PACMO
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PACMO clear sky chart from 2005-11-06 to 2025-04-11.

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	13% 8% 3 7% 5% 4 4 4 5% 7% 39%	113552
	Night	13% 9% 3 8% 5% 4 4 3 5% 6% 40%	114925
Transparency	Day	43% 6% 12% 23% 14% 2	117119
	Night	43% 5% 11% 23% 15% 2	119499
Seeing	Day	16% 12% 30% 25% 13% 4	16304
	Night	17% 11% 32% 29% 11% 1	26324

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	8% 13% 5% 16% 6% 6% 5% 4 5% 5% 27%	3617
	11th to 20th	Day	8% 10% 6% 11% 7% 5% 6% 4 7% 5% 30%	3620
	21st to end	Day	13% 16% 8% 9% 4 5% 4 3 4 5% 29%	4470
February	1st to 10th	Day	15% 13% 5% 9% 5% 4 5% 4 4 7% 29%	1800
	11th to 20th	Day	14% 10% 3 8% 6% 5% 4 3 4 7% 36%	1865
	21st to end	Day	13% 12% 4 10% 4 3 4 3 5% 5% 36%	1650
March	1st to 10th	Day	18% 11% 3 5% 3 2 3 2 4 5% 43%	4480
	11th to 20th	Day	9% 7% 2 6% 4 4 5% 4 6% 7% 47%	4980
	21st to end	Day	20% 10% 4 10% 5% 5% 4 3 5% 6% 27%	5781
April	1st to 10th	Day	15% 12% 4 7% 6% 4 4 3 5% 6% 34%	2400
	11th to 20th	Day	16% 11% 3 7% 4 5% 4 3 4 6% 37%	2254
	21st to end	Day	20% 9% 2 7% 5% 5% 4 3 4 6% 35%	2280
May	1st to 10th	Day	16% 7% 2 7% 5% 4 5% 5% 6% 8% 35%	4168
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 3 5% 7% 41%	4399
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 44%	4824
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 4 4 6% 10% 44%	2660
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 5% 8% 46%	2660
	21st to end	Day	10% 6% 3 6% 4 5% 5% 5% 7% 7% 41%	2660
July	1st to 10th	Day	9% 7% 3 8% 6% 5% 6% 5% 6% 10% 35%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 8% 52%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 11% 41%	4813
August	1st to 10th	Day	8% 4 2 6% 5% 5% 5% 5% 7% 11% 43%	4114
	11th to 20th	Day	11% 5% 2 6% 3 5% 5% 5% 7% 9% 42%	3829
	21st to end	Day	7% 4 2 6% 4 4 4 3 6% 9% 50%	4180
September	1st to 10th	Day	12% 6% 2 5% 4 4 5% 3 5% 8% 48%	2280
	11th to 20th	Day	11% 4 2 4 4 3 4 3 5% 8% 54%	2256
	21st to end	Day	10% 5% 1 6% 4 5% 3 4 6% 7% 47%	2073
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 5% 6% 50%	1900
	11th to 20th	Day	16% 8% 3 5% 4 4 4 3 4 6% 44%	1900
	21st to end	Day	18% 8% 3 6% 4 4 3 3 5% 6% 41%	1999
November	1st to 10th	Day	11% 9% 2 5% 4 4 4 3 5% 6% 46%	1762
	11th to 20th	Day	14% 12% 3 8% 4 3 4 4 6% 6% 36%	1798
	21st to end	Day	15% 12% 3 8% 5% 4 4 3 4 4 37%	1600
December	1st to 10th	Day	8% 10% 3 7% 5% 3 3 3 4 6% 47%	2610
	11th to 20th	Day	14% 12% 6% 16% 5% 5% 4 4 6% 6% 24%	3008
	21st to end	Day	23% 21% 8% 12% 5% 3 3 2 3 4 15%	3982
January	1st to 10th	Night	12% 12% 6% 13% 5% 4 4 3 4 5% 32%	6348
	11th to 20th	Night	11% 14% 5% 12% 7% 6% 5% 4 7% 5% 25%	6360
	21st to end	Night	15% 13% 6% 10% 5% 4 4 3 4 4 32%	6492
February	1st to 10th	Night	14% 12% 4 9% 6% 4 4 4 5% 5% 34%	2600
	11th to 20th	Night	12% 11% 4 7% 4 4 4 3 5% 5% 39%	2421
	21st to end	Night	15% 10% 3 8% 4 4 3 3 3 5% 41%	1980
March	1st to 10th	Night	18% 9% 3 5% 3 3 4 3 4 5% 43%	5373
	11th to 20th	Night	10% 6% 3 6% 4 4 4 3 5% 7% 48%	5009
	21st to end	Night	17% 12% 5% 10% 6% 4 4 4 5% 5% 28%	5147
April	1st to 10th	Night	16% 11% 3 6% 4 4 4 3 5% 6% 40%	2000
	11th to 20th	Night	18% 9% 2 6% 4 4 4 3 4 6% 42%	1881
	21st to end	Night	17% 7% 3 8% 4 4 4 4 6% 7% 37%	1900
May	1st to 10th	Night	18% 7% 3 7% 5% 5% 5% 4 6% 7% 34%	2716
	11th to 20th	Night	13% 6% 2 6% 5% 5% 5% 3 6% 8% 41%	2506
	21st to end	Night	7% 6% 2 6% 4 4 4 4 7% 8% 48%	2743
June	1st to 10th	Night	9% 5% 2 5% 4 3 4 3 6% 8% 51%	1520
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 7% 9% 46%	1520
	21st to end	Night	10% 5% 2 6% 5% 4 5% 4 5% 7% 48%	1520
July	1st to 10th	Night	14% 5% 2 6% 5% 4 5% 4 6% 8% 40%	2530
	11th to 20th	Night	5% 3 1 4 3 3 4 3 5% 9% 59%	2530
	21st to end	Night	5% 4 2 6% 5% 5% 5% 5% 6% 9% 46%	2750
August	1st to 10th	Night	5% 3 2 5% 3 4 4 3 7% 9% 55%	2823
	11th to 20th	Night	7% 4 2 6% 4 4 4 3 5% 8% 53%	3003
	21st to end	Night	6% 3 2 5% 4 3 4 3 6% 8% 55%	3476
September	1st to 10th	Night	10% 5% 1 5% 3 3 3 2 5% 7% 57%	1900
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 6% 7% 53%	1904
	21st to end	Night	9% 4 2 5% 4 4 5% 3 6% 8% 50%	2104
October	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 4 5% 55%	2280
	11th to 20th	Night	15% 7% 2 5% 4 3 4 2 3 7% 48%	2280
	21st to end	Night	17% 9% 3 6% 5% 4 4 3 5% 5% 39%	2598
November	1st to 10th	Night	11% 7% 2 5% 4 3 3 3 4 6% 52%	2536
	11th to 20th	Night	15% 8% 4 6% 4 4 4 3 5% 6% 40%	2600
	21st to end	Night	16% 11% 3 7% 5% 4 3 2 4 6% 39%	2800
December	1st to 10th	Night	8% 10% 3 7% 5% 3 4 3 5% 6% 48%	4580
	11th to 20th	Night	15% 13% 5% 9% 5% 4 3 3 4 5% 33%	5199
	21st to end	Night	23% 18% 6% 11% 5% 5% 4 4 4 4 16%	6996

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	59% 6% 9% 19% 6% 1	3618
	11th to 20th	Day	54% 5% 10% 13% 15% 4	3620
	21st to end	Day	58% 4 13% 17% 8% 1	4470
February	1st to 10th	Day	56% 4 15% 18% 7% 1	1800
	11th to 20th	Day	49% 6% 12% 21% 10% 2	1874
	21st to end	Day	49% 6% 11% 22% 9% 2	1650
March	1st to 10th	Day	44% 5% 9% 25% 15% 3	4480
	11th to 20th	Day	35% 6% 11% 33% 15% 1	4980
	21st to end	Day	56% 7% 13% 18% 6% 1	5783
April	1st to 10th	Day	50% 6% 12% 21% 11%	2400
	11th to 20th	Day	47% 6% 9% 22% 13% 2	2254
	21st to end	Day	50% 6% 10% 18% 13% 2	2280
May	1st to 10th	Day	43% 9% 13% 20% 15%	4205
	11th to 20th	Day	42% 8% 9% 29% 11%	4403
	21st to end	Day	36% 9% 12% 23% 18% 3	4837
June	1st to 10th	Day	34% 8% 15% 24% 16% 2	2660
	11th to 20th	Day	35% 6% 12% 22% 22% 3	2660
	21st to end	Day	37% 9% 14% 23% 14% 2	2660
July	1st to 10th	Day	44% 7% 12% 25% 10% 2	4371
	11th to 20th	Day	28% 6% 15% 31% 17% 2	4365
	21st to end	Day	35% 8% 19% 19% 17% 2	4662
August	1st to 10th	Day	33% 8% 17% 29% 12% 1	4099
	11th to 20th	Day	36% 8% 15% 25% 15% 1	3867
	21st to end	Day	31% 6% 13% 31% 17% 2	4179
September	1st to 10th	Day	34% 7% 10% 24% 20% 4	2280
	11th to 20th	Day	28% 5% 12% 27% 23% 3	2256
	21st to end	Day	34% 7% 11% 27% 20% 2	2073
October	1st to 10th	Day	23% 3 6% 29% 37% 1	3160
	11th to 20th	Day	28% 3 11% 23% 31% 4	3140
	21st to end	Day	40% 4 10% 28% 15% 4	3276
November	1st to 10th	Day	38% 6% 14% 25% 14% 3	1762
	11th to 20th	Day	48% 5% 13% 21% 11% 2	1798
	21st to end	Day	50% 5% 14% 21% 9%	1600
December	1st to 10th	Day	39% 4 14% 35% 8%	2610
	11th to 20th	Day	60% 5% 12% 15% 6% 2	3005
	21st to end	Day	74% 4 8% 10% 3	3982
January	1st to 10th	Night	54% 5% 11% 21% 8%	6348
	11th to 20th	Night	59% 4 12% 12% 13% 1	6360
	21st to end	Night	56% 4 9% 18% 12% 1	6492
February	1st to 10th	Night	52% 4 12% 19% 12% 1	2600
	11th to 20th	Night	46% 4 12% 23% 13% 1	2422
	21st to end	Night	46% 4 10% 23% 15% 2	1980
March	1st to 10th	Night	44% 5% 9% 25% 15% 3	5373
	11th to 20th	Night	35% 4 11% 29% 18% 3	5009
	21st to end	Night	56% 7% 11% 17% 8% 1	5147
April	1st to 10th	Night	44% 6% 10% 24% 14% 2	2000
	11th to 20th	Night	44% 5% 7% 23% 17% 4	1881
	21st to end	Night	45% 7% 11% 21% 13% 3	1900
May	1st to 10th	Night	46% 9% 12% 17% 14% 2	2743
	11th to 20th	Night	38% 10% 15% 26% 12% 1	2507
	21st to end	Night	30% 9% 12% 25% 21% 2	2751
June	1st to 10th	Night	30% 8% 15% 25% 19% 3	1520
	11th to 20th	Night	32% 8% 11% 22% 23% 4	1520
	21st to end	Night	35% 8% 15% 20% 20% 3	1520
July	1st to 10th	Night	41% 7% 14% 22% 13% 3	2490
	11th to 20th	Night	23% 6% 14% 34% 22% 2	2482
	21st to end	Night	31% 9% 16% 22% 18% 4	2662
August	1st to 10th	Night	24% 7% 18% 33% 17% 2	2814
	11th to 20th	Night	29% 6% 16% 31% 17% 1	3031
	21st to end	Night	27% 6% 14% 30% 19% 4	3476
September	1st to 10th	Night	28% 5% 11% 25% 27% 4	1900
	11th to 20th	Night	29% 7% 9% 29% 21% 5%	1904
	21st to end	Night	30% 7% 12% 25% 22% 3	2104
October	1st to 10th	Night	23% 3 7% 30% 33% 3	3800
	11th to 20th	Night	28% 4 8% 23% 33% 5%	3779
	21st to end	Night	43% 4 8% 22% 19% 4	4271
November	1st to 10th	Night	34% 5% 12% 26% 19% 4	2536
	11th to 20th	Night	45% 4 11% 24% 14% 2	2600
	21st to end	Night	48% 4 11% 24% 13% 1	2800
December	1st to 10th	Night	39% 4 12% 34% 11% 1	4580
	11th to 20th	Night	54% 4 11% 22% 8%	5201
	21st to end	Night	72% 6% 8% 10% 4	6996

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	21% 17% 32% 24% 5%	300
	11th to 20th	Day	21% 21% 40% 14% 4	300
	21st to end	Day	22% 21% 38% 16% 3 1	330
February	1st to 10th	Day	22% 20% 38% 16% 3	300
	11th to 20th	Day	25% 21% 33% 16% 4 1	300
	21st to end	Day	24% 17% 46% 10% 2	249
March	1st to 10th	Day	22% 22% 36% 17% 3	300
	11th to 20th	Day	23% 21% 35% 15% 5% 1	300
	21st to end	Day	30% 19% 31% 16% 4 1	468
April	1st to 10th	Day	23% 19% 33% 19% 7% 1	500
	11th to 20th	Day	21% 18% 31% 20% 8% 2	452
	21st to end	Day	24% 16% 32% 19% 7% 2	460
May	1st to 10th	Day	20% 11% 36% 24% 7% 2	547
	11th to 20th	Day	20% 13% 29% 23% 13% 2	675
	21st to end	Day	15% 10% 28% 26% 13% 7%	734
June	1st to 10th	Day	9% 7% 27% 34% 18% 6%	680
	11th to 20th	Day	9% 5% 32% 32% 17% 5%	680
	21st to end	Day	10% 10% 32% 29% 14% 5%	680
July	1st to 10th	Day	10% 4 21% 29% 24% 13%	671
	11th to 20th	Day	9% 5% 24% 34% 19% 9%	668
	21st to end	Day	6% 2 24% 37% 25% 5%	739
August	1st to 10th	Day	6% 3 22% 38% 24% 7%	676
	11th to 20th	Day	12% 5% 19% 30% 23% 11%	543
	21st to end	Day	7% 4 21% 32% 27% 9%	539
September	1st to 10th	Day	13% 6% 19% 33% 22% 7%	490
	11th to 20th	Day	9% 4 20% 34% 23% 11%	490
	21st to end	Day	12% 10% 26% 26% 19% 6%	473
October	1st to 10th	Day	17% 9% 27% 28% 15% 5%	300
	11th to 20th	Day	18% 23% 32% 19% 8%	300
	21st to end	Day	27% 17% 32% 19% 4 1	330
November	1st to 10th	Day	14% 15% 42% 18% 8% 3	300
	11th to 20th	Day	22% 19% 35% 16% 5% 2	300
	21st to end	Day	24% 13% 41% 19% 3	300
December	1st to 10th	Day	15% 19% 42% 17% 4 2	300
	11th to 20th	Day	18% 19% 37% 22% 4	300
	21st to end	Day	28% 19% 37% 14% 2	330
January	1st to 10th	Night	17% 15% 39% 22% 6%	1000
	11th to 20th	Night	18% 17% 37% 23% 5% 1	1000
	21st to end	Night	19% 14% 37% 25% 5%	1100
February	1st to 10th	Night	22% 14% 38% 22% 3	1000
	11th to 20th	Night	18% 15% 38% 25% 3	866
	21st to end	Night	22% 15% 40% 20% 3 1	660
March	1st to 10th	Night	22% 14% 37% 23% 4	800
	11th to 20th	Night	15% 14% 35% 29% 7%	616
	21st to end	Night	25% 12% 36% 21% 5%	660
April	1st to 10th	Night	21% 16% 37% 23% 4 1	600
	11th to 20th	Night	21% 15% 29% 28% 7% 1	565
	21st to end	Night	19% 11% 31% 32% 5% 1	570
May	1st to 10th	Night	18% 9% 38% 28% 6% 1	570
	11th to 20th	Night	19% 7% 32% 34% 9%	561
	21st to end	Night	12% 6% 32% 33% 16% 1	617
June	1st to 10th	Night	10% 4 27% 44% 14% 2	570
	11th to 20th	Night	9% 5% 31% 40% 15% 1	570
	21st to end	Night	10% 5% 29% 38% 16% 1	570
July	1st to 10th	Night	10% 3 15% 40% 27% 5%	555
	11th to 20th	Night	5% 2 25% 40% 25% 3	552
	21st to end	Night	6% 3 19% 42% 29% 1	624
August	1st to 10th	Night	4 4 16% 43% 29% 4	567
	11th to 20th	Night	6% 2 17% 42% 29% 5%	564
	21st to end	Night	9% 1 18% 44% 26% 3	627
September	1st to 10th	Night	12% 2 22% 38% 22% 4	570
	11th to 20th	Night	11% 3 21% 36% 24% 5%	574
	21st to end	Night	8% 9% 25% 34% 22% 2	760
October	1st to 10th	Night	12% 7% 32% 29% 19% 1	760
	11th to 20th	Night	18% 13% 30% 28% 11%	760
	21st to end	Night	23% 13% 33% 24% 7%	836
November	1st to 10th	Night	17% 10% 39% 26% 8% 1	780
	11th to 20th	Night	19% 16% 36% 24% 5%	800
	21st to end	Night	21% 14% 36% 24% 6%	1000
December	1st to 10th	Night	19% 14% 39% 23% 5%	1000
	11th to 20th	Night	23% 12% 34% 26% 5%	1000
	21st to end	Night	28% 16% 33% 21% 3	1100

Page generated at 2025-04-08 08:17:23. Version ac07

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