Plum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plum clear sky chart from 2005-11-06 to 2026-01-17.

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% 11% 4 9% 5% 5% 4 4 5% 7% 31%	127143
	Night	18% 11% 4 9% 5% 4 4 3 5% 6% 31%	124530
Transparency	Day	51% 6% 11% 20% 10% 1	130746
	Night	53% 5% 11% 19% 11% 2	128996
Seeing	Day	23% 15% 26% 20% 12% 4	16196
	Night	22% 12% 26% 28% 11% 1	25945

By Month

Percentage Cloud Cover SE (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	17% 18% 9% 23% 5% 3 3 2 3 4 14%	4076
	11th to 20th	Day	13% 22% 10% 17% 9% 6% 2 2 2 2 14%	4114
	21st to end	Day	21% 20% 6% 10% 7% 5% 4 2 4 4 17%	4466
February	1st to 10th	Day	20% 17% 6% 11% 7% 5% 3 3 3 4 21%	1800
	11th to 20th	Day	17% 16% 6% 13% 5% 4 4 4 5% 5% 21%	1753
	21st to end	Day	21% 15% 5% 11% 5% 3 4 4 4 5% 24%	1650
March	1st to 10th	Day	17% 13% 5% 8% 5% 4 4 3 4 5% 31%	4935
	11th to 20th	Day	14% 10% 5% 8% 5% 4 5% 4 5% 6% 35%	4980
	21st to end	Day	18% 10% 4 9% 5% 5% 4 3 5% 6% 30%	5465
April	1st to 10th	Day	17% 12% 3 9% 5% 5% 4 4 5% 7% 29%	2320
	11th to 20th	Day	16% 11% 4 7% 5% 4 5% 4 5% 6% 33%	2384
	21st to end	Day	20% 10% 2 7% 4 4 4 4 4 7% 34%	2600
May	1st to 10th	Day	22% 9% 3 7% 5% 5% 5% 4 6% 7% 27%	5862
	11th to 20th	Day	19% 7% 3 8% 5% 5% 5% 4 6% 9% 29%	5861
	21st to end	Day	20% 7% 2 6% 4 4 5% 4 6% 8% 32%	6880
June	1st to 10th	Day	14% 7% 2 6% 5% 6% 6% 5% 7% 9% 34%	2800
	11th to 20th	Day	15% 6% 3 6% 6% 5% 5% 5% 7% 9% 33%	2800
	21st to end	Day	13% 6% 2 7% 6% 5% 4 4 6% 10% 37%	2800
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 4 5% 8% 42%	6234
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 10% 33%	5900
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	6424
August	1st to 10th	Day	16% 7% 2 8% 5% 5% 6% 5% 7% 8% 31%	4234
	11th to 20th	Day	13% 6% 3 6% 5% 6% 6% 4 7% 8% 36%	4174
	21st to end	Day	9% 4 2 6% 5% 6% 5% 5% 7% 10% 41%	4421
September	1st to 10th	Day	15% 7% 2 5% 4 3 5% 3 5% 7% 45%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 5% 8% 45%	2230
	21st to end	Day	21% 8% 3 7% 5% 4 4 3 5% 6% 35%	2185
October	1st to 10th	Day	17% 6% 2 5% 5% 3 4 3 6% 8% 41%	2000
	11th to 20th	Day	18% 9% 3 6% 4 5% 5% 3 5% 6% 35%	2000
	21st to end	Day	22% 10% 2 7% 4 4 3 3 5% 7% 33%	2200
November	1st to 10th	Day	12% 9% 3 6% 6% 4 4 3 5% 6% 42%	1994
	11th to 20th	Day	18% 15% 3 6% 5% 5% 5% 3 4 5% 31%	1723
	21st to end	Day	20% 16% 6% 8% 4 4 5% 3 4 4 25%	1680
December	1st to 10th	Day	18% 15% 7% 18% 6% 5% 4 2 3 4 18%	2680
	11th to 20th	Day	24% 18% 6% 10% 4 3 3 2 4 5% 21%	3070
	21st to end	Day	23% 21% 6% 10% 5% 4 3 3 4 5% 14%	4048
January	1st to 10th	Night	15% 17% 7% 23% 6% 4 3 3 3 3 16%	6084
	11th to 20th	Night	15% 17% 8% 16% 10% 5% 4 2 4 4 15%	5979
	21st to end	Night	23% 16% 6% 11% 6% 4 5% 3 3 3 20%	6490
February	1st to 10th	Night	20% 14% 5% 10% 6% 5% 5% 3 4 5% 24%	2600
	11th to 20th	Night	19% 14% 5% 12% 5% 4 4 3 4 5% 24%	2533
	21st to end	Night	20% 14% 4 10% 5% 4 3 4 5% 5% 26%	1975
March	1st to 10th	Night	22% 12% 4 9% 4 3 3 3 4 5% 32%	4918
	11th to 20th	Night	14% 11% 4 6% 3 3 3 3 4 7% 41%	4980
	21st to end	Night	17% 10% 3 7% 5% 5% 4 3 4 5% 35%	5467
April	1st to 10th	Night	19% 13% 3 6% 4 4 3 3 4 5% 35%	2080
	11th to 20th	Night	17% 8% 2 7% 5% 4 4 4 6% 7% 37%	1975
	21st to end	Night	16% 9% 3 7% 4 4 5% 3 5% 8% 36%	1805
May	1st to 10th	Night	28% 10% 3 7% 5% 4 4 3 5% 6% 25%	4033
	11th to 20th	Night	18% 9% 3 7% 5% 4 3 3 4 7% 35%	4032
	21st to end	Night	21% 7% 3 6% 4 4 4 4 6% 8% 34%	3970
June	1st to 10th	Night	12% 7% 2 5% 5% 5% 4 3 5% 9% 43%	1600
	11th to 20th	Night	12% 6% 2 7% 5% 3 4 5% 6% 8% 40%	1600
	21st to end	Night	11% 5% 2 4 5% 5% 5% 4 7% 10% 41%	1600
July	1st to 10th	Night	13% 8% 3 7% 5% 4 4 3 5% 7% 42%	3705
	11th to 20th	Night	14% 6% 2 7% 4 5% 5% 4 7% 9% 35%	4057
	21st to end	Night	11% 7% 2 7% 5% 5% 5% 4 6% 8% 42%	4427
August	1st to 10th	Night	13% 8% 3 8% 4 5% 6% 4 7% 8% 35%	2915
	11th to 20th	Night	15% 6% 3 6% 4 4 4 3 5% 7% 42%	2875
	21st to end	Night	9% 5% 2 6% 4 5% 6% 5% 6% 8% 44%	3442
September	1st to 10th	Night	13% 5% 1 4 3 4 4 4 6% 6% 49%	2000
	11th to 20th	Night	10% 5% 2 4 3 4 4 3 6% 7% 53%	2166
	21st to end	Night	17% 8% 2 7% 6% 3 4 3 5% 6% 39%	2215
October	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 5% 6% 45%	2400
	11th to 20th	Night	18% 6% 3 6% 5% 5% 5% 4 5% 6% 40%	2400
	21st to end	Night	23% 8% 2 6% 3 3 3 2 4 7% 37%	2640
November	1st to 10th	Night	13% 10% 2 6% 4 3 4 3 5% 6% 44%	2519
	11th to 20th	Night	20% 12% 3 7% 5% 4 4 3 5% 6% 31%	2893
	21st to end	Night	20% 15% 4 6% 5% 4 4 3 4 6% 29%	2940
December	1st to 10th	Night	19% 16% 10% 14% 4 4 4 2 4 5% 18%	4720
	11th to 20th	Night	23% 16% 7% 10% 5% 3 3 3 3 5% 23%	5345
	21st to end	Night	23% 18% 5% 10% 5% 4 5% 3 4 5% 17%	7150

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	75% 4 8% 8% 4	4077
	11th to 20th	Day	79% 3 6% 8% 3 2	4114
	21st to end	Day	72% 4 8% 11% 5%	4466
February	1st to 10th	Day	68% 3 9% 14% 5% 1	1800
	11th to 20th	Day	65% 5% 11% 14% 6%	1762
	21st to end	Day	62% 5% 12% 14% 6% 1	1650
March	1st to 10th	Day	54% 5% 11% 22% 8% 1	4935
	11th to 20th	Day	49% 5% 12% 24% 8% 2	4980
	21st to end	Day	54% 5% 9% 19% 12% 1	5465
April	1st to 10th	Day	53% 6% 11% 17% 10% 2	2320
	11th to 20th	Day	50% 6% 12% 17% 13% 2	2384
	21st to end	Day	50% 6% 10% 22% 10% 2	2600
May	1st to 10th	Day	54% 6% 11% 18% 7% 3	5897
	11th to 20th	Day	50% 8% 11% 22% 9% 1	5865
	21st to end	Day	46% 9% 13% 20% 9% 2	6895
June	1st to 10th	Day	44% 9% 14% 21% 11% 1	2800
	11th to 20th	Day	45% 9% 14% 20% 12% 1	2800
	21st to end	Day	41% 8% 13% 25% 13% 1	2800
July	1st to 10th	Day	41% 5% 10% 27% 15% 2	6166
	11th to 20th	Day	45% 5% 15% 24% 11%	5831
	21st to end	Day	43% 5% 10% 22% 19% 1	6240
August	1st to 10th	Day	49% 8% 14% 21% 8%	4221
	11th to 20th	Day	42% 7% 12% 27% 11% 1	4213
	21st to end	Day	36% 7% 14% 28% 14% 2	4421
September	1st to 10th	Day	38% 7% 11% 21% 20% 3	2400
	11th to 20th	Day	36% 7% 14% 27% 16% 1	2230
	21st to end	Day	50% 5% 11% 20% 14% 1	2185
October	1st to 10th	Day	33% 5% 10% 32% 19% 1	3260
	11th to 20th	Day	43% 6% 11% 21% 14% 5%	3240
	21st to end	Day	37% 3 13% 28% 19% 1	3538
November	1st to 10th	Day	43% 6% 12% 21% 17% 2	1994
	11th to 20th	Day	54% 6% 11% 18% 10% 2	1723
	21st to end	Day	61% 5% 11% 17% 5% 1	1680
December	1st to 10th	Day	71% 4 11% 10% 4	2680
	11th to 20th	Day	66% 4 12% 14% 4	3066
	21st to end	Day	72% 4 8% 11% 4	4048
January	1st to 10th	Night	74% 4 7% 11% 4	6084
	11th to 20th	Night	73% 4 8% 10% 3 1	5979
	21st to end	Night	70% 4 8% 11% 5% 1	6490
February	1st to 10th	Night	63% 4 10% 16% 6%	2600
	11th to 20th	Night	62% 5% 10% 16% 7%	2535
	21st to end	Night	61% 5% 9% 15% 9% 1	1975
March	1st to 10th	Night	55% 4 9% 22% 8% 1	4918
	11th to 20th	Night	43% 4 13% 28% 8% 3	4980
	21st to end	Night	51% 5% 10% 17% 15% 1	5467
April	1st to 10th	Night	52% 5% 10% 18% 13% 2	2080
	11th to 20th	Night	46% 6% 12% 18% 15% 3	1975
	21st to end	Night	46% 6% 10% 23% 11% 3	1805
May	1st to 10th	Night	60% 5% 10% 13% 9% 3	4060
	11th to 20th	Night	48% 7% 9% 24% 11% 1	4033
	21st to end	Night	46% 8% 12% 21% 9% 3	3977
June	1st to 10th	Night	37% 9% 12% 24% 16% 2	1600
	11th to 20th	Night	40% 8% 14% 23% 14% 1	1600
	21st to end	Night	37% 9% 13% 23% 16% 2	1600
July	1st to 10th	Night	43% 5% 9% 23% 17% 2	3665
	11th to 20th	Night	45% 5% 18% 21% 11% 1	4006
	21st to end	Night	39% 6% 11% 19% 23% 2	4297
August	1st to 10th	Night	47% 7% 14% 21% 10% 1	2904
	11th to 20th	Night	40% 6% 10% 28% 15% 1	2902
	21st to end	Night	37% 6% 13% 29% 13% 2	3442
September	1st to 10th	Night	33% 8% 11% 23% 21% 4	2000
	11th to 20th	Night	30% 7% 14% 27% 22% 1	2166
	21st to end	Night	46% 6% 11% 20% 16% 1	2215
October	1st to 10th	Night	35% 5% 8% 27% 23% 2	3920
	11th to 20th	Night	38% 4 11% 24% 15% 8%	3899
	21st to end	Night	35% 4 9% 28% 22% 1	4252
November	1st to 10th	Night	41% 5% 11% 22% 17% 3	2519
	11th to 20th	Night	53% 5% 11% 19% 10% 2	2893
	21st to end	Night	56% 5% 11% 21% 7% 1	2940
December	1st to 10th	Night	69% 4 11% 13% 3	4720
	11th to 20th	Night	65% 3 14% 14% 3	5348
	21st to end	Night	69% 6% 10% 13% 2	7150

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	31% 27% 25% 13% 3 2	314
	11th to 20th	Day	36% 29% 26% 7% 2 1	318
	21st to end	Day	27% 32% 29% 8% 3 1	330
February	1st to 10th	Day	34% 24% 31% 10% 1	300
	11th to 20th	Day	35% 29% 27% 7% 1	300
	21st to end	Day	35% 25% 31% 8% 1	297
March	1st to 10th	Day	28% 21% 32% 15% 4	500
	11th to 20th	Day	32% 21% 30% 12% 5% 1	500
	21st to end	Day	31% 19% 29% 15% 5% 1	550
April	1st to 10th	Day	30% 20% 25% 17% 7% 1	500
	11th to 20th	Day	23% 23% 28% 15% 8% 3	492
	21st to end	Day	29% 17% 29% 17% 7% 2	500
May	1st to 10th	Day	28% 16% 28% 17% 9% 3	503
	11th to 20th	Day	22% 14% 28% 21% 11% 4	527
	21st to end	Day	18% 9% 25% 25% 17% 6%	573
June	1st to 10th	Day	16% 11% 29% 26% 14% 4	530
	11th to 20th	Day	18% 11% 26% 25% 16% 4	530
	21st to end	Day	13% 9% 29% 30% 15% 4	530
July	1st to 10th	Day	13% 7% 21% 28% 19% 11%	525
	11th to 20th	Day	11% 8% 27% 27% 19% 8%	524
	21st to end	Day	15% 5% 24% 31% 19% 7%	576
August	1st to 10th	Day	14% 6% 22% 28% 22% 8%	526
	11th to 20th	Day	16% 6% 19% 26% 23% 10%	519
	21st to end	Day	10% 5% 24% 26% 23% 11%	583
September	1st to 10th	Day	15% 9% 20% 25% 20% 10%	530
	11th to 20th	Day	13% 3 17% 31% 25% 11%	530
	21st to end	Day	26% 10% 23% 23% 14% 5%	530
October	1st to 10th	Day	17% 9% 26% 23% 20% 5%	530
	11th to 20th	Day	22% 16% 28% 23% 9% 2	530
	21st to end	Day	28% 17% 29% 17% 8% 1	583
November	1st to 10th	Day	21% 16% 30% 20% 10% 3	472
	11th to 20th	Day	33% 20% 29% 13% 5% 1	242
	21st to end	Day	30% 21% 36% 10% 2	220
December	1st to 10th	Day	31% 29% 25% 10% 4 1	220
	11th to 20th	Day	34% 21% 32% 10% 3	220
	21st to end	Day	33% 24% 27% 14% 3	242
January	1st to 10th	Night	29% 22% 30% 17% 3	840
	11th to 20th	Night	31% 19% 30% 18% 2	825
	21st to end	Night	29% 18% 30% 20% 3	880
February	1st to 10th	Night	30% 17% 28% 24% 2	800
	11th to 20th	Night	28% 20% 35% 16% 1	783
	21st to end	Night	28% 15% 36% 19% 2	660
March	1st to 10th	Night	25% 17% 32% 23% 4	800
	11th to 20th	Night	27% 17% 28% 22% 6% 1	800
	21st to end	Night	24% 15% 34% 21% 5% 1	880
April	1st to 10th	Night	26% 17% 29% 22% 5%	800
	11th to 20th	Night	23% 12% 32% 24% 9% 1	791
	21st to end	Night	18% 12% 32% 32% 6%	605
May	1st to 10th	Night	23% 10% 22% 35% 9% 1	600
	11th to 20th	Night	20% 9% 27% 32% 11%	591
	21st to end	Night	12% 4 23% 35% 24% 3	458
June	1st to 10th	Night	13% 5% 23% 42% 16% 1	400
	11th to 20th	Night	14% 4 24% 39% 19% 1	400
	21st to end	Night	13% 2 27% 38% 19% 1	400
July	1st to 10th	Night	10% 3 21% 43% 20% 5%	430
	11th to 20th	Night	11% 3 21% 33% 28% 5%	582
	21st to end	Night	11% 4 17% 40% 27% 2	657
August	1st to 10th	Night	9% 3 16% 42% 28% 2	597
	11th to 20th	Night	13% 3 13% 38% 29% 3	594
	21st to end	Night	11% 4 19% 34% 27% 5%	788
September	1st to 10th	Night	13% 4 20% 35% 24% 4	800
	11th to 20th	Night	11% 4 15% 38% 28% 4	800
	21st to end	Night	20% 9% 22% 27% 20% 3	800
October	1st to 10th	Night	17% 7% 22% 31% 22% 1	800
	11th to 20th	Night	21% 8% 30% 29% 11% 1	800
	21st to end	Night	26% 13% 24% 29% 7% 1	880
November	1st to 10th	Night	18% 12% 28% 31% 11%	820
	11th to 20th	Night	26% 15% 27% 24% 6% 1	840
	21st to end	Night	29% 13% 29% 22% 5%	840
December	1st to 10th	Night	31% 17% 28% 19% 5%	840
	11th to 20th	Night	29% 18% 29% 20% 3	840
	21st to end	Night	30% 17% 30% 22% 2	924

Page generated at 2026-01-14 01:14:32. Version ac07

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