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 2025-09-04.

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%	125811
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	122924
Transparency	Day	44% 5% 11% 21% 16% 3	129344
	Night	44% 5% 11% 21% 16% 3	127465
Seeing	Day	23% 17% 33% 19% 7% 2	16152
	Night	22% 14% 34% 23% 6%	25028

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% 7% 3 8% 4 4 4 3 5% 6% 46%	3597
	11th to 20th	Day	14% 9% 3 5% 3 3 4 3 4 5% 47%	3570
	21st to end	Day	18% 8% 2 6% 4 4 4 3 4 5% 43%	3960
February	1st to 10th	Day	18% 8% 3 5% 4 5% 5% 4 6% 6% 37%	1902
	11th to 20th	Day	14% 8% 3 6% 4 5% 5% 4 4 7% 41%	1948
	21st to end	Day	18% 10% 2 6% 4 4 4 4 4 6% 38%	1650
March	1st to 10th	Day	21% 10% 3 6% 4 3 3 3 4 6% 38%	4476
	11th to 20th	Day	14% 8% 2 5% 4 4 5% 3 5% 7% 42%	4745
	21st to end	Day	22% 11% 2 6% 4 4 4 4 6% 5% 32%	5818
April	1st to 10th	Day	19% 9% 3 6% 4 3 3 3 5% 6% 37%	2400
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 6% 6% 41%	2374
	21st to end	Day	21% 7% 2 6% 5% 4 4 3 4 6% 38%	2590
May	1st to 10th	Day	29% 10% 3 7% 4 3 4 3 5% 7% 25%	5886
	11th to 20th	Day	21% 9% 3 8% 5% 4 4 3 4 6% 33%	5942
	21st to end	Day	20% 7% 2 6% 5% 4 4 4 6% 7% 35%	6949
June	1st to 10th	Day	18% 7% 2 6% 4 5% 5% 4 7% 8% 35%	2800
	11th to 20th	Day	17% 7% 2 6% 5% 4 5% 4 6% 9% 36%	2800
	21st to end	Day	16% 7% 2 6% 4 5% 5% 4 7% 9% 36%	2800
July	1st to 10th	Day	16% 6% 3 7% 5% 5% 5% 5% 7% 9% 32%	6360
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 5% 8% 10% 34%	6360
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 7% 40%	6926
August	1st to 10th	Day	17% 7% 3 8% 4 5% 6% 4 6% 8% 32%	4273
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 4 6% 8% 39%	4053
	21st to end	Day	8% 4 2 5% 5% 5% 6% 5% 6% 9% 44%	4312
September	1st to 10th	Day	15% 6% 2 5% 4 5% 4 4 5% 5% 46%	2316
	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	15% 6% 2 6% 4 3 3 3 3 5% 49%	1761
	11th to 20th	Day	17% 7% 1 4 3 3 3 3 4 7% 47%	1800
	21st to end	Day	17% 7% 2 5% 4 3 3 3 5% 6% 45%	1800
December	1st to 10th	Day	15% 8% 2 5% 4 3 3 4 4 6% 47%	2675
	11th to 20th	Day	19% 6% 2 5% 3 3 3 2 4 5% 49%	2986
	21st to end	Day	19% 11% 3 6% 4 3 3 3 4 5% 40%	3960
January	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 5% 6% 50%	6349
	11th to 20th	Night	14% 10% 3 7% 4 4 4 3 4 5% 42%	6315
	21st to end	Night	17% 9% 3 6% 4 5% 5% 3 5% 5% 38%	6996
February	1st to 10th	Night	17% 8% 3 7% 4 4 4 3 5% 6% 40%	2492
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 6% 5% 40%	2338
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 4 5% 41%	1980
March	1st to 10th	Night	22% 9% 2 4 4 3 4 3 4 6% 39%	5364
	11th to 20th	Night	16% 10% 3 6% 4 4 3 3 4 7% 40%	5213
	21st to end	Night	23% 10% 2 5% 3 4 4 4 5% 5% 34%	5110
April	1st to 10th	Night	19% 8% 3 5% 5% 4 5% 4 6% 5% 37%	2000
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 5% 6% 38%	1975
	21st to end	Night	20% 8% 3 7% 3 3 3 3 5% 6% 40%	1810
May	1st to 10th	Night	29% 9% 3 7% 4 4 4 3 4 6% 27%	4054
	11th to 20th	Night	19% 9% 3 8% 6% 6% 4 4 6% 7% 28%	3993
	21st to end	Night	20% 8% 3 7% 5% 4 4 4 6% 6% 32%	3926
June	1st to 10th	Night	15% 9% 2 7% 4 5% 4 4 6% 7% 37%	1600
	11th to 20th	Night	19% 7% 3 6% 4 3 4 4 6% 8% 37%	1600
	21st to end	Night	15% 6% 2 6% 3 4 4 4 6% 9% 41%	1600
July	1st to 10th	Night	15% 9% 3 6% 4 4 4 4 6% 9% 36%	3600
	11th to 20th	Night	14% 8% 3 8% 4 4 5% 4 6% 9% 36%	3600
	21st to end	Night	14% 7% 2 6% 4 4 5% 4 4 6% 42%	3926
August	1st to 10th	Night	20% 7% 3 7% 5% 5% 5% 4 7% 7% 30%	2842
	11th to 20th	Night	11% 7% 3 8% 5% 6% 6% 5% 6% 7% 36%	2990
	21st to end	Night	9% 5% 2 4 3 3 3 3 5% 8% 56%	3586
September	1st to 10th	Night	12% 6% 3 7% 5% 4 5% 4 5% 6% 44%	1931
	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	13% 5% 2 5% 4 4 4 3 4 6% 49%	2537
	11th to 20th	Night	17% 7% 3 5% 4 4 3 2 4 5% 46%	2600
	21st to end	Night	16% 7% 2 4 4 3 3 4 5% 7% 45%	2600
December	1st to 10th	Night	16% 9% 2 6% 4 3 3 2 3 4 48%	4503
	11th to 20th	Night	22% 8% 2 5% 3 3 3 3 4 5% 41%	5208
	21st to end	Night	16% 10% 3 7% 4 3 3 3 4 5% 42%	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	38% 4 7% 27% 21% 4	3598
	11th to 20th	Day	41% 4 10% 20% 17% 8%	3600
	21st to end	Day	45% 4 9% 21% 19% 2	3960
February	1st to 10th	Day	46% 5% 13% 19% 14% 3	1902
	11th to 20th	Day	44% 4 11% 22% 16% 4	1957
	21st to end	Day	47% 4 10% 20% 16% 3	1650
March	1st to 10th	Day	48% 3 11% 14% 13% 11%	4479
	11th to 20th	Day	40% 5% 13% 28% 12% 2	4745
	21st to end	Day	52% 4 6% 15% 16% 7%	5818
April	1st to 10th	Day	47% 4 11% 18% 17% 3	2400
	11th to 20th	Day	42% 5% 10% 20% 19% 3	2374
	21st to end	Day	48% 4 8% 19% 17% 4	2590
May	1st to 10th	Day	58% 6% 13% 13% 8% 1	5897
	11th to 20th	Day	52% 6% 13% 16% 11% 2	5946
	21st to end	Day	47% 7% 11% 20% 14% 1	6949
June	1st to 10th	Day	46% 7% 12% 22% 13% 1	2800
	11th to 20th	Day	44% 6% 13% 21% 14% 2	2800
	21st to end	Day	43% 8% 13% 21% 15% 1	2800
July	1st to 10th	Day	47% 7% 11% 20% 13% 2	6290
	11th to 20th	Day	42% 6% 17% 22% 10% 1	6285
	21st to end	Day	42% 5% 10% 24% 17% 1	6728
August	1st to 10th	Day	49% 7% 15% 20% 9% 1	4259
	11th to 20th	Day	42% 7% 13% 27% 11% 1	4092
	21st to end	Day	34% 7% 12% 27% 18% 2	4312
September	1st to 10th	Day	39% 7% 10% 22% 21% 2	2316
	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	39% 5% 12% 21% 20% 2	1761
	11th to 20th	Day	38% 4 11% 21% 22% 4	1800
	21st to end	Day	42% 4 11% 27% 13% 3	1800
December	1st to 10th	Day	39% 4 14% 27% 15% 1	2675
	11th to 20th	Day	39% 3 14% 18% 19% 6%	2986
	21st to end	Day	49% 3 8% 21% 15% 5%	3960
January	1st to 10th	Night	35% 3 8% 33% 18% 3	6349
	11th to 20th	Night	45% 3 11% 21% 14% 6%	6360
	21st to end	Night	48% 5% 9% 17% 17% 5%	6996
February	1st to 10th	Night	46% 5% 12% 22% 14% 2	2492
	11th to 20th	Night	45% 5% 9% 19% 18% 4	2340
	21st to end	Night	46% 5% 8% 22% 15% 5%	1980
March	1st to 10th	Night	47% 4 9% 17% 16% 7%	5374
	11th to 20th	Night	45% 5% 16% 19% 12% 3	5214
	21st to end	Night	52% 3 6% 12% 22% 6%	5110
April	1st to 10th	Night	48% 5% 10% 18% 15% 4	2000
	11th to 20th	Night	46% 6% 9% 18% 19% 3	1975
	21st to end	Night	46% 5% 10% 19% 16% 5%	1810
May	1st to 10th	Night	58% 6% 13% 12% 9% 1	4060
	11th to 20th	Night	53% 7% 14% 15% 8% 3	3994
	21st to end	Night	50% 7% 13% 20% 10% 1	3927
June	1st to 10th	Night	45% 8% 12% 21% 14% 1	1600
	11th to 20th	Night	45% 6% 12% 24% 13% 1	1600
	21st to end	Night	40% 8% 15% 23% 14% 1	1600
July	1st to 10th	Night	45% 6% 12% 23% 13% 1	3560
	11th to 20th	Night	45% 6% 16% 23% 9% 1	3552
	21st to end	Night	42% 5% 11% 26% 15% 1	3809
August	1st to 10th	Night	52% 8% 12% 17% 9% 1	2832
	11th to 20th	Night	44% 9% 13% 25% 9%	3017
	21st to end	Night	27% 6% 14% 30% 22% 1	3586
September	1st to 10th	Night	41% 6% 10% 23% 18% 2	1931
	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	36% 6% 12% 22% 20% 4	2537
	11th to 20th	Night	42% 3 10% 21% 21% 4	2600
	21st to end	Night	39% 5% 13% 28% 14% 2	2600
December	1st to 10th	Night	41% 3 12% 28% 15% 1	4503
	11th to 20th	Night	46% 3 14% 14% 20% 2	5208
	21st to end	Night	46% 4 8% 21% 18% 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	28% 26% 29% 13% 4	200
	11th to 20th	Day	29% 26% 34% 8% 4 1	200
	21st to end	Day	24% 29% 34% 10% 3	220
February	1st to 10th	Day	32% 24% 31% 12% 1	424
	11th to 20th	Day	30% 28% 32% 8% 1	496
	21st to end	Day	29% 22% 34% 11% 4	414
March	1st to 10th	Day	30% 21% 33% 13% 2	500
	11th to 20th	Day	28% 23% 33% 13% 3 1	500
	21st to end	Day	31% 22% 31% 10% 4 1	550
April	1st to 10th	Day	32% 22% 27% 13% 5% 1	500
	11th to 20th	Day	28% 22% 34% 12% 3 1	492
	21st to end	Day	29% 20% 31% 15% 3 2	500
May	1st to 10th	Day	32% 21% 29% 14% 3 1	503
	11th to 20th	Day	21% 17% 42% 15% 5% 1	527
	21st to end	Day	19% 14% 35% 23% 6% 2	573
June	1st to 10th	Day	21% 12% 36% 20% 9% 1	530
	11th to 20th	Day	18% 15% 36% 22% 8% 1	530
	21st to end	Day	17% 14% 35% 25% 7% 2	530
July	1st to 10th	Day	17% 8% 33% 26% 14% 2	525
	11th to 20th	Day	15% 9% 30% 30% 15% 2	524
	21st to end	Day	14% 9% 41% 27% 9% 1	576
August	1st to 10th	Day	12% 10% 34% 30% 13% 2	526
	11th to 20th	Day	14% 7% 34% 30% 13% 3	519
	21st to end	Day	10% 11% 35% 25% 14% 5%	583
September	1st to 10th	Day	19% 9% 27% 25% 15% 4	502
	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% 34% 18% 8% 3	395
	11th to 20th	Day	25% 24% 30% 16% 5%	400
	21st to end	Day	26% 20% 33% 16% 6% 1	400
December	1st to 10th	Day	21% 28% 34% 12% 5%	249
	11th to 20th	Day	34% 25% 32% 8% 3	200
	21st to end	Day	24% 24% 35% 11% 4 2	220
January	1st to 10th	Night	22% 21% 39% 16% 3	800
	11th to 20th	Night	27% 20% 36% 15% 2	800
	21st to end	Night	20% 22% 37% 18% 3	880
February	1st to 10th	Night	24% 18% 39% 19% 1	800
	11th to 20th	Night	25% 23% 38% 13% 2	783
	21st to end	Night	27% 20% 36% 15% 2	660
March	1st to 10th	Night	25% 19% 36% 16% 3	800
	11th to 20th	Night	21% 20% 39% 18% 3	800
	21st to end	Night	26% 19% 35% 17% 3	880
April	1st to 10th	Night	25% 20% 35% 17% 3	800
	11th to 20th	Night	24% 19% 36% 18% 3	791
	21st to end	Night	24% 17% 39% 17% 3	610
May	1st to 10th	Night	29% 13% 33% 21% 4	600
	11th to 20th	Night	21% 16% 39% 21% 3	578
	21st to end	Night	21% 9% 34% 29% 6%	433
June	1st to 10th	Night	19% 8% 34% 31% 8%	400
	11th to 20th	Night	21% 8% 33% 33% 6%	400
	21st to end	Night	18% 5% 34% 37% 7%	400
July	1st to 10th	Night	14% 5% 31% 36% 12%	390
	11th to 20th	Night	15% 5% 30% 36% 13% 1	388
	21st to end	Night	15% 4 32% 40% 9%	438
August	1st to 10th	Night	14% 4 33% 40% 8%	550
	11th to 20th	Night	12% 5% 30% 39% 13%	658
	21st to end	Night	14% 6% 27% 37% 14% 2	880
September	1st to 10th	Night	16% 5% 32% 33% 12% 1	773
	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% 15% 35% 23% 10% 1	780
	11th to 20th	Night	23% 17% 36% 17% 6%	800
	21st to end	Night	24% 20% 32% 20% 4 1	800
December	1st to 10th	Night	25% 16% 37% 17% 4 1	800
	11th to 20th	Night	25% 18% 36% 18% 2	800
	21st to end	Night	24% 19% 36% 19% 3	880

Page generated at 2025-09-01 01:51:08. Version ac07

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