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 2024-03-23.

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	14% 5% 2 5% 5% 3 5% 5% 5% 8% 44%	79126
	Night	14% 6% 2 5% 5% 3 4 4 5% 7% 45%	80773
Transparency	Day	38% 6% 12% 20% 19% 5%	78849
	Night	38% 6% 11% 19% 20% 6%	80569
Seeing	Day	17% 9% 24% 24% 18% 8%	14383
	Night	16% 9% 23% 30% 19% 3	24563

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	18% 5% 2 4 4 2 3 3 4 6% 50%	2836
	11th to 20th	Day	14% 6% 2 5% 4 2 3 3 4 5% 52%	2840
	21st to end	Day	23% 8% 2 4 4 2 3 4 4 5% 42%	3124
February	1st to 10th	Day	16% 7% 2 4 5% 3 5% 5% 5% 8% 42%	1710
	11th to 20th	Day	21% 5% 2 4 4 2 4 3 3 5% 46%	1834
	21st to end	Day	20% 7% 2 6% 5% 3 4 4 3 5% 41%	1617
March	1st to 10th	Day	17% 4 2 3 4 2 3 4 4 6% 51%	2090
	11th to 20th	Day	17% 5% 3 5% 5% 2 5% 5% 5% 7% 41%	2090
	21st to end	Day	17% 5% 3 5% 5% 3 5% 5% 4 7% 41%	2200
April	1st to 10th	Day	12% 4 2 3 3 3 3 5% 5% 8% 51%	1980
	11th to 20th	Day	14% 5% 2 5% 4 3 5% 4 5% 7% 48%	2120
	21st to end	Day	13% 4 2 4 4 3 5% 4 4 7% 50%	2295
May	1st to 10th	Day	9% 4 2 6% 5% 3 6% 5% 6% 8% 46%	2340
	11th to 20th	Day	13% 5% 3 6% 5% 3 5% 6% 6% 8% 41%	2301
	21st to end	Day	11% 6% 2 6% 5% 4 5% 5% 6% 9% 40%	2531
June	1st to 10th	Day	10% 4 2 6% 5% 4 6% 6% 7% 10% 39%	2340
	11th to 20th	Day	8% 5% 2 5% 5% 4 6% 6% 6% 11% 43%	2340
	21st to end	Day	7% 3 2 5% 7% 5% 6% 7% 7% 11% 41%	2340
July	1st to 10th	Day	9% 5% 3 7% 7% 4 6% 6% 8% 11% 35%	2340
	11th to 20th	Day	8% 5% 2 6% 7% 5% 7% 7% 7% 12% 34%	2340
	21st to end	Day	7% 5% 3 6% 6% 4 7% 6% 8% 12% 36%	2547
August	1st to 10th	Day	12% 6% 3 5% 6% 4 7% 8% 8% 10% 32%	2327
	11th to 20th	Day	14% 6% 2 6% 7% 5% 6% 6% 7% 10% 31%	2303
	21st to end	Day	11% 5% 3 6% 6% 4 6% 6% 7% 10% 36%	2416
September	1st to 10th	Day	9% 4 2 4 6% 4 5% 6% 7% 9% 44%	1993
	11th to 20th	Day	12% 7% 3 4 5% 3 5% 5% 6% 8% 42%	1980
	21st to end	Day	18% 6% 2 5% 5% 3 5% 4 4 6% 42%	1980
October	1st to 10th	Day	15% 4 2 3 4 2 3 3 4 5% 54%	1875
	11th to 20th	Day	14% 5% 2 4 3 2 3 3 3 5% 57%	1800
	21st to end	Day	13% 5% 2 4 3 2 2 3 4 5% 57%	1980
November	1st to 10th	Day	14% 5% 2 4 4 2 2 2 4 5% 57%	1857
	11th to 20th	Day	13% 5% 2 3 4 2 3 3 4 7% 53%	1763
	21st to end	Day	16% 5% 1 4 4 2 5% 4 4 5% 50%	1710
December	1st to 10th	Day	19% 7% 3 5% 4 3 4 4 5% 7% 39%	1710
	11th to 20th	Day	17% 5% 2 4 3 2 2 3 4 6% 51%	2153
	21st to end	Day	19% 7% 2 4 4 2 4 5% 5% 6% 43%	3124
January	1st to 10th	Night	17% 6% 2 4 4 2 3 3 4 5% 49%	4109
	11th to 20th	Night	12% 7% 2 4 4 3 4 5% 5% 6% 48%	4120
	21st to end	Night	23% 7% 3 4 3 2 3 3 3 4 45%	4532
February	1st to 10th	Night	15% 7% 2 5% 5% 3 4 4 5% 5% 47%	2470
	11th to 20th	Night	21% 6% 2 5% 4 3 4 4 4 5% 43%	2228
	21st to end	Night	17% 6% 2 4 4 3 4 4 5% 6% 44%	1837
March	1st to 10th	Night	16% 6% 3 4 4 2 3 3 5% 6% 48%	2090
	11th to 20th	Night	15% 6% 2 5% 4 2 5% 4 5% 8% 43%	2090
	21st to end	Night	15% 6% 3 6% 5% 3 4 5% 5% 6% 42%	2201
April	1st to 10th	Night	13% 5% 3 4 5% 2 4 4 4 7% 50%	1980
	11th to 20th	Night	12% 4 2 4 4 3 3 3 5% 7% 52%	1788
	21st to end	Night	12% 5% 1 4 4 3 4 5% 4 7% 51%	1679
May	1st to 10th	Night	10% 4 2 5% 5% 4 5% 4 5% 8% 49%	1620
	11th to 20th	Night	12% 7% 2 6% 7% 4 6% 5% 7% 8% 36%	1592
	21st to end	Night	11% 5% 4 4 5% 4 5% 6% 5% 10% 41%	1744
June	1st to 10th	Night	10% 5% 3 6% 6% 4 7% 7% 8% 9% 35%	1620
	11th to 20th	Night	11% 4 2 5% 6% 5% 5% 7% 7% 11% 36%	1620
	21st to end	Night	7% 6% 3 6% 7% 4 6% 6% 8% 12% 35%	1620
July	1st to 10th	Night	8% 7% 2 7% 7% 4 7% 7% 9% 11% 31%	1620
	11th to 20th	Night	8% 6% 2 6% 5% 3 7% 8% 9% 12% 35%	1620
	21st to end	Night	7% 5% 3 6% 7% 4 8% 7% 8% 11% 34%	1771
August	1st to 10th	Night	12% 6% 2 6% 6% 3 6% 6% 9% 13% 31%	1611
	11th to 20th	Night	10% 8% 3 7% 5% 4 5% 6% 8% 9% 35%	1598
	21st to end	Night	10% 6% 3 7% 7% 4 5% 7% 8% 11% 32%	1921
September	1st to 10th	Night	9% 4 2 5% 6% 4 5% 5% 7% 9% 44%	1963
	11th to 20th	Night	13% 6% 2 6% 6% 3 5% 5% 6% 8% 41%	1980
	21st to end	Night	17% 6% 2 6% 6% 3 4 5% 5% 7% 40%	1980
October	1st to 10th	Night	15% 5% 2 3 4 2 3 3 4 5% 53%	2083
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 4 6% 53%	2160
	21st to end	Night	13% 5% 2 4 4 2 3 3 3 6% 56%	2376
November	1st to 10th	Night	14% 5% 2 4 3 1 3 3 3 6% 55%	2221
	11th to 20th	Night	13% 5% 2 4 5% 2 4 3 4 5% 52%	2415
	21st to end	Night	14% 4 3 4 4 3 4 4 5% 7% 49%	2470
December	1st to 10th	Night	19% 7% 2 5% 5% 2 4 4 3 5% 43%	2470
	11th to 20th	Night	17% 6% 2 4 4 2 3 3 3 5% 52%	3042
	21st to end	Night	20% 5% 2 4 4 3 4 4 5% 7% 43%	4532

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	36% 4 6% 25% 23% 7%	2837
	11th to 20th	Day	36% 3 7% 16% 31% 6%	2840
	21st to end	Day	45% 4 4 17% 25% 5%	3124
February	1st to 10th	Day	41% 4 10% 20% 19% 5%	1710
	11th to 20th	Day	42% 4 7% 18% 23% 6%	1844
	21st to end	Day	46% 4 7% 17% 19% 7%	1617
March	1st to 10th	Day	36% 4 7% 20% 24% 8%	2090
	11th to 20th	Day	42% 5% 10% 18% 18% 7%	2090
	21st to end	Day	44% 5% 10% 20% 18% 3	2200
April	1st to 10th	Day	31% 4 11% 22% 25% 6%	1980
	11th to 20th	Day	37% 4 9% 19% 21% 9%	2120
	21st to end	Day	33% 6% 11% 23% 22% 4	2295
May	1st to 10th	Day	34% 8% 14% 22% 19% 3	2340
	11th to 20th	Day	40% 6% 16% 20% 16% 2	2305
	21st to end	Day	40% 7% 14% 23% 14% 2	2531
June	1st to 10th	Day	38% 8% 14% 26% 12% 2	2340
	11th to 20th	Day	35% 9% 18% 22% 14% 2	2340
	21st to end	Day	35% 11% 17% 21% 15% 2	2340
July	1st to 10th	Day	40% 10% 20% 17% 10% 3	2275
	11th to 20th	Day	39% 11% 21% 19% 8% 1	2271
	21st to end	Day	37% 11% 20% 22% 9% 1	2405
August	1st to 10th	Day	43% 10% 18% 20% 8% 1	2314
	11th to 20th	Day	46% 11% 16% 17% 10% 1	2303
	21st to end	Day	41% 9% 18% 19% 9% 3	2416
September	1st to 10th	Day	35% 8% 15% 20% 18% 4	1993
	11th to 20th	Day	39% 8% 11% 17% 21% 4	1980
	21st to end	Day	44% 6% 7% 21% 17% 5%	1980
October	1st to 10th	Day	33% 5% 8% 20% 28% 5%	1875
	11th to 20th	Day	31% 3 9% 24% 24% 8%	1800
	21st to end	Day	31% 3 6% 22% 28% 9%	1980
November	1st to 10th	Day	32% 3 8% 18% 30% 9%	1857
	11th to 20th	Day	32% 3 8% 21% 28% 8%	1763
	21st to end	Day	36% 5% 8% 20% 23% 8%	1710
December	1st to 10th	Day	45% 4 8% 21% 18% 4	1710
	11th to 20th	Day	36% 2 7% 21% 26% 9%	2150
	21st to end	Day	41% 6% 8% 21% 21% 3	3124
January	1st to 10th	Night	37% 4 7% 19% 26% 7%	4109
	11th to 20th	Night	36% 5% 8% 16% 28% 7%	4120
	21st to end	Night	45% 3 4 17% 26% 6%	4532
February	1st to 10th	Night	40% 4 8% 20% 22% 6%	2470
	11th to 20th	Night	45% 2 6% 16% 23% 7%	2229
	21st to end	Night	41% 3 11% 17% 21% 7%	1837
March	1st to 10th	Night	38% 4 9% 17% 23% 10%	2090
	11th to 20th	Night	39% 5% 10% 17% 21% 8%	2090
	21st to end	Night	43% 5% 10% 19% 19% 5%	2201
April	1st to 10th	Night	35% 4 9% 19% 25% 7%	1980
	11th to 20th	Night	32% 4 9% 22% 23% 9%	1788
	21st to end	Night	33% 4 10% 27% 21% 4	1679
May	1st to 10th	Night	33% 7% 16% 22% 18% 4	1620
	11th to 20th	Night	43% 9% 12% 18% 16% 1	1593
	21st to end	Night	38% 9% 13% 24% 14% 2	1745
June	1st to 10th	Night	41% 10% 15% 20% 12% 2	1620
	11th to 20th	Night	40% 10% 16% 21% 11% 2	1620
	21st to end	Night	38% 12% 17% 18% 11% 3	1620
July	1st to 10th	Night	42% 11% 21% 15% 8% 3	1575
	11th to 20th	Night	36% 11% 25% 18% 7% 2	1566
	21st to end	Night	39% 13% 20% 19% 9% 1	1672
August	1st to 10th	Night	41% 11% 22% 19% 7% 1	1600
	11th to 20th	Night	43% 11% 20% 15% 11% 1	1598
	21st to end	Night	42% 10% 17% 20% 8% 2	1921
September	1st to 10th	Night	34% 10% 15% 20% 16% 5%	1963
	11th to 20th	Night	40% 8% 14% 16% 20% 2	1980
	21st to end	Night	43% 7% 10% 18% 17% 5%	1980
October	1st to 10th	Night	33% 5% 10% 20% 27% 5%	2083
	11th to 20th	Night	33% 4 8% 23% 22% 10%	2160
	21st to end	Night	31% 3 6% 21% 27% 12%	2376
November	1st to 10th	Night	33% 3 7% 18% 27% 12%	2221
	11th to 20th	Night	35% 4 6% 17% 25% 12%	2415
	21st to end	Night	35% 4 6% 21% 26% 7%	2470
December	1st to 10th	Night	45% 4 8% 17% 20% 6%	2470
	11th to 20th	Night	37% 3 7% 18% 25% 10%	3044
	21st to end	Night	41% 4 9% 21% 23% 3	4532

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	21% 20% 39% 17% 2	270
	11th to 20th	Day	27% 20% 31% 17% 4	270
	21st to end	Day	22% 22% 40% 11% 4	297
February	1st to 10th	Day	23% 21% 37% 13% 6%	270
	11th to 20th	Day	35% 19% 35% 9% 1	270
	21st to end	Day	28% 21% 33% 15% 3	272
March	1st to 10th	Day	23% 16% 32% 20% 7% 1	460
	11th to 20th	Day	22% 15% 40% 17% 5% 1	460
	21st to end	Day	23% 16% 35% 19% 6% 1	470
April	1st to 10th	Day	18% 17% 34% 20% 10% 1	420
	11th to 20th	Day	17% 16% 32% 20% 12% 3	412
	21st to end	Day	18% 8% 30% 25% 15% 4	420
May	1st to 10th	Day	11% 14% 35% 21% 13% 4	423
	11th to 20th	Day	17% 4 21% 26% 23% 9%	447
	21st to end	Day	15% 4 18% 25% 27% 12%	485
June	1st to 10th	Day	11% 3 16% 32% 27% 10%	450
	11th to 20th	Day	10% 4 19% 25% 28% 13%	450
	21st to end	Day	9% 2 16% 35% 28% 10%	450
July	1st to 10th	Day	9% 2 9% 30% 29% 21%	445
	11th to 20th	Day	9% 3 8% 30% 33% 17%	444
	21st to end	Day	6% 2 11% 31% 37% 13%	488
August	1st to 10th	Day	14% 2 13% 30% 22% 18%	446
	11th to 20th	Day	15% 3 10% 25% 27% 19%	439
	21st to end	Day	13% 3 11% 29% 26% 18%	495
September	1st to 10th	Day	10% 3 11% 30% 32% 15%	450
	11th to 20th	Day	16% 4 14% 25% 28% 14%	450
	21st to end	Day	19% 7% 15% 27% 26% 6%	450
October	1st to 10th	Day	17% 4 23% 24% 23% 9%	450
	11th to 20th	Day	12% 10% 24% 26% 24% 5%	450
	21st to end	Day	17% 9% 24% 27% 19% 4	495
November	1st to 10th	Day	20% 7% 25% 27% 16% 4	455
	11th to 20th	Day	18% 13% 36% 25% 7% 2	323
	21st to end	Day	16% 10% 36% 24% 11% 2	270
December	1st to 10th	Day	29% 12% 39% 15% 6%	270
	11th to 20th	Day	22% 12% 33% 22% 9% 1	270
	21st to end	Day	27% 14% 36% 14% 6% 3	297
January	1st to 10th	Night	21% 18% 28% 26% 6%	760
	11th to 20th	Night	20% 15% 30% 30% 5%	760
	21st to end	Night	20% 14% 35% 27% 4	836
February	1st to 10th	Night	19% 12% 36% 30% 4	760
	11th to 20th	Night	27% 14% 37% 21% 1	743
	21st to end	Night	21% 18% 35% 22% 4	628
March	1st to 10th	Night	19% 12% 33% 30% 6%	760
	11th to 20th	Night	19% 15% 32% 25% 8% 1	760
	21st to end	Night	19% 14% 32% 27% 7%	801
April	1st to 10th	Night	18% 15% 31% 26% 10%	720
	11th to 20th	Night	14% 13% 27% 31% 14% 1	711
	21st to end	Night	14% 8% 29% 34% 15% 1	599
May	1st to 10th	Night	11% 10% 29% 36% 14%	540
	11th to 20th	Night	13% 6% 23% 36% 21% 1	531
	21st to end	Night	14% 3 12% 37% 32% 2	584
June	1st to 10th	Night	11% 4 16% 39% 26% 5%	540
	11th to 20th	Night	14% 2 15% 29% 31% 9%	540
	21st to end	Night	10% 3 11% 38% 33% 6%	540
July	1st to 10th	Night	11% 2 10% 32% 36% 9%	525
	11th to 20th	Night	10% 2 6% 32% 41% 8%	522
	21st to end	Night	7% 3 10% 31% 42% 7%	591
August	1st to 10th	Night	14% 1 7% 31% 36% 10%	537
	11th to 20th	Night	13% 3 8% 29% 40% 7%	534
	21st to end	Night	12% 2 9% 27% 43% 7%	733
September	1st to 10th	Night	10% 3 9% 32% 41% 6%	720
	11th to 20th	Night	14% 3 12% 27% 38% 6%	720
	21st to end	Night	19% 5% 15% 31% 27% 3	720
October	1st to 10th	Night	16% 5% 17% 30% 26% 6%	720
	11th to 20th	Night	17% 6% 22% 31% 22% 3	720
	21st to end	Night	15% 9% 22% 31% 20% 3	792
November	1st to 10th	Night	16% 7% 24% 31% 19% 3	740
	11th to 20th	Night	15% 11% 29% 33% 11% 1	760
	21st to end	Night	15% 10% 26% 34% 14% 1	760
December	1st to 10th	Night	22% 11% 27% 33% 7% 1	760
	11th to 20th	Night	20% 10% 25% 33% 12% 1	760
	21st to end	Night	25% 13% 22% 31% 9%	836

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