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 2024-11-05.

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 8% 5% 5% 5% 4 5% 7% 31%	101333
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 31%	97679
Transparency	Day	50% 8% 12% 19% 10% 1	104844
	Night	50% 9% 12% 18% 10% 1	102257
Seeing	Day	25% 20% 27% 18% 8% 2	14871
	Night	24% 19% 25% 23% 8% 1	23872

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	13% 15% 11% 11% 6% 5% 5% 4 4 5% 23%	2517
	11th to 20th	Day	15% 15% 9% 12% 9% 4 4 3 4 4 20%	2520
	21st to end	Day	19% 18% 9% 11% 6% 4 4 3 3 4 20%	2808
February	1st to 10th	Day	13% 15% 5% 11% 8% 5% 4 3 5% 5% 23%	1719
	11th to 20th	Day	12% 13% 7% 14% 6% 6% 5% 3 5% 4 25%	1848
	21st to end	Day	15% 15% 5% 12% 7% 5% 4 2 4 4 26%	1570
March	1st to 10th	Day	21% 16% 5% 10% 6% 4 4 2 3 4 25%	3116
	11th to 20th	Day	11% 17% 7% 11% 6% 4 4 3 4 6% 27%	3256
	21st to end	Day	19% 10% 4 12% 5% 4 4 3 4 5% 31%	4166
April	1st to 10th	Day	18% 15% 4 8% 6% 4 4 3 4 5% 31%	2280
	11th to 20th	Day	15% 11% 3 8% 5% 5% 4 3 5% 6% 34%	2254
	21st to end	Day	18% 11% 4 8% 5% 5% 4 4 5% 5% 30%	2397
May	1st to 10th	Day	26% 11% 3 7% 4 4 4 3 4 6% 27%	4106
	11th to 20th	Day	19% 10% 3 8% 5% 4 5% 4 5% 7% 31%	4398
	21st to end	Day	13% 6% 2 7% 5% 4 5% 5% 7% 8% 39%	4837
June	1st to 10th	Day	18% 8% 3 6% 5% 5% 6% 5% 7% 8% 29%	2660
	11th to 20th	Day	19% 7% 2 7% 5% 5% 5% 4 7% 9% 32%	2660
	21st to end	Day	19% 8% 3 7% 5% 5% 5% 5% 5% 7% 31%	2660
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 7% 10% 35%	4440
	11th to 20th	Day	11% 5% 3 6% 5% 6% 6% 5% 8% 10% 34%	4440
	21st to end	Day	17% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	4814
August	1st to 10th	Day	18% 7% 3 8% 6% 5% 5% 4 6% 8% 32%	4361
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 9% 33%	3790
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 10% 38%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 4 5% 4 5% 7% 42%	2280
	11th to 20th	Day	15% 6% 2 5% 4 4 3 4 5% 7% 45%	2166
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 7% 40%	2090
October	1st to 10th	Day	21% 10% 3 8% 4 5% 4 3 5% 5% 34%	2024
	11th to 20th	Day	16% 10% 3 7% 4 4 4 3 5% 6% 38%	1878
	21st to end	Day	23% 14% 3 7% 4 4 4 3 4 6% 28%	1881
November	1st to 10th	Day	13% 11% 3 7% 4 3 5% 3 5% 6% 41%	1708
	11th to 20th	Day	16% 15% 4 6% 4 4 4 4 4 5% 34%	1710
	21st to end	Day	16% 14% 3 9% 5% 4 5% 3 5% 6% 30%	1601
December	1st to 10th	Day	17% 18% 6% 12% 6% 5% 4 3 4 4 21%	1520
	11th to 20th	Day	18% 18% 6% 9% 4 3 3 2 3 3 30%	1906
	21st to end	Day	18% 17% 5% 12% 6% 5% 4 3 5% 6% 20%	2772
January	1st to 10th	Night	13% 13% 8% 14% 5% 4 4 3 3 5% 27%	4429
	11th to 20th	Night	12% 16% 7% 14% 10% 6% 5% 3 4 4 19%	4440
	21st to end	Night	20% 17% 5% 11% 7% 6% 5% 3 4 4 19%	4846
February	1st to 10th	Night	14% 12% 5% 13% 8% 5% 5% 4 5% 4 24%	2460
	11th to 20th	Night	12% 12% 5% 12% 6% 7% 6% 5% 5% 5% 25%	2218
	21st to end	Night	15% 15% 4 12% 5% 5% 4 4 5% 5% 27%	1884
March	1st to 10th	Night	21% 13% 4 11% 5% 4 4 3 4 5% 25%	3724
	11th to 20th	Night	12% 16% 6% 8% 6% 5% 5% 3 4 5% 29%	3672
	21st to end	Night	17% 17% 5% 11% 4 4 3 2 3 4 30%	3466
April	1st to 10th	Night	18% 13% 4 6% 6% 4 4 4 6% 6% 29%	1900
	11th to 20th	Night	18% 11% 2 8% 4 4 4 4 4 6% 35%	1875
	21st to end	Night	19% 12% 3 7% 5% 3 4 3 5% 5% 34%	1781
May	1st to 10th	Night	25% 11% 3 7% 4 4 4 3 5% 5% 28%	2834
	11th to 20th	Night	16% 10% 4 8% 6% 5% 6% 4 6% 7% 28%	2509
	21st to end	Night	15% 8% 2 7% 5% 4 4 4 6% 7% 38%	2738
June	1st to 10th	Night	19% 8% 3 6% 5% 5% 4 3 5% 8% 35%	1520
	11th to 20th	Night	17% 8% 2 5% 4 4 4 4 5% 8% 39%	1520
	21st to end	Night	16% 5% 3 6% 5% 4 6% 4 6% 7% 39%	1520
July	1st to 10th	Night	12% 7% 2 8% 6% 5% 5% 4 6% 8% 37%	2520
	11th to 20th	Night	17% 6% 2 7% 4 4 5% 4 5% 9% 37%	2520
	21st to end	Night	18% 7% 3 6% 4 5% 4 3 6% 7% 37%	2739
August	1st to 10th	Night	26% 6% 2 6% 4 3 4 3 6% 7% 33%	2537
	11th to 20th	Night	18% 6% 1 6% 5% 5% 5% 3 6% 6% 37%	3033
	21st to end	Night	11% 7% 1 5% 4 4 4 4 6% 9% 46%	3476
September	1st to 10th	Night	16% 9% 2 6% 3 4 4 3 5% 6% 43%	1900
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 4 7% 50%	2013
	21st to end	Night	17% 7% 3 6% 5% 5% 4 2 4 6% 42%	2090
October	1st to 10th	Night	21% 8% 2 7% 5% 4 4 3 5% 6% 36%	2152
	11th to 20th	Night	15% 10% 2 7% 4 5% 4 4 5% 6% 38%	2300
	21st to end	Night	21% 13% 3 7% 5% 5% 3 3 4 6% 30%	2717
November	1st to 10th	Night	14% 11% 3 7% 5% 4 4 3 5% 6% 39%	2470
	11th to 20th	Night	16% 12% 4 7% 6% 5% 5% 4 4 5% 32%	2470
	21st to end	Night	14% 13% 4 8% 5% 4 5% 5% 5% 6% 30%	2574
December	1st to 10th	Night	18% 16% 6% 11% 6% 4 4 3 5% 5% 21%	2660
	11th to 20th	Night	19% 16% 6% 10% 5% 4 4 3 4 5% 24%	3288
	21st to end	Night	20% 16% 7% 12% 5% 5% 3 3 4 4 20%	4884

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	56% 12% 11% 17% 4	2518
	11th to 20th	Day	62% 11% 9% 12% 5% 1	2520
	21st to end	Day	67% 6% 8% 13% 4 1	2808
February	1st to 10th	Day	60% 8% 12% 13% 6% 1	1719
	11th to 20th	Day	58% 8% 8% 15% 9% 1	1857
	21st to end	Day	60% 6% 9% 15% 9% 1	1570
March	1st to 10th	Day	61% 7% 11% 12% 8% 1	3119
	11th to 20th	Day	50% 14% 14% 12% 8% 2	3256
	21st to end	Day	53% 7% 6% 14% 17% 2	4166
April	1st to 10th	Day	56% 5% 8% 20% 9% 2	2280
	11th to 20th	Day	49% 8% 10% 18% 13% 1	2254
	21st to end	Day	53% 7% 9% 17% 11% 3	2397
May	1st to 10th	Day	56% 9% 10% 15% 9% 1	4120
	11th to 20th	Day	50% 9% 15% 17% 8% 1	4402
	21st to end	Day	37% 12% 14% 25% 12% 1	4837
June	1st to 10th	Day	48% 10% 14% 18% 10%	2660
	11th to 20th	Day	47% 9% 13% 19% 11% 1	2660
	21st to end	Day	50% 9% 12% 19% 9%	2660
July	1st to 10th	Day	43% 9% 13% 23% 12% 1	4370
	11th to 20th	Day	42% 9% 16% 23% 10%	4365
	21st to end	Day	47% 7% 15% 18% 12%	4661
August	1st to 10th	Day	49% 9% 14% 19% 8%	4347
	11th to 20th	Day	46% 8% 17% 23% 6% 1	3828
	21st to end	Day	39% 8% 15% 28% 9% 1	4180
September	1st to 10th	Day	40% 7% 13% 26% 13% 1	2280
	11th to 20th	Day	39% 6% 10% 25% 19% 1	2166
	21st to end	Day	42% 8% 12% 23% 14% 2	2090
October	1st to 10th	Day	50% 6% 9% 17% 16% 1	3362
	11th to 20th	Day	43% 5% 12% 24% 15% 2	3094
	21st to end	Day	41% 6% 12% 27% 11% 2	3081
November	1st to 10th	Day	42% 8% 14% 21% 13% 1	1708
	11th to 20th	Day	51% 7% 11% 16% 13% 1	1710
	21st to end	Day	53% 7% 14% 17% 7% 1	1601
December	1st to 10th	Day	64% 8% 9% 11% 8% 1	1520
	11th to 20th	Day	59% 6% 11% 17% 7% 1	1906
	21st to end	Day	63% 9% 10% 11% 7%	2772
January	1st to 10th	Night	54% 11% 12% 14% 8% 1	4429
	11th to 20th	Night	62% 11% 10% 11% 5% 1	4440
	21st to end	Night	64% 11% 7% 11% 6% 1	4846
February	1st to 10th	Night	59% 9% 11% 13% 7% 2	2460
	11th to 20th	Night	56% 9% 11% 14% 9% 1	2220
	21st to end	Night	55% 8% 10% 16% 8% 2	1884
March	1st to 10th	Night	56% 11% 9% 16% 8% 1	3734
	11th to 20th	Night	50% 12% 12% 17% 8% 1	3673
	21st to end	Night	53% 9% 7% 11% 16% 3	3466
April	1st to 10th	Night	54% 7% 11% 18% 9% 1	1900
	11th to 20th	Night	49% 7% 11% 19% 13% 1	1875
	21st to end	Night	50% 7% 11% 16% 13% 3	1781
May	1st to 10th	Night	55% 10% 10% 15% 9% 1	2840
	11th to 20th	Night	50% 13% 14% 14% 7% 1	2510
	21st to end	Night	41% 12% 12% 20% 13% 1	2739
June	1st to 10th	Night	45% 9% 13% 22% 10%	1520
	11th to 20th	Night	43% 8% 16% 22% 11%	1520
	21st to end	Night	41% 9% 15% 23% 12%	1520
July	1st to 10th	Night	43% 9% 14% 19% 15%	2480
	11th to 20th	Night	43% 9% 13% 24% 11%	2472
	21st to end	Night	44% 7% 13% 23% 11% 1	2651
August	1st to 10th	Night	50% 8% 14% 21% 7%	2527
	11th to 20th	Night	46% 8% 18% 22% 8%	3061
	21st to end	Night	33% 8% 16% 32% 11% 1	3476
September	1st to 10th	Night	40% 8% 12% 25% 14% 1	1900
	11th to 20th	Night	34% 6% 13% 27% 19% 1	2013
	21st to end	Night	42% 7% 11% 23% 15% 1	2090
October	1st to 10th	Night	50% 6% 12% 19% 11% 1	3594
	11th to 20th	Night	40% 5% 12% 22% 18% 3	3823
	21st to end	Night	42% 7% 13% 28% 10% 1	4467
November	1st to 10th	Night	45% 8% 11% 21% 14% 2	2470
	11th to 20th	Night	51% 7% 12% 16% 11% 2	2470
	21st to end	Night	50% 10% 15% 17% 7%	2574
December	1st to 10th	Night	61% 10% 10% 12% 6%	2660
	11th to 20th	Night	59% 9% 12% 11% 9%	3288
	21st to end	Night	65% 8% 9% 13% 6%	4884

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% 29% 24% 11% 3	180
	11th to 20th	Day	32% 31% 26% 9% 2 1	180
	21st to end	Day	25% 30% 31% 10% 4	209
February	1st to 10th	Day	38% 32% 23% 4 2	279
	11th to 20th	Day	35% 29% 24% 10% 2	456
	21st to end	Day	34% 23% 29% 12% 2	382
March	1st to 10th	Day	34% 24% 25% 16% 1	460
	11th to 20th	Day	33% 27% 28% 9% 3 1	460
	21st to end	Day	35% 25% 21% 13% 5% 2	506
April	1st to 10th	Day	37% 22% 23% 12% 5% 1	460
	11th to 20th	Day	33% 25% 24% 10% 6% 2	452
	21st to end	Day	30% 22% 28% 13% 5% 2	460
May	1st to 10th	Day	34% 22% 24% 11% 7% 2	463
	11th to 20th	Day	23% 22% 32% 16% 5% 2	487
	21st to end	Day	20% 16% 31% 22% 9% 3	529
June	1st to 10th	Day	19% 17% 32% 21% 10% 1	490
	11th to 20th	Day	21% 14% 29% 23% 10% 1	490
	21st to end	Day	20% 20% 32% 17% 10% 1	490
July	1st to 10th	Day	15% 13% 31% 26% 13% 2	485
	11th to 20th	Day	12% 9% 38% 28% 12% 2	484
	21st to end	Day	16% 15% 35% 21% 11% 2	532
August	1st to 10th	Day	12% 14% 29% 29% 12% 5%	486
	11th to 20th	Day	18% 11% 27% 24% 13% 7%	479
	21st to end	Day	14% 14% 28% 24% 12% 7%	539
September	1st to 10th	Day	19% 12% 23% 25% 13% 7%	490
	11th to 20th	Day	17% 14% 22% 20% 16% 11%	490
	21st to end	Day	20% 14% 29% 21% 10% 6%	490
October	1st to 10th	Day	27% 16% 24% 18% 13% 3	490
	11th to 20th	Day	25% 19% 26% 21% 8% 2	477
	21st to end	Day	31% 19% 27% 17% 5% 1	429
November	1st to 10th	Day	27% 22% 20% 20% 11% 1	378
	11th to 20th	Day	33% 25% 22% 15% 4 1	370
	21st to end	Day	33% 21% 28% 15% 3 1	261
December	1st to 10th	Day	32% 28% 27% 10% 3 1	180
	11th to 20th	Day	39% 27% 23% 9% 1	180
	21st to end	Day	36% 26% 26% 10% 3	198
January	1st to 10th	Night	25% 29% 31% 13% 2	760
	11th to 20th	Night	28% 25% 27% 16% 2 1	760
	21st to end	Night	24% 27% 28% 19% 2	836
February	1st to 10th	Night	26% 26% 30% 16% 1	760
	11th to 20th	Night	26% 29% 29% 15% 2	743
	21st to end	Night	30% 25% 26% 17% 1	628
March	1st to 10th	Night	30% 23% 27% 18% 3	760
	11th to 20th	Night	28% 27% 25% 17% 3	760
	21st to end	Night	31% 21% 23% 18% 7%	836
April	1st to 10th	Night	29% 21% 28% 19% 3	760
	11th to 20th	Night	26% 23% 24% 19% 7%	751
	21st to end	Night	28% 20% 25% 22% 5%	641
May	1st to 10th	Night	29% 16% 26% 22% 6% 1	570
	11th to 20th	Night	20% 19% 26% 31% 5%	388
	21st to end	Night	17% 10% 27% 33% 13% 1	411
June	1st to 10th	Night	22% 12% 24% 30% 12%	380
	11th to 20th	Night	18% 11% 21% 38% 11%	380
	21st to end	Night	16% 8% 24% 41% 10% 1	380
July	1st to 10th	Night	15% 6% 23% 40% 16% 1	370
	11th to 20th	Night	14% 6% 21% 40% 17% 1	368
	21st to end	Night	14% 8% 22% 40% 16% 1	416
August	1st to 10th	Night	15% 4 21% 40% 18% 2	396
	11th to 20th	Night	16% 7% 20% 34% 19% 4	710
	21st to end	Night	16% 7% 22% 33% 21% 2	836
September	1st to 10th	Night	18% 10% 23% 30% 17% 2	760
	11th to 20th	Night	18% 10% 19% 27% 22% 5%	760
	21st to end	Night	19% 12% 23% 29% 14% 3	760
October	1st to 10th	Night	27% 12% 20% 27% 13% 1	760
	11th to 20th	Night	22% 16% 26% 26% 10% 1	760
	21st to end	Night	30% 19% 24% 22% 4	836
November	1st to 10th	Night	21% 23% 24% 25% 7% 1	760
	11th to 20th	Night	25% 22% 27% 17% 7% 1	760
	21st to end	Night	26% 22% 27% 19% 5% 1	760
December	1st to 10th	Night	32% 28% 23% 12% 5%	760
	11th to 20th	Night	32% 24% 25% 16% 3	760
	21st to end	Night	30% 28% 25% 16% 2	836

Page generated at 2024-11-03 02:11:17. Version ac07

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