Seminary Hill Observatory
Clear Sky Chart
History

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

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% 10% 4 9% 5% 5% 5% 4 6% 7% 32%	108645
	Night	16% 10% 4 9% 6% 5% 5% 4 5% 6% 31%	110350
Transparency	Day	49% 7% 12% 20% 12% 1	112174
	Night	51% 6% 11% 19% 11% 1	114959
Seeing	Day	24% 17% 28% 19% 9% 3	15179
	Night	22% 14% 27% 27% 9% 1	24264

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	11% 11% 7% 19% 7% 5% 4 3 5% 5% 24%	3597
	11th to 20th	Day	11% 12% 5% 14% 6% 5% 5% 4 5% 6% 27%	3570
	21st to end	Day	11% 13% 6% 10% 5% 5% 4 3 4 6% 31%	3960
February	1st to 10th	Day	13% 12% 4 9% 7% 6% 5% 4 5% 7% 28%	1775
	11th to 20th	Day	10% 11% 5% 12% 7% 5% 5% 4 5% 6% 29%	1948
	21st to end	Day	16% 12% 4 10% 6% 5% 3 3 4 7% 30%	1650
March	1st to 10th	Day	20% 16% 5% 11% 5% 4 3 3 4 4 24%	3216
	11th to 20th	Day	14% 12% 6% 9% 6% 5% 4 3 6% 6% 28%	3266
	21st to end	Day	18% 12% 4 11% 5% 4 4 3 3 5% 31%	4148
April	1st to 10th	Day	19% 12% 3 8% 4 4 3 3 5% 6% 34%	2280
	11th to 20th	Day	17% 10% 3 7% 5% 4 5% 5% 5% 6% 34%	2254
	21st to end	Day	22% 9% 3 8% 5% 4 3 4 5% 6% 30%	2460
May	1st to 10th	Day	23% 11% 3 8% 5% 4 4 3 4 6% 27%	4106
	11th to 20th	Day	16% 9% 4 8% 6% 5% 5% 4 6% 7% 29%	4380
	21st to end	Day	12% 6% 2 6% 4 5% 5% 4 7% 9% 40%	4837
June	1st to 10th	Day	18% 9% 3 8% 6% 6% 5% 4 6% 7% 29%	2660
	11th to 20th	Day	16% 8% 2 6% 5% 5% 4 3 6% 8% 36%	2660
	21st to end	Day	19% 7% 3 7% 6% 5% 5% 5% 6% 7% 28%	2660
July	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 7% 9% 36%	4440
	11th to 20th	Day	9% 6% 2 7% 5% 5% 7% 5% 9% 11% 34%	4440
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 5% 6% 8% 35%	4814
August	1st to 10th	Day	15% 6% 3 8% 5% 5% 6% 5% 7% 9% 31%	4329
	11th to 20th	Day	14% 9% 3 8% 5% 5% 5% 4 7% 8% 31%	3935
	21st to end	Day	9% 6% 3 7% 4 5% 5% 5% 7% 10% 40%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 6% 8% 44%	2280
	11th to 20th	Day	13% 5% 2 6% 4 4 3 3 5% 6% 48%	2134
	21st to end	Day	15% 6% 3 6% 4 4 4 4 5% 8% 42%	2090
October	1st to 10th	Day	21% 8% 2 8% 5% 4 4 3 5% 6% 34%	2070
	11th to 20th	Day	14% 9% 2 8% 5% 5% 5% 3 5% 7% 37%	1823
	21st to end	Day	23% 11% 3 7% 5% 4 4 4 5% 5% 29%	1881
November	1st to 10th	Day	13% 9% 3 7% 4 4 4 4 7% 7% 38%	1761
	11th to 20th	Day	16% 11% 2 7% 5% 4 4 4 5% 6% 36%	1800
	21st to end	Day	19% 12% 4 7% 5% 4 5% 3 5% 6% 30%	1695
December	1st to 10th	Day	15% 13% 5% 12% 8% 6% 5% 4 6% 7% 20%	2600
	11th to 20th	Day	20% 12% 4 9% 5% 4 4 3 5% 6% 29%	2986
	21st to end	Day	20% 14% 5% 10% 4 4 3 3 4 5% 27%	3960
January	1st to 10th	Night	12% 10% 8% 17% 7% 5% 4 3 4 5% 25%	6349
	11th to 20th	Night	9% 13% 5% 16% 7% 7% 6% 4 6% 6% 21%	6315
	21st to end	Night	13% 11% 4 11% 7% 6% 5% 4 5% 5% 28%	6996
February	1st to 10th	Night	12% 12% 4 10% 6% 6% 6% 4 6% 6% 27%	2620
	11th to 20th	Night	10% 10% 5% 10% 7% 6% 6% 5% 6% 6% 29%	2338
	21st to end	Night	12% 10% 4 11% 6% 4 5% 3 6% 7% 31%	1980
March	1st to 10th	Night	23% 12% 3 8% 6% 4 4 4 4 5% 27%	3844
	11th to 20th	Night	13% 14% 3 10% 6% 6% 5% 3 4 5% 31%	3722
	21st to end	Night	20% 13% 4 9% 4 4 3 3 4 5% 32%	3480
April	1st to 10th	Night	18% 10% 3 8% 6% 5% 4 3 5% 6% 32%	1900
	11th to 20th	Night	18% 10% 3 7% 6% 4 5% 4 5% 6% 35%	1875
	21st to end	Night	20% 9% 4 6% 5% 5% 4 3 5% 7% 34%	1720
May	1st to 10th	Night	25% 11% 3 8% 5% 5% 4 3 5% 6% 25%	2834
	11th to 20th	Night	17% 10% 3 10% 6% 6% 5% 4 6% 5% 28%	2541
	21st to end	Night	14% 5% 2 6% 5% 4 5% 4 6% 9% 39%	2738
June	1st to 10th	Night	19% 6% 3 7% 6% 5% 5% 5% 7% 8% 31%	1520
	11th to 20th	Night	18% 6% 2 7% 4 3 5% 4 5% 7% 39%	1520
	21st to end	Night	16% 7% 2 7% 7% 4 5% 4 6% 7% 34%	1520
July	1st to 10th	Night	12% 5% 3 6% 5% 6% 6% 5% 7% 8% 37%	2520
	11th to 20th	Night	15% 6% 2 6% 4 4 4 4 6% 8% 41%	2520
	21st to end	Night	15% 7% 2 6% 5% 5% 5% 4 6% 7% 39%	2739
August	1st to 10th	Night	20% 9% 2 8% 4 4 4 3 4 7% 34%	2570
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 4 6% 9% 37%	2876
	21st to end	Night	9% 6% 2 5% 4 4 4 4 7% 10% 46%	3476
September	1st to 10th	Night	13% 6% 2 4 3 3 4 3 5% 8% 48%	1900
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 6% 50%	2045
	21st to end	Night	14% 6% 2 5% 4 4 4 3 6% 7% 45%	2090
October	1st to 10th	Night	21% 7% 2 7% 5% 4 4 4 6% 6% 34%	2108
	11th to 20th	Night	16% 9% 3 6% 5% 4 5% 3 6% 6% 35%	2356
	21st to end	Night	22% 11% 3 6% 4 4 4 3 5% 7% 30%	2717
November	1st to 10th	Night	14% 8% 3 8% 5% 4 5% 4 5% 6% 38%	2537
	11th to 20th	Night	15% 9% 3 7% 6% 4 5% 4 5% 7% 37%	2600
	21st to end	Night	15% 11% 4 8% 5% 4 4 4 6% 7% 31%	2700
December	1st to 10th	Night	15% 14% 6% 15% 7% 6% 5% 4 4 5% 20%	4580
	11th to 20th	Night	22% 12% 4 8% 5% 4 4 3 4 5% 28%	5208
	21st to end	Night	19% 14% 5% 11% 6% 4 4 3 6% 5% 23%	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	60% 5% 9% 20% 5%	3598
	11th to 20th	Day	55% 7% 10% 16% 9% 3	3600
	21st to end	Day	53% 6% 8% 17% 15% 2	3960
February	1st to 10th	Day	55% 6% 12% 17% 9% 2	1775
	11th to 20th	Day	53% 7% 11% 18% 10% 2	1957
	21st to end	Day	55% 5% 10% 17% 12% 1	1650
March	1st to 10th	Day	63% 5% 8% 12% 10% 2	3219
	11th to 20th	Day	50% 10% 12% 14% 11% 2	3266
	21st to end	Day	53% 7% 6% 14% 17% 3	4148
April	1st to 10th	Day	53% 4 9% 20% 12% 3	2280
	11th to 20th	Day	48% 7% 10% 19% 15% 1	2254
	21st to end	Day	54% 6% 8% 17% 13% 2	2460
May	1st to 10th	Day	58% 7% 10% 17% 7% 1	4120
	11th to 20th	Day	50% 10% 14% 16% 9% 2	4384
	21st to end	Day	37% 9% 14% 25% 14% 1	4837
June	1st to 10th	Day	52% 8% 12% 18% 10%	2660
	11th to 20th	Day	46% 7% 11% 20% 14% 2	2660
	21st to end	Day	52% 8% 12% 17% 10% 1	2660
July	1st to 10th	Day	42% 7% 13% 22% 15% 1	4370
	11th to 20th	Day	41% 8% 18% 25% 9%	4365
	21st to end	Day	45% 6% 13% 23% 12% 1	4661
August	1st to 10th	Day	48% 7% 15% 20% 10%	4315
	11th to 20th	Day	49% 7% 15% 23% 6%	3974
	21st to end	Day	38% 6% 15% 26% 13% 1	4180
September	1st to 10th	Day	37% 7% 13% 28% 15% 1	2280
	11th to 20th	Day	37% 5% 10% 28% 19% 1	2134
	21st to end	Day	39% 7% 13% 26% 13% 2	2090
October	1st to 10th	Day	48% 5% 9% 22% 15% 1	3434
	11th to 20th	Day	44% 4 13% 24% 14% 1	3000
	21st to end	Day	43% 4 9% 30% 12% 2	3081
November	1st to 10th	Day	41% 7% 15% 21% 15% 1	1761
	11th to 20th	Day	48% 6% 11% 16% 17% 2	1800
	21st to end	Day	54% 6% 16% 17% 7% 1	1695
December	1st to 10th	Day	63% 5% 14% 15% 4	2600
	11th to 20th	Day	56% 6% 9% 16% 13% 1	2986
	21st to end	Day	58% 4 7% 14% 13% 3	3960
January	1st to 10th	Night	60% 6% 11% 16% 7% 1	6349
	11th to 20th	Night	58% 8% 11% 13% 8% 1	6360
	21st to end	Night	56% 6% 8% 13% 14% 3	6996
February	1st to 10th	Night	54% 7% 11% 18% 10% 1	2620
	11th to 20th	Night	51% 8% 11% 19% 10% 1	2340
	21st to end	Night	50% 7% 12% 20% 8% 3	1980
March	1st to 10th	Night	58% 5% 10% 15% 10% 2	3854
	11th to 20th	Night	51% 9% 11% 17% 11% 2	3723
	21st to end	Night	51% 7% 7% 13% 19% 4	3480
April	1st to 10th	Night	53% 5% 9% 18% 14% 2	1900
	11th to 20th	Night	49% 6% 9% 21% 14% 1	1875
	21st to end	Night	50% 6% 10% 17% 14% 3	1720
May	1st to 10th	Night	59% 7% 10% 15% 8% 1	2840
	11th to 20th	Night	54% 11% 11% 15% 8% 1	2542
	21st to end	Night	40% 9% 15% 21% 13% 2	2739
June	1st to 10th	Night	49% 8% 12% 18% 11% 1	1520
	11th to 20th	Night	45% 8% 14% 19% 12% 2	1520
	21st to end	Night	47% 8% 13% 20% 11% 1	1520
July	1st to 10th	Night	42% 7% 16% 21% 13%	2480
	11th to 20th	Night	41% 6% 15% 26% 12%	2472
	21st to end	Night	44% 6% 11% 27% 11% 1	2651
August	1st to 10th	Night	51% 6% 13% 21% 10%	2560
	11th to 20th	Night	44% 6% 19% 20% 10%	2903
	21st to end	Night	33% 6% 20% 28% 13% 1	3476
September	1st to 10th	Night	35% 6% 15% 28% 15% 1	1900
	11th to 20th	Night	35% 5% 12% 29% 18% 1	2045
	21st to end	Night	37% 7% 12% 27% 15% 2	2090
October	1st to 10th	Night	49% 5% 13% 22% 9% 1	3511
	11th to 20th	Night	43% 5% 12% 23% 15% 2	3905
	21st to end	Night	45% 5% 10% 28% 11% 1	4467
November	1st to 10th	Night	46% 6% 12% 22% 12% 2	2537
	11th to 20th	Night	46% 6% 11% 21% 15% 1	2600
	21st to end	Night	51% 7% 14% 20% 8%	2700
December	1st to 10th	Night	65% 5% 11% 15% 3	4580
	11th to 20th	Night	58% 5% 9% 12% 15% 1	5208
	21st to end	Night	61% 5% 7% 15% 8% 3	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	27% 25% 25% 18% 6%	200
	11th to 20th	Day	31% 26% 29% 13% 1 1	200
	21st to end	Day	24% 25% 30% 15% 6%	220
February	1st to 10th	Day	31% 29% 28% 10% 2	305
	11th to 20th	Day	30% 26% 29% 14% 2	496
	21st to end	Day	30% 21% 30% 16% 3	414
March	1st to 10th	Day	30% 20% 33% 15% 3	500
	11th to 20th	Day	31% 23% 29% 11% 5% 1	464
	21st to end	Day	32% 21% 25% 12% 7% 3	506
April	1st to 10th	Day	35% 19% 26% 12% 7% 2	460
	11th to 20th	Day	32% 21% 24% 14% 7% 1	452
	21st to end	Day	31% 21% 28% 12% 7% 1	460
May	1st to 10th	Day	33% 22% 25% 13% 6% 2	463
	11th to 20th	Day	22% 16% 34% 20% 5% 2	487
	21st to end	Day	19% 15% 33% 23% 9% 1	529
June	1st to 10th	Day	23% 14% 36% 17% 10% 1	490
	11th to 20th	Day	19% 16% 30% 22% 11% 2	490
	21st to end	Day	23% 16% 32% 18% 10% 2	490
July	1st to 10th	Day	16% 12% 33% 23% 12% 4	485
	11th to 20th	Day	13% 9% 34% 30% 12% 3	484
	21st to end	Day	16% 8% 38% 26% 11% 2	532
August	1st to 10th	Day	13% 12% 29% 25% 17% 3	486
	11th to 20th	Day	17% 9% 28% 25% 14% 8%	479
	21st to end	Day	13% 14% 25% 25% 17% 6%	539
September	1st to 10th	Day	17% 11% 24% 25% 15% 7%	490
	11th to 20th	Day	16% 13% 22% 21% 17% 10%	490
	21st to end	Day	15% 14% 29% 22% 12% 8%	490
October	1st to 10th	Day	26% 13% 24% 20% 12% 5%	490
	11th to 20th	Day	22% 16% 22% 27% 10% 3	449
	21st to end	Day	36% 13% 23% 20% 7%	429
November	1st to 10th	Day	24% 18% 23% 22% 13% 1	395
	11th to 20th	Day	32% 18% 24% 19% 7% 1	400
	21st to end	Day	36% 17% 25% 16% 5% 1	295
December	1st to 10th	Day	29% 23% 27% 16% 6%	200
	11th to 20th	Day	35% 23% 30% 9% 5%	200
	21st to end	Day	33% 19% 27% 15% 5%	220
January	1st to 10th	Night	23% 19% 33% 23% 3	800
	11th to 20th	Night	25% 19% 29% 24% 4	800
	21st to end	Night	20% 18% 35% 22% 4	880
February	1st to 10th	Night	24% 18% 34% 22% 3	800
	11th to 20th	Night	22% 19% 34% 22% 2	783
	21st to end	Night	23% 18% 32% 24% 3	660
March	1st to 10th	Night	26% 17% 31% 23% 4	800
	11th to 20th	Night	24% 19% 28% 24% 5%	767
	21st to end	Night	28% 16% 26% 22% 7% 1	836
April	1st to 10th	Night	25% 19% 27% 24% 5%	760
	11th to 20th	Night	24% 21% 24% 24% 7%	751
	21st to end	Night	27% 15% 30% 22% 6%	580
May	1st to 10th	Night	28% 13% 28% 24% 6% 1	570
	11th to 20th	Night	20% 12% 29% 31% 7%	407
	21st to end	Night	18% 9% 26% 35% 12% 1	411
June	1st to 10th	Night	21% 8% 28% 32% 10%	380
	11th to 20th	Night	19% 10% 24% 33% 12%	380
	21st to end	Night	18% 8% 25% 40% 9% 1	380
July	1st to 10th	Night	15% 5% 20% 44% 15% 1	370
	11th to 20th	Night	15% 5% 20% 42% 17% 2	368
	21st to end	Night	14% 6% 19% 44% 16% 1	416
August	1st to 10th	Night	15% 4 17% 42% 21% 1	416
	11th to 20th	Night	14% 6% 24% 36% 18% 3	617
	21st to end	Night	15% 6% 20% 35% 22% 2	836
September	1st to 10th	Night	14% 9% 24% 32% 19% 2	760
	11th to 20th	Night	17% 7% 21% 29% 22% 4	760
	21st to end	Night	17% 8% 24% 30% 19% 2	760
October	1st to 10th	Night	27% 9% 18% 29% 16% 2	760
	11th to 20th	Night	23% 12% 23% 31% 11% 1	760
	21st to end	Night	29% 14% 26% 24% 8%	836
November	1st to 10th	Night	20% 17% 26% 28% 9%	780
	11th to 20th	Night	24% 15% 28% 24% 8% 1	800
	21st to end	Night	25% 16% 27% 25% 6% 1	800
December	1st to 10th	Night	24% 19% 32% 22% 4	800
	11th to 20th	Night	27% 19% 29% 21% 4	800
	21st to end	Night	28% 18% 31% 19% 4	880

Page generated at 2025-03-09 08:08:42. Version ac07

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