Seagrave Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seagrave Memorial Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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% 8% 2 6% 4 4 4 4 5% 7% 39%	101179
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	97847
Transparency	Day	44% 6% 11% 21% 15% 2	104730
	Night	44% 5% 11% 22% 15% 2	102372
Seeing	Day	22% 16% 33% 19% 7% 2	15089
	Night	21% 14% 34% 23% 6%	24007

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% 8% 3 6% 4 4 4 3 5% 5% 45%	2517
	11th to 20th	Day	17% 9% 3 6% 4 4 5% 4 4 5% 40%	2520
	21st to end	Day	22% 11% 3 6% 4 3 4 3 4 5% 35%	2772
February	1st to 10th	Day	19% 8% 3 5% 4 5% 5% 4 5% 6% 37%	1806
	11th to 20th	Day	14% 8% 3 6% 4 5% 5% 4 4 7% 41%	1848
	21st to end	Day	19% 10% 2 6% 4 4 4 4 4 6% 37%	1570
March	1st to 10th	Day	25% 12% 3 7% 4 4 3 2 4 5% 32%	3116
	11th to 20th	Day	11% 5% 2 5% 4 5% 6% 4 6% 8% 44%	3315
	21st to end	Day	21% 10% 2 6% 4 4 4 3 5% 5% 36%	4056
April	1st to 10th	Day	18% 9% 3 6% 4 3 3 4 5% 6% 37%	2280
	11th to 20th	Day	17% 7% 2 5% 4 4 4 4 6% 6% 41%	2254
	21st to end	Day	21% 7% 2 6% 5% 4 4 3 4 6% 38%	2460
May	1st to 10th	Day	26% 11% 3 7% 4 4 4 3 4 7% 28%	4106
	11th to 20th	Day	23% 10% 2 7% 4 3 3 3 4 6% 33%	4134
	21st to end	Day	16% 5% 2 4 4 4 4 3 5% 8% 46%	4837
June	1st to 10th	Day	18% 7% 2 6% 4 5% 5% 4 7% 8% 35%	2660
	11th to 20th	Day	16% 7% 2 6% 5% 4 5% 3 6% 9% 36%	2660
	21st to end	Day	15% 7% 2 6% 5% 5% 5% 4 7% 9% 36%	2660
July	1st to 10th	Day	13% 6% 3 6% 6% 6% 6% 5% 6% 9% 34%	4440
	11th to 20th	Day	13% 6% 3 6% 4 5% 5% 4 7% 9% 38%	4440
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 7% 32%	4814
August	1st to 10th	Day	18% 7% 3 9% 4 5% 6% 4 6% 8% 31%	4142
	11th to 20th	Day	13% 6% 3 7% 5% 5% 4 4 6% 8% 39%	3927
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 6% 9% 44%	4180
September	1st to 10th	Day	15% 6% 2 5% 4 5% 4 4 5% 5% 46%	2280
	11th to 20th	Day	15% 5% 2 5% 3 3 4 3 5% 6% 50%	2094
	21st to end	Day	19% 7% 2 6% 3 4 4 3 3 6% 42%	2090
October	1st to 10th	Day	23% 6% 2 5% 3 3 3 3 4 5% 42%	2090
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 4 7% 48%	1867
	21st to end	Day	18% 6% 2 6% 4 4 4 3 4 5% 43%	1881
November	1st to 10th	Day	16% 6% 2 6% 4 4 3 3 3 5% 48%	1698
	11th to 20th	Day	18% 7% 1 4 3 4 3 3 5% 7% 45%	1710
	21st to end	Day	17% 7% 2 5% 4 3 3 3 5% 6% 45%	1710
December	1st to 10th	Day	18% 8% 2 6% 5% 3 4 5% 5% 5% 38%	1567
	11th to 20th	Day	21% 6% 2 5% 3 3 3 3 3 5% 46%	1906
	21st to end	Day	20% 10% 3 7% 4 3 4 3 4 5% 35%	2772
January	1st to 10th	Night	17% 7% 2 4 3 3 3 3 4 5% 48%	4429
	11th to 20th	Night	16% 11% 4 7% 4 4 3 3 4 4 40%	4440
	21st to end	Night	23% 12% 4 7% 4 4 4 3 4 5% 31%	4884
February	1st to 10th	Night	17% 8% 3 7% 4 4 4 3 5% 6% 40%	2368
	11th to 20th	Night	16% 7% 3 7% 5% 4 4 4 6% 5% 41%	2218
	21st to end	Night	18% 9% 2 6% 5% 3 4 3 4 5% 42%	1884
March	1st to 10th	Night	27% 11% 2 4 4 3 3 3 4 6% 33%	3724
	11th to 20th	Night	11% 9% 3 6% 4 5% 4 4 5% 7% 43%	3643
	21st to end	Night	25% 9% 2 5% 3 4 4 4 5% 5% 35%	3572
April	1st to 10th	Night	18% 8% 3 5% 6% 4 5% 4 6% 5% 37%	1900
	11th to 20th	Night	19% 7% 2 6% 4 4 4 4 5% 6% 38%	1875
	21st to end	Night	20% 8% 3 7% 3 3 3 3 5% 6% 40%	1720
May	1st to 10th	Night	25% 9% 3 7% 4 4 5% 3 5% 6% 28%	2834
	11th to 20th	Night	23% 9% 3 8% 5% 5% 4 4 6% 7% 26%	2787
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 7% 40%	2738
June	1st to 10th	Night	15% 9% 2 7% 4 5% 4 3 6% 7% 38%	1520
	11th to 20th	Night	18% 7% 3 6% 4 3 4 4 6% 7% 38%	1520
	21st to end	Night	15% 6% 2 6% 3 4 4 4 6% 9% 40%	1520
July	1st to 10th	Night	13% 7% 3 6% 5% 5% 5% 5% 7% 9% 35%	2520
	11th to 20th	Night	14% 7% 3 7% 4 4 5% 4 6% 9% 38%	2520
	21st to end	Night	19% 9% 3 7% 5% 4 5% 4 5% 7% 32%	2739
August	1st to 10th	Night	21% 7% 3 7% 5% 5% 5% 4 7% 7% 29%	2754
	11th to 20th	Night	11% 7% 3 8% 5% 6% 6% 5% 6% 7% 35%	2896
	21st to end	Night	9% 5% 2 4 3 3 3 3 5% 8% 56%	3476
September	1st to 10th	Night	12% 6% 3 7% 6% 4 5% 4 5% 6% 43%	1900
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 4 6% 51%	2085
	21st to end	Night	18% 8% 3 7% 4 3 4 3 4 4 42%	2090
October	1st to 10th	Night	20% 7% 2 6% 3 3 3 3 4 6% 41%	2090
	11th to 20th	Night	15% 5% 2 5% 3 4 3 3 5% 6% 48%	2308
	21st to end	Night	20% 7% 2 6% 4 3 4 3 4 6% 40%	2717
November	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 4 6% 49%	2453
	11th to 20th	Night	17% 8% 3 5% 4 4 3 2 4 5% 45%	2470
	21st to end	Night	16% 7% 2 4 4 3 3 4 5% 7% 45%	2470
December	1st to 10th	Night	18% 8% 2 7% 4 4 3 3 4 5% 41%	2611
	11th to 20th	Night	19% 7% 2 5% 4 4 4 3 5% 6% 42%	3288
	21st to end	Night	20% 11% 3 7% 3 3 3 3 5% 5% 37%	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	40% 4 9% 30% 15% 2	2518
	11th to 20th	Day	47% 5% 10% 23% 13% 2	2520
	21st to end	Day	52% 4 9% 22% 11% 2	2772
February	1st to 10th	Day	46% 5% 12% 19% 14% 3	1806
	11th to 20th	Day	44% 4 11% 22% 16% 3	1857
	21st to end	Day	48% 5% 10% 20% 15% 3	1570
March	1st to 10th	Day	55% 3 13% 15% 12% 3	3119
	11th to 20th	Day	34% 6% 14% 29% 14% 3	3315
	21st to end	Day	50% 4 7% 16% 17% 6%	4056
April	1st to 10th	Day	46% 4 11% 19% 17% 3	2280
	11th to 20th	Day	42% 5% 11% 20% 19% 3	2254
	21st to end	Day	48% 4 9% 18% 17% 4	2460
May	1st to 10th	Day	55% 6% 12% 14% 10% 2	4120
	11th to 20th	Day	53% 6% 13% 16% 10% 2	4138
	21st to end	Day	37% 7% 12% 23% 20% 2	4837
June	1st to 10th	Day	45% 7% 13% 22% 12% 1	2660
	11th to 20th	Day	44% 6% 13% 21% 14% 2	2660
	21st to end	Day	43% 8% 13% 21% 14% 1	2660
July	1st to 10th	Day	45% 8% 11% 20% 14% 1	4370
	11th to 20th	Day	42% 6% 15% 22% 12% 2	4365
	21st to end	Day	50% 6% 10% 18% 13% 2	4661
August	1st to 10th	Day	50% 7% 15% 19% 8% 1	4128
	11th to 20th	Day	42% 7% 13% 27% 10% 1	3966
	21st to end	Day	34% 7% 12% 27% 17% 2	4180
September	1st to 10th	Day	39% 7% 10% 21% 21% 2	2280
	11th to 20th	Day	34% 6% 8% 23% 25% 4	2094
	21st to end	Day	43% 5% 9% 19% 22% 2	2090
October	1st to 10th	Day	41% 4 8% 20% 22% 4	3480
	11th to 20th	Day	30% 3 7% 25% 30% 5%	3070
	21st to end	Day	32% 4 13% 27% 21% 3	3081
November	1st to 10th	Day	40% 5% 12% 21% 19% 2	1698
	11th to 20th	Day	40% 4 11% 21% 21% 3	1710
	21st to end	Day	41% 4 12% 26% 13% 3	1710
December	1st to 10th	Day	46% 6% 13% 16% 17% 2	1567
	11th to 20th	Day	43% 3 14% 18% 17% 5%	1906
	21st to end	Day	51% 5% 11% 22% 11% 1	2772
January	1st to 10th	Night	38% 4 10% 32% 12% 4	4429
	11th to 20th	Night	48% 4 11% 23% 11% 3	4440
	21st to end	Night	56% 5% 8% 17% 12% 2	4884
February	1st to 10th	Night	46% 5% 12% 22% 14% 2	2368
	11th to 20th	Night	45% 5% 10% 19% 18% 4	2220
	21st to end	Night	45% 5% 8% 23% 15% 4	1884
March	1st to 10th	Night	53% 4 8% 20% 12% 3	3734
	11th to 20th	Night	39% 6% 16% 20% 16% 3	3644
	21st to end	Night	51% 3 7% 13% 19% 6%	3572
April	1st to 10th	Night	48% 5% 10% 18% 15% 3	1900
	11th to 20th	Night	46% 6% 9% 18% 19% 3	1875
	21st to end	Night	46% 5% 10% 19% 15% 5%	1720
May	1st to 10th	Night	56% 7% 12% 15% 9% 2	2840
	11th to 20th	Night	56% 8% 13% 13% 8% 2	2788
	21st to end	Night	41% 8% 14% 23% 14% 1	2739
June	1st to 10th	Night	45% 8% 12% 21% 14% 1	1520
	11th to 20th	Night	44% 6% 12% 24% 13% 1	1520
	21st to end	Night	40% 8% 15% 24% 13% 1	1520
July	1st to 10th	Night	44% 8% 12% 22% 13% 1	2480
	11th to 20th	Night	42% 7% 16% 25% 10% 1	2472
	21st to end	Night	51% 6% 11% 16% 13% 2	2651
August	1st to 10th	Night	53% 8% 13% 17% 8% 1	2744
	11th to 20th	Night	44% 9% 13% 25% 8%	2923
	21st to end	Night	27% 6% 14% 30% 21% 1	3476
September	1st to 10th	Night	41% 6% 10% 22% 18% 2	1900
	11th to 20th	Night	35% 5% 9% 23% 26% 2	2085
	21st to end	Night	45% 5% 8% 19% 20% 3	2090
October	1st to 10th	Night	40% 5% 12% 23% 18% 3	3480
	11th to 20th	Night	27% 3 8% 27% 32% 3	3831
	21st to end	Night	34% 3 11% 28% 20% 3	4467
November	1st to 10th	Night	37% 6% 12% 22% 19% 4	2453
	11th to 20th	Night	43% 3 10% 22% 20% 3	2470
	21st to end	Night	39% 5% 13% 28% 14% 2	2470
December	1st to 10th	Night	46% 4 13% 21% 15% 1	2611
	11th to 20th	Night	42% 4 16% 18% 18% 3	3288
	21st to end	Night	50% 4 9% 24% 13% 1	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	27% 23% 31% 14% 4	180
	11th to 20th	Day	28% 27% 32% 9% 4 1	180
	21st to end	Day	22% 27% 35% 11% 4 1	198
February	1st to 10th	Day	31% 22% 32% 12% 2	388
	11th to 20th	Day	28% 29% 34% 9% 1	456
	21st to end	Day	31% 20% 34% 12% 4	382
March	1st to 10th	Day	28% 21% 34% 14% 3	460
	11th to 20th	Day	27% 23% 34% 13% 3 1	460
	21st to end	Day	30% 21% 32% 11% 5% 1	506
April	1st to 10th	Day	31% 22% 28% 14% 5% 1	460
	11th to 20th	Day	26% 22% 35% 13% 3 1	452
	21st to end	Day	29% 20% 30% 15% 3 2	460
May	1st to 10th	Day	32% 21% 30% 14% 3 1	463
	11th to 20th	Day	21% 15% 42% 16% 5%	487
	21st to end	Day	19% 13% 35% 24% 7% 2	529
June	1st to 10th	Day	21% 12% 35% 21% 9% 1	490
	11th to 20th	Day	16% 15% 37% 23% 8% 1	490
	21st to end	Day	17% 14% 35% 24% 7% 2	490
July	1st to 10th	Day	16% 8% 33% 27% 14% 2	485
	11th to 20th	Day	15% 9% 29% 30% 14% 2	484
	21st to end	Day	14% 9% 40% 27% 10% 1	532
August	1st to 10th	Day	12% 9% 35% 30% 12% 2	486
	11th to 20th	Day	14% 7% 34% 29% 12% 3	479
	21st to end	Day	11% 10% 33% 26% 14% 6%	539
September	1st to 10th	Day	19% 9% 28% 24% 15% 4	490
	11th to 20th	Day	16% 11% 28% 24% 12% 8%	490
	21st to end	Day	23% 10% 34% 21% 8% 3	490
October	1st to 10th	Day	27% 13% 25% 22% 9% 4	490
	11th to 20th	Day	18% 17% 32% 20% 9% 3	445
	21st to end	Day	22% 17% 33% 19% 7% 2	429
November	1st to 10th	Day	20% 17% 33% 19% 8% 3	374
	11th to 20th	Day	25% 23% 28% 17% 5%	370
	21st to end	Day	24% 18% 34% 16% 6% 1	370
December	1st to 10th	Day	22% 28% 33% 12% 5%	227
	11th to 20th	Day	32% 25% 33% 7% 3	180
	21st to end	Day	24% 26% 34% 11% 4 2	198
January	1st to 10th	Night	21% 20% 40% 16% 3	760
	11th to 20th	Night	27% 21% 36% 15% 2	760
	21st to end	Night	20% 22% 37% 18% 3	836
February	1st to 10th	Night	23% 18% 39% 19% 1	760
	11th to 20th	Night	23% 24% 39% 13% 2	743
	21st to end	Night	27% 20% 36% 15% 3	628
March	1st to 10th	Night	24% 19% 37% 16% 3	760
	11th to 20th	Night	19% 21% 39% 18% 3	760
	21st to end	Night	26% 19% 35% 17% 3	836
April	1st to 10th	Night	24% 20% 35% 18% 3	760
	11th to 20th	Night	23% 18% 37% 18% 3	751
	21st to end	Night	24% 16% 39% 17% 3	580
May	1st to 10th	Night	28% 14% 33% 21% 4	570
	11th to 20th	Night	22% 16% 38% 21% 3	549
	21st to end	Night	20% 8% 35% 29% 7%	411
June	1st to 10th	Night	19% 7% 33% 32% 8%	380
	11th to 20th	Night	20% 7% 33% 33% 6%	380
	21st to end	Night	18% 5% 34% 36% 7%	380
July	1st to 10th	Night	14% 5% 32% 35% 13%	370
	11th to 20th	Night	15% 5% 30% 36% 13% 1	368
	21st to end	Night	16% 4 31% 40% 9%	416
August	1st to 10th	Night	14% 4 34% 40% 7%	522
	11th to 20th	Night	13% 5% 29% 39% 13%	624
	21st to end	Night	14% 6% 25% 37% 15% 2	836
September	1st to 10th	Night	17% 6% 33% 33% 12% 1	760
	11th to 20th	Night	18% 7% 27% 30% 16% 2	760
	21st to end	Night	22% 7% 28% 29% 11% 2	760
October	1st to 10th	Night	25% 11% 27% 24% 12% 1	760
	11th to 20th	Night	19% 11% 34% 24% 12% 1	760
	21st to end	Night	25% 15% 34% 19% 7%	836
November	1st to 10th	Night	17% 14% 35% 23% 10% 1	755
	11th to 20th	Night	24% 16% 36% 17% 6%	760
	21st to end	Night	22% 20% 33% 20% 4 1	760
December	1st to 10th	Night	25% 15% 38% 17% 4 1	760
	11th to 20th	Night	24% 19% 36% 19% 2	760
	21st to end	Night	25% 18% 37% 18% 2	836

Page generated at 2024-11-01 22:47:29. Version ac07

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