Putah Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Putah Creek Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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	7% 3 1 3 2 2 3 2 4 5% 66%	112895
	Night	9% 4 1 3 3 3 3 2 4 5% 62%	114921
Transparency	Day	23% 2 7% 19% 25% 24%	112652
	Night	27% 2 7% 18% 23% 22%	114753
Seeing	Day	9% 11% 22% 29% 22% 7%	15667
	Night	10% 6% 14% 34% 30% 5%	25059

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% 6% 2 6% 4 4 4 3 5% 7% 48%	3793
	11th to 20th	Day	14% 6% 2 6% 3 3 3 2 2 4 55%	4100
	21st to end	Day	17% 6% 2 5% 3 3 3 2 4 6% 49%	4510
February	1st to 10th	Day	16% 5% 2 6% 4 4 3 3 5% 6% 46%	1800
	11th to 20th	Day	12% 7% 2 7% 4 4 3 3 6% 7% 45%	1807
	21st to end	Day	13% 7% 2 5% 4 3 3 3 5% 6% 49%	1650
March	1st to 10th	Day	15% 11% 3 8% 5% 5% 5% 4 6% 5% 32%	3492
	11th to 20th	Day	10% 5% 1 4 3 3 3 3 4 6% 58%	3503
	21st to end	Day	15% 8% 2 6% 4 5% 5% 4 5% 6% 39%	3875
April	1st to 10th	Day	12% 7% 2 5% 4 5% 5% 3 5% 7% 44%	2280
	11th to 20th	Day	8% 4 2 5% 3 4 4 4 5% 6% 56%	2249
	21st to end	Day	3 4 1 4 3 3 3 3 6% 8% 62%	2280
May	1st to 10th	Day	5% 3 1 2 2 2 2 2 3 5% 75%	4017
	11th to 20th	Day	2 1 2 1 2 2 2 3 5% 79%	4218
	21st to end	Day	2 1 1 2 2 2 2 2 4 5% 78%	4813
June	1st to 10th	Day	4 3 1 3 3 3 3 3 5% 7% 65%	2660
	11th to 20th	Day	1 1 1 1 2 3 2 3 5% 80%	2660
	21st to end	Day	2 1 2 1 2 2 2 3 85%	2660
July	1st to 10th	Day	1 1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	4215
	21st to end	Day	1 1 1 2 3 91%	4455
August	1st to 10th	Day	1 1 2 1 1 3 3 5% 5% 77%	4091
	11th to 20th	Day	1 1 1 1 2 3 89%	3886
	21st to end	Day	1 1 1 1 1 1 1 2 3 87%	4180
September	1st to 10th	Day	1 1 1 1 1 2 4 88%	2280
	11th to 20th	Day	2 1 1 2 2 2 2 2 4 6% 77%	2280
	21st to end	Day	2 1 1 2 1 1 2 2 4 4 79%	2006
October	1st to 10th	Day	2 2 1 3 2 2 2 2 3 5% 77%	3180
	11th to 20th	Day	4 2 1 3 2 2 3 2 3 5% 73%	3161
	21st to end	Day	5% 3 1 4 3 3 3 3 4 7% 64%	3448
November	1st to 10th	Day	8% 4 1 3 3 3 4 2 5% 7% 59%	1899
	11th to 20th	Day	11% 5% 2 5% 3 4 4 3 5% 7% 51%	1775
	21st to end	Day	10% 4 2 4 3 3 4 4 5% 6% 55%	1600
December	1st to 10th	Day	9% 4 1 3 3 2 4 3 5% 8% 57%	2620
	11th to 20th	Day	25% 9% 3 6% 3 3 4 3 6% 6% 32%	3028
	21st to end	Day	25% 8% 2 6% 3 4 4 4 6% 7% 31%	4004
January	1st to 10th	Night	10% 6% 2 4 3 3 4 3 6% 10% 50%	6157
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 3 4 54%	5880
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 5% 50%	6468
February	1st to 10th	Night	16% 6% 1 4 3 3 4 3 5% 5% 50%	2600
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 5% 7% 48%	2478
	21st to end	Night	10% 5% 1 4 4 3 3 3 4 6% 55%	1980
March	1st to 10th	Night	14% 11% 3 5% 5% 5% 4 4 5% 6% 38%	3587
	11th to 20th	Night	11% 5% 1 3 3 2 3 2 3 4 62%	3505
	21st to end	Night	12% 9% 3 7% 5% 5% 4 3 5% 6% 42%	3777
April	1st to 10th	Night	7% 5% 3 6% 4 4 4 3 4 6% 53%	1900
	11th to 20th	Night	7% 5% 1 3 3 3 2 2 5% 7% 61%	1879
	21st to end	Night	4 3 1 2 3 2 3 3 5% 8% 67%	1900
May	1st to 10th	Night	4 2 1 2 1 2 2 2 3 5% 77%	2938
	11th to 20th	Night	1 1 1 1 1 2 2 2 3 5% 81%	2677
	21st to end	Night	2 1 1 1 1 2 2 4 4 82%	2762
June	1st to 10th	Night	4 3 1 3 2 2 3 3 4 6% 69%	1520
	11th to 20th	Night	1 1 1 1 1 2 1 2 2 5% 83%	1520
	21st to end	Night	1 1 1 2 2 2 2 2 2 3 83%	1520
July	1st to 10th	Night	1 1 1 1 1 94%	2540
	11th to 20th	Night	1 1 1 1 1 1 1 2 3 87%	2748
	21st to end	Night	1 1 1 1 2 1 2 4 88%	3109
August	1st to 10th	Night	2 1 1 2 2 2 2 3 3 5% 78%	2848
	11th to 20th	Night	1 1 1 1 1 2 3 90%	2956
	21st to end	Night	2 1 2 1 1 1 1 1 3 87%	3498
September	1st to 10th	Night	1 1 1 1 1 3 4 88%	1900
	11th to 20th	Night	2 2 1 2 1 2 3 2 3 5% 76%	1900
	21st to end	Night	1 1 2 1 2 2 2 3 5% 81%	2170
October	1st to 10th	Night	2 2 1 3 2 2 2 2 3 4 79%	3800
	11th to 20th	Night	3 2 1 3 2 2 3 2 4 5% 74%	3777
	21st to end	Night	6% 2 1 3 2 3 3 3 5% 7% 66%	4121
November	1st to 10th	Night	7% 4 1 4 4 3 3 3 6% 7% 58%	2398
	11th to 20th	Night	9% 5% 2 4 3 3 3 3 6% 8% 55%	2620
	21st to end	Night	9% 4 2 4 3 4 3 3 4 6% 58%	2800
December	1st to 10th	Night	9% 4 1 3 3 3 3 3 5% 7% 58%	4560
	11th to 20th	Night	23% 8% 2 5% 4 4 4 4 6% 7% 32%	5176
	21st to end	Night	26% 10% 3 6% 4 4 3 3 4 5% 33%	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	39% 4 13% 23% 11% 10%	3793
	11th to 20th	Day	38% 1 6% 14% 24% 16%	4100
	21st to end	Day	40% 2 7% 15% 13% 23%	4510
February	1st to 10th	Day	41% 3 5% 20% 21% 9%	1800
	11th to 20th	Day	39% 4 9% 18% 22% 8%	1816
	21st to end	Day	38% 3 8% 24% 20% 8%	1650
March	1st to 10th	Day	55% 2 8% 15% 16% 3	3492
	11th to 20th	Day	29% 2 9% 27% 20% 12%	3503
	21st to end	Day	48% 2 12% 22% 14% 3	3875
April	1st to 10th	Day	40% 5% 10% 21% 18% 6%	2280
	11th to 20th	Day	30% 4 12% 24% 23% 7%	2252
	21st to end	Day	22% 3 13% 30% 25% 7%	2280
May	1st to 10th	Day	15% 2 10% 28% 25% 20%	4017
	11th to 20th	Day	12% 2 5% 24% 38% 19%	4221
	21st to end	Day	12% 2 11% 39% 27% 9%	4814
June	1st to 10th	Day	20% 3 9% 31% 27% 10%	2660
	11th to 20th	Day	8% 2 9% 21% 36% 23%	2660
	21st to end	Day	8% 2 2 14% 36% 37%	2660
July	1st to 10th	Day	3 1 1 7% 16% 71%	4351
	11th to 20th	Day	6% 1 3 7% 19% 65%	4142
	21st to end	Day	4 1 2 7% 25% 62%	4315
August	1st to 10th	Day	10% 1 2 11% 26% 50%	4076
	11th to 20th	Day	4 1 1 9% 44% 41%	3925
	21st to end	Day	6% 2 3 11% 37% 41%	4179
September	1st to 10th	Day	5% 1 2 8% 30% 54%	2280
	11th to 20th	Day	13% 3 4 24% 38% 18%	2280
	21st to end	Day	11% 2 3 21% 32% 30%	2006
October	1st to 10th	Day	13% 1 4 11% 26% 46%	3180
	11th to 20th	Day	17% 2 5% 24% 29% 24%	3161
	21st to end	Day	22% 2 7% 21% 33% 15%	3448
November	1st to 10th	Day	27% 4 6% 28% 23% 13%	1899
	11th to 20th	Day	33% 4 11% 23% 21% 8%	1775
	21st to end	Day	31% 4 8% 19% 23% 15%	1600
December	1st to 10th	Day	27% 3 6% 25% 28% 11%	2620
	11th to 20th	Day	54% 3 7% 18% 15% 3	3028
	21st to end	Day	54% 3 11% 17% 12% 3	4004
January	1st to 10th	Night	33% 5% 20% 21% 16% 5%	6157
	11th to 20th	Night	38% 2 8% 16% 23% 13%	5880
	21st to end	Night	40% 2 7% 11% 18% 22%	6468
February	1st to 10th	Night	38% 3 7% 18% 21% 13%	2600
	11th to 20th	Night	36% 4 8% 21% 21% 9%	2480
	21st to end	Night	32% 4 9% 24% 22% 8%	1980
March	1st to 10th	Night	51% 3 11% 16% 17% 3	3587
	11th to 20th	Night	29% 2 8% 26% 24% 11%	3505
	21st to end	Night	47% 2 10% 25% 12% 4	3777
April	1st to 10th	Night	34% 4 13% 21% 22% 7%	1900
	11th to 20th	Night	25% 3 12% 30% 18% 11%	1879
	21st to end	Night	18% 3 12% 33% 28% 7%	1900
May	1st to 10th	Night	14% 2 6% 28% 30% 19%	2938
	11th to 20th	Night	10% 2 5% 19% 38% 25%	2677
	21st to end	Night	9% 2 8% 40% 28% 12%	2762
June	1st to 10th	Night	19% 3 9% 26% 33% 11%	1520
	11th to 20th	Night	8% 2 6% 22% 37% 25%	1520
	21st to end	Night	10% 2 2 13% 33% 41%	1520
July	1st to 10th	Night	3 1 1 6% 16% 73%	2500
	11th to 20th	Night	7% 1 3 7% 15% 68%	2698
	21st to end	Night	5% 1 1 5% 25% 63%	3011
August	1st to 10th	Night	12% 1 1 7% 26% 52%	2839
	11th to 20th	Night	4 2 1 6% 41% 46%	2983
	21st to end	Night	8% 2 1 9% 38% 42%	3498
September	1st to 10th	Night	4 1 2 7% 29% 55%	1900
	11th to 20th	Night	14% 2 4 19% 39% 23%	1900
	21st to end	Night	9% 2 3 20% 30% 36%	2170
October	1st to 10th	Night	13% 1 3 11% 21% 51%	3800
	11th to 20th	Night	14% 3 9% 26% 24% 23%	3777
	21st to end	Night	20% 2 6% 23% 27% 21%	4121
November	1st to 10th	Night	27% 3 8% 25% 21% 15%	2398
	11th to 20th	Night	29% 3 11% 25% 21% 10%	2620
	21st to end	Night	29% 4 8% 21% 24% 14%	2800
December	1st to 10th	Night	27% 3 6% 21% 27% 16%	4560
	11th to 20th	Night	52% 4 8% 16% 16% 3	5176
	21st to end	Night	56% 2 10% 17% 12% 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	27% 14% 25% 20% 12% 3	263
	11th to 20th	Day	18% 14% 25% 27% 13% 4	400
	21st to end	Day	15% 15% 23% 26% 16% 6%	440
February	1st to 10th	Day	21% 15% 25% 22% 14% 3	400
	11th to 20th	Day	20% 19% 27% 20% 13% 2	422
	21st to end	Day	17% 14% 29% 27% 11% 3	414
March	1st to 10th	Day	22% 19% 25% 21% 11% 2	500
	11th to 20th	Day	21% 16% 24% 22% 14% 4	465
	21st to end	Day	18% 20% 29% 23% 9% 2	506
April	1st to 10th	Day	15% 18% 27% 24% 13% 4	460
	11th to 20th	Day	13% 19% 28% 21% 15% 5%	452
	21st to end	Day	6% 17% 26% 27% 18% 5%	460
May	1st to 10th	Day	11% 13% 32% 28% 12% 5%	463
	11th to 20th	Day	7% 12% 25% 28% 22% 7%	488
	21st to end	Day	5% 11% 23% 31% 24% 7%	529
June	1st to 10th	Day	6% 11% 22% 34% 21% 7%	490
	11th to 20th	Day	2 11% 26% 38% 18% 5%	490
	21st to end	Day	1 6% 22% 35% 28% 9%	490
July	1st to 10th	Day	1 3 14% 42% 33% 7%	486
	11th to 20th	Day	2 14% 38% 36% 10%	484
	21st to end	Day	1 1 12% 39% 37% 10%	531
August	1st to 10th	Day	2 2 17% 34% 33% 12%	487
	11th to 20th	Day	1 3 18% 34% 29% 15%	477
	21st to end	Day	1 1 14% 34% 36% 14%	539
September	1st to 10th	Day	2 15% 37% 36% 11%	490
	11th to 20th	Day	3 6% 17% 34% 31% 10%	490
	21st to end	Day	2 8% 20% 34% 27% 10%	490
October	1st to 10th	Day	3 11% 22% 27% 24% 13%	490
	11th to 20th	Day	6% 9% 21% 29% 25% 11%	490
	21st to end	Day	10% 9% 22% 29% 23% 6%	539
November	1st to 10th	Day	12% 13% 18% 22% 28% 8%	435
	11th to 20th	Day	10% 11% 19% 27% 23% 10%	287
	21st to end	Day	17% 10% 20% 31% 17% 6%	200
December	1st to 10th	Day	22% 11% 18% 26% 13% 11%	200
	11th to 20th	Day	24% 13% 21% 22% 16% 6%	200
	21st to end	Day	22% 16% 22% 21% 16% 3	220
January	1st to 10th	Night	20% 6% 17% 32% 20% 6%	800
	11th to 20th	Night	17% 8% 16% 36% 18% 4	800
	21st to end	Night	18% 9% 14% 33% 24% 3	880
February	1st to 10th	Night	19% 9% 15% 34% 20% 3	800
	11th to 20th	Night	14% 12% 16% 33% 21% 4	784
	21st to end	Night	13% 11% 22% 34% 18% 2	660
March	1st to 10th	Night	18% 11% 19% 30% 20% 4	800
	11th to 20th	Night	17% 8% 16% 31% 24% 3	766
	21st to end	Night	15% 11% 23% 33% 17% 2	836
April	1st to 10th	Night	9% 10% 21% 36% 20% 3	760
	11th to 20th	Night	9% 13% 20% 31% 22% 5%	751
	21st to end	Night	5% 10% 22% 34% 23% 5%	760
May	1st to 10th	Night	6% 8% 24% 31% 26% 5%	622
	11th to 20th	Night	3 6% 16% 33% 39% 4	470
	21st to end	Night	5% 6% 16% 34% 35% 4	411
June	1st to 10th	Night	4 8% 15% 38% 32% 2	380
	11th to 20th	Night	2 6% 14% 42% 34% 1	380
	21st to end	Night	1 2 12% 42% 41% 2	380
July	1st to 10th	Night	1 8% 35% 52% 3	370
	11th to 20th	Night	1 1 4 33% 54% 8%	483
	21st to end	Night	1 3 29% 58% 8%	624
August	1st to 10th	Night	2 1 6% 31% 52% 9%	567
	11th to 20th	Night	1 6% 37% 46% 10%	643
	21st to end	Night	1 5% 32% 53% 8%	836
September	1st to 10th	Night	1 2 7% 34% 49% 7%	760
	11th to 20th	Night	4 1 8% 36% 43% 8%	760
	21st to end	Night	1 3 11% 37% 37% 10%	760
October	1st to 10th	Night	4 5% 12% 33% 36% 9%	760
	11th to 20th	Night	5% 5% 12% 35% 36% 7%	760
	21st to end	Night	9% 3 15% 32% 30% 10%	836
November	1st to 10th	Night	8% 7% 15% 31% 32% 8%	780
	11th to 20th	Night	10% 6% 16% 36% 28% 4	800
	21st to end	Night	11% 8% 15% 38% 22% 7%	800
December	1st to 10th	Night	17% 6% 16% 33% 23% 5%	800
	11th to 20th	Night	22% 8% 13% 33% 21% 3	800
	21st to end	Night	20% 12% 17% 32% 18% 1	880

Page generated at 2025-03-09 05:08:46. Version ac07

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