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 2024-02-10.

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	8% 4 2 3 3 2 3 3 4 6% 62%	78426
	Night	10% 4 2 3 3 2 3 3 4 6% 59%	80576
Transparency	Day	26% 3 7% 20% 25% 19%	78143
	Night	29% 3 8% 19% 23% 18%	80380
Seeing	Day	9% 11% 21% 29% 22% 7%	14243
	Night	10% 6% 14% 33% 31% 6%	23639

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	14% 8% 3 7% 6% 3 4 4 4 6% 42%	2637
	11th to 20th	Day	20% 8% 4 8% 5% 3 4 4 3 6% 35%	2860
	21st to end	Day	22% 8% 3 6% 5% 3 4 3 5% 8% 34%	3146
February	1st to 10th	Day	15% 5% 2 5% 4 3 4 4 5% 6% 47%	1702
	11th to 20th	Day	11% 6% 2 6% 5% 3 3 3 5% 7% 49%	1622
	21st to end	Day	15% 6% 2 4 5% 2 3 3 5% 5% 50%	1480
March	1st to 10th	Day	15% 7% 3 5% 6% 4 4 5% 6% 7% 38%	1944
	11th to 20th	Day	16% 7% 2 5% 5% 2 4 5% 5% 7% 41%	1980
	21st to end	Day	13% 6% 2 4 4 3 5% 4 5% 6% 47%	2203
April	1st to 10th	Day	13% 6% 3 5% 5% 3 5% 4 5% 7% 44%	2160
	11th to 20th	Day	8% 4 2 4 4 3 4 4 5% 6% 57%	2129
	21st to end	Day	4 3 2 4 3 2 3 4 5% 9% 61%	2160
May	1st to 10th	Day	5% 4 1 3 2 2 3 4 4 7% 66%	2286
	11th to 20th	Day	4 2 1 2 3 2 3 3 5% 8% 66%	2383
	21st to end	Day	4 2 1 2 3 2 3 4 5% 8% 65%	2723
June	1st to 10th	Day	4 3 1 2 3 2 4 4 4 7% 65%	2520
	11th to 20th	Day	1 1 1 1 1 2 3 3 5% 80%	2520
	21st to end	Day	2 1 1 2 1 2 2 2 3 85%	2520
July	1st to 10th	Day	1 1 1 1 1 2 1 2 3 88%	2520
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 88%	2406
	21st to end	Day	1 1 1 1 1 2 2 4 87%	2546
August	1st to 10th	Day	2 1 1 1 1 2 2 3 4 82%	2328
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 86%	2223
	21st to end	Day	1 1 1 1 1 1 2 2 3 88%	2376
September	1st to 10th	Day	1 2 1 1 1 2 4 88%	2160
	11th to 20th	Day	2 1 2 2 1 2 2 4 6% 77%	2160
	21st to end	Day	2 2 1 1 2 1 2 2 4 5% 78%	1901
October	1st to 10th	Day	3 3 1 2 3 2 2 2 2 5% 74%	1800
	11th to 20th	Day	5% 3 1 3 2 2 3 3 4 5% 69%	1800
	21st to end	Day	8% 3 1 3 3 2 2 3 3 5% 66%	1980
November	1st to 10th	Day	9% 3 2 3 4 2 4 3 4 7% 58%	1802
	11th to 20th	Day	10% 4 2 4 4 3 4 4 5% 7% 52%	1687
	21st to end	Day	9% 4 2 4 4 2 5% 4 5% 6% 55%	1520
December	1st to 10th	Day	15% 7% 2 4 4 2 3 5% 3 6% 49%	1520
	11th to 20th	Day	29% 9% 4 6% 4 2 3 4 3 4 32%	1928
	21st to end	Day	24% 6% 3 4 3 2 4 4 5% 7% 37%	2794
January	1st to 10th	Night	14% 7% 2 4 4 2 4 4 5% 9% 45%	4309
	11th to 20th	Night	19% 8% 3 6% 6% 3 4 5% 5% 6% 34%	4110
	21st to end	Night	22% 8% 3 5% 5% 3 4 4 4 6% 35%	4521
February	1st to 10th	Night	15% 5% 2 3 4 2 4 4 5% 5% 51%	2462
	11th to 20th	Night	10% 4 2 4 4 3 4 5% 5% 8% 50%	2223
	21st to end	Night	12% 5% 2 3 4 2 4 4 4 6% 55%	1776
March	1st to 10th	Night	15% 8% 3 4 4 3 5% 5% 5% 6% 42%	2012
	11th to 20th	Night	13% 6% 2 5% 5% 3 4 4 6% 7% 46%	1980
	21st to end	Night	12% 5% 2 4 4 2 4 3 4 6% 53%	2152
April	1st to 10th	Night	8% 4 3 5% 5% 3 5% 4 4 6% 52%	1800
	11th to 20th	Night	7% 4 2 3 3 2 3 3 5% 8% 61%	1779
	21st to end	Night	4 2 1 2 3 1 3 4 5% 8% 66%	1800
May	1st to 10th	Night	4 2 1 2 3 2 2 4 4 7% 68%	1670
	11th to 20th	Night	3 2 1 2 2 1 3 4 5% 7% 69%	1507
	21st to end	Night	4 2 1 2 2 1 2 4 5% 7% 71%	1552
June	1st to 10th	Night	4 2 1 3 3 2 3 2 4 6% 69%	1440
	11th to 20th	Night	1 1 1 1 1 2 2 2 6% 82%	1440
	21st to end	Night	1 1 1 1 2 1 2 2 2 3 82%	1440
July	1st to 10th	Night	1 1 1 1 1 1 1 2 2 90%	1440
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 86%	1550
	21st to end	Night	1 1 1 1 1 2 2 2 4 85%	1773
August	1st to 10th	Night	2 1 1 2 1 2 2 2 4 84%	1611
	11th to 20th	Night	1 1 2 2 2 2 4 85%	1676
	21st to end	Night	1 1 1 1 1 1 2 4 86%	1980
September	1st to 10th	Night	1 1 1 1 1 1 3 4 88%	1800
	11th to 20th	Night	2 2 1 2 2 1 3 2 3 5% 77%	1800
	21st to end	Night	1 1 2 2 2 2 2 3 6% 80%	2055
October	1st to 10th	Night	3 2 1 3 2 2 3 2 3 4 75%	2160
	11th to 20th	Night	4 2 1 3 3 2 2 3 5% 6% 70%	2160
	21st to end	Night	7% 2 1 2 3 2 3 3 4 6% 67%	2376
November	1st to 10th	Night	7% 4 2 3 4 2 4 3 5% 7% 58%	2275
	11th to 20th	Night	9% 4 2 4 3 2 3 4 6% 7% 55%	2489
	21st to end	Night	8% 4 2 4 4 2 4 3 4 6% 60%	2660
December	1st to 10th	Night	15% 6% 2 4 4 3 3 3 5% 6% 49%	2660
	11th to 20th	Night	28% 7% 2 4 4 2 4 4 5% 6% 34%	3276
	21st to end	Night	20% 9% 3 5% 5% 3 4 4 4 6% 37%	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	47% 2 9% 29% 9% 3	2637
	11th to 20th	Day	54% 2 9% 16% 12% 7%	2860
	21st to end	Day	52% 3 9% 18% 13% 5%	3146
February	1st to 10th	Day	40% 4 6% 21% 21% 9%	1702
	11th to 20th	Day	37% 4 8% 19% 24% 9%	1631
	21st to end	Day	38% 3 7% 25% 19% 7%	1480
March	1st to 10th	Day	46% 4 11% 18% 17% 4	1944
	11th to 20th	Day	43% 4 9% 22% 16% 7%	1980
	21st to end	Day	38% 4 12% 20% 20% 5%	2203
April	1st to 10th	Day	40% 5% 10% 21% 18% 6%	2160
	11th to 20th	Day	29% 4 12% 24% 23% 8%	2132
	21st to end	Day	22% 3 13% 30% 24% 8%	2160
May	1st to 10th	Day	20% 3 8% 28% 30% 10%	2286
	11th to 20th	Day	19% 4 9% 31% 27% 12%	2386
	21st to end	Day	19% 3 14% 35% 22% 7%	2724
June	1st to 10th	Day	21% 3 9% 31% 27% 10%	2520
	11th to 20th	Day	8% 3 10% 22% 35% 23%	2520
	21st to end	Day	8% 2 2 15% 37% 36%	2520
July	1st to 10th	Day	6% 2 2 11% 29% 49%	2451
	11th to 20th	Day	5% 2 4 11% 31% 47%	2333
	21st to end	Day	6% 2 1 9% 27% 54%	2405
August	1st to 10th	Day	9% 2 1 16% 34% 37%	2312
	11th to 20th	Day	6% 2 1 12% 41% 39%	2223
	21st to end	Day	5% 2 1 13% 38% 41%	2376
September	1st to 10th	Day	5% 1 2 8% 31% 52%	2160
	11th to 20th	Day	13% 2 4 24% 38% 19%	2160
	21st to end	Day	12% 3 4 22% 32% 29%	1901
October	1st to 10th	Day	17% 2 5% 16% 33% 28%	1800
	11th to 20th	Day	20% 3 5% 21% 25% 26%	1800
	21st to end	Day	23% 2 6% 20% 29% 20%	1980
November	1st to 10th	Day	28% 4 5% 28% 22% 13%	1802
	11th to 20th	Day	33% 4 11% 23% 21% 8%	1687
	21st to end	Day	30% 4 9% 20% 23% 14%	1520
December	1st to 10th	Day	40% 4 7% 19% 21% 10%	1520
	11th to 20th	Day	58% 3 5% 13% 17% 5%	1928
	21st to end	Day	49% 2 12% 18% 14% 4	2794
January	1st to 10th	Night	40% 3 18% 26% 11% 4	4309
	11th to 20th	Night	54% 2 11% 13% 11% 8%	4110
	21st to end	Night	53% 2 9% 15% 14% 6%	4521
February	1st to 10th	Night	37% 3 7% 19% 21% 12%	2462
	11th to 20th	Night	33% 4 8% 21% 23% 10%	2225
	21st to end	Night	31% 5% 10% 24% 24% 7%	1776
March	1st to 10th	Night	44% 5% 11% 16% 20% 5%	2012
	11th to 20th	Night	39% 3 8% 22% 19% 9%	1980
	21st to end	Night	35% 4 12% 23% 19% 7%	2152
April	1st to 10th	Night	35% 4 13% 20% 21% 7%	1800
	11th to 20th	Night	24% 4 12% 29% 19% 12%	1779
	21st to end	Night	18% 3 12% 32% 28% 7%	1800
May	1st to 10th	Night	19% 4 7% 25% 32% 13%	1670
	11th to 20th	Night	15% 4 10% 29% 32% 12%	1507
	21st to end	Night	14% 3 14% 36% 26% 7%	1552
June	1st to 10th	Night	19% 3 9% 25% 33% 10%	1440
	11th to 20th	Night	9% 2 6% 23% 36% 25%	1440
	21st to end	Night	10% 2 2 13% 34% 39%	1440
July	1st to 10th	Night	5% 2 1 11% 29% 53%	1400
	11th to 20th	Night	6% 1 3 11% 22% 56%	1500
	21st to end	Night	8% 2 8% 23% 59%	1674
August	1st to 10th	Night	10% 2 1 12% 32% 44%	1602
	11th to 20th	Night	7% 3 1 9% 34% 46%	1676
	21st to end	Night	7% 3 1 11% 33% 44%	1980
September	1st to 10th	Night	5% 2 2 8% 31% 53%	1800
	11th to 20th	Night	14% 2 3 18% 39% 24%	1800
	21st to end	Night	10% 2 3 21% 29% 36%	2055
October	1st to 10th	Night	16% 2 5% 18% 30% 30%	2160
	11th to 20th	Night	16% 4 6% 19% 25% 29%	2160
	21st to end	Night	21% 3 5% 21% 25% 25%	2376
November	1st to 10th	Night	28% 3 8% 24% 21% 15%	2275
	11th to 20th	Night	29% 3 10% 25% 21% 11%	2489
	21st to end	Night	28% 4 8% 21% 25% 14%	2660
December	1st to 10th	Night	39% 4 8% 18% 19% 12%	2660
	11th to 20th	Night	53% 4 8% 14% 17% 4	3276
	21st to end	Night	51% 2 11% 18% 14% 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	28% 14% 24% 19% 13% 3	239
	11th to 20th	Day	18% 13% 25% 27% 13% 4	370
	21st to end	Day	15% 15% 23% 26% 15% 5%	407
February	1st to 10th	Day	19% 16% 24% 22% 15% 3	368
	11th to 20th	Day	19% 20% 25% 21% 13% 2	357
	21st to end	Day	18% 14% 29% 25% 12% 2	346
March	1st to 10th	Day	23% 18% 25% 21% 11% 2	420
	11th to 20th	Day	21% 16% 24% 23% 12% 4	420
	21st to end	Day	16% 20% 29% 24% 10% 2	462
April	1st to 10th	Day	15% 19% 27% 24% 12% 3	420
	11th to 20th	Day	14% 19% 29% 20% 13% 5%	412
	21st to end	Day	6% 18% 26% 27% 18% 5%	420
May	1st to 10th	Day	11% 13% 30% 29% 12% 5%	423
	11th to 20th	Day	7% 13% 26% 27% 20% 7%	448
	21st to end	Day	5% 12% 22% 30% 24% 7%	485
June	1st to 10th	Day	6% 11% 22% 34% 21% 7%	450
	11th to 20th	Day	2 11% 25% 39% 18% 4	450
	21st to end	Day	6% 20% 36% 29% 8%	450
July	1st to 10th	Day	1 3 15% 42% 33% 7%	446
	11th to 20th	Day	2 14% 36% 37% 10%	444
	21st to end	Day	1 1 11% 39% 37% 11%	487
August	1st to 10th	Day	2 2 16% 34% 34% 11%	447
	11th to 20th	Day	1 2 17% 34% 31% 15%	437
	21st to end	Day	1 1 13% 35% 37% 14%	495
September	1st to 10th	Day	2 14% 36% 36% 11%	450
	11th to 20th	Day	3 5% 17% 34% 31% 10%	450
	21st to end	Day	2 9% 21% 33% 26% 9%	450
October	1st to 10th	Day	3 12% 22% 28% 23% 13%	450
	11th to 20th	Day	6% 9% 20% 29% 26% 10%	450
	21st to end	Day	11% 9% 20% 29% 23% 7%	495
November	1st to 10th	Day	12% 11% 17% 22% 29% 8%	398
	11th to 20th	Day	8% 10% 17% 29% 25% 11%	259
	21st to end	Day	15% 9% 21% 31% 18% 6%	180
December	1st to 10th	Day	23% 12% 17% 26% 13% 9%	180
	11th to 20th	Day	22% 14% 22% 22% 14% 6%	180
	21st to end	Day	22% 15% 21% 22% 17% 3	198
January	1st to 10th	Night	20% 6% 16% 31% 21% 6%	760
	11th to 20th	Night	18% 8% 16% 36% 18% 4	760
	21st to end	Night	19% 8% 15% 32% 23% 3	836
February	1st to 10th	Night	18% 9% 14% 35% 21% 3	758
	11th to 20th	Night	13% 12% 16% 34% 22% 4	704
	21st to end	Night	15% 11% 22% 31% 18% 2	592
March	1st to 10th	Night	18% 10% 18% 29% 21% 4	720
	11th to 20th	Night	17% 8% 16% 32% 24% 3	720
	21st to end	Night	15% 10% 23% 32% 18% 2	792
April	1st to 10th	Night	9% 10% 22% 36% 20% 3	720
	11th to 20th	Night	9% 14% 21% 30% 22% 5%	711
	21st to end	Night	6% 10% 21% 34% 24% 5%	720
May	1st to 10th	Night	6% 8% 23% 31% 27% 5%	590
	11th to 20th	Night	3 6% 16% 32% 38% 4	445
	21st to end	Night	5% 6% 16% 33% 35% 5%	389
June	1st to 10th	Night	5% 8% 14% 37% 34% 2	360
	11th to 20th	Night	2 6% 13% 43% 34% 1	360
	21st to end	Night	1 3 12% 42% 41% 2	360
July	1st to 10th	Night	1 9% 35% 52% 3	350
	11th to 20th	Night	1 4 32% 55% 7%	456
	21st to end	Night	1 3 29% 57% 9%	591
August	1st to 10th	Night	1 1 6% 30% 53% 9%	537
	11th to 20th	Night	1 5% 36% 47% 11%	608
	21st to end	Night	1 5% 32% 54% 8%	792
September	1st to 10th	Night	1 2 7% 32% 50% 8%	720
	11th to 20th	Night	4 1 7% 35% 44% 8%	720
	21st to end	Night	1 3 11% 37% 37% 11%	720
October	1st to 10th	Night	5% 6% 13% 33% 35% 9%	720
	11th to 20th	Night	5% 5% 12% 35% 37% 7%	720
	21st to end	Night	9% 4 14% 31% 31% 11%	792
November	1st to 10th	Night	8% 7% 14% 30% 32% 8%	740
	11th to 20th	Night	10% 5% 15% 36% 29% 5%	760
	21st to end	Night	10% 8% 14% 38% 23% 7%	760
December	1st to 10th	Night	18% 6% 16% 33% 22% 5%	760
	11th to 20th	Night	22% 8% 14% 32% 21% 3	760
	21st to end	Night	19% 12% 17% 33% 19% 2	836

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