Hopkins Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hopkins Observatory 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	17% 10% 4 9% 5% 5% 4 4 5% 6% 31%	126563
	Night	17% 12% 4 10% 5% 4 4 3 5% 6% 30%	123120
Transparency	Day	51% 7% 11% 19% 10% 1	130056
	Night	52% 8% 12% 17% 10% 1	127714
Seeing	Day	26% 20% 27% 17% 8% 2	16099
	Night	25% 19% 25% 23% 8% 1	25061

By Month

Percentage Cloud Cover NE (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	10% 13% 10% 19% 9% 5% 4 3 3 4 19%	3597
	11th to 20th	Day	12% 15% 7% 17% 8% 4 4 3 4 4 22%	3570
	21st to end	Day	16% 15% 9% 10% 6% 4 4 3 3 4 27%	4024
February	1st to 10th	Day	13% 15% 6% 13% 8% 6% 4 3 5% 5% 22%	1810
	11th to 20th	Day	12% 13% 6% 15% 7% 6% 4 3 5% 5% 25%	1948
	21st to end	Day	15% 15% 5% 12% 7% 5% 4 2 4 4 26%	1650
March	1st to 10th	Day	19% 15% 6% 11% 5% 4 4 2 3 5% 26%	4476
	11th to 20th	Day	13% 13% 6% 9% 5% 4 4 3 4 5% 33%	4658
	21st to end	Day	19% 11% 4 11% 4 4 4 3 4 5% 31%	5970
April	1st to 10th	Day	18% 15% 4 8% 6% 5% 4 3 4 5% 31%	2400
	11th to 20th	Day	16% 12% 3 7% 5% 5% 4 3 5% 5% 34%	2374
	21st to end	Day	18% 11% 4 8% 5% 5% 4 4 5% 5% 31%	2523
May	1st to 10th	Day	32% 12% 3 7% 4 4 4 3 4 6% 21%	5886
	11th to 20th	Day	18% 11% 3 9% 5% 5% 5% 4 5% 6% 29%	6318
	21st to end	Day	20% 9% 3 7% 5% 5% 5% 4 6% 7% 30%	6949
June	1st to 10th	Day	18% 7% 3 6% 5% 5% 6% 5% 7% 8% 29%	2800
	11th to 20th	Day	19% 7% 2 7% 5% 5% 4 4 7% 8% 32%	2800
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 5% 5% 7% 32%	2800
July	1st to 10th	Day	13% 6% 2 8% 6% 5% 6% 5% 7% 10% 33%	6360
	11th to 20th	Day	11% 5% 3 7% 5% 6% 6% 5% 8% 10% 34%	6360
	21st to end	Day	16% 6% 2 6% 4 4 4 4 5% 7% 40%	6926
August	1st to 10th	Day	18% 7% 3 8% 5% 5% 5% 4 6% 8% 32%	4499
	11th to 20th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 9% 33%	3912
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 10% 38%	4312
September	1st to 10th	Day	15% 6% 2 6% 4 4 5% 4 5% 7% 43%	2400
	11th to 20th	Day	15% 6% 2 5% 4 4 3 4 5% 7% 46%	2280
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 7% 40%	2200
October	1st to 10th	Day	20% 9% 3 7% 4 5% 4 3 4 5% 36%	2130
	11th to 20th	Day	17% 10% 3 8% 4 3 4 3 5% 6% 38%	1958
	21st to end	Day	23% 14% 3 7% 4 4 4 3 4 6% 28%	1881
November	1st to 10th	Day	12% 11% 3 7% 4 3 5% 3 5% 6% 42%	1761
	11th to 20th	Day	16% 14% 4 6% 4 4 4 4 4 5% 35%	1800
	21st to end	Day	17% 15% 3 9% 5% 4 5% 3 5% 6% 29%	1685
December	1st to 10th	Day	12% 15% 7% 15% 6% 5% 5% 3 3 5% 23%	2600
	11th to 20th	Day	17% 17% 8% 10% 4 4 3 2 3 4 29%	2986
	21st to end	Day	16% 16% 5% 12% 5% 4 3 3 4 6% 27%	3960
January	1st to 10th	Night	11% 12% 8% 22% 8% 5% 4 2 3 4 21%	6349
	11th to 20th	Night	10% 15% 7% 20% 9% 5% 4 3 4 4 18%	6315
	21st to end	Night	14% 13% 4 10% 7% 6% 5% 4 5% 6% 26%	6930
February	1st to 10th	Night	14% 12% 5% 14% 8% 5% 5% 4 5% 4 24%	2589
	11th to 20th	Night	12% 12% 5% 12% 7% 7% 6% 5% 5% 6% 24%	2338
	21st to end	Night	14% 15% 4 12% 5% 5% 4 4 5% 5% 26%	1980
March	1st to 10th	Night	18% 11% 5% 12% 6% 4 4 3 4 5% 27%	5364
	11th to 20th	Night	15% 12% 5% 7% 5% 4 4 3 4 6% 33%	5270
	21st to end	Night	20% 16% 5% 11% 4 3 3 2 3 4 28%	4962
April	1st to 10th	Night	18% 13% 4 6% 6% 4 5% 4 6% 6% 28%	2000
	11th to 20th	Night	18% 12% 2 8% 4 4 3 4 4 6% 34%	1975
	21st to end	Night	18% 12% 3 7% 5% 3 4 3 5% 5% 35%	1875
May	1st to 10th	Night	33% 12% 3 6% 4 4 4 3 4 5% 22%	4054
	11th to 20th	Night	17% 13% 3 8% 6% 5% 5% 3 5% 6% 28%	3603
	21st to end	Night	23% 11% 3 7% 4 4 4 3 5% 6% 30%	3926
June	1st to 10th	Night	19% 8% 2 6% 5% 4 4 3 5% 8% 35%	1600
	11th to 20th	Night	18% 8% 2 5% 5% 4 4 4 6% 7% 38%	1600
	21st to end	Night	15% 5% 3 6% 5% 4 6% 4 5% 7% 40%	1600
July	1st to 10th	Night	12% 8% 2 7% 5% 5% 4 4 6% 8% 39%	3600
	11th to 20th	Night	17% 6% 3 7% 5% 5% 5% 4 5% 8% 35%	3600
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 6% 46%	3926
August	1st to 10th	Night	25% 6% 2 6% 4 3 4 3 6% 7% 33%	2618
	11th to 20th	Night	18% 6% 1 6% 5% 5% 5% 3 6% 6% 38%	3131
	21st to end	Night	10% 7% 2 5% 4 4 4 4 6% 9% 46%	3586
September	1st to 10th	Night	16% 8% 2 6% 3 4 4 3 5% 6% 44%	2000
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 4 7% 50%	2119
	21st to end	Night	17% 7% 3 6% 4 5% 4 2 4 6% 42%	2200
October	1st to 10th	Night	20% 8% 2 7% 4 4 4 3 5% 6% 38%	2265
	11th to 20th	Night	15% 10% 2 6% 4 5% 4 4 5% 6% 38%	2397
	21st to end	Night	21% 13% 3 7% 5% 5% 3 3 4 6% 30%	2717
November	1st to 10th	Night	14% 10% 3 7% 5% 4 4 3 5% 6% 40%	2537
	11th to 20th	Night	16% 12% 4 8% 6% 5% 5% 4 4 5% 33%	2600
	21st to end	Night	14% 14% 4 8% 5% 4 5% 5% 5% 6% 30%	2710
December	1st to 10th	Night	15% 17% 6% 16% 5% 4 4 3 4 5% 20%	4580
	11th to 20th	Night	21% 17% 6% 9% 4 4 3 3 3 4 26%	5208
	21st to end	Night	17% 16% 6% 11% 5% 5% 4 3 4 4 24%	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	64% 9% 8% 15% 4	3598
	11th to 20th	Day	62% 8% 8% 10% 10% 2	3600
	21st to end	Day	60% 5% 7% 17% 9% 1	4024
February	1st to 10th	Day	60% 8% 12% 13% 7% 1	1810
	11th to 20th	Day	59% 8% 9% 15% 9% 1	1957
	21st to end	Day	60% 6% 10% 15% 9% 1	1650
March	1st to 10th	Day	61% 5% 10% 12% 8% 4	4479
	11th to 20th	Day	48% 10% 15% 17% 7% 3	4658
	21st to end	Day	55% 7% 6% 12% 18% 3	5970
April	1st to 10th	Day	57% 5% 8% 19% 9% 2	2400
	11th to 20th	Day	50% 8% 9% 18% 13% 1	2374
	21st to end	Day	53% 7% 9% 17% 12% 3	2523
May	1st to 10th	Day	63% 8% 9% 13% 7% 1	5897
	11th to 20th	Day	54% 8% 14% 16% 7% 1	6322
	21st to end	Day	50% 10% 11% 21% 8% 1	6949
June	1st to 10th	Day	48% 10% 14% 18% 10%	2800
	11th to 20th	Day	47% 9% 13% 19% 11% 1	2800
	21st to end	Day	50% 8% 13% 19% 10% 1	2800
July	1st to 10th	Day	46% 8% 13% 23% 11%	6290
	11th to 20th	Day	42% 10% 15% 22% 11% 1	6285
	21st to end	Day	41% 6% 13% 26% 13%	6728
August	1st to 10th	Day	49% 8% 14% 20% 9%	4485
	11th to 20th	Day	46% 8% 17% 23% 6% 1	3950
	21st to end	Day	38% 8% 14% 29% 10% 1	4312
September	1st to 10th	Day	39% 7% 12% 27% 14% 1	2400
	11th to 20th	Day	38% 6% 10% 26% 19% 1	2280
	21st to end	Day	42% 8% 12% 24% 14% 2	2200
October	1st to 10th	Day	49% 6% 9% 18% 17% 1	3468
	11th to 20th	Day	43% 5% 12% 24% 15% 2	3174
	21st to end	Day	41% 6% 12% 27% 11% 2	3081
November	1st to 10th	Day	42% 8% 15% 22% 13% 1	1761
	11th to 20th	Day	50% 7% 10% 16% 15% 2	1800
	21st to end	Day	54% 7% 14% 16% 7% 1	1685
December	1st to 10th	Day	64% 6% 10% 15% 4	2600
	11th to 20th	Day	60% 5% 9% 16% 10% 1	2986
	21st to end	Day	58% 7% 8% 13% 11% 3	3960
January	1st to 10th	Night	63% 9% 10% 12% 6% 1	6349
	11th to 20th	Night	65% 9% 10% 9% 6% 1	6360
	21st to end	Night	54% 9% 11% 11% 12% 2	6930
February	1st to 10th	Night	60% 9% 10% 13% 6% 1	2589
	11th to 20th	Night	57% 9% 11% 13% 9% 1	2340
	21st to end	Night	56% 8% 10% 16% 7% 2	1980
March	1st to 10th	Night	56% 8% 11% 14% 10% 1	5374
	11th to 20th	Night	48% 10% 16% 16% 8% 2	5271
	21st to end	Night	58% 7% 5% 12% 16% 2	4962
April	1st to 10th	Night	55% 7% 11% 17% 9% 1	2000
	11th to 20th	Night	50% 7% 10% 18% 13% 1	1975
	21st to end	Night	50% 7% 11% 16% 13% 3	1875
May	1st to 10th	Night	63% 8% 9% 12% 7% 1	4060
	11th to 20th	Night	54% 10% 12% 15% 8% 1	3604
	21st to end	Night	53% 10% 9% 17% 10% 1	3927
June	1st to 10th	Night	46% 8% 13% 22% 10% 1	1600
	11th to 20th	Night	44% 8% 16% 22% 11%	1600
	21st to end	Night	41% 9% 15% 23% 12%	1600
July	1st to 10th	Night	42% 7% 15% 22% 14%	3560
	11th to 20th	Night	46% 9% 12% 20% 12% 1	3552
	21st to end	Night	38% 6% 14% 28% 12% 1	3809
August	1st to 10th	Night	49% 8% 14% 21% 8%	2608
	11th to 20th	Night	45% 8% 17% 22% 8%	3159
	21st to end	Night	33% 7% 16% 32% 11% 1	3586
September	1st to 10th	Night	39% 8% 12% 25% 15% 1	2000
	11th to 20th	Night	34% 6% 12% 28% 19% 1	2119
	21st to end	Night	43% 7% 11% 23% 15% 1	2200
October	1st to 10th	Night	49% 6% 12% 20% 12% 2	3707
	11th to 20th	Night	40% 5% 12% 22% 17% 3	3920
	21st to end	Night	42% 7% 13% 28% 10% 1	4467
November	1st to 10th	Night	44% 8% 11% 21% 14% 2	2537
	11th to 20th	Night	51% 7% 12% 16% 13% 2	2600
	21st to end	Night	51% 9% 15% 17% 7%	2710
December	1st to 10th	Night	66% 7% 11% 12% 4	4580
	11th to 20th	Night	61% 7% 9% 11% 12%	5208
	21st to end	Night	61% 6% 9% 10% 10% 4	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	35% 31% 22% 10% 3	200
	11th to 20th	Day	34% 30% 26% 10% 2 1	200
	21st to end	Day	29% 30% 29% 9% 4	233
February	1st to 10th	Day	41% 32% 21% 4 2	310
	11th to 20th	Day	38% 29% 23% 10% 1	496
	21st to end	Day	35% 24% 29% 11% 1	414
March	1st to 10th	Day	36% 24% 23% 15% 1	500
	11th to 20th	Day	33% 27% 28% 9% 3 1	500
	21st to end	Day	36% 26% 21% 12% 4 1	550
April	1st to 10th	Day	40% 22% 21% 11% 5% 1	500
	11th to 20th	Day	35% 25% 23% 10% 6% 1	492
	21st to end	Day	30% 22% 28% 14% 4 1	500
May	1st to 10th	Day	36% 22% 24% 10% 7% 2	503
	11th to 20th	Day	23% 23% 32% 16% 4 2	527
	21st to end	Day	22% 16% 31% 21% 8% 2	573
June	1st to 10th	Day	20% 16% 32% 21% 9% 1	530
	11th to 20th	Day	23% 14% 30% 22% 10% 1	530
	21st to end	Day	20% 20% 33% 16% 9% 1	530
July	1st to 10th	Day	16% 13% 31% 25% 12% 2	525
	11th to 20th	Day	12% 9% 37% 28% 12% 2	524
	21st to end	Day	16% 15% 34% 22% 11% 2	576
August	1st to 10th	Day	12% 13% 29% 30% 12% 5%	526
	11th to 20th	Day	17% 11% 27% 24% 14% 7%	519
	21st to end	Day	15% 14% 29% 24% 11% 7%	583
September	1st to 10th	Day	19% 12% 24% 25% 13% 6%	530
	11th to 20th	Day	16% 14% 23% 21% 16% 11%	530
	21st to end	Day	21% 13% 29% 21% 10% 6%	530
October	1st to 10th	Day	25% 16% 23% 18% 14% 3	530
	11th to 20th	Day	25% 20% 25% 20% 8% 2	509
	21st to end	Day	31% 19% 27% 17% 5% 1	429
November	1st to 10th	Day	28% 22% 20% 19% 10% 1	395
	11th to 20th	Day	32% 25% 23% 15% 4 1	400
	21st to end	Day	36% 21% 26% 13% 2 1	285
December	1st to 10th	Day	30% 30% 28% 10% 3 1	200
	11th to 20th	Day	41% 27% 23% 10% 1	200
	21st to end	Day	35% 24% 28% 11% 3	220
January	1st to 10th	Night	26% 30% 30% 12% 2	800
	11th to 20th	Night	29% 25% 27% 16% 2 1	800
	21st to end	Night	24% 26% 29% 19% 2	880
February	1st to 10th	Night	28% 25% 29% 16% 1	800
	11th to 20th	Night	28% 28% 28% 14% 2	783
	21st to end	Night	30% 25% 26% 17% 1	660
March	1st to 10th	Night	30% 23% 26% 17% 3	800
	11th to 20th	Night	28% 27% 25% 17% 3	800
	21st to end	Night	31% 22% 24% 17% 6%	880
April	1st to 10th	Night	30% 22% 27% 19% 3	800
	11th to 20th	Night	27% 24% 24% 18% 7%	791
	21st to end	Night	28% 20% 25% 21% 4	675
May	1st to 10th	Night	30% 16% 25% 22% 6% 1	600
	11th to 20th	Night	20% 19% 25% 31% 5%	409
	21st to end	Night	18% 10% 26% 33% 12% 1	433
June	1st to 10th	Night	22% 12% 24% 31% 12%	400
	11th to 20th	Night	18% 11% 22% 38% 11%	400
	21st to end	Night	16% 9% 24% 40% 11% 1	400
July	1st to 10th	Night	14% 6% 23% 41% 15% 1	390
	11th to 20th	Night	15% 6% 21% 39% 17% 1	388
	21st to end	Night	13% 7% 23% 41% 15% 1	438
August	1st to 10th	Night	15% 4 20% 40% 19% 1	417
	11th to 20th	Night	16% 6% 20% 34% 20% 3	748
	21st to end	Night	16% 7% 23% 33% 20% 2	880
September	1st to 10th	Night	18% 10% 23% 31% 17% 2	800
	11th to 20th	Night	17% 9% 19% 28% 23% 5%	800
	21st to end	Night	19% 12% 23% 29% 14% 3	800
October	1st to 10th	Night	26% 12% 20% 27% 14% 1	800
	11th to 20th	Night	23% 16% 26% 25% 10% 1	793
	21st to end	Night	30% 19% 24% 22% 4	836
November	1st to 10th	Night	21% 24% 24% 25% 7% 1	780
	11th to 20th	Night	26% 22% 27% 17% 7% 2	800
	21st to end	Night	27% 23% 27% 19% 5% 1	800
December	1st to 10th	Night	32% 27% 24% 13% 5%	800
	11th to 20th	Night	32% 23% 25% 16% 3	800
	21st to end	Night	30% 27% 25% 16% 2	880

Page generated at 2025-10-16 01:50:30. Version ac07

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