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 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	8% 7% 2 6% 4 4 4 3 6% 9% 45%	117696
	Night	10% 10% 3 7% 4 4 4 3 5% 6% 46%	119809
Transparency	Day	19% 42% 5% 12% 13% 8%	117458
	Night	23% 41% 5% 11% 12% 8%	119637
Seeing	Day	11% 11% 28% 32% 14% 3	16007
	Night	13% 7% 17% 32% 26% 6%	25462

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	13% 6% 2 5% 3 3 3 4 7% 9% 45%	3961
	11th to 20th	Day	12% 7% 3 7% 4 4 3 3 4 5% 49%	4100
	21st to end	Day	17% 10% 3 7% 4 4 4 3 4 6% 38%	4510
February	1st to 10th	Day	18% 7% 2 6% 3 3 4 3 4 5% 44%	1800
	11th to 20th	Day	14% 8% 3 7% 3 4 4 3 7% 8% 39%	1825
	21st to end	Day	12% 8% 2 6% 3 3 4 3 5% 8% 47%	1650
March	1st to 10th	Day	13% 8% 3 9% 6% 6% 6% 4 7% 8% 31%	4925
	11th to 20th	Day	10% 7% 3 7% 4 4 4 4 6% 9% 43%	5000
	21st to end	Day	11% 7% 3 9% 6% 5% 5% 4 7% 10% 33%	5525
April	1st to 10th	Day	13% 8% 2 6% 4 4 4 4 5% 8% 43%	2400
	11th to 20th	Day	8% 7% 3 5% 4 4 3 4 6% 9% 48%	2369
	21st to end	Day	6% 6% 1 5% 3 3 4 3 7% 11% 50%	2342
May	1st to 10th	Day	6% 4 2 4 2 2 2 2 4 6% 65%	3970
	11th to 20th	Day	5% 5% 2 4 3 4 4 4 7% 12% 50%	4097
	21st to end	Day	6% 11% 2 6% 5% 3 3 3 5% 6% 52%	4813
June	1st to 10th	Day	7% 8% 2 5% 4 4 4 4 6% 10% 46%	2660
	11th to 20th	Day	2 5% 2 4 3 3 3 3 6% 10% 58%	2660
	21st to end	Day	4 7% 3 7% 4 4 3 4 6% 9% 49%	2660
July	1st to 10th	Day	1 6% 2 4 2 3 2 2 6% 10% 61%	4367
	11th to 20th	Day	2 5% 3 7% 5% 5% 5% 4 8% 17% 39%	4110
	21st to end	Day	1 11% 3 7% 6% 7% 5% 4 8% 14% 33%	4455
August	1st to 10th	Day	3 9% 4 9% 9% 9% 7% 5% 9% 14% 22%	4091
	11th to 20th	Day	1 8% 3 7% 5% 4 3 3 6% 12% 47%	3855
	21st to end	Day	2 7% 3 7% 5% 5% 4 5% 8% 13% 42%	4180
September	1st to 10th	Day	3 6% 2 5% 4 3 3 3 6% 12% 54%	2280
	11th to 20th	Day	5% 9% 3 6% 4 4 3 4 7% 13% 42%	2280
	21st to end	Day	4 3 1 3 3 3 3 3 6% 10% 61%	2000
October	1st to 10th	Day	3 3 2 4 3 2 2 3 5% 7% 65%	3180
	11th to 20th	Day	6% 9% 3 6% 4 4 3 2 5% 7% 51%	3161
	21st to end	Day	7% 5% 2 5% 3 3 3 3 6% 10% 52%	3448
November	1st to 10th	Day	8% 5% 2 5% 3 3 4 4 6% 8% 52%	1954
	11th to 20th	Day	10% 8% 3 6% 4 4 4 4 5% 7% 46%	1800
	21st to end	Day	12% 7% 2 4 3 2 3 4 5% 7% 51%	1616
December	1st to 10th	Day	8% 4 2 5% 3 3 4 3 6% 10% 52%	2620
	11th to 20th	Day	21% 9% 3 6% 4 4 5% 4 8% 8% 28%	3028
	21st to end	Day	23% 11% 3 7% 4 4 4 3 6% 7% 28%	4004
January	1st to 10th	Night	10% 7% 4 6% 4 3 4 4 6% 8% 44%	5982
	11th to 20th	Night	13% 8% 3 7% 4 4 4 3 4 4 45%	5880
	21st to end	Night	16% 12% 3 7% 4 4 4 2 4 6% 39%	6468
February	1st to 10th	Night	18% 7% 2 5% 4 3 4 2 5% 6% 45%	2600
	11th to 20th	Night	13% 9% 3 7% 4 3 4 3 6% 7% 41%	2461
	21st to end	Night	13% 7% 2 6% 4 3 3 3 5% 8% 46%	1980
March	1st to 10th	Night	14% 11% 3 8% 5% 5% 5% 4 5% 6% 34%	4952
	11th to 20th	Night	14% 8% 3 6% 4 4 3 2 4 7% 46%	5000
	21st to end	Night	14% 11% 3 8% 5% 3 4 3 4 7% 37%	5449
April	1st to 10th	Night	10% 8% 3 6% 4 4 4 3 5% 7% 46%	2000
	11th to 20th	Night	10% 9% 3 5% 3 3 4 3 4 6% 51%	1979
	21st to end	Night	6% 6% 2 4 3 4 3 3 5% 8% 54%	1954
May	1st to 10th	Night	6% 7% 2 4 3 2 2 2 3 4 64%	3001
	11th to 20th	Night	3 10% 5% 8% 4 6% 2 2 3 6% 51%	2802
	21st to end	Night	5% 11% 2 4 3 2 2 2 3 5% 62%	2762
June	1st to 10th	Night	5% 9% 3 7% 4 4 4 3 5% 11% 45%	1520
	11th to 20th	Night	2 7% 3 5% 3 4 3 2 4 7% 60%	1520
	21st to end	Night	4 10% 4 6% 4 4 2 2 3 9% 53%	1520
July	1st to 10th	Night	1 8% 3 4 3 2 2 1 3 8% 64%	2590
	11th to 20th	Night	2 11% 4 10% 5% 5% 4 3 4 6% 45%	2860
	21st to end	Night	1 26% 4 9% 5% 4 4 3 4 7% 32%	3109
August	1st to 10th	Night	4 22% 12% 11% 5% 3 2 2 4 7% 26%	2848
	11th to 20th	Night	2 14% 3 7% 4 5% 5% 3 4 7% 48%	2990
	21st to end	Night	2 14% 3 9% 6% 5% 4 4 4 7% 42%	3498
September	1st to 10th	Night	2 8% 2 7% 5% 3 4 4 5% 9% 53%	1900
	11th to 20th	Night	5% 13% 3 6% 4 4 4 2 5% 8% 46%	1900
	21st to end	Night	3 5% 2 6% 4 3 2 2 4 8% 61%	2175
October	1st to 10th	Night	3 4 1 5% 3 3 3 3 4 5% 65%	3800
	11th to 20th	Night	5% 16% 4 8% 4 3 3 2 3 4 49%	3777
	21st to end	Night	7% 5% 2 6% 3 3 4 3 4 5% 57%	4121
November	1st to 10th	Night	9% 7% 2 6% 3 3 3 3 5% 8% 52%	2343
	11th to 20th	Night	10% 7% 3 6% 4 4 4 4 6% 8% 46%	2600
	21st to end	Night	10% 5% 1 4 5% 3 3 3 5% 7% 53%	2780
December	1st to 10th	Night	9% 4 2 5% 4 4 4 3 5% 6% 54%	4560
	11th to 20th	Night	22% 10% 3 6% 4 4 5% 4 7% 8% 28%	5176
	21st to end	Night	23% 9% 4 8% 5% 5% 4 3 4 6% 29%	6952

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	24% 30% 14% 17% 12% 3	3961
	11th to 20th	Day	31% 28% 6% 8% 15% 11%	4100
	21st to end	Day	34% 31% 4 9% 7% 15%	4510
February	1st to 10th	Day	19% 62% 3 8% 6% 2	1800
	11th to 20th	Day	18% 61% 3 7% 7% 2	1834
	21st to end	Day	13% 62% 4 13% 6% 3	1650
March	1st to 10th	Day	35% 33% 6% 13% 12% 1	4925
	11th to 20th	Day	22% 30% 13% 21% 7% 7%	5000
	21st to end	Day	30% 34% 12% 17% 6% 1	5525
April	1st to 10th	Day	16% 62% 4 11% 7% 1	2400
	11th to 20th	Day	11% 62% 7% 11% 7% 1	2372
	21st to end	Day	9% 64% 6% 10% 9% 1	2342
May	1st to 10th	Day	10% 40% 8% 19% 15% 8%	3970
	11th to 20th	Day	12% 39% 5% 15% 24% 6%	4100
	21st to end	Day	13% 47% 3 16% 19% 1	4814
June	1st to 10th	Day	8% 65% 4 11% 9% 3	2660
	11th to 20th	Day	6% 65% 3 10% 13% 3	2660
	21st to end	Day	10% 63% 1 4 12% 9%	2660
July	1st to 10th	Day	9% 35% 1 4 10% 42%	4299
	11th to 20th	Day	13% 36% 2 7% 15% 26%	4041
	21st to end	Day	19% 35% 3 7% 18% 17%	4315
August	1st to 10th	Day	27% 37% 4 12% 14% 6%	4076
	11th to 20th	Day	18% 37% 2 8% 25% 10%	3894
	21st to end	Day	16% 38% 4 13% 17% 11%	4179
September	1st to 10th	Day	10% 60% 1 8% 13% 9%	2280
	11th to 20th	Day	12% 60% 2 10% 11% 5%	2280
	21st to end	Day	9% 59% 2 11% 13% 6%	2000
October	1st to 10th	Day	8% 36% 1 7% 18% 30%	3180
	11th to 20th	Day	20% 37% 7% 10% 18% 8%	3161
	21st to end	Day	17% 37% 6% 17% 17% 7%	3448
November	1st to 10th	Day	8% 56% 4 16% 11% 4	1954
	11th to 20th	Day	18% 56% 5% 11% 8% 2	1800
	21st to end	Day	17% 57% 4 8% 11% 3	1616
December	1st to 10th	Day	17% 36% 4 20% 20% 3	2620
	11th to 20th	Day	38% 35% 6% 7% 12% 2	3028
	21st to end	Day	42% 29% 8% 14% 7% 1	4004
January	1st to 10th	Night	23% 31% 17% 16% 11% 3	5982
	11th to 20th	Night	32% 30% 7% 9% 13% 8%	5880
	21st to end	Night	34% 30% 4 8% 13% 11%	6468
February	1st to 10th	Night	20% 61% 4 6% 6% 3	2600
	11th to 20th	Night	18% 61% 4 8% 7% 3	2463
	21st to end	Night	12% 62% 4 14% 6% 3	1980
March	1st to 10th	Night	34% 35% 6% 13% 11% 1	4952
	11th to 20th	Night	25% 31% 8% 17% 12% 6%	5000
	21st to end	Night	32% 35% 9% 17% 7%	5449
April	1st to 10th	Night	14% 62% 4 10% 8% 1	2000
	11th to 20th	Night	15% 62% 4 10% 7% 2	1979
	21st to end	Night	11% 64% 4 11% 8% 2	1954
May	1st to 10th	Night	13% 42% 6% 17% 14% 9%	3001
	11th to 20th	Night	20% 40% 7% 7% 15% 11%	2803
	21st to end	Night	13% 43% 4 20% 19% 1	2762
June	1st to 10th	Night	13% 65% 1 7% 11% 3	1520
	11th to 20th	Night	9% 64% 2 8% 13% 3	1520
	21st to end	Night	10% 63% 1 3 11% 12%	1520
July	1st to 10th	Night	13% 35% 1 3 5% 43%	2549
	11th to 20th	Night	23% 36% 1 3 6% 30%	2806
	21st to end	Night	30% 36% 2 12% 20%	3011
August	1st to 10th	Night	40% 39% 1 4 11% 5%	2839
	11th to 20th	Night	20% 37% 6% 29% 8%	3017
	21st to end	Night	22% 39% 4 6% 16% 12%	3498
September	1st to 10th	Night	13% 60% 2 5% 12% 9%	1900
	11th to 20th	Night	13% 60% 2 9% 10% 6%	1900
	21st to end	Night	11% 59% 2 10% 10% 8%	2175
October	1st to 10th	Night	9% 37% 1 4 16% 33%	3800
	11th to 20th	Night	23% 41% 4 9% 12% 12%	3777
	21st to end	Night	18% 36% 4 14% 16% 11%	4121
November	1st to 10th	Night	11% 55% 3 14% 11% 5%	2343
	11th to 20th	Night	16% 57% 5% 13% 7% 2	2600
	21st to end	Night	16% 57% 4 10% 9% 5%	2780
December	1st to 10th	Night	17% 36% 4 18% 18% 5%	4560
	11th to 20th	Night	37% 35% 6% 9% 10% 3	5176
	21st to end	Night	42% 29% 9% 15% 5% 1	6952

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	24% 12% 28% 22% 13% 1	340
	11th to 20th	Day	18% 10% 25% 33% 14% 2	400
	21st to end	Day	13% 9% 29% 32% 15% 2	440
February	1st to 10th	Day	18% 12% 27% 26% 16% 2	400
	11th to 20th	Day	17% 17% 31% 24% 11%	430
	21st to end	Day	13% 16% 30% 28% 12% 2	414
March	1st to 10th	Day	21% 18% 28% 22% 9% 1	500
	11th to 20th	Day	21% 13% 27% 24% 12% 4	500
	21st to end	Day	15% 19% 31% 25% 8% 1	550
April	1st to 10th	Day	16% 18% 32% 21% 10% 3	500
	11th to 20th	Day	13% 20% 31% 24% 10% 3	492
	21st to end	Day	7% 20% 33% 25% 10% 5%	481
May	1st to 10th	Day	10% 17% 34% 27% 10% 2	463
	11th to 20th	Day	12% 14% 34% 29% 9% 2	488
	21st to end	Day	12% 15% 30% 27% 13% 2	529
June	1st to 10th	Day	8% 13% 36% 30% 11% 3	490
	11th to 20th	Day	4 12% 37% 35% 11% 2	490
	21st to end	Day	6% 7% 35% 40% 11% 2	490
July	1st to 10th	Day	8% 5% 30% 45% 12% 1	486
	11th to 20th	Day	2 4 25% 51% 16% 2	484
	21st to end	Day	5% 3 27% 50% 13% 3	531
August	1st to 10th	Day	7% 6% 30% 47% 9% 2	487
	11th to 20th	Day	4 5% 30% 41% 16% 3	477
	21st to end	Day	5% 4 23% 44% 18% 5%	539
September	1st to 10th	Day	5% 3 26% 39% 21% 6%	490
	11th to 20th	Day	7% 11% 27% 36% 15% 5%	490
	21st to end	Day	4 9% 25% 34% 23% 5%	490
October	1st to 10th	Day	5% 8% 26% 31% 21% 9%	490
	11th to 20th	Day	8% 10% 23% 30% 22% 8%	490
	21st to end	Day	11% 7% 23% 30% 22% 7%	539
November	1st to 10th	Day	9% 11% 23% 29% 23% 6%	490
	11th to 20th	Day	10% 10% 16% 32% 26% 6%	300
	21st to end	Day	13% 7% 28% 33% 18% 2	207
December	1st to 10th	Day	17% 10% 20% 29% 20% 5%	200
	11th to 20th	Day	16% 15% 22% 34% 13% 1	200
	21st to end	Day	23% 13% 27% 28% 9%	220
January	1st to 10th	Night	20% 8% 15% 31% 22% 5%	800
	11th to 20th	Night	16% 7% 14% 32% 25% 6%	800
	21st to end	Night	18% 6% 13% 34% 25% 4	880
February	1st to 10th	Night	19% 7% 16% 30% 23% 4	800
	11th to 20th	Night	16% 12% 16% 30% 20% 5%	784
	21st to end	Night	14% 12% 20% 32% 20% 3	660
March	1st to 10th	Night	21% 13% 19% 26% 19% 3	800
	11th to 20th	Night	20% 8% 16% 31% 21% 4	800
	21st to end	Night	15% 10% 27% 28% 17% 2	880
April	1st to 10th	Night	13% 9% 27% 31% 17% 4	800
	11th to 20th	Night	14% 14% 22% 28% 17% 4	791
	21st to end	Night	7% 11% 29% 28% 21% 4	782
May	1st to 10th	Night	8% 7% 26% 31% 24% 4	659
	11th to 20th	Night	9% 4 22% 33% 30% 3	543
	21st to end	Night	14% 10% 19% 33% 18% 4	411
June	1st to 10th	Night	10% 6% 23% 34% 25% 3	380
	11th to 20th	Night	4 7% 22% 42% 22% 3	380
	21st to end	Night	9% 3 16% 38% 30% 4	380
July	1st to 10th	Night	7% 3 13% 42% 28% 7%	393
	11th to 20th	Night	9% 3 9% 38% 31% 10%	552
	21st to end	Night	12% 5% 9% 33% 33% 8%	624
August	1st to 10th	Night	19% 3 13% 30% 29% 6%	567
	11th to 20th	Night	10% 3 16% 34% 30% 8%	664
	21st to end	Night	8% 4 12% 37% 31% 9%	836
September	1st to 10th	Night	4 2 14% 33% 36% 11%	760
	11th to 20th	Night	10% 5% 16% 31% 31% 7%	760
	21st to end	Night	4 5% 13% 30% 37% 11%	760
October	1st to 10th	Night	8% 4 13% 32% 33% 10%	760
	11th to 20th	Night	8% 5% 14% 28% 35% 10%	760
	21st to end	Night	10% 5% 11% 27% 33% 13%	836
November	1st to 10th	Night	10% 5% 14% 28% 34% 9%	780
	11th to 20th	Night	12% 5% 14% 33% 31% 7%	800
	21st to end	Night	11% 7% 12% 38% 27% 5%	800
December	1st to 10th	Night	18% 7% 13% 33% 23% 6%	800
	11th to 20th	Night	25% 7% 14% 29% 21% 4	800
	21st to end	Night	22% 11% 18% 29% 19% 1	880

Page generated at 2025-04-23 06:25:05. Version ac07

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