Cal Poly Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cal Poly Observatory clear sky chart from 2005-11-06 to 2024-04-25.

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% 4 2 3 3 2 3 3 4 7% 62%	83584
	Night	8% 5% 2 3 3 2 3 4 4 6% 59%	85614
Transparency	Day	23% 5% 10% 22% 24% 16%	83305
	Night	27% 5% 10% 21% 22% 16%	85416
Seeing	Day	8% 9% 24% 35% 19% 4	14814
	Night	8% 4 14% 33% 33% 9%	24513

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	9% 4 2 4 4 3 5% 3 5% 6% 55%	2713
	11th to 20th	Day	14% 7% 2 4 5% 2 4 5% 6% 8% 44%	2860
	21st to end	Day	14% 6% 3 5% 5% 3 5% 6% 6% 8% 38%	3146
February	1st to 10th	Day	15% 4 2 3 4 2 2 3 4 6% 55%	1777
	11th to 20th	Day	11% 4 2 4 5% 2 3 3 5% 7% 53%	1850
	21st to end	Day	13% 5% 3 4 4 2 4 4 5% 6% 51%	1570
March	1st to 10th	Day	14% 7% 2 6% 5% 3 4 4 6% 8% 41%	3247
	11th to 20th	Day	7% 3 2 3 4 2 3 3 4 6% 63%	3490
	21st to end	Day	11% 6% 2 5% 5% 3 5% 6% 6% 8% 43%	3996
April	1st to 10th	Day	9% 4 2 4 3 2 3 4 4 7% 57%	2280
	11th to 20th	Day	7% 5% 2 3 3 2 3 4 5% 7% 59%	2249
	21st to end	Day	7% 3 2 3 2 2 3 4 4 6% 64%	2210
May	1st to 10th	Day	5% 4 1 2 2 1 2 3 4 7% 67%	2160
	11th to 20th	Day	3 2 1 3 3 2 3 3 4 7% 69%	2193
	21st to end	Day	4 4 2 3 2 2 3 3 4 7% 66%	2723
June	1st to 10th	Day	4 3 1 2 2 2 3 3 5% 9% 66%	2520
	11th to 20th	Day	1 2 1 2 2 1 2 2 3 6% 77%	2520
	21st to end	Day	1 1 1 2 2 1 2 3 3 7% 78%	2520
July	1st to 10th	Day	1 1 2 2 1 2 3 4 8% 75%	2520
	11th to 20th	Day	1 1 1 2 3 2 3 3 4 5% 75%	2347
	21st to end	Day	1 1 1 2 2 2 3 4 6% 9% 69%	2546
August	1st to 10th	Day	1 3 1 2 3 1 3 3 4 8% 71%	2228
	11th to 20th	Day	1 1 1 2 2 1 2 2 3 6% 79%	2122
	21st to end	Day	1 2 1 2 2 1 2 3 4 6% 76%	2376
September	1st to 10th	Day	1 2 1 2 1 2 3 3 7% 79%	2160
	11th to 20th	Day	2 2 1 2 1 1 2 2 2 7% 78%	2159
	21st to end	Day	2 2 1 2 2 2 2 2 4 6% 76%	1979
October	1st to 10th	Day	3 2 1 1 2 2 3 3 4 5% 73%	1800
	11th to 20th	Day	4 2 2 3 3 2 2 3 3 6% 71%	1800
	21st to end	Day	4 2 2 3 3 2 3 3 3 6% 69%	1980
November	1st to 10th	Day	6% 4 1 3 3 1 3 3 4 6% 66%	1791
	11th to 20th	Day	6% 3 1 3 3 2 3 3 4 5% 66%	1710
	21st to end	Day	8% 3 2 4 4 2 4 4 4 6% 60%	1710
December	1st to 10th	Day	10% 5% 2 4 4 2 4 4 4 7% 53%	1610
	11th to 20th	Day	25% 6% 3 5% 3 2 4 4 4 6% 38%	1928
	21st to end	Day	16% 8% 3 4 5% 3 4 4 5% 7% 43%	2794
January	1st to 10th	Night	11% 5% 2 4 4 3 4 3 4 6% 56%	4220
	11th to 20th	Night	13% 8% 3 5% 4 2 5% 4 5% 7% 45%	4110
	21st to end	Night	17% 8% 2 4 5% 3 4 4 4 6% 42%	4521
February	1st to 10th	Night	13% 5% 2 4 4 1 3 4 4 5% 56%	2400
	11th to 20th	Night	10% 5% 1 4 3 2 4 3 4 7% 57%	2218
	21st to end	Night	12% 6% 2 4 4 2 4 3 3 5% 56%	1884
March	1st to 10th	Night	16% 9% 3 5% 4 3 4 5% 6% 7% 39%	3624
	11th to 20th	Night	7% 4 1 4 3 2 3 4 4 6% 62%	3490
	21st to end	Night	15% 8% 2 4 3 2 3 4 4 7% 48%	3657
April	1st to 10th	Night	8% 5% 2 3 4 2 3 3 4 8% 59%	1900
	11th to 20th	Night	6% 5% 2 3 4 2 2 3 3 5% 65%	1879
	21st to end	Night	6% 6% 2 2 2 2 2 3 4 7% 63%	1850
May	1st to 10th	Night	5% 5% 1 3 3 1 4 3 4 8% 63%	1800
	11th to 20th	Night	4 4 1 4 3 2 3 4 5% 9% 62%	1694
	21st to end	Night	3 5% 3 5% 4 3 2 4 3 7% 61%	1552
June	1st to 10th	Night	3 4 1 4 3 2 4 5% 4 8% 63%	1440
	11th to 20th	Night	1 3 1 3 2 2 3 4 4 7% 70%	1440
	21st to end	Night	1 3 1 3 3 1 2 3 3 6% 75%	1440
July	1st to 10th	Night	1 1 2 4 3 1 3 4 5% 8% 69%	1440
	11th to 20th	Night	1 2 1 3 2 1 3 4 4 7% 72%	1608
	21st to end	Night	1 2 1 2 3 2 5% 6% 6% 7% 65%	1773
August	1st to 10th	Night	1 2 1 3 3 2 3 4 5% 8% 69%	1708
	11th to 20th	Night	1 1 1 2 2 1 2 4 4 7% 75%	1780
	21st to end	Night	1 1 1 2 3 2 2 3 4 6% 76%	1980
September	1st to 10th	Night	2 2 1 2 2 1 3 3 3 4 77%	1800
	11th to 20th	Night	2 3 1 3 2 1 2 4 4 5% 75%	1800
	21st to end	Night	1 2 1 2 2 1 3 2 2 5% 79%	1980
October	1st to 10th	Night	3 3 1 2 2 2 2 4 3 5% 71%	2158
	11th to 20th	Night	4 3 2 3 3 2 3 3 3 5% 69%	2160
	21st to end	Night	4 4 2 3 2 2 3 3 3 5% 69%	2376
November	1st to 10th	Night	5% 3 2 2 3 2 3 3 5% 7% 66%	2286
	11th to 20th	Night	4 4 2 3 3 2 3 4 4 7% 65%	2470
	21st to end	Night	5% 5% 2 3 3 3 4 5% 5% 5% 60%	2470
December	1st to 10th	Night	12% 5% 2 4 3 2 3 4 5% 6% 56%	2568
	11th to 20th	Night	23% 6% 2 4 4 3 4 4 5% 5% 39%	3276
	21st to end	Night	15% 8% 3 5% 5% 3 4 5% 5% 6% 41%	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	26% 7% 10% 35% 18% 4	2713
	11th to 20th	Day	36% 7% 11% 19% 16% 11%	2860
	21st to end	Day	39% 8% 11% 20% 14% 9%	3146
February	1st to 10th	Day	32% 3 6% 19% 22% 18%	1777
	11th to 20th	Day	29% 6% 9% 22% 23% 10%	1860
	21st to end	Day	33% 6% 10% 21% 23% 7%	1570
March	1st to 10th	Day	35% 10% 14% 20% 17% 5%	3247
	11th to 20th	Day	22% 7% 11% 32% 16% 12%	3490
	21st to end	Day	31% 11% 16% 24% 13% 4	3996
April	1st to 10th	Day	27% 6% 12% 27% 20% 8%	2280
	11th to 20th	Day	24% 6% 13% 25% 24% 8%	2252
	21st to end	Day	22% 6% 12% 26% 27% 8%	2210
May	1st to 10th	Day	20% 5% 11% 25% 30% 9%	2160
	11th to 20th	Day	17% 5% 11% 29% 28% 10%	2196
	21st to end	Day	21% 6% 14% 30% 26% 4	2724
June	1st to 10th	Day	18% 6% 10% 27% 27% 12%	2520
	11th to 20th	Day	12% 3 11% 18% 35% 21%	2520
	21st to end	Day	13% 2 8% 18% 31% 29%	2520
July	1st to 10th	Day	12% 2 7% 18% 28% 33%	2451
	11th to 20th	Day	15% 2 9% 17% 31% 27%	2277
	21st to end	Day	15% 4 10% 19% 29% 22%	2405
August	1st to 10th	Day	18% 2 8% 17% 25% 31%	2212
	11th to 20th	Day	11% 1 9% 17% 34% 27%	2122
	21st to end	Day	13% 2 9% 18% 32% 27%	2376
September	1st to 10th	Day	11% 2 8% 15% 29% 36%	2160
	11th to 20th	Day	13% 2 8% 20% 36% 21%	2159
	21st to end	Day	11% 4 8% 18% 35% 25%	1979
October	1st to 10th	Day	14% 5% 9% 20% 24% 29%	1800
	11th to 20th	Day	17% 4 7% 18% 28% 26%	1800
	21st to end	Day	19% 4 6% 15% 28% 28%	1980
November	1st to 10th	Day	21% 4 8% 18% 29% 20%	1791
	11th to 20th	Day	21% 5% 8% 19% 27% 21%	1710
	21st to end	Day	25% 5% 5% 17% 25% 22%	1710
December	1st to 10th	Day	31% 6% 8% 17% 23% 16%	1610
	11th to 20th	Day	47% 5% 8% 16% 16% 9%	1928
	21st to end	Day	39% 7% 9% 25% 14% 6%	2794
January	1st to 10th	Night	28% 6% 11% 26% 23% 5%	4220
	11th to 20th	Night	36% 8% 9% 23% 12% 11%	4110
	21st to end	Night	42% 6% 10% 17% 15% 10%	4521
February	1st to 10th	Night	31% 3 8% 17% 24% 16%	2400
	11th to 20th	Night	27% 5% 9% 22% 24% 14%	2219
	21st to end	Night	31% 5% 8% 23% 23% 9%	1884
March	1st to 10th	Night	39% 10% 11% 21% 15% 5%	3624
	11th to 20th	Night	22% 7% 12% 26% 22% 10%	3490
	21st to end	Night	34% 9% 14% 23% 16% 4	3657
April	1st to 10th	Night	27% 5% 13% 26% 18% 10%	1900
	11th to 20th	Night	24% 4 12% 29% 23% 9%	1879
	21st to end	Night	24% 4 12% 26% 27% 8%	1850
May	1st to 10th	Night	22% 5% 14% 23% 25% 11%	1800
	11th to 20th	Night	23% 5% 16% 23% 23% 10%	1696
	21st to end	Night	27% 3 17% 23% 24% 6%	1552
June	1st to 10th	Night	22% 6% 13% 23% 24% 13%	1440
	11th to 20th	Night	18% 3 9% 19% 29% 22%	1440
	21st to end	Night	16% 2 11% 16% 28% 28%	1440
July	1st to 10th	Night	16% 2 8% 20% 21% 34%	1400
	11th to 20th	Night	16% 3 11% 22% 22% 26%	1555
	21st to end	Night	20% 3 11% 22% 25% 20%	1674
August	1st to 10th	Night	19% 1 11% 22% 20% 26%	1699
	11th to 20th	Night	11% 2 12% 20% 28% 28%	1780
	21st to end	Night	12% 1 11% 19% 26% 31%	1980
September	1st to 10th	Night	13% 2 7% 15% 24% 40%	1800
	11th to 20th	Night	15% 2 10% 18% 28% 27%	1800
	21st to end	Night	12% 4 8% 16% 34% 26%	1980
October	1st to 10th	Night	16% 3 9% 20% 22% 29%	2158
	11th to 20th	Night	21% 4 10% 16% 24% 26%	2160
	21st to end	Night	21% 2 6% 16% 27% 27%	2376
November	1st to 10th	Night	20% 4 9% 20% 25% 23%	2286
	11th to 20th	Night	20% 4 10% 18% 26% 22%	2470
	21st to end	Night	25% 5% 9% 17% 22% 23%	2470
December	1st to 10th	Night	29% 5% 7% 18% 22% 19%	2568
	11th to 20th	Night	46% 6% 7% 15% 17% 9%	3276
	21st to end	Night	40% 8% 10% 21% 14% 7%	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	25% 13% 29% 19% 13% 1	289
	11th to 20th	Day	15% 13% 28% 28% 16% 1	370
	21st to end	Day	12% 15% 29% 27% 14% 2	407
February	1st to 10th	Day	16% 12% 28% 27% 13% 3	402
	11th to 20th	Day	15% 15% 30% 26% 13% 2	455
	21st to end	Day	10% 19% 32% 28% 8% 2	382
March	1st to 10th	Day	17% 17% 34% 25% 6%	460
	11th to 20th	Day	15% 16% 30% 28% 10% 1	460
	21st to end	Day	15% 21% 32% 26% 7% 1	506
April	1st to 10th	Day	13% 13% 35% 25% 12% 2	460
	11th to 20th	Day	11% 17% 30% 29% 11% 2	452
	21st to end	Day	6% 14% 34% 28% 14% 4	437
May	1st to 10th	Day	9% 11% 32% 33% 11% 3	423
	11th to 20th	Day	4 11% 29% 38% 14% 3	448
	21st to end	Day	10% 12% 26% 32% 16% 3	485
June	1st to 10th	Day	6% 6% 21% 42% 20% 5%	450
	11th to 20th	Day	3 6% 25% 40% 22% 5%	450
	21st to end	Day	2 3 20% 42% 26% 7%	450
July	1st to 10th	Day	1 1 14% 47% 31% 6%	446
	11th to 20th	Day	2 7% 45% 34% 11%	444
	21st to end	Day	1 1 7% 46% 35% 10%	487
August	1st to 10th	Day	3 1 16% 47% 27% 6%	447
	11th to 20th	Day	3 1 10% 40% 37% 9%	437
	21st to end	Day	2 2 11% 47% 31% 8%	495
September	1st to 10th	Day	3 3 14% 42% 29% 8%	450
	11th to 20th	Day	3 5% 23% 41% 24% 5%	450
	21st to end	Day	1 7% 18% 40% 29% 5%	450
October	1st to 10th	Day	3 8% 26% 41% 18% 4	450
	11th to 20th	Day	5% 7% 26% 38% 20% 4	450
	21st to end	Day	5% 6% 26% 39% 20% 4	495
November	1st to 10th	Day	6% 10% 26% 32% 23% 4	387
	11th to 20th	Day	8% 12% 25% 33% 19% 3	270
	21st to end	Day	10% 7% 29% 29% 21% 5%	270
December	1st to 10th	Day	15% 13% 28% 31% 12% 2	222
	11th to 20th	Day	14% 12% 26% 27% 18% 3	180
	21st to end	Day	20% 16% 27% 23% 10% 4	198
January	1st to 10th	Night	16% 8% 18% 31% 21% 6%	760
	11th to 20th	Night	12% 4 19% 37% 22% 6%	760
	21st to end	Night	15% 5% 18% 39% 18% 5%	836
February	1st to 10th	Night	16% 5% 15% 34% 24% 6%	760
	11th to 20th	Night	12% 8% 18% 35% 23% 4	744
	21st to end	Night	15% 6% 20% 37% 18% 3	628
March	1st to 10th	Night	15% 6% 21% 37% 18% 4	760
	11th to 20th	Night	12% 5% 17% 38% 22% 7%	760
	21st to end	Night	13% 8% 18% 34% 23% 3	836
April	1st to 10th	Night	9% 6% 19% 38% 25% 3	760
	11th to 20th	Night	8% 6% 21% 33% 26% 6%	751
	21st to end	Night	7% 5% 24% 31% 24% 9%	740
May	1st to 10th	Night	8% 4 15% 35% 30% 9%	720
	11th to 20th	Night	6% 4 14% 34% 35% 7%	631
	21st to end	Night	5% 3 16% 38% 30% 8%	389
June	1st to 10th	Night	4 2 11% 35% 40% 9%	360
	11th to 20th	Night	3 2 10% 30% 46% 9%	360
	21st to end	Night	2 1 5% 29% 47% 16%	360
July	1st to 10th	Night	1 4 19% 61% 15%	350
	11th to 20th	Night	2 2 13% 58% 26%	511
	21st to end	Night	1 2 17% 54% 25%	591
August	1st to 10th	Night	1 5% 30% 49% 14%	634
	11th to 20th	Night	1 1 2 23% 60% 14%	712
	21st to end	Night	1 1 3 23% 57% 16%	792
September	1st to 10th	Night	3 1 3 24% 54% 15%	720
	11th to 20th	Night	3 1 6% 33% 45% 12%	720
	21st to end	Night	3 1 9% 30% 46% 12%	720
October	1st to 10th	Night	5% 1 9% 36% 40% 8%	720
	11th to 20th	Night	6% 2 11% 35% 37% 9%	720
	21st to end	Night	7% 2 13% 35% 32% 11%	792
November	1st to 10th	Night	6% 4 14% 37% 31% 8%	740
	11th to 20th	Night	6% 3 16% 38% 29% 7%	760
	21st to end	Night	7% 5% 17% 42% 25% 5%	760
December	1st to 10th	Night	13% 6% 17% 35% 24% 4	760
	11th to 20th	Night	17% 4 19% 37% 19% 4	760
	21st to end	Night	14% 7% 22% 34% 19% 3	836

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