Catoctin Mountain Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Catoctin Mountain Park clear sky chart from 2005-11-06 to 2025-07-21.

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% 6% 2 6% 4 4 5% 4 6% 8% 39%	120260
	Night	15% 6% 2 6% 4 4 4 4 6% 7% 42%	118853
Transparency	Day	40% 7% 13% 23% 15% 3	123897
	Night	39% 6% 12% 23% 16% 3	123353
Seeing	Day	19% 13% 25% 23% 14% 5%	15786
	Night	18% 10% 29% 30% 12% 1	25268

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 7% 4 6% 6% 4 5% 6% 41%	3667
	11th to 20th	Day	12% 9% 3 9% 5% 5% 5% 4 5% 6% 38%	4060
	21st to end	Day	14% 8% 3 6% 4 4 5% 4 6% 6% 39%	4466
February	1st to 10th	Day	13% 9% 3 8% 6% 4 5% 5% 5% 6% 36%	1800
	11th to 20th	Day	15% 8% 3 8% 5% 5% 4 3 5% 6% 38%	1778
	21st to end	Day	17% 8% 3 6% 5% 4 4 3 6% 5% 39%	1650
March	1st to 10th	Day	18% 6% 1 4 4 4 5% 3 5% 7% 42%	4881
	11th to 20th	Day	11% 6% 3 6% 4 5% 4 4 5% 7% 45%	4980
	21st to end	Day	14% 7% 2 6% 5% 4 5% 4 5% 7% 40%	5543
April	1st to 10th	Day	15% 7% 2 7% 6% 5% 5% 4 6% 7% 36%	2400
	11th to 20th	Day	13% 7% 2 6% 4 5% 4 3 5% 7% 43%	2374
	21st to end	Day	18% 6% 2 6% 4 4 4 3 6% 6% 40%	2404
May	1st to 10th	Day	23% 6% 2 6% 4 4 5% 5% 7% 9% 29%	5862
	11th to 20th	Day	24% 6% 2 6% 4 4 4 4 7% 9% 30%	6175
	21st to end	Day	16% 5% 2 6% 5% 4 5% 4 7% 9% 37%	6934
June	1st to 10th	Day	13% 5% 2 5% 4 4 5% 5% 8% 10% 40%	2800
	11th to 20th	Day	13% 5% 2 5% 5% 5% 5% 4 8% 9% 39%	2800
	21st to end	Day	11% 4 1 5% 4 4 5% 4 6% 11% 45%	2800
July	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 10% 43%	4580
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 11% 41%	4463
	21st to end	Day	10% 7% 3 7% 6% 6% 5% 5% 6% 8% 37%	4466
August	1st to 10th	Day	17% 6% 2 6% 5% 6% 5% 5% 6% 9% 34%	4104
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	3925
	21st to end	Day	13% 5% 2 5% 4 4 4 4 6% 8% 46%	4180
September	1st to 10th	Day	16% 6% 1 5% 3 4 4 3 5% 6% 46%	2280
	11th to 20th	Day	13% 5% 2 5% 4 3 4 4 5% 8% 47%	2280
	21st to end	Day	23% 6% 2 5% 4 4 3 3 5% 6% 38%	1986
October	1st to 10th	Day	17% 6% 2 5% 4 4 4 3 5% 6% 45%	1900
	11th to 20th	Day	16% 5% 1 5% 3 3 3 3 6% 6% 47%	1900
	21st to end	Day	13% 6% 2 7% 3 3 4 4 5% 6% 46%	2090
November	1st to 10th	Day	11% 6% 2 4 3 3 4 3 6% 7% 50%	1852
	11th to 20th	Day	15% 5% 2 4 4 4 5% 4 5% 6% 46%	1730
	21st to end	Day	15% 7% 2 7% 5% 3 4 4 6% 7% 41%	1600
December	1st to 10th	Day	13% 6% 2 5% 4 5% 6% 5% 6% 8% 40%	2600
	11th to 20th	Day	19% 9% 2 6% 3 3 4 4 5% 6% 38%	2990
	21st to end	Day	16% 7% 3 8% 5% 4 5% 4 6% 9% 32%	3960
January	1st to 10th	Night	11% 7% 2 6% 4 4 5% 4 6% 9% 43%	6275
	11th to 20th	Night	14% 7% 2 6% 4 4 5% 4 5% 6% 45%	5900
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 44%	6490
February	1st to 10th	Night	14% 7% 3 7% 4 4 5% 4 6% 7% 38%	2600
	11th to 20th	Night	17% 7% 2 6% 5% 4 5% 4 4 6% 39%	2508
	21st to end	Night	16% 9% 2 7% 5% 5% 5% 4 5% 7% 36%	1980
March	1st to 10th	Night	16% 7% 2 4 4 4 4 3 5% 6% 46%	4967
	11th to 20th	Night	11% 5% 3 6% 4 3 4 3 4 6% 50%	4980
	21st to end	Night	11% 8% 2 6% 4 4 4 4 5% 7% 45%	5388
April	1st to 10th	Night	16% 8% 2 7% 5% 5% 4 4 6% 6% 36%	2000
	11th to 20th	Night	13% 6% 2 6% 5% 3 4 3 6% 8% 43%	1975
	21st to end	Night	17% 6% 2 5% 4 4 4 4 6% 7% 42%	2000
May	1st to 10th	Night	26% 8% 2 7% 5% 4 4 4 5% 8% 27%	4045
	11th to 20th	Night	25% 8% 2 6% 4 3 4 3 6% 8% 32%	3716
	21st to end	Night	19% 6% 2 6% 5% 4 5% 4 7% 8% 35%	3920
June	1st to 10th	Night	12% 4 2 5% 4 3 5% 3 6% 8% 47%	1600
	11th to 20th	Night	13% 5% 2 7% 4 5% 5% 5% 7% 8% 40%	1600
	21st to end	Night	9% 4 2 6% 5% 5% 5% 4 7% 10% 44%	1600
July	1st to 10th	Night	7% 4 2 6% 6% 5% 5% 5% 7% 11% 42%	2600
	11th to 20th	Night	7% 5% 3 7% 6% 5% 5% 6% 6% 10% 40%	2699
	21st to end	Night	12% 7% 2 6% 4 5% 5% 4 6% 9% 39%	3087
August	1st to 10th	Night	17% 5% 3 7% 4 4 5% 4 6% 8% 35%	2825
	11th to 20th	Night	10% 6% 2 8% 6% 4 5% 4 6% 7% 41%	2871
	21st to end	Night	14% 4 1 5% 4 4 4 4 5% 8% 48%	3476
September	1st to 10th	Night	14% 5% 2 5% 4 4 4 4 5% 6% 47%	1900
	11th to 20th	Night	13% 5% 2 6% 3 3 3 3 4 7% 53%	1900
	21st to end	Night	23% 5% 2 4 3 4 4 3 4 6% 42%	2189
October	1st to 10th	Night	20% 5% 3 5% 3 4 4 3 3 6% 44%	2280
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 4 6% 51%	2280
	21st to end	Night	15% 6% 1 5% 3 3 3 3 6% 7% 48%	2508
November	1st to 10th	Night	10% 6% 2 5% 3 4 3 3 5% 6% 53%	2446
	11th to 20th	Night	14% 8% 2 4 4 4 4 4 5% 6% 46%	2667
	21st to end	Night	13% 5% 2 6% 5% 4 5% 4 6% 7% 43%	2800
December	1st to 10th	Night	10% 6% 2 5% 4 5% 5% 4 6% 8% 45%	4580
	11th to 20th	Night	20% 7% 2 5% 4 4 5% 4 5% 6% 37%	5205
	21st to end	Night	16% 8% 2 7% 5% 5% 5% 5% 7% 8% 32%	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	40% 4 12% 25% 16% 4	3668
	11th to 20th	Day	44% 6% 10% 18% 17% 5%	4060
	21st to end	Day	43% 6% 12% 24% 13% 3	4466
February	1st to 10th	Day	47% 7% 12% 23% 11% 1	1800
	11th to 20th	Day	46% 6% 10% 22% 14% 2	1787
	21st to end	Day	47% 5% 12% 20% 13% 4	1650
March	1st to 10th	Day	41% 5% 10% 22% 15% 6%	4881
	11th to 20th	Day	36% 5% 12% 26% 15% 5%	4980
	21st to end	Day	43% 5% 11% 19% 18% 4	5543
April	1st to 10th	Day	46% 6% 11% 18% 15% 4	2400
	11th to 20th	Day	40% 6% 12% 21% 18% 4	2374
	21st to end	Day	43% 6% 10% 21% 17% 3	2404
May	1st to 10th	Day	49% 7% 14% 19% 10% 2	5897
	11th to 20th	Day	48% 9% 11% 23% 9% 1	6179
	21st to end	Day	41% 8% 13% 22% 15% 2	6949
June	1st to 10th	Day	36% 9% 15% 23% 16% 1	2800
	11th to 20th	Day	39% 10% 13% 23% 13% 2	2800
	21st to end	Day	33% 8% 14% 27% 16% 1	2800
July	1st to 10th	Day	33% 10% 17% 23% 12% 5%	4510
	11th to 20th	Day	34% 9% 16% 28% 12% 1	4388
	21st to end	Day	43% 7% 11% 20% 17% 1	4324
August	1st to 10th	Day	44% 9% 14% 24% 8%	4091
	11th to 20th	Day	37% 8% 12% 28% 14% 1	3964
	21st to end	Day	35% 6% 13% 27% 17% 2	4180
September	1st to 10th	Day	39% 6% 12% 19% 21% 4	2280
	11th to 20th	Day	34% 6% 11% 25% 21% 3	2280
	21st to end	Day	47% 5% 9% 18% 19% 2	1986
October	1st to 10th	Day	36% 5% 8% 20% 26% 5%	3160
	11th to 20th	Day	27% 4 12% 27% 21% 9%	3140
	21st to end	Day	25% 4 12% 30% 26% 3	3428
November	1st to 10th	Day	33% 5% 14% 24% 20% 4	1852
	11th to 20th	Day	37% 5% 14% 22% 19% 3	1730
	21st to end	Day	41% 6% 12% 28% 12% 1	1600
December	1st to 10th	Day	38% 7% 17% 28% 7% 3	2600
	11th to 20th	Day	47% 5% 13% 25% 8% 2	2986
	21st to end	Day	47% 7% 17% 15% 13% 1	3960
January	1st to 10th	Night	36% 6% 11% 26% 19% 2	6275
	11th to 20th	Night	38% 5% 12% 24% 16% 6%	5900
	21st to end	Night	39% 5% 12% 26% 15% 2	6490
February	1st to 10th	Night	43% 7% 13% 23% 12% 1	2600
	11th to 20th	Night	44% 5% 10% 21% 17% 2	2510
	21st to end	Night	46% 6% 12% 20% 13% 4	1980
March	1st to 10th	Night	38% 5% 9% 27% 12% 8%	4967
	11th to 20th	Night	35% 5% 11% 29% 14% 5%	4980
	21st to end	Night	38% 4 11% 21% 22% 4	5388
April	1st to 10th	Night	46% 7% 11% 17% 15% 3	2000
	11th to 20th	Night	38% 6% 12% 21% 18% 5%	1975
	21st to end	Night	39% 6% 11% 24% 16% 4	2000
May	1st to 10th	Night	54% 6% 10% 15% 11% 3	4072
	11th to 20th	Night	48% 7% 11% 22% 10% 1	3717
	21st to end	Night	45% 8% 13% 24% 9% 1	3927
June	1st to 10th	Night	35% 8% 10% 26% 17% 3	1600
	11th to 20th	Night	39% 9% 12% 24% 14% 2	1600
	21st to end	Night	35% 8% 13% 24% 18% 2	1600
July	1st to 10th	Night	36% 10% 18% 17% 13% 5%	2560
	11th to 20th	Night	37% 9% 15% 23% 14% 1	2651
	21st to end	Night	39% 9% 14% 19% 18% 1	2988
August	1st to 10th	Night	45% 8% 17% 22% 8% 1	2814
	11th to 20th	Night	39% 7% 12% 27% 14% 1	2898
	21st to end	Night	35% 5% 13% 29% 16% 2	3476
September	1st to 10th	Night	37% 7% 12% 19% 21% 4	1900
	11th to 20th	Night	33% 5% 9% 27% 23% 3	1900
	21st to end	Night	44% 6% 10% 20% 18% 2	2189
October	1st to 10th	Night	39% 5% 10% 22% 22% 2	3800
	11th to 20th	Night	24% 5% 11% 25% 28% 7%	3779
	21st to end	Night	25% 4 12% 31% 24% 5%	4120
November	1st to 10th	Night	32% 6% 11% 26% 21% 5%	2446
	11th to 20th	Night	38% 6% 10% 23% 19% 4	2667
	21st to end	Night	38% 7% 13% 24% 16% 2	2800
December	1st to 10th	Night	36% 5% 15% 31% 12% 1	4580
	11th to 20th	Night	46% 6% 13% 19% 13% 2	5208
	21st to end	Night	46% 9% 15% 17% 13%	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	28% 28% 26% 13% 4	216
	11th to 20th	Day	29% 29% 29% 10% 3	300
	21st to end	Day	21% 30% 33% 12% 3 1	330
February	1st to 10th	Day	25% 26% 35% 11% 3	300
	11th to 20th	Day	34% 26% 32% 6% 1	325
	21st to end	Day	27% 22% 31% 17% 3	414
March	1st to 10th	Day	24% 22% 27% 22% 4	500
	11th to 20th	Day	24% 21% 30% 18% 5% 1	500
	21st to end	Day	25% 20% 31% 18% 5% 1	550
April	1st to 10th	Day	23% 20% 29% 21% 6% 2	500
	11th to 20th	Day	20% 18% 29% 17% 13% 3	492
	21st to end	Day	24% 16% 28% 21% 8% 3	500
May	1st to 10th	Day	26% 14% 29% 19% 9% 3	503
	11th to 20th	Day	22% 14% 27% 21% 13% 3	527
	21st to end	Day	18% 9% 21% 24% 21% 7%	573
June	1st to 10th	Day	15% 9% 27% 24% 18% 6%	530
	11th to 20th	Day	17% 6% 26% 29% 17% 5%	530
	21st to end	Day	12% 6% 24% 30% 22% 6%	530
July	1st to 10th	Day	11% 4 19% 31% 23% 12%	525
	11th to 20th	Day	8% 5% 19% 29% 29% 11%	524
	21st to end	Day	11% 5% 19% 32% 25% 9%	532
August	1st to 10th	Day	12% 3 19% 26% 29% 12%	486
	11th to 20th	Day	12% 3 20% 30% 22% 13%	479
	21st to end	Day	12% 4 16% 29% 24% 14%	539
September	1st to 10th	Day	17% 7% 17% 24% 23% 12%	490
	11th to 20th	Day	15% 3 17% 28% 26% 12%	490
	21st to end	Day	28% 8% 22% 22% 16% 5%	490
October	1st to 10th	Day	20% 8% 22% 25% 19% 6%	490
	11th to 20th	Day	17% 14% 26% 27% 14% 2	490
	21st to end	Day	15% 12% 34% 27% 9% 2	539
November	1st to 10th	Day	16% 13% 29% 28% 12% 2	442
	11th to 20th	Day	19% 17% 30% 23% 9% 1	330
	21st to end	Day	22% 24% 32% 17% 7% 1	200
December	1st to 10th	Day	24% 24% 32% 17% 3 2	200
	11th to 20th	Day	24% 23% 29% 19% 6%	200
	21st to end	Day	23% 19% 30% 22% 6%	220
January	1st to 10th	Night	18% 19% 41% 20% 2	800
	11th to 20th	Night	23% 17% 33% 25% 3	800
	21st to end	Night	18% 16% 41% 24% 1	880
February	1st to 10th	Night	19% 17% 35% 27% 3	800
	11th to 20th	Night	25% 21% 36% 17% 1	783
	21st to end	Night	24% 18% 37% 19% 2	660
March	1st to 10th	Night	20% 16% 38% 22% 4	800
	11th to 20th	Night	23% 17% 33% 22% 5%	800
	21st to end	Night	18% 16% 38% 22% 5% 1	880
April	1st to 10th	Night	21% 18% 34% 24% 4	800
	11th to 20th	Night	15% 14% 33% 30% 8%	791
	21st to end	Night	21% 11% 32% 29% 8%	800
May	1st to 10th	Night	25% 8% 24% 31% 12%	612
	11th to 20th	Night	22% 7% 25% 34% 12% 1	457
	21st to end	Night	16% 3 23% 36% 20% 1	433
June	1st to 10th	Night	14% 2 27% 33% 23% 1	400
	11th to 20th	Night	14% 5% 19% 45% 16% 2	400
	21st to end	Night	8% 4 20% 48% 20%	400
July	1st to 10th	Night	12% 3 16% 40% 26% 3	390
	11th to 20th	Night	8% 2 16% 39% 33% 3	448
	21st to end	Night	10% 2 14% 42% 29% 3	624
August	1st to 10th	Night	12% 1 9% 44% 32% 2	567
	11th to 20th	Night	12% 3 14% 41% 28% 3	611
	21st to end	Night	14% 2 12% 40% 28% 4	836
September	1st to 10th	Night	16% 2 19% 34% 27% 3	760
	11th to 20th	Night	14% 3 17% 34% 29% 4	760
	21st to end	Night	26% 3 18% 33% 19% 1	760
October	1st to 10th	Night	21% 4 21% 29% 21% 3	760
	11th to 20th	Night	16% 9% 29% 33% 12% 1	760
	21st to end	Night	19% 8% 31% 30% 10% 1	836
November	1st to 10th	Night	13% 8% 33% 35% 9% 2	780
	11th to 20th	Night	19% 15% 31% 28% 8%	800
	21st to end	Night	16% 14% 35% 29% 6%	800
December	1st to 10th	Night	21% 14% 37% 22% 5%	800
	11th to 20th	Night	19% 11% 40% 26% 5%	800
	21st to end	Night	19% 12% 35% 31% 3	880

Page generated at 2025-07-18 01:10:03. Version ac07

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