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 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	16% 13% 4 9% 6% 5% 5% 4 6% 6% 24%	115491
	Night	12% 15% 5% 10% 6% 5% 5% 4 6% 6% 27%	121497
Transparency	Day	57% 10% 14% 12% 5% 1	115181
	Night	54% 9% 12% 13% 9% 2	121455
Seeing	Day	21% 11% 22% 26% 16% 3	18328
	Night	18% 12% 25% 27% 15% 2	23093

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	6% 17% 4 8% 6% 5% 5% 5% 6% 7% 32%	1819
	11th to 20th	Day	8% 16% 5% 10% 4 4 5% 4 5% 7% 32%	2303
	21st to end	Day	14% 15% 4 9% 7% 6% 6% 3 4 5% 28%	2992
February	1st to 10th	Day	5% 14% 3 9% 5% 3 3 3 6% 6% 42%	1349
	11th to 20th	Day	7% 25% 5% 10% 5% 5% 5% 5% 4 7% 23%	1559
	21st to end	Day	7% 12% 6% 8% 6% 6% 5% 5% 6% 5% 34%	1409
March	1st to 10th	Day	5% 18% 6% 12% 7% 5% 5% 3 4 6% 28%	4427
	11th to 20th	Day	3 12% 5% 11% 7% 7% 7% 5% 7% 7% 28%	4630
	21st to end	Day	8% 17% 4 8% 5% 5% 6% 5% 6% 7% 27%	5969
April	1st to 10th	Day	7% 15% 5% 11% 6% 6% 7% 6% 5% 6% 24%	2453
	11th to 20th	Day	6% 12% 4 9% 5% 5% 5% 5% 7% 8% 35%	2631
	21st to end	Day	7% 13% 5% 10% 7% 5% 6% 5% 6% 7% 29%	2712
May	1st to 10th	Day	15% 20% 5% 13% 8% 6% 5% 4 4 4 16%	5002
	11th to 20th	Day	8% 13% 5% 11% 7% 6% 6% 4 7% 7% 25%	5044
	21st to end	Day	20% 15% 4 9% 6% 5% 5% 4 5% 5% 21%	6032
June	1st to 10th	Day	15% 10% 4 9% 7% 6% 6% 6% 7% 8% 22%	3420
	11th to 20th	Day	17% 11% 4 8% 6% 7% 5% 5% 7% 7% 23%	3420
	21st to end	Day	16% 11% 4 9% 7% 6% 6% 5% 7% 7% 21%	3420
July	1st to 10th	Day	28% 10% 3 9% 6% 5% 5% 4 7% 7% 18%	5700
	11th to 20th	Day	25% 12% 3 9% 6% 5% 6% 4 5% 6% 17%	5624
	21st to end	Day	23% 10% 3 7% 5% 5% 5% 4 6% 7% 24%	5512
August	1st to 10th	Day	32% 11% 4 9% 5% 4 4 3 4 5% 17%	5046
	11th to 20th	Day	29% 10% 3 7% 4 5% 5% 4 6% 7% 19%	4433
	21st to end	Day	22% 11% 4 9% 5% 6% 5% 5% 6% 7% 20%	4884
September	1st to 10th	Day	27% 11% 4 9% 6% 5% 5% 4 5% 5% 17%	2358
	11th to 20th	Day	29% 14% 4 9% 5% 4 4 3 3 4 20%	2272
	21st to end	Day	20% 12% 3 8% 6% 6% 4 3 5% 6% 26%	2033
October	1st to 10th	Day	15% 15% 3 9% 6% 6% 6% 5% 6% 7% 22%	3103
	11th to 20th	Day	9% 9% 3 7% 4 4 5% 4 6% 7% 41%	2817
	21st to end	Day	16% 14% 3 6% 3 4 4 4 6% 7% 32%	2636
November	1st to 10th	Day	8% 14% 4 11% 5% 6% 6% 5% 7% 6% 29%	1371
	11th to 20th	Day	8% 11% 4 8% 5% 5% 4 4 6% 6% 38%	1185
	21st to end	Day	7% 14% 7% 13% 5% 5% 4 4 5% 6% 30%	1000
December	1st to 10th	Day	16% 13% 4 11% 6% 4 5% 4 6% 6% 24%	1407
	11th to 20th	Day	6% 21% 7% 10% 6% 6% 4 4 6% 7% 23%	1517
	21st to end	Day	3 10% 6% 12% 6% 5% 5% 4 6% 6% 35%	2002
January	1st to 10th	Night	6% 16% 4 9% 6% 5% 5% 4 7% 7% 32%	8146
	11th to 20th	Night	11% 17% 5% 11% 6% 5% 4 3 4 5% 30%	7620
	21st to end	Night	11% 14% 4 9% 7% 5% 5% 4 6% 5% 31%	7986
February	1st to 10th	Night	5% 13% 4 8% 5% 3 3 3 4 6% 44%	3048
	11th to 20th	Night	9% 21% 6% 10% 5% 5% 5% 4 5% 7% 23%	2728
	21st to end	Night	6% 12% 5% 12% 7% 6% 5% 5% 6% 6% 30%	2220
March	1st to 10th	Night	5% 18% 6% 12% 6% 4 5% 3 4 5% 31%	5426
	11th to 20th	Night	4 18% 6% 10% 5% 5% 6% 5% 7% 7% 29%	5313
	21st to end	Night	7% 18% 5% 8% 5% 5% 5% 4 7% 8% 29%	4962
April	1st to 10th	Night	7% 19% 6% 10% 7% 6% 6% 4 6% 5% 24%	1945
	11th to 20th	Night	7% 13% 3 8% 6% 5% 6% 5% 6% 7% 35%	1504
	21st to end	Night	7% 13% 5% 9% 7% 5% 5% 4 5% 7% 34%	1468
May	1st to 10th	Night	19% 18% 4 12% 7% 4 6% 4 5% 5% 17%	1965
	11th to 20th	Night	13% 18% 5% 10% 7% 6% 5% 4 4 6% 22%	1860
	21st to end	Night	25% 12% 4 11% 5% 4 5% 3 4 7% 21%	1546
June	1st to 10th	Night	16% 13% 4 7% 7% 8% 7% 4 5% 7% 21%	760
	11th to 20th	Night	19% 11% 5% 11% 7% 5% 5% 4 6% 5% 23%	760
	21st to end	Night	19% 9% 5% 11% 6% 7% 6% 4 6% 7% 20%	760
July	1st to 10th	Night	37% 9% 3 9% 5% 4 5% 4 6% 6% 15%	1270
	11th to 20th	Night	32% 12% 4 10% 5% 5% 3 3 4 5% 18%	1334
	21st to end	Night	29% 11% 4 8% 5% 5% 4 4 4 7% 19%	2053
August	1st to 10th	Night	35% 11% 3 8% 4 5% 4 2 3 5% 20%	1894
	11th to 20th	Night	31% 12% 3 9% 5% 5% 6% 5% 4 5% 14%	2393
	21st to end	Night	29% 13% 4 10% 7% 5% 5% 3 4 4 17%	2783
September	1st to 10th	Night	31% 13% 3 9% 5% 6% 5% 4 5% 5% 15%	1819
	11th to 20th	Night	31% 13% 4 8% 4 5% 4 3 4 5% 19%	1908
	21st to end	Night	22% 12% 4 9% 7% 5% 4 3 6% 6% 21%	2145
October	1st to 10th	Night	17% 15% 4 10% 7% 7% 6% 5% 6% 6% 18%	3859
	11th to 20th	Night	8% 8% 3 9% 5% 5% 5% 4 7% 8% 38%	4109
	21st to end	Night	13% 14% 4 9% 6% 5% 4 4 6% 7% 27%	4923
November	1st to 10th	Night	10% 14% 4 10% 6% 5% 6% 4 5% 6% 30%	2927
	11th to 20th	Night	8% 12% 3 8% 5% 5% 4 5% 6% 7% 38%	3211
	21st to end	Night	8% 15% 5% 11% 6% 5% 5% 4 6% 6% 29%	3400
December	1st to 10th	Night	13% 19% 6% 10% 6% 4 5% 4 6% 6% 22%	5776
	11th to 20th	Night	5% 20% 6% 11% 6% 5% 6% 5% 7% 7% 22%	6700
	21st to end	Night	3 13% 5% 12% 6% 6% 5% 5% 7% 7% 31%	8976

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	45% 11% 11% 14% 15% 4	1819
	11th to 20th	Day	49% 8% 10% 17% 12% 4	2318
	21st to end	Day	57% 7% 7% 9% 15% 5%	2992
February	1st to 10th	Day	42% 7% 13% 16% 15% 6%	1349
	11th to 20th	Day	58% 10% 11% 10% 9% 2	1566
	21st to end	Day	48% 10% 11% 16% 12% 2	1409
March	1st to 10th	Day	54% 8% 8% 12% 15% 4	4427
	11th to 20th	Day	47% 11% 12% 14% 15% 2	4630
	21st to end	Day	51% 10% 13% 15% 10% 2	5969
April	1st to 10th	Day	58% 10% 14% 13% 5% 1	2453
	11th to 20th	Day	45% 12% 15% 19% 9%	2631
	21st to end	Day	52% 10% 13% 17% 8% 1	2712
May	1st to 10th	Day	69% 10% 9% 10% 2	5002
	11th to 20th	Day	51% 13% 14% 19% 3	5047
	21st to end	Day	62% 11% 13% 13% 2	6033
June	1st to 10th	Day	58% 13% 15% 12% 1	3420
	11th to 20th	Day	59% 13% 16% 10% 3	3420
	21st to end	Day	61% 13% 19% 7%	3420
July	1st to 10th	Day	65% 10% 18% 6%	5611
	11th to 20th	Day	67% 10% 19% 5%	5525
	21st to end	Day	59% 12% 21% 8%	5338
August	1st to 10th	Day	70% 9% 15% 6% 1	5027
	11th to 20th	Day	63% 11% 14% 10% 1	4476
	21st to end	Day	63% 10% 19% 7% 1	4883
September	1st to 10th	Day	68% 13% 10% 7% 2	2358
	11th to 20th	Day	68% 10% 11% 10% 1	2272
	21st to end	Day	59% 11% 14% 12% 4	2033
October	1st to 10th	Day	59% 8% 16% 15% 3	3103
	11th to 20th	Day	40% 9% 15% 22% 12% 1	2817
	21st to end	Day	49% 8% 12% 25% 5% 1	2636
November	1st to 10th	Day	52% 9% 15% 13% 7% 3	1371
	11th to 20th	Day	45% 8% 11% 23% 12% 1	1185
	21st to end	Day	54% 8% 12% 11% 14% 2	1000
December	1st to 10th	Day	57% 8% 11% 14% 8% 1	1407
	11th to 20th	Day	58% 8% 13% 16% 4 1	1520
	21st to end	Day	42% 10% 13% 18% 11% 7%	2002
January	1st to 10th	Night	46% 10% 10% 14% 14% 6%	8146
	11th to 20th	Night	54% 8% 9% 14% 13% 2	7668
	21st to end	Night	50% 8% 7% 10% 18% 6%	7986
February	1st to 10th	Night	40% 7% 11% 18% 16% 7%	3048
	11th to 20th	Night	59% 9% 10% 12% 9% 2	2732
	21st to end	Night	51% 10% 9% 16% 12% 2	2220
March	1st to 10th	Night	51% 7% 8% 14% 14% 5%	5426
	11th to 20th	Night	47% 10% 11% 13% 18% 2	5315
	21st to end	Night	47% 10% 12% 19% 10% 1	4962
April	1st to 10th	Night	58% 9% 12% 14% 5% 1	1945
	11th to 20th	Night	47% 9% 16% 18% 9%	1504
	21st to end	Night	47% 10% 16% 19% 8%	1468
May	1st to 10th	Night	65% 11% 12% 11% 1	1965
	11th to 20th	Night	57% 14% 12% 14% 3	1861
	21st to end	Night	57% 14% 15% 12% 1	1546
June	1st to 10th	Night	60% 14% 16% 10% 1	760
	11th to 20th	Night	62% 12% 16% 8% 2	760
	21st to end	Night	61% 16% 18% 5%	760
July	1st to 10th	Night	68% 9% 17% 5% 1	1250
	11th to 20th	Night	68% 9% 19% 4	1310
	21st to end	Night	65% 10% 19% 5%	1988
August	1st to 10th	Night	69% 9% 18% 4	1888
	11th to 20th	Night	69% 10% 13% 7% 1	2415
	21st to end	Night	71% 8% 13% 7% 1	2783
September	1st to 10th	Night	70% 12% 9% 6% 2	1819
	11th to 20th	Night	67% 11% 12% 9% 1	1908
	21st to end	Night	63% 9% 14% 12% 3	2145
October	1st to 10th	Night	63% 8% 15% 10% 3	3859
	11th to 20th	Night	40% 9% 17% 20% 13% 1	4109
	21st to end	Night	56% 6% 13% 20% 6% 1	4923
November	1st to 10th	Night	54% 9% 12% 14% 8% 2	2927
	11th to 20th	Night	43% 10% 12% 23% 12% 1	3211
	21st to end	Night	52% 10% 11% 13% 11% 3	3400
December	1st to 10th	Night	59% 8% 10% 16% 7%	5776
	11th to 20th	Night	57% 9% 15% 13% 6%	6696
	21st to end	Night	44% 10% 14% 15% 11% 5%	8976

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% 19% 26% 23% 15% 6%	100
	11th to 20th	Day	17% 17% 30% 22% 11% 3	254
	21st to end	Day	18% 14% 23% 27% 15% 3	330
February	1st to 10th	Day	10% 15% 23% 28% 16% 8%	305
	11th to 20th	Day	17% 16% 29% 21% 14% 2	395
	21st to end	Day	12% 17% 27% 26% 11% 7%	331
March	1st to 10th	Day	13% 13% 30% 27% 15% 3	400
	11th to 20th	Day	9% 13% 31% 26% 15% 7%	445
	21st to end	Day	10% 15% 25% 25% 22% 3	660
April	1st to 10th	Day	12% 14% 34% 26% 13% 1	623
	11th to 20th	Day	13% 11% 22% 34% 16% 3	642
	21st to end	Day	15% 10% 24% 31% 19% 1	650
May	1st to 10th	Day	20% 15% 25% 26% 12% 1	653
	11th to 20th	Day	16% 13% 24% 27% 18% 2	675
	21st to end	Day	21% 11% 25% 29% 13% 1	832
June	1st to 10th	Day	21% 10% 20% 24% 21% 4	870
	11th to 20th	Day	21% 9% 17% 29% 19% 4	870
	21st to end	Day	19% 10% 17% 28% 22% 4	870
July	1st to 10th	Day	22% 10% 18% 29% 18% 3	856
	11th to 20th	Day	27% 6% 17% 31% 16% 3	852
	21st to end	Day	25% 8% 18% 27% 18% 3	752
August	1st to 10th	Day	32% 10% 20% 23% 13% 2	676
	11th to 20th	Day	29% 12% 23% 20% 13% 2	665
	21st to end	Day	31% 10% 18% 26% 12% 3	748
September	1st to 10th	Day	28% 12% 23% 23% 12% 3	613
	11th to 20th	Day	37% 10% 17% 21% 12% 2	572
	21st to end	Day	28% 11% 17% 22% 17% 5%	390
October	1st to 10th	Day	30% 11% 21% 20% 12% 6%	390
	11th to 20th	Day	21% 8% 20% 28% 18% 5%	390
	21st to end	Day	23% 9% 21% 26% 16% 5%	429
November	1st to 10th	Day	16% 14% 24% 22% 17% 7%	395
	11th to 20th	Day	16% 8% 20% 25% 25% 7%	285
	21st to end	Day	7% 19% 20% 30% 12% 12%	100
December	1st to 10th	Day	22% 15% 13% 31% 19%	100
	11th to 20th	Day	13% 14% 37% 22% 9% 5%	100
	21st to end	Day	13% 20% 26% 25% 12% 4	110
January	1st to 10th	Night	10% 15% 31% 28% 14% 3	1000
	11th to 20th	Night	16% 20% 30% 22% 11% 1	966
	21st to end	Night	16% 13% 30% 28% 13% 1	990
February	1st to 10th	Night	9% 14% 33% 27% 15% 2	900
	11th to 20th	Night	17% 16% 34% 24% 8%	884
	21st to end	Night	12% 13% 36% 25% 11% 3	653
March	1st to 10th	Night	10% 14% 31% 34% 10% 1	700
	11th to 20th	Night	8% 13% 28% 32% 17% 2	700
	21st to end	Night	10% 9% 29% 28% 22% 1	660
April	1st to 10th	Night	12% 12% 27% 29% 19% 1	600
	11th to 20th	Night	12% 8% 24% 30% 24% 2	564
	21st to end	Night	11% 7% 22% 38% 20% 1	570
May	1st to 10th	Night	16% 11% 21% 33% 19% 1	419
	11th to 20th	Night	16% 8% 20% 39% 15% 2	365
	21st to end	Night	19% 8% 18% 34% 19%	206
June	1st to 10th	Night	17% 9% 15% 33% 24% 2	190
	11th to 20th	Night	25% 5% 12% 32% 25% 3	190
	21st to end	Night	24% 8% 13% 26% 26% 3	190
July	1st to 10th	Night	30% 5% 12% 18% 31% 3	185
	11th to 20th	Night	36% 3 13% 21% 26% 1	222
	21st to end	Night	31% 9% 12% 21% 23% 4	416
August	1st to 10th	Night	36% 5% 13% 26% 18% 2	378
	11th to 20th	Night	36% 10% 19% 21% 14%	522
	21st to end	Night	32% 7% 14% 27% 18% 1	627
September	1st to 10th	Night	34% 10% 17% 23% 14% 2	570
	11th to 20th	Night	37% 8% 14% 23% 16% 2	570
	21st to end	Night	28% 10% 16% 24% 19% 2	600
October	1st to 10th	Night	30% 11% 22% 19% 15% 3	703
	11th to 20th	Night	18% 7% 24% 29% 19% 3	860
	21st to end	Night	20% 11% 23% 28% 16% 1	946
November	1st to 10th	Night	18% 10% 30% 29% 12% 1	880
	11th to 20th	Night	12% 12% 28% 31% 16% 2	900
	21st to end	Night	16% 15% 27% 28% 12% 3	900
December	1st to 10th	Night	20% 16% 25% 25% 11% 2	967
	11th to 20th	Night	13% 12% 31% 30% 11% 4	1000
	21st to end	Night	15% 17% 29% 25% 12% 3	1100

Page generated at 2025-04-08 05:17:29. Version ac07

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