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 2024-11-25.

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% 10% 4 8% 5% 5% 4 4 5% 7% 31%	101313
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 33%	98102
Transparency	Day	50% 6% 12% 20% 10% 1	104960
	Night	50% 6% 11% 20% 11% 2	102594
Seeing	Day	22% 14% 27% 21% 12% 4	14775
	Night	22% 12% 26% 28% 12% 1	24449

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	20% 20% 10% 12% 4 3 3 2 3 4 18%	2781
	11th to 20th	Day	13% 23% 9% 17% 9% 3 3 2 2 3 17%	2840
	21st to end	Day	26% 21% 7% 13% 6% 4 3 2 3 3 12%	3124
February	1st to 10th	Day	20% 17% 5% 11% 7% 5% 4 3 3 4 21%	1710
	11th to 20th	Day	16% 16% 6% 13% 5% 4 4 4 5% 5% 21%	1663
	21st to end	Day	21% 15% 5% 11% 5% 3 4 4 4 5% 24%	1569
March	1st to 10th	Day	21% 12% 4 8% 5% 4 4 3 5% 6% 29%	3435
	11th to 20th	Day	15% 12% 5% 8% 6% 4 5% 4 4 5% 32%	3480
	21st to end	Day	19% 10% 4 8% 5% 4 5% 3 5% 7% 31%	3815
April	1st to 10th	Day	16% 12% 3 9% 5% 5% 4 4 5% 7% 30%	2204
	11th to 20th	Day	16% 11% 4 7% 5% 4 5% 4 5% 6% 33%	2263
	21st to end	Day	21% 10% 2 7% 4 4 4 4 4 7% 33%	2470
May	1st to 10th	Day	26% 9% 2 6% 4 5% 4 4 6% 6% 28%	4082
	11th to 20th	Day	20% 8% 3 8% 5% 5% 5% 4 6% 7% 30%	4081
	21st to end	Day	14% 6% 2 6% 5% 4 5% 4 7% 9% 38%	4783
June	1st to 10th	Day	14% 8% 2 6% 5% 5% 6% 4 7% 9% 34%	2660
	11th to 20th	Day	14% 6% 3 6% 6% 5% 5% 5% 7% 9% 34%	2660
	21st to end	Day	13% 6% 2 7% 6% 5% 4 4 6% 10% 36%	2660
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 4 6% 8% 41%	4356
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 10% 39%	4120
	21st to end	Day	13% 7% 3 8% 5% 6% 5% 4 6% 8% 36%	4466
August	1st to 10th	Day	16% 8% 2 8% 5% 5% 5% 5% 7% 8% 30%	4104
	11th to 20th	Day	13% 6% 3 6% 5% 6% 6% 4 7% 8% 36%	4044
	21st to end	Day	10% 4 2 6% 5% 6% 5% 5% 7% 10% 41%	4286
September	1st to 10th	Day	16% 7% 2 5% 4 3 5% 3 5% 6% 44%	2280
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 8% 45%	2119
	21st to end	Day	20% 8% 3 7% 5% 4 4 3 5% 6% 36%	2076
October	1st to 10th	Day	17% 6% 2 6% 5% 3 4 3 6% 8% 39%	1900
	11th to 20th	Day	19% 9% 3 6% 4 5% 5% 3 5% 6% 35%	1900
	21st to end	Day	22% 10% 2 7% 4 4 3 3 5% 7% 33%	2090
November	1st to 10th	Day	12% 9% 3 6% 6% 4 4 3 5% 6% 42%	1897
	11th to 20th	Day	17% 15% 3 6% 5% 5% 5% 3 4 6% 32%	1641
	21st to end	Day	20% 16% 6% 8% 5% 4 4 3 5% 4 26%	1552
December	1st to 10th	Day	21% 19% 6% 11% 7% 4 4 2 4 5% 18%	1520
	11th to 20th	Day	20% 18% 8% 10% 5% 3 3 3 4 6% 20%	1910
	21st to end	Day	25% 23% 7% 9% 5% 4 3 2 4 4 14%	2772
January	1st to 10th	Night	17% 20% 8% 12% 5% 4 4 3 4 4 19%	4159
	11th to 20th	Night	17% 18% 7% 13% 7% 6% 4 3 4 4 16%	4120
	21st to end	Night	28% 17% 7% 11% 6% 4 3 2 3 3 17%	4532
February	1st to 10th	Night	20% 14% 5% 10% 6% 5% 4 3 4 5% 24%	2470
	11th to 20th	Night	18% 15% 5% 12% 5% 4 4 3 4 5% 25%	2403
	21st to end	Night	21% 14% 4 10% 5% 4 3 4 5% 5% 26%	1880
March	1st to 10th	Night	23% 12% 3 7% 4 3 3 3 4 5% 33%	3418
	11th to 20th	Night	18% 14% 5% 7% 3 3 3 3 4 6% 34%	3480
	21st to end	Night	18% 10% 3 6% 5% 4 4 3 4 5% 39%	3817
April	1st to 10th	Night	18% 13% 3 6% 4 4 3 3 4 5% 36%	1976
	11th to 20th	Night	17% 8% 2 7% 4 4 4 4 6% 7% 38%	1875
	21st to end	Night	16% 9% 3 7% 4 4 5% 3 5% 7% 36%	1715
May	1st to 10th	Night	28% 10% 2 6% 4 4 4 3 5% 6% 27%	2813
	11th to 20th	Night	18% 9% 3 7% 5% 5% 3 4 4 7% 35%	2812
	21st to end	Night	13% 6% 2 6% 4 4 5% 4 6% 10% 39%	2768
June	1st to 10th	Night	11% 6% 2 5% 5% 5% 4 3 5% 9% 43%	1520
	11th to 20th	Night	12% 6% 2 7% 5% 3 4 5% 6% 8% 41%	1520
	21st to end	Night	12% 5% 2 4 4 5% 5% 4 7% 10% 41%	1520
July	1st to 10th	Night	13% 8% 3 7% 4 4 4 3 5% 8% 41%	2599
	11th to 20th	Night	13% 5% 2 6% 4 5% 5% 4 6% 9% 41%	2840
	21st to end	Night	12% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	3087
August	1st to 10th	Night	14% 8% 3 8% 4 5% 5% 4 7% 8% 34%	2825
	11th to 20th	Night	15% 6% 3 6% 5% 4 4 3 6% 7% 42%	2785
	21st to end	Night	9% 5% 2 6% 4 5% 6% 5% 6% 8% 44%	3336
September	1st to 10th	Night	13% 5% 1 5% 3 4 4 4 6% 7% 48%	1900
	11th to 20th	Night	10% 5% 2 4 3 3 4 3 6% 7% 52%	2057
	21st to end	Night	17% 8% 2 6% 5% 3 4 3 5% 6% 40%	2104
October	1st to 10th	Night	15% 7% 2 6% 4 3 5% 3 5% 6% 44%	2280
	11th to 20th	Night	18% 6% 3 6% 5% 5% 5% 4 5% 6% 39%	2280
	21st to end	Night	23% 8% 2 6% 3 3 3 2 4 7% 37%	2508
November	1st to 10th	Night	13% 10% 2 6% 4 3 4 3 4 6% 44%	2396
	11th to 20th	Night	19% 12% 3 7% 5% 4 4 4 5% 6% 31%	2755
	21st to end	Night	20% 15% 4 7% 5% 4 4 3 4 6% 30%	2723
December	1st to 10th	Night	23% 17% 6% 10% 5% 4 4 2 5% 6% 19%	2660
	11th to 20th	Night	20% 17% 6% 10% 6% 4 4 3 3 5% 24%	3285
	21st to end	Night	25% 19% 6% 9% 6% 4 4 3 4 5% 15%	4884

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	69% 5% 10% 11% 4	2782
	11th to 20th	Day	74% 3 8% 11% 3 1	2840
	21st to end	Day	77% 5% 7% 8% 2	3124
February	1st to 10th	Day	68% 3 9% 13% 6% 1	1710
	11th to 20th	Day	64% 5% 11% 14% 6%	1672
	21st to end	Day	62% 5% 11% 14% 6% 1	1569
March	1st to 10th	Day	55% 6% 11% 22% 6%	3435
	11th to 20th	Day	53% 6% 12% 22% 6% 1	3480
	21st to end	Day	51% 7% 12% 19% 10% 1	3815
April	1st to 10th	Day	52% 6% 12% 18% 10% 2	2204
	11th to 20th	Day	50% 7% 11% 16% 14% 2	2263
	21st to end	Day	51% 6% 11% 21% 9% 2	2470
May	1st to 10th	Day	54% 7% 10% 20% 7% 1	4120
	11th to 20th	Day	50% 9% 12% 20% 8% 1	4085
	21st to end	Day	38% 11% 16% 22% 11% 3	4797
June	1st to 10th	Day	44% 9% 14% 21% 10% 1	2660
	11th to 20th	Day	43% 9% 14% 21% 12% 1	2660
	21st to end	Day	42% 8% 12% 25% 12% 1	2660
July	1st to 10th	Day	41% 7% 12% 23% 15% 3	4288
	11th to 20th	Day	39% 6% 13% 26% 15% 1	4051
	21st to end	Day	45% 5% 12% 19% 17% 1	4324
August	1st to 10th	Day	49% 8% 14% 20% 8%	4091
	11th to 20th	Day	43% 7% 12% 27% 11% 1	4083
	21st to end	Day	37% 7% 14% 27% 14% 2	4286
September	1st to 10th	Day	39% 7% 11% 20% 19% 3	2280
	11th to 20th	Day	37% 7% 13% 26% 15% 1	2119
	21st to end	Day	48% 5% 11% 21% 13% 1	2076
October	1st to 10th	Day	34% 5% 10% 32% 19% 1	3160
	11th to 20th	Day	44% 6% 11% 21% 14% 5%	3140
	21st to end	Day	37% 3 13% 28% 19% 1	3428
November	1st to 10th	Day	42% 6% 12% 21% 17% 3	1897
	11th to 20th	Day	53% 6% 11% 18% 11% 2	1641
	21st to end	Day	60% 6% 11% 17% 5% 1	1552
December	1st to 10th	Day	70% 4 10% 11% 4	1520
	11th to 20th	Day	65% 5% 11% 15% 3	1906
	21st to end	Day	73% 5% 9% 10% 3	2772
January	1st to 10th	Night	68% 5% 10% 13% 3	4159
	11th to 20th	Night	71% 5% 11% 10% 2	4120
	21st to end	Night	74% 4 7% 12% 3	4532
February	1st to 10th	Night	62% 4 10% 16% 7%	2470
	11th to 20th	Night	61% 5% 10% 16% 7%	2405
	21st to end	Night	61% 5% 10% 15% 9% 1	1880
March	1st to 10th	Night	53% 6% 9% 24% 8%	3418
	11th to 20th	Night	51% 6% 12% 25% 6% 1	3480
	21st to end	Night	46% 6% 11% 22% 14% 2	3817
April	1st to 10th	Night	51% 5% 10% 18% 13% 2	1976
	11th to 20th	Night	45% 6% 12% 18% 15% 3	1875
	21st to end	Night	46% 6% 10% 23% 11% 3	1715
May	1st to 10th	Night	58% 6% 9% 16% 10% 2	2840
	11th to 20th	Night	48% 8% 11% 22% 9% 1	2813
	21st to end	Night	36% 11% 14% 22% 12% 4	2775
June	1st to 10th	Night	36% 9% 13% 25% 15% 2	1520
	11th to 20th	Night	38% 8% 15% 24% 14% 1	1520
	21st to end	Night	37% 9% 13% 24% 16% 2	1520
July	1st to 10th	Night	43% 6% 10% 22% 16% 3	2557
	11th to 20th	Night	38% 6% 15% 24% 15% 1	2786
	21st to end	Night	42% 7% 11% 16% 21% 2	2988
August	1st to 10th	Night	47% 7% 15% 21% 10%	2814
	11th to 20th	Night	41% 6% 10% 28% 14% 1	2812
	21st to end	Night	37% 6% 13% 29% 13% 2	3336
September	1st to 10th	Night	33% 8% 11% 22% 21% 4	1900
	11th to 20th	Night	30% 7% 14% 26% 23% 1	2057
	21st to end	Night	44% 6% 12% 21% 16% 1	2104
October	1st to 10th	Night	36% 5% 9% 27% 22% 2	3800
	11th to 20th	Night	38% 4 10% 24% 15% 8%	3779
	21st to end	Night	35% 4 9% 28% 22% 1	4120
November	1st to 10th	Night	41% 5% 11% 22% 17% 4	2396
	11th to 20th	Night	53% 5% 11% 18% 11% 3	2755
	21st to end	Night	56% 5% 11% 21% 7% 1	2723
December	1st to 10th	Night	66% 6% 11% 12% 4	2660
	11th to 20th	Night	64% 4 11% 17% 3	3288
	21st to end	Night	71% 7% 10% 10% 2	4884

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	29% 26% 27% 12% 4 2	256
	11th to 20th	Day	32% 29% 28% 8% 2 1	270
	21st to end	Day	27% 31% 29% 9% 3 1	297
February	1st to 10th	Day	34% 21% 33% 11% 1	270
	11th to 20th	Day	33% 30% 28% 8% 1	270
	21st to end	Day	36% 24% 30% 9% 1	270
March	1st to 10th	Day	28% 20% 32% 16% 4	460
	11th to 20th	Day	32% 21% 30% 12% 5% 1	460
	21st to end	Day	30% 19% 29% 16% 5% 1	506
April	1st to 10th	Day	28% 20% 26% 18% 7% 1	460
	11th to 20th	Day	21% 22% 29% 15% 9% 4	452
	21st to end	Day	28% 18% 28% 17% 7% 2	460
May	1st to 10th	Day	29% 16% 27% 16% 9% 3	463
	11th to 20th	Day	23% 13% 28% 21% 11% 5%	487
	21st to end	Day	16% 8% 25% 27% 18% 6%	529
June	1st to 10th	Day	16% 11% 28% 26% 15% 4	490
	11th to 20th	Day	17% 11% 26% 26% 16% 4	490
	21st to end	Day	14% 10% 30% 29% 14% 3	490
July	1st to 10th	Day	13% 7% 21% 28% 19% 12%	485
	11th to 20th	Day	12% 7% 27% 27% 19% 8%	484
	21st to end	Day	15% 5% 24% 31% 18% 7%	532
August	1st to 10th	Day	15% 6% 21% 28% 22% 8%	486
	11th to 20th	Day	16% 6% 20% 26% 21% 10%	479
	21st to end	Day	11% 5% 23% 27% 22% 11%	539
September	1st to 10th	Day	16% 9% 20% 26% 20% 10%	490
	11th to 20th	Day	14% 4 17% 31% 23% 11%	490
	21st to end	Day	26% 11% 24% 21% 13% 5%	490
October	1st to 10th	Day	18% 9% 27% 22% 19% 4	490
	11th to 20th	Day	22% 16% 27% 23% 10% 2	490
	21st to end	Day	29% 16% 29% 18% 8% 1	539
November	1st to 10th	Day	21% 15% 29% 21% 10% 3	435
	11th to 20th	Day	29% 21% 30% 13% 5% 1	220
	21st to end	Day	24% 24% 39% 10% 3 1	188
December	1st to 10th	Day	31% 26% 27% 10% 5% 1	180
	11th to 20th	Day	32% 21% 32% 11% 4	180
	21st to end	Day	31% 25% 26% 14% 4	198
January	1st to 10th	Night	28% 22% 29% 17% 4	760
	11th to 20th	Night	31% 17% 30% 19% 2	760
	21st to end	Night	29% 18% 30% 21% 3	836
February	1st to 10th	Night	29% 16% 28% 24% 2	760
	11th to 20th	Night	28% 20% 35% 16% 1	743
	21st to end	Night	29% 15% 35% 20% 2	628
March	1st to 10th	Night	24% 17% 31% 24% 4	760
	11th to 20th	Night	27% 17% 28% 21% 6% 1	760
	21st to end	Night	24% 14% 34% 22% 5% 1	836
April	1st to 10th	Night	25% 17% 30% 23% 5%	760
	11th to 20th	Night	21% 11% 33% 25% 9% 1	751
	21st to end	Night	18% 11% 32% 32% 6%	575
May	1st to 10th	Night	22% 11% 22% 35% 9% 1	570
	11th to 20th	Night	20% 9% 28% 33% 11%	561
	21st to end	Night	10% 3 23% 36% 25% 3	435
June	1st to 10th	Night	12% 4 23% 42% 17% 1	380
	11th to 20th	Night	13% 4 23% 40% 19% 1	380
	21st to end	Night	13% 2 27% 38% 19% 1	380
July	1st to 10th	Night	10% 3 21% 42% 20% 5%	408
	11th to 20th	Night	10% 3 22% 33% 27% 5%	552
	21st to end	Night	11% 4 17% 40% 27% 2	624
August	1st to 10th	Night	10% 3 15% 42% 28% 2	567
	11th to 20th	Night	13% 3 13% 38% 29% 3	564
	21st to end	Night	11% 4 19% 34% 28% 5%	748
September	1st to 10th	Night	14% 4 19% 34% 25% 4	760
	11th to 20th	Night	11% 4 15% 37% 28% 4	760
	21st to end	Night	19% 9% 23% 27% 20% 3	760
October	1st to 10th	Night	18% 7% 22% 30% 22% 1	760
	11th to 20th	Night	22% 9% 29% 29% 11% 1	760
	21st to end	Night	26% 13% 24% 29% 8% 1	836
November	1st to 10th	Night	17% 12% 27% 32% 12%	780
	11th to 20th	Night	26% 15% 27% 24% 6% 2	800
	21st to end	Night	29% 13% 29% 23% 6% 1	779
December	1st to 10th	Night	32% 17% 28% 19% 5%	760
	11th to 20th	Night	28% 18% 29% 21% 4	760
	21st to end	Night	30% 16% 30% 22% 2	836

Page generated at 2024-11-22 08:02:28. Version ac07

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