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 2025-02-06.

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	11% 14% 5% 11% 6% 5% 5% 4 5% 6% 28%	111914
	Night	11% 16% 5% 12% 7% 6% 5% 4 5% 6% 24%	114984
Transparency	Day	57% 7% 11% 15% 8% 2	111661
	Night	61% 7% 12% 13% 6% 1	114883
Seeing	Day	21% 17% 27% 21% 10% 3	15602
	Night	22% 12% 29% 26% 11% 1	23900

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	11% 26% 8% 16% 6% 5% 5% 4 5% 5% 9%	3179
	11th to 20th	Day	10% 15% 5% 9% 6% 6% 5% 4 6% 9% 26%	3159
	21st to end	Day	17% 24% 5% 9% 5% 4 4 3 3 3 22%	3837
February	1st to 10th	Day	12% 24% 5% 15% 7% 4 5% 4 5% 5% 13%	1731
	11th to 20th	Day	8% 19% 7% 14% 7% 6% 5% 4 4 6% 19%	1665
	21st to end	Day	6% 21% 6% 18% 8% 7% 6% 4 6% 6% 13%	1422
March	1st to 10th	Day	5% 20% 7% 18% 11% 8% 5% 4 5% 4 14%	3217
	11th to 20th	Day	7% 21% 4 14% 8% 6% 4 4 5% 5% 22%	3490
	21st to end	Day	7% 20% 7% 17% 9% 6% 6% 4 4 5% 15%	3943
April	1st to 10th	Day	9% 25% 7% 14% 9% 5% 6% 4 5% 5% 12%	2280
	11th to 20th	Day	11% 19% 7% 15% 8% 6% 5% 4 4 5% 17%	2359
	21st to end	Day	12% 19% 5% 14% 8% 6% 5% 4 6% 6% 15%	2561
May	1st to 10th	Day	9% 16% 5% 10% 6% 5% 6% 5% 5% 6% 29%	4420
	11th to 20th	Day	9% 11% 4 10% 6% 6% 5% 4 7% 9% 28%	4377
	21st to end	Day	15% 19% 6% 12% 7% 5% 6% 4 5% 5% 17%	5022
June	1st to 10th	Day	20% 14% 5% 10% 6% 4 5% 4 4 6% 21%	2850
	11th to 20th	Day	19% 13% 4 11% 7% 6% 6% 4 5% 6% 19%	2850
	21st to end	Day	14% 11% 4 9% 6% 5% 5% 4 5% 7% 30%	2850
July	1st to 10th	Day	6% 4 2 4 3 3 3 2 4 5% 64%	4740
	11th to 20th	Day	5% 3 1 3 3 3 3 3 4 5% 67%	4740
	21st to end	Day	5% 5% 1 4 4 4 4 4 6% 10% 51%	4962
August	1st to 10th	Day	5% 4 2 5% 3 4 5% 4 5% 8% 56%	4401
	11th to 20th	Day	5% 6% 3 7% 5% 5% 5% 4 7% 9% 46%	4065
	21st to end	Day	12% 7% 3 8% 4 5% 5% 4 7% 8% 37%	4436
September	1st to 10th	Day	10% 7% 2 7% 5% 4 4 3 5% 6% 46%	2280
	11th to 20th	Day	14% 12% 3 9% 5% 5% 4 3 4 4 38%	2280
	21st to end	Day	18% 12% 5% 8% 5% 5% 3 4 4 6% 30%	2000
October	1st to 10th	Day	12% 8% 5% 11% 6% 7% 7% 5% 6% 8% 24%	3180
	11th to 20th	Day	24% 11% 4 10% 4 4 4 3 5% 6% 24%	3161
	21st to end	Day	14% 12% 6% 13% 7% 7% 7% 6% 6% 6% 17%	3091
November	1st to 10th	Day	23% 19% 5% 11% 7% 4 4 4 4 5% 13%	1568
	11th to 20th	Day	16% 26% 6% 12% 7% 5% 4 3 4 4 14%	1600
	21st to end	Day	15% 23% 6% 15% 9% 6% 4 3 4 3 12%	1600
December	1st to 10th	Day	10% 13% 5% 13% 10% 9% 7% 4 5% 6% 18%	2455
	11th to 20th	Day	13% 23% 8% 16% 9% 8% 5% 3 4 3 6%	2645
	21st to end	Day	15% 21% 8% 22% 8% 5% 4 3 3 4 7%	3498
January	1st to 10th	Night	11% 25% 9% 18% 10% 8% 5% 3 3 3 6%	6766
	11th to 20th	Night	11% 14% 5% 11% 6% 7% 6% 3 6% 9% 22%	6739
	21st to end	Night	17% 23% 6% 12% 6% 4 3 2 3 4 20%	7118
February	1st to 10th	Night	13% 24% 6% 16% 8% 5% 5% 3 6% 4 9%	2559
	11th to 20th	Night	8% 21% 7% 16% 7% 6% 5% 5% 5% 4 16%	2402
	21st to end	Night	9% 20% 7% 18% 7% 6% 4 4 5% 4 16%	2032
March	1st to 10th	Night	7% 19% 8% 18% 9% 7% 6% 4 5% 6% 12%	3654
	11th to 20th	Night	6% 20% 5% 12% 7% 6% 6% 4 6% 6% 22%	3489
	21st to end	Night	8% 19% 6% 15% 8% 6% 5% 4 5% 6% 19%	3710
April	1st to 10th	Night	8% 21% 6% 13% 7% 6% 5% 4 5% 7% 17%	1900
	11th to 20th	Night	9% 16% 5% 11% 8% 5% 5% 4 6% 7% 23%	1767
	21st to end	Night	12% 13% 4 11% 5% 7% 6% 6% 7% 7% 21%	1618
May	1st to 10th	Night	7% 11% 4 9% 5% 5% 4 4 5% 6% 40%	2540
	11th to 20th	Night	7% 8% 3 8% 6% 5% 5% 4 7% 11% 36%	2516
	21st to end	Night	14% 15% 4 9% 6% 7% 6% 4 6% 7% 21%	2550
June	1st to 10th	Night	16% 12% 4 8% 5% 6% 4 4 6% 7% 27%	1330
	11th to 20th	Night	13% 12% 4 8% 6% 6% 5% 4 7% 8% 28%	1330
	21st to end	Night	10% 9% 3 8% 6% 5% 5% 6% 6% 8% 33%	1330
July	1st to 10th	Night	4 4 2 4 2 3 3 2 4 5% 68%	2220
	11th to 20th	Night	3 2 1 3 2 2 3 2 3 5% 74%	2220
	21st to end	Night	4 3 2 5% 3 3 5% 5% 8% 8% 56%	2589
August	1st to 10th	Night	4 3 2 4 3 2 4 3 6% 9% 59%	2529
	11th to 20th	Night	5% 4 2 6% 3 3 4 4 7% 12% 49%	2769
	21st to end	Night	11% 5% 2 6% 4 4 5% 3 6% 8% 46%	3233
September	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 6% 8% 50%	1900
	11th to 20th	Night	11% 9% 3 7% 5% 5% 5% 3 6% 7% 39%	1900
	21st to end	Night	15% 11% 3 9% 5% 5% 4 4 4 7% 33%	2177
October	1st to 10th	Night	12% 8% 4 11% 6% 6% 6% 5% 7% 8% 27%	3800
	11th to 20th	Night	15% 10% 4 10% 6% 6% 5% 3 6% 7% 28%	3777
	21st to end	Night	15% 12% 6% 13% 8% 6% 5% 5% 6% 6% 19%	4452
November	1st to 10th	Night	21% 19% 7% 14% 7% 5% 4 3 5% 4 11%	2730
	11th to 20th	Night	17% 23% 7% 14% 7% 5% 4 3 4 3 12%	2800
	21st to end	Night	15% 23% 9% 16% 9% 6% 5% 3 3 2 9%	2800
December	1st to 10th	Night	11% 16% 6% 16% 9% 11% 7% 4 4 5% 11%	4720
	11th to 20th	Night	15% 24% 8% 18% 9% 8% 5% 3 4 3 5%	5560
	21st to end	Night	16% 23% 8% 20% 8% 5% 5% 3 3 3 5%	7458

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% 9% 7% 7% 2 1	3179
	11th to 20th	Day	56% 7% 14% 14% 4 4	3180
	21st to end	Day	69% 4 9% 15% 3	3837
February	1st to 10th	Day	72% 8% 7% 6% 7% 1	1731
	11th to 20th	Day	66% 9% 7% 10% 6% 1	1674
	21st to end	Day	73% 8% 7% 8% 3 1	1422
March	1st to 10th	Day	69% 11% 8% 7% 5%	3217
	11th to 20th	Day	59% 11% 8% 17% 4	3490
	21st to end	Day	69% 10% 10% 7% 4	3943
April	1st to 10th	Day	74% 7% 10% 7% 1	2280
	11th to 20th	Day	70% 6% 8% 9% 6%	2359
	21st to end	Day	69% 9% 10% 9% 3	2561
May	1st to 10th	Day	55% 8% 14% 18% 6%	4420
	11th to 20th	Day	51% 11% 19% 16% 3	4380
	21st to end	Day	71% 6% 10% 11% 2	5023
June	1st to 10th	Day	65% 8% 10% 11% 5% 2	2850
	11th to 20th	Day	67% 8% 12% 10% 3	2850
	21st to end	Day	56% 8% 12% 16% 7% 1	2850
July	1st to 10th	Day	26% 4 8% 19% 28% 16%	4666
	11th to 20th	Day	21% 5% 10% 26% 29% 8%	4659
	21st to end	Day	30% 4 17% 26% 18% 5%	4805
August	1st to 10th	Day	28% 4 10% 36% 19% 3	4384
	11th to 20th	Day	36% 4 26% 22% 10% 2	4105
	21st to end	Day	46% 6% 18% 21% 7% 1	4435
September	1st to 10th	Day	40% 7% 13% 24% 12% 3	2280
	11th to 20th	Day	51% 5% 10% 17% 14% 3	2280
	21st to end	Day	57% 7% 13% 14% 7% 2	2000
October	1st to 10th	Day	56% 7% 12% 17% 7%	3180
	11th to 20th	Day	64% 4 11% 17% 4	3161
	21st to end	Day	69% 5% 10% 13% 4	3091
November	1st to 10th	Day	74% 6% 8% 8% 4	1568
	11th to 20th	Day	74% 5% 7% 7% 6% 1	1600
	21st to end	Day	76% 7% 4 7% 4 2	1600
December	1st to 10th	Day	68% 6% 14% 9% 3	2455
	11th to 20th	Day	80% 8% 5% 4 2 1	2648
	21st to end	Day	77% 8% 5% 8% 2	3498
January	1st to 10th	Night	78% 7% 8% 6% 2	6766
	11th to 20th	Night	60% 8% 12% 12% 4 3	6780
	21st to end	Night	69% 4 12% 12% 2	7118
February	1st to 10th	Night	76% 6% 6% 6% 6% 1	2559
	11th to 20th	Night	70% 6% 9% 8% 6% 1	2404
	21st to end	Night	71% 6% 7% 9% 5% 1	2032
March	1st to 10th	Night	66% 10% 9% 9% 6%	3654
	11th to 20th	Night	55% 13% 13% 17% 2	3490
	21st to end	Night	61% 13% 12% 9% 4 1	3710
April	1st to 10th	Night	65% 10% 13% 10% 1	1900
	11th to 20th	Night	60% 10% 12% 12% 7%	1767
	21st to end	Night	60% 10% 13% 13% 4	1618
May	1st to 10th	Night	45% 8% 17% 23% 6%	2540
	11th to 20th	Night	43% 11% 24% 20% 2	2516
	21st to end	Night	60% 10% 15% 13% 2	2550
June	1st to 10th	Night	56% 9% 15% 12% 6% 1	1330
	11th to 20th	Night	56% 10% 16% 14% 4	1330
	21st to end	Night	48% 11% 17% 17% 6% 1	1330
July	1st to 10th	Night	22% 5% 12% 25% 27% 10%	2185
	11th to 20th	Night	15% 5% 16% 37% 25% 1	2178
	21st to end	Night	28% 5% 17% 27% 17% 5%	2507
August	1st to 10th	Night	25% 5% 14% 37% 18% 2	2521
	11th to 20th	Night	32% 6% 30% 22% 8% 1	2795
	21st to end	Night	38% 8% 26% 22% 5% 1	3233
September	1st to 10th	Night	33% 9% 19% 25% 11% 3	1900
	11th to 20th	Night	45% 8% 12% 19% 14% 2	1900
	21st to end	Night	52% 7% 16% 15% 7% 2	2177
October	1st to 10th	Night	54% 8% 14% 18% 6%	3800
	11th to 20th	Night	56% 6% 19% 15% 3	3777
	21st to end	Night	69% 4 11% 13% 4	4452
November	1st to 10th	Night	77% 6% 7% 6% 3 1	2730
	11th to 20th	Night	76% 6% 6% 6% 5% 1	2800
	21st to end	Night	79% 5% 5% 6% 4 1	2800
December	1st to 10th	Night	73% 6% 12% 7% 2	4720
	11th to 20th	Night	83% 6% 7% 3 2	5556
	21st to end	Night	81% 7% 5% 5% 2	7458

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% 18% 22% 24% 13% 1	200
	11th to 20th	Day	27% 15% 19% 24% 12% 3	200
	21st to end	Day	21% 14% 22% 25% 15% 3	220
February	1st to 10th	Day	26% 15% 32% 18% 8% 2	361
	11th to 20th	Day	17% 21% 28% 17% 13% 4	365
	21st to end	Day	19% 21% 26% 21% 11% 2	312
March	1st to 10th	Day	17% 22% 26% 21% 11% 2	460
	11th to 20th	Day	24% 16% 26% 16% 13% 5%	460
	21st to end	Day	24% 19% 26% 18% 9% 3	506
April	1st to 10th	Day	26% 22% 24% 20% 7% 1	460
	11th to 20th	Day	21% 22% 21% 21% 7% 7%	452
	21st to end	Day	22% 20% 25% 18% 10% 5%	460
May	1st to 10th	Day	19% 15% 25% 24% 14% 3	463
	11th to 20th	Day	18% 13% 28% 26% 10% 4	488
	21st to end	Day	21% 17% 29% 22% 9% 3	647
June	1st to 10th	Day	26% 21% 29% 17% 5% 1	680
	11th to 20th	Day	25% 21% 29% 18% 6% 1	680
	21st to end	Day	17% 20% 32% 21% 9% 2	680
July	1st to 10th	Day	14% 18% 32% 25% 9% 2	671
	11th to 20th	Day	12% 18% 35% 25% 8% 2	668
	21st to end	Day	9% 14% 34% 28% 9% 5%	638
August	1st to 10th	Day	13% 11% 24% 30% 15% 7%	487
	11th to 20th	Day	9% 13% 31% 27% 14% 5%	477
	21st to end	Day	17% 13% 34% 22% 9% 4	539
September	1st to 10th	Day	16% 20% 30% 20% 12% 3	490
	11th to 20th	Day	23% 16% 24% 25% 11% 2	490
	21st to end	Day	26% 19% 26% 20% 9% 1	490
October	1st to 10th	Day	25% 17% 23% 18% 12% 4	490
	11th to 20th	Day	33% 16% 23% 14% 12% 2	490
	21st to end	Day	31% 14% 27% 17% 9% 2	358
November	1st to 10th	Day	31% 15% 27% 17% 10% 2	200
	11th to 20th	Day	31% 11% 23% 17% 13% 7%	200
	21st to end	Day	23% 17% 26% 17% 10% 8%	200
December	1st to 10th	Day	24% 11% 17% 23% 22% 5%	200
	11th to 20th	Day	27% 9% 24% 24% 13% 4	200
	21st to end	Day	21% 13% 25% 30% 9% 1	220
January	1st to 10th	Night	32% 14% 24% 22% 7%	900
	11th to 20th	Night	29% 13% 27% 23% 7%	900
	21st to end	Night	24% 13% 29% 25% 9%	917
February	1st to 10th	Night	26% 14% 33% 22% 5% 1	780
	11th to 20th	Night	18% 13% 35% 24% 10% 1	744
	21st to end	Night	18% 18% 32% 22% 9% 1	628
March	1st to 10th	Night	22% 16% 29% 21% 10% 1	760
	11th to 20th	Night	22% 17% 31% 18% 11% 1	760
	21st to end	Night	20% 15% 31% 23% 9% 1	836
April	1st to 10th	Night	21% 13% 30% 25% 9%	760
	11th to 20th	Night	19% 14% 31% 22% 10% 3	639
	21st to end	Night	20% 8% 35% 25% 11% 2	478
May	1st to 10th	Night	14% 7% 32% 32% 14% 2	380
	11th to 20th	Night	14% 8% 23% 37% 19%	374
	21st to end	Night	20% 7% 29% 33% 12%	411
June	1st to 10th	Night	21% 12% 32% 27% 7%	380
	11th to 20th	Night	15% 10% 34% 32% 8%	380
	21st to end	Night	14% 8% 34% 33% 12% 1	380
July	1st to 10th	Night	9% 7% 31% 39% 13% 1	370
	11th to 20th	Night	6% 8% 30% 44% 12%	368
	21st to end	Night	5% 7% 28% 40% 20%	416
August	1st to 10th	Night	8% 3 20% 48% 20% 1	378
	11th to 20th	Night	8% 3 27% 39% 22% 1	533
	21st to end	Night	14% 8% 29% 34% 13% 1	675
September	1st to 10th	Night	10% 11% 32% 31% 16% 1	760
	11th to 20th	Night	17% 9% 23% 28% 21% 2	760
	21st to end	Night	23% 11% 31% 23% 12%	760
October	1st to 10th	Night	22% 12% 28% 24% 12%	760
	11th to 20th	Night	26% 11% 27% 24% 11% 1	760
	21st to end	Night	28% 17% 27% 19% 9%	836
November	1st to 10th	Night	33% 14% 26% 21% 5% 1	780
	11th to 20th	Night	30% 17% 27% 18% 8% 1	800
	21st to end	Night	28% 15% 25% 22% 9% 2	800
December	1st to 10th	Night	23% 13% 27% 26% 10% 1	847
	11th to 20th	Night	30% 13% 27% 22% 8% 1	900
	21st to end	Night	27% 14% 29% 24% 5%	990

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