PACMO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PACMO clear sky chart from 2005-11-06 to 2026-08-15.

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% 8% 3 7% 5% 4 5% 4 5% 7% 39%	144318
	Night	12% 9% 3 8% 5% 4 4 3 5% 6% 41%	137887
Transparency	Day	42% 6% 12% 24% 14% 2	147840
	Night	43% 5% 11% 23% 16% 2	142442
Seeing	Day	16% 12% 30% 25% 13% 4	18292
	Night	17% 11% 32% 29% 11% 1	28053

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	8% 13% 5% 14% 6% 6% 5% 4 5% 6% 27%	4691
	11th to 20th	Day	7% 10% 6% 10% 7% 6% 6% 5% 7% 6% 31%	4710
	21st to end	Day	11% 14% 7% 9% 5% 6% 5% 4 5% 5% 30%	5794
February	1st to 10th	Day	14% 13% 5% 9% 5% 4 5% 4 4 7% 30%	1890
	11th to 20th	Day	14% 10% 3 8% 6% 5% 4 3 4 7% 36%	1961
	21st to end	Day	13% 12% 4 10% 4 3 4 3 5% 5% 36%	1730
March	1st to 10th	Day	18% 11% 3 5% 3 2 3 2 4 5% 43%	4580
	11th to 20th	Day	9% 7% 2 6% 4 4 5% 4 6% 7% 47%	5090
	21st to end	Day	20% 10% 4 10% 5% 5% 4 3 5% 6% 28%	5909
April	1st to 10th	Day	15% 13% 4 7% 6% 4 4 3 5% 6% 33%	2520
	11th to 20th	Day	16% 10% 3 7% 4 4 4 3 4 6% 36%	2494
	21st to end	Day	19% 9% 2 7% 5% 4 4 3 4 6% 35%	2520
May	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 6% 8% 38%	7771
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 3 5% 7% 43%	8222
	21st to end	Day	14% 7% 3 8% 4 4 5% 4 6% 8% 38%	9047
June	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 5% 10% 44%	2940
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 5% 8% 46%	2940
	21st to end	Day	10% 6% 2 6% 4 5% 5% 5% 7% 7% 42%	2940
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 5% 4 6% 10% 41%	8280
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 49%	8280
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	8990
August	1st to 10th	Day	8% 4 2 6% 5% 5% 5% 5% 7% 11% 43%	4374
	11th to 20th	Day	11% 5% 2 6% 3 5% 5% 4 7% 9% 42%	4004
	21st to end	Day	7% 4 2 6% 4 4 4 4 6% 9% 50%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 5% 3 5% 8% 48%	2400
	11th to 20th	Day	10% 4 2 4 3 3 4 3 5% 8% 54%	2375
	21st to end	Day	10% 5% 2 6% 4 5% 3 4 6% 7% 48%	2182
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 5% 6% 50%	2000
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 4 6% 44%	2000
	21st to end	Day	18% 8% 3 6% 4 4 3 3 5% 6% 42%	2104
November	1st to 10th	Day	11% 9% 2 5% 4 4 5% 3 5% 6% 45%	1852
	11th to 20th	Day	14% 12% 3 8% 4 3 4 4 6% 7% 36%	1888
	21st to end	Day	15% 12% 3 8% 5% 4 4 3 4 4 37%	1680
December	1st to 10th	Day	8% 10% 3 8% 5% 3 4 3 4 6% 46%	2690
	11th to 20th	Day	14% 12% 5% 16% 5% 5% 4 4 6% 6% 24%	3088
	21st to end	Day	23% 20% 8% 12% 5% 3 3 2 3 4 15%	4070
January	1st to 10th	Night	12% 13% 5% 12% 5% 4 4 3 4 5% 33%	8232
	11th to 20th	Night	9% 12% 5% 12% 7% 7% 5% 4 6% 5% 27%	8280
	21st to end	Night	12% 13% 5% 10% 5% 4 4 3 4 5% 35%	8404
February	1st to 10th	Night	14% 12% 4 8% 5% 4 4 4 5% 5% 35%	2730
	11th to 20th	Night	12% 11% 4 7% 4 4 4 3 5% 5% 40%	2545
	21st to end	Night	14% 11% 3 8% 4 4 3 3 3 5% 41%	2076
March	1st to 10th	Night	18% 9% 3 5% 3 3 4 3 4 5% 43%	5493
	11th to 20th	Night	11% 6% 3 6% 4 4 4 2 5% 7% 48%	5120
	21st to end	Night	17% 12% 5% 10% 6% 4 4 3 5% 5% 28%	5261
April	1st to 10th	Night	16% 11% 3 7% 4 3 4 3 5% 6% 39%	2100
	11th to 20th	Night	17% 9% 2 6% 4 4 4 3 4 6% 41%	2075
	21st to end	Night	16% 7% 3 8% 4 4 4 3 5% 7% 38%	2100
May	1st to 10th	Night	15% 7% 2 7% 5% 4 5% 3 5% 6% 40%	5067
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 4 7% 9% 43%	4670
	21st to end	Night	12% 8% 3 7% 5% 4 4 3 6% 7% 42%	5139
June	1st to 10th	Night	9% 5% 2 5% 4 3 4 3 6% 9% 50%	1680
	11th to 20th	Night	8% 5% 2 6% 5% 4 4 4 6% 9% 46%	1680
	21st to end	Night	10% 6% 2 6% 5% 4 5% 3 5% 7% 48%	1680
July	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 7% 10% 45%	4708
	11th to 20th	Night	7% 4 2 5% 3 3 3 3 5% 8% 57%	4707
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 6% 8% 45%	5117
August	1st to 10th	Night	5% 3 2 5% 3 4 4 4 7% 9% 54%	3002
	11th to 20th	Night	7% 4 2 6% 4 4 4 3 5% 8% 53%	3136
	21st to end	Night	6% 3 2 5% 4 3 4 3 6% 8% 55%	3586
September	1st to 10th	Night	10% 4 1 5% 3 3 3 2 5% 7% 58%	2000
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 6% 7% 53%	2004
	21st to end	Night	9% 4 2 5% 4 4 5% 3 6% 8% 50%	2215
October	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 4 5% 55%	2400
	11th to 20th	Night	15% 7% 2 5% 4 3 4 2 4 7% 47%	2400
	21st to end	Night	17% 9% 3 6% 5% 4 4 3 5% 5% 39%	2735
November	1st to 10th	Night	11% 7% 2 5% 3 2 3 3 4 6% 52%	2666
	11th to 20th	Night	15% 8% 4 6% 4 4 4 3 5% 6% 40%	2730
	21st to end	Night	16% 11% 3 8% 5% 4 3 2 4 5% 39%	2940
December	1st to 10th	Night	8% 10% 3 7% 5% 3 4 3 5% 5% 47%	4720
	11th to 20th	Night	15% 13% 5% 9% 5% 4 3 3 4 5% 32%	5339
	21st to end	Night	23% 18% 6% 11% 5% 5% 4 4 4 4 16%	7150

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	57% 5% 13% 18% 6% 1	4692
	11th to 20th	Day	52% 4 8% 19% 14% 3	4710
	21st to end	Day	56% 3 13% 20% 7% 1	5794
February	1st to 10th	Day	55% 4 15% 18% 7% 1	1890
	11th to 20th	Day	49% 5% 12% 22% 10% 2	1970
	21st to end	Day	50% 6% 11% 23% 9% 2	1730
March	1st to 10th	Day	44% 5% 9% 25% 14% 3	4580
	11th to 20th	Day	35% 6% 11% 33% 15% 1	5090
	21st to end	Day	55% 7% 13% 18% 6%	5911
April	1st to 10th	Day	51% 6% 11% 21% 10%	2520
	11th to 20th	Day	48% 6% 10% 22% 13% 2	2494
	21st to end	Day	49% 6% 11% 20% 13% 2	2520
May	1st to 10th	Day	42% 6% 11% 22% 18% 1	7807
	11th to 20th	Day	40% 6% 10% 26% 15% 3	8226
	21st to end	Day	43% 6% 11% 21% 16% 3	9061
June	1st to 10th	Day	35% 8% 14% 24% 17% 2	2940
	11th to 20th	Day	35% 6% 12% 23% 22% 3	2940
	21st to end	Day	37% 9% 14% 24% 14% 2	2940
July	1st to 10th	Day	38% 6% 13% 27% 15% 1	8211
	11th to 20th	Day	32% 6% 13% 31% 17% 1	8205
	21st to end	Day	39% 7% 17% 25% 11% 1	8794
August	1st to 10th	Day	33% 8% 17% 29% 12% 1	4359
	11th to 20th	Day	37% 7% 15% 26% 15% 1	4042
	21st to end	Day	31% 6% 13% 31% 17% 2	4311
September	1st to 10th	Day	34% 7% 10% 26% 20% 4	2400
	11th to 20th	Day	28% 6% 13% 28% 23% 3	2375
	21st to end	Day	33% 6% 11% 27% 20% 2	2182
October	1st to 10th	Day	23% 3 6% 29% 37% 1	3260
	11th to 20th	Day	29% 3 11% 23% 31% 4	3240
	21st to end	Day	39% 4 9% 28% 16% 3	3381
November	1st to 10th	Day	38% 6% 14% 25% 14% 3	1852
	11th to 20th	Day	48% 5% 13% 22% 11% 1	1888
	21st to end	Day	51% 5% 13% 21% 9%	1680
December	1st to 10th	Day	40% 4 14% 34% 8% 1	2690
	11th to 20th	Day	60% 5% 12% 16% 6% 1	3085
	21st to end	Day	74% 4 8% 10% 3	4070
January	1st to 10th	Night	53% 5% 15% 20% 7%	8233
	11th to 20th	Night	57% 4 11% 14% 14% 1	8280
	21st to end	Night	52% 4 12% 20% 11% 1	8404
February	1st to 10th	Night	51% 4 12% 20% 11% 1	2730
	11th to 20th	Night	46% 4 12% 24% 13% 1	2546
	21st to end	Night	46% 4 10% 23% 14% 2	2076
March	1st to 10th	Night	44% 5% 9% 25% 15% 3	5493
	11th to 20th	Night	35% 4 11% 29% 18% 4	5120
	21st to end	Night	56% 7% 11% 17% 8% 1	5261
April	1st to 10th	Night	46% 6% 10% 24% 13% 2	2100
	11th to 20th	Night	44% 5% 8% 23% 16% 4	2075
	21st to end	Night	45% 7% 10% 22% 13% 3	2100
May	1st to 10th	Night	43% 7% 9% 20% 20% 2	5094
	11th to 20th	Night	36% 8% 14% 23% 16% 4	4671
	21st to end	Night	40% 6% 10% 23% 18% 3	5149
June	1st to 10th	Night	31% 7% 15% 26% 19% 3	1680
	11th to 20th	Night	32% 7% 11% 22% 24% 4	1680
	21st to end	Night	35% 7% 14% 21% 19% 3	1680
July	1st to 10th	Night	34% 7% 15% 25% 17% 2	4670
	11th to 20th	Night	26% 5% 12% 30% 24% 1	4662
	21st to end	Night	36% 7% 14% 28% 14% 2	5002
August	1st to 10th	Night	25% 7% 17% 32% 17% 2	2993
	11th to 20th	Night	30% 6% 16% 30% 17% 1	3164
	21st to end	Night	27% 6% 14% 30% 19% 4	3586
September	1st to 10th	Night	27% 5% 11% 27% 27% 3	2000
	11th to 20th	Night	29% 7% 10% 29% 21% 5%	2004
	21st to end	Night	30% 7% 12% 25% 23% 3	2215
October	1st to 10th	Night	23% 3 7% 30% 33% 3	3920
	11th to 20th	Night	28% 4 8% 23% 33% 4	3899
	21st to end	Night	43% 4 8% 22% 20% 4	4408
November	1st to 10th	Night	34% 5% 12% 27% 19% 4	2666
	11th to 20th	Night	45% 4 11% 25% 14% 2	2730
	21st to end	Night	48% 4 10% 24% 12% 1	2940
December	1st to 10th	Night	40% 3 12% 33% 11% 1	4720
	11th to 20th	Night	54% 4 11% 22% 8% 1	5341
	21st to end	Night	71% 6% 9% 10% 4	7150

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	20% 16% 34% 25% 5%	330
	11th to 20th	Day	20% 22% 42% 13% 3	330
	21st to end	Day	20% 22% 39% 16% 3 1	363
February	1st to 10th	Day	20% 21% 38% 17% 3	330
	11th to 20th	Day	25% 21% 34% 16% 4 1	330
	21st to end	Day	23% 18% 47% 10% 2	273
March	1st to 10th	Day	23% 21% 36% 18% 3	330
	11th to 20th	Day	23% 20% 36% 15% 5% 1	330
	21st to end	Day	29% 19% 32% 15% 4 1	508
April	1st to 10th	Day	23% 19% 33% 18% 6% 1	540
	11th to 20th	Day	21% 20% 31% 20% 8% 2	532
	21st to end	Day	22% 16% 33% 21% 7% 1	540
May	1st to 10th	Day	18% 12% 35% 25% 7% 2	636
	11th to 20th	Day	19% 14% 29% 23% 12% 3	775
	21st to end	Day	16% 10% 29% 26% 13% 7%	844
June	1st to 10th	Day	9% 8% 27% 34% 18% 5%	780
	11th to 20th	Day	10% 7% 31% 31% 16% 4	780
	21st to end	Day	10% 9% 32% 31% 13% 4	780
July	1st to 10th	Day	9% 4 19% 30% 25% 12%	771
	11th to 20th	Day	10% 5% 23% 34% 20% 9%	768
	21st to end	Day	8% 3 24% 36% 24% 4	849
August	1st to 10th	Day	7% 3 21% 38% 24% 7%	776
	11th to 20th	Day	12% 5% 18% 30% 23% 11%	606
	21st to end	Day	6% 4 21% 33% 27% 9%	583
September	1st to 10th	Day	13% 7% 19% 32% 22% 7%	530
	11th to 20th	Day	8% 4 19% 33% 25% 12%	530
	21st to end	Day	11% 9% 25% 27% 20% 8%	512
October	1st to 10th	Day	16% 8% 28% 27% 16% 5%	330
	11th to 20th	Day	17% 23% 32% 21% 8%	330
	21st to end	Day	25% 17% 33% 19% 4 1	363
November	1st to 10th	Day	14% 16% 42% 18% 7% 3	330
	11th to 20th	Day	22% 19% 35% 17% 5% 2	330
	21st to end	Day	25% 14% 39% 19% 4	330
December	1st to 10th	Day	15% 19% 42% 17% 5% 2	330
	11th to 20th	Day	19% 18% 37% 22% 4	330
	21st to end	Day	28% 19% 37% 13% 2	363
January	1st to 10th	Night	18% 15% 38% 22% 6%	1050
	11th to 20th	Night	18% 17% 38% 22% 5%	1050
	21st to end	Night	19% 14% 38% 24% 5%	1155
February	1st to 10th	Night	21% 14% 38% 22% 4	1050
	11th to 20th	Night	18% 15% 39% 26% 3	910
	21st to end	Night	22% 15% 40% 20% 3 1	692
March	1st to 10th	Night	22% 14% 37% 22% 4	840
	11th to 20th	Night	16% 14% 35% 28% 6%	647
	21st to end	Night	24% 13% 37% 21% 5%	693
April	1st to 10th	Night	21% 16% 36% 23% 3	630
	11th to 20th	Night	21% 16% 29% 27% 6%	623
	21st to end	Night	18% 12% 33% 31% 5% 1	630
May	1st to 10th	Night	17% 9% 38% 28% 7% 1	630
	11th to 20th	Night	18% 8% 32% 33% 10%	621
	21st to end	Night	13% 6% 32% 33% 15% 1	683
June	1st to 10th	Night	10% 4 27% 44% 13% 1	630
	11th to 20th	Night	9% 6% 30% 39% 15% 1	630
	21st to end	Night	11% 5% 29% 39% 16% 1	630
July	1st to 10th	Night	10% 3 16% 40% 28% 4	615
	11th to 20th	Night	6% 2 23% 40% 25% 3	612
	21st to end	Night	7% 3 19% 42% 28% 1	690
August	1st to 10th	Night	5% 3 16% 44% 29% 4	627
	11th to 20th	Night	6% 2 16% 42% 29% 4	606
	21st to end	Night	8% 1 18% 45% 26% 3	660
September	1st to 10th	Night	12% 3 22% 37% 22% 4	600
	11th to 20th	Night	10% 3 21% 36% 25% 5%	604
	21st to end	Night	8% 9% 25% 34% 23% 2	800
October	1st to 10th	Night	12% 7% 32% 30% 19% 1	800
	11th to 20th	Night	18% 12% 31% 28% 11%	800
	21st to end	Night	23% 13% 33% 24% 7%	880
November	1st to 10th	Night	17% 10% 39% 26% 7% 1	820
	11th to 20th	Night	18% 17% 36% 24% 5%	840
	21st to end	Night	21% 14% 36% 24% 5%	1050
December	1st to 10th	Night	19% 14% 39% 23% 5%	1050
	11th to 20th	Night	23% 13% 34% 25% 5%	1050
	21st to end	Night	28% 16% 33% 21% 3	1155

Page generated at 2026-08-12 02:37:20. Version ac07

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