Diablo Valley College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Diablo Valley College clear sky chart from 2005-11-06 to 2026-08-30.

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% 7% 65%	152429
	Night	6% 4 2 4 3 3 3 3 5% 7% 59%	147269
Transparency	Day	21% 5% 8% 19% 25% 22%	147348
	Night	25% 6% 11% 18% 22% 18%	143338
Seeing	Day	7% 10% 28% 34% 17% 4	17732
	Night	8% 6% 21% 39% 22% 3	27008

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	11% 7% 3 6% 4 4 5% 4 6% 8% 41%	4962
	11th to 20th	Day	7% 6% 3 6% 4 3 3 3 5% 7% 54%	5340
	21st to end	Day	11% 7% 4 9% 5% 4 4 4 6% 7% 39%	5843
February	1st to 10th	Day	12% 8% 3 5% 4 4 4 4 5% 7% 45%	1890
	11th to 20th	Day	11% 6% 2 8% 5% 5% 4 4 6% 8% 40%	1939
	21st to end	Day	10% 6% 3 6% 5% 4 5% 3 5% 7% 47%	1730
March	1st to 10th	Day	8% 7% 3 9% 6% 5% 5% 4 6% 8% 38%	4963
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 8% 51%	5110
	21st to end	Day	7% 6% 2 6% 5% 5% 6% 5% 7% 11% 39%	5696
April	1st to 10th	Day	10% 6% 2 6% 4 4 4 3 6% 8% 46%	2520
	11th to 20th	Day	7% 4 1 5% 3 3 4 4 4 7% 57%	2489
	21st to end	Day	3 5% 2 4 3 3 4 3 6% 9% 61%	2520
May	1st to 10th	Day	3 3 1 3 2 2 2 2 4 5% 73%	7299
	11th to 20th	Day	1 1 1 1 1 1 2 3 6% 82%	7836
	21st to end	Day	1 1 2 1 2 2 2 4 6% 79%	8992
June	1st to 10th	Day	3 3 1 3 2 3 3 3 6% 8% 66%	2940
	11th to 20th	Day	1 1 1 1 1 2 2 5% 7% 78%	2940
	21st to end	Day	1 1 1 2 2 1 1 2 3 6% 80%	2940
July	1st to 10th	Day	1 1 1 1 1 3 5% 87%	8219
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 6% 77%	7787
	21st to end	Day	1 1 1 2 2 4 6% 82%	8304
August	1st to 10th	Day	1 1 1 1 1 2 2 4 6% 81%	5907
	11th to 20th	Day	1 1 1 1 2 3 5% 87%	5643
	21st to end	Day	1 1 1 1 1 2 2 3 7% 81%	6093
September	1st to 10th	Day	1 1 1 1 2 1 2 2 3 5% 81%	2400
	11th to 20th	Day	2 2 1 3 3 2 3 3 5% 8% 69%	2400
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 77%	2125
October	1st to 10th	Day	1 2 1 3 3 2 2 2 4 6% 74%	3280
	11th to 20th	Day	3 2 1 3 2 2 3 2 4 7% 70%	3261
	21st to end	Day	3 3 1 4 3 3 4 3 6% 9% 63%	3558
November	1st to 10th	Day	6% 5% 2 4 3 3 4 3 6% 7% 58%	2000
	11th to 20th	Day	8% 6% 3 7% 5% 6% 5% 4 6% 7% 45%	1890
	21st to end	Day	10% 6% 2 5% 3 4 3 4 6% 6% 53%	1713
December	1st to 10th	Day	6% 5% 2 4 3 3 3 4 7% 11% 53%	2700
	11th to 20th	Day	14% 11% 4 9% 7% 6% 7% 5% 7% 6% 23%	3108
	21st to end	Day	13% 10% 4 10% 7% 5% 6% 4 5% 7% 29%	4092
January	1st to 10th	Night	11% 7% 3 6% 4 4 4 4 6% 8% 44%	7927
	11th to 20th	Night	8% 5% 2 6% 3 3 3 3 5% 8% 54%	7650
	21st to end	Night	10% 8% 3 7% 5% 4 4 3 5% 8% 41%	8371
February	1st to 10th	Night	12% 6% 3 5% 5% 4 3 3 4 6% 49%	2730
	11th to 20th	Night	10% 5% 2 7% 5% 5% 5% 4 5% 8% 45%	2567
	21st to end	Night	9% 6% 2 5% 4 4 4 3 5% 7% 52%	2076
March	1st to 10th	Night	9% 8% 3 8% 4 4 5% 4 6% 7% 42%	5149
	11th to 20th	Night	10% 5% 2 5% 4 4 3 3 4 6% 54%	5110
	21st to end	Night	10% 7% 3 6% 4 4 5% 4 5% 8% 45%	5508
April	1st to 10th	Night	6% 6% 1 6% 5% 4 4 3 6% 8% 52%	2100
	11th to 20th	Night	5% 5% 2 5% 4 3 3 3 5% 8% 57%	2079
	21st to end	Night	3 2 1 3 3 3 4 3 7% 9% 62%	2100
May	1st to 10th	Night	3 3 1 3 2 3 3 3 4 5% 70%	5649
	11th to 20th	Night	1 1 2 2 2 2 2 4 6% 78%	5039
	21st to end	Night	1 1 1 1 2 2 3 4 6% 77%	5181
June	1st to 10th	Night	3 2 1 4 3 3 2 5% 6% 9% 63%	1680
	11th to 20th	Night	1 1 2 2 2 2 3 4 7% 77%	1680
	21st to end	Night	1 1 2 2 2 2 3 4 7% 75%	1680
July	1st to 10th	Night	1 1 1 1 2 5% 87%	4740
	11th to 20th	Night	2 1 1 2 2 2 3 2 4 8% 73%	5178
	21st to end	Night	1 2 2 2 3 4 8% 9% 70%	5806
August	1st to 10th	Night	2 1 1 1 2 2 2 3 5% 7% 76%	4125
	11th to 20th	Night	1 1 1 2 3 4 6% 82%	4424
	21st to end	Night	1 1 1 1 2 3 3 5% 7% 76%	5105
September	1st to 10th	Night	1 1 1 2 2 2 3 5% 8% 76%	2000
	11th to 20th	Night	2 3 1 3 2 3 3 4 7% 10% 63%	2000
	21st to end	Night	1 1 1 3 2 3 2 2 4 6% 75%	2272
October	1st to 10th	Night	2 1 1 3 2 2 2 2 5% 7% 73%	3920
	11th to 20th	Night	2 3 1 4 2 2 3 3 5% 8% 65%	3897
	21st to end	Night	3 2 1 4 3 4 4 4 6% 8% 62%	4253
November	1st to 10th	Night	4 4 2 5% 4 3 4 3 6% 9% 56%	2518
	11th to 20th	Night	9% 6% 2 5% 3 4 3 4 6% 8% 51%	2730
	21st to end	Night	8% 4 2 4 4 3 3 3 5% 8% 55%	2903
December	1st to 10th	Night	6% 4 2 5% 3 3 4 3 5% 7% 57%	4700
	11th to 20th	Night	15% 9% 3 8% 5% 5% 6% 5% 8% 9% 27%	5316
	21st to end	Night	15% 9% 3 9% 5% 5% 5% 4 5% 6% 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	37% 8% 17% 21% 13% 3	4963
	11th to 20th	Day	30% 4 7% 15% 27% 18%	5340
	21st to end	Day	42% 6% 7% 17% 15% 12%	5843
February	1st to 10th	Day	39% 5% 8% 20% 20% 8%	1890
	11th to 20th	Day	41% 7% 9% 18% 20% 5%	1948
	21st to end	Day	37% 6% 9% 24% 18% 7%	1730
March	1st to 10th	Day	42% 8% 9% 19% 20% 2	4963
	11th to 20th	Day	30% 6% 15% 29% 11% 8%	5110
	21st to end	Day	36% 10% 16% 26% 10% 2	5696
April	1st to 10th	Day	37% 6% 13% 20% 20% 5%	2520
	11th to 20th	Day	28% 7% 14% 24% 21% 7%	2492
	21st to end	Day	22% 6% 12% 30% 26% 4	2520
May	1st to 10th	Day	16% 3 7% 24% 33% 16%	7299
	11th to 20th	Day	7% 4 5% 26% 44% 14%	7839
	21st to end	Day	9% 7% 8% 28% 41% 7%	8994
June	1st to 10th	Day	18% 7% 10% 26% 29% 10%	2940
	11th to 20th	Day	7% 6% 8% 19% 37% 22%	2940
	21st to end	Day	9% 4 6% 12% 32% 37%	2940
July	1st to 10th	Day	4 2 3 7% 23% 61%	8151
	11th to 20th	Day	12% 2 4 6% 9% 67%	7715
	21st to end	Day	5% 3 6% 12% 25% 49%	8119
August	1st to 10th	Day	10% 3 6% 9% 25% 47%	4336
	11th to 20th	Day	6% 3 5% 10% 39% 39%	4113
	21st to end	Day	8% 4 6% 11% 35% 36%	4420
September	1st to 10th	Day	8% 4 6% 9% 31% 42%	2400
	11th to 20th	Day	16% 7% 8% 20% 33% 17%	2400
	21st to end	Day	12% 4 6% 19% 35% 25%	2125
October	1st to 10th	Day	13% 3 4 13% 27% 40%	3280
	11th to 20th	Day	15% 4 8% 24% 31% 18%	3261
	21st to end	Day	19% 4 9% 23% 31% 14%	3558
November	1st to 10th	Day	25% 5% 10% 27% 21% 11%	2000
	11th to 20th	Day	36% 7% 10% 22% 21% 5%	1890
	21st to end	Day	31% 5% 10% 20% 24% 10%	1713
December	1st to 10th	Day	25% 5% 7% 32% 26% 6%	2700
	11th to 20th	Day	53% 7% 10% 14% 13% 3	3108
	21st to end	Day	51% 9% 12% 17% 10% 2	4092
January	1st to 10th	Night	35% 9% 18% 22% 13% 3	7927
	11th to 20th	Night	29% 5% 8% 14% 28% 16%	7650
	21st to end	Night	40% 5% 10% 15% 18% 12%	8371
February	1st to 10th	Night	37% 5% 8% 19% 20% 11%	2730
	11th to 20th	Night	38% 6% 11% 19% 19% 8%	2569
	21st to end	Night	32% 6% 11% 27% 18% 5%	2076
March	1st to 10th	Night	40% 10% 10% 19% 18% 3	5149
	11th to 20th	Night	31% 9% 13% 23% 16% 8%	5110
	21st to end	Night	36% 12% 14% 23% 13% 3	5508
April	1st to 10th	Night	30% 8% 15% 21% 20% 6%	2100
	11th to 20th	Night	27% 8% 13% 25% 19% 8%	2079
	21st to end	Night	20% 8% 12% 30% 24% 6%	2100
May	1st to 10th	Night	17% 7% 8% 24% 28% 16%	5647
	11th to 20th	Night	10% 7% 14% 21% 32% 16%	5039
	21st to end	Night	10% 7% 14% 23% 38% 8%	5182
June	1st to 10th	Night	18% 10% 12% 21% 27% 12%	1680
	11th to 20th	Night	9% 7% 10% 17% 33% 24%	1680
	21st to end	Night	9% 4 14% 12% 26% 34%	1680
July	1st to 10th	Night	4 1 6% 7% 25% 56%	4700
	11th to 20th	Night	12% 2 6% 9% 11% 59%	5127
	21st to end	Night	8% 3 12% 15% 23% 39%	5679
August	1st to 10th	Night	14% 5% 9% 7% 22% 43%	3019
	11th to 20th	Night	7% 5% 10% 10% 31% 37%	3223
	21st to end	Night	10% 5% 13% 10% 29% 34%	3698
September	1st to 10th	Night	9% 4 12% 10% 23% 43%	2000
	11th to 20th	Night	19% 8% 12% 20% 24% 17%	2000
	21st to end	Night	13% 5% 7% 19% 29% 27%	2272
October	1st to 10th	Night	13% 4 7% 11% 18% 47%	3920
	11th to 20th	Night	18% 4 11% 25% 20% 22%	3897
	21st to end	Night	20% 5% 8% 21% 25% 21%	4253
November	1st to 10th	Night	24% 8% 11% 25% 18% 14%	2518
	11th to 20th	Night	32% 5% 11% 24% 20% 7%	2730
	21st to end	Night	29% 5% 9% 20% 24% 12%	2903
December	1st to 10th	Night	27% 4 7% 26% 26% 11%	4700
	11th to 20th	Night	49% 6% 10% 16% 16% 3	5316
	21st to end	Night	50% 9% 13% 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	22% 13% 29% 21% 13% 2	304
	11th to 20th	Day	15% 10% 21% 33% 17% 3	430
	21st to end	Day	12% 11% 25% 30% 20% 3	473
February	1st to 10th	Day	16% 13% 21% 27% 18% 4	430
	11th to 20th	Day	15% 18% 28% 23% 15% 2	475
	21st to end	Day	12% 14% 26% 33% 13% 2	446
March	1st to 10th	Day	16% 16% 32% 24% 10% 3	540
	11th to 20th	Day	17% 14% 24% 22% 19% 4	540
	21st to end	Day	12% 18% 32% 25% 12% 2	594
April	1st to 10th	Day	12% 16% 30% 25% 14% 4	540
	11th to 20th	Day	11% 17% 32% 26% 10% 5%	532
	21st to end	Day	5% 19% 28% 31% 12% 5%	540
May	1st to 10th	Day	7% 15% 31% 32% 13% 1	543
	11th to 20th	Day	5% 13% 35% 32% 13% 2	568
	21st to end	Day	4 10% 32% 38% 14% 2	617
June	1st to 10th	Day	4 12% 34% 34% 14% 2	570
	11th to 20th	Day	2 11% 36% 37% 12% 1	570
	21st to end	Day	1 7% 34% 43% 14% 2	570
July	1st to 10th	Day	4 33% 48% 14% 1	566
	11th to 20th	Day	1 3 24% 54% 18% 1	564
	21st to end	Day	1 1 30% 49% 17% 2	619
August	1st to 10th	Day	1 4 27% 48% 16% 4	567
	11th to 20th	Day	1 4 28% 45% 20% 3	557
	21st to end	Day	1 2 24% 47% 22% 3	620
September	1st to 10th	Day	1 4 24% 44% 23% 5%	530
	11th to 20th	Day	3 7% 27% 40% 15% 8%	530
	21st to end	Day	2 7% 26% 32% 27% 5%	530
October	1st to 10th	Day	3 6% 27% 31% 24% 8%	530
	11th to 20th	Day	5% 8% 22% 31% 27% 8%	530
	21st to end	Day	8% 8% 23% 29% 26% 6%	583
November	1st to 10th	Day	8% 10% 20% 23% 31% 8%	476
	11th to 20th	Day	8% 12% 18% 35% 23% 5%	330
	21st to end	Day	12% 6% 25% 33% 25% 1	236
December	1st to 10th	Day	11% 10% 24% 30% 20% 5%	220
	11th to 20th	Day	18% 10% 21% 27% 21% 2	220
	21st to end	Day	18% 12% 28% 26% 14% 1	242
January	1st to 10th	Night	17% 7% 16% 33% 22% 5%	840
	11th to 20th	Night	12% 5% 17% 36% 25% 4	840
	21st to end	Night	15% 5% 16% 37% 23% 3	924
February	1st to 10th	Night	15% 8% 17% 38% 20% 3	840
	11th to 20th	Night	13% 9% 21% 34% 20% 4	824
	21st to end	Night	11% 9% 22% 36% 20% 2	692
March	1st to 10th	Night	16% 9% 23% 31% 19% 2	840
	11th to 20th	Night	14% 9% 22% 33% 20% 3	840
	21st to end	Night	12% 9% 28% 35% 15% 1	924
April	1st to 10th	Night	8% 9% 28% 37% 16% 2	840
	11th to 20th	Night	7% 13% 29% 32% 16% 3	831
	21st to end	Night	5% 8% 32% 36% 18% 2	840
May	1st to 10th	Night	5% 7% 29% 41% 16% 2	750
	11th to 20th	Night	2 5% 26% 46% 18% 1	534
	21st to end	Night	3 7% 34% 41% 14% 1	455
June	1st to 10th	Night	3 4 34% 48% 11% 1	420
	11th to 20th	Night	1 4 32% 51% 12%	420
	21st to end	Night	4 33% 45% 19%	420
July	1st to 10th	Night	1 1 26% 54% 18%	410
	11th to 20th	Night	1 1 23% 51% 21% 3	557
	21st to end	Night	1 2 23% 46% 26% 1	690
August	1st to 10th	Night	3 3 24% 45% 23% 2	627
	11th to 20th	Night	2 25% 50% 21% 2	750
	21st to end	Night	1 1 21% 50% 24% 3	916
September	1st to 10th	Night	1 2 18% 47% 28% 4	800
	11th to 20th	Night	3 3 23% 44% 24% 4	800
	21st to end	Night	2 3 19% 40% 31% 5%	800
October	1st to 10th	Night	4 3 18% 39% 30% 5%	800
	11th to 20th	Night	6% 6% 16% 37% 30% 6%	800
	21st to end	Night	7% 4 14% 34% 32% 9%	880
November	1st to 10th	Night	6% 6% 16% 32% 33% 7%	820
	11th to 20th	Night	11% 4 16% 36% 27% 5%	840
	21st to end	Night	9% 6% 13% 38% 27% 5%	840
December	1st to 10th	Night	11% 7% 15% 38% 24% 5%	840
	11th to 20th	Night	20% 7% 13% 36% 22% 3	840
	21st to end	Night	19% 8% 21% 32% 18% 1	924

Page generated at 2026-08-27 01:07:16. Version ac07

Copyright © 2026 Attilla Danko.