Seaman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seaman Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	13% 6% 2 6% 4 4 4 4 6% 8% 42%	100745
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 43%	98289
Transparency	Day	39% 5% 12% 22% 18% 5%	104391
	Night	39% 5% 10% 21% 19% 6%	102782
Seeing	Day	15% 7% 22% 26% 20% 9%	15340
	Night	16% 6% 25% 33% 17% 2	25267

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	17% 5% 1 4 4 3 3 2 3 5% 51%	2836
	11th to 20th	Day	13% 7% 3 5% 4 3 3 2 4 5% 51%	2840
	21st to end	Day	17% 9% 3 7% 3 3 4 3 4 6% 43%	3124
February	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 6% 44%	1710
	11th to 20th	Day	21% 6% 2 4 4 4 3 2 3 4 47%	1739
	21st to end	Day	17% 7% 3 6% 5% 3 4 3 5% 7% 40%	1631
March	1st to 10th	Day	23% 9% 3 5% 3 4 4 4 6% 6% 35%	3435
	11th to 20th	Day	11% 5% 2 5% 3 3 3 2 5% 6% 55%	3480
	21st to end	Day	19% 6% 2 5% 3 3 4 3 5% 5% 44%	3815
April	1st to 10th	Day	11% 5% 2 4 3 3 3 4 5% 8% 52%	2263
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 6% 8% 50%	2254
	21st to end	Day	13% 6% 2 5% 4 4 4 3 5% 6% 46%	2280
May	1st to 10th	Day	9% 6% 3 8% 6% 6% 6% 4 8% 10% 35%	3893
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 8% 34%	4081
	21st to end	Day	7% 4 2 5% 4 4 5% 4 6% 10% 49%	4525
June	1st to 10th	Day	9% 4 2 7% 5% 6% 5% 5% 8% 10% 40%	2660
	11th to 20th	Day	8% 5% 2 5% 6% 4 5% 4 7% 10% 43%	2660
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 10% 41%	2581
July	1st to 10th	Day	6% 4 3 6% 5% 6% 6% 6% 9% 13% 37%	4120
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 4 8% 10% 32%	4120
	21st to end	Day	10% 7% 3 9% 6% 6% 7% 5% 8% 9% 30%	4466
August	1st to 10th	Day	19% 9% 3 9% 6% 5% 5% 4 8% 10% 22%	4104
	11th to 20th	Day	9% 6% 2 8% 6% 5% 6% 5% 7% 10% 37%	3804
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 6% 10% 47%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 5% 5% 5% 6% 8% 43%	2280
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 3 6% 8% 42%	2252
	21st to end	Day	19% 7% 2 6% 5% 5% 4 3 5% 7% 36%	1995
October	1st to 10th	Day	14% 6% 2 6% 4 3 4 3 4 5% 49%	1900
	11th to 20th	Day	13% 5% 2 5% 3 2 3 4 3 5% 55%	1900
	21st to end	Day	11% 5% 2 6% 4 4 3 2 3 4 55%	2090
November	1st to 10th	Day	13% 6% 2 5% 3 2 3 2 4 6% 52%	1927
	11th to 20th	Day	13% 7% 2 4 3 4 3 3 4 6% 51%	1814
	21st to end	Day	14% 5% 2 4 5% 3 3 2 5% 6% 52%	1587
December	1st to 10th	Day	15% 8% 2 5% 5% 4 4 3 6% 7% 41%	1520
	11th to 20th	Day	15% 5% 2 4 3 3 2 2 4 5% 54%	1910
	21st to end	Day	16% 7% 3 6% 4 4 4 3 5% 6% 43%	2969
January	1st to 10th	Night	14% 7% 2 6% 3 4 4 3 5% 6% 46%	4109
	11th to 20th	Night	15% 8% 3 6% 4 3 4 3 5% 5% 43%	4120
	21st to end	Night	18% 6% 3 6% 4 4 4 3 4 5% 43%	4532
February	1st to 10th	Night	17% 7% 2 5% 3 3 4 3 5% 5% 45%	2470
	11th to 20th	Night	20% 7% 2 6% 4 4 3 4 4 5% 41%	2327
	21st to end	Night	16% 7% 2 4 4 3 4 4 6% 6% 46%	1820
March	1st to 10th	Night	24% 8% 3 7% 4 4 4 3 4 4 33%	3418
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 5% 6% 51%	3480
	21st to end	Night	16% 6% 1 4 3 3 3 2 4 5% 51%	3817
April	1st to 10th	Night	13% 5% 2 6% 4 4 3 4 6% 6% 48%	1915
	11th to 20th	Night	12% 5% 2 5% 4 4 4 2 5% 7% 49%	1875
	21st to end	Night	11% 5% 2 5% 4 4 4 3 6% 8% 49%	1900
May	1st to 10th	Night	11% 8% 3 9% 6% 5% 4 3 5% 8% 39%	3016
	11th to 20th	Night	21% 8% 3 7% 5% 5% 4 3 4 6% 36%	2812
	21st to end	Night	9% 4 2 6% 4 4 5% 3 6% 11% 45%	3026
June	1st to 10th	Night	9% 5% 2 6% 4 4 5% 5% 9% 11% 39%	1520
	11th to 20th	Night	9% 6% 1 7% 6% 5% 5% 6% 7% 9% 39%	1520
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 6% 9% 11% 39%	1589
July	1st to 10th	Night	7% 6% 3 6% 5% 5% 5% 5% 8% 12% 38%	2840
	11th to 20th	Night	13% 6% 3 9% 5% 5% 5% 4 7% 11% 31%	2840
	21st to end	Night	17% 8% 4 9% 5% 5% 5% 4 6% 7% 30%	3087
August	1st to 10th	Night	22% 9% 3 8% 5% 4 5% 4 7% 8% 24%	2825
	11th to 20th	Night	8% 6% 2 8% 6% 5% 6% 5% 8% 10% 34%	2990
	21st to end	Night	6% 5% 2 6% 4 4 5% 5% 8% 10% 46%	3476
September	1st to 10th	Night	10% 5% 2 6% 3 4 4 4 6% 10% 46%	1900
	11th to 20th	Night	13% 6% 3 7% 4 4 5% 3 4 7% 43%	1924
	21st to end	Night	19% 7% 2 8% 5% 4 4 3 4 7% 35%	2185
October	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 5% 6% 48%	2280
	11th to 20th	Night	13% 5% 2 5% 3 2 4 2 4 6% 54%	2280
	21st to end	Night	12% 4 2 5% 3 3 3 3 4 6% 56%	2508
November	1st to 10th	Night	14% 6% 2 6% 4 3 3 3 4 5% 50%	2312
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 5% 5% 50%	2362
	21st to end	Night	13% 4 2 5% 3 3 4 3 5% 5% 52%	2590
December	1st to 10th	Night	16% 7% 3 7% 4 4 4 4 4 6% 42%	2660
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 5% 48%	3285
	21st to end	Night	15% 7% 2 5% 3 3 4 3 5% 6% 47%	4679

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	38% 2 5% 26% 24% 5%	2837
	11th to 20th	Day	38% 3 4 18% 32% 6%	2840
	21st to end	Day	45% 3 6% 19% 19% 8%	3124
February	1st to 10th	Day	40% 4 10% 22% 19% 5%	1710
	11th to 20th	Day	43% 2 7% 20% 22% 6%	1748
	21st to end	Day	44% 4 11% 16% 19% 6%	1631
March	1st to 10th	Day	50% 4 8% 18% 13% 7%	3435
	11th to 20th	Day	32% 3 8% 20% 24% 12%	3480
	21st to end	Day	43% 4 9% 23% 17% 5%	3815
April	1st to 10th	Day	31% 3 10% 26% 23% 7%	2263
	11th to 20th	Day	33% 4 12% 23% 22% 6%	2254
	21st to end	Day	38% 5% 12% 23% 18% 4	2280
May	1st to 10th	Day	43% 5% 19% 20% 11% 2	3928
	11th to 20th	Day	48% 5% 11% 26% 11% 1	4085
	21st to end	Day	31% 6% 15% 27% 18% 3	4539
June	1st to 10th	Day	37% 7% 14% 23% 16% 2	2660
	11th to 20th	Day	34% 8% 17% 26% 14% 1	2660
	21st to end	Day	35% 10% 17% 22% 15% 2	2581
July	1st to 10th	Day	35% 10% 21% 21% 11% 2	4055
	11th to 20th	Day	45% 8% 24% 15% 6% 1	4051
	21st to end	Day	47% 9% 16% 16% 12%	4324
August	1st to 10th	Day	56% 7% 19% 14% 5%	4091
	11th to 20th	Day	40% 8% 15% 26% 10% 1	3842
	21st to end	Day	32% 6% 16% 29% 15% 2	4180
September	1st to 10th	Day	37% 8% 15% 17% 19% 4	2280
	11th to 20th	Day	40% 5% 11% 22% 20% 2	2252
	21st to end	Day	48% 4 10% 21% 13% 4	1995
October	1st to 10th	Day	32% 5% 9% 23% 25% 6%	3160
	11th to 20th	Day	22% 3 5% 20% 30% 21%	3140
	21st to end	Day	25% 3 6% 25% 27% 14%	3428
November	1st to 10th	Day	35% 3 7% 24% 26% 5%	1927
	11th to 20th	Day	35% 3 10% 20% 25% 7%	1814
	21st to end	Day	34% 3 7% 24% 26% 5%	1587
December	1st to 10th	Day	43% 3 12% 21% 17% 4	1520
	11th to 20th	Day	34% 3 6% 28% 20% 9%	1906
	21st to end	Day	42% 3 8% 24% 21% 2	2969
January	1st to 10th	Night	38% 3 6% 22% 26% 5%	4109
	11th to 20th	Night	44% 3 6% 18% 23% 5%	4120
	21st to end	Night	45% 3 5% 19% 21% 7%	4532
February	1st to 10th	Night	42% 3 8% 20% 21% 5%	2470
	11th to 20th	Night	45% 3 7% 16% 21% 7%	2329
	21st to end	Night	39% 4 7% 22% 19% 8%	1820
March	1st to 10th	Night	55% 3 7% 15% 15% 5%	3418
	11th to 20th	Night	35% 4 8% 13% 23% 16%	3480
	21st to end	Night	36% 3 8% 25% 22% 6%	3817
April	1st to 10th	Night	35% 5% 9% 18% 25% 7%	1915
	11th to 20th	Night	35% 5% 9% 24% 21% 7%	1875
	21st to end	Night	34% 5% 11% 29% 18% 5%	1900
May	1st to 10th	Night	43% 5% 15% 23% 12% 2	3046
	11th to 20th	Night	50% 5% 9% 21% 14% 1	2813
	21st to end	Night	33% 6% 16% 25% 18% 2	3033
June	1st to 10th	Night	35% 8% 18% 22% 16% 2	1520
	11th to 20th	Night	38% 10% 16% 19% 16% 2	1520
	21st to end	Night	37% 10% 14% 23% 14% 2	1589
July	1st to 10th	Night	37% 12% 20% 18% 10% 2	2795
	11th to 20th	Night	46% 10% 23% 16% 5% 1	2786
	21st to end	Night	53% 8% 12% 16% 10%	2988
August	1st to 10th	Night	56% 7% 17% 15% 4	2814
	11th to 20th	Night	41% 10% 20% 21% 7% 1	3018
	21st to end	Night	31% 7% 12% 31% 17% 2	3476
September	1st to 10th	Night	34% 7% 17% 21% 15% 5%	1900
	11th to 20th	Night	41% 7% 10% 20% 20% 3	1924
	21st to end	Night	49% 5% 10% 17% 16% 3	2185
October	1st to 10th	Night	38% 5% 7% 22% 23% 4	3800
	11th to 20th	Night	24% 3 5% 17% 27% 23%	3779
	21st to end	Night	22% 3 7% 29% 24% 16%	4120
November	1st to 10th	Night	38% 4 6% 20% 27% 5%	2312
	11th to 20th	Night	36% 4 9% 16% 25% 10%	2362
	21st to end	Night	34% 4 7% 23% 26% 7%	2590
December	1st to 10th	Night	44% 4 10% 20% 19% 4	2660
	11th to 20th	Night	39% 2 8% 20% 23% 7%	3288
	21st to end	Night	39% 3 7% 25% 23% 3	4679

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	19% 18% 37% 20% 6%	270
	11th to 20th	Day	19% 23% 33% 18% 7%	270
	21st to end	Day	19% 18% 39% 20% 4 1	297
February	1st to 10th	Day	18% 16% 36% 24% 6% 1	270
	11th to 20th	Day	31% 13% 37% 17% 3	346
	21st to end	Day	23% 15% 32% 23% 6%	382
March	1st to 10th	Day	19% 13% 33% 26% 8% 1	460
	11th to 20th	Day	19% 17% 34% 22% 7% 1	460
	21st to end	Day	20% 11% 35% 24% 9% 2	506
April	1st to 10th	Day	17% 15% 30% 25% 12% 2	460
	11th to 20th	Day	13% 12% 28% 28% 15% 3	452
	21st to end	Day	16% 9% 25% 28% 19% 2	460
May	1st to 10th	Day	10% 11% 34% 23% 14% 7%	463
	11th to 20th	Day	16% 4 22% 30% 19% 9%	487
	21st to end	Day	15% 4 14% 26% 26% 15%	529
June	1st to 10th	Day	11% 2 15% 31% 28% 13%	490
	11th to 20th	Day	11% 3 17% 24% 26% 18%	490
	21st to end	Day	8% 3 12% 31% 32% 13%	490
July	1st to 10th	Day	9% 1 7% 25% 35% 22%	485
	11th to 20th	Day	10% 2 6% 27% 35% 21%	484
	21st to end	Day	9% 2 11% 31% 33% 15%	532
August	1st to 10th	Day	14% 3 12% 27% 28% 17%	486
	11th to 20th	Day	11% 1 10% 25% 30% 22%	479
	21st to end	Day	11% 2 9% 27% 27% 25%	539
September	1st to 10th	Day	13% 2 13% 21% 28% 22%	490
	11th to 20th	Day	15% 4 12% 26% 29% 14%	490
	21st to end	Day	22% 6% 19% 27% 19% 8%	490
October	1st to 10th	Day	16% 4 20% 23% 25% 12%	490
	11th to 20th	Day	14% 6% 21% 31% 22% 6%	490
	21st to end	Day	12% 6% 25% 29% 21% 6%	539
November	1st to 10th	Day	19% 6% 24% 31% 15% 6%	483
	11th to 20th	Day	18% 7% 32% 29% 12% 2	419
	21st to end	Day	17% 10% 25% 28% 17% 4	247
December	1st to 10th	Day	19% 12% 39% 18% 9% 2	180
	11th to 20th	Day	18% 8% 37% 20% 15% 2	180
	21st to end	Day	22% 12% 31% 19% 13% 3	255
January	1st to 10th	Night	19% 11% 36% 28% 6% 1	760
	11th to 20th	Night	19% 11% 34% 29% 7%	760
	21st to end	Night	18% 8% 41% 29% 4	836
February	1st to 10th	Night	21% 8% 38% 30% 3	760
	11th to 20th	Night	24% 13% 41% 21% 1	743
	21st to end	Night	20% 10% 37% 31% 3	628
March	1st to 10th	Night	16% 11% 35% 32% 6%	760
	11th to 20th	Night	20% 10% 38% 25% 7% 1	760
	21st to end	Night	19% 9% 35% 30% 7%	836
April	1st to 10th	Night	14% 11% 36% 32% 6%	760
	11th to 20th	Night	15% 9% 34% 31% 11%	751
	21st to end	Night	14% 6% 31% 41% 9% 1	760
May	1st to 10th	Night	13% 8% 32% 35% 12%	698
	11th to 20th	Night	18% 6% 21% 39% 15% 1	561
	21st to end	Night	12% 2 14% 40% 31% 2	589
June	1st to 10th	Night	9% 4 17% 42% 27% 1	380
	11th to 20th	Night	9% 4 15% 41% 28% 4	380
	21st to end	Night	6% 3 13% 45% 30% 3	449
July	1st to 10th	Night	9% 1 10% 37% 39% 4	555
	11th to 20th	Night	8% 2 6% 39% 38% 6%	552
	21st to end	Night	9% 2 12% 41% 33% 4	624
August	1st to 10th	Night	12% 2 9% 38% 36% 3	567
	11th to 20th	Night	11% 2 8% 36% 38% 5%	679
	21st to end	Night	13% 3 8% 31% 39% 6%	836
September	1st to 10th	Night	12% 2 9% 34% 38% 6%	760
	11th to 20th	Night	16% 3 12% 34% 31% 5%	760
	21st to end	Night	23% 3 14% 33% 26% 1	760
October	1st to 10th	Night	16% 3 16% 34% 26% 4	760
	11th to 20th	Night	15% 5% 23% 35% 19% 2	760
	21st to end	Night	13% 4 27% 33% 21% 2	836
November	1st to 10th	Night	17% 4 26% 33% 17% 3	771
	11th to 20th	Night	17% 7% 29% 33% 13% 1	760
	21st to end	Night	15% 6% 29% 34% 15% 1	760
December	1st to 10th	Night	19% 8% 32% 33% 8%	760
	11th to 20th	Night	18% 7% 31% 35% 8%	760
	21st to end	Night	18% 8% 32% 32% 9%	836

Page generated at 2024-11-05 08:15:48. Version ac07

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