PAC Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PAC Observatory clear sky chart from 2005-11-06 to 2024-02-25.

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	10% 7% 3 6% 5% 3 4 4 5% 7% 45%	78473
	Night	10% 7% 3 6% 4 2 4 4 5% 7% 49%	80845
Transparency	Day	39% 6% 13% 25% 15% 2	78196
	Night	35% 5% 13% 26% 18% 3	80642
Seeing	Day	13% 10% 31% 29% 15% 3	14711
	Night	13% 9% 34% 33% 10% 1	24805

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	9% 11% 4 10% 10% 4 6% 5% 4 6% 30%	2847
	11th to 20th	Day	8% 11% 5% 8% 5% 3 8% 5% 4 5% 38%	2850
	21st to end	Day	7% 12% 6% 12% 7% 3 4 4 4 5% 35%	3135
February	1st to 10th	Day	9% 11% 4 7% 7% 3 4 5% 6% 8% 36%	1710
	11th to 20th	Day	8% 8% 4 8% 5% 2 5% 4 5% 8% 43%	1742
	21st to end	Day	9% 7% 4 6% 5% 4 4 4 5% 7% 44%	1520
March	1st to 10th	Day	9% 8% 4 6% 4 3 4 4 5% 6% 47%	1934
	11th to 20th	Day	16% 9% 4 5% 5% 2 4 3 4 6% 42%	1980
	21st to end	Day	16% 12% 4 7% 7% 3 5% 5% 4 6% 32%	2178
April	1st to 10th	Day	14% 7% 3 5% 4 2 3 3 4 6% 47%	2002
	11th to 20th	Day	13% 8% 2 7% 5% 3 4 5% 4 6% 42%	2256
	21st to end	Day	20% 6% 3 5% 4 3 4 4 4 7% 39%	2340
May	1st to 10th	Day	15% 8% 3 5% 6% 3 5% 4 5% 7% 39%	2340
	11th to 20th	Day	14% 6% 3 6% 5% 2 4 4 6% 8% 42%	2302
	21st to end	Day	14% 7% 3 6% 6% 4 5% 5% 5% 7% 35%	2531
June	1st to 10th	Day	6% 5% 2 4 5% 3 5% 4 6% 10% 50%	2402
	11th to 20th	Day	9% 4 2 4 4 3 5% 5% 6% 9% 48%	2520
	21st to end	Day	6% 5% 2 4 5% 4 5% 5% 6% 9% 48%	2344
July	1st to 10th	Day	8% 5% 2 4 4 3 5% 4 5% 9% 50%	2340
	11th to 20th	Day	5% 3 2 4 5% 3 4 5% 5% 8% 55%	2340
	21st to end	Day	6% 4 2 4 5% 4 5% 6% 7% 10% 47%	2546
August	1st to 10th	Day	7% 4 3 6% 5% 4 6% 6% 8% 11% 42%	2328
	11th to 20th	Day	7% 4 2 4 4 3 4 5% 5% 10% 52%	2301
	21st to end	Day	8% 4 2 5% 4 2 3 5% 5% 7% 55%	2546
September	1st to 10th	Day	14% 4 2 3 3 2 3 3 5% 6% 54%	2044
	11th to 20th	Day	10% 5% 2 4 4 3 3 4 5% 8% 53%	1980
	21st to end	Day	10% 4 2 3 4 3 4 4 4 7% 56%	1980
October	1st to 10th	Day	15% 5% 2 4 4 2 4 4 4 5% 51%	1868
	11th to 20th	Day	11% 4 2 5% 4 2 3 4 3 5% 57%	1800
	21st to end	Day	13% 6% 3 6% 4 3 3 3 4 6% 50%	1980
November	1st to 10th	Day	8% 5% 1 5% 4 2 3 4 4 6% 58%	1686
	11th to 20th	Day	8% 6% 2 5% 5% 2 3 4 5% 6% 53%	1710
	21st to end	Day	13% 7% 3 6% 4 2 3 3 5% 7% 47%	1573
December	1st to 10th	Day	8% 7% 2 5% 5% 3 4 5% 6% 8% 46%	1520
	11th to 20th	Day	8% 10% 3 7% 6% 3 4 4 5% 8% 42%	1918
	21st to end	Day	14% 12% 5% 13% 7% 3 5% 4 4 6% 27%	3080
January	1st to 10th	Night	7% 10% 4 13% 8% 3 4 5% 5% 6% 35%	4108
	11th to 20th	Night	8% 8% 3 6% 5% 3 8% 5% 5% 5% 43%	4120
	21st to end	Night	12% 12% 5% 9% 4 3 3 4 4 6% 39%	4532
February	1st to 10th	Night	11% 9% 4 6% 6% 3 5% 5% 5% 6% 39%	2470
	11th to 20th	Night	9% 7% 3 6% 6% 4 4 5% 6% 8% 43%	2342
	21st to end	Night	9% 7% 3 6% 5% 2 3 4 4 6% 50%	1830
March	1st to 10th	Night	9% 7% 3 6% 5% 2 4 3 4 7% 50%	2026
	11th to 20th	Night	15% 9% 3 4 4 2 3 4 5% 7% 44%	1980
	21st to end	Night	15% 10% 4 7% 5% 3 5% 4 6% 5% 35%	2178
April	1st to 10th	Night	12% 6% 2 5% 5% 3 4 4 4 6% 49%	1958
	11th to 20th	Night	11% 7% 3 5% 4 2 4 4 5% 8% 46%	1652
	21st to end	Night	16% 6% 2 4 4 3 3 4 4 8% 45%	1620
May	1st to 10th	Night	14% 7% 3 6% 5% 2 5% 4 6% 8% 41%	1620
	11th to 20th	Night	14% 5% 3 5% 5% 3 4 4 5% 7% 47%	1593
	21st to end	Night	12% 5% 2 5% 5% 3 5% 4 5% 9% 44%	1745
June	1st to 10th	Night	4 3 2 4 4 3 5% 4 7% 10% 55%	1548
	11th to 20th	Night	5% 4 1 4 4 3 4 5% 7% 10% 54%	1440
	21st to end	Night	4 2 2 2 4 2 3 4 6% 9% 61%	1603
July	1st to 10th	Night	7% 4 2 4 4 2 4 4 6% 10% 53%	1620
	11th to 20th	Night	3 3 1 3 4 2 4 5% 6% 9% 60%	1620
	21st to end	Night	4 2 1 4 3 3 5% 6% 7% 11% 55%	1772
August	1st to 10th	Night	4 3 2 3 5% 3 4 5% 7% 10% 54%	1611
	11th to 20th	Night	7% 3 2 2 4 3 4 4 5% 10% 57%	1600
	21st to end	Night	6% 3 1 3 3 2 4 4 4 7% 62%	1808
September	1st to 10th	Night	9% 4 2 3 3 2 3 4 4 6% 60%	1912
	11th to 20th	Night	9% 3 1 3 3 3 4 4 5% 8% 58%	1980
	21st to end	Night	8% 3 2 4 3 2 3 3 4 6% 61%	1980
October	1st to 10th	Night	12% 5% 2 4 4 2 4 4 6% 6% 52%	2073
	11th to 20th	Night	12% 5% 2 3 3 2 3 3 3 5% 58%	2160
	21st to end	Night	13% 7% 2 3 4 2 3 3 4 6% 54%	2376
November	1st to 10th	Night	6% 3 2 4 4 2 3 4 4 6% 60%	2391
	11th to 20th	Night	9% 6% 2 4 4 3 4 4 5% 6% 54%	2470
	21st to end	Night	10% 7% 3 4 3 2 2 3 4 7% 53%	2603
December	1st to 10th	Night	8% 6% 3 6% 5% 3 4 5% 4 6% 51%	2660
	11th to 20th	Night	9% 8% 3 6% 4 2 3 3 4 6% 52%	3279
	21st to end	Night	16% 13% 5% 10% 5% 2 3 3 4 5% 34%	4565

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	55% 5% 13% 17% 8% 1	2848
	11th to 20th	Day	48% 4 17% 19% 11% 1	2850
	21st to end	Day	51% 5% 13% 18% 12%	3135
February	1st to 10th	Day	45% 6% 16% 21% 10% 2	1710
	11th to 20th	Day	40% 6% 11% 25% 15% 3	1751
	21st to end	Day	40% 5% 13% 25% 15% 3	1520
March	1st to 10th	Day	39% 4 13% 26% 15% 3	1934
	11th to 20th	Day	45% 5% 9% 24% 15% 2	1980
	21st to end	Day	53% 6% 14% 18% 8% 1	2178
April	1st to 10th	Day	39% 4 13% 26% 16% 2	2002
	11th to 20th	Day	41% 5% 13% 24% 14% 2	2256
	21st to end	Day	46% 6% 10% 26% 12% 1	2340
May	1st to 10th	Day	46% 6% 13% 20% 14% 1	2340
	11th to 20th	Day	41% 7% 15% 25% 12%	2306
	21st to end	Day	46% 8% 14% 20% 10% 1	2531
June	1st to 10th	Day	29% 8% 15% 33% 13% 2	2402
	11th to 20th	Day	31% 7% 15% 27% 18% 2	2520
	21st to end	Day	31% 8% 15% 28% 14% 5%	2344
July	1st to 10th	Day	30% 6% 15% 30% 17% 3	2276
	11th to 20th	Day	26% 6% 14% 29% 20% 5%	2271
	21st to end	Day	29% 8% 15% 32% 15% 1	2405
August	1st to 10th	Day	33% 9% 20% 24% 11% 2	2314
	11th to 20th	Day	28% 7% 14% 29% 20% 1	2301
	21st to end	Day	29% 6% 12% 26% 22% 6%	2546
September	1st to 10th	Day	31% 4 12% 27% 20% 5%	2044
	11th to 20th	Day	31% 5% 11% 29% 22% 2	1980
	21st to end	Day	29% 4 10% 33% 24% 2	1980
October	1st to 10th	Day	36% 5% 11% 23% 24% 1	1868
	11th to 20th	Day	31% 4 8% 26% 24% 7%	1800
	21st to end	Day	36% 3 13% 22% 20% 5%	1980
November	1st to 10th	Day	29% 4 14% 28% 21% 3	1686
	11th to 20th	Day	32% 4 13% 28% 21% 2	1710
	21st to end	Day	38% 4 11% 28% 16% 2	1573
December	1st to 10th	Day	36% 5% 15% 29% 14% 1	1520
	11th to 20th	Day	42% 4 15% 29% 10%	1915
	21st to end	Day	59% 4 11% 17% 9% 1	3080
January	1st to 10th	Night	49% 5% 13% 20% 12% 1	4108
	11th to 20th	Night	43% 4 13% 26% 14%	4120
	21st to end	Night	47% 5% 14% 19% 14% 1	4532
February	1st to 10th	Night	46% 5% 12% 20% 14% 4	2470
	11th to 20th	Night	39% 5% 13% 25% 16% 3	2343
	21st to end	Night	35% 4 11% 24% 19% 6%	1830
March	1st to 10th	Night	35% 5% 11% 28% 17% 4	2026
	11th to 20th	Night	41% 4 10% 24% 18% 3	1980
	21st to end	Night	50% 5% 14% 19% 10% 2	2178
April	1st to 10th	Night	35% 5% 10% 27% 19% 3	1958
	11th to 20th	Night	37% 5% 12% 28% 16% 3	1652
	21st to end	Night	39% 4 13% 25% 18% 1	1620
May	1st to 10th	Night	42% 6% 14% 23% 13% 2	1620
	11th to 20th	Night	38% 6% 14% 25% 16% 1	1594
	21st to end	Night	38% 6% 19% 22% 13% 2	1746
June	1st to 10th	Night	23% 7% 16% 33% 18% 2	1548
	11th to 20th	Night	24% 8% 15% 31% 18% 5%	1440
	21st to end	Night	20% 5% 16% 33% 22% 4	1603
July	1st to 10th	Night	26% 7% 15% 29% 18% 5%	1575
	11th to 20th	Night	19% 7% 16% 27% 25% 6%	1566
	21st to end	Night	20% 7% 17% 35% 19% 2	1673
August	1st to 10th	Night	24% 8% 21% 30% 17% 2	1601
	11th to 20th	Night	24% 6% 13% 35% 19% 3	1600
	21st to end	Night	23% 5% 12% 28% 22% 9%	1808
September	1st to 10th	Night	24% 4 13% 29% 22% 8%	1912
	11th to 20th	Night	25% 6% 10% 32% 24% 4	1980
	21st to end	Night	26% 4 7% 29% 31% 3	1980
October	1st to 10th	Night	32% 6% 11% 24% 24% 3	2073
	11th to 20th	Night	30% 3 8% 27% 25% 8%	2160
	21st to end	Night	33% 4 10% 25% 25% 4	2376
November	1st to 10th	Night	25% 4 12% 27% 26% 6%	2391
	11th to 20th	Night	31% 4 10% 30% 22% 3	2470
	21st to end	Night	33% 4 11% 27% 22% 3	2603
December	1st to 10th	Night	33% 5% 14% 30% 16% 1	2660
	11th to 20th	Night	35% 3 12% 35% 14%	3281
	21st to end	Night	54% 4 10% 22% 8% 1	4565

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	18% 16% 37% 21% 6% 2	270
	11th to 20th	Day	21% 16% 34% 21% 7%	270
	21st to end	Day	15% 10% 47% 22% 6%	297
February	1st to 10th	Day	14% 21% 44% 18% 3	270
	11th to 20th	Day	14% 14% 43% 19% 9%	270
	21st to end	Day	12% 20% 45% 19% 4	210
March	1st to 10th	Day	17% 14% 44% 20% 5% 1	240
	11th to 20th	Day	28% 13% 34% 19% 6% 1	240
	21st to end	Day	30% 17% 36% 13% 3	264
April	1st to 10th	Day	21% 18% 39% 17% 4 1	262
	11th to 20th	Day	17% 17% 38% 22% 6% 1	534
	21st to end	Day	22% 19% 29% 20% 9% 1	600
May	1st to 10th	Day	18% 13% 35% 23% 9% 1	603
	11th to 20th	Day	17% 11% 32% 25% 12% 2	625
	21st to end	Day	18% 8% 30% 29% 13% 2	679
June	1st to 10th	Day	5% 5% 23% 36% 25% 6%	630
	11th to 20th	Day	7% 5% 32% 33% 19% 3	630
	21st to end	Day	6% 8% 34% 35% 15% 2	630
July	1st to 10th	Day	9% 3 18% 36% 27% 7%	621
	11th to 20th	Day	6% 4 21% 38% 24% 7%	618
	21st to end	Day	7% 1 17% 41% 30% 4	684
August	1st to 10th	Day	7% 2 21% 41% 25% 4	626
	11th to 20th	Day	9% 3 19% 42% 23% 3	615
	21st to end	Day	10% 3 21% 35% 26% 6%	665
September	1st to 10th	Day	14% 4 24% 35% 19% 4	334
	11th to 20th	Day	6% 4 29% 34% 21% 5%	270
	21st to end	Day	15% 9% 32% 28% 13% 3	270
October	1st to 10th	Day	24% 7% 24% 25% 15% 5%	270
	11th to 20th	Day	8% 19% 43% 21% 8% 1	270
	21st to end	Day	17% 18% 35% 20% 8% 2	297
November	1st to 10th	Day	13% 14% 39% 23% 9% 1	270
	11th to 20th	Day	13% 15% 43% 21% 7% 1	270
	21st to end	Day	23% 12% 33% 24% 8%	270
December	1st to 10th	Day	9% 12% 44% 27% 7% 1	270
	11th to 20th	Day	15% 15% 36% 25% 9%	270
	21st to end	Day	19% 15% 42% 22% 1	297
January	1st to 10th	Night	12% 12% 38% 32% 6%	950
	11th to 20th	Night	14% 12% 39% 28% 7%	950
	21st to end	Night	15% 9% 43% 28% 5%	1045
February	1st to 10th	Night	15% 11% 43% 27% 4	950
	11th to 20th	Night	13% 14% 39% 29% 4	868
	21st to end	Night	14% 14% 42% 24% 5%	610
March	1st to 10th	Night	14% 14% 40% 25% 6% 1	586
	11th to 20th	Night	20% 12% 34% 27% 7%	540
	21st to end	Night	20% 12% 39% 23% 5%	594
April	1st to 10th	Night	16% 14% 42% 23% 5%	540
	11th to 20th	Night	16% 12% 39% 29% 4 1	533
	21st to end	Night	18% 11% 35% 27% 8% 1	540
May	1st to 10th	Night	17% 10% 33% 31% 8%	540
	11th to 20th	Night	16% 10% 32% 33% 9% 1	531
	21st to end	Night	15% 8% 31% 32% 14% 1	584
June	1st to 10th	Night	6% 4 26% 43% 20% 1	540
	11th to 20th	Night	8% 4 31% 42% 14% 1	540
	21st to end	Night	5% 4 35% 43% 13%	540
July	1st to 10th	Night	9% 2 16% 46% 27%	525
	11th to 20th	Night	5% 1 21% 47% 25% 2	522
	21st to end	Night	3 1 17% 52% 26% 1	591
August	1st to 10th	Night	4 2 17% 51% 22% 3	537
	11th to 20th	Night	7% 3 18% 50% 21% 2	534
	21st to end	Night	9% 1 22% 42% 25% 2	594
September	1st to 10th	Night	11% 3 26% 40% 17% 1	540
	11th to 20th	Night	10% 3 27% 41% 18% 1	540
	21st to end	Night	9% 6% 34% 37% 14% 1	540
October	1st to 10th	Night	16% 8% 30% 32% 13% 2	633
	11th to 20th	Night	14% 12% 37% 29% 8%	720
	21st to end	Night	17% 12% 31% 31% 9%	792
November	1st to 10th	Night	9% 11% 35% 35% 10%	911
	11th to 20th	Night	12% 12% 41% 29% 6%	950
	21st to end	Night	15% 12% 36% 28% 9% 1	950
December	1st to 10th	Night	12% 11% 40% 30% 6%	950
	11th to 20th	Night	16% 11% 36% 31% 7%	950
	21st to end	Night	18% 13% 36% 29% 5%	1045

Page generated at 2024-02-22 05:05:46. Version ac07

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