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 2026-08-30.

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% 2 7% 4 4 4 3 5% 7% 46%	143227
	Night	9% 6% 2 6% 4 4 4 3 5% 7% 50%	139324
Transparency	Day	36% 5% 12% 26% 18% 3	146877
	Night	34% 5% 12% 26% 20% 3	143741
Seeing	Day	13% 10% 31% 29% 14% 3	18313
	Night	13% 9% 34% 33% 10% 1	28075

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	10% 11% 4 12% 6% 5% 5% 3 4 6% 35%	5289
	11th to 20th	Day	5% 8% 4 8% 5% 6% 7% 4 5% 5% 42%	5310
	21st to end	Day	6% 12% 5% 10% 5% 4 4 3 5% 5% 41%	5812
February	1st to 10th	Day	9% 12% 3 8% 6% 4 4 3 6% 8% 37%	1890
	11th to 20th	Day	8% 9% 4 9% 4 4 4 3 5% 7% 42%	1931
	21st to end	Day	8% 7% 3 6% 4 5% 3 3 5% 7% 49%	1730
March	1st to 10th	Day	12% 8% 2 5% 3 3 3 2 3 4 54%	4948
	11th to 20th	Day	10% 6% 2 5% 4 3 3 3 4 7% 52%	5090
	21st to end	Day	14% 10% 3 8% 5% 4 4 3 5% 7% 36%	5583
April	1st to 10th	Day	14% 8% 3 6% 4 4 3 2 5% 7% 45%	2336
	11th to 20th	Day	13% 8% 2 7% 4 4 4 4 4 6% 44%	2637
	21st to end	Day	20% 7% 3 6% 4 4 4 3 4 7% 39%	2730
May	1st to 10th	Day	12% 7% 2 6% 4 3 4 3 5% 6% 47%	7617
	11th to 20th	Day	12% 6% 2 5% 3 4 3 3 5% 6% 50%	7627
	21st to end	Day	18% 7% 3 6% 4 4 5% 3 5% 6% 39%	8392
June	1st to 10th	Day	7% 5% 2 5% 4 4 4 4 6% 9% 49%	2802
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 6% 9% 48%	2940
	21st to end	Day	7% 5% 2 5% 5% 5% 5% 4 6% 9% 47%	2734
July	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 48%	7680
	11th to 20th	Day	5% 3 2 5% 4 4 4 3 6% 7% 56%	7680
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 6% 8% 45%	8337
August	1st to 10th	Day	6% 4 2 7% 4 5% 5% 4 7% 9% 47%	4364
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 8% 45%	4301
	21st to end	Day	6% 4 2 6% 4 4 4 4 6% 8% 52%	4734
September	1st to 10th	Day	14% 4 2 4 3 3 3 3 5% 6% 54%	2270
	11th to 20th	Day	9% 5% 1 5% 3 3 3 3 5% 8% 54%	2200
	21st to end	Day	10% 5% 2 3 3 3 4 3 4 6% 57%	2200
October	1st to 10th	Day	14% 5% 2 5% 3 3 3 3 4 5% 54%	2075
	11th to 20th	Day	10% 4 2 5% 4 2 3 3 3 5% 58%	2000
	21st to end	Day	13% 7% 2 6% 3 4 3 3 4 5% 50%	2200
November	1st to 10th	Day	9% 6% 2 6% 3 3 3 3 4 6% 57%	1866
	11th to 20th	Day	8% 6% 2 6% 4 3 3 3 6% 6% 53%	1890
	21st to end	Day	13% 8% 3 6% 4 3 3 3 5% 6% 46%	1737
December	1st to 10th	Day	5% 6% 2 5% 4 4 4 3 6% 7% 54%	2690
	11th to 20th	Day	7% 9% 3 9% 5% 4 3 2 5% 7% 46%	3088
	21st to end	Day	16% 11% 4 14% 4 4 3 3 4 5% 32%	4517
January	1st to 10th	Night	9% 10% 4 14% 6% 4 5% 3 5% 6% 36%	7634
	11th to 20th	Night	5% 6% 2 7% 4 4 6% 4 6% 7% 48%	7680
	21st to end	Night	9% 11% 4 10% 4 4 3 2 4 5% 44%	8402
February	1st to 10th	Night	11% 10% 4 7% 5% 4 5% 4 5% 6% 41%	2730
	11th to 20th	Night	9% 8% 2 8% 5% 5% 5% 4 6% 8% 41%	2595
	21st to end	Night	9% 7% 3 6% 4 3 4 3 4 6% 52%	2076
March	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 4 6% 53%	5130
	11th to 20th	Night	9% 6% 1 4 3 3 3 3 5% 6% 58%	5090
	21st to end	Night	12% 9% 2 7% 4 5% 4 4 6% 7% 39%	5588
April	1st to 10th	Night	13% 8% 2 5% 4 3 3 4 5% 6% 48%	2283
	11th to 20th	Night	11% 7% 2 6% 3 3 4 4 5% 7% 47%	1930
	21st to end	Night	16% 6% 2 5% 4 3 3 4 4 8% 44%	1890
May	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 5% 5% 53%	5266
	11th to 20th	Night	13% 4 2 4 3 3 3 2 4 5% 56%	5265
	21st to end	Night	17% 6% 3 5% 3 4 4 3 4 7% 44%	5793
June	1st to 10th	Night	6% 4 2 5% 4 4 5% 3 7% 9% 52%	1806
	11th to 20th	Night	5% 4 1 4 3 4 4 4 7% 10% 54%	1680
	21st to end	Night	4 4 1 3 4 3 3 4 6% 9% 59%	1871
July	1st to 10th	Night	7% 5% 2 6% 4 3 4 4 6% 9% 50%	5308
	11th to 20th	Night	5% 3 2 4 3 3 4 4 5% 7% 61%	5307
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 10% 49%	5770
August	1st to 10th	Night	3 3 1 4 4 4 4 4 6% 10% 57%	3016
	11th to 20th	Night	12% 5% 2 4 3 4 4 3 5% 8% 49%	2978
	21st to end	Night	6% 4 1 4 3 3 4 3 5% 9% 58%	3369
September	1st to 10th	Night	9% 4 2 3 2 2 3 3 5% 6% 61%	2125
	11th to 20th	Night	8% 4 1 4 2 4 3 3 5% 8% 58%	2200
	21st to end	Night	8% 4 2 4 3 3 3 3 4 6% 60%	2200
October	1st to 10th	Night	12% 5% 2 4 3 3 4 3 6% 6% 52%	2304
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 4 5% 59%	2400
	21st to end	Night	13% 7% 2 4 3 2 3 3 4 5% 54%	2640
November	1st to 10th	Night	7% 4 2 5% 3 3 3 3 5% 6% 59%	2650
	11th to 20th	Night	8% 6% 2 4 3 3 3 3 5% 6% 54%	2730
	21st to end	Night	10% 8% 3 6% 3 2 3 3 4 6% 52%	2878
December	1st to 10th	Night	5% 5% 2 5% 4 4 4 3 4 6% 59%	4720
	11th to 20th	Night	9% 7% 2 8% 4 3 4 3 5% 6% 49%	5339
	21st to end	Night	15% 12% 4 10% 4 3 3 3 4 5% 36%	6681

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	51% 4 11% 22% 12% 1	5290
	11th to 20th	Day	42% 4 14% 20% 19% 2	5310
	21st to end	Day	45% 4 12% 22% 16% 1	5812
February	1st to 10th	Day	45% 6% 17% 21% 10% 2	1890
	11th to 20th	Day	42% 5% 10% 25% 14% 3	1940
	21st to end	Day	36% 5% 12% 26% 19% 3	1730
March	1st to 10th	Day	36% 2 9% 25% 20% 8%	4948
	11th to 20th	Day	32% 4 9% 24% 25% 6%	5090
	21st to end	Day	47% 5% 21% 19% 6% 2	5585
April	1st to 10th	Day	39% 4 12% 25% 16% 3	2336
	11th to 20th	Day	39% 5% 13% 24% 16% 3	2637
	21st to end	Day	46% 6% 10% 25% 12% 1	2730
May	1st to 10th	Day	36% 6% 9% 27% 20% 2	7655
	11th to 20th	Day	35% 5% 10% 22% 24% 4	7631
	21st to end	Day	44% 6% 10% 26% 13% 1	8405
June	1st to 10th	Day	30% 7% 16% 32% 13% 2	2802
	11th to 20th	Day	31% 6% 14% 28% 19% 2	2940
	21st to end	Day	32% 8% 14% 27% 14% 5%	2734
July	1st to 10th	Day	32% 6% 14% 31% 17% 1	7616
	11th to 20th	Day	26% 6% 12% 30% 20% 6%	7611
	21st to end	Day	35% 7% 14% 28% 15% 2	8155
August	1st to 10th	Day	31% 7% 17% 25% 17% 3	4350
	11th to 20th	Day	37% 6% 16% 23% 17% 1	4340
	21st to end	Day	29% 6% 16% 26% 17% 7%	4733
September	1st to 10th	Day	31% 4 12% 27% 20% 5%	2270
	11th to 20th	Day	30% 5% 12% 29% 22% 2	2200
	21st to end	Day	28% 4 9% 32% 24% 3	2200
October	1st to 10th	Day	22% 3 6% 18% 47% 3	3374
	11th to 20th	Day	20% 3 11% 28% 29% 9%	3240
	21st to end	Day	32% 3 11% 27% 24% 3	3538
November	1st to 10th	Day	29% 4 13% 28% 21% 3	1866
	11th to 20th	Day	32% 4 13% 28% 22% 2	1890
	21st to end	Day	39% 4 11% 28% 15% 2	1737
December	1st to 10th	Day	30% 4 14% 33% 18% 1	2690
	11th to 20th	Day	39% 3 16% 30% 11% 1	3085
	21st to end	Day	55% 3 10% 22% 8% 1	4517
January	1st to 10th	Night	50% 4 12% 19% 13% 2	7635
	11th to 20th	Night	34% 4 11% 28% 20% 3	7680
	21st to end	Night	44% 4 12% 19% 19% 3	8402
February	1st to 10th	Night	44% 5% 12% 21% 15% 3	2730
	11th to 20th	Night	40% 5% 13% 24% 15% 3	2596
	21st to end	Night	34% 4 10% 24% 21% 7%	2076
March	1st to 10th	Night	33% 3 8% 26% 26% 3	5130
	11th to 20th	Night	28% 3 8% 22% 30% 9%	5090
	21st to end	Night	42% 5% 20% 24% 7% 1	5588
April	1st to 10th	Night	36% 5% 10% 27% 18% 4	2283
	11th to 20th	Night	35% 6% 11% 27% 17% 4	1930
	21st to end	Night	39% 5% 12% 26% 17% 1	1890
May	1st to 10th	Night	33% 5% 9% 26% 24% 3	5293
	11th to 20th	Night	31% 5% 9% 25% 25% 5%	5266
	21st to end	Night	41% 6% 12% 23% 18% 1	5805
June	1st to 10th	Night	26% 7% 16% 31% 18% 2	1806
	11th to 20th	Night	24% 7% 14% 31% 19% 5%	1680
	21st to end	Night	22% 5% 16% 32% 22% 3	1871
July	1st to 10th	Night	30% 6% 14% 30% 17% 3	5265
	11th to 20th	Night	22% 6% 12% 28% 25% 7%	5256
	21st to end	Night	29% 6% 18% 26% 19% 1	5641
August	1st to 10th	Night	21% 6% 18% 28% 22% 4	3006
	11th to 20th	Night	34% 6% 14% 30% 16% 2	3005
	21st to end	Night	24% 5% 16% 29% 17% 9%	3369
September	1st to 10th	Night	24% 4 13% 28% 23% 7%	2125
	11th to 20th	Night	25% 6% 11% 32% 23% 4	2200
	21st to end	Night	26% 4 7% 28% 31% 3	2200
October	1st to 10th	Night	21% 4 7% 17% 44% 6%	3772
	11th to 20th	Night	21% 3 8% 32% 26% 10%	3899
	21st to end	Night	30% 3 8% 31% 25% 2	4252
November	1st to 10th	Night	26% 4 12% 27% 25% 5%	2650
	11th to 20th	Night	30% 5% 10% 30% 21% 3	2730
	21st to end	Night	35% 4 12% 27% 21% 3	2878
December	1st to 10th	Night	27% 4 15% 35% 18% 1	4720
	11th to 20th	Night	37% 3 15% 30% 15% 1	5341
	21st to end	Night	51% 4 11% 24% 9% 1	6681

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% 15% 39% 22% 5% 2	330
	11th to 20th	Day	18% 18% 38% 20% 6%	330
	21st to end	Day	13% 11% 48% 22% 6%	363
February	1st to 10th	Day	14% 21% 45% 17% 2	330
	11th to 20th	Day	17% 15% 41% 19% 8%	330
	21st to end	Day	10% 19% 47% 20% 3	273
March	1st to 10th	Day	17% 14% 42% 22% 4 1	330
	11th to 20th	Day	26% 13% 33% 20% 6% 2	330
	21st to end	Day	28% 18% 37% 13% 3	363
April	1st to 10th	Day	22% 20% 37% 16% 4 1	356
	11th to 20th	Day	16% 19% 38% 21% 5%	675
	21st to end	Day	23% 18% 30% 20% 8% 1	750
May	1st to 10th	Day	17% 12% 35% 25% 10% 2	753
	11th to 20th	Day	16% 12% 32% 25% 13% 2	775
	21st to end	Day	19% 8% 31% 28% 11% 2	844
June	1st to 10th	Day	7% 5% 24% 35% 23% 6%	780
	11th to 20th	Day	7% 7% 33% 32% 18% 3	780
	21st to end	Day	8% 8% 35% 35% 13% 2	780
July	1st to 10th	Day	9% 4 20% 35% 25% 7%	771
	11th to 20th	Day	5% 4 19% 38% 25% 9%	768
	21st to end	Day	8% 2 18% 39% 29% 4	849
August	1st to 10th	Day	7% 3 23% 41% 22% 3	776
	11th to 20th	Day	9% 4 20% 43% 22% 3	765
	21st to end	Day	10% 3 21% 37% 24% 6%	816
September	1st to 10th	Day	14% 4 23% 36% 20% 4	400
	11th to 20th	Day	7% 5% 28% 36% 19% 5%	330
	21st to end	Day	15% 8% 29% 28% 15% 4	330
October	1st to 10th	Day	21% 7% 25% 29% 15% 5%	330
	11th to 20th	Day	8% 17% 45% 22% 8% 1	330
	21st to end	Day	18% 18% 35% 20% 7% 2	363
November	1st to 10th	Day	14% 15% 42% 21% 7% 1	330
	11th to 20th	Day	12% 16% 42% 22% 7% 1	330
	21st to end	Day	23% 13% 35% 21% 7% 1	330
December	1st to 10th	Day	8% 13% 45% 27% 6% 1	330
	11th to 20th	Day	15% 15% 38% 23% 8%	330
	21st to end	Day	19% 14% 42% 22% 3	363
January	1st to 10th	Night	12% 11% 39% 32% 5%	1050
	11th to 20th	Night	13% 13% 40% 27% 6%	1050
	21st to end	Night	14% 9% 45% 27% 5%	1155
February	1st to 10th	Night	15% 11% 43% 27% 4	1050
	11th to 20th	Night	14% 15% 39% 28% 4	961
	21st to end	Night	13% 14% 43% 25% 5%	692
March	1st to 10th	Night	14% 14% 40% 25% 5% 1	683
	11th to 20th	Night	19% 13% 36% 26% 6%	630
	21st to end	Night	19% 13% 41% 22% 4	693
April	1st to 10th	Night	18% 14% 42% 22% 4	630
	11th to 20th	Night	15% 14% 40% 27% 4 1	623
	21st to end	Night	17% 11% 37% 27% 7% 1	630
May	1st to 10th	Night	17% 9% 33% 31% 9%	630
	11th to 20th	Night	15% 10% 31% 34% 9%	621
	21st to end	Night	15% 8% 31% 32% 12% 1	683
June	1st to 10th	Night	7% 5% 27% 42% 18% 1	630
	11th to 20th	Night	8% 5% 33% 40% 14%	630
	21st to end	Night	6% 4 37% 41% 12%	630
July	1st to 10th	Night	7% 3 17% 46% 27%	615
	11th to 20th	Night	5% 1 20% 47% 26% 1	612
	21st to end	Night	4 1 18% 51% 26% 1	690
August	1st to 10th	Night	4 3 19% 51% 21% 2	627
	11th to 20th	Night	7% 3 19% 50% 21% 2	624
	21st to end	Night	9% 1 21% 44% 23% 2	687
September	1st to 10th	Night	11% 3 26% 41% 17% 1	600
	11th to 20th	Night	10% 3 28% 41% 18% 1	600
	21st to end	Night	9% 6% 31% 39% 15% 1	600
October	1st to 10th	Night	15% 8% 30% 33% 13% 2	704
	11th to 20th	Night	13% 12% 38% 30% 7%	800
	21st to end	Night	17% 12% 32% 29% 9%	880
November	1st to 10th	Night	10% 11% 35% 34% 9%	1010
	11th to 20th	Night	12% 12% 41% 29% 5%	1050
	21st to end	Night	15% 12% 35% 28% 8% 1	1050
December	1st to 10th	Night	12% 11% 42% 30% 5%	1050
	11th to 20th	Night	15% 12% 37% 30% 6%	1050
	21st to end	Night	17% 12% 36% 29% 5%	1155

Page generated at 2026-08-27 03:07:12. Version ac07

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