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 2024-10-25.

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	17% 13% 4 9% 6% 5% 5% 4 6% 6% 24%	103096
	Night	13% 14% 4 10% 6% 5% 5% 4 5% 6% 28%	95814
Transparency	Day	58% 11% 14% 11% 5% 1	102786
	Night	54% 10% 12% 12% 9% 3	95772
Seeing	Day	21% 11% 22% 26% 16% 3	17944
	Night	18% 12% 25% 28% 15% 2	22280

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	4 13% 3 7% 6% 5% 6% 6% 8% 8% 33%	1269
	11th to 20th	Day	8% 14% 3 7% 4 4 4 3 5% 6% 42%	1589
	21st to end	Day	5% 12% 4 11% 7% 7% 6% 4 5% 7% 32%	2090
February	1st to 10th	Day	5% 15% 4 9% 5% 3 3 4 6% 6% 40%	1281
	11th to 20th	Day	7% 25% 5% 9% 5% 5% 4 5% 4 7% 23%	1479
	21st to end	Day	7% 12% 6% 8% 6% 6% 5% 5% 6% 5% 35%	1340
March	1st to 10th	Day	4 16% 6% 12% 7% 5% 5% 3 4 5% 33%	3081
	11th to 20th	Day	4 8% 3 9% 6% 7% 8% 6% 8% 8% 33%	3242
	21st to end	Day	12% 23% 6% 8% 5% 4 5% 4 5% 6% 23%	4165
April	1st to 10th	Day	7% 15% 5% 11% 6% 7% 7% 5% 5% 6% 24%	2330
	11th to 20th	Day	6% 12% 4 9% 5% 5% 5% 5% 7% 8% 36%	2626
	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	23% 14% 3 9% 6% 4 4 4 5% 6% 23%	1867
	11th to 20th	Day	14% 12% 4 8% 5% 5% 5% 4 6% 6% 29%	1705
	21st to end	Day	12% 13% 4 7% 5% 5% 4 3 6% 8% 33%	1561
November	1st to 10th	Day	8% 15% 4 10% 4 6% 6% 5% 7% 6% 29%	1301
	11th to 20th	Day	9% 12% 4 8% 5% 5% 4 4 6% 6% 39%	1126
	21st to end	Day	8% 14% 7% 13% 5% 5% 4 3 5% 6% 30%	950
December	1st to 10th	Day	14% 13% 4 12% 7% 4 4 5% 6% 7% 25%	818
	11th to 20th	Day	4 21% 9% 11% 5% 5% 4 3 5% 7% 24%	967
	21st to end	Day	4 10% 4 10% 8% 6% 6% 5% 6% 6% 36%	1397
January	1st to 10th	Night	5% 12% 4 10% 7% 5% 5% 5% 7% 7% 32%	5686
	11th to 20th	Night	8% 13% 4 8% 5% 4 4 3 4 5% 41%	5320
	21st to end	Night	5% 12% 4 9% 6% 5% 5% 4 6% 6% 36%	5577
February	1st to 10th	Night	6% 14% 4 8% 5% 4 4 3 4 6% 42%	2897
	11th to 20th	Night	9% 21% 6% 10% 5% 5% 5% 4 5% 7% 23%	2588
	21st to end	Night	6% 12% 5% 12% 6% 6% 4 4 7% 6% 31%	2113
March	1st to 10th	Night	5% 17% 6% 12% 6% 4 4 3 4 6% 34%	3772
	11th to 20th	Night	5% 12% 4 8% 5% 5% 6% 5% 7% 8% 35%	3715
	21st to end	Night	10% 23% 5% 8% 4 4 3 3 5% 6% 28%	3466
April	1st to 10th	Night	7% 19% 6% 10% 7% 6% 6% 4 5% 5% 24%	1848
	11th to 20th	Night	7% 13% 3 8% 6% 5% 6% 5% 6% 7% 35%	1503
	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	24% 15% 3 8% 6% 6% 4 3 4 5% 21%	2313
	11th to 20th	Night	14% 12% 4 10% 6% 5% 4 4 6% 7% 28%	2473
	21st to end	Night	12% 14% 5% 9% 6% 5% 5% 4 6% 6% 30%	2900
November	1st to 10th	Night	10% 14% 3 9% 6% 5% 6% 4 5% 6% 31%	2777
	11th to 20th	Night	7% 11% 4 8% 5% 5% 5% 4 6% 7% 39%	3050
	21st to end	Night	8% 15% 5% 11% 6% 5% 5% 4 6% 6% 28%	3230
December	1st to 10th	Night	12% 18% 4 10% 6% 4 4 4 5% 6% 26%	3358
	11th to 20th	Night	5% 20% 7% 12% 6% 5% 6% 4 6% 6% 24%	4240
	21st to end	Night	4 14% 4 11% 7% 5% 5% 5% 6% 6% 33%	6270

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	40% 14% 13% 17% 13% 3	1269
	11th to 20th	Day	41% 7% 8% 20% 18% 5%	1604
	21st to end	Day	47% 9% 9% 10% 17% 7%	2090
February	1st to 10th	Day	44% 7% 12% 15% 15% 6%	1281
	11th to 20th	Day	58% 10% 12% 10% 8% 2	1486
	21st to end	Day	48% 10% 11% 15% 13% 3	1340
March	1st to 10th	Day	50% 8% 10% 11% 15% 6%	3081
	11th to 20th	Day	39% 13% 16% 15% 16% 2	3242
	21st to end	Day	58% 12% 13% 8% 7% 2	4165
April	1st to 10th	Day	58% 10% 13% 13% 5% 1	2330
	11th to 20th	Day	45% 12% 15% 19% 9%	2626
	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	62% 9% 15% 10% 3 1	1867
	11th to 20th	Day	53% 13% 14% 14% 5% 2	1705
	21st to end	Day	48% 12% 14% 17% 7% 2	1561
November	1st to 10th	Day	51% 10% 16% 13% 7% 3	1301
	11th to 20th	Day	45% 8% 11% 23% 12% 1	1126
	21st to end	Day	54% 8% 12% 11% 13% 2	950
December	1st to 10th	Day	58% 9% 13% 10% 10% 1	818
	11th to 20th	Day	58% 9% 11% 14% 6% 1	970
	21st to end	Day	40% 13% 12% 12% 14% 10%	1397
January	1st to 10th	Night	45% 12% 12% 17% 11% 4	5686
	11th to 20th	Night	43% 8% 10% 17% 18% 4	5368
	21st to end	Night	43% 9% 9% 12% 19% 8%	5577
February	1st to 10th	Night	42% 8% 10% 17% 17% 7%	2897
	11th to 20th	Night	59% 9% 10% 12% 8% 2	2592
	21st to end	Night	51% 11% 9% 16% 12% 2	2113
March	1st to 10th	Night	50% 7% 10% 13% 13% 8%	3772
	11th to 20th	Night	40% 12% 15% 13% 19% 2	3717
	21st to end	Night	52% 11% 14% 12% 8% 2	3466
April	1st to 10th	Night	59% 10% 12% 14% 5% 1	1848
	11th to 20th	Night	47% 9% 16% 18% 9%	1503
	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	65% 9% 11% 11% 4	2313
	11th to 20th	Night	53% 11% 15% 14% 7% 1	2473
	21st to end	Night	54% 9% 12% 14% 9% 1	2900
November	1st to 10th	Night	53% 9% 13% 15% 9% 2	2777
	11th to 20th	Night	43% 10% 11% 23% 12%	3050
	21st to end	Night	53% 10% 10% 13% 11% 3	3230
December	1st to 10th	Night	58% 9% 11% 12% 10% 1	3358
	11th to 20th	Night	58% 11% 12% 12% 8% 1	4236
	21st to end	Night	45% 12% 10% 10% 15% 8%	6270

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	8% 18% 28% 24% 16% 7%	90
	11th to 20th	Day	15% 17% 29% 24% 12% 3	236
	21st to end	Day	15% 15% 24% 27% 16% 4	308
February	1st to 10th	Day	11% 16% 24% 27% 14% 8%	284
	11th to 20th	Day	18% 17% 28% 21% 14% 2	365
	21st to end	Day	12% 17% 27% 27% 10% 6%	307
March	1st to 10th	Day	13% 12% 28% 28% 15% 4	370
	11th to 20th	Day	9% 14% 31% 26% 15% 6%	413
	21st to end	Day	10% 16% 26% 24% 21% 2	616
April	1st to 10th	Day	12% 14% 34% 25% 14% 1	581
	11th to 20th	Day	13% 11% 22% 34% 16% 3	640
	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% 10% 20% 25% 17% 5%	410
November	1st to 10th	Day	16% 13% 24% 22% 18% 7%	365
	11th to 20th	Day	16% 9% 21% 24% 23% 6%	266
	21st to end	Day	7% 21% 20% 30% 9% 13%	90
December	1st to 10th	Day	18% 17% 13% 33% 19%	90
	11th to 20th	Day	13% 11% 38% 22% 10% 6%	90
	21st to end	Day	11% 21% 26% 26% 12% 3	99
January	1st to 10th	Night	9% 15% 31% 29% 14% 3	940
	11th to 20th	Night	14% 19% 31% 23% 12% 2	910
	21st to end	Night	15% 12% 31% 29% 13% 1	935
February	1st to 10th	Night	10% 14% 34% 26% 13% 2	850
	11th to 20th	Night	18% 17% 33% 24% 8%	834
	21st to end	Night	12% 12% 37% 26% 11% 3	618
March	1st to 10th	Night	10% 13% 31% 34% 10% 1	660
	11th to 20th	Night	8% 13% 28% 32% 17% 2	660
	21st to end	Night	11% 9% 30% 28% 22% 1	627
April	1st to 10th	Night	12% 11% 26% 30% 19% 1	570
	11th to 20th	Night	12% 7% 24% 30% 24% 2	563
	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% 22% 28% 17% 2	916
November	1st to 10th	Night	18% 9% 30% 30% 12% 1	830
	11th to 20th	Night	12% 12% 28% 31% 15% 2	850
	21st to end	Night	16% 15% 27% 27% 12% 3	850
December	1st to 10th	Night	19% 15% 25% 27% 12% 2	910
	11th to 20th	Night	13% 10% 31% 31% 12% 4	940
	21st to end	Night	15% 16% 29% 26% 12% 3	1034

Page generated at 2024-10-22 16:20:58. Version ac07

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