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 2025-05-26.

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% 12% 4 9% 5% 4 4 4 5% 6% 28%	114093
	Night	18% 13% 5% 11% 6% 4 4 3 4 5% 26%	115184
Transparency	Day	55% 5% 12% 19% 8% 1	117632
	Night	59% 5% 11% 16% 8% 1	119809
Seeing	Day	27% 18% 28% 17% 8% 2	15337
	Night	26% 17% 27% 22% 7% 1	24590

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	11% 21% 8% 24% 6% 4 3 2 3 4 14%	3597
	11th to 20th	Day	13% 21% 11% 20% 8% 4 3 3 4 3 12%	3570
	21st to end	Day	15% 20% 8% 12% 8% 5% 5% 3 4 4 15%	4300
February	1st to 10th	Day	16% 18% 5% 17% 9% 5% 5% 4 4 3 15%	1800
	11th to 20th	Day	14% 18% 7% 17% 8% 5% 4 3 4 4 17%	1825
	21st to end	Day	19% 18% 6% 15% 6% 4 4 3 3 4 18%	1650
March	1st to 10th	Day	22% 15% 6% 12% 6% 5% 3 2 3 3 21%	4476
	11th to 20th	Day	13% 19% 5% 8% 4 4 3 3 5% 5% 31%	4970
	21st to end	Day	18% 13% 5% 11% 4 4 3 3 4 5% 29%	5786
April	1st to 10th	Day	19% 16% 4 9% 5% 4 3 3 4 5% 29%	2400
	11th to 20th	Day	19% 13% 3 6% 4 4 4 3 6% 5% 33%	2374
	21st to end	Day	22% 12% 3 7% 5% 4 3 3 5% 6% 31%	2410
May	1st to 10th	Day	26% 10% 3 6% 4 3 4 3 4 6% 30%	4530
	11th to 20th	Day	18% 9% 3 7% 5% 5% 5% 4 6% 8% 31%	4538
	21st to end	Day	14% 6% 3 6% 4 4 5% 4 6% 9% 38%	4907
June	1st to 10th	Day	18% 8% 3 7% 5% 5% 4 4 6% 8% 31%	2660
	11th to 20th	Day	19% 5% 2 6% 4 5% 6% 4 6% 9% 33%	2660
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 7% 8% 31%	2660
July	1st to 10th	Day	13% 6% 2 6% 5% 5% 6% 5% 7% 11% 34%	4440
	11th to 20th	Day	11% 6% 3 8% 5% 6% 5% 5% 8% 10% 35%	4440
	21st to end	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 8% 36%	4814
August	1st to 10th	Day	18% 7% 2 6% 4 5% 5% 5% 6% 8% 35%	4239
	11th to 20th	Day	16% 7% 3 8% 5% 5% 6% 5% 8% 9% 30%	3867
	21st to end	Day	17% 6% 3 6% 5% 4 4 4 6% 8% 39%	4180
September	1st to 10th	Day	15% 6% 2 5% 4 4 5% 4 6% 8% 40%	2280
	11th to 20th	Day	15% 7% 2 6% 3 4 3 3 5% 7% 44%	2261
	21st to end	Day	17% 6% 3 7% 5% 5% 5% 4 6% 7% 35%	2062
October	1st to 10th	Day	20% 8% 3 8% 5% 5% 4 3 5% 7% 32%	1900
	11th to 20th	Day	20% 9% 3 7% 5% 4 5% 4 5% 6% 33%	1900
	21st to end	Day	27% 13% 3 7% 4 4 4 3 5% 6% 23%	1966
November	1st to 10th	Day	16% 12% 3 8% 5% 5% 5% 4 5% 6% 31%	1761
	11th to 20th	Day	16% 17% 4 9% 6% 4 3 3 4 4 28%	1724
	21st to end	Day	19% 19% 6% 10% 5% 4 4 3 5% 6% 21%	1600
December	1st to 10th	Day	20% 22% 11% 17% 6% 4 4 2 3 4 9%	2600
	11th to 20th	Day	19% 20% 10% 14% 6% 4 3 2 3 3 16%	2986
	21st to end	Day	22% 19% 7% 10% 5% 4 3 3 4 5% 18%	3960
January	1st to 10th	Night	14% 20% 10% 26% 7% 4 3 2 3 3 8%	6349
	11th to 20th	Night	10% 20% 9% 22% 8% 5% 4 3 4 4 11%	6315
	21st to end	Night	15% 17% 6% 17% 9% 6% 5% 3 4 4 15%	6651
February	1st to 10th	Night	16% 18% 6% 16% 8% 5% 4 4 4 4 16%	2600
	11th to 20th	Night	14% 15% 7% 16% 8% 6% 5% 4 4 5% 17%	2459
	21st to end	Night	17% 17% 6% 16% 7% 5% 4 2 4 5% 17%	1980
March	1st to 10th	Night	19% 11% 8% 14% 9% 4 4 3 4 4 21%	5364
	11th to 20th	Night	18% 17% 5% 8% 4 3 3 3 4 6% 29%	4988
	21st to end	Night	23% 13% 5% 12% 4 4 3 3 4 4 26%	5146
April	1st to 10th	Night	22% 15% 4 7% 4 4 4 3 5% 6% 27%	2000
	11th to 20th	Night	21% 11% 3 7% 5% 5% 4 4 4 6% 32%	1975
	21st to end	Night	21% 10% 3 7% 4 3 4 3 6% 5% 35%	1990
May	1st to 10th	Night	27% 9% 4 7% 5% 4 3 3 4 5% 28%	2660
	11th to 20th	Night	16% 11% 4 8% 6% 5% 4 4 5% 6% 32%	2575
	21st to end	Night	16% 6% 2 7% 5% 5% 4 5% 7% 7% 37%	2784
June	1st to 10th	Night	19% 6% 2 7% 4 5% 5% 3 5% 7% 36%	1520
	11th to 20th	Night	19% 6% 2 5% 4 4 4 3 6% 8% 39%	1520
	21st to end	Night	16% 6% 3 7% 4 4 5% 5% 6% 7% 39%	1520
July	1st to 10th	Night	12% 6% 2 7% 7% 6% 5% 5% 7% 7% 37%	2520
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	2520
	21st to end	Night	14% 7% 2 6% 4 4 4 4 5% 8% 42%	2739
August	1st to 10th	Night	22% 8% 2 6% 3 3 3 3 6% 7% 37%	2687
	11th to 20th	Night	16% 7% 3 7% 5% 5% 4 3 4 7% 38%	2928
	21st to end	Night	13% 6% 2 6% 4 4 5% 3 6% 7% 42%	3476
September	1st to 10th	Night	15% 6% 2 6% 4 4 4 4 5% 8% 43%	1900
	11th to 20th	Night	17% 6% 2 4 3 4 4 3 4 5% 48%	1918
	21st to end	Night	19% 6% 2 6% 4 4 5% 3 6% 8% 38%	2114
October	1st to 10th	Night	23% 7% 3 8% 5% 4 5% 3 5% 6% 32%	2280
	11th to 20th	Night	22% 9% 2 7% 4 4 5% 4 4 6% 31%	2280
	21st to end	Night	28% 13% 3 7% 5% 4 4 3 4 5% 24%	2631
November	1st to 10th	Night	17% 11% 3 9% 5% 5% 4 3 5% 5% 32%	2537
	11th to 20th	Night	19% 15% 4 8% 4 4 4 3 5% 6% 28%	2674
	21st to end	Night	17% 19% 6% 9% 6% 4 4 4 5% 6% 21%	2800
December	1st to 10th	Night	18% 23% 10% 14% 5% 5% 4 3 4 3 11%	4580
	11th to 20th	Night	23% 19% 7% 12% 5% 3 3 3 3 4 18%	5208
	21st to end	Night	23% 19% 7% 10% 6% 4 4 2 3 4 17%	6996

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% 3 8% 11% 3	3598
	11th to 20th	Day	78% 3 7% 7% 4 1	3600
	21st to end	Day	71% 3 10% 10% 4 1	4300
February	1st to 10th	Day	74% 4 7% 11% 3	1800
	11th to 20th	Day	71% 5% 7% 12% 4	1834
	21st to end	Day	71% 3 8% 11% 6%	1650
March	1st to 10th	Day	69% 3 10% 12% 6% 1	4479
	11th to 20th	Day	56% 2 18% 13% 8% 2	4970
	21st to end	Day	59% 3 7% 15% 15% 1	5786
April	1st to 10th	Day	59% 3 7% 20% 9% 2	2400
	11th to 20th	Day	53% 4 10% 20% 12% 1	2374
	21st to end	Day	55% 3 8% 20% 11% 2	2410
May	1st to 10th	Day	56% 3 10% 18% 11% 1	4544
	11th to 20th	Day	53% 3 12% 24% 7% 1	4542
	21st to end	Day	42% 5% 14% 28% 11% 1	4907
June	1st to 10th	Day	51% 6% 12% 19% 11% 1	2660
	11th to 20th	Day	47% 5% 14% 23% 9% 1	2660
	21st to end	Day	49% 6% 14% 21% 9%	2660
July	1st to 10th	Day	41% 7% 16% 25% 11%	4370
	11th to 20th	Day	40% 8% 16% 28% 8%	4365
	21st to end	Day	43% 8% 14% 25% 9%	4661
August	1st to 10th	Day	47% 6% 14% 21% 11%	4225
	11th to 20th	Day	47% 8% 19% 19% 6%	3906
	21st to end	Day	43% 6% 14% 31% 6% 1	4180
September	1st to 10th	Day	41% 6% 12% 29% 11% 1	2280
	11th to 20th	Day	42% 3 11% 27% 16% 1	2261
	21st to end	Day	48% 6% 10% 24% 11% 1	2062
October	1st to 10th	Day	46% 6% 12% 28% 8% 1	3160
	11th to 20th	Day	46% 4 14% 22% 13%	3140
	21st to end	Day	47% 6% 15% 23% 8% 1	3217
November	1st to 10th	Day	53% 5% 11% 20% 10% 1	1761
	11th to 20th	Day	59% 4 10% 17% 8% 2	1724
	21st to end	Day	65% 5% 14% 12% 4	1600
December	1st to 10th	Day	81% 4 7% 7% 2	2600
	11th to 20th	Day	71% 6% 10% 10% 4	2986
	21st to end	Day	69% 4 10% 12% 6%	3960
January	1st to 10th	Night	83% 3 7% 5% 2	6349
	11th to 20th	Night	76% 4 7% 7% 5%	6360
	21st to end	Night	71% 5% 8% 7% 7% 1	6651
February	1st to 10th	Night	72% 4 8% 12% 5%	2600
	11th to 20th	Night	71% 4 7% 12% 5%	2461
	21st to end	Night	71% 3 9% 11% 6%	1980
March	1st to 10th	Night	66% 3 11% 12% 6%	5374
	11th to 20th	Night	58% 3 16% 14% 6% 3	4989
	21st to end	Night	62% 4 4 19% 9% 1	5146
April	1st to 10th	Night	60% 5% 9% 18% 8% 2	2000
	11th to 20th	Night	54% 5% 9% 18% 13% 1	1975
	21st to end	Night	51% 4 11% 19% 13% 3	1990
May	1st to 10th	Night	59% 2 11% 16% 10% 2	2666
	11th to 20th	Night	51% 5% 14% 20% 8% 1	2576
	21st to end	Night	45% 4 13% 21% 16% 1	2785
June	1st to 10th	Night	47% 5% 11% 24% 12% 1	1520
	11th to 20th	Night	42% 6% 16% 23% 13% 1	1520
	21st to end	Night	44% 6% 12% 27% 11% 1	1520
July	1st to 10th	Night	44% 6% 14% 21% 15% 1	2480
	11th to 20th	Night	39% 8% 15% 25% 13%	2472
	21st to end	Night	40% 5% 12% 28% 14% 1	2651
August	1st to 10th	Night	47% 6% 14% 24% 9%	2677
	11th to 20th	Night	45% 6% 16% 25% 8%	2955
	21st to end	Night	37% 8% 14% 31% 9% 1	3476
September	1st to 10th	Night	41% 5% 13% 28% 12% 1	1900
	11th to 20th	Night	38% 4 12% 28% 17% 1	1918
	21st to end	Night	45% 5% 11% 26% 11% 1	2114
October	1st to 10th	Night	51% 7% 12% 22% 8%	3800
	11th to 20th	Night	47% 5% 14% 18% 15% 1	3779
	21st to end	Night	51% 6% 13% 23% 7%	4330
November	1st to 10th	Night	54% 5% 10% 17% 11% 2	2537
	11th to 20th	Night	57% 5% 10% 19% 8% 2	2674
	21st to end	Night	64% 5% 13% 14% 4	2800
December	1st to 10th	Night	77% 6% 9% 7% 1	4580
	11th to 20th	Night	70% 5% 7% 12% 6%	5208
	21st to end	Night	72% 4 8% 10% 6% 1	6996

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	33% 24% 28% 12% 5%	200
	11th to 20th	Day	41% 24% 28% 6% 2 1	200
	21st to end	Day	31% 27% 31% 9% 2	293
February	1st to 10th	Day	43% 27% 23% 6%	300
	11th to 20th	Day	37% 32% 25% 6%	372
	21st to end	Day	39% 23% 27% 9% 2	414
March	1st to 10th	Day	33% 25% 27% 12% 2	500
	11th to 20th	Day	37% 22% 27% 12% 3	500
	21st to end	Day	38% 23% 23% 12% 3 1	550
April	1st to 10th	Day	37% 25% 22% 11% 4	500
	11th to 20th	Day	35% 24% 22% 11% 6% 3	492
	21st to end	Day	37% 17% 26% 13% 6% 1	500
May	1st to 10th	Day	34% 17% 27% 13% 7% 2	503
	11th to 20th	Day	25% 14% 33% 18% 8% 2	527
	21st to end	Day	20% 15% 30% 22% 10% 3	549
June	1st to 10th	Day	23% 14% 29% 23% 8% 2	490
	11th to 20th	Day	20% 13% 31% 23% 12% 2	490
	21st to end	Day	20% 17% 35% 20% 6% 2	490
July	1st to 10th	Day	14% 9% 32% 28% 13% 5%	485
	11th to 20th	Day	15% 8% 31% 31% 13% 3	484
	21st to end	Day	15% 10% 36% 26% 10% 3	532
August	1st to 10th	Day	16% 11% 28% 25% 15% 6%	486
	11th to 20th	Day	18% 9% 27% 28% 13% 5%	479
	21st to end	Day	16% 12% 28% 26% 12% 5%	539
September	1st to 10th	Day	18% 11% 26% 23% 16% 6%	490
	11th to 20th	Day	15% 10% 25% 22% 18% 10%	490
	21st to end	Day	21% 13% 28% 21% 13% 4	490
October	1st to 10th	Day	26% 13% 26% 18% 12% 5%	490
	11th to 20th	Day	26% 20% 29% 15% 9% 1	490
	21st to end	Day	37% 19% 24% 15% 5%	473
November	1st to 10th	Day	31% 20% 25% 18% 7%	395
	11th to 20th	Day	30% 27% 24% 14% 4	324
	21st to end	Day	41% 20% 31% 9%	200
December	1st to 10th	Day	36% 24% 29% 10% 1 1	200
	11th to 20th	Day	38% 27% 29% 6% 1	200
	21st to end	Day	36% 23% 32% 8% 1	220
January	1st to 10th	Night	31% 27% 29% 12% 1	800
	11th to 20th	Night	31% 25% 28% 14% 2 1	800
	21st to end	Night	24% 25% 32% 17% 2	880
February	1st to 10th	Night	31% 24% 29% 15% 1	800
	11th to 20th	Night	30% 26% 29% 13% 2	783
	21st to end	Night	31% 24% 28% 15% 2	660
March	1st to 10th	Night	28% 23% 30% 18% 2	800
	11th to 20th	Night	31% 22% 28% 17% 2	800
	21st to end	Night	31% 21% 24% 18% 5%	880
April	1st to 10th	Night	29% 19% 31% 19% 2	800
	11th to 20th	Night	29% 18% 28% 17% 7% 1	791
	21st to end	Night	29% 14% 27% 24% 6%	790
May	1st to 10th	Night	30% 11% 24% 28% 7%	440
	11th to 20th	Night	20% 8% 36% 28% 7% 1	394
	21st to end	Night	18% 7% 33% 32% 10% 1	423
June	1st to 10th	Night	18% 9% 25% 36% 12% 1	380
	11th to 20th	Night	19% 9% 23% 37% 11%	380
	21st to end	Night	16% 7% 28% 40% 10%	380
July	1st to 10th	Night	11% 5% 24% 43% 16% 1	370
	11th to 20th	Night	13% 4 24% 43% 15% 1	368
	21st to end	Night	16% 4 22% 43% 14% 1	416
August	1st to 10th	Night	15% 4 26% 34% 20% 1	481
	11th to 20th	Night	15% 5% 22% 33% 22% 3	642
	21st to end	Night	14% 7% 24% 35% 19% 1	836
September	1st to 10th	Night	17% 8% 25% 31% 18% 2	760
	11th to 20th	Night	19% 7% 20% 29% 22% 3	760
	21st to end	Night	19% 11% 25% 29% 14% 2	760
October	1st to 10th	Night	26% 11% 23% 22% 16% 2	760
	11th to 20th	Night	27% 16% 27% 24% 6%	760
	21st to end	Night	36% 15% 28% 19% 2	836
November	1st to 10th	Night	26% 18% 30% 19% 6%	780
	11th to 20th	Night	29% 21% 28% 16% 6% 1	800
	21st to end	Night	30% 23% 28% 15% 4	800
December	1st to 10th	Night	34% 25% 25% 14% 3	800
	11th to 20th	Night	36% 22% 27% 13% 3	800
	21st to end	Night	33% 25% 28% 12% 2	880

Page generated at 2025-05-23 09:21:02. Version ac07

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