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 2024-02-05.

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	14% 15% 6% 10% 7% 4 5% 4 4 5% 25%	77733
	Night	15% 17% 7% 11% 7% 4 4 4 4 5% 21%	81075
Transparency	Day	62% 7% 10% 13% 7% 1	77440
	Night	65% 8% 10% 11% 5% 1	80948
Seeing	Day	21% 18% 27% 21% 10% 3	14298
	Night	22% 12% 29% 25% 10% 1	22629

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	15% 30% 9% 14% 6% 3 4 4 4 4 8%	2219
	11th to 20th	Day	15% 20% 7% 10% 9% 4 4 3 3 4 21%	2199
	21st to end	Day	25% 32% 6% 10% 6% 3 4 3 2 2 7%	2669
February	1st to 10th	Day	13% 23% 6% 13% 9% 4 5% 4 4 5% 14%	1633
	11th to 20th	Day	9% 18% 9% 13% 9% 5% 5% 5% 4 6% 18%	1575
	21st to end	Day	7% 18% 7% 16% 10% 5% 6% 5% 6% 6% 14%	1340
March	1st to 10th	Day	9% 20% 5% 14% 11% 5% 5% 5% 5% 5% 16%	1840
	11th to 20th	Day	11% 26% 6% 12% 9% 5% 4 5% 4 4 14%	1980
	21st to end	Day	10% 22% 8% 14% 10% 5% 5% 4 4 5% 12%	2245
April	1st to 10th	Day	10% 23% 8% 12% 10% 4 6% 5% 5% 5% 12%	2160
	11th to 20th	Day	12% 18% 8% 13% 9% 5% 6% 5% 4 5% 16%	2233
	21st to end	Day	13% 17% 6% 12% 9% 5% 5% 5% 5% 6% 16%	2426
May	1st to 10th	Day	13% 17% 6% 10% 8% 4 6% 5% 5% 5% 23%	2520
	11th to 20th	Day	12% 12% 5% 10% 8% 5% 5% 5% 6% 9% 25%	2477
	21st to end	Day	16% 14% 6% 10% 8% 3 5% 5% 5% 5% 23%	2834
June	1st to 10th	Day	21% 13% 5% 9% 6% 3 5% 5% 4 6% 21%	2700
	11th to 20th	Day	20% 12% 5% 10% 9% 4 6% 5% 5% 6% 18%	2700
	21st to end	Day	14% 10% 5% 9% 7% 4 5% 5% 4 6% 30%	2700
July	1st to 10th	Day	11% 7% 3 7% 6% 4 5% 5% 5% 6% 42%	2700
	11th to 20th	Day	9% 4 3 5% 6% 3 5% 5% 5% 7% 47%	2700
	21st to end	Day	7% 5% 2 5% 5% 4 4 5% 5% 8% 51%	2843
August	1st to 10th	Day	9% 5% 3 4 5% 3 5% 5% 5% 7% 50%	2507
	11th to 20th	Day	7% 5% 2 4 4 2 5% 4 5% 6% 55%	2325
	21st to end	Day	11% 7% 3 7% 6% 4 6% 6% 6% 8% 38%	2528
September	1st to 10th	Day	10% 8% 2 7% 6% 4 4 4 5% 6% 45%	2160
	11th to 20th	Day	15% 11% 4 8% 6% 4 4 4 3 4 38%	2160
	21st to end	Day	19% 12% 5% 7% 6% 3 4 4 4 5% 31%	1896
October	1st to 10th	Day	16% 11% 5% 9% 6% 3 4 5% 5% 7% 29%	1800
	11th to 20th	Day	22% 15% 6% 9% 6% 3 3 3 4 5% 24%	1800
	21st to end	Day	22% 15% 7% 9% 7% 4 5% 4 3 4 20%	1784
November	1st to 10th	Day	24% 18% 6% 9% 8% 4 4 5% 4 5% 13%	1488
	11th to 20th	Day	18% 23% 9% 10% 8% 3 4 3 4 4 14%	1520
	21st to end	Day	17% 21% 7% 14% 11% 5% 4 4 3 3 12%	1520
December	1st to 10th	Day	14% 17% 8% 12% 10% 5% 6% 4 4 4 17%	1425
	11th to 20th	Day	18% 26% 10% 15% 8% 4 5% 3 3 3 6%	1685
	21st to end	Day	20% 22% 9% 17% 9% 4 3 3 3 3 8%	2442
January	1st to 10th	Night	14% 31% 11% 15% 8% 3 4 3 3 3 5%	4726
	11th to 20th	Night	17% 18% 7% 13% 8% 3 4 3 4 5% 18%	4699
	21st to end	Night	24% 31% 9% 13% 7% 3 2 3 2 2 5%	4986
February	1st to 10th	Night	14% 24% 7% 16% 8% 3 4 5% 5% 4 10%	2420
	11th to 20th	Night	9% 20% 8% 15% 10% 4 6% 6% 5% 4 15%	2272
	21st to end	Night	10% 17% 8% 15% 9% 5% 5% 5% 5% 4 16%	1916
March	1st to 10th	Night	11% 20% 7% 15% 9% 4 6% 4 4 5% 15%	2117
	11th to 20th	Night	10% 22% 7% 11% 8% 4 6% 5% 5% 5% 15%	1979
	21st to end	Night	11% 18% 6% 13% 11% 5% 5% 5% 5% 6% 16%	2111
April	1st to 10th	Night	9% 20% 7% 11% 9% 4 5% 6% 6% 7% 17%	1800
	11th to 20th	Night	10% 15% 6% 10% 8% 5% 6% 5% 6% 7% 22%	1673
	21st to end	Night	13% 12% 5% 10% 6% 5% 6% 7% 7% 7% 22%	1533
May	1st to 10th	Night	10% 10% 5% 9% 6% 3 6% 6% 5% 8% 32%	1440
	11th to 20th	Night	10% 8% 3 7% 8% 4 6% 7% 7% 9% 31%	1416
	21st to end	Night	15% 11% 5% 6% 6% 3 6% 5% 5% 7% 30%	1438
June	1st to 10th	Night	17% 11% 5% 7% 6% 5% 4 5% 6% 7% 27%	1260
	11th to 20th	Night	14% 10% 5% 8% 6% 4 5% 5% 6% 8% 27%	1260
	21st to end	Night	11% 8% 4 7% 7% 4 5% 7% 6% 9% 34%	1260
July	1st to 10th	Night	7% 7% 3 5% 4 3 5% 3 6% 6% 52%	1260
	11th to 20th	Night	5% 3 2 4 5% 3 5% 5% 5% 7% 56%	1260
	21st to end	Night	5% 3 1 3 3 2 3 5% 7% 7% 60%	1473
August	1st to 10th	Night	7% 3 2 4 3 2 3 4 5% 9% 59%	1432
	11th to 20th	Night	5% 3 1 3 3 1 3 4 6% 9% 61%	1572
	21st to end	Night	9% 5% 2 5% 5% 4 5% 5% 6% 8% 46%	1827
September	1st to 10th	Night	7% 5% 2 5% 5% 3 5% 5% 6% 8% 49%	1800
	11th to 20th	Night	11% 9% 4 6% 6% 4 5% 4 5% 8% 39%	1800
	21st to end	Night	16% 10% 3 7% 6% 4 4 5% 5% 7% 33%	2061
October	1st to 10th	Night	15% 11% 4 8% 5% 3 4 3 4 6% 35%	2160
	11th to 20th	Night	21% 12% 5% 8% 6% 4 5% 4 5% 5% 25%	2160
	21st to end	Night	20% 14% 7% 13% 8% 3 4 3 4 5% 19%	2570
November	1st to 10th	Night	23% 18% 8% 12% 8% 4 4 4 4 4 11%	2590
	11th to 20th	Night	18% 21% 9% 13% 8% 4 4 4 4 3 13%	2660
	21st to end	Night	16% 22% 10% 14% 10% 5% 4 4 3 2 9%	2660
December	1st to 10th	Night	14% 18% 10% 13% 11% 5% 6% 6% 4 4 8%	2750
	11th to 20th	Night	19% 28% 9% 16% 8% 4 4 3 3 2 4	3520
	21st to end	Night	21% 26% 9% 16% 9% 3 3 3 2 2 5%	5214

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% 12% 5% 5% 2 1	2219
	11th to 20th	Day	69% 3 9% 8% 5% 6%	2220
	21st to end	Day	84% 4 3 5% 3	2669
February	1st to 10th	Day	72% 8% 7% 5% 6% 1	1633
	11th to 20th	Day	68% 8% 8% 10% 5%	1584
	21st to end	Day	72% 8% 7% 8% 3 1	1340
March	1st to 10th	Day	71% 7% 9% 7% 6%	1840
	11th to 20th	Day	75% 7% 7% 7% 3	1980
	21st to end	Day	74% 9% 9% 7% 1	2245
April	1st to 10th	Day	75% 7% 10% 7% 1	2160
	11th to 20th	Day	72% 6% 8% 9% 5%	2233
	21st to end	Day	69% 9% 10% 9% 3	2426
May	1st to 10th	Day	64% 8% 12% 11% 5%	2520
	11th to 20th	Day	57% 10% 16% 14% 2	2480
	21st to end	Day	64% 7% 11% 14% 4	2835
June	1st to 10th	Day	65% 8% 10% 10% 4 2	2700
	11th to 20th	Day	68% 8% 12% 9% 3	2700
	21st to end	Day	56% 8% 12% 16% 7% 1	2700
July	1st to 10th	Day	44% 7% 12% 21% 12% 3	2626
	11th to 20th	Day	37% 8% 16% 25% 14% 1	2619
	21st to end	Day	33% 7% 13% 27% 18% 3	2685
August	1st to 10th	Day	35% 7% 13% 28% 14% 3	2490
	11th to 20th	Day	30% 7% 15% 28% 17% 3	2325
	21st to end	Day	44% 8% 15% 23% 8% 2	2528
September	1st to 10th	Day	42% 7% 13% 24% 11% 3	2160
	11th to 20th	Day	51% 5% 10% 16% 15% 4	2160
	21st to end	Day	57% 7% 12% 14% 8% 2	1896
October	1st to 10th	Day	56% 8% 11% 17% 9%	1800
	11th to 20th	Day	65% 6% 9% 14% 6%	1800
	21st to end	Day	68% 5% 8% 13% 5%	1784
November	1st to 10th	Day	74% 6% 8% 8% 4	1488
	11th to 20th	Day	75% 5% 6% 6% 6% 1	1520
	21st to end	Day	77% 7% 4 7% 4 2	1520
December	1st to 10th	Day	70% 7% 7% 10% 5%	1425
	11th to 20th	Day	81% 10% 4 2 2 1	1688
	21st to end	Day	77% 9% 4 7% 3	2442
January	1st to 10th	Night	79% 9% 6% 4 2 1	4726
	11th to 20th	Night	68% 4 9% 9% 5% 5%	4740
	21st to end	Night	85% 5% 4 3 2	4986
February	1st to 10th	Night	77% 5% 6% 6% 5% 1	2420
	11th to 20th	Night	71% 6% 9% 8% 5% 1	2274
	21st to end	Night	70% 6% 7% 10% 5% 1	1916
March	1st to 10th	Night	70% 8% 8% 9% 5%	2117
	11th to 20th	Night	73% 7% 8% 9% 3	1980
	21st to end	Night	68% 11% 11% 8% 1	2111
April	1st to 10th	Night	66% 9% 13% 11% 1	1800
	11th to 20th	Night	62% 9% 11% 12% 5%	1673
	21st to end	Night	60% 10% 14% 12% 4	1533
May	1st to 10th	Night	51% 11% 16% 16% 5% 1	1440
	11th to 20th	Night	50% 13% 19% 14% 3	1416
	21st to end	Night	54% 9% 17% 15% 4	1438
June	1st to 10th	Night	56% 9% 16% 11% 6% 2	1260
	11th to 20th	Night	57% 10% 16% 13% 4	1260
	21st to end	Night	47% 12% 18% 17% 6% 1	1260
July	1st to 10th	Night	35% 9% 17% 24% 11% 4	1225
	11th to 20th	Night	27% 9% 24% 28% 11% 1	1218
	21st to end	Night	22% 8% 19% 32% 15% 3	1391
August	1st to 10th	Night	27% 7% 17% 31% 16% 3	1424
	11th to 20th	Night	22% 9% 22% 30% 15% 2	1572
	21st to end	Night	37% 9% 21% 26% 7% 1	1827
September	1st to 10th	Night	34% 10% 18% 25% 11% 3	1800
	11th to 20th	Night	45% 8% 11% 19% 15% 2	1800
	21st to end	Night	51% 8% 16% 15% 8% 2	2061
October	1st to 10th	Night	52% 8% 13% 20% 7%	2160
	11th to 20th	Night	60% 8% 10% 15% 6%	2160
	21st to end	Night	69% 5% 7% 13% 6%	2570
November	1st to 10th	Night	78% 6% 7% 6% 3 1	2590
	11th to 20th	Night	76% 6% 6% 6% 5% 1	2660
	21st to end	Night	79% 5% 4 6% 4 1	2660
December	1st to 10th	Night	75% 7% 6% 8% 3 1	2750
	11th to 20th	Night	83% 7% 4 3 2 1	3516
	21st to end	Night	81% 9% 3 5% 3	5214

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	26% 20% 20% 21% 13% 1	180
	11th to 20th	Day	30% 17% 16% 23% 12% 3	180
	21st to end	Day	23% 15% 21% 24% 15% 2	198
February	1st to 10th	Day	28% 16% 32% 18% 6%	329
	11th to 20th	Day	18% 22% 30% 16% 12% 3	335
	21st to end	Day	17% 20% 27% 23% 11% 1	284
March	1st to 10th	Day	18% 23% 26% 21% 10% 2	420
	11th to 20th	Day	26% 16% 26% 16% 10% 5%	420
	21st to end	Day	25% 20% 27% 16% 9% 3	462
April	1st to 10th	Day	26% 21% 24% 21% 7% 1	420
	11th to 20th	Day	22% 24% 21% 19% 7% 7%	412
	21st to end	Day	22% 21% 24% 18% 10% 5%	420
May	1st to 10th	Day	20% 16% 25% 24% 13% 2	423
	11th to 20th	Day	18% 13% 29% 26% 10% 4	448
	21st to end	Day	19% 16% 29% 23% 9% 4	596
June	1st to 10th	Day	25% 22% 29% 17% 6% 1	630
	11th to 20th	Day	24% 21% 30% 18% 6%	630
	21st to end	Day	15% 20% 32% 21% 9% 2	630
July	1st to 10th	Day	14% 18% 32% 26% 9% 2	621
	11th to 20th	Day	13% 18% 34% 25% 8% 2	618
	21st to end	Day	8% 14% 34% 30% 9% 5%	589
August	1st to 10th	Day	14% 11% 24% 30% 14% 7%	447
	11th to 20th	Day	8% 14% 32% 26% 14% 5%	437
	21st to end	Day	16% 14% 34% 22% 10% 4	495
September	1st to 10th	Day	17% 22% 30% 18% 11% 3	450
	11th to 20th	Day	23% 16% 24% 24% 10% 2	450
	21st to end	Day	25% 19% 25% 20% 9% 1	450
October	1st to 10th	Day	25% 18% 22% 17% 13% 4	450
	11th to 20th	Day	32% 16% 22% 15% 12% 2	450
	21st to end	Day	34% 15% 24% 15% 10% 2	326
November	1st to 10th	Day	31% 16% 26% 14% 11% 2	180
	11th to 20th	Day	31% 12% 22% 16% 13% 7%	180
	21st to end	Day	25% 18% 23% 15% 11% 8%	180
December	1st to 10th	Day	24% 12% 17% 21% 23% 4	180
	11th to 20th	Day	29% 10% 22% 23% 13% 3	180
	21st to end	Day	22% 14% 24% 30% 8% 2	198
January	1st to 10th	Night	34% 15% 25% 20% 7%	850
	11th to 20th	Night	31% 14% 25% 22% 8%	850
	21st to end	Night	25% 14% 29% 23% 9%	870
February	1st to 10th	Night	28% 14% 33% 21% 4	738
	11th to 20th	Night	19% 13% 36% 23% 9%	704
	21st to end	Night	18% 16% 34% 22% 9% 1	592
March	1st to 10th	Night	23% 16% 30% 20% 10% 1	720
	11th to 20th	Night	23% 17% 32% 18% 9% 1	720
	21st to end	Night	20% 16% 32% 22% 9% 1	792
April	1st to 10th	Night	22% 13% 30% 26% 10%	720
	11th to 20th	Night	20% 15% 30% 21% 10% 3	605
	21st to end	Night	20% 8% 35% 25% 11% 2	453
May	1st to 10th	Night	14% 8% 32% 31% 14% 2	360
	11th to 20th	Night	14% 7% 22% 37% 19%	354
	21st to end	Night	21% 6% 28% 33% 12%	389
June	1st to 10th	Night	22% 11% 32% 27% 8%	360
	11th to 20th	Night	16% 10% 34% 33% 7%	360
	21st to end	Night	14% 7% 33% 33% 12% 1	360
July	1st to 10th	Night	9% 6% 31% 40% 13% 1	350
	11th to 20th	Night	6% 7% 30% 44% 12%	348
	21st to end	Night	5% 6% 28% 41% 21%	394
August	1st to 10th	Night	8% 3 21% 49% 18% 1	358
	11th to 20th	Night	7% 3 29% 40% 21% 1	504
	21st to end	Night	13% 9% 30% 34% 13% 1	639
September	1st to 10th	Night	10% 11% 33% 30% 15% 1	720
	11th to 20th	Night	17% 9% 23% 28% 21% 2	720
	21st to end	Night	22% 11% 31% 23% 13%	720
October	1st to 10th	Night	23% 12% 28% 24% 13%	720
	11th to 20th	Night	27% 11% 28% 23% 11% 1	720
	21st to end	Night	29% 17% 28% 18% 9%	792
November	1st to 10th	Night	34% 14% 26% 20% 5% 1	740
	11th to 20th	Night	30% 17% 26% 16% 8% 1	760
	21st to end	Night	29% 16% 26% 20% 8% 2	760
December	1st to 10th	Night	23% 13% 27% 25% 10% 1	802
	11th to 20th	Night	31% 14% 28% 20% 7% 1	850
	21st to end	Night	28% 15% 30% 22% 5%	935

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