Reisterstown
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Reisterstown clear sky chart from 2005-11-06 to 2025-10-19.

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 4 4 6% 8% 41%	126614
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	123086
Transparency	Day	40% 6% 12% 23% 16% 3	130208
	Night	39% 5% 11% 23% 17% 3	127562
Seeing	Day	20% 15% 29% 22% 11% 3	16202
	Night	19% 14% 30% 26% 11% 1	25607

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 4 3 3 4 4 5% 7% 49%	3597
	11th to 20th	Day	13% 8% 4 7% 5% 4 4 3 4 5% 43%	4037
	21st to end	Day	16% 8% 3 6% 4 4 4 4 4 5% 42%	4466
February	1st to 10th	Day	14% 8% 3 7% 5% 5% 5% 4 7% 6% 38%	1800
	11th to 20th	Day	18% 8% 1 6% 5% 4 4 4 5% 6% 39%	1821
	21st to end	Day	17% 9% 3 6% 4 4 4 2 4 5% 41%	1650
March	1st to 10th	Day	19% 5% 2 4 4 4 4 3 5% 7% 44%	4773
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 5% 7% 48%	4980
	21st to end	Day	16% 6% 2 6% 5% 4 5% 4 6% 6% 41%	5601
April	1st to 10th	Day	15% 8% 2 5% 5% 5% 4 4 7% 8% 38%	2400
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 3 5% 6% 45%	2374
	21st to end	Day	17% 6% 2 5% 4 3 5% 3 6% 7% 41%	2400
May	1st to 10th	Day	24% 6% 2 5% 4 4 5% 4 6% 9% 30%	5741
	11th to 20th	Day	24% 6% 3 6% 5% 4 4 4 6% 8% 30%	6272
	21st to end	Day	16% 5% 2 6% 4 5% 5% 4 6% 7% 39%	6934
June	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 10% 41%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 6% 5% 4 6% 8% 40%	2800
	21st to end	Day	10% 4 1 5% 4 5% 4 5% 7% 9% 46%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	6360
	11th to 20th	Day	10% 6% 2 7% 6% 6% 6% 5% 8% 11% 32%	6323
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 6% 10% 40%	6424
August	1st to 10th	Day	18% 5% 2 7% 5% 5% 5% 6% 7% 8% 33%	4234
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 8% 40%	3982
	21st to end	Day	12% 5% 2 5% 4 4 4 4 5% 7% 47%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 3 5% 7% 46%	2400
	11th to 20th	Day	12% 5% 2 6% 4 3 4 4 5% 8% 47%	2400
	21st to end	Day	21% 8% 2 7% 4 4 3 4 5% 7% 36%	2117
October	1st to 10th	Day	17% 6% 1 5% 3 3 3 4 5% 7% 46%	2000
	11th to 20th	Day	17% 5% 2 4 4 3 4 4 4 5% 48%	1980
	21st to end	Day	14% 6% 2 6% 4 4 4 3 4 5% 48%	2090
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 6% 7% 51%	1811
	11th to 20th	Day	14% 6% 1 4 3 3 4 3 6% 6% 49%	1785
	21st to end	Day	15% 7% 2 5% 4 4 4 3 5% 5% 47%	1600
December	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 6% 8% 44%	2600
	11th to 20th	Day	19% 9% 2 6% 4 3 3 3 4 6% 41%	2990
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 8% 37%	3960
January	1st to 10th	Night	12% 7% 2 5% 4 4 4 3 5% 6% 48%	6349
	11th to 20th	Night	14% 7% 2 5% 3 3 3 3 4 6% 50%	5918
	21st to end	Night	16% 8% 3 5% 3 3 3 3 4 5% 46%	6490
February	1st to 10th	Night	16% 8% 3 6% 5% 3 4 3 6% 7% 40%	2600
	11th to 20th	Night	18% 8% 3 6% 4 3 3 3 5% 5% 42%	2463
	21st to end	Night	15% 8% 2 7% 6% 4 3 3 5% 5% 42%	1980
March	1st to 10th	Night	17% 6% 2 5% 3 3 4 3 4 5% 48%	5062
	11th to 20th	Night	10% 6% 2 5% 3 4 3 3 5% 6% 51%	4980
	21st to end	Night	13% 8% 3 6% 4 4 4 3 4 6% 45%	5330
April	1st to 10th	Night	16% 8% 3 5% 4 6% 3 3 6% 7% 39%	2000
	11th to 20th	Night	14% 6% 3 7% 4 4 3 3 6% 6% 45%	1975
	21st to end	Night	17% 6% 2 4 3 4 4 4 6% 7% 43%	2000
May	1st to 10th	Night	24% 9% 2 6% 4 4 4 4 6% 7% 30%	4183
	11th to 20th	Night	24% 8% 3 6% 4 3 4 3 6% 8% 31%	3604
	21st to end	Night	19% 6% 2 6% 4 4 4 4 6% 8% 36%	3920
June	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 7% 8% 45%	1600
	11th to 20th	Night	12% 6% 2 6% 4 5% 4 5% 7% 9% 40%	1600
	21st to end	Night	11% 4 2 5% 4 5% 5% 4 7% 11% 42%	1600
July	1st to 10th	Night	8% 6% 3 7% 4 6% 6% 5% 6% 10% 39%	3598
	11th to 20th	Night	10% 7% 3 8% 6% 6% 7% 4 8% 10% 31%	3630
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 10% 44%	4427
August	1st to 10th	Night	19% 6% 2 6% 4 4 5% 4 7% 10% 34%	2915
	11th to 20th	Night	12% 7% 3 7% 5% 6% 5% 5% 6% 7% 37%	3034
	21st to end	Night	12% 5% 2 5% 4 3 4 4 5% 8% 48%	3586
September	1st to 10th	Night	13% 6% 2 5% 3 3 3 4 5% 8% 47%	2000
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 4 8% 51%	2000
	21st to end	Night	20% 7% 3 6% 4 4 4 3 5% 5% 38%	2279
October	1st to 10th	Night	20% 6% 2 5% 4 4 3 3 4 5% 46%	2400
	11th to 20th	Night	16% 5% 2 4 3 3 3 2 5% 6% 52%	2377
	21st to end	Night	15% 6% 1 5% 3 3 3 3 5% 6% 49%	2508
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 6% 53%	2485
	11th to 20th	Night	14% 8% 2 5% 3 3 4 3 4 6% 48%	2612
	21st to end	Night	13% 5% 2 5% 5% 3 4 4 6% 7% 47%	2800
December	1st to 10th	Night	11% 6% 2 5% 3 4 4 4 5% 7% 49%	4580
	11th to 20th	Night	21% 6% 2 4 4 4 4 3 5% 5% 40%	5205
	21st to end	Night	17% 7% 2 5% 4 4 5% 4 6% 8% 37%	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	35% 4 8% 27% 21% 5%	3598
	11th to 20th	Day	45% 4 8% 20% 17% 6%	4037
	21st to end	Day	43% 5% 10% 26% 13% 3	4466
February	1st to 10th	Day	44% 6% 12% 26% 11% 2	1800
	11th to 20th	Day	45% 5% 10% 23% 14% 3	1830
	21st to end	Day	46% 4 10% 22% 13% 4	1650
March	1st to 10th	Day	39% 4 10% 23% 17% 6%	4773
	11th to 20th	Day	35% 6% 10% 26% 17% 6%	4980
	21st to end	Day	44% 4 9% 18% 19% 5%	5601
April	1st to 10th	Day	42% 7% 12% 19% 16% 5%	2400
	11th to 20th	Day	39% 5% 11% 21% 18% 5%	2374
	21st to end	Day	42% 5% 10% 21% 19% 3	2400
May	1st to 10th	Day	49% 7% 14% 18% 10% 2	5775
	11th to 20th	Day	49% 8% 11% 21% 10% 1	6276
	21st to end	Day	41% 7% 13% 22% 16% 2	6949
June	1st to 10th	Day	37% 8% 14% 23% 15% 2	2800
	11th to 20th	Day	41% 7% 13% 23% 13% 2	2800
	21st to end	Day	33% 8% 15% 24% 18% 2	2800
July	1st to 10th	Day	36% 9% 16% 20% 16% 3	6290
	11th to 20th	Day	43% 10% 16% 23% 8% 1	6248
	21st to end	Day	38% 7% 11% 21% 22% 1	6240
August	1st to 10th	Day	46% 8% 12% 25% 9%	4221
	11th to 20th	Day	38% 7% 11% 28% 15% 1	4021
	21st to end	Day	35% 6% 12% 28% 18% 2	4312
September	1st to 10th	Day	38% 6% 11% 21% 20% 4	2400
	11th to 20th	Day	34% 5% 10% 27% 21% 3	2400
	21st to end	Day	48% 5% 9% 18% 18% 2	2117
October	1st to 10th	Day	37% 4 7% 19% 27% 5%	3260
	11th to 20th	Day	28% 4 10% 26% 22% 10%	3220
	21st to end	Day	25% 3 10% 29% 29% 4	3428
November	1st to 10th	Day	33% 5% 12% 26% 21% 4	1811
	11th to 20th	Day	36% 5% 11% 23% 22% 3	1785
	21st to end	Day	40% 5% 11% 29% 14% 2	1600
December	1st to 10th	Day	38% 5% 16% 27% 11% 4	2600
	11th to 20th	Day	45% 4 11% 28% 10% 1	2986
	21st to end	Day	44% 6% 16% 18% 14% 1	3960
January	1st to 10th	Night	37% 4 9% 25% 22% 4	6349
	11th to 20th	Night	37% 3 10% 25% 18% 6%	5918
	21st to end	Night	40% 4 8% 27% 18% 2	6490
February	1st to 10th	Night	43% 5% 14% 24% 13% 1	2600
	11th to 20th	Night	44% 4 9% 21% 18% 4	2465
	21st to end	Night	45% 4 10% 21% 16% 5%	1980
March	1st to 10th	Night	39% 5% 8% 26% 12% 10%	5062
	11th to 20th	Night	33% 5% 12% 30% 14% 6%	4980
	21st to end	Night	40% 3 10% 19% 23% 4	5330
April	1st to 10th	Night	44% 5% 10% 19% 18% 4	2000
	11th to 20th	Night	39% 5% 12% 20% 19% 5%	1975
	21st to end	Night	39% 6% 11% 22% 18% 4	2000
May	1st to 10th	Night	53% 6% 11% 16% 11% 3	4212
	11th to 20th	Night	50% 7% 10% 20% 11% 1	3605
	21st to end	Night	44% 7% 14% 23% 10% 2	3927
June	1st to 10th	Night	35% 8% 11% 26% 18% 2	1600
	11th to 20th	Night	38% 9% 13% 25% 14% 1	1600
	21st to end	Night	35% 7% 15% 24% 17% 2	1600
July	1st to 10th	Night	40% 10% 17% 18% 13% 3	3560
	11th to 20th	Night	47% 10% 16% 19% 8% 1	3585
	21st to end	Night	36% 7% 12% 24% 20% 1	4297
August	1st to 10th	Night	45% 8% 18% 20% 9% 1	2904
	11th to 20th	Night	43% 6% 10% 26% 14% 1	3061
	21st to end	Night	34% 6% 11% 30% 17% 2	3586
September	1st to 10th	Night	36% 6% 11% 22% 20% 5%	2000
	11th to 20th	Night	34% 4 10% 26% 24% 3	2000
	21st to end	Night	46% 6% 9% 17% 18% 3	2279
October	1st to 10th	Night	42% 5% 10% 19% 21% 4	3920
	11th to 20th	Night	25% 4 10% 25% 29% 7%	3876
	21st to end	Night	25% 3 11% 32% 24% 5%	4120
November	1st to 10th	Night	32% 4 10% 26% 22% 5%	2485
	11th to 20th	Night	39% 4 9% 24% 20% 5%	2612
	21st to end	Night	36% 5% 12% 25% 19% 2	2800
December	1st to 10th	Night	35% 4 14% 30% 15% 2	4580
	11th to 20th	Night	45% 4 13% 18% 17% 3	5208
	21st to end	Night	44% 7% 13% 19% 17% 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% 27% 29% 8% 4	200
	11th to 20th	Day	34% 23% 33% 9%	295
	21st to end	Day	25% 31% 30% 11% 3	330
February	1st to 10th	Day	28% 28% 34% 8% 2	300
	11th to 20th	Day	38% 26% 32% 4 1	368
	21st to end	Day	31% 26% 30% 11% 2	414
March	1st to 10th	Day	24% 27% 31% 15% 3	500
	11th to 20th	Day	27% 22% 32% 14% 3 1	500
	21st to end	Day	25% 23% 32% 15% 4 1	550
April	1st to 10th	Day	25% 23% 35% 12% 4 1	500
	11th to 20th	Day	22% 21% 30% 16% 9% 2	492
	21st to end	Day	26% 15% 37% 13% 8% 1	500
May	1st to 10th	Day	27% 17% 31% 17% 6% 2	503
	11th to 20th	Day	25% 15% 28% 22% 8% 2	527
	21st to end	Day	19% 8% 26% 27% 18% 2	573
June	1st to 10th	Day	15% 14% 31% 22% 14% 4	530
	11th to 20th	Day	18% 9% 29% 28% 13% 3	530
	21st to end	Day	12% 8% 31% 31% 14% 4	530
July	1st to 10th	Day	12% 6% 24% 32% 18% 8%	525
	11th to 20th	Day	7% 3 24% 36% 24% 6%	524
	21st to end	Day	12% 5% 26% 33% 19% 4	576
August	1st to 10th	Day	13% 7% 22% 31% 20% 8%	526
	11th to 20th	Day	14% 5% 22% 34% 18% 7%	519
	21st to end	Day	11% 5% 25% 31% 19% 9%	583
September	1st to 10th	Day	16% 8% 21% 25% 20% 8%	530
	11th to 20th	Day	12% 5% 24% 31% 20% 8%	530
	21st to end	Day	27% 12% 22% 25% 11% 3	530
October	1st to 10th	Day	21% 11% 24% 23% 15% 6%	530
	11th to 20th	Day	18% 18% 26% 24% 11% 2	522
	21st to end	Day	17% 17% 36% 21% 9% 2	539
November	1st to 10th	Day	17% 16% 33% 22% 11% 2	421
	11th to 20th	Day	18% 22% 33% 18% 8%	385
	21st to end	Day	29% 18% 33% 16% 5%	200
December	1st to 10th	Day	27% 24% 37% 10% 3 1	200
	11th to 20th	Day	30% 24% 31% 13% 4	200
	21st to end	Day	26% 22% 30% 16% 5%	220
January	1st to 10th	Night	21% 24% 36% 17% 2	800
	11th to 20th	Night	24% 21% 34% 18% 3	800
	21st to end	Night	20% 22% 40% 17% 1	880
February	1st to 10th	Night	20% 21% 36% 21% 2	800
	11th to 20th	Night	28% 25% 34% 13% 1	783
	21st to end	Night	25% 21% 37% 14% 2	660
March	1st to 10th	Night	20% 22% 37% 18% 3	800
	11th to 20th	Night	23% 21% 34% 19% 4	800
	21st to end	Night	20% 19% 41% 16% 4 1	880
April	1st to 10th	Night	22% 22% 35% 17% 4	800
	11th to 20th	Night	17% 19% 35% 23% 6% 1	791
	21st to end	Night	21% 17% 31% 24% 6%	800
May	1st to 10th	Night	22% 10% 32% 27% 9% 1	674
	11th to 20th	Night	22% 9% 29% 31% 9% 1	410
	21st to end	Night	17% 7% 24% 34% 17% 1	433
June	1st to 10th	Night	12% 3 31% 34% 20%	400
	11th to 20th	Night	14% 5% 27% 38% 16% 1	400
	21st to end	Night	11% 3 25% 44% 16% 1	400
July	1st to 10th	Night	9% 3 20% 41% 24% 3	390
	11th to 20th	Night	8% 3 19% 39% 28% 2	395
	21st to end	Night	10% 3 20% 39% 25% 3	657
August	1st to 10th	Night	14% 3 14% 42% 26% 2	597
	11th to 20th	Night	14% 5% 15% 40% 24% 2	688
	21st to end	Night	14% 4 19% 36% 24% 4	880
September	1st to 10th	Night	15% 4 21% 33% 24% 3	800
	11th to 20th	Night	12% 5% 17% 37% 25% 4	800
	21st to end	Night	24% 6% 21% 32% 17% 1	800
October	1st to 10th	Night	21% 9% 20% 27% 18% 4	800
	11th to 20th	Night	18% 12% 29% 28% 12% 1	793
	21st to end	Night	20% 13% 32% 23% 12%	836
November	1st to 10th	Night	14% 13% 33% 29% 9% 1	780
	11th to 20th	Night	20% 20% 30% 23% 6% 1	800
	21st to end	Night	17% 19% 36% 22% 6% 1	800
December	1st to 10th	Night	24% 16% 36% 17% 6%	800
	11th to 20th	Night	20% 17% 39% 20% 4	800
	21st to end	Night	21% 18% 35% 23% 3	880

Page generated at 2025-10-16 01:50:28. Version ac07

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