Pickersville Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pickersville 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	12% 6% 2 6% 4 4 4 4 6% 7% 43%	113939
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	115317
Transparency	Day	38% 7% 11% 19% 19% 6%	117586
	Night	37% 6% 10% 18% 21% 7%	119809
Seeing	Day	15% 8% 23% 25% 20% 9%	16154
	Night	15% 7% 25% 34% 18% 2	26508

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	14% 5% 2 5% 3 2 3 2 3 4 57%	4056
	11th to 20th	Day	13% 5% 1 5% 3 2 3 2 3 4 58%	4060
	21st to end	Day	13% 7% 2 7% 4 4 4 4 4 5% 45%	4466
February	1st to 10th	Day	15% 6% 2 6% 4 4 4 3 6% 8% 43%	1800
	11th to 20th	Day	21% 6% 2 5% 3 3 3 2 4 5% 44%	1798
	21st to end	Day	18% 7% 2 5% 4 4 4 3 4 6% 43%	1790
March	1st to 10th	Day	20% 7% 2 6% 4 4 4 4 5% 6% 39%	4935
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 5% 6% 53%	4980
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 6% 49%	5465
April	1st to 10th	Day	10% 5% 1 4 4 4 4 3 5% 7% 54%	2335
	11th to 20th	Day	10% 4 2 5% 3 3 4 4 6% 7% 51%	2374
	21st to end	Day	11% 5% 2 5% 4 4 4 3 6% 7% 49%	2400
May	1st to 10th	Day	9% 6% 3 8% 6% 6% 6% 4 7% 10% 36%	4080
	11th to 20th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 7% 35%	4211
	21st to end	Day	6% 4 2 5% 5% 5% 5% 4 7% 10% 48%	4541
June	1st to 10th	Day	9% 6% 3 7% 6% 6% 5% 5% 7% 9% 38%	2470
	11th to 20th	Day	9% 5% 1 5% 5% 5% 6% 5% 7% 10% 42%	2470
	21st to end	Day	8% 5% 2 6% 6% 5% 6% 5% 8% 10% 39%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 5% 9% 12% 33%	4120
	11th to 20th	Day	10% 7% 3 9% 6% 6% 7% 5% 9% 11% 28%	4120
	21st to end	Day	9% 7% 4 9% 6% 6% 6% 5% 8% 11% 28%	4466
August	1st to 10th	Day	20% 10% 4 9% 6% 6% 5% 5% 8% 9% 20%	4104
	11th to 20th	Day	10% 6% 3 9% 6% 5% 6% 4 7% 10% 34%	3858
	21st to end	Day	6% 4 2 5% 4 5% 5% 4 7% 10% 47%	4180
September	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	2280
	11th to 20th	Day	14% 6% 2 7% 4 5% 4 3 6% 7% 41%	2176
	21st to end	Day	18% 7% 3 6% 5% 5% 4 4 6% 7% 36%	2034
October	1st to 10th	Day	15% 6% 3 5% 3 3 3 3 5% 6% 47%	1900
	11th to 20th	Day	11% 5% 2 5% 3 3 2 2 4 5% 57%	1900
	21st to end	Day	11% 5% 2 5% 4 3 2 2 4 5% 57%	2090
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 4 6% 52%	1957
	11th to 20th	Day	14% 6% 2 5% 3 3 3 4 4 6% 51%	2000
	21st to end	Day	14% 6% 1 4 3 3 3 3 5% 7% 50%	1746
December	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 5% 52%	2600
	11th to 20th	Day	16% 6% 2 7% 3 3 4 3 5% 6% 46%	3241
	21st to end	Day	16% 6% 2 6% 4 3 4 3 5% 6% 44%	4466
January	1st to 10th	Night	12% 5% 2 5% 3 3 3 2 4 5% 56%	5889
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 4 52%	5900
	21st to end	Night	13% 8% 2 6% 4 4 3 3 4 6% 46%	6490
February	1st to 10th	Night	17% 7% 2 5% 3 4 4 3 5% 7% 44%	2600
	11th to 20th	Night	20% 6% 2 6% 4 3 4 4 4 6% 42%	2487
	21st to end	Night	15% 6% 2 5% 3 4 4 3 5% 6% 47%	1835
March	1st to 10th	Night	19% 8% 3 7% 4 4 3 3 4 5% 41%	4918
	11th to 20th	Night	12% 5% 2 4 3 4 3 3 5% 7% 52%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 5% 50%	5467
April	1st to 10th	Night	13% 5% 2 6% 3 3 4 3 5% 6% 49%	2065
	11th to 20th	Night	11% 4 2 6% 4 5% 4 3 5% 6% 51%	1975
	21st to end	Night	11% 3 2 5% 4 4 5% 4 7% 7% 50%	2000
May	1st to 10th	Night	10% 7% 3 8% 5% 4 5% 4 6% 9% 39%	3051
	11th to 20th	Night	16% 8% 4 7% 5% 5% 4 4 4 7% 36%	2902
	21st to end	Night	7% 5% 2 5% 4 5% 5% 4 8% 11% 44%	3130
June	1st to 10th	Night	10% 5% 3 5% 6% 5% 5% 5% 7% 11% 38%	1710
	11th to 20th	Night	10% 5% 2 6% 6% 4 5% 5% 6% 9% 41%	1710
	21st to end	Night	8% 5% 2 6% 6% 6% 6% 5% 9% 11% 38%	1710
July	1st to 10th	Night	9% 6% 3 8% 6% 6% 6% 4 10% 12% 30%	2840
	11th to 20th	Night	12% 7% 3 7% 6% 5% 6% 5% 8% 12% 29%	2840
	21st to end	Night	16% 10% 4 10% 5% 6% 5% 5% 6% 8% 27%	3087
August	1st to 10th	Night	19% 9% 4 9% 6% 5% 6% 4 6% 8% 24%	2825
	11th to 20th	Night	8% 6% 3 8% 6% 6% 7% 5% 7% 11% 32%	2936
	21st to end	Night	6% 4 2 5% 4 5% 5% 5% 8% 10% 45%	3476
September	1st to 10th	Night	10% 4 2 6% 5% 5% 4 3 6% 9% 46%	1900
	11th to 20th	Night	13% 6% 3 7% 5% 5% 5% 4 6% 7% 40%	2000
	21st to end	Night	18% 7% 3 8% 6% 5% 4 3 4 6% 38%	2144
October	1st to 10th	Night	15% 6% 2 5% 4 3 4 2 4 7% 49%	2280
	11th to 20th	Night	13% 4 2 5% 3 2 3 3 4 4 56%	2280
	21st to end	Night	11% 4 2 5% 4 3 3 3 4 6% 56%	2508
November	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 3 5% 53%	2341
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 6% 49%	2400
	21st to end	Night	13% 4 2 5% 3 3 3 3 5% 7% 52%	2640
December	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 54%	4580
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 3 4 5% 41%	4931
	21st to end	Night	14% 6% 2 6% 3 4 4 3 5% 6% 48%	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	34% 3 5% 19% 31% 8%	4057
	11th to 20th	Day	32% 3 4 16% 34% 11%	4060
	21st to end	Day	42% 4 6% 15% 22% 11%	4466
February	1st to 10th	Day	40% 4 12% 19% 19% 6%	1800
	11th to 20th	Day	43% 4 6% 17% 24% 6%	1807
	21st to end	Day	43% 5% 8% 17% 20% 7%	1790
March	1st to 10th	Day	45% 4 7% 15% 19% 10%	4935
	11th to 20th	Day	31% 5% 8% 14% 27% 14%	4980
	21st to end	Day	38% 4 9% 22% 21% 7%	5465
April	1st to 10th	Day	29% 5% 10% 24% 24% 7%	2335
	11th to 20th	Day	31% 6% 10% 23% 21% 8%	2374
	21st to end	Day	34% 5% 14% 23% 20% 4	2400
May	1st to 10th	Day	41% 9% 17% 19% 12% 2	4115
	11th to 20th	Day	45% 8% 12% 21% 14% 1	4215
	21st to end	Day	29% 10% 15% 23% 20% 3	4554
June	1st to 10th	Day	40% 11% 14% 19% 15% 1	2470
	11th to 20th	Day	33% 10% 16% 23% 15% 2	2470
	21st to end	Day	35% 12% 16% 21% 12% 3	2470
July	1st to 10th	Day	37% 12% 18% 20% 10% 2	4055
	11th to 20th	Day	45% 14% 23% 13% 4 1	4051
	21st to end	Day	47% 13% 15% 15% 9% 1	4324
August	1st to 10th	Day	58% 10% 16% 11% 5%	4091
	11th to 20th	Day	43% 11% 13% 22% 10%	3897
	21st to end	Day	30% 8% 16% 31% 14% 1	4180
September	1st to 10th	Day	36% 10% 15% 17% 17% 5%	2280
	11th to 20th	Day	41% 7% 11% 15% 21% 4	2176
	21st to end	Day	46% 6% 10% 18% 15% 4	2034
October	1st to 10th	Day	35% 5% 9% 21% 24% 7%	3160
	11th to 20th	Day	20% 3 6% 19% 25% 27%	3140
	21st to end	Day	25% 3 6% 23% 26% 17%	3428
November	1st to 10th	Day	34% 4 8% 19% 26% 8%	1957
	11th to 20th	Day	35% 4 8% 20% 25% 8%	2000
	21st to end	Day	34% 4 7% 23% 24% 8%	1746
December	1st to 10th	Day	35% 5% 8% 19% 24% 10%	2600
	11th to 20th	Day	41% 4 11% 18% 21% 5%	3238
	21st to end	Day	40% 4 10% 23% 21% 2	4466
January	1st to 10th	Night	31% 3 6% 16% 36% 8%	5889
	11th to 20th	Night	35% 5% 6% 14% 33% 8%	5900
	21st to end	Night	40% 4 6% 16% 27% 8%	6490
February	1st to 10th	Night	41% 4 9% 20% 20% 7%	2600
	11th to 20th	Night	44% 5% 9% 15% 20% 7%	2489
	21st to end	Night	38% 5% 8% 17% 23% 8%	1835
March	1st to 10th	Night	47% 5% 6% 12% 23% 8%	4918
	11th to 20th	Night	32% 5% 7% 12% 26% 17%	4980
	21st to end	Night	36% 5% 9% 22% 21% 6%	5467
April	1st to 10th	Night	36% 5% 11% 17% 22% 9%	2065
	11th to 20th	Night	33% 5% 10% 23% 21% 8%	1975
	21st to end	Night	31% 8% 13% 25% 19% 4	2000
May	1st to 10th	Night	39% 10% 16% 20% 12% 2	3081
	11th to 20th	Night	46% 7% 11% 21% 15% 1	2903
	21st to end	Night	29% 10% 16% 25% 17% 2	3138
June	1st to 10th	Night	36% 11% 19% 21% 11% 2	1710
	11th to 20th	Night	37% 9% 18% 21% 13% 2	1710
	21st to end	Night	37% 12% 17% 22% 10% 2	1710
July	1st to 10th	Night	43% 13% 18% 19% 7% 1	2795
	11th to 20th	Night	44% 14% 24% 13% 5% 1	2786
	21st to end	Night	54% 9% 13% 14% 9%	2988
August	1st to 10th	Night	57% 9% 18% 13% 3	2814
	11th to 20th	Night	42% 12% 19% 19% 7% 1	2963
	21st to end	Night	30% 9% 14% 30% 15% 2	3476
September	1st to 10th	Night	36% 10% 14% 19% 15% 5%	1900
	11th to 20th	Night	41% 8% 13% 17% 18% 3	2000
	21st to end	Night	48% 7% 11% 17% 15% 3	2144
October	1st to 10th	Night	37% 5% 7% 21% 23% 6%	3800
	11th to 20th	Night	22% 3 6% 17% 26% 27%	3779
	21st to end	Night	24% 4 7% 25% 23% 17%	4120
November	1st to 10th	Night	34% 4 7% 18% 27% 10%	2341
	11th to 20th	Night	36% 4 8% 16% 25% 11%	2400
	21st to end	Night	33% 4 8% 22% 26% 8%	2640
December	1st to 10th	Night	32% 4 7% 15% 31% 10%	4580
	11th to 20th	Night	45% 5% 9% 16% 17% 8%	4933
	21st to end	Night	37% 5% 10% 24% 22% 2	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	16% 19% 41% 19% 4	300
	11th to 20th	Day	18% 23% 32% 20% 7%	300
	21st to end	Day	15% 20% 39% 20% 6%	330
February	1st to 10th	Day	18% 14% 40% 22% 5% 1	300
	11th to 20th	Day	34% 12% 37% 16% 1	345
	21st to end	Day	21% 15% 34% 21% 8% 1	414
March	1st to 10th	Day	17% 16% 31% 25% 9% 1	500
	11th to 20th	Day	17% 14% 38% 24% 6% 1	500
	21st to end	Day	16% 14% 35% 26% 8% 1	550
April	1st to 10th	Day	16% 16% 30% 25% 11% 2	500
	11th to 20th	Day	11% 11% 31% 25% 17% 5%	492
	21st to end	Day	15% 9% 26% 24% 22% 5%	500
May	1st to 10th	Day	10% 10% 36% 23% 15% 6%	503
	11th to 20th	Day	16% 4 21% 26% 25% 9%	527
	21st to end	Day	13% 5% 18% 25% 24% 15%	549
June	1st to 10th	Day	12% 3 15% 34% 24% 12%	490
	11th to 20th	Day	11% 2 14% 26% 30% 16%	490
	21st to end	Day	8% 3 13% 27% 33% 16%	490
July	1st to 10th	Day	10% 3 8% 23% 33% 23%	485
	11th to 20th	Day	9% 2 8% 21% 37% 22%	484
	21st to end	Day	7% 2 11% 28% 38% 15%	532
August	1st to 10th	Day	14% 2 13% 22% 29% 19%	486
	11th to 20th	Day	12% 2 10% 23% 29% 24%	479
	21st to end	Day	12% 2 9% 24% 27% 26%	539
September	1st to 10th	Day	10% 2 14% 26% 26% 22%	490
	11th to 20th	Day	18% 4 11% 23% 30% 14%	490
	21st to end	Day	21% 3 18% 28% 22% 7%	490
October	1st to 10th	Day	16% 7% 18% 24% 24% 11%	490
	11th to 20th	Day	13% 6% 24% 29% 22% 6%	490
	21st to end	Day	12% 8% 21% 30% 23% 6%	539
November	1st to 10th	Day	18% 6% 23% 32% 16% 5%	495
	11th to 20th	Day	17% 8% 31% 31% 11% 2	500
	21st to end	Day	12% 12% 31% 25% 18% 2	295
December	1st to 10th	Day	17% 12% 40% 21% 11% 1	200
	11th to 20th	Day	18% 8% 39% 19% 13% 3	260
	21st to end	Day	22% 14% 32% 20% 9% 3	330
January	1st to 10th	Night	18% 12% 36% 27% 7% 1	800
	11th to 20th	Night	17% 11% 34% 31% 7%	800
	21st to end	Night	19% 9% 39% 30% 4	880
February	1st to 10th	Night	20% 10% 36% 31% 4	800
	11th to 20th	Night	23% 12% 40% 23% 1	783
	21st to end	Night	16% 12% 38% 29% 5%	660
March	1st to 10th	Night	18% 12% 33% 32% 6%	800
	11th to 20th	Night	17% 10% 39% 27% 7% 1	800
	21st to end	Night	16% 10% 38% 30% 6%	880
April	1st to 10th	Night	15% 13% 36% 30% 7%	800
	11th to 20th	Night	13% 9% 34% 33% 11% 1	791
	21st to end	Night	13% 7% 29% 41% 10% 1	800
May	1st to 10th	Night	9% 8% 33% 38% 12%	699
	11th to 20th	Night	15% 5% 22% 40% 17%	591
	21st to end	Night	11% 2 13% 41% 31% 2	634
June	1st to 10th	Night	11% 2 16% 44% 25% 2	570
	11th to 20th	Night	11% 2 14% 39% 29% 6%	570
	21st to end	Night	8% 2 13% 41% 34% 2	570
July	1st to 10th	Night	8% 2 9% 34% 42% 5%	555
	11th to 20th	Night	7% 2 7% 35% 43% 5%	552
	21st to end	Night	9% 3 10% 38% 38% 2	624
August	1st to 10th	Night	13% 2 9% 34% 38% 4	567
	11th to 20th	Night	12% 2 10% 33% 38% 5%	650
	21st to end	Night	11% 2 8% 34% 38% 6%	836
September	1st to 10th	Night	11% 2 11% 30% 39% 6%	760
	11th to 20th	Night	14% 4 12% 31% 35% 4	760
	21st to end	Night	22% 4 16% 32% 24% 2	760
October	1st to 10th	Night	17% 4 16% 31% 28% 4	760
	11th to 20th	Night	14% 4 26% 33% 22% 2	760
	21st to end	Night	14% 4 24% 34% 21% 2	836
November	1st to 10th	Night	15% 6% 23% 37% 17% 3	780
	11th to 20th	Night	14% 11% 27% 34% 13% 1	800
	21st to end	Night	15% 6% 29% 34% 16% 1	800
December	1st to 10th	Night	18% 10% 29% 36% 7%	800
	11th to 20th	Night	19% 7% 28% 36% 9% 1	800
	21st to end	Night	18% 9% 28% 35% 8%	880

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