Peters Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peters Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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% 4 4 4 5% 7% 29%	149820
	Night	18% 13% 5% 11% 6% 4 4 3 5% 5% 26%	141592
Transparency	Day	54% 5% 12% 19% 8% 1	153313
	Night	59% 5% 11% 17% 8% 1	146188
Seeing	Day	27% 18% 28% 17% 8% 2	17117
	Night	26% 17% 27% 22% 7% 1	26196

By Month

Percentage Cloud Cover NE (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	13% 21% 8% 24% 8% 4 3 2 2 4 12%	4661
	11th to 20th	Day	12% 20% 10% 21% 9% 5% 4 3 4 3 10%	4650
	21st to end	Day	13% 18% 7% 12% 11% 7% 6% 3 4 5% 14%	5559
February	1st to 10th	Day	15% 17% 5% 18% 9% 6% 5% 4 4 4 15%	1890
	11th to 20th	Day	14% 19% 7% 16% 8% 5% 4 3 4 4 17%	1919
	21st to end	Day	18% 18% 6% 16% 7% 5% 4 3 3 4 17%	1730
March	1st to 10th	Day	22% 15% 6% 12% 6% 5% 3 3 3 3 22%	4576
	11th to 20th	Day	14% 19% 5% 8% 4 4 3 3 5% 5% 30%	5079
	21st to end	Day	18% 13% 5% 11% 4 4 3 3 4 5% 29%	5914
April	1st to 10th	Day	19% 16% 4 9% 5% 4 3 3 4 5% 29%	2520
	11th to 20th	Day	19% 13% 3 6% 5% 4 4 3 6% 5% 33%	2494
	21st to end	Day	22% 12% 3 7% 5% 4 3 3 5% 5% 31%	2530
May	1st to 10th	Day	29% 9% 3 6% 4 4 4 3 5% 6% 26%	8174
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 4 5% 7% 32%	8221
	21st to end	Day	23% 8% 3 7% 4 4 5% 4 6% 8% 29%	9060
June	1st to 10th	Day	18% 8% 3 7% 5% 5% 4 4 6% 8% 32%	2940
	11th to 20th	Day	20% 6% 2 6% 4 5% 6% 4 6% 9% 32%	2940
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 7% 9% 32%	2940
July	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 10% 35%	8280
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 10% 41%	8280
	21st to end	Day	15% 6% 2 6% 5% 4 5% 4 6% 7% 39%	8993
August	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 6% 7% 32%	6209
	11th to 20th	Day	16% 6% 2 7% 5% 5% 6% 5% 8% 9% 33%	5700
	21st to end	Day	16% 6% 2 7% 5% 5% 4 5% 6% 9% 35%	6080
September	1st to 10th	Day	15% 6% 2 5% 4 4 5% 4 6% 8% 41%	2520
	11th to 20th	Day	15% 7% 2 7% 4 4 3 3 5% 7% 44%	2499
	21st to end	Day	17% 6% 3 7% 5% 5% 5% 4 6% 7% 35%	2258
October	1st to 10th	Day	20% 8% 3 8% 5% 4 4 3 5% 7% 34%	2000
	11th to 20th	Day	20% 9% 3 7% 5% 4 5% 4 5% 6% 33%	2000
	21st to end	Day	26% 13% 3 7% 4 4 4 3 5% 6% 24%	2069
November	1st to 10th	Day	17% 13% 3 8% 4 5% 5% 4 4 6% 31%	1851
	11th to 20th	Day	15% 18% 5% 9% 6% 4 3 3 4 4 28%	1810
	21st to end	Day	19% 19% 6% 10% 5% 4 4 3 5% 5% 20%	1680
December	1st to 10th	Day	20% 22% 10% 17% 6% 4 4 2 3 4 10%	2680
	11th to 20th	Day	19% 19% 10% 15% 6% 4 3 2 3 3 16%	3066
	21st to end	Day	22% 19% 7% 10% 5% 4 4 3 4 5% 18%	4048
January	1st to 10th	Night	15% 21% 9% 25% 7% 4 3 2 3 3 8%	8234
	11th to 20th	Night	11% 21% 9% 21% 8% 5% 4 3 4 4 11%	8235
	21st to end	Night	12% 15% 5% 15% 11% 8% 6% 3 4 4 15%	8604
February	1st to 10th	Night	15% 17% 6% 16% 8% 5% 4 4 4 4 17%	2730
	11th to 20th	Night	14% 15% 7% 16% 8% 5% 5% 4 4 5% 16%	2585
	21st to end	Night	16% 17% 7% 16% 7% 5% 4 2 4 5% 18%	2076
March	1st to 10th	Night	19% 11% 8% 13% 8% 4 4 3 4 4 21%	5484
	11th to 20th	Night	18% 17% 5% 8% 4 3 3 3 4 6% 28%	5098
	21st to end	Night	23% 13% 5% 12% 4 4 3 3 4 4 26%	5260
April	1st to 10th	Night	22% 15% 4 7% 4 4 4 3 5% 5% 27%	2100
	11th to 20th	Night	22% 11% 3 7% 5% 5% 4 4 4 6% 32%	2075
	21st to end	Night	21% 10% 3 7% 4 3 4 3 6% 5% 35%	2090
May	1st to 10th	Night	29% 11% 4 7% 4 4 4 3 4 5% 24%	4780
	11th to 20th	Night	17% 10% 3 8% 5% 4 4 3 6% 7% 31%	4639
	21st to end	Night	24% 9% 2 7% 4 5% 4 4 6% 6% 29%	5114
June	1st to 10th	Night	19% 6% 2 7% 4 5% 5% 3 5% 7% 37%	1680
	11th to 20th	Night	20% 6% 2 5% 4 4 4 3 5% 8% 37%	1680
	21st to end	Night	16% 6% 2 7% 4 4 5% 4 6% 7% 38%	1680
July	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 8% 35%	4680
	11th to 20th	Night	14% 5% 2 6% 4 4 5% 4 6% 8% 43%	4680
	21st to end	Night	14% 6% 2 6% 3 4 4 4 6% 8% 43%	5084
August	1st to 10th	Night	20% 9% 2 6% 4 4 4 3 6% 7% 34%	3923
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 4 7% 38%	4298
	21st to end	Night	14% 7% 2 7% 5% 5% 5% 4 6% 7% 39%	5045
September	1st to 10th	Night	15% 6% 2 6% 4 4 4 3 5% 7% 42%	2100
	11th to 20th	Night	16% 6% 2 5% 3 4 4 3 4 5% 47%	2120
	21st to end	Night	19% 6% 2 6% 4 4 5% 3 6% 7% 38%	2315
October	1st to 10th	Night	22% 6% 3 7% 5% 4 4 3 5% 6% 34%	2400
	11th to 20th	Night	22% 9% 3 7% 4 4 5% 4 4 6% 32%	2400
	21st to end	Night	27% 13% 3 7% 5% 4 4 3 4 5% 25%	2770
November	1st to 10th	Night	18% 12% 3 8% 5% 5% 4 3 5% 4 32%	2667
	11th to 20th	Night	19% 16% 4 8% 4 4 4 3 4 6% 27%	2808
	21st to end	Night	18% 19% 6% 9% 5% 4 4 4 5% 6% 21%	2940
December	1st to 10th	Night	18% 23% 10% 14% 5% 5% 4 3 4 3 11%	4720
	11th to 20th	Night	23% 19% 7% 13% 5% 3 3 3 3 4 18%	5348
	21st to end	Night	23% 19% 7% 10% 6% 4 3 2 3 4 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	79% 3 8% 9% 2	4662
	11th to 20th	Day	78% 4 7% 6% 4 1	4680
	21st to end	Day	71% 4 10% 11% 3 1	5559
February	1st to 10th	Day	74% 4 7% 11% 3	1890
	11th to 20th	Day	72% 5% 7% 12% 4	1928
	21st to end	Day	72% 3 8% 11% 6%	1730
March	1st to 10th	Day	68% 3 10% 12% 6% 1	4579
	11th to 20th	Day	56% 2 18% 13% 8% 2	5079
	21st to end	Day	59% 3 7% 15% 15% 1	5914
April	1st to 10th	Day	60% 3 7% 20% 9% 2	2520
	11th to 20th	Day	53% 4 10% 20% 11% 1	2494
	21st to end	Day	55% 3 8% 20% 11% 2	2530
May	1st to 10th	Day	59% 4 10% 16% 10% 1	8186
	11th to 20th	Day	51% 5% 11% 24% 8% 1	8225
	21st to end	Day	53% 5% 12% 22% 8%	9061
June	1st to 10th	Day	49% 6% 12% 20% 12% 1	2940
	11th to 20th	Day	48% 5% 14% 23% 9% 1	2940
	21st to end	Day	48% 6% 14% 21% 9% 1	2940
July	1st to 10th	Day	41% 8% 15% 25% 10%	8210
	11th to 20th	Day	35% 8% 16% 30% 10%	8205
	21st to end	Day	42% 7% 12% 28% 11%	8795
August	1st to 10th	Day	50% 7% 12% 18% 11% 1	6195
	11th to 20th	Day	44% 8% 22% 21% 5%	5739
	21st to end	Day	43% 8% 16% 28% 5%	6080
September	1st to 10th	Day	40% 6% 12% 30% 11%	2520
	11th to 20th	Day	41% 4 10% 28% 16% 1	2499
	21st to end	Day	48% 6% 11% 23% 12% 1	2258
October	1st to 10th	Day	45% 6% 12% 28% 9% 1	3260
	11th to 20th	Day	46% 5% 14% 22% 13%	3240
	21st to end	Day	47% 6% 15% 23% 9% 1	3320
November	1st to 10th	Day	54% 5% 11% 20% 10% 1	1851
	11th to 20th	Day	60% 4 10% 17% 8% 2	1810
	21st to end	Day	66% 5% 14% 11% 3	1680
December	1st to 10th	Day	80% 4 7% 7% 2	2680
	11th to 20th	Day	71% 6% 10% 10% 3	3066
	21st to end	Day	69% 4 9% 11% 6%	4048
January	1st to 10th	Night	83% 3 7% 5% 2	8234
	11th to 20th	Night	76% 5% 7% 7% 6%	8280
	21st to end	Night	70% 5% 10% 8% 6% 1	8604
February	1st to 10th	Night	71% 4 8% 12% 5%	2730
	11th to 20th	Night	71% 4 7% 12% 5%	2587
	21st to end	Night	71% 3 9% 11% 6%	2076
March	1st to 10th	Night	66% 3 11% 13% 7%	5494
	11th to 20th	Night	58% 3 16% 14% 5% 3	5099
	21st to end	Night	62% 4 4 19% 9% 1	5260
April	1st to 10th	Night	59% 5% 9% 17% 8% 1	2100
	11th to 20th	Night	54% 5% 9% 19% 12% 1	2075
	21st to end	Night	51% 4 11% 20% 13% 3	2090
May	1st to 10th	Night	62% 4 9% 14% 10% 1	4786
	11th to 20th	Night	51% 6% 12% 22% 8% 1	4640
	21st to end	Night	54% 5% 11% 19% 10% 1	5115
June	1st to 10th	Night	47% 5% 10% 24% 14% 1	1680
	11th to 20th	Night	44% 6% 14% 22% 12% 1	1680
	21st to end	Night	44% 6% 12% 26% 11% 1	1680
July	1st to 10th	Night	44% 8% 14% 21% 12%	4640
	11th to 20th	Night	37% 7% 13% 26% 15% 1	4632
	21st to end	Night	38% 6% 14% 26% 16% 1	4967
August	1st to 10th	Night	49% 6% 13% 21% 10% 1	3913
	11th to 20th	Night	45% 6% 16% 26% 6%	4325
	21st to end	Night	40% 9% 15% 26% 9%	5045
September	1st to 10th	Night	41% 5% 13% 28% 12% 1	2100
	11th to 20th	Night	39% 4 12% 28% 17% 1	2120
	21st to end	Night	46% 5% 11% 25% 12% 2	2315
October	1st to 10th	Night	50% 7% 11% 22% 9%	3920
	11th to 20th	Night	47% 5% 14% 18% 15% 1	3899
	21st to end	Night	51% 6% 13% 23% 7%	4469
November	1st to 10th	Night	54% 5% 10% 17% 11% 2	2667
	11th to 20th	Night	58% 5% 10% 19% 7% 2	2808
	21st to end	Night	65% 4 13% 14% 3	2940
December	1st to 10th	Night	76% 6% 9% 7% 1	4720
	11th to 20th	Night	70% 5% 7% 12% 6%	5348
	21st to end	Night	72% 4 8% 10% 6% 1	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	34% 23% 28% 11% 4	220
	11th to 20th	Day	41% 25% 27% 5% 1	220
	21st to end	Day	33% 27% 30% 9% 2	322
February	1st to 10th	Day	41% 27% 24% 8%	330
	11th to 20th	Day	38% 31% 25% 6%	406
	21st to end	Day	38% 24% 28% 8% 2	446
March	1st to 10th	Day	34% 25% 26% 12% 2	540
	11th to 20th	Day	39% 21% 25% 11% 3	540
	21st to end	Day	38% 23% 23% 12% 3 1	594
April	1st to 10th	Day	38% 25% 22% 10% 4	540
	11th to 20th	Day	34% 24% 23% 11% 5% 2	532
	21st to end	Day	36% 18% 26% 13% 6% 1	540
May	1st to 10th	Day	35% 17% 28% 12% 6% 2	543
	11th to 20th	Day	25% 15% 33% 18% 8% 2	567
	21st to end	Day	21% 15% 31% 21% 10% 3	617
June	1st to 10th	Day	23% 15% 28% 24% 8% 1	570
	11th to 20th	Day	22% 14% 31% 21% 11% 2	570
	21st to end	Day	20% 17% 34% 21% 6% 2	570
July	1st to 10th	Day	13% 9% 33% 28% 13% 4	565
	11th to 20th	Day	13% 9% 31% 30% 13% 3	564
	21st to end	Day	15% 10% 35% 26% 10% 3	620
August	1st to 10th	Day	16% 11% 26% 25% 17% 6%	566
	11th to 20th	Day	17% 10% 27% 28% 13% 5%	559
	21st to end	Day	15% 13% 30% 25% 12% 5%	627
September	1st to 10th	Day	17% 12% 27% 22% 15% 6%	570
	11th to 20th	Day	15% 11% 26% 22% 17% 10%	570
	21st to end	Day	21% 13% 29% 22% 13% 3	562
October	1st to 10th	Day	25% 12% 25% 19% 14% 5%	530
	11th to 20th	Day	26% 21% 29% 15% 8% 1	530
	21st to end	Day	36% 19% 25% 15% 5%	510
November	1st to 10th	Day	33% 20% 24% 16% 7%	425
	11th to 20th	Day	30% 28% 23% 14% 4	350
	21st to end	Day	42% 21% 29% 8%	220
December	1st to 10th	Day	36% 24% 29% 10% 1	220
	11th to 20th	Day	39% 27% 28% 5% 1	220
	21st to end	Day	37% 23% 31% 7% 1	242
January	1st to 10th	Night	31% 27% 29% 11% 1	840
	11th to 20th	Night	32% 25% 27% 14% 2 1	840
	21st to end	Night	25% 25% 32% 17% 2	924
February	1st to 10th	Night	30% 23% 30% 16% 1	840
	11th to 20th	Night	30% 26% 30% 13% 2	823
	21st to end	Night	31% 24% 28% 15% 2	692
March	1st to 10th	Night	28% 22% 30% 18% 2	840
	11th to 20th	Night	32% 22% 27% 17% 2	840
	21st to end	Night	31% 21% 24% 18% 5%	924
April	1st to 10th	Night	29% 19% 31% 18% 2	840
	11th to 20th	Night	29% 19% 28% 17% 7%	831
	21st to end	Night	30% 14% 27% 23% 6%	830
May	1st to 10th	Night	31% 11% 24% 29% 6%	462
	11th to 20th	Night	20% 8% 36% 29% 6%	414
	21st to end	Night	18% 7% 34% 31% 9% 1	455
June	1st to 10th	Night	19% 9% 26% 34% 12% 1	420
	11th to 20th	Night	20% 11% 24% 35% 10%	420
	21st to end	Night	15% 8% 27% 40% 9%	420
July	1st to 10th	Night	12% 5% 24% 43% 15% 1	410
	11th to 20th	Night	14% 4 24% 43% 14% 1	408
	21st to end	Night	16% 4 22% 43% 14% 1	460
August	1st to 10th	Night	14% 5% 25% 36% 18% 1	532
	11th to 20th	Night	14% 5% 22% 35% 21% 3	710
	21st to end	Night	15% 7% 25% 35% 17% 1	924
September	1st to 10th	Night	17% 8% 24% 32% 17% 1	840
	11th to 20th	Night	18% 7% 19% 30% 22% 3	840
	21st to end	Night	19% 11% 25% 30% 14% 1	833
October	1st to 10th	Night	26% 11% 22% 23% 16% 2	800
	11th to 20th	Night	27% 16% 28% 24% 6%	800
	21st to end	Night	35% 14% 28% 20% 2	880
November	1st to 10th	Night	28% 18% 30% 18% 6%	820
	11th to 20th	Night	29% 22% 27% 16% 6% 1	840
	21st to end	Night	30% 23% 28% 15% 4	840
December	1st to 10th	Night	34% 25% 25% 14% 3	840
	11th to 20th	Night	36% 22% 27% 12% 3	840
	21st to end	Night	34% 25% 27% 12% 2	924

Page generated at 2026-09-26 04:18:41. Version ac07

Copyright © 2026 Attilla Danko.