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 2025-04-26.

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% 62%	117716
	Night	7% 4 2 5% 3 3 3 3 5% 7% 57%	119771
Transparency	Day	23% 6% 9% 19% 23% 20%	117473
	Night	26% 7% 12% 18% 20% 17%	119602
Seeing	Day	7% 10% 28% 34% 17% 3	15914
	Night	8% 6% 21% 39% 23% 3	25363

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 3 5% 4 5% 8% 43%	3823
	11th to 20th	Day	9% 8% 4 7% 4 4 3 2 3 4 51%	4100
	21st to end	Day	14% 7% 4 8% 4 4 4 3 5% 6% 41%	4510
February	1st to 10th	Day	13% 9% 3 5% 4 4 4 4 5% 6% 45%	1800
	11th to 20th	Day	11% 6% 3 7% 5% 5% 5% 4 6% 8% 42%	1844
	21st to end	Day	10% 6% 2 6% 5% 4 4 3 4 6% 49%	1650
March	1st to 10th	Day	9% 7% 3 9% 6% 5% 5% 4 6% 8% 37%	4856
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 8% 51%	5000
	21st to end	Day	7% 6% 2 7% 5% 5% 6% 5% 7% 10% 39%	5573
April	1st to 10th	Day	11% 6% 2 6% 4 4 4 3 6% 8% 46%	2400
	11th to 20th	Day	7% 4 1 5% 3 3 4 4 4 7% 58%	2369
	21st to end	Day	3 4 2 4 3 2 3 3 6% 9% 62%	2342
May	1st to 10th	Day	5% 2 1 2 1 2 2 2 4 4 75%	3938
	11th to 20th	Day	2 1 1 1 2 1 1 2 4 6% 79%	4208
	21st to end	Day	2 1 1 2 2 2 2 2 4 6% 77%	4813
June	1st to 10th	Day	3 3 1 3 2 3 4 3 7% 8% 64%	2660
	11th to 20th	Day	1 1 1 1 1 2 3 6% 7% 77%	2660
	21st to end	Day	1 1 1 2 2 2 2 2 3 6% 79%	2660
July	1st to 10th	Day	1 1 1 1 1 3 4 88%	4420
	11th to 20th	Day	1 1 1 1 2 2 4 6% 82%	4182
	21st to end	Day	1 1 1 1 1 2 4 6% 84%	4455
August	1st to 10th	Day	1 1 2 2 2 2 3 6% 8% 74%	4091
	11th to 20th	Day	1 1 1 2 2 3 5% 85%	3827
	21st to end	Day	1 1 1 1 2 2 2 3 6% 82%	4180
September	1st to 10th	Day	1 1 1 1 1 1 2 2 3 4 83%	2280
	11th to 20th	Day	2 2 1 3 3 3 3 3 5% 8% 69%	2280
	21st to end	Day	2 1 1 2 2 2 2 2 3 5% 79%	2019
October	1st to 10th	Day	1 2 1 3 3 2 2 2 4 6% 74%	3180
	11th to 20th	Day	3 2 1 3 2 2 3 3 4 7% 70%	3161
	21st to end	Day	3 3 1 3 3 3 4 3 6% 9% 63%	3448
November	1st to 10th	Day	6% 5% 2 4 3 3 3 3 6% 7% 58%	1903
	11th to 20th	Day	8% 6% 3 7% 4 5% 5% 3 6% 7% 47%	1800
	21st to end	Day	10% 6% 2 4 2 4 2 4 6% 6% 53%	1632
December	1st to 10th	Day	6% 5% 2 4 3 3 3 4 6% 11% 53%	2620
	11th to 20th	Day	14% 11% 4 9% 8% 6% 7% 5% 7% 6% 24%	3028
	21st to end	Day	13% 10% 4 10% 7% 5% 6% 4 5% 7% 29%	4004
January	1st to 10th	Night	8% 6% 3 5% 3 3 4 4 6% 9% 48%	6113
	11th to 20th	Night	10% 7% 3 7% 4 4 4 3 4 6% 49%	5880
	21st to end	Night	12% 8% 3 7% 4 4 4 3 5% 6% 43%	6468
February	1st to 10th	Night	12% 7% 3 5% 5% 4 3 3 4 6% 49%	2600
	11th to 20th	Night	10% 5% 2 7% 5% 4 4 4 5% 8% 46%	2442
	21st to end	Night	9% 5% 2 4 4 4 4 3 5% 7% 53%	1980
March	1st to 10th	Night	9% 8% 3 8% 4 4 5% 4 6% 8% 41%	5036
	11th to 20th	Night	11% 5% 2 5% 4 4 3 3 4 6% 53%	5000
	21st to end	Night	10% 7% 3 7% 4 4 5% 4 5% 7% 44%	5389
April	1st to 10th	Night	6% 6% 1 6% 5% 5% 4 3 6% 7% 51%	2000
	11th to 20th	Night	5% 5% 2 4 4 3 3 3 5% 8% 58%	1979
	21st to end	Night	3 2 1 3 3 3 3 3 6% 9% 63%	1954
May	1st to 10th	Night	3 3 1 3 2 2 2 3 4 5% 74%	3034
	11th to 20th	Night	1 1 1 2 2 2 2 2 5% 7% 75%	2689
	21st to end	Night	2 1 2 2 2 2 3 4 6% 76%	2762
June	1st to 10th	Night	3 2 1 4 3 3 3 5% 6% 10% 61%	1520
	11th to 20th	Night	1 1 2 2 2 2 3 4 8% 77%	1520
	21st to end	Night	1 1 1 2 2 2 2 3 4 7% 75%	1520
July	1st to 10th	Night	1 1 1 1 3 6% 85%	2540
	11th to 20th	Night	1 1 2 1 2 2 2 5% 9% 76%	2782
	21st to end	Night	1 1 1 2 3 4 9% 10% 69%	3109
August	1st to 10th	Night	2 1 1 1 2 2 3 4 7% 10% 66%	2848
	11th to 20th	Night	1 1 1 2 3 5% 8% 78%	3004
	21st to end	Night	2 1 1 2 1 1 3 2 5% 8% 76%	3498
September	1st to 10th	Night	1 1 2 2 2 3 4 8% 76%	1900
	11th to 20th	Night	2 2 1 3 2 3 3 4 7% 10% 63%	1900
	21st to end	Night	1 1 1 2 3 3 2 1 4 7% 75%	2158
October	1st to 10th	Night	2 1 1 3 2 2 2 2 4 7% 73%	3800
	11th to 20th	Night	2 3 1 4 2 2 3 4 5% 9% 65%	3777
	21st to end	Night	3 2 1 4 3 4 4 4 6% 7% 63%	4121
November	1st to 10th	Night	5% 4 2 5% 4 4 4 3 6% 9% 56%	2395
	11th to 20th	Night	8% 6% 2 4 3 4 3 4 6% 8% 52%	2600
	21st to end	Night	8% 5% 2 3 4 3 3 3 5% 7% 56%	2765
December	1st to 10th	Night	6% 4 2 4 3 3 4 3 4 7% 58%	4560
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 5% 8% 9% 27%	5176
	21st to end	Night	15% 9% 3 9% 5% 5% 5% 4 5% 6% 32%	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	36% 7% 18% 19% 15% 4	3823
	11th to 20th	Day	37% 5% 8% 15% 22% 14%	4100
	21st to end	Day	43% 6% 7% 13% 16% 15%	4510
February	1st to 10th	Day	40% 5% 8% 19% 19% 8%	1800
	11th to 20th	Day	40% 7% 9% 18% 20% 6%	1853
	21st to end	Day	36% 6% 8% 24% 19% 7%	1650
March	1st to 10th	Day	43% 8% 9% 19% 19% 2	4856
	11th to 20th	Day	31% 7% 15% 29% 10% 8%	5000
	21st to end	Day	36% 10% 16% 26% 10% 2	5573
April	1st to 10th	Day	37% 6% 13% 20% 20% 4	2400
	11th to 20th	Day	27% 6% 14% 24% 22% 7%	2372
	21st to end	Day	21% 6% 13% 30% 26% 4	2342
May	1st to 10th	Day	14% 5% 10% 30% 25% 16%	3938
	11th to 20th	Day	10% 7% 7% 24% 40% 13%	4211
	21st to end	Day	12% 11% 9% 31% 33% 4	4814
June	1st to 10th	Day	19% 8% 11% 26% 27% 9%	2660
	11th to 20th	Day	8% 7% 9% 21% 36% 19%	2660
	21st to end	Day	10% 5% 7% 12% 31% 36%	2660
July	1st to 10th	Day	4 3 5% 5% 14% 70%	4351
	11th to 20th	Day	8% 3 6% 7% 13% 63%	4110
	21st to end	Day	4 3 5% 6% 26% 56%	4315
August	1st to 10th	Day	11% 3 6% 9% 24% 48%	4076
	11th to 20th	Day	5% 3 4 9% 40% 40%	3865
	21st to end	Day	7% 4 6% 11% 35% 37%	4179
September	1st to 10th	Day	8% 4 5% 8% 31% 44%	2280
	11th to 20th	Day	16% 7% 8% 20% 32% 17%	2280
	21st to end	Day	11% 4 6% 20% 35% 25%	2019
October	1st to 10th	Day	13% 3 4 12% 27% 41%	3180
	11th to 20th	Day	15% 4 8% 24% 31% 19%	3161
	21st to end	Day	19% 4 9% 22% 32% 14%	3448
November	1st to 10th	Day	26% 5% 10% 27% 20% 12%	1903
	11th to 20th	Day	35% 6% 9% 23% 22% 6%	1800
	21st to end	Day	31% 5% 10% 19% 24% 10%	1632
December	1st to 10th	Day	25% 5% 7% 31% 26% 6%	2620
	11th to 20th	Day	53% 7% 10% 14% 13% 2	3028
	21st to end	Day	51% 9% 12% 17% 9% 2	4004
January	1st to 10th	Night	31% 9% 20% 22% 15% 4	6113
	11th to 20th	Night	36% 6% 10% 14% 22% 12%	5880
	21st to end	Night	40% 6% 10% 11% 19% 14%	6468
February	1st to 10th	Night	38% 4 8% 18% 20% 11%	2600
	11th to 20th	Night	37% 6% 11% 18% 20% 8%	2444
	21st to end	Night	31% 6% 11% 27% 19% 6%	1980
March	1st to 10th	Night	41% 11% 10% 18% 18% 3	5036
	11th to 20th	Night	32% 9% 13% 23% 15% 8%	5000
	21st to end	Night	36% 12% 14% 23% 12% 2	5389
April	1st to 10th	Night	31% 8% 15% 21% 20% 5%	2000
	11th to 20th	Night	26% 8% 13% 25% 19% 9%	1979
	21st to end	Night	19% 8% 13% 30% 25% 6%	1954
May	1st to 10th	Night	13% 8% 11% 25% 28% 14%	3034
	11th to 20th	Night	11% 11% 22% 15% 27% 14%	2689
	21st to end	Night	11% 11% 17% 28% 29% 3	2762
June	1st to 10th	Night	20% 11% 13% 21% 25% 10%	1520
	11th to 20th	Night	9% 8% 10% 19% 33% 20%	1520
	21st to end	Night	10% 5% 15% 12% 26% 33%	1520
July	1st to 10th	Night	5% 2 9% 4 13% 66%	2500
	11th to 20th	Night	9% 4 10% 9% 13% 56%	2731
	21st to end	Night	9% 4 10% 13% 22% 42%	3011
August	1st to 10th	Night	15% 5% 10% 7% 20% 43%	2839
	11th to 20th	Night	6% 5% 9% 9% 32% 38%	3031
	21st to end	Night	10% 5% 13% 10% 28% 35%	3498
September	1st to 10th	Night	9% 4 11% 8% 23% 44%	1900
	11th to 20th	Night	19% 8% 12% 19% 24% 17%	1900
	21st to end	Night	12% 5% 7% 19% 30% 27%	2158
October	1st to 10th	Night	13% 4 6% 10% 18% 48%	3800
	11th to 20th	Night	17% 4 11% 24% 20% 23%	3777
	21st to end	Night	20% 5% 8% 22% 25% 21%	4121
November	1st to 10th	Night	25% 8% 11% 25% 17% 14%	2395
	11th to 20th	Night	30% 5% 11% 24% 21% 8%	2600
	21st to end	Night	29% 5% 10% 19% 25% 12%	2765
December	1st to 10th	Night	27% 4 7% 25% 26% 11%	4560
	11th to 20th	Night	50% 6% 10% 16% 16% 2	5176
	21st to end	Night	50% 9% 13% 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	24% 13% 27% 21% 14% 2	280
	11th to 20th	Day	16% 11% 22% 30% 17% 4	400
	21st to end	Day	12% 11% 24% 30% 21% 3	440
February	1st to 10th	Day	18% 14% 20% 27% 18% 5%	400
	11th to 20th	Day	14% 18% 28% 22% 16% 3	440
	21st to end	Day	12% 14% 25% 33% 14% 2	414
March	1st to 10th	Day	17% 15% 31% 24% 10% 3	500
	11th to 20th	Day	18% 16% 26% 21% 16% 4	500
	21st to end	Day	12% 19% 33% 24% 11% 1	550
April	1st to 10th	Day	13% 16% 31% 25% 12% 3	500
	11th to 20th	Day	11% 17% 32% 25% 10% 5%	492
	21st to end	Day	5% 20% 29% 28% 13% 5%	481
May	1st to 10th	Day	8% 16% 32% 32% 11% 1	463
	11th to 20th	Day	5% 13% 36% 32% 13% 1	488
	21st to end	Day	4 11% 31% 38% 14% 2	529
June	1st to 10th	Day	4 13% 33% 33% 14% 2	490
	11th to 20th	Day	2 11% 36% 38% 11% 1	490
	21st to end	Day	1 6% 35% 43% 14% 2	490
July	1st to 10th	Day	4 30% 50% 14% 1	486
	11th to 20th	Day	2 23% 55% 19% 1	484
	21st to end	Day	1 1 29% 49% 18% 2	531
August	1st to 10th	Day	1 5% 29% 49% 14% 2	487
	11th to 20th	Day	1 4 29% 44% 18% 3	477
	21st to end	Day	1 2 25% 47% 21% 4	539
September	1st to 10th	Day	1 3 23% 44% 23% 6%	490
	11th to 20th	Day	3 8% 29% 40% 14% 7%	490
	21st to end	Day	2 7% 25% 33% 27% 6%	490
October	1st to 10th	Day	3 6% 27% 32% 24% 8%	490
	11th to 20th	Day	5% 8% 22% 31% 26% 8%	490
	21st to end	Day	7% 7% 24% 29% 26% 6%	539
November	1st to 10th	Day	9% 10% 21% 23% 30% 8%	439
	11th to 20th	Day	7% 11% 17% 36% 24% 5%	300
	21st to end	Day	13% 6% 26% 31% 24% 1	215
December	1st to 10th	Day	12% 12% 24% 28% 20% 6%	200
	11th to 20th	Day	19% 10% 23% 27% 20% 2	200
	21st to end	Day	18% 11% 27% 28% 15% 1	220
January	1st to 10th	Night	17% 6% 16% 33% 23% 5%	800
	11th to 20th	Night	13% 6% 18% 36% 23% 4	800
	21st to end	Night	16% 5% 16% 36% 24% 4	880
February	1st to 10th	Night	15% 8% 17% 37% 20% 3	800
	11th to 20th	Night	12% 9% 20% 34% 21% 4	784
	21st to end	Night	12% 10% 21% 35% 20% 2	660
March	1st to 10th	Night	17% 10% 22% 31% 19% 3	800
	11th to 20th	Night	15% 9% 23% 32% 19% 3	800
	21st to end	Night	12% 9% 28% 34% 15% 1	880
April	1st to 10th	Night	9% 9% 28% 37% 16% 2	800
	11th to 20th	Night	7% 14% 29% 32% 16% 3	791
	21st to end	Night	4 9% 32% 35% 18% 2	782
May	1st to 10th	Night	5% 7% 29% 41% 17% 2	679
	11th to 20th	Night	2 5% 24% 48% 19% 2	482
	21st to end	Night	3 8% 34% 41% 14% 1	411
June	1st to 10th	Night	3 4 32% 48% 12% 1	380
	11th to 20th	Night	1 5% 28% 53% 13%	380
	21st to end	Night	3 31% 45% 20%	380
July	1st to 10th	Night	1 22% 56% 19%	370
	11th to 20th	Night	1 1 21% 51% 23% 3	503
	21st to end	Night	1 2 21% 46% 28% 1	624
August	1st to 10th	Night	3 3 24% 44% 23% 2	567
	11th to 20th	Night	2 25% 48% 23% 2	678
	21st to end	Night	1 1 20% 50% 25% 3	836
September	1st to 10th	Night	1 1 18% 47% 29% 4	760
	11th to 20th	Night	3 3 23% 44% 24% 3	760
	21st to end	Night	1 3 18% 40% 32% 5%	760
October	1st to 10th	Night	4 3 18% 39% 31% 6%	760
	11th to 20th	Night	5% 6% 15% 37% 31% 6%	760
	21st to end	Night	6% 4 14% 35% 32% 9%	836
November	1st to 10th	Night	6% 6% 17% 31% 32% 7%	780
	11th to 20th	Night	10% 4 17% 36% 28% 6%	800
	21st to end	Night	10% 7% 14% 38% 27% 6%	800
December	1st to 10th	Night	12% 8% 15% 37% 23% 5%	800
	11th to 20th	Night	20% 7% 14% 36% 22% 3	800
	21st to end	Night	18% 8% 21% 32% 19% 1	880

Page generated at 2025-04-23 06:25:07. Version ac07

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