Harford Community College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Harford Community College Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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%	127056
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	123637
Transparency	Day	39% 7% 11% 23% 16% 3	130644
	Night	40% 5% 11% 23% 17% 4	128118
Seeing	Day	20% 15% 28% 23% 12% 3	16362
	Night	19% 13% 29% 27% 11% 1	25754

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% 4 4 4 3 5% 6% 51%	3597
	11th to 20th	Day	13% 9% 3 8% 5% 5% 4 3 3 5% 43%	3940
	21st to end	Day	16% 8% 3 6% 4 4 4 3 4 6% 43%	4466
February	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 5% 7% 40%	1800
	11th to 20th	Day	17% 8% 2 6% 4 4 4 4 5% 7% 39%	1852
	21st to end	Day	18% 11% 3 5% 4 3 3 3 5% 5% 41%	1650
March	1st to 10th	Day	19% 5% 1 4 4 4 4 3 5% 5% 47%	4680
	11th to 20th	Day	12% 7% 2 6% 4 4 4 3 5% 6% 47%	4980
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 6% 42%	5666
April	1st to 10th	Day	16% 7% 3 6% 5% 4 5% 3 5% 7% 40%	2400
	11th to 20th	Day	13% 7% 2 5% 5% 4 5% 3 6% 7% 43%	2374
	21st to end	Day	17% 6% 1 5% 4 4 4 4 5% 8% 41%	2400
May	1st to 10th	Day	25% 6% 2 5% 4 4 5% 4 6% 8% 31%	5730
	11th to 20th	Day	24% 7% 2 6% 4 4 4 4 6% 7% 31%	6318
	21st to end	Day	17% 5% 2 5% 4 4 4 4 7% 8% 37%	6934
June	1st to 10th	Day	12% 5% 2 6% 6% 5% 4 4 6% 10% 40%	2800
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 4 8% 9% 39%	2800
	21st to end	Day	10% 4 2 4 4 4 5% 6% 7% 11% 44%	2800
July	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 5% 8% 11% 42%	6360
	11th to 20th	Day	10% 6% 2 7% 5% 5% 7% 6% 9% 11% 31%	6360
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 5% 8% 11% 40%	6522
August	1st to 10th	Day	17% 6% 2 6% 4 4 6% 4 7% 9% 34%	4234
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 8% 9% 40%	3927
	21st to end	Day	10% 4 2 5% 4 4 5% 4 7% 8% 48%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	12% 4 1 5% 4 4 4 3 5% 7% 50%	2400
	21st to end	Day	20% 8% 3 6% 4 4 4 3 5% 7% 37%	2136
October	1st to 10th	Day	16% 6% 2 6% 4 3 4 4 5% 7% 46%	2000
	11th to 20th	Day	17% 4 2 4 3 4 4 3 5% 7% 48%	2000
	21st to end	Day	15% 5% 2 6% 3 4 4 3 4 6% 48%	2200
November	1st to 10th	Day	13% 5% 1 4 3 3 3 4 5% 7% 52%	1861
	11th to 20th	Day	14% 6% 2 4 3 5% 3 4 5% 6% 48%	1890
	21st to end	Day	15% 6% 3 6% 4 3 4 4 4 6% 46%	1701
December	1st to 10th	Day	13% 7% 3 5% 4 4 4 3 5% 6% 46%	2616
	11th to 20th	Day	20% 8% 2 5% 3 4 3 3 5% 6% 40%	2990
	21st to end	Day	15% 6% 3 7% 4 5% 4 4 6% 7% 39%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 4 3 3 5% 6% 49%	6349
	11th to 20th	Night	15% 6% 3 5% 3 3 3 2 4 5% 50%	6016
	21st to end	Night	17% 8% 2 5% 3 3 3 3 4 6% 46%	6490
February	1st to 10th	Night	16% 8% 3 6% 5% 3 4 4 4 6% 40%	2600
	11th to 20th	Night	17% 9% 2 6% 4 4 3 3 4 5% 43%	2433
	21st to end	Night	16% 8% 3 6% 5% 4 3 3 4 5% 42%	1980
March	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 3 5% 48%	5157
	11th to 20th	Night	11% 7% 2 5% 4 4 3 3 4 6% 51%	4980
	21st to end	Night	14% 8% 2 5% 4 4 4 3 4 6% 45%	5251
April	1st to 10th	Night	17% 7% 2 5% 4 5% 4 3 5% 7% 39%	2000
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 6% 8% 44%	1975
	21st to end	Night	19% 6% 2 4 3 4 4 3 5% 6% 45%	2000
May	1st to 10th	Night	24% 9% 3 6% 4 4 4 4 6% 7% 30%	4192
	11th to 20th	Night	25% 9% 3 6% 4 3 4 3 6% 7% 30%	3575
	21st to end	Night	20% 7% 2 6% 3 3 4 4 5% 9% 37%	3920
June	1st to 10th	Night	13% 5% 3 6% 5% 3 4 3 5% 8% 45%	1600
	11th to 20th	Night	12% 6% 3 6% 5% 5% 5% 4 7% 9% 40%	1600
	21st to end	Night	11% 4 2 6% 4 4 5% 5% 7% 10% 42%	1600
July	1st to 10th	Night	9% 6% 2 7% 6% 5% 6% 5% 7% 10% 38%	3598
	11th to 20th	Night	12% 7% 3 8% 6% 5% 6% 5% 7% 10% 30%	3597
	21st to end	Night	10% 6% 3 6% 4 4 5% 4 6% 9% 42%	4325
August	1st to 10th	Night	19% 5% 2 7% 5% 4 4 4 7% 10% 34%	2915
	11th to 20th	Night	13% 6% 3 8% 5% 4 5% 4 6% 8% 38%	3089
	21st to end	Night	12% 5% 2 6% 3 4 4 4 5% 8% 48%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 46%	2000
	11th to 20th	Night	11% 6% 2 6% 3 3 3 3 4 7% 51%	2000
	21st to end	Night	20% 8% 2 6% 4 4 4 3 4 6% 39%	2259
October	1st to 10th	Night	18% 7% 2 6% 4 3 3 3 4 5% 43%	2400
	11th to 20th	Night	17% 5% 1 5% 3 3 3 2 4 6% 51%	2400
	21st to end	Night	17% 5% 2 4 3 3 4 3 4 6% 50%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 4 5% 6% 53%	2653
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 5% 5% 48%	2730
	21st to end	Night	13% 5% 2 5% 5% 4 4 3 5% 7% 47%	2917
December	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 49%	4609
	11th to 20th	Night	22% 6% 2 5% 3 3 4 3 5% 6% 40%	5205
	21st to end	Night	17% 7% 3 6% 4 4 5% 4 6% 7% 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	34% 4 8% 25% 23% 5%	3598
	11th to 20th	Day	45% 4 7% 21% 16% 6%	3940
	21st to end	Day	42% 6% 11% 26% 13% 2	4466
February	1st to 10th	Day	43% 5% 13% 25% 12% 2	1800
	11th to 20th	Day	44% 4 9% 24% 15% 3	1861
	21st to end	Day	46% 5% 10% 21% 14% 4	1650
March	1st to 10th	Day	39% 4 9% 24% 17% 7%	4680
	11th to 20th	Day	36% 6% 9% 27% 17% 6%	4980
	21st to end	Day	44% 5% 8% 18% 19% 6%	5666
April	1st to 10th	Day	42% 7% 11% 19% 16% 5%	2400
	11th to 20th	Day	39% 6% 12% 21% 17% 5%	2374
	21st to end	Day	40% 5% 11% 20% 20% 4	2400
May	1st to 10th	Day	49% 8% 13% 17% 11% 2	5762
	11th to 20th	Day	49% 8% 10% 21% 10% 1	6322
	21st to end	Day	42% 7% 12% 23% 15% 2	6949
June	1st to 10th	Day	38% 9% 12% 23% 16% 2	2800
	11th to 20th	Day	39% 9% 12% 24% 13% 3	2800
	21st to end	Day	32% 10% 15% 24% 18% 1	2800
July	1st to 10th	Day	35% 11% 16% 19% 16% 3	6290
	11th to 20th	Day	42% 12% 17% 22% 7% 1	6285
	21st to end	Day	37% 7% 11% 22% 22% 1	6335
August	1st to 10th	Day	44% 9% 14% 24% 9%	4221
	11th to 20th	Day	38% 8% 11% 28% 14% 1	3965
	21st to end	Day	32% 7% 12% 29% 18% 2	4312
September	1st to 10th	Day	37% 7% 11% 21% 19% 4	2400
	11th to 20th	Day	33% 6% 9% 28% 21% 4	2400
	21st to end	Day	47% 6% 8% 18% 18% 2	2136
October	1st to 10th	Day	36% 5% 7% 19% 28% 5%	3260
	11th to 20th	Day	27% 4 10% 25% 25% 9%	3240
	21st to end	Day	25% 3 10% 30% 28% 5%	3538
November	1st to 10th	Day	32% 5% 14% 25% 21% 4	1861
	11th to 20th	Day	36% 5% 10% 23% 22% 3	1890
	21st to end	Day	39% 5% 10% 30% 13% 2	1701
December	1st to 10th	Day	40% 4 16% 25% 12% 3	2616
	11th to 20th	Day	45% 4 12% 28% 10% 1	2986
	21st to end	Day	42% 7% 14% 19% 16% 2	3960
January	1st to 10th	Night	37% 4 8% 24% 22% 5%	6349
	11th to 20th	Night	37% 4 9% 25% 18% 7%	6016
	21st to end	Night	41% 4 8% 27% 18% 2	6490
February	1st to 10th	Night	44% 5% 13% 24% 13% 1	2600
	11th to 20th	Night	44% 4 9% 21% 18% 4	2435
	21st to end	Night	45% 4 10% 20% 16% 5%	1980
March	1st to 10th	Night	40% 4 9% 25% 12% 10%	5157
	11th to 20th	Night	33% 5% 12% 28% 15% 6%	4980
	21st to end	Night	40% 5% 8% 19% 25% 4	5251
April	1st to 10th	Night	44% 6% 10% 18% 19% 4	2000
	11th to 20th	Night	39% 5% 12% 20% 19% 5%	1975
	21st to end	Night	39% 6% 9% 21% 20% 5%	2000
May	1st to 10th	Night	52% 6% 11% 16% 11% 3	4222
	11th to 20th	Night	51% 8% 10% 19% 11% 1	3576
	21st to end	Night	43% 8% 11% 23% 12% 2	3927
June	1st to 10th	Night	37% 7% 12% 24% 19% 2	1600
	11th to 20th	Night	40% 8% 13% 24% 15% 1	1600
	21st to end	Night	35% 8% 13% 26% 15% 3	1600
July	1st to 10th	Night	41% 9% 17% 17% 14% 3	3560
	11th to 20th	Night	46% 10% 19% 17% 8% 1	3552
	21st to end	Night	37% 7% 13% 23% 19% 1	4198
August	1st to 10th	Night	45% 8% 17% 20% 9% 1	2904
	11th to 20th	Night	41% 7% 10% 27% 13% 1	3117
	21st to end	Night	34% 6% 10% 31% 17% 2	3586
September	1st to 10th	Night	38% 6% 10% 21% 20% 5%	2000
	11th to 20th	Night	33% 5% 9% 25% 25% 3	2000
	21st to end	Night	46% 5% 10% 18% 19% 3	2259
October	1st to 10th	Night	44% 4 9% 19% 21% 3	3920
	11th to 20th	Night	26% 3 11% 26% 29% 6%	3899
	21st to end	Night	25% 4 10% 32% 24% 6%	4252
November	1st to 10th	Night	32% 5% 10% 26% 22% 5%	2653
	11th to 20th	Night	38% 4 9% 23% 20% 5%	2730
	21st to end	Night	37% 5% 11% 27% 18% 2	2917
December	1st to 10th	Night	37% 4 12% 30% 15% 2	4609
	11th to 20th	Night	45% 4 13% 19% 16% 4	5208
	21st to end	Night	44% 7% 12% 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	28% 29% 28% 13% 2 1	200
	11th to 20th	Day	29% 31% 27% 12% 1	273
	21st to end	Day	22% 31% 31% 12% 4 1	330
February	1st to 10th	Day	24% 29% 34% 9% 3	300
	11th to 20th	Day	34% 24% 35% 6% 1	399
	21st to end	Day	27% 26% 31% 13% 3	414
March	1st to 10th	Day	24% 24% 31% 17% 3	500
	11th to 20th	Day	26% 24% 30% 14% 5% 1	500
	21st to end	Day	25% 23% 32% 16% 4 1	550
April	1st to 10th	Day	24% 24% 34% 13% 5% 1	500
	11th to 20th	Day	22% 23% 27% 18% 7% 2	492
	21st to end	Day	25% 17% 34% 17% 6% 1	500
May	1st to 10th	Day	30% 15% 29% 16% 8% 2	503
	11th to 20th	Day	23% 14% 30% 22% 9% 2	527
	21st to end	Day	21% 8% 24% 26% 17% 4	573
June	1st to 10th	Day	14% 11% 31% 26% 14% 4	530
	11th to 20th	Day	18% 8% 26% 28% 15% 4	530
	21st to end	Day	13% 8% 29% 31% 14% 5%	530
July	1st to 10th	Day	12% 2 20% 35% 23% 8%	525
	11th to 20th	Day	5% 4 22% 39% 24% 7%	524
	21st to end	Day	11% 7% 23% 33% 22% 4	576
August	1st to 10th	Day	13% 4 22% 33% 21% 7%	526
	11th to 20th	Day	13% 5% 17% 36% 22% 8%	519
	21st to end	Day	11% 5% 25% 31% 19% 9%	583
September	1st to 10th	Day	17% 8% 20% 26% 20% 8%	530
	11th to 20th	Day	15% 5% 23% 31% 18% 8%	530
	21st to end	Day	27% 12% 21% 24% 14% 2	530
October	1st to 10th	Day	22% 9% 24% 23% 15% 7%	530
	11th to 20th	Day	20% 17% 25% 24% 12% 2	530
	21st to end	Day	18% 17% 34% 20% 8% 2	583
November	1st to 10th	Day	18% 13% 37% 20% 10% 3	430
	11th to 20th	Day	19% 21% 31% 22% 6% 1	430
	21st to end	Day	25% 21% 30% 17% 6%	241
December	1st to 10th	Day	24% 23% 37% 12% 3	204
	11th to 20th	Day	30% 23% 31% 13% 4	200
	21st to end	Day	25% 19% 32% 17% 7%	220
January	1st to 10th	Night	21% 21% 39% 16% 3	800
	11th to 20th	Night	26% 18% 31% 22% 4	800
	21st to end	Night	20% 21% 38% 20% 1	880
February	1st to 10th	Night	21% 19% 36% 22% 2	800
	11th to 20th	Night	25% 23% 36% 14% 1	783
	21st to end	Night	23% 21% 36% 17% 3	660
March	1st to 10th	Night	21% 21% 36% 18% 4	800
	11th to 20th	Night	22% 19% 33% 20% 5%	800
	21st to end	Night	20% 15% 39% 20% 5%	880
April	1st to 10th	Night	23% 20% 34% 18% 5%	800
	11th to 20th	Night	18% 14% 35% 26% 6%	791
	21st to end	Night	22% 15% 29% 28% 7%	800
May	1st to 10th	Night	23% 11% 28% 29% 8% 1	684
	11th to 20th	Night	22% 9% 31% 29% 9%	394
	21st to end	Night	19% 5% 21% 36% 18% 1	433
June	1st to 10th	Night	14% 5% 27% 39% 16% 1	400
	11th to 20th	Night	15% 6% 23% 38% 17% 1	400
	21st to end	Night	11% 2 25% 44% 18% 1	400
July	1st to 10th	Night	11% 4 19% 40% 22% 3	390
	11th to 20th	Night	9% 3 16% 40% 29% 3	388
	21st to end	Night	9% 2 16% 45% 24% 3	607
August	1st to 10th	Night	13% 2 15% 39% 27% 3	597
	11th to 20th	Night	15% 3 16% 34% 30% 3	718
	21st to end	Night	14% 4 17% 38% 25% 3	880
September	1st to 10th	Night	16% 5% 21% 32% 24% 3	800
	11th to 20th	Night	14% 4 17% 36% 25% 4	800
	21st to end	Night	23% 6% 21% 31% 18% 2	800
October	1st to 10th	Night	22% 7% 23% 25% 19% 5%	800
	11th to 20th	Night	20% 11% 27% 30% 12% 1	800
	21st to end	Night	20% 14% 30% 24% 12%	880
November	1st to 10th	Night	15% 12% 33% 28% 11% 1	820
	11th to 20th	Night	20% 17% 31% 24% 7% 1	840
	21st to end	Night	17% 19% 32% 24% 8% 1	840
December	1st to 10th	Night	24% 16% 33% 19% 7% 1	809
	11th to 20th	Night	21% 16% 36% 23% 5%	800
	21st to end	Night	20% 17% 32% 27% 3	880

Page generated at 2025-11-30 01:00:56. Version ac07

Copyright © 2025 Attilla Danko.