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 2024-10-25.

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 7% 4 4 4 3 5% 7% 46%	100840
	Night	10% 7% 2 6% 4 3 4 3 5% 7% 50%	98036
Transparency	Day	37% 6% 13% 25% 16% 2	100652
	Night	34% 5% 13% 26% 19% 3	97895
Seeing	Day	13% 10% 31% 29% 14% 3	15728
	Night	13% 9% 34% 33% 10% 1	25563

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% 12% 4 13% 8% 5% 6% 4 4 6% 29%	2847
	11th to 20th	Day	8% 11% 4 9% 4 5% 7% 4 4 5% 38%	2850
	21st to end	Day	7% 13% 6% 13% 5% 4 4 3 4 6% 35%	3135
February	1st to 10th	Day	9% 12% 3 8% 6% 4 5% 3 6% 8% 36%	1710
	11th to 20th	Day	8% 9% 3 9% 5% 4 4 3 5% 8% 42%	1742
	21st to end	Day	9% 7% 4 6% 4 5% 3 4 5% 7% 45%	1570
March	1st to 10th	Day	11% 8% 2 6% 4 3 3 2 4 5% 52%	3367
	11th to 20th	Day	11% 7% 3 5% 4 3 4 3 4 6% 51%	3480
	21st to end	Day	16% 10% 3 9% 5% 4 4 3 5% 7% 33%	3814
April	1st to 10th	Day	14% 8% 3 5% 4 4 3 2 5% 6% 47%	2114
	11th to 20th	Day	13% 8% 2 7% 4 4 4 4 4 6% 43%	2383
	21st to end	Day	20% 6% 3 6% 4 4 4 3 5% 7% 39%	2470
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	12% 7% 2 6% 3 4 3 3 4 6% 51%	2026
November	1st to 10th	Day	8% 5% 1 6% 3 3 3 3 5% 6% 58%	1686
	11th to 20th	Day	8% 6% 2 6% 5% 2 4 3 6% 6% 52%	1710
	21st to end	Day	12% 8% 3 6% 3 3 3 3 6% 6% 46%	1573
December	1st to 10th	Day	8% 7% 2 6% 5% 4 4 4 6% 8% 46%	1520
	11th to 20th	Day	8% 11% 3 8% 6% 4 4 3 5% 8% 41%	1918
	21st to end	Day	14% 12% 5% 15% 5% 4 4 3 4 6% 27%	3080
January	1st to 10th	Night	7% 10% 4 15% 6% 4 5% 4 5% 6% 35%	4108
	11th to 20th	Night	8% 9% 3 7% 4 4 7% 4 5% 5% 43%	4120
	21st to end	Night	11% 12% 5% 10% 4 3 3 3 4 6% 39%	4532
February	1st to 10th	Night	11% 11% 4 6% 5% 4 5% 4 5% 6% 39%	2470
	11th to 20th	Night	8% 8% 2 7% 5% 5% 4 4 6% 8% 43%	2342
	21st to end	Night	9% 7% 3 6% 4 3 4 3 4 6% 50%	1884
March	1st to 10th	Night	10% 5% 2 5% 3 3 3 2 4 7% 54%	3489
	11th to 20th	Night	10% 6% 2 4 3 3 3 3 4 6% 57%	3480
	21st to end	Night	17% 9% 3 8% 4 4 4 3 6% 6% 37%	3817
April	1st to 10th	Night	12% 7% 2 5% 4 3 3 4 4 6% 49%	2066
	11th to 20th	Night	11% 7% 2 6% 3 3 4 3 5% 8% 47%	1744
	21st to end	Night	16% 6% 2 5% 4 3 3 3 4 8% 45%	1710
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% 55%	2436
November	1st to 10th	Night	6% 4 2 5% 3 3 3 3 5% 6% 60%	2391
	11th to 20th	Night	9% 6% 2 4 3 4 3 3 5% 6% 54%	2470
	21st to end	Night	10% 8% 3 5% 3 2 2 3 4 7% 53%	2603
December	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 5% 6% 51%	2660
	11th to 20th	Night	9% 8% 3 7% 3 2 3 3 4 6% 51%	3279
	21st to end	Night	16% 13% 5% 11% 4 3 3 2 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	38% 5% 12% 25% 16% 3	1570
March	1st to 10th	Day	36% 3 10% 26% 21% 4	3367
	11th to 20th	Day	34% 5% 12% 28% 20% 1	3480
	21st to end	Day	50% 6% 21% 17% 5% 1	3814
April	1st to 10th	Day	38% 4 13% 26% 17% 2	2114
	11th to 20th	Day	41% 5% 13% 23% 15% 3	2383
	21st to end	Day	45% 6% 11% 26% 12% 1	2470
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	34% 5% 10% 23% 27% 1	1971
	11th to 20th	Day	29% 4 8% 27% 24% 7%	1900
	21st to end	Day	36% 4 13% 23% 20% 5%	2026
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% 23% 19% 7%	1884
March	1st to 10th	Night	31% 4 9% 28% 24% 3	3489
	11th to 20th	Night	30% 3 9% 28% 24% 5%	3480
	21st to end	Night	46% 6% 22% 20% 6% 1	3817
April	1st to 10th	Night	35% 5% 10% 27% 19% 4	2066
	11th to 20th	Night	36% 6% 11% 28% 16% 3	1744
	21st to end	Night	39% 4 12% 26% 17% 1	1710
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	30% 5% 11% 24% 26% 3	2189
	11th to 20th	Night	29% 3 8% 27% 25% 8%	2280
	21st to end	Night	32% 4 10% 26% 25% 4	2436
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% 19% 45% 20% 4	225
March	1st to 10th	Day	17% 14% 43% 21% 4 1	270
	11th to 20th	Day	26% 13% 33% 20% 6% 1	270
	21st to end	Day	30% 16% 37% 13% 3	297
April	1st to 10th	Day	21% 19% 38% 18% 3 1	294
	11th to 20th	Day	17% 18% 38% 22% 6% 1	581
	21st to end	Day	22% 18% 30% 21% 9% 1	650
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% 37% 20% 8% 2	311
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%	628
March	1st to 10th	Night	14% 14% 40% 25% 6% 1	618
	11th to 20th	Night	19% 12% 35% 27% 7%	570
	21st to end	Night	20% 13% 39% 23% 5%	627
April	1st to 10th	Night	16% 14% 42% 23% 5%	570
	11th to 20th	Night	16% 12% 39% 28% 4 1	563
	21st to end	Night	18% 11% 36% 27% 7% 1	570
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%	812
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 2024-10-22 19:21:00. Version ac07

Copyright © 2024 Attilla Danko.