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 2024-03-01.

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	15% 12% 4 8% 7% 4 6% 6% 6% 7% 27%	76652
	Night	11% 13% 5% 8% 7% 4 5% 5% 5% 6% 30%	82752
Transparency	Day	56% 12% 14% 12% 6% 1	76299
	Night	52% 10% 12% 13% 10% 3	82688
Seeing	Day	21% 11% 22% 26% 16% 3	16838
	Night	18% 11% 25% 28% 16% 2	21661

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 12% 4 6% 7% 4 6% 8% 8% 8% 33%	1269
	11th to 20th	Day	8% 12% 5% 6% 5% 3 4 5% 5% 6% 42%	1589
	21st to end	Day	5% 12% 4 9% 8% 5% 6% 6% 5% 6% 32%	2090
February	1st to 10th	Day	6% 14% 4 9% 6% 2 4 4 5% 6% 40%	1281
	11th to 20th	Day	7% 25% 5% 9% 5% 3 5% 5% 5% 8% 24%	1479
	21st to end	Day	7% 11% 7% 7% 7% 4 6% 6% 5% 6% 35%	1340
March	1st to 10th	Day	7% 13% 5% 8% 6% 3 5% 3 4 5% 41%	1782
	11th to 20th	Day	3 8% 3 7% 6% 3 6% 6% 7% 7% 43%	1840
	21st to end	Day	5% 13% 5% 8% 6% 4 5% 6% 6% 9% 35%	2376
April	1st to 10th	Day	7% 14% 6% 11% 6% 5% 8% 7% 5% 6% 25%	2206
	11th to 20th	Day	6% 10% 4 7% 7% 4 6% 6% 7% 9% 35%	2486
	21st to end	Day	8% 12% 6% 9% 7% 4 7% 6% 5% 7% 29%	2569
May	1st to 10th	Day	11% 13% 6% 11% 9% 5% 5% 7% 6% 6% 22%	2840
	11th to 20th	Day	12% 10% 4 7% 7% 4 6% 6% 6% 7% 30%	2841
	21st to end	Day	16% 10% 4 6% 6% 4 5% 5% 6% 7% 31%	3396
June	1st to 10th	Day	15% 9% 4 8% 8% 4 7% 7% 7% 8% 23%	3240
	11th to 20th	Day	17% 10% 4 8% 7% 5% 6% 7% 7% 7% 22%	3240
	21st to end	Day	18% 11% 4 8% 8% 5% 6% 7% 6% 7% 20%	3240
July	1st to 10th	Day	18% 9% 4 8% 8% 4 6% 6% 7% 8% 22%	3240
	11th to 20th	Day	21% 10% 4 8% 7% 4 6% 6% 6% 7% 20%	3203
	21st to end	Day	22% 10% 4 7% 7% 3 6% 5% 5% 7% 24%	3150
August	1st to 10th	Day	26% 10% 5% 8% 8% 4 5% 5% 5% 6% 19%	2865
	11th to 20th	Day	28% 11% 4 7% 5% 4 6% 5% 6% 6% 18%	2534
	21st to end	Day	25% 10% 3 7% 6% 4 5% 6% 6% 6% 21%	2772
September	1st to 10th	Day	27% 11% 4 8% 7% 4 6% 5% 5% 5% 18%	2235
	11th to 20th	Day	28% 13% 4 8% 6% 3 4 5% 3 5% 21%	2153
	21st to end	Day	21% 11% 3 7% 7% 4 4 4 5% 6% 26%	1926
October	1st to 10th	Day	24% 13% 3 7% 7% 3 5% 4 5% 6% 22%	1769
	11th to 20th	Day	15% 12% 5% 7% 6% 4 5% 5% 6% 7% 28%	1616
	21st to end	Day	12% 12% 4 6% 6% 4 4 5% 6% 9% 33%	1526
November	1st to 10th	Day	8% 14% 5% 8% 6% 5% 6% 6% 7% 7% 29%	1301
	11th to 20th	Day	9% 11% 4 7% 6% 4 4 4 6% 7% 39%	1126
	21st to end	Day	8% 13% 7% 12% 5% 4 5% 4 5% 6% 30%	950
December	1st to 10th	Day	14% 12% 5% 10% 8% 3 4 5% 6% 7% 25%	818
	11th to 20th	Day	5% 20% 9% 10% 6% 4 5% 4 5% 7% 24%	967
	21st to end	Day	4 10% 5% 9% 9% 4 6% 6% 7% 6% 36%	1397
January	1st to 10th	Night	5% 12% 4 9% 7% 4 6% 6% 7% 8% 32%	5686
	11th to 20th	Night	9% 12% 5% 7% 6% 3 4 4 4 5% 41%	5320
	21st to end	Night	6% 12% 5% 8% 7% 4 5% 5% 6% 6% 37%	5577
February	1st to 10th	Night	6% 13% 5% 8% 6% 2 4 4 4 6% 43%	2897
	11th to 20th	Night	10% 21% 6% 9% 6% 4 5% 5% 5% 7% 23%	2588
	21st to end	Night	6% 12% 5% 11% 8% 6% 5% 5% 6% 6% 31%	2113
March	1st to 10th	Night	7% 13% 4 9% 6% 3 4 4 5% 6% 39%	2183
	11th to 20th	Night	3 11% 4 7% 6% 3 5% 5% 7% 7% 42%	2117
	21st to end	Night	5% 13% 5% 8% 6% 3 4 5% 5% 8% 38%	1980
April	1st to 10th	Night	7% 18% 7% 9% 7% 4 7% 6% 6% 5% 25%	1752
	11th to 20th	Night	8% 11% 4 7% 6% 4 6% 6% 6% 7% 35%	1423
	21st to end	Night	7% 13% 5% 8% 8% 3 6% 4 5% 7% 33%	1391
May	1st to 10th	Night	11% 13% 6% 11% 8% 4 7% 6% 5% 6% 22%	1117
	11th to 20th	Night	13% 12% 4 9% 6% 5% 6% 6% 5% 6% 30%	1054
	21st to end	Night	16% 8% 3 9% 5% 3 5% 5% 6% 10% 31%	876
June	1st to 10th	Night	17% 11% 5% 5% 8% 5% 7% 6% 5% 7% 22%	720
	11th to 20th	Night	20% 10% 5% 9% 8% 4 5% 4 5% 6% 23%	720
	21st to end	Night	20% 9% 5% 10% 7% 5% 6% 5% 6% 7% 19%	720
July	1st to 10th	Night	24% 9% 3 8% 7% 4 6% 5% 6% 7% 21%	720
	11th to 20th	Night	29% 11% 5% 9% 6% 3 4 4 5% 4 19%	756
	21st to end	Night	28% 10% 5% 7% 5% 4 5% 5% 5% 5% 20%	1169
August	1st to 10th	Night	27% 10% 4 7% 7% 5% 7% 4 4 7% 19%	1074
	11th to 20th	Night	30% 13% 4 9% 8% 4 5% 5% 3 5% 15%	1366
	21st to end	Night	28% 11% 4 8% 7% 4 6% 4 4 5% 19%	1584
September	1st to 10th	Night	31% 12% 4 8% 6% 4 6% 4 5% 5% 15%	1722
	11th to 20th	Night	30% 13% 4 7% 6% 4 4 4 4 5% 20%	1807
	21st to end	Night	23% 12% 4 8% 8% 3 5% 4 6% 6% 21%	2032
October	1st to 10th	Night	26% 15% 4 7% 6% 4 4 4 4 5% 21%	2191
	11th to 20th	Night	15% 12% 5% 8% 8% 4 5% 5% 5% 7% 27%	2342
	21st to end	Night	12% 13% 5% 8% 6% 3 5% 5% 6% 6% 30%	2830
November	1st to 10th	Night	10% 14% 4 8% 7% 4 6% 6% 5% 7% 31%	2777
	11th to 20th	Night	8% 11% 3 7% 6% 3 5% 5% 5% 7% 39%	3050
	21st to end	Night	8% 14% 5% 9% 8% 4 5% 5% 6% 7% 29%	3230
December	1st to 10th	Night	12% 17% 5% 9% 7% 3 5% 5% 5% 7% 26%	3358
	11th to 20th	Night	6% 19% 8% 10% 7% 3 6% 6% 6% 6% 24%	4240
	21st to end	Night	5% 13% 5% 9% 8% 4 6% 6% 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% 11% 11% 16% 13% 3	1340
March	1st to 10th	Day	46% 6% 12% 15% 15% 6%	1782
	11th to 20th	Day	37% 11% 12% 19% 17% 4	1840
	21st to end	Day	46% 12% 16% 11% 12% 4	2376
April	1st to 10th	Day	58% 10% 13% 13% 5% 1	2206
	11th to 20th	Day	45% 13% 15% 19% 9%	2486
	21st to end	Day	53% 9% 13% 16% 8% 1	2569
May	1st to 10th	Day	61% 13% 13% 11% 3	2840
	11th to 20th	Day	50% 12% 14% 19% 5%	2844
	21st to end	Day	52% 10% 17% 18% 3	3397
June	1st to 10th	Day	59% 13% 16% 12% 1	3240
	11th to 20th	Day	60% 14% 14% 9% 3	3240
	21st to end	Day	62% 14% 18% 6%	3240
July	1st to 10th	Day	59% 13% 18% 10% 1	3151
	11th to 20th	Day	65% 13% 19% 3	3104
	21st to end	Day	60% 15% 19% 6%	2975
August	1st to 10th	Day	67% 13% 15% 4 1	2846
	11th to 20th	Day	66% 13% 13% 6% 1	2534
	21st to end	Day	65% 12% 14% 7% 2	2772
September	1st to 10th	Day	68% 13% 10% 7% 2	2235
	11th to 20th	Day	67% 11% 12% 10% 1	2153
	21st to end	Day	59% 11% 14% 12% 4	1926
October	1st to 10th	Day	64% 9% 14% 9% 3 1	1769
	11th to 20th	Day	54% 13% 14% 13% 4 2	1616
	21st to end	Day	48% 12% 14% 16% 7% 2	1526
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% 10% 9% 16% 12% 2	2113
March	1st to 10th	Night	45% 7% 11% 14% 15% 7%	2183
	11th to 20th	Night	39% 9% 10% 16% 20% 4	2119
	21st to end	Night	44% 10% 15% 13% 14% 4	1980
April	1st to 10th	Night	59% 9% 12% 14% 5% 1	1752
	11th to 20th	Night	47% 9% 16% 18% 10%	1423
	21st to end	Night	49% 10% 16% 17% 9%	1391
May	1st to 10th	Night	60% 12% 16% 11% 2	1117
	11th to 20th	Night	53% 11% 12% 18% 5%	1055
	21st to end	Night	49% 11% 21% 17% 2	876
June	1st to 10th	Night	60% 14% 16% 10% 1	720
	11th to 20th	Night	63% 12% 15% 8% 2	720
	21st to end	Night	63% 16% 16% 4	720
July	1st to 10th	Night	60% 13% 18% 8% 1	700
	11th to 20th	Night	67% 11% 19% 3	732
	21st to end	Night	65% 16% 16% 4	1104
August	1st to 10th	Night	66% 13% 17% 4 1	1068
	11th to 20th	Night	71% 11% 11% 6% 1	1366
	21st to end	Night	68% 11% 13% 7% 2	1584
September	1st to 10th	Night	70% 13% 9% 6% 2	1722
	11th to 20th	Night	65% 11% 13% 10% 1	1807
	21st to end	Night	63% 9% 13% 11% 3	2032
October	1st to 10th	Night	66% 9% 11% 10% 4	2191
	11th to 20th	Night	55% 11% 15% 12% 6% 1	2342
	21st to end	Night	54% 9% 13% 14% 9% 1	2830
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% 28% 10% 6%	307
March	1st to 10th	Day	14% 11% 29% 28% 15% 4	341
	11th to 20th	Day	9% 12% 30% 27% 16% 6%	380
	21st to end	Day	8% 16% 26% 26% 22% 2	572
April	1st to 10th	Day	12% 14% 34% 25% 14% 1	538
	11th to 20th	Day	12% 11% 23% 35% 17% 3	590
	21st to end	Day	16% 11% 24% 30% 18% 1	600
May	1st to 10th	Day	19% 15% 26% 27% 12% 1	603
	11th to 20th	Day	16% 13% 25% 27% 17% 2	625
	21st to end	Day	21% 11% 24% 29% 13% 1	771
June	1st to 10th	Day	21% 10% 20% 24% 22% 4	810
	11th to 20th	Day	22% 9% 18% 28% 19% 4	810
	21st to end	Day	21% 10% 18% 27% 21% 4	810
July	1st to 10th	Day	21% 10% 18% 30% 18% 3	796
	11th to 20th	Day	26% 6% 17% 32% 16% 3	792
	21st to end	Day	25% 8% 18% 27% 19% 3	697
August	1st to 10th	Day	31% 10% 20% 24% 12% 2	626
	11th to 20th	Day	29% 13% 23% 19% 14% 2	615
	21st to end	Day	31% 10% 18% 26% 12% 2	693
September	1st to 10th	Day	28% 12% 24% 23% 11% 3	570
	11th to 20th	Day	35% 10% 18% 22% 13% 2	533
	21st to end	Day	29% 12% 17% 23% 16% 4	360
October	1st to 10th	Day	32% 11% 22% 20% 12% 3	360
	11th to 20th	Day	21% 8% 19% 29% 18% 5%	360
	21st to end	Day	22% 10% 20% 25% 17% 5%	396
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% 36% 26% 11% 3	618
March	1st to 10th	Night	11% 12% 31% 35% 10% 1	621
	11th to 20th	Night	8% 12% 28% 32% 17% 2	620
	21st to end	Night	10% 9% 30% 28% 22% 1	594
April	1st to 10th	Night	13% 11% 25% 30% 20% 1	540
	11th to 20th	Night	12% 7% 23% 31% 25% 2	533
	21st to end	Night	12% 7% 23% 36% 20% 1	540
May	1st to 10th	Night	15% 10% 21% 34% 19% 1	397
	11th to 20th	Night	15% 8% 21% 39% 15% 2	346
	21st to end	Night	19% 7% 18% 35% 21% 1	195
June	1st to 10th	Night	18% 8% 15% 33% 24% 2	180
	11th to 20th	Night	26% 4 12% 31% 24% 2	180
	21st to end	Night	25% 8% 12% 26% 26% 3	180
July	1st to 10th	Night	29% 4 13% 19% 33% 3	175
	11th to 20th	Night	35% 3 13% 21% 27% 1	210
	21st to end	Night	30% 9% 13% 21% 23% 4	394
August	1st to 10th	Night	34% 6% 13% 27% 17% 2	358
	11th to 20th	Night	35% 10% 19% 21% 14%	494
	21st to end	Night	32% 7% 14% 27% 18% 1	594
September	1st to 10th	Night	34% 10% 17% 23% 14% 2	540
	11th to 20th	Night	36% 8% 14% 24% 17% 2	540
	21st to end	Night	29% 10% 17% 23% 19% 2	567
October	1st to 10th	Night	31% 11% 23% 19% 15% 2	661
	11th to 20th	Night	18% 8% 23% 29% 20% 3	810
	21st to end	Night	19% 11% 22% 28% 17% 2	891
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 2024-02-27 02:30:19. Version ac07

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