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 2025-07-06.

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 4 2 4 3 3 3 3 5% 7% 61%	123781
	Night	5% 5% 2 5% 4 4 4 3 5% 7% 56%	123909
Transparency	Day	23% 4 10% 20% 25% 18%	123542
	Night	27% 5% 11% 19% 22% 16%	123736
Seeing	Day	7% 10% 25% 32% 20% 7%	16291
	Night	8% 5% 16% 35% 30% 6%	25660

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 4 4 6% 9% 45%	3933
	11th to 20th	Day	7% 6% 3 8% 5% 4 4 3 4 5% 52%	4100
	21st to end	Day	10% 8% 3 8% 5% 4 5% 4 5% 7% 39%	4510
February	1st to 10th	Day	9% 8% 3 7% 4 5% 4 4 6% 6% 44%	1800
	11th to 20th	Day	9% 7% 3 9% 5% 4 5% 3 8% 7% 42%	1854
	21st to end	Day	9% 7% 2 8% 5% 4 3 4 6% 6% 46%	1650
March	1st to 10th	Day	8% 7% 3 9% 6% 7% 7% 5% 8% 7% 32%	4868
	11th to 20th	Day	5% 4 2 6% 5% 6% 6% 4 7% 9% 46%	5000
	21st to end	Day	6% 6% 2 7% 6% 6% 6% 6% 8% 10% 36%	5567
April	1st to 10th	Day	9% 7% 3 6% 4 4 4 3 6% 8% 46%	2400
	11th to 20th	Day	7% 5% 2 4 4 3 4 3 6% 7% 55%	2369
	21st to end	Day	5% 5% 2 4 4 3 3 3 5% 9% 57%	2400
May	1st to 10th	Day	3 2 1 2 1 1 2 2 4 5% 76%	5572
	11th to 20th	Day	2 2 1 2 1 1 2 2 3 6% 79%	5927
	21st to end	Day	2 3 1 3 2 2 2 2 3 7% 74%	6902
June	1st to 10th	Day	4 4 1 3 3 2 3 3 5% 8% 64%	2800
	11th to 20th	Day	2 1 1 1 1 1 2 2 4 7% 78%	2800
	21st to end	Day	1 1 1 2 1 2 2 2 3 5% 80%	2800
July	1st to 10th	Day	1 1 1 1 1 1 3 5% 86%	4472
	11th to 20th	Day	1 2 1 2 1 2 2 2 4 7% 77%	4110
	21st to end	Day	1 1 1 1 2 2 4 8% 80%	4455
August	1st to 10th	Day	1 2 1 2 2 2 3 3 6% 10% 69%	4091
	11th to 20th	Day	1 1 1 1 1 2 2 3 6% 82%	3816
	21st to end	Day	1 2 2 2 3 3 5% 8% 75%	4180
September	1st to 10th	Day	1 1 1 2 1 2 2 2 3 6% 80%	2280
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 8% 67%	2280
	21st to end	Day	2 2 1 2 1 2 2 2 4 6% 76%	2015
October	1st to 10th	Day	2 2 1 3 3 2 3 2 4 6% 72%	3180
	11th to 20th	Day	3 3 1 3 3 3 3 3 6% 7% 65%	3161
	21st to end	Day	2 3 1 3 3 4 3 3 6% 9% 62%	3448
November	1st to 10th	Day	5% 5% 2 5% 4 4 5% 3 5% 7% 55%	1934
	11th to 20th	Day	6% 6% 3 6% 5% 4 5% 4 5% 7% 50%	1800
	21st to end	Day	8% 6% 2 5% 4 3 4 4 6% 7% 52%	1655
December	1st to 10th	Day	4 3 2 6% 5% 4 4 3 7% 10% 52%	2620
	11th to 20th	Day	9% 7% 3 9% 6% 6% 6% 5% 8% 9% 31%	3028
	21st to end	Day	9% 9% 4 10% 6% 7% 6% 5% 6% 8% 29%	4004
January	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 7% 10% 46%	6022
	11th to 20th	Night	7% 7% 4 8% 5% 4 4 3 5% 5% 48%	5880
	21st to end	Night	9% 9% 3 8% 5% 5% 5% 4 4 6% 42%	6468
February	1st to 10th	Night	9% 8% 3 6% 5% 5% 4 3 4 6% 48%	2600
	11th to 20th	Night	8% 7% 2 7% 5% 4 5% 4 6% 9% 43%	2433
	21st to end	Night	7% 6% 2 6% 4 4 4 3 5% 8% 49%	1980
March	1st to 10th	Night	9% 8% 4 10% 6% 5% 5% 5% 6% 6% 36%	5011
	11th to 20th	Night	9% 6% 2 6% 5% 4 4 3 5% 7% 48%	5000
	21st to end	Night	8% 9% 3 8% 6% 5% 5% 4 5% 7% 41%	5407
April	1st to 10th	Night	7% 6% 2 6% 5% 4 4 3 7% 7% 48%	2000
	11th to 20th	Night	5% 6% 2 6% 4 3 5% 2 5% 8% 53%	1979
	21st to end	Night	4 3 1 3 3 3 4 4 5% 9% 60%	2000
May	1st to 10th	Night	2 4 1 3 2 2 2 2 4 5% 73%	4404
	11th to 20th	Night	1 2 1 2 2 3 3 3 5% 7% 71%	3965
	21st to end	Night	1 2 1 2 2 2 2 3 4 6% 75%	3972
June	1st to 10th	Night	3 3 1 4 3 2 3 3 6% 10% 63%	1600
	11th to 20th	Night	1 1 2 2 2 1 2 4 6% 80%	1600
	21st to end	Night	1 2 1 2 2 2 2 2 3 5% 78%	1600
July	1st to 10th	Night	1 1 1 1 1 2 3 4 86%	2595
	11th to 20th	Night	1 1 1 2 3 4 3 5% 6% 72%	2860
	21st to end	Night	1 1 2 2 3 3 4 7% 12% 67%	3109
August	1st to 10th	Night	2 2 1 3 3 3 4 4 9% 11% 58%	2848
	11th to 20th	Night	1 1 2 2 2 2 2 4 7% 78%	3030
	21st to end	Night	1 2 2 2 2 3 2 7% 8% 71%	3498
September	1st to 10th	Night	1 1 1 2 2 2 2 3 6% 7% 75%	1900
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 7% 65%	1900
	21st to end	Night	1 2 1 2 2 1 2 2 4 8% 74%	2158
October	1st to 10th	Night	2 2 2 2 2 3 2 4 6% 74%	3800
	11th to 20th	Night	3 4 1 4 2 3 3 3 4 7% 66%	3777
	21st to end	Night	3 3 1 4 3 3 4 3 6% 7% 63%	4121
November	1st to 10th	Night	5% 5% 2 4 3 3 3 4 6% 8% 56%	2364
	11th to 20th	Night	6% 6% 3 5% 4 3 4 3 6% 8% 52%	2600
	21st to end	Night	6% 5% 2 4 4 3 4 4 5% 7% 55%	2740
December	1st to 10th	Night	5% 3 2 5% 4 4 5% 3 5% 7% 57%	4560
	11th to 20th	Night	10% 8% 3 8% 6% 6% 5% 5% 8% 11% 29%	5176
	21st to end	Night	10% 9% 3 10% 7% 6% 6% 5% 6% 7% 31%	6952

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	34% 7% 18% 21% 16% 4	3933
	11th to 20th	Day	36% 5% 8% 13% 21% 17%	4100
	21st to end	Day	42% 5% 8% 12% 16% 17%	4510
February	1st to 10th	Day	39% 5% 9% 19% 17% 11%	1800
	11th to 20th	Day	39% 5% 11% 18% 20% 7%	1863
	21st to end	Day	39% 6% 8% 22% 19% 7%	1650
March	1st to 10th	Day	45% 8% 10% 17% 18% 2	4868
	11th to 20th	Day	31% 7% 16% 27% 12% 7%	5000
	21st to end	Day	39% 8% 18% 23% 10% 1	5567
April	1st to 10th	Day	36% 5% 14% 22% 19% 4	2400
	11th to 20th	Day	29% 5% 16% 24% 20% 6%	2372
	21st to end	Day	25% 5% 15% 28% 22% 5%	2400
May	1st to 10th	Day	13% 3 7% 29% 33% 15%	5572
	11th to 20th	Day	10% 2 10% 32% 36% 10%	5930
	21st to end	Day	12% 3 13% 30% 38% 3	6904
June	1st to 10th	Day	20% 5% 14% 26% 26% 8%	2800
	11th to 20th	Day	9% 3 10% 24% 36% 18%	2800
	21st to end	Day	10% 2 7% 17% 33% 32%	2800
July	1st to 10th	Day	5% 1 3 10% 16% 65%	4403
	11th to 20th	Day	8% 2 6% 11% 17% 57%	4041
	21st to end	Day	4 2 3 12% 35% 43%	4315
August	1st to 10th	Day	11% 2 6% 14% 34% 33%	4076
	11th to 20th	Day	5% 2 4 13% 43% 33%	3854
	21st to end	Day	8% 2 7% 15% 39% 29%	4179
September	1st to 10th	Day	9% 2 5% 13% 31% 41%	2280
	11th to 20th	Day	17% 3 10% 25% 28% 17%	2280
	21st to end	Day	11% 2 7% 21% 35% 23%	2015
October	1st to 10th	Day	14% 3 5% 12% 28% 39%	3180
	11th to 20th	Day	16% 4 9% 25% 30% 16%	3161
	21st to end	Day	18% 5% 8% 24% 29% 16%	3448
November	1st to 10th	Day	28% 5% 10% 23% 22% 12%	1934
	11th to 20th	Day	32% 6% 11% 22% 22% 8%	1800
	21st to end	Day	31% 4 10% 16% 29% 9%	1655
December	1st to 10th	Day	27% 5% 8% 22% 32% 7%	2620
	11th to 20th	Day	45% 7% 11% 16% 15% 5%	3028
	21st to end	Day	51% 7% 13% 17% 10% 2	4004
January	1st to 10th	Night	33% 7% 20% 22% 15% 3	6022
	11th to 20th	Night	38% 5% 10% 12% 22% 12%	5880
	21st to end	Night	43% 5% 8% 10% 18% 16%	6468
February	1st to 10th	Night	38% 5% 9% 18% 19% 12%	2600
	11th to 20th	Night	38% 5% 11% 19% 21% 6%	2435
	21st to end	Night	34% 5% 12% 28% 16% 5%	1980
March	1st to 10th	Night	45% 7% 13% 16% 16% 3	5011
	11th to 20th	Night	35% 6% 12% 23% 17% 7%	5000
	21st to end	Night	43% 8% 14% 22% 11% 2	5407
April	1st to 10th	Night	34% 6% 16% 22% 17% 4	2000
	11th to 20th	Night	29% 7% 14% 25% 18% 8%	1979
	21st to end	Night	21% 6% 15% 31% 21% 6%	2000
May	1st to 10th	Night	16% 4 10% 25% 29% 17%	4401
	11th to 20th	Night	12% 4 12% 34% 25% 13%	3966
	21st to end	Night	13% 3 14% 24% 41% 4	3972
June	1st to 10th	Night	18% 7% 13% 26% 28% 8%	1600
	11th to 20th	Night	9% 3 11% 22% 33% 22%	1600
	21st to end	Night	12% 1 6% 17% 31% 33%	1600
July	1st to 10th	Night	5% 1 3 10% 18% 63%	2555
	11th to 20th	Night	8% 2 9% 15% 16% 51%	2806
	21st to end	Night	7% 1 7% 14% 31% 39%	3011
August	1st to 10th	Night	14% 2 9% 16% 30% 28%	2839
	11th to 20th	Night	8% 2 4 12% 39% 35%	3058
	21st to end	Night	9% 2 8% 15% 36% 29%	3498
September	1st to 10th	Night	9% 3 7% 15% 25% 42%	1900
	11th to 20th	Night	18% 4 8% 26% 28% 17%	1900
	21st to end	Night	11% 3 8% 20% 31% 26%	2158
October	1st to 10th	Night	13% 3 6% 11% 23% 45%	3800
	11th to 20th	Night	18% 4 8% 25% 27% 19%	3777
	21st to end	Night	20% 4 7% 21% 26% 21%	4121
November	1st to 10th	Night	25% 5% 12% 25% 20% 13%	2364
	11th to 20th	Night	30% 6% 11% 24% 20% 8%	2600
	21st to end	Night	29% 4 11% 20% 23% 13%	2740
December	1st to 10th	Night	27% 5% 7% 22% 28% 11%	4560
	11th to 20th	Night	46% 6% 10% 14% 18% 4	5176
	21st to end	Night	51% 7% 15% 16% 9% 3	6952

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	20% 14% 27% 22% 13% 2	325
	11th to 20th	Day	13% 11% 29% 30% 14% 4	400
	21st to end	Day	13% 10% 29% 27% 17% 4	440
February	1st to 10th	Day	14% 20% 25% 24% 14% 5%	400
	11th to 20th	Day	15% 19% 29% 24% 11% 2	445
	21st to end	Day	11% 16% 30% 30% 11% 1	414
March	1st to 10th	Day	15% 18% 32% 25% 9% 1	500
	11th to 20th	Day	16% 19% 27% 21% 13% 3	500
	21st to end	Day	12% 19% 35% 23% 9% 2	550
April	1st to 10th	Day	11% 16% 34% 25% 11% 3	500
	11th to 20th	Day	11% 17% 29% 25% 13% 4	492
	21st to end	Day	5% 19% 30% 28% 12% 6%	500
May	1st to 10th	Day	6% 13% 34% 29% 14% 5%	503
	11th to 20th	Day	7% 13% 32% 29% 14% 5%	528
	21st to end	Day	5% 12% 29% 32% 17% 6%	573
June	1st to 10th	Day	4 10% 29% 32% 19% 7%	530
	11th to 20th	Day	2 9% 31% 35% 19% 4	530
	21st to end	Day	1 4 24% 38% 25% 8%	530
July	1st to 10th	Day	2 3 17% 42% 30% 7%	507
	11th to 20th	Day	1 1 15% 43% 32% 9%	484
	21st to end	Day	1 14% 44% 31% 10%	531
August	1st to 10th	Day	2 2 21% 40% 25% 9%	487
	11th to 20th	Day	1 2 22% 40% 25% 11%	477
	21st to end	Day	2 17% 42% 28% 10%	539
September	1st to 10th	Day	1 3 14% 40% 31% 10%	490
	11th to 20th	Day	3 7% 24% 37% 21% 8%	490
	21st to end	Day	2 8% 18% 35% 28% 9%	490
October	1st to 10th	Day	2 8% 20% 34% 24% 11%	490
	11th to 20th	Day	5% 9% 20% 29% 26% 10%	490
	21st to end	Day	6% 6% 25% 32% 24% 7%	539
November	1st to 10th	Day	7% 11% 24% 25% 24% 9%	470
	11th to 20th	Day	5% 11% 19% 31% 24% 10%	300
	21st to end	Day	9% 7% 26% 30% 23% 5%	227
December	1st to 10th	Day	6% 12% 24% 26% 23% 9%	200
	11th to 20th	Day	11% 17% 21% 21% 22% 10%	200
	21st to end	Day	15% 19% 25% 24% 14% 3	220
January	1st to 10th	Night	16% 7% 18% 32% 22% 5%	800
	11th to 20th	Night	13% 7% 16% 35% 23% 7%	800
	21st to end	Night	15% 6% 14% 36% 23% 5%	880
February	1st to 10th	Night	13% 9% 18% 35% 21% 5%	800
	11th to 20th	Night	13% 9% 20% 34% 20% 4	784
	21st to end	Night	11% 9% 22% 36% 18% 3	660
March	1st to 10th	Night	13% 12% 22% 31% 20% 3	800
	11th to 20th	Night	14% 10% 18% 33% 22% 4	800
	21st to end	Night	11% 10% 24% 36% 18% 2	880
April	1st to 10th	Night	10% 9% 23% 36% 21% 3	800
	11th to 20th	Night	9% 12% 21% 34% 20% 4	791
	21st to end	Night	5% 7% 26% 33% 22% 6%	800
May	1st to 10th	Night	5% 4 26% 37% 25% 4	735
	11th to 20th	Night	3 3 19% 36% 32% 5%	548
	21st to end	Night	4 4 24% 36% 29% 3	433
June	1st to 10th	Night	3 4 18% 44% 29% 3	400
	11th to 20th	Night	1 3 18% 47% 31% 1	400
	21st to end	Night	1 1 14% 39% 43% 3	400
July	1st to 10th	Night	1 1 10% 35% 50% 3	382
	11th to 20th	Night	1 3 36% 48% 12%	552
	21st to end	Night	1 4 31% 55% 10%	624
August	1st to 10th	Night	4 1 7% 32% 49% 7%	567
	11th to 20th	Night	1 1 10% 39% 42% 6%	692
	21st to end	Night	2 9% 34% 45% 10%	836
September	1st to 10th	Night	1 1 8% 37% 43% 10%	760
	11th to 20th	Night	4 3 13% 37% 36% 8%	760
	21st to end	Night	3 2 11% 33% 41% 11%	760
October	1st to 10th	Night	4 2 12% 37% 36% 9%	760
	11th to 20th	Night	6% 4 11% 31% 39% 9%	760
	21st to end	Night	5% 5% 11% 35% 31% 13%	836
November	1st to 10th	Night	7% 4 17% 30% 33% 9%	780
	11th to 20th	Night	9% 4 15% 35% 30% 7%	800
	21st to end	Night	10% 4 15% 39% 25% 6%	800
December	1st to 10th	Night	10% 6% 19% 34% 25% 6%	800
	11th to 20th	Night	18% 5% 20% 33% 21% 4	800
	21st to end	Night	15% 11% 23% 30% 18% 3	880

Page generated at 2025-07-02 22:30:12. Version ac07

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