Perkins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Perkins Observatory clear sky chart from 2005-11-06 to 2025-02-06.

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	15% 10% 4 8% 5% 5% 5% 4 5% 7% 32%	109582
	Night	15% 11% 4 8% 5% 4 4 3 5% 6% 33%	108700
Transparency	Day	51% 3 11% 22% 13% 1	113270
	Night	50% 3 10% 22% 13% 2	113151
Seeing	Day	20% 8% 25% 24% 16% 8%	15537
	Night	20% 6% 25% 32% 15% 2	25243

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% 19% 8% 17% 8% 5% 4 3 4 4 13%	4056
	11th to 20th	Day	14% 22% 6% 12% 7% 6% 6% 3 3 3 19%	4060
	21st to end	Day	22% 17% 9% 10% 8% 7% 4 3 4 4 13%	4466
February	1st to 10th	Day	20% 16% 4 10% 7% 5% 4 3 4 5% 23%	1755
	11th to 20th	Day	16% 12% 5% 14% 7% 5% 4 3 5% 5% 24%	1829
	21st to end	Day	18% 15% 4 9% 5% 5% 4 4 4 6% 27%	1570
March	1st to 10th	Day	21% 15% 6% 10% 6% 4 4 3 5% 5% 22%	3246
	11th to 20th	Day	13% 10% 4 8% 5% 5% 5% 3 5% 6% 37%	3480
	21st to end	Day	20% 10% 5% 8% 5% 4 4 3 5% 6% 30%	3815
April	1st to 10th	Day	15% 13% 3 8% 6% 4 4 3 4 7% 33%	2151
	11th to 20th	Day	18% 9% 3 7% 4 5% 4 3 5% 6% 36%	2394
	21st to end	Day	18% 9% 3 8% 6% 4 5% 4 5% 6% 33%	2470
May	1st to 10th	Day	21% 10% 3 8% 5% 4 4 4 5% 7% 29%	4082
	11th to 20th	Day	20% 9% 3 7% 4 3 4 4 5% 7% 34%	4081
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 5% 8% 10% 34%	4476
June	1st to 10th	Day	11% 8% 2 7% 6% 5% 5% 4 6% 9% 37%	2470
	11th to 20th	Day	10% 7% 3 6% 5% 5% 5% 4 8% 9% 39%	2551
	21st to end	Day	11% 5% 2 6% 5% 6% 5% 6% 8% 10% 38%	2660
July	1st to 10th	Day	9% 7% 3 8% 5% 5% 5% 4 7% 9% 38%	4418
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 7% 12% 43%	4120
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 5% 8% 10% 36%	4466
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	4104
	11th to 20th	Day	8% 6% 2 6% 5% 5% 6% 5% 8% 11% 39%	4044
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 7% 10% 45%	4445
September	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 5% 6% 48%	2106
	11th to 20th	Day	9% 5% 2 6% 5% 4 4 4 6% 9% 46%	2090
	21st to end	Day	17% 8% 2 6% 4 4 4 3 4 6% 42%	2090
October	1st to 10th	Day	13% 6% 3 7% 4 4 4 4 6% 7% 43%	2042
	11th to 20th	Day	15% 8% 2 6% 4 4 3 4 5% 7% 42%	1900
	21st to end	Day	20% 10% 3 7% 4 4 4 4 5% 6% 32%	2014
November	1st to 10th	Day	11% 9% 3 7% 4 5% 3 3 5% 6% 45%	1762
	11th to 20th	Day	17% 13% 4 7% 6% 4 4 4 4 6% 34%	1800
	21st to end	Day	18% 14% 5% 9% 5% 4 4 2 5% 5% 30%	1800
December	1st to 10th	Day	20% 13% 6% 14% 4 5% 3 3 4 4 25%	2930
	11th to 20th	Day	21% 17% 7% 10% 4 4 4 4 3 5% 21%	3373
	21st to end	Day	25% 21% 6% 11% 6% 5% 4 3 4 4 13%	4466
January	1st to 10th	Night	16% 17% 8% 17% 6% 4 4 3 4 5% 16%	5889
	11th to 20th	Night	13% 18% 8% 16% 7% 6% 6% 3 4 4 15%	5900
	21st to end	Night	22% 15% 6% 9% 6% 4 3 3 4 4 23%	6490
February	1st to 10th	Night	21% 14% 4 9% 6% 4 4 3 5% 4 26%	2536
	11th to 20th	Night	17% 12% 4 9% 7% 4 5% 4 5% 6% 27%	2233
	21st to end	Night	20% 15% 5% 11% 4 5% 3 3 4 5% 26%	1884
March	1st to 10th	Night	21% 14% 4 7% 5% 4 4 2 4 5% 31%	3606
	11th to 20th	Night	14% 9% 3 6% 4 4 4 3 5% 7% 41%	3480
	21st to end	Night	18% 9% 3 7% 4 4 4 3 4 5% 39%	3817
April	1st to 10th	Night	15% 9% 3 7% 5% 4 5% 3 6% 8% 35%	2046
	11th to 20th	Night	15% 8% 3 7% 5% 4 5% 5% 5% 7% 37%	1735
	21st to end	Night	16% 8% 3 7% 4 4 4 4 5% 8% 37%	1710
May	1st to 10th	Night	24% 10% 3 7% 5% 4 5% 4 5% 6% 27%	2813
	11th to 20th	Night	18% 8% 3 7% 3 4 4 3 5% 7% 39%	2812
	21st to end	Night	9% 5% 2 5% 5% 4 5% 4 7% 10% 44%	3079
June	1st to 10th	Night	9% 6% 2 5% 4 4 5% 4 6% 8% 46%	1710
	11th to 20th	Night	8% 4 2 6% 4 5% 5% 4 8% 10% 44%	1621
	21st to end	Night	9% 4 2 5% 4 4 5% 5% 7% 10% 45%	1520
July	1st to 10th	Night	11% 6% 2 6% 4 4 3 4 6% 7% 46%	2547
	11th to 20th	Night	7% 4 2 5% 5% 4 5% 4 7% 9% 49%	2840
	21st to end	Night	8% 6% 3 7% 5% 5% 5% 4 6% 9% 41%	3087
August	1st to 10th	Night	9% 7% 3 7% 5% 5% 5% 5% 7% 9% 37%	2825
	11th to 20th	Night	7% 8% 3 9% 5% 5% 5% 3 5% 7% 43%	2785
	21st to end	Night	4 4 2 6% 4 5% 6% 5% 7% 9% 49%	3208
September	1st to 10th	Night	12% 5% 2 4 3 3 4 3 4 7% 52%	2053
	11th to 20th	Night	9% 5% 2 4 3 4 3 3 6% 6% 55%	2090
	21st to end	Night	15% 6% 2 6% 4 4 4 3 4 6% 45%	2090
October	1st to 10th	Night	10% 5% 2 6% 5% 3 4 3 5% 7% 51%	2137
	11th to 20th	Night	17% 7% 3 5% 4 4 5% 3 5% 6% 42%	2280
	21st to end	Night	19% 8% 3 8% 4 4 4 3 4 5% 38%	2579
November	1st to 10th	Night	14% 7% 2 5% 4 4 5% 3 5% 6% 46%	2536
	11th to 20th	Night	17% 10% 3 7% 5% 5% 4 3 5% 6% 35%	2600
	21st to end	Night	20% 11% 4 9% 4 5% 4 4 5% 6% 28%	2600
December	1st to 10th	Night	18% 11% 5% 10% 6% 6% 5% 3 5% 6% 25%	4250
	11th to 20th	Night	18% 18% 6% 12% 5% 4 4 3 4 5% 21%	4822
	21st to end	Night	24% 20% 6% 10% 5% 4 4 3 4 5% 15%	6490

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% 2 6% 12% 4	4057
	11th to 20th	Day	72% 2 8% 8% 10% 1	4060
	21st to end	Day	76% 2 8% 9% 5% 1	4466
February	1st to 10th	Day	65% 3 10% 14% 8%	1755
	11th to 20th	Day	63% 3 8% 17% 9% 1	1839
	21st to end	Day	59% 2 11% 19% 7% 2	1570
March	1st to 10th	Day	66% 1 8% 20% 4	3246
	11th to 20th	Day	50% 1 8% 29% 11% 1	3480
	21st to end	Day	57% 1 10% 24% 8%	3815
April	1st to 10th	Day	53% 2 11% 19% 13% 1	2151
	11th to 20th	Day	50% 2 12% 20% 13% 2	2394
	21st to end	Day	52% 3 9% 22% 12% 2	2470
May	1st to 10th	Day	57% 1 11% 22% 8% 1	4120
	11th to 20th	Day	51% 2 13% 23% 11% 1	4085
	21st to end	Day	43% 3 13% 24% 13% 3	4489
June	1st to 10th	Day	44% 3 10% 27% 15% 2	2470
	11th to 20th	Day	40% 4 14% 24% 16% 1	2551
	21st to end	Day	39% 5% 15% 22% 16% 3	2660
July	1st to 10th	Day	42% 5% 11% 23% 16% 3	4349
	11th to 20th	Day	32% 5% 13% 28% 20% 1	4051
	21st to end	Day	39% 5% 13% 21% 18% 4	4324
August	1st to 10th	Day	39% 5% 15% 26% 14% 1	4091
	11th to 20th	Day	35% 5% 11% 32% 15% 1	4083
	21st to end	Day	33% 5% 13% 32% 14% 3	4444
September	1st to 10th	Day	36% 4 10% 25% 21% 4	2106
	11th to 20th	Day	36% 3 10% 27% 21% 3	2090
	21st to end	Day	45% 2 8% 23% 21% 1	2090
October	1st to 10th	Day	27% 3 10% 35% 24% 1	3393
	11th to 20th	Day	35% 3 10% 28% 23% 2	3140
	21st to end	Day	42% 3 10% 26% 19% 1	3303
November	1st to 10th	Day	40% 3 10% 27% 18% 2	1762
	11th to 20th	Day	53% 2 9% 22% 12% 2	1800
	21st to end	Day	58% 2 10% 20% 10% 1	1800
December	1st to 10th	Day	63% 2 14% 17% 4	2930
	11th to 20th	Day	67% 2 10% 15% 5%	3370
	21st to end	Day	77% 2 7% 9% 4	4466
January	1st to 10th	Night	71% 2 8% 15% 4	5889
	11th to 20th	Night	74% 2 7% 10% 6% 1	5900
	21st to end	Night	66% 2 11% 14% 6% 2	6490
February	1st to 10th	Night	62% 2 10% 17% 8% 1	2536
	11th to 20th	Night	58% 3 9% 18% 11% 1	2234
	21st to end	Night	62% 2 9% 16% 9% 2	1884
March	1st to 10th	Night	58% 1 8% 22% 11% 1	3606
	11th to 20th	Night	44% 2 8% 32% 12% 2	3480
	21st to end	Night	49% 2 7% 29% 12% 1	3817
April	1st to 10th	Night	47% 3 12% 22% 15% 2	2046
	11th to 20th	Night	45% 3 11% 18% 19% 3	1735
	21st to end	Night	47% 3 11% 22% 16% 2	1710
May	1st to 10th	Night	58% 2 9% 21% 8% 2	2840
	11th to 20th	Night	46% 3 11% 27% 11% 1	2813
	21st to end	Night	36% 3 13% 32% 15% 3	3087
June	1st to 10th	Night	36% 4 13% 24% 21% 2	1710
	11th to 20th	Night	34% 5% 13% 27% 17% 3	1621
	21st to end	Night	34% 5% 15% 26% 17% 4	1520
July	1st to 10th	Night	38% 4 11% 25% 17% 6%	2507
	11th to 20th	Night	30% 6% 12% 32% 19% 1	2786
	21st to end	Night	38% 5% 13% 22% 17% 5%	2988
August	1st to 10th	Night	42% 5% 14% 25% 13% 1	2814
	11th to 20th	Night	39% 5% 10% 26% 19%	2812
	21st to end	Night	30% 6% 14% 30% 18% 2	3208
September	1st to 10th	Night	34% 2 12% 26% 20% 6%	2053
	11th to 20th	Night	30% 2 8% 29% 26% 4	2090
	21st to end	Night	41% 3 8% 22% 25% 2	2090
October	1st to 10th	Night	26% 2 7% 31% 32% 2	3566
	11th to 20th	Night	32% 3 10% 29% 24% 3	3779
	21st to end	Night	36% 3 7% 27% 23% 5%	4240
November	1st to 10th	Night	40% 2 11% 25% 20% 2	2536
	11th to 20th	Night	50% 3 7% 23% 15% 3	2600
	21st to end	Night	57% 3 10% 22% 8% 1	2600
December	1st to 10th	Night	59% 3 12% 20% 6%	4250
	11th to 20th	Night	66% 2 12% 14% 6%	4824
	21st to end	Night	73% 2 9% 12% 3	6490

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	29% 13% 36% 15% 6% 2	300
	11th to 20th	Day	28% 16% 37% 15% 3 1	300
	21st to end	Day	31% 14% 34% 16% 5%	330
February	1st to 10th	Day	32% 15% 32% 17% 4	285
	11th to 20th	Day	25% 16% 40% 16% 3	270
	21st to end	Day	27% 15% 42% 15% 1	225
March	1st to 10th	Day	23% 17% 32% 22% 6%	344
	11th to 20th	Day	26% 13% 35% 19% 6% 1	460
	21st to end	Day	31% 9% 32% 22% 6% 1	506
April	1st to 10th	Day	24% 10% 32% 21% 10% 2	460
	11th to 20th	Day	22% 13% 28% 21% 11% 6%	452
	21st to end	Day	23% 9% 34% 21% 11% 2	460
May	1st to 10th	Day	26% 6% 29% 24% 11% 4	463
	11th to 20th	Day	24% 4 25% 23% 16% 7%	487
	21st to end	Day	14% 6% 20% 27% 23% 12%	529
June	1st to 10th	Day	13% 4 20% 34% 20% 9%	490
	11th to 20th	Day	13% 4 21% 30% 24% 9%	571
	21st to end	Day	13% 5% 25% 28% 21% 8%	680
July	1st to 10th	Day	14% 3 13% 27% 25% 19%	662
	11th to 20th	Day	13% 2 18% 24% 25% 19%	484
	21st to end	Day	11% 1 15% 27% 28% 18%	532
August	1st to 10th	Day	13% 3 10% 25% 29% 20%	486
	11th to 20th	Day	13% 3 16% 28% 23% 18%	479
	21st to end	Day	10% 3 13% 25% 30% 19%	539
September	1st to 10th	Day	13% 3 14% 26% 26% 17%	490
	11th to 20th	Day	11% 1 11% 24% 31% 21%	490
	21st to end	Day	22% 8% 17% 23% 21% 9%	490
October	1st to 10th	Day	12% 5% 21% 31% 20% 12%	490
	11th to 20th	Day	16% 10% 29% 26% 14% 5%	490
	21st to end	Day	26% 10% 32% 23% 9% 1	463
November	1st to 10th	Day	16% 12% 33% 24% 11% 4	300
	11th to 20th	Day	24% 13% 31% 20% 10% 2	300
	21st to end	Day	26% 14% 35% 21% 4	300
December	1st to 10th	Day	24% 14% 36% 18% 5% 2	300
	11th to 20th	Day	26% 12% 41% 16% 5%	300
	21st to end	Day	36% 12% 26% 22% 3	330
January	1st to 10th	Night	25% 12% 36% 22% 4 1	800
	11th to 20th	Night	30% 12% 32% 22% 4 1	800
	21st to end	Night	28% 9% 32% 28% 3	880
February	1st to 10th	Night	29% 8% 31% 28% 4	781
	11th to 20th	Night	25% 9% 36% 28% 2	743
	21st to end	Night	28% 9% 37% 23% 3	628
March	1st to 10th	Night	24% 10% 31% 28% 7%	760
	11th to 20th	Night	23% 10% 32% 27% 8% 1	760
	21st to end	Night	24% 8% 31% 28% 9%	836
April	1st to 10th	Night	21% 10% 32% 30% 6% 1	716
	11th to 20th	Night	19% 8% 32% 28% 12% 1	563
	21st to end	Night	19% 5% 35% 31% 9% 1	570
May	1st to 10th	Night	25% 3 26% 35% 11% 1	570
	11th to 20th	Night	21% 4 23% 35% 16% 1	561
	21st to end	Night	9% 4 20% 39% 27% 3	617
June	1st to 10th	Night	9% 2 22% 46% 19% 2	570
	11th to 20th	Night	10% 2 17% 45% 23% 2	570
	21st to end	Night	11% 2 17% 42% 26% 2	570
July	1st to 10th	Night	12% 2 9% 39% 29% 8%	555
	11th to 20th	Night	9% 3 10% 39% 34% 4	552
	21st to end	Night	10% 2 9% 40% 37% 1	624
August	1st to 10th	Night	8% 2 9% 43% 35% 4	567
	11th to 20th	Night	10% 2 8% 42% 32% 5%	564
	21st to end	Night	6% 1 9% 40% 37% 7%	627
September	1st to 10th	Night	14% 4 11% 35% 31% 5%	723
	11th to 20th	Night	11% 3 10% 35% 37% 5%	760
	21st to end	Night	17% 5% 17% 33% 25% 2	760
October	1st to 10th	Night	13% 4 22% 35% 22% 4	760
	11th to 20th	Night	21% 4 29% 32% 14% 1	760
	21st to end	Night	24% 7% 28% 31% 10% 1	836
November	1st to 10th	Night	17% 6% 27% 33% 16% 1	780
	11th to 20th	Night	23% 11% 30% 27% 9% 1	800
	21st to end	Night	25% 8% 29% 31% 8% 1	800
December	1st to 10th	Night	29% 8% 31% 26% 6% 1	800
	11th to 20th	Night	27% 9% 30% 28% 5% 1	800
	21st to end	Night	33% 9% 28% 26% 4	880

Page generated at 2025-02-03 03:05:52. Version ac07

Copyright © 2025 Attilla Danko.