Pemberton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pemberton clear sky chart from 2005-11-06 to 2026-06-01.

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% 13% 4 11% 6% 5% 5% 4 5% 7% 30%	135748
	Night	10% 15% 5% 12% 7% 6% 5% 4 5% 6% 24%	136068
Transparency	Day	55% 6% 13% 16% 8% 2	135453
	Night	59% 7% 13% 14% 5% 1	135942
Seeing	Day	20% 17% 28% 22% 10% 3	17343
	Night	21% 12% 29% 27% 11% 1	25543

By Month

Percentage Cloud Cover NW (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% 25% 8% 17% 6% 6% 5% 5% 5% 5% 8%	4125
	11th to 20th	Day	8% 12% 4 9% 7% 6% 5% 4 6% 9% 30%	4119
	21st to end	Day	15% 22% 5% 11% 5% 4 5% 3 3 4 22%	4963
February	1st to 10th	Day	12% 23% 5% 14% 8% 4 5% 3 5% 6% 15%	1864
	11th to 20th	Day	7% 18% 7% 15% 8% 7% 5% 4 4 6% 20%	1845
	21st to end	Day	5% 22% 6% 18% 8% 7% 6% 4 6% 6% 13%	1567
March	1st to 10th	Day	7% 17% 6% 16% 11% 8% 5% 4 5% 5% 16%	4725
	11th to 20th	Day	5% 18% 5% 14% 9% 7% 6% 5% 6% 7% 18%	5110
	21st to end	Day	6% 19% 7% 17% 10% 7% 6% 5% 5% 5% 13%	5781
April	1st to 10th	Day	8% 23% 6% 14% 8% 6% 6% 4 5% 6% 14%	2520
	11th to 20th	Day	10% 17% 7% 14% 7% 6% 5% 4 4 6% 20%	2611
	21st to end	Day	11% 18% 5% 13% 8% 6% 5% 4 5% 6% 19%	2831
May	1st to 10th	Day	7% 13% 4 9% 6% 7% 6% 4 5% 8% 33%	6460
	11th to 20th	Day	7% 10% 4 11% 7% 7% 6% 5% 8% 10% 25%	6417
	21st to end	Day	12% 14% 5% 10% 7% 6% 6% 4 6% 8% 23%	7356
June	1st to 10th	Day	19% 14% 5% 9% 5% 4 5% 4 4 6% 24%	3001
	11th to 20th	Day	18% 13% 4 11% 7% 6% 6% 4 6% 6% 19%	3000
	21st to end	Day	13% 11% 4 9% 6% 5% 5% 4 5% 7% 30%	3000
July	1st to 10th	Day	5% 4 2 5% 3 3 3 3 5% 7% 62%	6780
	11th to 20th	Day	4 3 1 3 3 3 3 3 4 6% 67%	6780
	21st to end	Day	4 4 1 4 3 4 4 4 6% 9% 56%	7116
August	1st to 10th	Day	5% 4 2 5% 3 4 5% 4 5% 8% 56%	4541
	11th to 20th	Day	6% 6% 3 7% 5% 5% 5% 4 7% 9% 45%	4196
	21st to end	Day	11% 7% 3 8% 4 5% 5% 4 7% 8% 37%	4576
September	1st to 10th	Day	9% 7% 2 7% 5% 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	14% 11% 3 9% 5% 5% 4 3 4 4 38%	2400
	21st to end	Day	18% 12% 4 9% 5% 5% 4 4 4 6% 30%	2105
October	1st to 10th	Day	12% 9% 5% 11% 6% 7% 7% 5% 6% 8% 25%	3280
	11th to 20th	Day	23% 11% 4 10% 5% 4 4 3 5% 6% 25%	3261
	21st to end	Day	14% 12% 6% 13% 8% 7% 6% 6% 6% 6% 16%	3190
November	1st to 10th	Day	22% 19% 5% 11% 6% 4 5% 4 5% 5% 14%	1648
	11th to 20th	Day	17% 26% 6% 12% 7% 5% 4 3 4 4 14%	1680
	21st to end	Day	15% 22% 6% 15% 9% 6% 5% 3 4 3 12%	1680
December	1st to 10th	Day	10% 13% 5% 13% 10% 9% 7% 5% 5% 6% 17%	2530
	11th to 20th	Day	13% 23% 8% 16% 9% 8% 5% 4 4 3 6%	2715
	21st to end	Day	15% 21% 8% 22% 8% 5% 4 3 3 4 7%	3575
January	1st to 10th	Night	9% 23% 8% 17% 10% 8% 6% 4 4 4 7%	8770
	11th to 20th	Night	11% 12% 4 12% 6% 6% 5% 3 6% 10% 24%	8779
	21st to end	Night	13% 19% 6% 12% 6% 5% 5% 3 3 5% 20%	9216
February	1st to 10th	Night	12% 23% 6% 16% 8% 5% 5% 4 6% 5% 11%	2755
	11th to 20th	Night	8% 20% 7% 17% 8% 6% 5% 5% 5% 4 17%	2662
	21st to end	Night	9% 21% 7% 17% 8% 6% 4 4 5% 4 16%	2239
March	1st to 10th	Night	8% 17% 6% 15% 8% 7% 6% 4 5% 6% 18%	5373
	11th to 20th	Night	5% 18% 5% 13% 9% 7% 7% 5% 7% 7% 19%	5109
	21st to end	Night	7% 17% 6% 16% 9% 7% 6% 4 5% 6% 15%	5436
April	1st to 10th	Night	7% 20% 6% 12% 8% 6% 6% 5% 6% 8% 17%	2100
	11th to 20th	Night	9% 15% 5% 11% 7% 5% 5% 4 6% 7% 25%	1955
	21st to end	Night	12% 12% 4 12% 5% 6% 5% 5% 7% 7% 24%	1788
May	1st to 10th	Night	6% 10% 3 9% 6% 6% 5% 4 5% 8% 38%	3720
	11th to 20th	Night	5% 7% 3 9% 6% 6% 6% 6% 9% 12% 31%	3696
	21st to end	Night	10% 11% 3 8% 6% 7% 7% 5% 7% 8% 27%	3744
June	1st to 10th	Night	15% 12% 4 8% 5% 6% 4 4 6% 7% 30%	1400
	11th to 20th	Night	13% 12% 4 9% 6% 6% 5% 4 7% 8% 29%	1400
	21st to end	Night	10% 9% 3 8% 6% 5% 5% 6% 6% 8% 33%	1400
July	1st to 10th	Night	3 4 2 4 3 3 3 3 5% 6% 64%	3180
	11th to 20th	Night	2 2 1 3 2 2 2 2 4 5% 76%	3180
	21st to end	Night	3 3 1 4 3 3 4 4 7% 8% 59%	3712
August	1st to 10th	Night	5% 3 2 4 3 2 4 3 6% 9% 59%	2609
	11th to 20th	Night	5% 4 2 6% 3 4 4 4 7% 12% 48%	2858
	21st to end	Night	10% 5% 2 6% 4 4 4 3 6% 8% 47%	3335
September	1st to 10th	Night	7% 5% 2 6% 4 4 5% 3 6% 8% 50%	2000
	11th to 20th	Night	11% 9% 3 7% 5% 4 4 4 6% 7% 40%	2000
	21st to end	Night	15% 10% 3 9% 5% 6% 4 4 4 7% 33%	2292
October	1st to 10th	Night	12% 8% 4 11% 6% 6% 5% 5% 7% 8% 27%	3920
	11th to 20th	Night	15% 10% 4 9% 6% 6% 5% 3 6% 7% 28%	3897
	21st to end	Night	15% 12% 6% 13% 8% 6% 5% 5% 6% 6% 19%	4595
November	1st to 10th	Night	21% 18% 7% 14% 7% 6% 4 3 5% 4 11%	2870
	11th to 20th	Night	16% 23% 7% 14% 7% 6% 4 3 4 3 12%	2940
	21st to end	Night	15% 23% 9% 15% 9% 6% 5% 3 3 2 9%	2940
December	1st to 10th	Night	11% 16% 6% 16% 9% 11% 7% 4 4 5% 11%	4865
	11th to 20th	Night	15% 23% 8% 17% 8% 8% 5% 4 4 3 5%	5710
	21st to end	Night	15% 23% 8% 20% 8% 5% 5% 3 4 3 5%	7623

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	76% 7% 9% 6% 1 1	4125
	11th to 20th	Day	52% 7% 13% 19% 5% 3	4140
	21st to end	Day	69% 4 8% 15% 3	4964
February	1st to 10th	Day	70% 8% 7% 6% 8% 1	1864
	11th to 20th	Day	66% 8% 8% 12% 6% 1	1854
	21st to end	Day	73% 7% 8% 8% 4 1	1567
March	1st to 10th	Day	68% 8% 9% 10% 6%	4725
	11th to 20th	Day	61% 9% 8% 15% 7%	5110
	21st to end	Day	71% 7% 11% 7% 4	5781
April	1st to 10th	Day	71% 7% 12% 9% 1	2520
	11th to 20th	Day	67% 6% 10% 11% 6%	2611
	21st to end	Day	65% 8% 11% 12% 4	2831
May	1st to 10th	Day	50% 6% 16% 22% 6%	6460
	11th to 20th	Day	55% 9% 18% 17% 2	6420
	21st to end	Day	61% 5% 19% 12% 3	7357
June	1st to 10th	Day	62% 8% 10% 12% 5% 2	3001
	11th to 20th	Day	66% 8% 12% 11% 3	3000
	21st to end	Day	56% 8% 13% 16% 7% 1	3000
July	1st to 10th	Day	26% 4 14% 23% 22% 11%	6706
	11th to 20th	Day	21% 5% 14% 30% 24% 6%	6699
	21st to end	Day	27% 3 20% 29% 18% 3	6916
August	1st to 10th	Day	28% 4 11% 36% 18% 3	4524
	11th to 20th	Day	37% 4 25% 22% 10% 2	4236
	21st to end	Day	45% 6% 18% 21% 8% 2	4575
September	1st to 10th	Day	40% 7% 14% 24% 12% 3	2400
	11th to 20th	Day	50% 5% 10% 18% 14% 3	2400
	21st to end	Day	57% 7% 13% 14% 7% 2	2105
October	1st to 10th	Day	56% 7% 13% 17% 7%	3280
	11th to 20th	Day	64% 4 10% 18% 4	3261
	21st to end	Day	69% 4 10% 13% 4	3190
November	1st to 10th	Day	73% 6% 9% 8% 3	1648
	11th to 20th	Day	75% 5% 7% 7% 5% 1	1680
	21st to end	Day	77% 7% 5% 7% 4 1	1680
December	1st to 10th	Day	68% 6% 14% 9% 3	2530
	11th to 20th	Day	80% 8% 5% 4 2 1	2718
	21st to end	Day	77% 8% 6% 7% 2	3575
January	1st to 10th	Night	76% 7% 9% 6% 2	8770
	11th to 20th	Night	58% 9% 11% 16% 4 3	8820
	21st to end	Night	66% 5% 11% 15% 3	9216
February	1st to 10th	Night	74% 5% 6% 7% 7% 1	2755
	11th to 20th	Night	69% 6% 9% 9% 6% 1	2664
	21st to end	Night	72% 6% 8% 9% 5% 1	2239
March	1st to 10th	Night	62% 7% 12% 13% 6%	5373
	11th to 20th	Night	58% 10% 10% 16% 5%	5110
	21st to end	Night	67% 10% 11% 8% 4	5436
April	1st to 10th	Night	64% 9% 15% 10% 1	2100
	11th to 20th	Night	57% 10% 14% 12% 7%	1955
	21st to end	Night	58% 10% 15% 14% 4	1788
May	1st to 10th	Night	44% 7% 21% 20% 8%	3717
	11th to 20th	Night	49% 9% 24% 16% 2	3696
	21st to end	Night	53% 8% 22% 15% 2	3744
June	1st to 10th	Night	54% 9% 16% 13% 7% 1	1400
	11th to 20th	Night	56% 10% 15% 15% 4	1400
	21st to end	Night	49% 10% 18% 16% 6% 1	1400
July	1st to 10th	Night	24% 4 22% 24% 20% 7%	3145
	11th to 20th	Night	13% 5% 24% 37% 19% 1	3138
	21st to end	Night	25% 4 26% 27% 14% 3	3608
August	1st to 10th	Night	25% 4 15% 36% 18% 1	2601
	11th to 20th	Night	32% 6% 30% 22% 8% 1	2884
	21st to end	Night	38% 8% 26% 22% 6% 1	3335
September	1st to 10th	Night	33% 9% 19% 25% 11% 2	2000
	11th to 20th	Night	44% 7% 13% 20% 14% 2	2000
	21st to end	Night	52% 7% 17% 15% 7% 2	2292
October	1st to 10th	Night	54% 7% 15% 18% 6%	3920
	11th to 20th	Night	56% 6% 19% 15% 3	3897
	21st to end	Night	69% 4 11% 13% 3	4595
November	1st to 10th	Night	77% 6% 7% 6% 3 1	2870
	11th to 20th	Night	76% 6% 6% 6% 5% 1	2940
	21st to end	Night	79% 5% 5% 6% 4 1	2940
December	1st to 10th	Night	73% 6% 11% 7% 2	4865
	11th to 20th	Night	82% 6% 7% 3 2	5706
	21st to end	Night	81% 7% 5% 5% 2	7623

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% 17% 23% 24% 12% 1	220
	11th to 20th	Day	25% 14% 20% 27% 12% 3	220
	21st to end	Day	20% 12% 24% 25% 16% 3	242
February	1st to 10th	Day	24% 14% 31% 22% 7% 2	404
	11th to 20th	Day	16% 19% 30% 20% 12% 4	425
	21st to end	Day	21% 19% 29% 20% 10% 1	361
March	1st to 10th	Day	19% 20% 27% 23% 9% 2	540
	11th to 20th	Day	25% 15% 27% 17% 11% 5%	540
	21st to end	Day	25% 18% 27% 18% 9% 3	594
April	1st to 10th	Day	24% 20% 25% 21% 8% 2	540
	11th to 20th	Day	19% 21% 21% 23% 8% 8%	532
	21st to end	Day	19% 18% 27% 20% 11% 5%	540
May	1st to 10th	Day	18% 15% 28% 24% 13% 2	543
	11th to 20th	Day	18% 14% 29% 25% 10% 4	568
	21st to end	Day	20% 18% 29% 21% 8% 3	747
June	1st to 10th	Day	24% 21% 31% 17% 5% 1	731
	11th to 20th	Day	24% 21% 30% 19% 6% 1	730
	21st to end	Day	17% 19% 33% 21% 8% 2	730
July	1st to 10th	Day	15% 19% 32% 24% 8% 2	721
	11th to 20th	Day	12% 19% 36% 24% 8% 2	718
	21st to end	Day	9% 14% 34% 28% 10% 5%	687
August	1st to 10th	Day	13% 10% 25% 31% 15% 7%	527
	11th to 20th	Day	10% 15% 31% 26% 13% 5%	517
	21st to end	Day	16% 12% 33% 23% 10% 5%	583
September	1st to 10th	Day	15% 19% 28% 20% 14% 3	530
	11th to 20th	Day	22% 15% 23% 25% 11% 3	530
	21st to end	Day	27% 20% 25% 19% 8% 1	530
October	1st to 10th	Day	25% 16% 25% 18% 12% 3	530
	11th to 20th	Day	32% 15% 24% 15% 12% 2	530
	21st to end	Day	32% 14% 27% 17% 8% 2	391
November	1st to 10th	Day	31% 13% 30% 15% 9% 2	220
	11th to 20th	Day	31% 10% 25% 17% 12% 6%	220
	21st to end	Day	22% 16% 27% 18% 10% 7%	220
December	1st to 10th	Day	23% 10% 19% 23% 20% 5%	220
	11th to 20th	Day	29% 9% 24% 24% 11% 4	220
	21st to end	Day	21% 12% 26% 31% 8% 2	242
January	1st to 10th	Night	31% 14% 24% 23% 7%	950
	11th to 20th	Night	28% 12% 27% 25% 7%	950
	21st to end	Night	23% 13% 29% 26% 9%	965
February	1st to 10th	Night	25% 13% 33% 23% 5% 1	840
	11th to 20th	Night	17% 12% 34% 25% 11% 1	824
	21st to end	Night	19% 17% 31% 24% 9% 1	692
March	1st to 10th	Night	22% 16% 29% 22% 10% 1	840
	11th to 20th	Night	23% 16% 30% 20% 10% 1	840
	21st to end	Night	20% 15% 31% 24% 9% 1	924
April	1st to 10th	Night	20% 13% 30% 27% 10%	840
	11th to 20th	Night	19% 14% 31% 24% 10% 3	707
	21st to end	Night	19% 7% 34% 26% 11% 3	528
May	1st to 10th	Night	13% 8% 32% 32% 15% 1	420
	11th to 20th	Night	13% 8% 24% 37% 18%	414
	21st to end	Night	19% 8% 30% 32% 11%	455
June	1st to 10th	Night	20% 12% 32% 28% 8%	400
	11th to 20th	Night	15% 10% 35% 32% 8%	400
	21st to end	Night	14% 8% 33% 34% 11% 1	400
July	1st to 10th	Night	9% 8% 32% 37% 13% 1	390
	11th to 20th	Night	6% 9% 32% 43% 12%	388
	21st to end	Night	5% 7% 28% 40% 21%	438
August	1st to 10th	Night	8% 3 21% 47% 20% 1	398
	11th to 20th	Night	8% 3 28% 39% 21% 1	562
	21st to end	Night	13% 8% 28% 34% 16% 1	711
September	1st to 10th	Night	10% 10% 31% 31% 17% 1	800
	11th to 20th	Night	17% 9% 23% 29% 21% 2	800
	21st to end	Night	23% 12% 30% 23% 12%	800
October	1st to 10th	Night	22% 12% 29% 25% 12%	800
	11th to 20th	Night	26% 11% 26% 25% 11% 1	800
	21st to end	Night	28% 16% 27% 20% 9%	880
November	1st to 10th	Night	33% 14% 26% 22% 5%	820
	11th to 20th	Night	29% 16% 26% 19% 8% 1	840
	21st to end	Night	28% 14% 25% 22% 9% 1	840
December	1st to 10th	Night	24% 12% 26% 26% 10% 1	892
	11th to 20th	Night	31% 13% 27% 21% 7% 1	950
	21st to end	Night	26% 14% 29% 26% 5%	1045

Page generated at 2026-05-28 22:53:24. Version ac07

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