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 2024-11-03.

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	5% 4 2 4 3 3 3 3 5% 7% 61%	101060
	Night	6% 5% 2 5% 4 4 4 3 5% 7% 55%	98126
Transparency	Day	24% 4 10% 19% 24% 19%	100820
	Night	28% 5% 11% 19% 21% 16%	97956
Seeing	Day	7% 10% 25% 32% 20% 6%	15452
	Night	8% 5% 16% 35% 30% 6%	24787

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	12% 7% 2 9% 5% 4 5% 4 6% 8% 37%	2735
	11th to 20th	Day	10% 9% 4 11% 7% 6% 6% 4 5% 6% 32%	2860
	21st to end	Day	13% 10% 4 10% 6% 4 5% 4 6% 8% 28%	3146
February	1st to 10th	Day	10% 8% 3 6% 4 5% 4 4 6% 6% 45%	1710
	11th to 20th	Day	9% 7% 3 8% 4 4 5% 3 7% 6% 43%	1759
	21st to end	Day	10% 7% 2 7% 5% 4 3 4 6% 6% 47%	1570
March	1st to 10th	Day	8% 8% 4 10% 8% 7% 7% 5% 8% 7% 28%	3384
	11th to 20th	Day	6% 4 2 6% 4 5% 4 4 5% 8% 53%	3490
	21st to end	Day	8% 8% 2 9% 6% 6% 6% 5% 8% 8% 34%	3880
April	1st to 10th	Day	10% 7% 3 6% 4 4 4 3 6% 8% 46%	2280
	11th to 20th	Day	8% 5% 2 4 4 3 3 3 6% 7% 55%	2249
	21st to end	Day	5% 4 2 4 4 3 3 3 5% 9% 58%	2280
May	1st to 10th	Day	4 3 1 3 2 2 2 3 4 5% 73%	3893
	11th to 20th	Day	2 2 1 2 2 1 2 2 3 6% 78%	4131
	21st to end	Day	2 4 1 4 2 2 3 3 3 7% 69%	4813
June	1st to 10th	Day	4 4 1 3 3 3 3 4 5% 8% 63%	2660
	11th to 20th	Day	2 2 1 2 1 1 2 2 4 8% 77%	2660
	21st to end	Day	2 1 1 2 2 2 2 2 3 5% 79%	2660
July	1st to 10th	Day	1 1 1 1 1 1 3 4 86%	4401
	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	3 3 1 3 3 3 3 2 4 6% 69%	1900
	11th to 20th	Day	4 3 1 3 3 3 3 3 5% 7% 64%	1900
	21st to end	Day	4 4 2 3 3 4 3 3 5% 7% 61%	2090
November	1st to 10th	Day	5% 5% 2 5% 4 4 5% 4 5% 7% 54%	1857
	11th to 20th	Day	6% 6% 3 6% 5% 4 5% 4 5% 7% 51%	1710
	21st to end	Day	7% 5% 2 5% 4 3 4 4 5% 7% 53%	1573
December	1st to 10th	Day	7% 5% 2 8% 7% 5% 5% 4 6% 8% 43%	1520
	11th to 20th	Day	13% 10% 3 9% 6% 5% 5% 4 6% 7% 31%	1928
	21st to end	Day	9% 10% 5% 9% 6% 6% 5% 4 6% 8% 33%	2794
January	1st to 10th	Night	11% 8% 3 7% 4 4 4 4 7% 8% 41%	4213
	11th to 20th	Night	11% 9% 5% 11% 7% 6% 6% 4 6% 6% 31%	4110
	21st to end	Night	13% 12% 3 10% 6% 6% 6% 4 4 6% 30%	4521
February	1st to 10th	Night	9% 8% 3 6% 5% 4 4 3 4 6% 49%	2470
	11th to 20th	Night	8% 7% 2 6% 5% 4 5% 4 6% 9% 44%	2308
	21st to end	Night	8% 7% 2 6% 4 4 4 3 5% 7% 49%	1884
March	1st to 10th	Night	8% 8% 4 10% 7% 5% 6% 5% 6% 6% 34%	3475
	11th to 20th	Night	7% 6% 2 6% 4 4 3 3 5% 6% 53%	3490
	21st to end	Night	8% 11% 3 8% 7% 6% 6% 5% 5% 6% 36%	3772
April	1st to 10th	Night	8% 6% 2 6% 5% 4 4 4 7% 7% 47%	1900
	11th to 20th	Night	6% 7% 2 5% 4 3 4 2 5% 8% 54%	1879
	21st to end	Night	4 3 1 3 3 3 3 4 5% 9% 61%	1900
May	1st to 10th	Night	2 4 1 4 3 2 3 2 5% 6% 68%	3066
	11th to 20th	Night	1 2 1 3 2 2 3 3 5% 7% 72%	2753
	21st to end	Night	1 2 1 2 2 2 3 3 4 7% 71%	2762
June	1st to 10th	Night	3 3 1 4 3 3 3 3 5% 10% 62%	1520
	11th to 20th	Night	1 1 2 2 2 1 2 4 6% 79%	1520
	21st to end	Night	1 2 1 2 2 2 2 2 3 5% 78%	1520
July	1st to 10th	Night	1 1 1 1 1 2 3 4 86%	2555
	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	3 3 1 3 2 2 4 2 4 6% 69%	2280
	11th to 20th	Night	4 4 1 5% 3 3 3 3 4 7% 63%	2280
	21st to end	Night	3 3 1 4 3 3 3 3 5% 7% 63%	2508
November	1st to 10th	Night	5% 5% 2 4 4 3 3 4 6% 9% 55%	2266
	11th to 20th	Night	6% 6% 3 5% 4 3 4 4 6% 8% 51%	2470
	21st to end	Night	6% 5% 2 4 4 3 4 3 6% 7% 57%	2603
December	1st to 10th	Night	8% 5% 3 7% 5% 4 5% 3 5% 7% 47%	2660
	11th to 20th	Night	14% 10% 3 8% 7% 5% 5% 4 5% 7% 31%	3276
	21st to end	Night	10% 8% 3 9% 6% 6% 6% 5% 6% 8% 35%	4862

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% 8% 12% 26% 8% 3	2735
	11th to 20th	Day	51% 7% 11% 13% 10% 8%	2860
	21st to end	Day	52% 7% 9% 15% 13% 5%	3146
February	1st to 10th	Day	39% 5% 9% 20% 17% 10%	1710
	11th to 20th	Day	39% 5% 10% 18% 21% 7%	1768
	21st to end	Day	38% 6% 8% 22% 19% 6%	1570
March	1st to 10th	Day	51% 9% 11% 14% 13% 3	3384
	11th to 20th	Day	29% 5% 13% 26% 16% 10%	3490
	21st to end	Day	46% 7% 17% 18% 10% 2	3880
April	1st to 10th	Day	37% 5% 14% 20% 19% 4	2280
	11th to 20th	Day	29% 5% 16% 24% 21% 5%	2252
	21st to end	Day	24% 5% 15% 28% 23% 5%	2280
May	1st to 10th	Day	16% 4 10% 30% 27% 13%	3893
	11th to 20th	Day	11% 2 10% 26% 36% 14%	4134
	21st to end	Day	17% 4 18% 27% 30% 4	4814
June	1st to 10th	Day	21% 6% 15% 25% 25% 9%	2660
	11th to 20th	Day	10% 3 11% 24% 35% 16%	2660
	21st to end	Day	10% 2 7% 17% 31% 33%	2660
July	1st to 10th	Day	5% 1 4 10% 15% 65%	4332
	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	18% 3 8% 17% 28% 26%	1900
	11th to 20th	Day	20% 5% 10% 20% 23% 23%	1900
	21st to end	Day	22% 4 9% 21% 24% 19%	2090
November	1st to 10th	Day	29% 6% 10% 22% 21% 12%	1857
	11th to 20th	Day	32% 6% 11% 21% 22% 8%	1710
	21st to end	Day	31% 4 10% 17% 30% 9%	1573
December	1st to 10th	Day	37% 7% 11% 18% 16% 10%	1520
	11th to 20th	Day	54% 6% 10% 13% 13% 5%	1928
	21st to end	Day	48% 6% 13% 19% 11% 3	2794
January	1st to 10th	Night	41% 7% 17% 22% 10% 3	4213
	11th to 20th	Night	53% 6% 14% 11% 9% 7%	4110
	21st to end	Night	54% 6% 10% 11% 12% 7%	4521
February	1st to 10th	Night	37% 4 9% 19% 19% 12%	2470
	11th to 20th	Night	37% 5% 11% 19% 22% 7%	2310
	21st to end	Night	34% 5% 12% 29% 16% 4	1884
March	1st to 10th	Night	49% 7% 15% 13% 13% 3	3475
	11th to 20th	Night	32% 5% 11% 21% 21% 10%	3490
	21st to end	Night	49% 7% 13% 17% 10% 3	3772
April	1st to 10th	Night	35% 6% 16% 22% 17% 5%	1900
	11th to 20th	Night	30% 6% 13% 25% 18% 7%	1879
	21st to end	Night	21% 6% 15% 31% 21% 7%	1900
May	1st to 10th	Night	19% 5% 13% 25% 25% 13%	3066
	11th to 20th	Night	13% 3 12% 29% 24% 19%	2754
	21st to end	Night	15% 5% 20% 26% 30% 5%	2762
June	1st to 10th	Night	19% 7% 13% 26% 26% 9%	1520
	11th to 20th	Night	10% 3 11% 23% 33% 21%	1520
	21st to end	Night	12% 1 6% 18% 30% 33%	1520
July	1st to 10th	Night	5% 1 3 10% 18% 63%	2515
	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	17% 4 9% 18% 25% 28%	2280
	11th to 20th	Night	22% 4 8% 20% 22% 24%	2280
	21st to end	Night	22% 4 9% 20% 25% 21%	2508
November	1st to 10th	Night	26% 5% 12% 24% 19% 13%	2266
	11th to 20th	Night	30% 6% 11% 24% 21% 9%	2470
	21st to end	Night	27% 4 11% 21% 23% 13%	2603
December	1st to 10th	Night	37% 7% 9% 18% 17% 12%	2660
	11th to 20th	Night	53% 5% 10% 14% 15% 4	3276
	21st to end	Night	48% 7% 15% 16% 11% 4	4862

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% 25% 23% 14% 2	298
	11th to 20th	Day	14% 11% 27% 30% 14% 4	370
	21st to end	Day	14% 11% 28% 27% 17% 4	407
February	1st to 10th	Day	13% 19% 25% 24% 14% 5%	370
	11th to 20th	Day	14% 20% 30% 24% 11% 2	410
	21st to end	Day	12% 16% 31% 28% 12% 2	382
March	1st to 10th	Day	16% 18% 33% 24% 9% 1	460
	11th to 20th	Day	15% 19% 27% 21% 14% 3	460
	21st to end	Day	11% 19% 36% 24% 8% 2	506
April	1st to 10th	Day	12% 16% 34% 24% 11% 3	460
	11th to 20th	Day	12% 18% 29% 26% 12% 4	452
	21st to end	Day	5% 20% 30% 28% 11% 6%	460
May	1st to 10th	Day	6% 13% 35% 29% 13% 4	463
	11th to 20th	Day	7% 13% 32% 30% 14% 5%	488
	21st to end	Day	5% 12% 29% 32% 16% 6%	529
June	1st to 10th	Day	4 10% 30% 31% 18% 6%	490
	11th to 20th	Day	2 9% 31% 35% 19% 5%	490
	21st to end	Day	1 4 25% 39% 24% 7%	490
July	1st to 10th	Day	2 2 17% 43% 30% 7%	486
	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% 8%	441
	11th to 20th	Day	4 9% 19% 31% 26% 11%	270
	21st to end	Day	6% 8% 24% 31% 25% 6%	205
December	1st to 10th	Day	7% 13% 23% 26% 23% 8%	180
	11th to 20th	Day	11% 17% 22% 21% 21% 9%	180
	21st to end	Day	16% 18% 22% 25% 15% 4	198
January	1st to 10th	Night	17% 7% 18% 31% 22% 5%	760
	11th to 20th	Night	14% 8% 15% 34% 23% 7%	760
	21st to end	Night	16% 7% 15% 34% 23% 6%	836
February	1st to 10th	Night	13% 8% 17% 35% 22% 5%	760
	11th to 20th	Night	13% 9% 19% 33% 21% 5%	744
	21st to end	Night	12% 10% 22% 35% 19% 3	628
March	1st to 10th	Night	13% 12% 21% 31% 20% 3	760
	11th to 20th	Night	14% 9% 17% 33% 23% 4	760
	21st to end	Night	11% 10% 24% 36% 18% 2	836
April	1st to 10th	Night	10% 9% 23% 35% 21% 3	760
	11th to 20th	Night	9% 12% 20% 33% 20% 5%	751
	21st to end	Night	5% 7% 27% 32% 22% 6%	760
May	1st to 10th	Night	5% 4 26% 36% 25% 5%	698
	11th to 20th	Night	4 3 17% 37% 33% 6%	520
	21st to end	Night	4 4 22% 37% 29% 3	411
June	1st to 10th	Night	3 4 19% 43% 28% 3	380
	11th to 20th	Night	1 3 16% 47% 32% 1	380
	21st to end	Night	1 1 14% 39% 43% 3	380
July	1st to 10th	Night	1 1 10% 35% 51% 3	372
	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 16% 31% 33% 9%	748
	11th to 20th	Night	9% 3 14% 35% 31% 7%	760
	21st to end	Night	9% 4 15% 39% 26% 7%	760
December	1st to 10th	Night	11% 6% 19% 34% 25% 6%	760
	11th to 20th	Night	18% 5% 20% 32% 21% 4	760
	21st to end	Night	15% 11% 22% 31% 19% 3	836

Page generated at 2024-10-31 00:39:25. Version ac07

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