Perry Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perry Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	16% 7% 2 6% 4 4 4 3 5% 7% 41%	127127
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 41%	124558
Transparency	Day	41% 6% 13% 22% 16% 2	130669
	Night	43% 5% 11% 22% 16% 3	129103
Seeing	Day	18% 9% 26% 29% 15% 3	16562
	Night	20% 9% 29% 28% 13% 2	25702

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% 7% 2 6% 3 4 4 3 4 5% 50%	3677
	11th to 20th	Day	14% 8% 2 6% 4 3 4 3 5% 5% 46%	3618
	21st to end	Day	18% 9% 4 7% 4 4 3 3 4 4 40%	4121
February	1st to 10th	Day	17% 7% 2 7% 4 4 3 4 6% 6% 39%	1894
	11th to 20th	Day	16% 6% 3 6% 4 4 4 3 4 6% 45%	1948
	21st to end	Day	19% 8% 3 5% 4 3 4 3 5% 6% 39%	1650
March	1st to 10th	Day	20% 9% 2 5% 3 3 3 2 4 6% 43%	4476
	11th to 20th	Day	11% 9% 3 5% 4 3 4 4 6% 8% 43%	4651
	21st to end	Day	19% 9% 2 6% 3 4 4 3 5% 6% 40%	5970
April	1st to 10th	Day	18% 9% 2 6% 4 4 4 3 5% 6% 39%	2400
	11th to 20th	Day	17% 7% 2 5% 4 3 5% 4 5% 6% 43%	2374
	21st to end	Day	19% 6% 2 5% 4 4 3 3 6% 6% 43%	2496
May	1st to 10th	Day	28% 9% 2 5% 3 3 3 3 5% 9% 29%	5886
	11th to 20th	Day	22% 9% 3 6% 4 3 3 2 4 6% 38%	6042
	21st to end	Day	20% 7% 2 5% 3 4 4 4 6% 8% 37%	6949
June	1st to 10th	Day	17% 6% 2 5% 4 4 4 4 6% 9% 40%	2800
	11th to 20th	Day	15% 6% 2 5% 4 5% 4 4 6% 8% 40%	2800
	21st to end	Day	11% 7% 2 5% 4 4 4 4 6% 11% 43%	2800
July	1st to 10th	Day	13% 8% 3 7% 4 4 5% 4 6% 10% 36%	6360
	11th to 20th	Day	12% 7% 2 7% 5% 5% 5% 4 7% 10% 35%	6360
	21st to end	Day	14% 7% 2 6% 4 4 4 4 6% 8% 43%	6926
August	1st to 10th	Day	17% 6% 2 6% 5% 5% 4 3 6% 9% 36%	4325
	11th to 20th	Day	11% 5% 3 7% 5% 4 4 3 5% 8% 45%	3850
	21st to end	Day	7% 4 2 5% 4 4 5% 4 7% 8% 50%	4312
September	1st to 10th	Day	14% 5% 2 6% 4 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	11% 6% 2 5% 4 3 3 3 4 7% 52%	2255
	21st to end	Day	18% 7% 2 7% 4 5% 4 3 4 6% 39%	2200
October	1st to 10th	Day	20% 5% 2 5% 3 3 3 2 5% 7% 45%	2181
	11th to 20th	Day	14% 5% 2 5% 3 4 4 3 5% 8% 46%	1991
	21st to end	Day	18% 5% 2 6% 4 4 3 3 5% 7% 42%	1980
November	1st to 10th	Day	13% 6% 2 5% 4 4 4 2 4 5% 50%	1851
	11th to 20th	Day	15% 6% 2 4 4 4 3 2 4 7% 49%	1890
	21st to end	Day	16% 7% 3 5% 3 4 4 4 5% 6% 44%	1890
December	1st to 10th	Day	14% 7% 2 6% 4 4 4 3 5% 7% 44%	2690
	11th to 20th	Day	23% 7% 2 3 2 2 2 2 3 5% 47%	3066
	21st to end	Day	17% 10% 4 8% 4 4 4 3 4 5% 38%	4048
January	1st to 10th	Night	14% 5% 2 4 3 4 3 3 5% 6% 51%	6489
	11th to 20th	Night	15% 8% 3 7% 4 3 3 3 4 4 47%	6400
	21st to end	Night	17% 9% 3 7% 4 4 4 3 5% 6% 38%	6830
February	1st to 10th	Night	17% 9% 3 6% 5% 4 4 3 4 6% 40%	2505
	11th to 20th	Night	18% 8% 2 5% 4 4 4 4 4 6% 41%	2338
	21st to end	Night	20% 6% 2 4 5% 5% 4 3 4 6% 41%	1980
March	1st to 10th	Night	20% 10% 2 5% 3 3 3 3 4 5% 42%	5364
	11th to 20th	Night	16% 10% 3 6% 3 4 4 3 5% 5% 42%	5302
	21st to end	Night	19% 9% 3 6% 4 4 4 4 5% 5% 38%	4962
April	1st to 10th	Night	19% 8% 2 6% 5% 5% 4 3 4 6% 38%	2000
	11th to 20th	Night	18% 8% 2 5% 4 3 3 3 4 7% 41%	1975
	21st to end	Night	19% 8% 2 5% 4 4 4 3 4 6% 41%	1902
May	1st to 10th	Night	24% 11% 4 7% 4 4 4 3 5% 7% 27%	4054
	11th to 20th	Night	24% 10% 3 6% 4 3 4 3 5% 6% 32%	3881
	21st to end	Night	22% 10% 2 7% 4 4 4 3 6% 8% 31%	3926
June	1st to 10th	Night	17% 8% 2 5% 4 4 5% 3 5% 8% 39%	1600
	11th to 20th	Night	17% 6% 2 5% 3 4 4 4 7% 9% 40%	1600
	21st to end	Night	11% 6% 3 7% 5% 5% 6% 4 6% 9% 39%	1600
July	1st to 10th	Night	14% 11% 4 8% 5% 4 5% 4 6% 7% 33%	3600
	11th to 20th	Night	18% 11% 3 7% 4 4 4 4 6% 9% 30%	3600
	21st to end	Night	14% 7% 3 6% 4 4 4 3 6% 7% 41%	3926
August	1st to 10th	Night	22% 7% 3 6% 4 4 5% 4 6% 8% 31%	2786
	11th to 20th	Night	11% 7% 3 6% 4 5% 5% 5% 7% 8% 40%	3197
	21st to end	Night	8% 4 2 4 4 3 4 4 6% 8% 53%	3586
September	1st to 10th	Night	14% 6% 2 7% 5% 4 5% 3 5% 7% 43%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 3 4 4 6% 52%	2144
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 6% 40%	2200
October	1st to 10th	Night	18% 7% 3 7% 4 3 4 2 4 7% 42%	2215
	11th to 20th	Night	16% 6% 2 6% 3 2 4 3 4 6% 49%	2407
	21st to end	Night	19% 6% 3 6% 4 3 3 3 4 6% 42%	2860
November	1st to 10th	Night	12% 6% 3 6% 5% 4 3 3 4 7% 48%	2667
	11th to 20th	Night	18% 7% 2 5% 4 3 3 3 4 5% 47%	2730
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 6% 46%	2730
December	1st to 10th	Night	15% 7% 2 5% 4 3 4 3 4 5% 48%	4704
	11th to 20th	Night	25% 7% 2 4 3 3 3 2 4 4 43%	5348
	21st to end	Night	17% 9% 3 6% 4 4 3 3 4 6% 42%	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	36% 4 10% 29% 21% 2	3678
	11th to 20th	Day	40% 4 12% 21% 17% 6%	3648
	21st to end	Day	47% 6% 10% 21% 15% 1	4121
February	1st to 10th	Day	45% 6% 15% 20% 13% 1	1894
	11th to 20th	Day	42% 5% 10% 25% 17% 2	1957
	21st to end	Day	45% 5% 11% 20% 15% 3	1650
March	1st to 10th	Day	42% 3 13% 18% 15% 9%	4479
	11th to 20th	Day	37% 6% 15% 27% 14% 1	4651
	21st to end	Day	46% 3 9% 15% 22% 5%	5970
April	1st to 10th	Day	44% 5% 11% 19% 17% 4	2400
	11th to 20th	Day	41% 5% 13% 20% 19% 3	2374
	21st to end	Day	41% 6% 10% 21% 19% 4	2496
May	1st to 10th	Day	52% 8% 16% 13% 9% 1	5897
	11th to 20th	Day	49% 5% 11% 20% 13% 2	6046
	21st to end	Day	45% 8% 11% 21% 14% 1	6949
June	1st to 10th	Day	41% 8% 13% 22% 15% 1	2800
	11th to 20th	Day	40% 8% 13% 25% 13% 2	2800
	21st to end	Day	36% 8% 16% 23% 15% 1	2800
July	1st to 10th	Day	45% 9% 12% 19% 14% 1	6290
	11th to 20th	Day	43% 8% 18% 21% 9% 1	6285
	21st to end	Day	38% 6% 12% 24% 18% 1	6728
August	1st to 10th	Day	44% 7% 17% 22% 9%	4311
	11th to 20th	Day	36% 8% 15% 32% 10% 1	3886
	21st to end	Day	28% 8% 15% 29% 19% 1	4312
September	1st to 10th	Day	37% 7% 14% 21% 19% 2	2400
	11th to 20th	Day	32% 6% 12% 24% 23% 2	2255
	21st to end	Day	47% 6% 9% 18% 18% 2	2200
October	1st to 10th	Day	33% 5% 13% 22% 23% 5%	3558
	11th to 20th	Day	27% 4 12% 23% 32% 3	3219
	21st to end	Day	32% 6% 15% 26% 20% 2	3180
November	1st to 10th	Day	37% 5% 14% 22% 20% 2	1851
	11th to 20th	Day	37% 5% 12% 22% 21% 2	1890
	21st to end	Day	42% 5% 13% 28% 10% 2	1890
December	1st to 10th	Day	41% 5% 15% 27% 12% 1	2690
	11th to 20th	Day	42% 3 13% 19% 19% 3	3066
	21st to end	Day	48% 5% 10% 20% 15% 3	4048
January	1st to 10th	Night	35% 3 9% 26% 23% 3	6489
	11th to 20th	Night	43% 3 12% 21% 16% 5%	6445
	21st to end	Night	48% 5% 9% 19% 15% 3	6830
February	1st to 10th	Night	45% 7% 12% 22% 12% 2	2505
	11th to 20th	Night	45% 5% 10% 21% 16% 4	2340
	21st to end	Night	45% 5% 8% 22% 17% 4	1980
March	1st to 10th	Night	46% 3 11% 19% 13% 8%	5374
	11th to 20th	Night	44% 5% 12% 24% 12% 4	5303
	21st to end	Night	48% 4 7% 12% 23% 5%	4962
April	1st to 10th	Night	47% 5% 9% 19% 16% 4	2000
	11th to 20th	Night	43% 6% 10% 19% 21% 2	1975
	21st to end	Night	44% 5% 9% 21% 17% 5%	1902
May	1st to 10th	Night	56% 6% 14% 13% 9% 2	4060
	11th to 20th	Night	53% 6% 12% 15% 11% 3	3882
	21st to end	Night	51% 6% 13% 18% 10% 1	3927
June	1st to 10th	Night	46% 6% 9% 20% 17% 2	1600
	11th to 20th	Night	40% 6% 14% 25% 14% 1	1600
	21st to end	Night	41% 7% 13% 25% 12% 1	1600
July	1st to 10th	Night	50% 7% 10% 21% 11% 1	3560
	11th to 20th	Night	49% 7% 14% 20% 9% 1	3552
	21st to end	Night	42% 5% 14% 23% 15% 1	3809
August	1st to 10th	Night	50% 8% 15% 18% 9% 1	2776
	11th to 20th	Night	38% 8% 15% 29% 9%	3227
	21st to end	Night	27% 6% 14% 33% 19% 1	3586
September	1st to 10th	Night	40% 7% 10% 25% 17% 2	2000
	11th to 20th	Night	33% 6% 11% 25% 23% 3	2144
	21st to end	Night	46% 7% 8% 18% 18% 3	2200
October	1st to 10th	Night	39% 5% 11% 24% 18% 3	3618
	11th to 20th	Night	26% 4 9% 26% 30% 4	3918
	21st to end	Night	32% 5% 13% 26% 21% 2	4610
November	1st to 10th	Night	38% 6% 12% 23% 18% 4	2667
	11th to 20th	Night	41% 5% 9% 22% 19% 5%	2730
	21st to end	Night	38% 5% 14% 28% 13% 2	2730
December	1st to 10th	Night	40% 4 12% 28% 13% 2	4704
	11th to 20th	Night	47% 3 10% 23% 15% 2	5348
	21st to end	Night	44% 4 11% 20% 17% 3	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	17% 22% 32% 22% 6%	220
	11th to 20th	Day	17% 25% 39% 15% 5%	212
	21st to end	Day	21% 13% 38% 19% 8% 1	255
February	1st to 10th	Day	22% 14% 34% 24% 5%	394
	11th to 20th	Day	22% 18% 38% 20% 2	496
	21st to end	Day	23% 14% 35% 23% 5% 1	414
March	1st to 10th	Day	17% 17% 36% 24% 6%	500
	11th to 20th	Day	16% 13% 39% 25% 6% 1	500
	21st to end	Day	21% 13% 37% 21% 7% 1	550
April	1st to 10th	Day	23% 11% 35% 22% 9% 1	500
	11th to 20th	Day	20% 11% 30% 29% 9% 1	492
	21st to end	Day	23% 9% 29% 29% 8% 2	500
May	1st to 10th	Day	28% 7% 29% 26% 8% 2	503
	11th to 20th	Day	21% 8% 25% 34% 12% 1	527
	21st to end	Day	20% 3 18% 32% 22% 4	573
June	1st to 10th	Day	16% 4 19% 38% 20% 3	530
	11th to 20th	Day	17% 4 21% 37% 18% 3	530
	21st to end	Day	14% 4 19% 38% 21% 4	530
July	1st to 10th	Day	15% 3 13% 38% 24% 6%	525
	11th to 20th	Day	12% 3 12% 39% 27% 7%	524
	21st to end	Day	14% 2 15% 41% 25% 3	576
August	1st to 10th	Day	12% 3 14% 38% 27% 6%	526
	11th to 20th	Day	13% 3 11% 39% 27% 8%	519
	21st to end	Day	11% 4 17% 36% 24% 8%	583
September	1st to 10th	Day	19% 4 22% 26% 22% 8%	530
	11th to 20th	Day	13% 2 18% 34% 21% 11%	530
	21st to end	Day	22% 4 23% 31% 17% 3	530
October	1st to 10th	Day	23% 6% 20% 24% 20% 8%	530
	11th to 20th	Day	15% 8% 31% 29% 14% 4	524
	21st to end	Day	21% 11% 30% 23% 12% 2	462
November	1st to 10th	Day	15% 10% 29% 29% 14% 2	425
	11th to 20th	Day	18% 17% 31% 25% 10%	430
	21st to end	Day	18% 16% 32% 24% 9% 1	430
December	1st to 10th	Day	17% 16% 35% 24% 7%	230
	11th to 20th	Day	23% 23% 30% 19% 4	220
	21st to end	Day	23% 14% 38% 16% 9% 2	242
January	1st to 10th	Night	19% 15% 41% 19% 5%	840
	11th to 20th	Night	24% 18% 32% 22% 4	825
	21st to end	Night	19% 14% 42% 21% 4	880
February	1st to 10th	Night	21% 13% 39% 25% 3	800
	11th to 20th	Night	25% 15% 40% 19% 2	783
	21st to end	Night	23% 12% 39% 21% 5%	660
March	1st to 10th	Night	20% 14% 36% 24% 6%	800
	11th to 20th	Night	19% 12% 38% 24% 6% 1	800
	21st to end	Night	20% 9% 39% 24% 7% 1	880
April	1st to 10th	Night	23% 9% 34% 27% 7%	800
	11th to 20th	Night	21% 7% 30% 31% 9%	791
	21st to end	Night	23% 4 31% 33% 9% 1	702
May	1st to 10th	Night	24% 6% 25% 32% 13% 2	600
	11th to 20th	Night	23% 4 28% 34% 10%	531
	21st to end	Night	22% 3 13% 37% 23% 2	433
June	1st to 10th	Night	21% 3 19% 34% 22% 2	400
	11th to 20th	Night	18% 2 15% 42% 20% 2	400
	21st to end	Night	14% 3 18% 40% 24% 1	400
July	1st to 10th	Night	13% 5% 13% 41% 25% 3	390
	11th to 20th	Night	14% 2 14% 40% 27% 4	388
	21st to end	Night	13% 3 15% 45% 22% 1	438
August	1st to 10th	Night	11% 2 13% 44% 29% 2	507
	11th to 20th	Night	14% 2 14% 37% 29% 4	790
	21st to end	Night	13% 2 12% 39% 27% 7%	880
September	1st to 10th	Night	16% 2 21% 33% 23% 5%	800
	11th to 20th	Night	17% 3 16% 33% 24% 8%	800
	21st to end	Night	23% 4 21% 31% 19% 2	800
October	1st to 10th	Night	22% 6% 25% 22% 20% 5%	800
	11th to 20th	Night	19% 9% 27% 28% 15% 2	800
	21st to end	Night	23% 10% 30% 26% 10% 1	880
November	1st to 10th	Night	16% 9% 32% 28% 13% 2	820
	11th to 20th	Night	21% 14% 32% 23% 10% 1	840
	21st to end	Night	16% 16% 32% 26% 8% 1	840
December	1st to 10th	Night	23% 15% 32% 23% 6% 1	840
	11th to 20th	Night	23% 16% 33% 23% 4	840
	21st to end	Night	23% 14% 36% 23% 5%	924

Page generated at 2026-01-14 01:14:24. Version ac07

Copyright © 2026 Attilla Danko.