26% 20%	4017
	21st to end	Night	21% 4 7% 22% 27% 20%	4349
November	1st to 10th	Night	25% 5% 12% 25% 21% 13%	2485
	11th to 20th	Night	32% 6% 11% 24% 20% 8%	2730
	21st to end	Night	28% 5% 11% 21% 23% 13%	2877
December	1st to 10th	Night	26% 5% 7% 23% 28% 11%	4700
	11th to 20th	Night	46% 6% 11% 14% 18% 5%	5316
	21st to end	Night	51% 7% 15% 16% 9% 3	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	21% 15% 28% 21% 13% 2	351
	11th to 20th	Day	12% 10% 27% 30% 16% 5%	430
	21st to end	Day	12% 10% 29% 27% 17% 4	473
February	1st to 10th	Day	13% 19% 25% 25% 13% 5%	430
	11th to 20th	Day	16% 19% 29% 24% 10% 1	480
	21st to end	Day	11% 16% 30% 31% 11% 1	446
March	1st to 10th	Day	15% 18% 32% 26% 9% 1	540
	11th to 20th	Day	15% 18% 26% 22% 16% 3	540
	21st to end	Day	11% 18% 34% 24% 10% 2	594
April	1st to 10th	Day	11% 16% 33% 25% 13% 4	540
	11th to 20th	Day	11% 17% 29% 25% 13% 4	532
	21st to end	Day	5% 19% 29% 28% 12% 6%	540
May	1st to 10th	Day	6% 12% 34% 29% 15% 5%	543
	11th to 20th	Day	7% 14% 32% 29% 14% 5%	568
	21st to end	Day	5% 11% 29% 31% 17% 7%	617
June	1st to 10th	Day	4 9% 29% 32% 19% 7%	570
	11th to 20th	Day	2 9% 30% 35% 19% 5%	570
	21st to end	Day	1 5% 24% 37% 24% 9%	570
July	1st to 10th	Day	1 3 17% 42% 29% 7%	566
	11th to 20th	Day	1 1 15% 42% 31% 10%	564
	21st to end	Day	1 1 16% 43% 29% 10%	619
August	1st to 10th	Day	2 2 21% 38% 26% 13%	567
	11th to 20th	Day	1 1 21% 40% 25% 11%	557
	21st to end	Day	2 16% 41% 29% 11%	627
September	1st to 10th	Day	1 4 17% 39% 30% 10%	570
	11th to 20th	Day	3 7% 25% 35% 22% 9%	570
	21st to end	Day	3 8% 19% 34% 27% 10%	570
October	1st to 10th	Day	2 8% 20% 33% 25% 12%	570
	11th to 20th	Day	5% 9% 20% 31% 25% 10%	570
	21st to end	Day	6% 6% 25% 32% 24% 7%	616
November	1st to 10th	Day	7% 11% 23% 25% 25% 10%	508
	11th to 20th	Day	6% 12% 19% 32% 22% 9%	330
	21st to end	Day	8% 7% 25% 30% 23% 7%	250
December	1st to 10th	Day	5% 11% 23% 27% 25% 9%	220
	11th to 20th	Day	10% 16% 21% 21% 21% 11%	220
	21st to end	Day	16% 21% 24% 23% 14% 3	242
January	1st to 10th	Night	17% 7% 19% 32% 21% 5%	840
	11th to 20th	Night	12% 7% 15% 35% 24% 7%	840
	21st to end	Night	15% 6% 14% 37% 23% 5%	924
February	1st to 10th	Night	13% 9% 18% 36% 20% 5%	840
	11th to 20th	Night	13% 9% 21% 33% 19% 4	824
	21st to end	Night	11% 9% 23% 37% 17% 2	692
March	1st to 10th	Night	12% 12% 22% 32% 19% 3	840
	11th to 20th	Night	13% 9% 18% 33% 23% 3	840
	21st to end	Night	10% 9% 24% 37% 18% 2	924
April	1st to 10th	Night	10% 8% 23% 36% 21% 2	840
	11th to 20th	Night	9% 12% 22% 35% 19% 4	831
	21st to end	Night	6% 7% 27% 33% 22% 6%	840
May	1st to 10th	Night	5% 4 25% 37% 25% 4	772
	11th to 20th	Night	3 4 20% 36% 31% 5%	576
	21st to end	Night	4 4 24% 36% 29% 4	455
June	1st to 10th	Night	3 3 20% 43% 29% 3	420
	11th to 20th	Night	1 3 17% 47% 31% 1	420
	21st to end	Night	1 1 15% 38% 42% 3	420
July	1st to 10th	Night	1 11% 36% 49% 3	412
	11th to 20th	Night	1 3 36% 48% 11%	612
	21st to end	Night	1 4 33% 53% 9%	690
August	1st to 10th	Night	3 1 7% 32% 49% 8%	627
	11th to 20th	Night	1 1 10% 39% 43% 6%	766
	21st to end	Night	1 9% 36% 45% 9%	924
September	1st to 10th	Night	1 2 9% 37% 42% 9%	840
	11th to 20th	Night	4 3 14% 38% 35% 7%	840
	21st to end	Night	2 2 11% 35% 40% 10%	840
October	1st to 10th	Night	4 2 11% 38% 36% 9%	840
	11th to 20th	Night	5% 4 13% 32% 37% 9%	840
	21st to end	Night	5% 5% 11% 36% 31% 12%	912
November	1st to 10th	Night	7% 4 17% 30% 34% 9%	820
	11th to 20th	Night	10% 4 15% 35% 29% 7%	840
	21st to end	Night	9% 4 15% 40% 26% 6%	840
December	1st to 10th	Night	10% 5% 19% 35% 26% 6%	840
	11th to 20th	Night	17% 5% 19% 33% 22% 3	840
	21st to end	Night	16% 12% 23% 30% 17% 2	924

Page generated at 2026-10-26 01:37:50. Version ac07

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