The Peninsula Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Peninsula Observatory clear sky chart from 2005-11-06 to 2024-07-05.

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	12% 6% 2 6% 4 4 4 4 6% 8% 42%	89504
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 43%	89690
Transparency	Day	18% 55% 6% 11% 8% 2	89270
	Night	20% 54% 5% 11% 8% 2	89528
Seeing	Day	16% 13% 28% 25% 14% 4	14884
	Night	16% 11% 29% 30% 13% 2	24575

By Month

Percentage Cloud Cover SE (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% 7% 1 5% 3 3 3 2 4 5% 51%	2772
	11th to 20th	Day	12% 7% 3 6% 4 3 3 3 4 6% 49%	2840
	21st to end	Day	13% 10% 3 7% 5% 4 4 3 4 6% 40%	3124
February	1st to 10th	Day	15% 6% 2 7% 4 4 4 3 6% 8% 41%	1710
	11th to 20th	Day	19% 6% 2 5% 4 3 3 3 5% 5% 44%	1828
	21st to end	Day	15% 8% 2 6% 4 4 4 4 5% 6% 41%	1570
March	1st to 10th	Day	23% 8% 2 6% 4 3 3 3 4 6% 37%	3343
	11th to 20th	Day	11% 5% 2 5% 3 3 3 3 4 6% 55%	3480
	21st to end	Day	16% 6% 3 6% 4 3 3 3 5% 5% 47%	3924
April	1st to 10th	Day	12% 6% 1 5% 4 3 3 2 6% 6% 53%	2280
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 5% 8% 51%	2254
	21st to end	Day	10% 5% 2 5% 5% 4 4 4 6% 7% 47%	2280
May	1st to 10th	Day	9% 7% 3 7% 5% 5% 7% 5% 7% 9% 36%	3765
	11th to 20th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 8% 32%	4060
	21st to end	Day	7% 6% 3 6% 4 5% 6% 4 7% 10% 42%	4823
June	1st to 10th	Day	9% 6% 1 6% 5% 5% 5% 6% 8% 10% 39%	2660
	11th to 20th	Day	10% 5% 2 4 4 5% 5% 6% 8% 11% 41%	2660
	21st to end	Day	6% 4 2 6% 6% 5% 6% 6% 9% 11% 38%	2660
July	1st to 10th	Day	8% 6% 2 5% 6% 4 6% 5% 9% 12% 37%	2550
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 6% 10% 12% 34%	2340
	21st to end	Day	8% 5% 2 6% 6% 6% 7% 6% 9% 12% 34%	2547
August	1st to 10th	Day	11% 7% 3 7% 5% 6% 6% 5% 8% 10% 33%	2292
	11th to 20th	Day	11% 6% 3 7% 5% 6% 6% 6% 8% 10% 33%	2126
	21st to end	Day	11% 6% 2 7% 5% 6% 6% 5% 7% 9% 37%	2376
September	1st to 10th	Day	12% 6% 2 7% 4 5% 5% 5% 7% 9% 39%	2160
	11th to 20th	Day	12% 8% 2 7% 4 4 4 4 6% 7% 41%	2160
	21st to end	Day	19% 8% 3 7% 5% 4 4 4 6% 7% 34%	1927
October	1st to 10th	Day	14% 7% 2 6% 4 3 4 3 5% 7% 44%	1800
	11th to 20th	Day	12% 6% 2 7% 4 3 4 3 5% 6% 49%	1800
	21st to end	Day	12% 6% 2 6% 4 3 3 3 5% 5% 51%	1980
November	1st to 10th	Day	16% 6% 2 5% 3 3 4 4 5% 6% 48%	1810
	11th to 20th	Day	15% 7% 2 4 3 2 3 3 4 7% 51%	1710
	21st to end	Day	13% 5% 2 4 4 3 4 3 5% 7% 49%	1691
December	1st to 10th	Day	17% 7% 2 6% 5% 4 4 4 5% 7% 39%	1520
	11th to 20th	Day	16% 6% 1 5% 3 2 3 2 3 5% 53%	1910
	21st to end	Day	15% 7% 2 5% 4 4 4 3 4 7% 44%	2772
January	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 5% 6% 46%	4169
	11th to 20th	Night	15% 7% 3 7% 5% 4 3 4 6% 6% 41%	4120
	21st to end	Night	14% 8% 3 7% 5% 4 4 3 5% 5% 42%	4532
February	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 5% 7% 41%	2470
	11th to 20th	Night	19% 9% 2 6% 4 3 3 3 4 6% 41%	2237
	21st to end	Night	15% 8% 2 6% 4 3 3 3 5% 7% 45%	1884
March	1st to 10th	Night	24% 10% 2 7% 4 4 4 4 4 5% 32%	3493
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 5% 51%	3480
	21st to end	Night	17% 7% 2 5% 3 3 3 2 4 5% 48%	3706
April	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 5% 7% 46%	1900
	11th to 20th	Night	10% 7% 2 6% 4 4 5% 3 5% 7% 47%	1875
	21st to end	Night	12% 6% 2 4 4 4 3 3 6% 7% 47%	1900
May	1st to 10th	Night	10% 7% 2 8% 6% 5% 6% 5% 4 8% 38%	3130
	11th to 20th	Night	19% 7% 3 6% 5% 3 5% 4 5% 7% 36%	2858
	21st to end	Night	8% 5% 2 7% 4 4 4 5% 7% 11% 42%	2732
June	1st to 10th	Night	10% 7% 2 5% 5% 4 4 5% 7% 11% 40%	1520
	11th to 20th	Night	7% 5% 2 6% 6% 4 6% 5% 9% 12% 38%	1520
	21st to end	Night	8% 5% 2 6% 5% 4 4 5% 9% 11% 40%	1520
July	1st to 10th	Night	6% 6% 2 7% 6% 6% 6% 5% 8% 12% 35%	1499
	11th to 20th	Night	8% 5% 1 6% 4 4 6% 5% 8% 13% 40%	1620
	21st to end	Night	7% 6% 2 7% 5% 6% 5% 6% 9% 10% 36%	1771
August	1st to 10th	Night	11% 6% 3 7% 5% 5% 5% 4 6% 10% 38%	1627
	11th to 20th	Night	8% 6% 3 9% 6% 6% 5% 4 7% 11% 36%	1774
	21st to end	Night	11% 7% 2 7% 4 4 6% 5% 7% 8% 36%	1980
September	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 5% 6% 9% 42%	1800
	11th to 20th	Night	13% 7% 3 5% 4 4 4 3 7% 8% 42%	1800
	21st to end	Night	17% 8% 3 7% 5% 4 5% 4 7% 7% 33%	2029
October	1st to 10th	Night	13% 6% 2 6% 3 4 3 4 5% 6% 48%	2160
	11th to 20th	Night	13% 6% 2 5% 3 3 4 2 5% 6% 51%	2160
	21st to end	Night	12% 5% 2 5% 3 3 4 3 5% 6% 53%	2376
November	1st to 10th	Night	16% 5% 2 5% 3 3 3 3 5% 6% 49%	2264
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 5% 6% 48%	2470
	21st to end	Night	12% 6% 2 5% 3 4 3 2 6% 7% 50%	2485
December	1st to 10th	Night	15% 8% 3 7% 4 4 5% 4 4 6% 40%	2660
	11th to 20th	Night	17% 6% 2 6% 4 3 3 3 5% 7% 46%	3285
	21st to end	Night	14% 6% 2 5% 3 3 4 3 6% 7% 45%	4884

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	22% 38% 3 20% 15% 3	2773
	11th to 20th	Day	22% 38% 4 10% 21% 5%	2840
	21st to end	Day	30% 42% 5% 9% 9% 5%	3124
February	1st to 10th	Day	15% 65% 5% 9% 6% 1	1710
	11th to 20th	Day	17% 64% 3 6% 7% 3	1837
	21st to end	Day	16% 65% 3 6% 7% 2	1570
March	1st to 10th	Day	36% 43% 6% 7% 4 4	3343
	11th to 20th	Day	16% 41% 3 10% 19% 11%	3480
	21st to end	Day	23% 40% 6% 15% 12% 4	3924
April	1st to 10th	Day	12% 65% 5% 8% 9% 1	2280
	11th to 20th	Day	12% 66% 3 10% 6% 2	2254
	21st to end	Day	13% 66% 5% 10% 6% 1	2280
May	1st to 10th	Day	27% 47% 10% 10% 5% 1	3800
	11th to 20th	Day	31% 43% 7% 14% 5%	4064
	21st to end	Day	18% 43% 8% 20% 9% 1	4837
June	1st to 10th	Day	13% 68% 5% 7% 6% 1	2660
	11th to 20th	Day	9% 67% 8% 9% 6%	2660
	21st to end	Day	11% 66% 8% 10% 5%	2660
July	1st to 10th	Day	15% 64% 9% 8% 4	2481
	11th to 20th	Day	14% 65% 9% 10% 2	2271
	21st to end	Day	15% 63% 7% 9% 6% 1	2405
August	1st to 10th	Day	17% 65% 8% 7% 3	2279
	11th to 20th	Day	14% 67% 8% 8% 4	2126
	21st to end	Day	14% 66% 8% 7% 5% 1	2376
September	1st to 10th	Day	15% 67% 8% 6% 3 1	2160
	11th to 20th	Day	15% 65% 6% 7% 5% 2	2160
	21st to end	Day	21% 64% 5% 8% 2	1927
October	1st to 10th	Day	14% 64% 6% 10% 7%	1800
	11th to 20th	Day	12% 63% 3 11% 9% 1	1800
	21st to end	Day	11% 62% 3 9% 13% 4	1980
November	1st to 10th	Day	17% 59% 5% 9% 8% 2	1810
	11th to 20th	Day	15% 59% 3 8% 10% 4	1710
	21st to end	Day	13% 61% 3 11% 10% 2	1691
December	1st to 10th	Day	22% 60% 4 7% 4 2	1520
	11th to 20th	Day	16% 47% 3 16% 10% 6%	1906
	21st to end	Day	26% 39% 4 17% 14% 1	2772
January	1st to 10th	Night	24% 38% 4 16% 14% 3	4169
	11th to 20th	Night	27% 38% 3 12% 16% 4	4120
	21st to end	Night	28% 42% 4 9% 12% 5%	4532
February	1st to 10th	Night	13% 65% 4 10% 5% 1	2470
	11th to 20th	Night	20% 64% 3 5% 7% 2	2239
	21st to end	Night	15% 65% 2 9% 7% 2	1884
March	1st to 10th	Night	41% 43% 5% 5% 4 1	3493
	11th to 20th	Night	18% 41% 4 8% 17% 12%	3480
	21st to end	Night	23% 41% 4 19% 11% 4	3706
April	1st to 10th	Night	14% 67% 5% 5% 8% 2	1900
	11th to 20th	Night	15% 66% 4 9% 5% 1	1875
	21st to end	Night	13% 66% 5% 11% 6% 1	1900
May	1st to 10th	Night	28% 45% 11% 12% 4 1	3160
	11th to 20th	Night	29% 43% 6% 17% 5%	2859
	21st to end	Night	19% 45% 13% 15% 7% 1	2739
June	1st to 10th	Night	14% 67% 6% 8% 5% 1	1520
	11th to 20th	Night	9% 68% 9% 10% 5%	1520
	21st to end	Night	11% 65% 7% 12% 5%	1520
July	1st to 10th	Night	16% 65% 7% 8% 3 1	1458
	11th to 20th	Night	14% 64% 8% 12% 1	1566
	21st to end	Night	14% 64% 7% 9% 5% 1	1672
August	1st to 10th	Night	17% 64% 8% 9% 2	1616
	11th to 20th	Night	13% 66% 9% 8% 3	1774
	21st to end	Night	15% 64% 8% 8% 3 2	1980
September	1st to 10th	Night	15% 66% 9% 6% 3 1	1800
	11th to 20th	Night	15% 65% 5% 7% 6% 2	1800
	21st to end	Night	21% 65% 5% 7% 2	2029
October	1st to 10th	Night	13% 64% 4 12% 6% 1	2160
	11th to 20th	Night	9% 63% 5% 10% 10% 2	2160
	21st to end	Night	10% 62% 3 11% 11% 3	2376
November	1st to 10th	Night	19% 59% 3 9% 8% 2	2264
	11th to 20th	Night	17% 60% 4 5% 9% 5%	2470
	21st to end	Night	13% 60% 3 12% 10% 2	2485
December	1st to 10th	Night	22% 61% 3 7% 5% 1	2660
	11th to 20th	Night	19% 49% 3 12% 13% 4	3288
	21st to end	Night	23% 39% 7% 16% 13% 1	4884

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	19% 34% 34% 13% 1	252
	11th to 20th	Day	23% 27% 34% 15% 1	270
	21st to end	Day	21% 28% 38% 10% 2 1	297
February	1st to 10th	Day	19% 26% 40% 13% 2	270
	11th to 20th	Day	29% 24% 34% 11% 1	436
	21st to end	Day	23% 25% 32% 16% 3 1	382
March	1st to 10th	Day	18% 25% 34% 18% 5%	460
	11th to 20th	Day	21% 24% 35% 14% 4 2	460
	21st to end	Day	21% 21% 39% 14% 6%	506
April	1st to 10th	Day	18% 26% 35% 14% 5% 2	460
	11th to 20th	Day	14% 20% 35% 20% 8% 3	452
	21st to end	Day	15% 15% 34% 27% 7% 2	460
May	1st to 10th	Day	14% 17% 36% 24% 7% 2	463
	11th to 20th	Day	15% 11% 33% 28% 11% 2	487
	21st to end	Day	14% 7% 26% 29% 18% 6%	529
June	1st to 10th	Day	11% 6% 29% 30% 22% 2	490
	11th to 20th	Day	11% 8% 20% 32% 21% 7%	490
	21st to end	Day	7% 4 26% 33% 24% 6%	490
July	1st to 10th	Day	9% 4 15% 35% 29% 8%	461
	11th to 20th	Day	9% 2 12% 36% 32% 8%	444
	21st to end	Day	9% 3 21% 39% 23% 6%	488
August	1st to 10th	Day	13% 4 19% 34% 21% 9%	446
	11th to 20th	Day	12% 2 13% 32% 28% 13%	439
	21st to end	Day	11% 3 17% 33% 24% 11%	495
September	1st to 10th	Day	13% 3 24% 27% 21% 13%	450
	11th to 20th	Day	14% 6% 22% 30% 18% 9%	450
	21st to end	Day	22% 7% 24% 25% 16% 6%	450
October	1st to 10th	Day	17% 8% 23% 28% 16% 8%	450
	11th to 20th	Day	15% 12% 27% 27% 16% 3	450
	21st to end	Day	12% 15% 31% 24% 16% 3	495
November	1st to 10th	Day	19% 12% 33% 20% 11% 3	433
	11th to 20th	Day	19% 16% 32% 24% 9% 1	370
	21st to end	Day	16% 17% 28% 24% 14% 1	351
December	1st to 10th	Day	19% 23% 40% 11% 5% 2	180
	11th to 20th	Day	17% 24% 33% 18% 7%	180
	21st to end	Day	23% 19% 32% 15% 10% 1	198
January	1st to 10th	Night	20% 22% 31% 23% 4	760
	11th to 20th	Night	20% 17% 33% 24% 6%	760
	21st to end	Night	18% 18% 41% 21% 2	836
February	1st to 10th	Night	21% 17% 34% 25% 3	760
	11th to 20th	Night	25% 20% 36% 17% 2	743
	21st to end	Night	17% 21% 36% 22% 4	628
March	1st to 10th	Night	18% 17% 38% 21% 5%	760
	11th to 20th	Night	21% 18% 38% 18% 5% 1	760
	21st to end	Night	19% 17% 37% 22% 5%	836
April	1st to 10th	Night	17% 18% 40% 22% 3	760
	11th to 20th	Night	14% 15% 40% 23% 8%	751
	21st to end	Night	14% 10% 42% 29% 5% 1	760
May	1st to 10th	Night	14% 12% 37% 30% 7%	760
	11th to 20th	Night	17% 8% 29% 35% 10%	594
	21st to end	Night	11% 2 20% 45% 21% 1	411
June	1st to 10th	Night	12% 4 26% 42% 15% 1	380
	11th to 20th	Night	7% 3 24% 44% 19% 3	380
	21st to end	Night	8% 3 21% 51% 17% 1	380
July	1st to 10th	Night	7% 2 17% 47% 24% 2	380
	11th to 20th	Night	8% 2 11% 43% 30% 6%	522
	21st to end	Night	9% 3 16% 46% 25% 2	591
August	1st to 10th	Night	13% 3 12% 41% 27% 4	553
	11th to 20th	Night	10% 3 15% 41% 28% 4	710
	21st to end	Night	12% 4 13% 34% 31% 5%	792
September	1st to 10th	Night	11% 2 14% 38% 27% 8%	720
	11th to 20th	Night	15% 4 18% 35% 24% 3	720
	21st to end	Night	21% 6% 18% 32% 21% 3	720
October	1st to 10th	Night	14% 5% 24% 29% 23% 5%	720
	11th to 20th	Night	16% 9% 28% 28% 17% 2	720
	21st to end	Night	15% 9% 29% 29% 16% 3	792
November	1st to 10th	Night	18% 8% 28% 30% 12% 4	740
	11th to 20th	Night	20% 11% 32% 26% 10% 1	760
	21st to end	Night	14% 14% 29% 27% 14% 1	760
December	1st to 10th	Night	20% 14% 34% 24% 7%	760
	11th to 20th	Night	18% 13% 36% 26% 8%	760
	21st to end	Night	18% 16% 33% 27% 7%	836

Page generated at 2024-07-02 06:41:28. Version ac07

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