Palmer
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Palmer clear sky chart from 2005-11-06 to 2026-08-15.

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	16% 14% 4 10% 6% 5% 5% 4 6% 6% 23%	150008
	Night	13% 15% 5% 10% 6% 5% 5% 4 5% 6% 26%	140729
Transparency	Day	59% 10% 14% 12% 5% 1	149651
	Night	55% 9% 12% 13% 9% 2	140670
Seeing	Day	21% 11% 22% 26% 16% 4	20351
	Night	19% 12% 25% 27% 15% 2	24599

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	5% 17% 4 9% 6% 4 5% 4 5% 7% 33%	2361
	11th to 20th	Day	11% 16% 5% 11% 4 4 5% 3 5% 6% 28%	3017
	21st to end	Day	13% 14% 4 8% 6% 6% 5% 4 5% 6% 29%	3871
February	1st to 10th	Day	6% 14% 4 9% 5% 3 3 3 5% 7% 41%	1417
	11th to 20th	Day	7% 24% 5% 10% 5% 5% 5% 5% 4 7% 23%	1639
	21st to end	Day	6% 11% 6% 8% 6% 6% 5% 5% 6% 5% 35%	1477
March	1st to 10th	Day	5% 18% 6% 13% 7% 5% 5% 3 4 6% 28%	4526
	11th to 20th	Day	3 12% 5% 11% 7% 7% 7% 5% 7% 7% 28%	4732
	21st to end	Day	8% 16% 4 8% 5% 5% 6% 5% 6% 7% 29%	6101
April	1st to 10th	Day	7% 16% 5% 11% 6% 6% 7% 6% 5% 6% 25%	2575
	11th to 20th	Day	6% 13% 4 9% 6% 5% 5% 5% 7% 8% 33%	2906
	21st to end	Day	9% 14% 5% 10% 7% 6% 6% 5% 6% 7% 27%	2998
May	1st to 10th	Day	15% 21% 5% 12% 6% 6% 5% 4 5% 5% 15%	9297
	11th to 20th	Day	14% 15% 5% 12% 7% 6% 6% 4 6% 6% 19%	9415
	21st to end	Day	15% 18% 5% 12% 7% 6% 5% 4 6% 6% 17%	11276
June	1st to 10th	Day	16% 10% 4 9% 7% 6% 6% 5% 7% 8% 22%	3780
	11th to 20th	Day	16% 10% 3 8% 6% 6% 5% 5% 7% 8% 25%	3780
	21st to end	Day	17% 11% 4 9% 7% 6% 6% 6% 7% 7% 20%	3780
July	1st to 10th	Day	23% 10% 4 10% 6% 6% 5% 4 6% 7% 19%	10620
	11th to 20th	Day	28% 13% 4 10% 6% 5% 6% 4 5% 6% 14%	10492
	21st to end	Day	23% 9% 3 7% 5% 5% 5% 4 6% 7% 23%	10278
August	1st to 10th	Day	31% 11% 4 9% 5% 4 4 4 5% 5% 18%	5366
	11th to 20th	Day	29% 10% 3 7% 4 6% 5% 4 5% 7% 20%	4636
	21st to end	Day	22% 11% 4 9% 5% 6% 5% 5% 6% 7% 19%	5038
September	1st to 10th	Day	28% 11% 4 9% 6% 6% 5% 4 5% 5% 17%	2482
	11th to 20th	Day	29% 14% 4 9% 5% 4 4 3 3 4 20%	2391
	21st to end	Day	20% 12% 3 9% 6% 6% 4 3 5% 6% 26%	2140
October	1st to 10th	Day	15% 14% 3 9% 6% 6% 6% 5% 6% 7% 22%	3201
	11th to 20th	Day	10% 9% 3 8% 4 5% 5% 4 6% 7% 40%	2907
	21st to end	Day	15% 14% 3 7% 4 4 4 4 6% 7% 31%	2721
November	1st to 10th	Day	8% 14% 4 11% 5% 6% 5% 5% 7% 6% 28%	1441
	11th to 20th	Day	8% 11% 4 8% 5% 6% 4 4 6% 7% 37%	1244
	21st to end	Day	7% 14% 7% 13% 5% 5% 4 4 5% 6% 30%	1050
December	1st to 10th	Day	16% 13% 4 11% 6% 4 5% 4 6% 6% 24%	1450
	11th to 20th	Day	6% 20% 7% 10% 6% 6% 4 4 6% 7% 25%	1557
	21st to end	Day	3 11% 6% 11% 6% 5% 5% 4 6% 6% 36%	2046
January	1st to 10th	Night	5% 16% 4 9% 5% 4 4 4 6% 7% 34%	10564
	11th to 20th	Night	12% 18% 6% 11% 6% 5% 5% 4 4 4 26%	9920
	21st to end	Night	10% 14% 4 8% 6% 5% 5% 4 7% 7% 30%	10342
February	1st to 10th	Night	6% 13% 4 8% 5% 4 4 3 5% 6% 44%	3200
	11th to 20th	Night	9% 21% 5% 10% 5% 5% 5% 4 5% 7% 24%	2868
	21st to end	Night	6% 11% 5% 12% 7% 6% 5% 5% 6% 6% 32%	2328
March	1st to 10th	Night	5% 18% 6% 12% 6% 4 4 3 4 5% 31%	5547
	11th to 20th	Night	4 17% 5% 10% 5% 5% 6% 5% 6% 7% 29%	5431
	21st to end	Night	7% 17% 5% 8% 5% 4 5% 4 7% 8% 31%	5072
April	1st to 10th	Night	7% 19% 6% 10% 7% 6% 6% 4 6% 5% 25%	2043
	11th to 20th	Night	8% 14% 3 8% 6% 5% 6% 4 5% 6% 33%	1663
	21st to end	Night	9% 14% 5% 9% 7% 5% 5% 4 5% 7% 31%	1622
May	1st to 10th	Night	19% 20% 5% 12% 6% 5% 5% 4 5% 6% 13%	3649
	11th to 20th	Night	20% 20% 5% 10% 6% 5% 5% 4 4 5% 17%	3474
	21st to end	Night	22% 15% 5% 12% 6% 6% 6% 3 4 6% 15%	2908
June	1st to 10th	Night	16% 13% 4 8% 7% 7% 7% 4 5% 7% 22%	840
	11th to 20th	Night	18% 11% 4 10% 7% 6% 5% 3 6% 6% 23%	840
	21st to end	Night	20% 9% 5% 11% 6% 7% 6% 4 7% 7% 19%	840
July	1st to 10th	Night	35% 13% 4 9% 5% 4 5% 3 4 5% 13%	2370
	11th to 20th	Night	35% 12% 4 10% 5% 6% 3 3 4 4 13%	2490
	21st to end	Night	28% 11% 3 9% 6% 6% 4 4 4 6% 20%	3826
August	1st to 10th	Night	35% 11% 3 8% 4 4 4 2 3 5% 20%	2014
	11th to 20th	Night	31% 12% 3 9% 5% 5% 6% 5% 4 5% 14%	2499
	21st to end	Night	30% 12% 4 9% 6% 5% 5% 3 4 4 17%	2871
September	1st to 10th	Night	32% 13% 3 9% 5% 6% 5% 3 5% 5% 14%	1915
	11th to 20th	Night	31% 14% 4 8% 4 5% 4 3 4 5% 19%	2009
	21st to end	Night	22% 13% 4 9% 6% 5% 4 3 6% 6% 21%	2258
October	1st to 10th	Night	17% 15% 4 9% 7% 7% 6% 5% 6% 6% 18%	3981
	11th to 20th	Night	9% 9% 4 9% 5% 5% 5% 4 7% 8% 38%	4239
	21st to end	Night	13% 14% 4 10% 6% 5% 4 4 6% 7% 26%	5080
November	1st to 10th	Night	9% 14% 4 10% 6% 5% 6% 4 5% 6% 29%	3077
	11th to 20th	Night	7% 12% 3 8% 5% 5% 5% 5% 5% 7% 38%	3372
	21st to end	Night	7% 14% 5% 10% 6% 5% 5% 4 6% 7% 29%	3570
December	1st to 10th	Night	13% 18% 6% 10% 6% 4 5% 4 5% 6% 22%	5953
	11th to 20th	Night	5% 20% 6% 11% 6% 5% 6% 5% 7% 6% 24%	6880
	21st to end	Night	3 13% 5% 11% 6% 6% 5% 5% 7% 7% 32%	9174

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	47% 9% 9% 11% 20% 4	2361
	11th to 20th	Day	55% 6% 9% 16% 12% 3	3032
	21st to end	Day	52% 7% 8% 16% 12% 4	3872
February	1st to 10th	Day	43% 7% 13% 17% 14% 6%	1417
	11th to 20th	Day	58% 10% 11% 9% 9% 2	1646
	21st to end	Day	47% 10% 11% 16% 14% 3	1477
March	1st to 10th	Day	53% 7% 8% 12% 15% 5%	4526
	11th to 20th	Day	47% 10% 12% 14% 16% 2	4732
	21st to end	Day	50% 10% 13% 16% 10% 2	6101
April	1st to 10th	Day	57% 9% 13% 15% 5% 1	2575
	11th to 20th	Day	47% 12% 15% 18% 8%	2906
	21st to end	Day	54% 9% 13% 16% 7% 1	2998
May	1st to 10th	Day	66% 9% 7% 14% 4	9297
	11th to 20th	Day	60% 9% 12% 17% 2	9418
	21st to end	Day	64% 9% 12% 13% 1	11277
June	1st to 10th	Day	58% 12% 15% 13% 1	3780
	11th to 20th	Day	56% 13% 18% 11% 2	3780
	21st to end	Day	61% 13% 19% 7%	3780
July	1st to 10th	Day	64% 9% 19% 8%	10531
	11th to 20th	Day	70% 9% 16% 5%	10393
	21st to end	Day	60% 10% 21% 10%	10056
August	1st to 10th	Day	69% 10% 15% 6% 1	5347
	11th to 20th	Day	63% 11% 15% 10% 1	4679
	21st to end	Day	63% 10% 19% 7% 1	5037
September	1st to 10th	Day	68% 13% 10% 7% 2	2482
	11th to 20th	Day	68% 10% 12% 10% 1	2391
	21st to end	Day	59% 10% 14% 13% 4	2140
October	1st to 10th	Day	59% 8% 15% 15% 3	3201
	11th to 20th	Day	41% 9% 15% 21% 12% 1	2907
	21st to end	Day	49% 8% 12% 25% 5% 1	2721
November	1st to 10th	Day	53% 9% 14% 13% 7% 3	1441
	11th to 20th	Day	44% 8% 11% 23% 13% 1	1244
	21st to end	Day	54% 8% 11% 12% 13% 2	1050
December	1st to 10th	Day	57% 8% 11% 14% 8% 1	1450
	11th to 20th	Day	57% 8% 13% 15% 6% 1	1560
	21st to end	Day	42% 10% 13% 18% 11% 7%	2046
January	1st to 10th	Night	46% 8% 8% 12% 18% 8%	10564
	11th to 20th	Night	59% 7% 9% 12% 12% 2	9968
	21st to end	Night	48% 7% 10% 16% 14% 5%	10342
February	1st to 10th	Night	41% 7% 11% 19% 16% 7%	3200
	11th to 20th	Night	59% 9% 10% 12% 9% 2	2872
	21st to end	Night	50% 10% 9% 15% 12% 4	2328
March	1st to 10th	Night	51% 7% 8% 13% 15% 6%	5547
	11th to 20th	Night	47% 10% 11% 13% 18% 2	5433
	21st to end	Night	46% 10% 12% 20% 10% 2	5072
April	1st to 10th	Night	58% 9% 12% 16% 5% 1	2043
	11th to 20th	Night	49% 9% 15% 17% 9%	1663
	21st to end	Night	50% 10% 15% 18% 8%	1622
May	1st to 10th	Night	66% 10% 9% 14% 2	3646
	11th to 20th	Night	65% 10% 11% 12% 2	3475
	21st to end	Night	63% 12% 13% 11% 1	2912
June	1st to 10th	Night	60% 13% 15% 11% 1	840
	11th to 20th	Night	60% 12% 16% 9% 2	840
	21st to end	Night	62% 15% 18% 5%	840
July	1st to 10th	Night	73% 8% 12% 6%	2350
	11th to 20th	Night	73% 7% 15% 4	2466
	21st to end	Night	66% 8% 17% 8% 1	3743
August	1st to 10th	Night	68% 9% 19% 4	2008
	11th to 20th	Night	69% 10% 13% 7% 1	2521
	21st to end	Night	71% 8% 13% 7% 1	2871
September	1st to 10th	Night	71% 12% 9% 6% 2	1915
	11th to 20th	Night	67% 11% 12% 9% 1	2009
	21st to end	Night	63% 9% 14% 12% 3	2258
October	1st to 10th	Night	63% 8% 15% 10% 3	3981
	11th to 20th	Night	41% 9% 17% 20% 13%	4239
	21st to end	Night	56% 6% 13% 19% 5% 1	5080
November	1st to 10th	Night	54% 9% 12% 15% 8% 2	3077
	11th to 20th	Night	43% 10% 11% 23% 13% 1	3372
	21st to end	Night	52% 9% 10% 14% 11% 3	3570
December	1st to 10th	Night	59% 8% 9% 15% 7% 1	5953
	11th to 20th	Night	55% 9% 14% 13% 7% 1	6876
	21st to end	Night	44% 10% 14% 15% 11% 6%	9174

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	11% 17% 25% 24% 17% 5%	110
	11th to 20th	Day	18% 17% 28% 23% 11% 3	272
	21st to end	Day	17% 14% 22% 26% 16% 5%	352
February	1st to 10th	Day	10% 15% 23% 28% 16% 8%	326
	11th to 20th	Day	18% 15% 30% 21% 14% 2	425
	21st to end	Day	13% 17% 28% 26% 10% 6%	355
March	1st to 10th	Day	13% 12% 29% 27% 16% 4	430
	11th to 20th	Day	8% 12% 31% 27% 15% 7%	477
	21st to end	Day	9% 16% 25% 26% 21% 4	704
April	1st to 10th	Day	12% 14% 33% 26% 14% 2	665
	11th to 20th	Day	15% 12% 22% 32% 16% 3	740
	21st to end	Day	18% 10% 23% 30% 18% 1	750
May	1st to 10th	Day	21% 16% 24% 26% 12% 1	753
	11th to 20th	Day	18% 13% 24% 26% 17% 2	775
	21st to end	Day	21% 11% 23% 29% 14% 2	952
June	1st to 10th	Day	21% 10% 21% 24% 20% 4	990
	11th to 20th	Day	20% 8% 16% 29% 22% 5%	990
	21st to end	Day	20% 10% 16% 28% 22% 5%	990
July	1st to 10th	Day	22% 9% 17% 30% 18% 3	976
	11th to 20th	Day	28% 7% 16% 30% 16% 3	972
	21st to end	Day	25% 8% 18% 28% 18% 3	863
August	1st to 10th	Day	32% 9% 18% 23% 15% 3	776
	11th to 20th	Day	29% 11% 21% 22% 14% 3	736
	21st to end	Day	31% 10% 18% 26% 12% 3	803
September	1st to 10th	Day	30% 11% 22% 23% 11% 3	656
	11th to 20th	Day	36% 10% 18% 21% 13% 2	611
	21st to end	Day	27% 11% 17% 23% 18% 5%	420
October	1st to 10th	Day	31% 10% 21% 20% 12% 5%	420
	11th to 20th	Day	22% 8% 20% 28% 17% 5%	420
	21st to end	Day	23% 9% 21% 26% 16% 5%	462
November	1st to 10th	Day	15% 14% 24% 23% 18% 7%	425
	11th to 20th	Day	15% 9% 19% 25% 26% 7%	304
	21st to end	Day	7% 18% 21% 27% 13% 14%	110
December	1st to 10th	Day	24% 15% 13% 31% 17% 1	110
	11th to 20th	Day	14% 15% 36% 22% 8% 5%	110
	21st to end	Day	13% 19% 26% 26% 11% 4	121
January	1st to 10th	Night	10% 15% 30% 29% 13% 3	1060
	11th to 20th	Night	16% 19% 30% 23% 11% 1	1022
	21st to end	Night	17% 13% 29% 27% 13% 2	1045
February	1st to 10th	Night	11% 13% 33% 27% 14% 2	950
	11th to 20th	Night	16% 16% 35% 24% 8%	934
	21st to end	Night	13% 12% 36% 25% 11% 3	689
March	1st to 10th	Night	10% 14% 30% 33% 11% 1	740
	11th to 20th	Night	8% 13% 28% 32% 18% 2	740
	21st to end	Night	10% 9% 29% 29% 22% 1	693
April	1st to 10th	Night	13% 11% 27% 29% 20% 1	630
	11th to 20th	Night	15% 8% 23% 30% 22% 2	623
	21st to end	Night	13% 8% 23% 37% 19% 1	630
May	1st to 10th	Night	18% 11% 22% 30% 18% 1	463
	11th to 20th	Night	18% 8% 21% 37% 14% 2	404
	21st to end	Night	20% 7% 19% 35% 19%	228
June	1st to 10th	Night	18% 9% 16% 33% 23% 2	210
	11th to 20th	Night	23% 6% 11% 33% 24% 2	210
	21st to end	Night	24% 8% 13% 27% 26% 3	210
July	1st to 10th	Night	31% 5% 12% 21% 29% 2	205
	11th to 20th	Night	37% 4 12% 22% 25% 1	246
	21st to end	Night	31% 8% 12% 23% 22% 3	460
August	1st to 10th	Night	35% 6% 12% 26% 20% 2	418
	11th to 20th	Night	34% 10% 18% 23% 15%	560
	21st to end	Night	33% 7% 14% 27% 18% 1	660
September	1st to 10th	Night	35% 10% 18% 22% 13% 2	600
	11th to 20th	Night	37% 8% 14% 24% 16% 2	600
	21st to end	Night	28% 10% 16% 24% 20% 2	633
October	1st to 10th	Night	31% 11% 23% 19% 14% 3	745
	11th to 20th	Night	19% 8% 24% 28% 18% 2	910
	21st to end	Night	20% 11% 22% 27% 17% 2	1001
November	1st to 10th	Night	17% 10% 29% 29% 12% 1	930
	11th to 20th	Night	12% 12% 27% 31% 16% 2	950
	21st to end	Night	16% 15% 27% 27% 12% 3	950
December	1st to 10th	Night	22% 16% 25% 24% 11% 2	1024
	11th to 20th	Night	14% 12% 31% 30% 11% 3	1060
	21st to end	Night	15% 17% 29% 25% 12% 2	1166

Page generated at 2026-08-11 23:37:15. Version ac07

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