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 2025-04-26.

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 7% 4 4 4 3 5% 7% 46%	113689
	Night	9% 7% 2 6% 4 3 4 3 5% 7% 50%	115117
Transparency	Day	36% 5% 13% 25% 18% 3	117380
	Night	33% 5% 12% 26% 20% 3	119555
Seeing	Day	13% 10% 31% 29% 14% 3	16304
	Night	13% 9% 34% 33% 10% 1	26347

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 14% 7% 5% 5% 4 4 6% 31%	4077
	11th to 20th	Day	6% 9% 4 8% 4 5% 7% 4 5% 5% 43%	4080
	21st to end	Day	5% 10% 5% 11% 5% 4 4 3 5% 6% 42%	4488
February	1st to 10th	Day	9% 12% 3 8% 6% 4 4 3 6% 8% 36%	1800
	11th to 20th	Day	8% 10% 4 9% 5% 4 5% 3 5% 7% 41%	1837
	21st to end	Day	8% 7% 3 6% 4 5% 3 3 5% 7% 48%	1650
March	1st to 10th	Day	12% 8% 2 5% 3 2 3 2 3 5% 54%	4841
	11th to 20th	Day	10% 6% 2 5% 4 3 3 3 4 7% 52%	4980
	21st to end	Day	15% 10% 3 8% 5% 4 4 3 5% 7% 36%	5462
April	1st to 10th	Day	14% 8% 3 6% 4 3 3 2 5% 7% 47%	2225
	11th to 20th	Day	13% 8% 2 7% 4 4 4 4 4 6% 44%	2510
	21st to end	Day	20% 7% 3 6% 4 4 4 3 5% 7% 38%	2536
May	1st to 10th	Day	15% 8% 2 6% 5% 4 4 3 5% 7% 40%	4082
	11th to 20th	Day	14% 7% 3 6% 4 4 3 3 5% 6% 44%	4082
	21st to end	Day	13% 6% 2 6% 4 4 5% 4 6% 7% 42%	4477
June	1st to 10th	Day	6% 5% 2 5% 4 4 4 4 6% 10% 50%	2536
	11th to 20th	Day	9% 5% 2 6% 3 5% 4 4 7% 9% 47%	2660
	21st to end	Day	6% 5% 2 5% 5% 5% 5% 4 7% 9% 48%	2474
July	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 5% 8% 46%	4120
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 8% 52%	4120
	21st to end	Day	5% 4 1 4 4 4 4 4 7% 10% 53%	4465
August	1st to 10th	Day	6% 4 2 7% 4 5% 5% 4 7% 10% 47%	4104
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 6% 8% 45%	4041
	21st to end	Day	5% 3 2 6% 4 4 4 4 6% 8% 53%	4476
September	1st to 10th	Day	13% 4 2 4 3 3 3 3 5% 6% 55%	2157
	11th to 20th	Day	10% 5% 1 5% 3 3 3 3 5% 8% 54%	2090
	21st to end	Day	10% 4 2 4 3 3 4 3 4 6% 57%	2090
October	1st to 10th	Day	14% 5% 2 5% 3 3 3 3 4 5% 53%	1971
	11th to 20th	Day	10% 5% 2 5% 4 2 3 3 4 5% 58%	1900
	21st to end	Day	13% 6% 2 6% 3 4 3 3 4 6% 51%	2090
November	1st to 10th	Day	9% 6% 2 6% 3 3 3 3 4 6% 56%	1776
	11th to 20th	Day	8% 6% 2 6% 5% 3 4 3 6% 6% 53%	1800
	21st to end	Day	12% 8% 2 6% 3 3 3 3 6% 6% 47%	1655
December	1st to 10th	Day	5% 5% 2 5% 4 4 4 3 6% 7% 54%	2610
	11th to 20th	Day	7% 9% 3 9% 5% 3 3 2 5% 7% 46%	3008
	21st to end	Day	16% 11% 4 14% 4 4 3 3 4 5% 32%	4419
January	1st to 10th	Night	9% 10% 4 16% 7% 4 5% 3 5% 6% 32%	5888
	11th to 20th	Night	6% 7% 2 7% 4 4 6% 4 6% 6% 46%	5900
	21st to end	Night	9% 11% 4 10% 4 3 3 2 4 5% 45%	6490
February	1st to 10th	Night	11% 11% 4 6% 5% 4 5% 4 5% 6% 40%	2600
	11th to 20th	Night	9% 8% 2 8% 5% 5% 4 4 6% 8% 42%	2468
	21st to end	Night	9% 7% 3 6% 4 3 4 3 4 6% 52%	1980
March	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 6% 53%	5017
	11th to 20th	Night	9% 6% 2 4 3 3 3 3 4 6% 58%	4980
	21st to end	Night	13% 9% 3 7% 4 5% 4 4 6% 7% 39%	5467
April	1st to 10th	Night	12% 7% 2 5% 4 4 3 4 5% 6% 48%	2174
	11th to 20th	Night	11% 7% 2 6% 3 3 4 3 5% 8% 47%	1837
	21st to end	Night	17% 6% 2 5% 4 4 3 3 4 7% 44%	1760
May	1st to 10th	Night	18% 7% 2 5% 4 3 4 3 4 5% 45%	2823
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 5% 6% 50%	2823
	21st to end	Night	14% 5% 2 5% 4 4 4 3 5% 8% 46%	3090
June	1st to 10th	Night	5% 3 2 4 4 4 5% 4 6% 9% 54%	1634
	11th to 20th	Night	6% 4 1 4 3 4 4 4 7% 10% 53%	1520
	21st to end	Night	4 3 1 3 4 3 3 4 6% 9% 60%	1693
July	1st to 10th	Night	9% 6% 2 6% 4 3 4 3 5% 8% 50%	2850
	11th to 20th	Night	4 3 1 3 3 3 3 4 6% 9% 61%	2850
	21st to end	Night	3 2 1 3 2 3 4 4 5% 10% 62%	3098
August	1st to 10th	Night	3 3 2 4 4 4 4 4 6% 10% 58%	2836
	11th to 20th	Night	12% 6% 2 5% 3 4 4 3 5% 8% 49%	2798
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 8% 59%	3187
September	1st to 10th	Night	8% 4 2 3 2 2 3 3 5% 6% 61%	2019
	11th to 20th	Night	8% 4 1 4 2 4 3 3 5% 8% 59%	2090
	21st to end	Night	8% 4 2 4 3 3 3 3 4 6% 61%	2090
October	1st to 10th	Night	12% 5% 1 4 3 3 4 3 6% 6% 53%	2189
	11th to 20th	Night	12% 5% 2 3 3 3 3 2 3 5% 60%	2280
	21st to end	Night	13% 7% 2 4 3 2 3 3 4 5% 54%	2508
November	1st to 10th	Night	8% 4 2 5% 3 3 3 3 5% 6% 58%	2521
	11th to 20th	Night	9% 7% 2 4 3 4 3 3 5% 6% 54%	2600
	21st to end	Night	10% 8% 3 5% 3 2 2 3 4 6% 53%	2741
December	1st to 10th	Night	5% 5% 2 5% 4 3 4 3 4 6% 60%	4580
	11th to 20th	Night	9% 7% 2 8% 4 3 4 3 5% 6% 49%	5199
	21st to end	Night	15% 12% 4 10% 4 3 3 3 4 5% 35%	6537

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	54% 5% 11% 20% 10% 1	4078
	11th to 20th	Day	42% 4 13% 20% 18% 2	4080
	21st to end	Day	44% 4 12% 21% 18% 1	4488
February	1st to 10th	Day	46% 6% 16% 21% 10% 2	1800
	11th to 20th	Day	42% 5% 10% 24% 14% 3	1846
	21st to end	Day	37% 5% 12% 25% 18% 3	1650
March	1st to 10th	Day	36% 2 9% 25% 20% 8%	4841
	11th to 20th	Day	33% 4 9% 24% 25% 6%	4980
	21st to end	Day	47% 5% 21% 19% 6% 2	5464
April	1st to 10th	Day	38% 4 12% 26% 17% 3	2225
	11th to 20th	Day	40% 5% 13% 24% 15% 3	2510
	21st to end	Day	45% 6% 10% 26% 12% 1	2536
May	1st to 10th	Day	42% 7% 11% 24% 13% 4	4120
	11th to 20th	Day	41% 6% 12% 23% 19%	4086
	21st to end	Day	39% 8% 12% 27% 13% 1	4489
June	1st to 10th	Day	28% 8% 16% 33% 14% 2	2536
	11th to 20th	Day	31% 7% 14% 28% 18% 2	2660
	21st to end	Day	30% 8% 15% 28% 14% 5%	2474
July	1st to 10th	Day	36% 5% 13% 27% 17% 2	4056
	11th to 20th	Day	28% 6% 13% 32% 18% 3	4051
	21st to end	Day	23% 8% 15% 29% 22% 4	4325
August	1st to 10th	Day	30% 7% 17% 26% 17% 3	4090
	11th to 20th	Day	37% 6% 16% 22% 18% 1	4080
	21st to end	Day	28% 6% 16% 26% 18% 7%	4475
September	1st to 10th	Day	31% 4 12% 27% 21% 5%	2157
	11th to 20th	Day	30% 5% 11% 29% 23% 2	2090
	21st to end	Day	28% 4 9% 32% 24% 2	2090
October	1st to 10th	Day	22% 3 7% 18% 47% 4	3270
	11th to 20th	Day	20% 3 11% 28% 29% 9%	3140
	21st to end	Day	31% 3 11% 27% 24% 3	3428
November	1st to 10th	Day	30% 4 14% 28% 20% 3	1776
	11th to 20th	Day	32% 4 12% 27% 23% 2	1800
	21st to end	Day	38% 4 11% 29% 16% 2	1655
December	1st to 10th	Day	29% 4 14% 33% 18% 2	2610
	11th to 20th	Day	40% 3 17% 30% 10%	3005
	21st to end	Day	56% 3 10% 22% 8% 1	4419
January	1st to 10th	Night	53% 5% 11% 18% 13%	5888
	11th to 20th	Night	37% 4 12% 26% 19% 3	5900
	21st to end	Night	43% 4 11% 19% 20% 4	6490
February	1st to 10th	Night	45% 5% 12% 20% 15% 3	2600
	11th to 20th	Night	40% 5% 13% 24% 15% 3	2469
	21st to end	Night	34% 4 11% 24% 20% 7%	1980
March	1st to 10th	Night	32% 4 8% 27% 26% 3	5017
	11th to 20th	Night	27% 3 8% 22% 31% 9%	4980
	21st to end	Night	43% 6% 20% 24% 6% 1	5467
April	1st to 10th	Night	36% 5% 10% 27% 19% 4	2174
	11th to 20th	Night	36% 5% 11% 28% 16% 3	1837
	21st to end	Night	39% 5% 12% 25% 17% 1	1760
May	1st to 10th	Night	40% 6% 11% 23% 18% 3	2850
	11th to 20th	Night	34% 6% 12% 27% 20% 1	2824
	21st to end	Night	36% 6% 15% 21% 20% 1	3099
June	1st to 10th	Night	25% 7% 16% 32% 18% 2	1634
	11th to 20th	Night	24% 8% 15% 31% 18% 5%	1520
	21st to end	Night	20% 5% 16% 33% 22% 4	1693
July	1st to 10th	Night	32% 6% 12% 26% 19% 4	2805
	11th to 20th	Night	20% 6% 14% 32% 25% 4	2796
	21st to end	Night	16% 7% 18% 29% 28% 2	2999
August	1st to 10th	Night	21% 6% 18% 28% 23% 4	2826
	11th to 20th	Night	34% 6% 14% 29% 16% 2	2825
	21st to end	Night	23% 5% 16% 29% 18% 9%	3187
September	1st to 10th	Night	23% 4 13% 28% 24% 8%	2019
	11th to 20th	Night	24% 6% 11% 32% 24% 4	2090
	21st to end	Night	26% 4 7% 29% 31% 3	2090
October	1st to 10th	Night	20% 4 7% 17% 45% 7%	3657
	11th to 20th	Night	20% 3 8% 32% 26% 11%	3779
	21st to end	Night	29% 4 8% 31% 26% 2	4120
November	1st to 10th	Night	26% 4 12% 27% 25% 5%	2521
	11th to 20th	Night	30% 5% 10% 30% 22% 3	2600
	21st to end	Night	33% 4 12% 27% 21% 3	2741
December	1st to 10th	Night	26% 4 15% 35% 18% 1	4580
	11th to 20th	Night	37% 3 15% 30% 14% 1	5201
	21st to end	Night	52% 4 11% 24% 9% 1	6537

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% 38% 22% 6% 2	300
	11th to 20th	Day	19% 17% 36% 22% 7%	300
	21st to end	Day	14% 11% 48% 22% 6%	330
February	1st to 10th	Day	15% 22% 44% 17% 2	300
	11th to 20th	Day	16% 16% 41% 19% 8%	300
	21st to end	Day	11% 19% 46% 21% 3	249
March	1st to 10th	Day	16% 15% 42% 21% 4 1	300
	11th to 20th	Day	26% 13% 33% 21% 6% 2	300
	21st to end	Day	30% 16% 37% 14% 3	330
April	1st to 10th	Day	22% 20% 36% 17% 4 1	325
	11th to 20th	Day	16% 19% 38% 21% 5%	628
	21st to end	Day	22% 18% 30% 21% 8% 1	676
May	1st to 10th	Day	18% 13% 36% 23% 9% 1	653
	11th to 20th	Day	17% 11% 31% 26% 13% 2	675
	21st to end	Day	17% 9% 31% 29% 12% 2	734
June	1st to 10th	Day	6% 5% 24% 36% 24% 6%	680
	11th to 20th	Day	7% 6% 33% 32% 18% 3	680
	21st to end	Day	6% 8% 35% 35% 14% 2	680
July	1st to 10th	Day	10% 4 19% 35% 26% 7%	671
	11th to 20th	Day	6% 4 20% 39% 23% 7%	668
	21st to end	Day	6% 2 17% 39% 31% 5%	739
August	1st to 10th	Day	7% 2 22% 41% 24% 3	676
	11th to 20th	Day	9% 3 20% 43% 22% 3	665
	21st to end	Day	10% 3 21% 36% 25% 6%	718
September	1st to 10th	Day	13% 3 23% 37% 20% 4	367
	11th to 20th	Day	6% 4 28% 37% 20% 5%	300
	21st to end	Day	15% 9% 30% 28% 16% 3	300
October	1st to 10th	Day	21% 7% 24% 27% 16% 5%	300
	11th to 20th	Day	7% 18% 43% 22% 9% 1	300
	21st to end	Day	16% 17% 36% 20% 8% 2	330
November	1st to 10th	Day	15% 15% 39% 22% 8% 1	300
	11th to 20th	Day	13% 17% 41% 21% 7% 1	300
	21st to end	Day	20% 13% 35% 23% 8% 1	300
December	1st to 10th	Day	8% 13% 45% 27% 7% 1	300
	11th to 20th	Day	15% 15% 38% 23% 9%	300
	21st to end	Day	19% 15% 41% 22% 3	330
January	1st to 10th	Night	12% 12% 38% 32% 5%	1000
	11th to 20th	Night	13% 13% 39% 28% 7%	1000
	21st to end	Night	14% 9% 44% 27% 5%	1100
February	1st to 10th	Night	15% 11% 43% 26% 4	1000
	11th to 20th	Night	14% 15% 39% 28% 4	914
	21st to end	Night	13% 14% 42% 25% 5%	660
March	1st to 10th	Night	14% 14% 41% 25% 5% 1	650
	11th to 20th	Night	19% 12% 35% 27% 6%	600
	21st to end	Night	20% 13% 40% 22% 5%	660
April	1st to 10th	Night	17% 14% 42% 23% 5%	600
	11th to 20th	Night	16% 13% 40% 27% 4 1	593
	21st to end	Night	18% 11% 36% 27% 7% 1	587
May	1st to 10th	Night	18% 10% 33% 31% 8%	570
	11th to 20th	Night	15% 9% 31% 35% 9% 1	561
	21st to end	Night	14% 8% 32% 32% 13% 1	617
June	1st to 10th	Night	6% 5% 26% 43% 19% 1	570
	11th to 20th	Night	8% 5% 32% 41% 14% 1	570
	21st to end	Night	5% 4 36% 42% 13%	570
July	1st to 10th	Night	8% 3 16% 46% 26%	555
	11th to 20th	Night	5% 1 20% 48% 24% 1	552
	21st to end	Night	3 1 17% 51% 27% 1	624
August	1st to 10th	Night	4 2 18% 51% 22% 2	567
	11th to 20th	Night	7% 3 18% 50% 20% 2	564
	21st to end	Night	9% 22% 42% 24% 2	627
September	1st to 10th	Night	11% 3 26% 41% 18% 1	570
	11th to 20th	Night	10% 3 27% 42% 17% 1	570
	21st to end	Night	9% 6% 32% 37% 14% 1	570
October	1st to 10th	Night	15% 8% 29% 32% 13% 2	669
	11th to 20th	Night	14% 11% 37% 30% 8%	760
	21st to end	Night	16% 12% 32% 30% 9%	836
November	1st to 10th	Night	10% 10% 34% 35% 10%	961
	11th to 20th	Night	12% 13% 40% 29% 6%	1000
	21st to end	Night	14% 12% 36% 28% 8% 1	1000
December	1st to 10th	Night	12% 11% 41% 30% 6%	1000
	11th to 20th	Night	16% 11% 36% 31% 6%	1000
	21st to end	Night	18% 12% 36% 30% 5%	1100

Page generated at 2025-04-23 08:25:06. Version ac07

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