Peppermill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peppermill Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	149179
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	142571
Transparency	Day	38% 7% 13% 20% 18% 5%	152838
	Night	38% 6% 11% 19% 19% 6%	146984
Seeing	Day	17% 10% 24% 24% 18% 7%	17912
	Night	16% 9% 23% 31% 18% 3	27937

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	15% 6% 2 5% 3 3 3 2 4 5% 51%	5258
	11th to 20th	Day	12% 5% 2 4 3 3 2 2 3 4 60%	5280
	21st to end	Day	19% 9% 2 6% 4 3 4 3 4 4 40%	5780
February	1st to 10th	Day	15% 7% 2 5% 4 4 4 4 5% 7% 43%	1890
	11th to 20th	Day	22% 6% 2 4 3 3 3 3 4 5% 45%	2034
	21st to end	Day	19% 8% 2 6% 4 4 4 3 4 5% 41%	1782
March	1st to 10th	Day	20% 7% 2 6% 4 3 3 3 5% 5% 41%	5045
	11th to 20th	Day	10% 4 2 6% 4 3 4 3 4 6% 54%	5090
	21st to end	Day	17% 6% 2 6% 4 4 3 3 3 5% 47%	5586
April	1st to 10th	Day	12% 4 2 4 3 3 3 5% 6% 8% 49%	2310
	11th to 20th	Day	12% 4 2 5% 4 4 5% 4 5% 6% 50%	2478
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 7% 49%	2679
May	1st to 10th	Day	13% 7% 2 7% 4 4 5% 4 7% 9% 37%	7616
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	7626
	21st to end	Day	14% 7% 3 8% 5% 5% 5% 4 7% 8% 35%	8391
June	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 8% 11% 39%	2730
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 5% 7% 12% 42%	2730
	21st to end	Day	6% 3 2 5% 6% 6% 6% 5% 8% 11% 43%	2730
July	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 5% 8% 11% 41%	7680
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 6% 8% 12% 33%	7680
	21st to end	Day	11% 6% 3 7% 5% 6% 6% 5% 8% 12% 30%	8340
August	1st to 10th	Day	18% 7% 3 7% 6% 6% 7% 6% 8% 11% 21%	6014
	11th to 20th	Day	8% 4 2 5% 4 5% 4 4 7% 12% 45%	5951
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 6% 9% 48%	6176
September	1st to 10th	Day	8% 4 2 4 5% 5% 5% 5% 7% 9% 47%	2324
	11th to 20th	Day	12% 7% 3 5% 4 4 5% 4 6% 8% 41%	2310
	21st to end	Day	18% 6% 2 6% 4 4 5% 3 5% 7% 41%	2310
October	1st to 10th	Day	14% 4 2 4 3 4 3 3 4 5% 53%	2187
	11th to 20th	Day	14% 5% 2 4 3 3 3 2 3 4 59%	2030
	21st to end	Day	13% 5% 1 4 3 3 2 2 4 5% 58%	2200
November	1st to 10th	Day	14% 5% 2 5% 4 3 3 2 5% 4 55%	2057
	11th to 20th	Day	13% 6% 1 3 3 3 3 3 5% 7% 54%	1947
	21st to end	Day	15% 5% 1 5% 4 3 4 3 4 6% 51%	1890
December	1st to 10th	Day	15% 6% 3 5% 3 3 3 2 4 5% 50%	3020
	11th to 20th	Day	17% 8% 2 6% 4 4 3 3 5% 6% 42%	3463
	21st to end	Day	21% 6% 2 4 3 3 4 3 5% 6% 45%	4565
January	1st to 10th	Night	13% 7% 3 5% 3 3 2 2 3 4 55%	7635
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 4 5% 56%	7680
	21st to end	Night	19% 8% 3 5% 3 3 3 2 4 5% 44%	8401
February	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 5% 5% 47%	2730
	11th to 20th	Night	21% 7% 2 5% 4 3 4 3 4 4 43%	2468
	21st to end	Night	17% 7% 2 5% 4 3 4 4 5% 6% 45%	2024
March	1st to 10th	Night	20% 7% 2 6% 4 3 3 3 4 6% 43%	5028
	11th to 20th	Night	10% 5% 2 4 3 3 4 4 5% 8% 54%	5090
	21st to end	Night	15% 5% 2 5% 4 4 4 4 5% 6% 48%	5588
April	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 7% 49%	2310
	11th to 20th	Night	11% 4 2 4 4 3 3 3 5% 7% 53%	2089
	21st to end	Night	11% 5% 1 5% 3 4 4 3 5% 7% 51%	1958
May	1st to 10th	Night	14% 6% 3 7% 5% 4 4 3 5% 9% 40%	5237
	11th to 20th	Night	16% 6% 2 6% 4 4 4 4 7% 8% 40%	5234
	21st to end	Night	17% 7% 3 7% 4 5% 5% 4 6% 9% 33%	5761
June	1st to 10th	Night	10% 6% 3 7% 6% 6% 6% 5% 8% 9% 36%	1890
	11th to 20th	Night	11% 5% 2 6% 6% 5% 5% 6% 7% 11% 37%	1890
	21st to end	Night	6% 6% 2 7% 6% 5% 6% 5% 8% 12% 37%	1890
July	1st to 10th	Night	9% 7% 2 8% 6% 6% 5% 5% 8% 11% 33%	5278
	11th to 20th	Night	9% 6% 2 8% 6% 6% 8% 6% 10% 12% 26%	5277
	21st to end	Night	13% 8% 4 9% 6% 6% 7% 5% 7% 10% 24%	5736
August	1st to 10th	Night	21% 9% 3 8% 6% 5% 6% 5% 8% 9% 19%	4135
	11th to 20th	Night	8% 6% 2 6% 5% 5% 6% 5% 8% 12% 36%	4095
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 4 5% 8% 46%	4902
September	1st to 10th	Night	9% 4 2 5% 5% 5% 5% 4 7% 9% 46%	2291
	11th to 20th	Night	13% 6% 2 7% 5% 4 5% 4 6% 8% 39%	2310
	21st to end	Night	18% 5% 2 6% 5% 4 4 4 5% 6% 40%	2310
October	1st to 10th	Night	14% 5% 2 4 3 3 3 2 4 6% 52%	2431
	11th to 20th	Night	13% 5% 2 5% 3 3 3 2 4 6% 54%	2437
	21st to end	Night	13% 5% 1 4 3 3 2 2 3 5% 57%	2640
November	1st to 10th	Night	14% 6% 2 4 3 3 3 3 3 6% 53%	2461
	11th to 20th	Night	12% 5% 2 4 4 3 3 3 5% 5% 53%	2670
	21st to end	Night	13% 4 2 4 3 4 4 3 5% 7% 50%	2730
December	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 4 5% 51%	4380
	11th to 20th	Night	18% 7% 2 6% 4 3 3 3 4 6% 44%	4952
	21st to end	Night	20% 4 2 5% 3 3 3 3 4 5% 46%	6633

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	37% 3 9% 21% 25% 5%	5259
	11th to 20th	Day	30% 3 4 12% 38% 13%	5280
	21st to end	Day	47% 3 4 16% 20% 10%	5780
February	1st to 10th	Day	40% 4 10% 21% 20% 5%	1890
	11th to 20th	Day	43% 3 7% 19% 22% 5%	2044
	21st to end	Day	45% 4 7% 16% 20% 7%	1782
March	1st to 10th	Day	45% 4 6% 14% 20% 11%	5045
	11th to 20th	Day	31% 4 7% 14% 25% 18%	5090
	21st to end	Day	41% 4 6% 21% 23% 6%	5586
April	1st to 10th	Day	32% 4 11% 22% 25% 6%	2310
	11th to 20th	Day	35% 4 9% 21% 22% 8%	2478
	21st to end	Day	34% 6% 12% 22% 22% 4	2679
May	1st to 10th	Day	41% 7% 15% 22% 13% 1	7651
	11th to 20th	Day	41% 7% 13% 22% 15% 2	7630
	21st to end	Day	45% 8% 13% 17% 14% 2	8405
June	1st to 10th	Day	37% 8% 15% 25% 12% 2	2730
	11th to 20th	Day	33% 9% 19% 23% 14% 1	2730
	21st to end	Day	33% 10% 17% 23% 15% 2	2730
July	1st to 10th	Day	33% 11% 22% 27% 6% 1	7615
	11th to 20th	Day	39% 13% 27% 17% 4	7611
	21st to end	Day	44% 11% 16% 20% 9% 1	8156
August	1st to 10th	Day	52% 13% 19% 12% 3	6001
	11th to 20th	Day	29% 9% 23% 27% 10% 1	5990
	21st to end	Day	30% 8% 17% 29% 14% 2	6176
September	1st to 10th	Day	32% 8% 15% 21% 20% 4	2324
	11th to 20th	Day	39% 8% 11% 18% 20% 3	2310
	21st to end	Day	43% 6% 9% 20% 17% 5%	2310
October	1st to 10th	Day	30% 5% 7% 20% 29% 9%	3499
	11th to 20th	Day	20% 2 6% 18% 29% 25%	3270
	21st to end	Day	26% 3 5% 24% 26% 16%	3538
November	1st to 10th	Day	33% 3 8% 18% 28% 9%	2057
	11th to 20th	Day	30% 3 8% 23% 28% 8%	1947
	21st to end	Day	35% 4 8% 21% 24% 7%	1890
December	1st to 10th	Day	37% 3 6% 18% 27% 9%	3020
	11th to 20th	Day	44% 4 7% 18% 21% 6%	3460
	21st to end	Day	40% 5% 8% 25% 20% 3	4565
January	1st to 10th	Night	34% 3 9% 21% 27% 6%	7636
	11th to 20th	Night	32% 3 5% 15% 34% 11%	7680
	21st to end	Night	44% 3 3 17% 24% 9%	8401
February	1st to 10th	Night	40% 4 9% 21% 21% 6%	2730
	11th to 20th	Night	45% 3 7% 16% 23% 6%	2469
	21st to end	Night	40% 3 10% 16% 22% 8%	2024
March	1st to 10th	Night	42% 5% 7% 13% 26% 7%	5028
	11th to 20th	Night	29% 4 7% 14% 28% 18%	5090
	21st to end	Night	38% 4 7% 22% 21% 8%	5588
April	1st to 10th	Night	35% 4 11% 19% 24% 7%	2310
	11th to 20th	Night	30% 4 10% 23% 24% 9%	2089
	21st to end	Night	33% 5% 11% 27% 21% 4	1958
May	1st to 10th	Night	42% 6% 14% 23% 12% 2	5264
	11th to 20th	Night	40% 7% 14% 24% 13% 2	5235
	21st to end	Night	46% 9% 13% 20% 11% 2	5771
June	1st to 10th	Night	41% 10% 14% 19% 13% 2	1890
	11th to 20th	Night	39% 10% 17% 21% 11% 1	1890
	21st to end	Night	35% 12% 17% 20% 12% 3	1890
July	1st to 10th	Night	41% 11% 21% 21% 6% 1	5235
	11th to 20th	Night	44% 16% 24% 10% 5% 1	5226
	21st to end	Night	51% 11% 17% 14% 6% 2	5606
August	1st to 10th	Night	57% 11% 16% 12% 3	4124
	11th to 20th	Night	35% 11% 27% 20% 6%	4122
	21st to end	Night	34% 8% 12% 28% 16% 2	4902
September	1st to 10th	Night	33% 9% 14% 22% 17% 5%	2291
	11th to 20th	Night	41% 8% 13% 17% 20% 2	2310
	21st to end	Night	43% 7% 11% 17% 17% 5%	2310
October	1st to 10th	Night	34% 5% 10% 19% 26% 6%	3899
	11th to 20th	Night	23% 3 5% 18% 28% 24%	3936
	21st to end	Night	28% 3 6% 21% 26% 17%	4252
November	1st to 10th	Night	34% 3 7% 18% 27% 11%	2461
	11th to 20th	Night	34% 4 6% 18% 27% 12%	2670
	21st to end	Night	34% 4 6% 22% 26% 8%	2730
December	1st to 10th	Night	36% 3 6% 19% 28% 8%	4380
	11th to 20th	Night	43% 4 10% 15% 18% 9%	4954
	21st to end	Night	40% 3 10% 23% 22% 2	6633

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	22% 22% 38% 16% 2	330
	11th to 20th	Day	26% 22% 32% 16% 4	330
	21st to end	Day	23% 23% 39% 11% 3	363
February	1st to 10th	Day	20% 21% 41% 13% 5%	330
	11th to 20th	Day	37% 20% 34% 8% 1	330
	21st to end	Day	28% 21% 33% 14% 3	325
March	1st to 10th	Day	24% 17% 32% 20% 6% 1	540
	11th to 20th	Day	22% 16% 38% 19% 5% 1	540
	21st to end	Day	21% 18% 36% 19% 6% 1	594
April	1st to 10th	Day	19% 17% 33% 21% 9% 1	540
	11th to 20th	Day	15% 15% 33% 22% 11% 3	532
	21st to end	Day	16% 9% 29% 27% 14% 4	540
May	1st to 10th	Day	12% 15% 34% 22% 13% 4	543
	11th to 20th	Day	18% 5% 23% 27% 20% 7%	567
	21st to end	Day	15% 8% 20% 25% 23% 10%	617
June	1st to 10th	Day	10% 4 18% 33% 25% 9%	570
	11th to 20th	Day	9% 5% 21% 26% 27% 12%	570
	21st to end	Day	7% 2 16% 35% 29% 10%	570
July	1st to 10th	Day	8% 1 10% 29% 30% 21%	565
	11th to 20th	Day	9% 3 7% 30% 35% 16%	564
	21st to end	Day	7% 3 12% 31% 35% 12%	620
August	1st to 10th	Day	18% 4 11% 30% 21% 15%	566
	11th to 20th	Day	14% 3 10% 26% 27% 20%	559
	21st to end	Day	11% 3 11% 31% 27% 17%	627
September	1st to 10th	Day	9% 4 12% 32% 30% 13%	570
	11th to 20th	Day	16% 4 14% 25% 28% 14%	570
	21st to end	Day	20% 7% 16% 26% 25% 6%	570
October	1st to 10th	Day	16% 5% 23% 25% 24% 8%	570
	11th to 20th	Day	12% 9% 25% 27% 23% 5%	542
	21st to end	Day	17% 9% 25% 26% 18% 5%	583
November	1st to 10th	Day	19% 8% 27% 28% 15% 3	535
	11th to 20th	Day	18% 13% 35% 25% 6% 2	387
	21st to end	Day	17% 12% 35% 24% 11% 2	330
December	1st to 10th	Day	29% 14% 37% 15% 5%	330
	11th to 20th	Day	23% 11% 35% 22% 8% 1	330
	21st to end	Day	26% 14% 36% 15% 7% 2	363
January	1st to 10th	Night	21% 18% 29% 25% 6%	840
	11th to 20th	Night	20% 15% 31% 29% 5%	840
	21st to end	Night	20% 15% 36% 26% 4	924
February	1st to 10th	Night	18% 12% 35% 30% 4	840
	11th to 20th	Night	26% 15% 37% 21% 1	823
	21st to end	Night	21% 17% 35% 23% 4	692
March	1st to 10th	Night	18% 13% 33% 30% 6%	840
	11th to 20th	Night	18% 15% 34% 25% 7% 1	840
	21st to end	Night	19% 14% 34% 27% 7%	924
April	1st to 10th	Night	17% 15% 33% 26% 9%	840
	11th to 20th	Night	13% 13% 28% 31% 14% 1	831
	21st to end	Night	13% 7% 29% 36% 15%	698
May	1st to 10th	Night	11% 10% 30% 35% 14%	630
	11th to 20th	Night	13% 6% 23% 36% 20% 1	621
	21st to end	Night	15% 4 15% 35% 29% 2	683
June	1st to 10th	Night	11% 4 17% 39% 25% 5%	630
	11th to 20th	Night	13% 3 15% 30% 30% 8%	630
	21st to end	Night	8% 3 11% 39% 34% 6%	630
July	1st to 10th	Night	10% 2 11% 32% 37% 8%	615
	11th to 20th	Night	10% 2 7% 33% 40% 8%	612
	21st to end	Night	8% 3 11% 30% 41% 7%	690
August	1st to 10th	Night	17% 2 8% 30% 35% 8%	627
	11th to 20th	Night	12% 3 8% 29% 39% 8%	624
	21st to end	Night	11% 2 10% 30% 41% 7%	856
September	1st to 10th	Night	9% 3 10% 32% 39% 6%	840
	11th to 20th	Night	15% 3 11% 28% 37% 6%	840
	21st to end	Night	20% 5% 16% 31% 25% 3	840
October	1st to 10th	Night	15% 5% 16% 32% 26% 5%	840
	11th to 20th	Night	17% 7% 22% 31% 20% 2	813
	21st to end	Night	16% 9% 22% 31% 20% 3	880
November	1st to 10th	Night	16% 7% 25% 33% 17% 3	820
	11th to 20th	Night	15% 11% 29% 33% 11% 1	840
	21st to end	Night	14% 11% 28% 33% 13% 1	840
December	1st to 10th	Night	22% 12% 26% 33% 6% 1	840
	11th to 20th	Night	20% 11% 25% 33% 11% 1	840
	21st to end	Night	24% 13% 23% 31% 9%	924

Page generated at 2026-10-11 05:06:28. Version ac07

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