Pacifica
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pacifica 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	7% 8% 3 6% 5% 5% 4 3 6% 10% 44%	152298
	Night	9% 11% 3 7% 4 4 4 3 4 6% 45%	147433
Transparency	Day	21% 35% 6% 13% 16% 9%	147210
	Night	26% 35% 5% 11% 14% 9%	143512
Seeing	Day	11% 11% 29% 32% 14% 3	17830
	Night	13% 7% 17% 32% 25% 6%	27118

By Month

Percentage Cloud Cover SW (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	15% 6% 2 5% 4 3 4 4 6% 9% 42%	5142
	11th to 20th	Day	9% 5% 2 6% 4 3 3 3 5% 6% 54%	5340
	21st to end	Day	14% 10% 3 7% 4 4 5% 4 5% 7% 37%	5843
February	1st to 10th	Day	18% 6% 2 6% 3 3 4 3 5% 5% 44%	1890
	11th to 20th	Day	15% 8% 3 7% 4 4 4 3 6% 8% 38%	1919
	21st to end	Day	12% 8% 2 6% 3 3 4 3 5% 8% 45%	1730
March	1st to 10th	Day	12% 8% 3 9% 6% 6% 6% 4 7% 8% 31%	5034
	11th to 20th	Day	10% 7% 3 7% 4 4 4 3 6% 9% 44%	5110
	21st to end	Day	11% 7% 3 9% 6% 5% 5% 4 7% 10% 33%	5647
April	1st to 10th	Day	13% 8% 2 6% 4 4 5% 4 5% 8% 42%	2520
	11th to 20th	Day	8% 7% 2 5% 4 4 3 4 6% 9% 48%	2489
	21st to end	Day	7% 7% 2 5% 3 3 3 3 7% 11% 49%	2520
May	1st to 10th	Day	5% 8% 3 7% 4 4 3 3 5% 7% 52%	7357
	11th to 20th	Day	3 4 2 3 3 3 3 2 5% 10% 62%	7647
	21st to end	Day	4 9% 2 7% 5% 5% 4 3 5% 8% 48%	8992
June	1st to 10th	Day	6% 8% 2 5% 4 4 4 4 6% 10% 47%	2940
	11th to 20th	Day	2 5% 2 5% 4 4 4 3 7% 11% 56%	2940
	21st to end	Day	4 7% 3 7% 4 4 3 4 6% 9% 49%	2940
July	1st to 10th	Day	1 6% 3 7% 5% 5% 4 4 7% 13% 45%	8114
	11th to 20th	Day	3 6% 3 8% 6% 6% 6% 4 8% 14% 35%	7650
	21st to end	Day	1 16% 4 8% 7% 8% 5% 4 7% 12% 29%	8304
August	1st to 10th	Day	2 7% 3 8% 8% 8% 6% 4 8% 12% 35%	5907
	11th to 20th	Day	2 10% 3 7% 5% 5% 4 3 6% 13% 42%	5685
	21st to end	Day	2 6% 3 7% 5% 5% 5% 5% 9% 15% 39%	6093
September	1st to 10th	Day	3 6% 3 5% 4 3 3 3 6% 11% 53%	2400
	11th to 20th	Day	5% 10% 3 6% 4 4 3 4 7% 12% 42%	2400
	21st to end	Day	4 3 1 4 3 3 3 2 6% 10% 60%	2105
October	1st to 10th	Day	3 4 2 4 3 3 2 3 5% 7% 65%	3280
	11th to 20th	Day	6% 9% 3 6% 4 4 3 2 5% 7% 51%	3261
	21st to end	Day	7% 5% 2 6% 3 3 3 3 6% 9% 52%	3558
November	1st to 10th	Day	8% 5% 2 5% 3 3 4 4 6% 8% 51%	2054
	11th to 20th	Day	11% 8% 3 6% 4 4 4 3 5% 7% 44%	1890
	21st to end	Day	11% 6% 2 4 3 2 3 4 5% 7% 52%	1697
December	1st to 10th	Day	8% 4 2 5% 3 3 4 3 6% 10% 52%	2700
	11th to 20th	Day	21% 9% 3 6% 4 4 5% 4 7% 8% 28%	3108
	21st to end	Day	24% 12% 3 7% 4 4 4 3 5% 7% 28%	4092
January	1st to 10th	Night	15% 7% 4 6% 4 4 4 4 6% 7% 41%	7757
	11th to 20th	Night	10% 6% 2 6% 4 4 3 3 4 6% 51%	7650
	21st to end	Night	14% 12% 3 7% 4 4 4 3 5% 7% 37%	8371
February	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 5% 6% 45%	2730
	11th to 20th	Night	13% 9% 3 7% 4 4 4 3 6% 7% 40%	2587
	21st to end	Night	13% 7% 2 6% 4 3 3 3 5% 8% 45%	2076
March	1st to 10th	Night	14% 11% 3 8% 5% 5% 5% 4 5% 5% 34%	5063
	11th to 20th	Night	14% 8% 3 5% 4 4 3 2 4 7% 47%	5110
	21st to end	Night	14% 11% 3 8% 5% 4 4 3 4 7% 37%	5569
April	1st to 10th	Night	10% 9% 3 7% 4 4 4 3 5% 6% 45%	2100
	11th to 20th	Night	10% 9% 3 5% 3 3 4 3 4 6% 50%	2079
	21st to end	Night	6% 7% 2 4 3 4 3 3 5% 8% 53%	2100
May	1st to 10th	Night	9% 14% 3 4 2 2 2 1 2 4 55%	5588
	11th to 20th	Night	2 8% 3 6% 3 5% 3 2 3 5% 61%	5236
	21st to end	Night	5% 15% 3 5% 3 4 3 2 2 4 54%	5181
June	1st to 10th	Night	5% 10% 3 7% 4 5% 4 3 5% 10% 45%	1680
	11th to 20th	Night	2 8% 3 5% 3 4 3 2 4 7% 59%	1680
	21st to end	Night	4 10% 4 6% 4 4 2 2 3 8% 53%	1680
July	1st to 10th	Night	1 13% 5% 8% 5% 6% 3 1 2 5% 50%	4832
	11th to 20th	Night	4 17% 5% 12% 5% 5% 3 2 3 4 38%	5340
	21st to end	Night	2 32% 5% 7% 4 3 3 2 3 5% 35%	5806
August	1st to 10th	Night	3 18% 9% 8% 4 4 3 2 4 6% 39%	4125
	11th to 20th	Night	1 19% 4 8% 4 5% 4 2 3 5% 43%	4395
	21st to end	Night	2 15% 5% 9% 6% 5% 4 3 4 6% 40%	5105
September	1st to 10th	Night	2 9% 2 7% 5% 3 4 3 4 8% 52%	2000
	11th to 20th	Night	5% 14% 4 6% 4 4 4 2 5% 8% 45%	2000
	21st to end	Night	3 6% 2 6% 4 3 2 2 4 8% 61%	2289
October	1st to 10th	Night	4 5% 2 5% 3 3 3 3 4 5% 65%	3920
	11th to 20th	Night	5% 16% 4 8% 4 3 3 2 3 4 50%	3897
	21st to end	Night	7% 6% 2 6% 4 3 4 3 4 5% 57%	4253
November	1st to 10th	Night	8% 7% 2 6% 3 3 3 3 5% 8% 52%	2463
	11th to 20th	Night	11% 7% 3 6% 4 4 4 4 5% 8% 45%	2730
	21st to end	Night	10% 5% 1 4 5% 3 3 3 5% 7% 53%	2919
December	1st to 10th	Night	9% 4 2 5% 4 4 4 3 5% 7% 54%	4700
	11th to 20th	Night	22% 10% 3 6% 4 4 5% 4 7% 8% 28%	5316
	21st to end	Night	23% 9% 4 8% 5% 5% 4 3 4 6% 29%	7106

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	29% 25% 13% 19% 11% 2	5143
	11th to 20th	Day	26% 22% 5% 8% 21% 17%	5340
	21st to end	Day	35% 25% 5% 13% 10% 12%	5843
February	1st to 10th	Day	20% 59% 4 8% 7% 3	1890
	11th to 20th	Day	21% 59% 3 8% 7% 2	1928
	21st to end	Day	15% 59% 5% 12% 6% 3	1730
March	1st to 10th	Day	35% 33% 6% 13% 12% 1	5034
	11th to 20th	Day	21% 29% 13% 21% 8% 7%	5110
	21st to end	Day	30% 34% 12% 17% 6% 1	5647
April	1st to 10th	Day	17% 59% 5% 10% 8% 1	2520
	11th to 20th	Day	13% 60% 7% 11% 7% 1	2492
	21st to end	Day	11% 60% 7% 12% 9% 1	2520
May	1st to 10th	Day	20% 24% 9% 17% 21% 9%	7357
	11th to 20th	Day	10% 22% 6% 22% 31% 9%	7650
	21st to end	Day	17% 28% 8% 18% 28% 2	8994
June	1st to 10th	Day	10% 59% 5% 11% 12% 4	2940
	11th to 20th	Day	7% 59% 4 11% 15% 4	2940
	21st to end	Day	12% 57% 2 6% 14% 10%	2940
July	1st to 10th	Day	18% 21% 5% 12% 18% 27%	8047
	11th to 20th	Day	25% 21% 3 10% 17% 24%	7581
	21st to end	Day	30% 21% 5% 9% 15% 20%	8119
August	1st to 10th	Day	26% 35% 4 12% 15% 7%	4336
	11th to 20th	Day	20% 35% 3 8% 24% 10%	4144
	21st to end	Day	17% 36% 4 13% 18% 11%	4420
September	1st to 10th	Day	12% 57% 1 8% 13% 9%	2400
	11th to 20th	Day	13% 58% 3 10% 11% 5%	2400
	21st to end	Day	11% 56% 2 11% 13% 6%	2105
October	1st to 10th	Day	9% 35% 2 8% 18% 29%	3280
	11th to 20th	Day	20% 36% 7% 10% 18% 8%	3261
	21st to end	Day	17% 36% 6% 17% 17% 7%	3558
November	1st to 10th	Day	9% 53% 5% 17% 12% 4	2054
	11th to 20th	Day	20% 54% 5% 11% 7% 2	1890
	21st to end	Day	17% 55% 5% 9% 12% 3	1697
December	1st to 10th	Day	16% 35% 5% 21% 20% 3	2700
	11th to 20th	Day	38% 34% 6% 8% 12% 3	3108
	21st to end	Day	42% 28% 8% 14% 7% 1	4092
January	1st to 10th	Night	31% 25% 15% 16% 11% 2	7757
	11th to 20th	Night	27% 24% 6% 10% 21% 14%	7650
	21st to end	Night	37% 25% 4 11% 14% 9%	8371
February	1st to 10th	Night	20% 59% 4 7% 7% 4	2730
	11th to 20th	Night	20% 58% 4 8% 7% 3	2589
	21st to end	Night	14% 59% 4 14% 6% 3	2076
March	1st to 10th	Night	34% 35% 6% 13% 11% 1	5063
	11th to 20th	Night	24% 31% 8% 17% 13% 7%	5110
	21st to end	Night	32% 34% 9% 17% 8% 1	5569
April	1st to 10th	Night	16% 59% 5% 10% 9% 2	2100
	11th to 20th	Night	16% 59% 5% 10% 7% 2	2079
	21st to end	Night	13% 60% 5% 12% 8% 2	2100
May	1st to 10th	Night	27% 25% 4 16% 20% 8%	5586
	11th to 20th	Night	17% 23% 6% 18% 24% 12%	5237
	21st to end	Night	22% 27% 5% 18% 27% 2	5182
June	1st to 10th	Night	15% 59% 1 8% 13% 4	1680
	11th to 20th	Night	12% 58% 2 9% 14% 5%	1680
	21st to end	Night	12% 57% 1 4 12% 13%	1680
July	1st to 10th	Night	29% 21% 2 7% 14% 26%	4791
	11th to 20th	Night	40% 20% 2 3 6% 28%	5286
	21st to end	Night	38% 22% 1 4 14% 21%	5679
August	1st to 10th	Night	39% 37% 1 4 13% 6%	3019
	11th to 20th	Night	22% 35% 1 6% 28% 8%	3207
	21st to end	Night	23% 38% 4 7% 16% 12%	3698
September	1st to 10th	Night	15% 57% 2 5% 13% 8%	2000
	11th to 20th	Night	15% 57% 2 10% 11% 6%	2000
	21st to end	Night	12% 56% 2 10% 10% 9%	2289
October	1st to 10th	Night	10% 36% 1 4 16% 33%	3920
	11th to 20th	Night	23% 39% 4 10% 13% 11%	3897
	21st to end	Night	19% 35% 4 14% 17% 11%	4253
November	1st to 10th	Night	12% 53% 3 14% 12% 5%	2463
	11th to 20th	Night	18% 54% 5% 13% 7% 2	2730
	21st to end	Night	16% 55% 4 11% 9% 5%	2919
December	1st to 10th	Night	17% 36% 5% 19% 18% 5%	4700
	11th to 20th	Night	37% 34% 6% 9% 10% 4	5316
	21st to end	Night	42% 28% 8% 15% 5% 2	7106

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	25% 13% 29% 21% 13% 1	367
	11th to 20th	Day	17% 9% 24% 36% 13% 1	430
	21st to end	Day	13% 9% 31% 33% 14% 1	473
February	1st to 10th	Day	17% 12% 28% 26% 15% 2	430
	11th to 20th	Day	19% 17% 31% 23% 10%	464
	21st to end	Day	12% 16% 31% 28% 11% 2	446
March	1st to 10th	Day	21% 18% 29% 22% 9% 1	540
	11th to 20th	Day	19% 13% 26% 25% 14% 4	540
	21st to end	Day	14% 19% 32% 25% 9% 2	594
April	1st to 10th	Day	15% 17% 32% 23% 10% 3	540
	11th to 20th	Day	13% 19% 31% 24% 9% 3	532
	21st to end	Day	8% 20% 32% 26% 10% 5%	540
May	1st to 10th	Day	10% 16% 33% 29% 10% 2	543
	11th to 20th	Day	12% 15% 35% 28% 9% 2	568
	21st to end	Day	12% 14% 32% 27% 12% 3	617
June	1st to 10th	Day	7% 12% 36% 32% 11% 3	570
	11th to 20th	Day	4 12% 38% 34% 10% 2	570
	21st to end	Day	7% 8% 34% 40% 10% 2	570
July	1st to 10th	Day	7% 5% 32% 43% 11% 1	566
	11th to 20th	Day	3 5% 27% 49% 15% 2	564
	21st to end	Day	7% 4 29% 45% 12% 3	619
August	1st to 10th	Day	6% 6% 29% 46% 11% 2	567
	11th to 20th	Day	7% 5% 30% 40% 15% 3	557
	21st to end	Day	6% 4 24% 44% 17% 5%	620
September	1st to 10th	Day	6% 5% 27% 38% 19% 5%	530
	11th to 20th	Day	8% 11% 27% 35% 15% 5%	530
	21st to end	Day	5% 9% 25% 33% 22% 5%	530
October	1st to 10th	Day	5% 8% 26% 31% 20% 10%	530
	11th to 20th	Day	8% 10% 22% 29% 23% 7%	530
	21st to end	Day	11% 7% 23% 31% 22% 7%	583
November	1st to 10th	Day	9% 10% 22% 28% 24% 6%	530
	11th to 20th	Day	12% 10% 17% 32% 24% 5%	330
	21st to end	Day	11% 6% 28% 34% 18% 2	228
December	1st to 10th	Day	15% 9% 21% 31% 19% 4	220
	11th to 20th	Day	16% 15% 20% 34% 15% 1	220
	21st to end	Day	25% 13% 27% 26% 9%	242
January	1st to 10th	Night	22% 8% 15% 31% 21% 5%	840
	11th to 20th	Night	16% 7% 14% 33% 25% 5%	840
	21st to end	Night	18% 6% 13% 35% 24% 3	924
February	1st to 10th	Night	19% 7% 16% 31% 23% 4	840
	11th to 20th	Night	17% 13% 17% 30% 19% 5%	824
	21st to end	Night	14% 12% 20% 33% 19% 2	692
March	1st to 10th	Night	20% 12% 20% 26% 19% 3	840
	11th to 20th	Night	19% 8% 16% 31% 22% 3	840
	21st to end	Night	15% 10% 27% 29% 17% 2	924
April	1st to 10th	Night	13% 9% 26% 31% 17% 3	840
	11th to 20th	Night	13% 14% 23% 29% 17% 4	831
	21st to end	Night	8% 11% 27% 29% 21% 4	840
May	1st to 10th	Night	9% 7% 25% 33% 22% 4	728
	11th to 20th	Night	9% 5% 23% 33% 28% 3	601
	21st to end	Night	13% 11% 21% 33% 18% 4	455
June	1st to 10th	Night	10% 5% 24% 35% 24% 2	420
	11th to 20th	Night	5% 7% 24% 41% 20% 3	420
	21st to end	Night	9% 3 18% 38% 29% 4	420
July	1st to 10th	Night	7% 3 16% 42% 26% 6%	436
	11th to 20th	Night	9% 3 11% 38% 30% 9%	612
	21st to end	Night	13% 5% 11% 32% 30% 8%	690
August	1st to 10th	Night	17% 3 13% 31% 30% 6%	627
	11th to 20th	Night	12% 3 17% 34% 28% 7%	734
	21st to end	Night	8% 3 13% 37% 31% 8%	916
September	1st to 10th	Night	6% 2 14% 33% 34% 11%	800
	11th to 20th	Night	11% 5% 16% 31% 30% 7%	800
	21st to end	Night	4 5% 14% 30% 36% 11%	800
October	1st to 10th	Night	8% 4 14% 32% 32% 10%	800
	11th to 20th	Night	8% 5% 15% 28% 35% 10%	800
	21st to end	Night	10% 5% 12% 27% 33% 13%	880
November	1st to 10th	Night	10% 5% 14% 27% 35% 9%	820
	11th to 20th	Night	13% 5% 14% 32% 29% 7%	840
	21st to end	Night	11% 6% 12% 39% 27% 5%	840
December	1st to 10th	Night	17% 7% 13% 34% 24% 5%	840
	11th to 20th	Night	25% 7% 13% 29% 22% 4	840
	21st to end	Night	23% 11% 18% 29% 18% 1	924

Page generated at 2026-08-27 01:07:12. Version ac07

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