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 2024-03-28.

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% 7% 3 5% 5% 3 4 4 5% 7% 40%	78628
	Night	17% 7% 3 5% 5% 3 4 4 5% 6% 41%	81356
Transparency	Day	44% 6% 11% 21% 16% 3	78333
	Night	44% 5% 11% 22% 15% 3	81185
Seeing	Day	22% 17% 33% 19% 7% 2	14224
	Night	22% 14% 34% 23% 6%	23314

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 5% 5% 3 4 4 4 5% 45%	2517
	11th to 20th	Day	17% 8% 3 6% 5% 2 5% 5% 4 5% 40%	2520
	21st to end	Day	23% 11% 3 6% 4 3 3 4 4 5% 35%	2772
February	1st to 10th	Day	19% 7% 3 4 5% 3 6% 5% 5% 7% 37%	1806
	11th to 20th	Day	14% 7% 3 6% 4 3 6% 4 3 7% 41%	1848
	21st to end	Day	20% 9% 3 5% 5% 2 5% 4 3 6% 37%	1570
March	1st to 10th	Day	18% 7% 2 5% 4 2 4 4 5% 7% 40%	1897
	11th to 20th	Day	16% 6% 2 5% 5% 3 5% 5% 5% 8% 40%	1990
	21st to end	Day	18% 9% 3 6% 5% 3 4 3 4 5% 40%	2389
April	1st to 10th	Day	18% 8% 3 5% 5% 2 4 4 5% 7% 38%	2160
	11th to 20th	Day	17% 6% 2 5% 5% 3 4 4 5% 7% 42%	2134
	21st to end	Day	22% 7% 2 5% 5% 3 5% 4 4 6% 37%	2330
May	1st to 10th	Day	26% 8% 3 5% 5% 2 4 4 4 7% 32%	2340
	11th to 20th	Day	18% 7% 3 5% 5% 2 4 4 4 7% 41%	2326
	21st to end	Day	15% 6% 2 4 5% 3 4 4 5% 7% 45%	2725
June	1st to 10th	Day	19% 7% 3 5% 4 4 5% 5% 6% 8% 34%	2520
	11th to 20th	Day	17% 6% 3 5% 6% 3 5% 5% 6% 9% 36%	2520
	21st to end	Day	16% 6% 2 5% 5% 4 5% 5% 7% 9% 36%	2520
July	1st to 10th	Day	13% 5% 3 5% 6% 4 7% 5% 5% 7% 40%	2520
	11th to 20th	Day	14% 6% 3 4 5% 4 5% 5% 7% 9% 37%	2520
	21st to end	Day	12% 5% 3 5% 6% 3 5% 6% 7% 8% 39%	2744
August	1st to 10th	Day	11% 6% 3 6% 6% 4 6% 6% 7% 9% 37%	2350
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 5% 6% 8% 38%	2242
	21st to end	Day	13% 4 3 5% 5% 4 5% 6% 5% 8% 42%	2376
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 5% 5% 5% 45%	2160
	11th to 20th	Day	15% 4 2 4 4 3 4 4 5% 6% 50%	1984
	21st to end	Day	18% 7% 2 5% 4 3 3 4 4 7% 45%	1980
October	1st to 10th	Day	24% 6% 2 5% 4 2 3 3 3 5% 43%	1980
	11th to 20th	Day	15% 6% 2 4 4 2 4 4 4 7% 47%	1770
	21st to end	Day	19% 6% 2 6% 5% 3 4 4 4 5% 42%	1782
November	1st to 10th	Day	16% 6% 2 5% 5% 3 3 3 3 6% 48%	1671
	11th to 20th	Day	18% 7% 2 3 4 3 3 4 5% 7% 45%	1710
	21st to end	Day	17% 6% 3 5% 4 3 4 4 5% 6% 45%	1710
December	1st to 10th	Day	19% 7% 2 6% 5% 2 5% 5% 5% 6% 39%	1567
	11th to 20th	Day	22% 6% 2 4 4 2 3 3 3 5% 46%	1906
	21st to end	Day	20% 10% 3 7% 5% 2 4 3 4 5% 36%	2772
January	1st to 10th	Night	17% 7% 2 4 4 2 3 4 3 5% 49%	4429
	11th to 20th	Night	16% 10% 4 6% 5% 3 4 3 4 4 40%	4440
	21st to end	Night	23% 11% 4 6% 5% 3 4 3 4 5% 31%	4884
February	1st to 10th	Night	17% 8% 3 6% 5% 3 4 4 4 6% 41%	2368
	11th to 20th	Night	16% 7% 3 6% 5% 3 5% 4 6% 5% 41%	2218
	21st to end	Night	18% 8% 3 6% 5% 3 3 4 4 5% 42%	1884
March	1st to 10th	Night	17% 6% 2 4 5% 3 4 5% 5% 7% 41%	2270
	11th to 20th	Night	15% 8% 3 5% 6% 3 4 5% 5% 7% 39%	2188
	21st to end	Night	18% 9% 3 5% 4 3 4 4 3 6% 41%	2118
April	1st to 10th	Night	19% 7% 3 5% 5% 4 5% 5% 5% 5% 38%	1800
	11th to 20th	Night	18% 6% 3 5% 5% 3 4 5% 5% 6% 39%	1775
	21st to end	Night	21% 8% 3 6% 4 2 3 4 4 6% 39%	1630
May	1st to 10th	Night	23% 8% 4 6% 4 3 5% 4 5% 7% 32%	1620
	11th to 20th	Night	19% 5% 3 6% 5% 3 4 4 5% 8% 37%	1581
	21st to end	Night	17% 7% 3 6% 5% 3 4 5% 5% 7% 38%	1550
June	1st to 10th	Night	16% 8% 3 6% 5% 4 4 5% 5% 8% 37%	1440
	11th to 20th	Night	19% 6% 3 6% 5% 2 4 5% 5% 7% 37%	1440
	21st to end	Night	15% 6% 2 5% 4 3 4 5% 6% 9% 41%	1440
July	1st to 10th	Night	13% 6% 4 5% 6% 3 5% 6% 6% 9% 39%	1440
	11th to 20th	Night	14% 5% 3 6% 5% 4 5% 7% 6% 9% 36%	1440
	21st to end	Night	13% 5% 2 5% 5% 3 6% 5% 5% 8% 41%	1574
August	1st to 10th	Night	9% 6% 3 6% 6% 3 5% 5% 8% 7% 43%	1568
	11th to 20th	Night	10% 5% 2 7% 6% 4 6% 6% 6% 8% 39%	1654
	21st to end	Night	14% 5% 2 4 4 2 3 4 5% 8% 49%	1980
September	1st to 10th	Night	12% 6% 3 6% 6% 3 4 6% 5% 6% 42%	1800
	11th to 20th	Night	15% 5% 2 5% 3 2 3 4 4 6% 51%	1975
	21st to end	Night	17% 7% 3 5% 5% 2 4 5% 4 5% 44%	1980
October	1st to 10th	Night	21% 7% 3 5% 4 3 3 4 4 6% 41%	1980
	11th to 20th	Night	16% 5% 2 5% 4 3 3 4 5% 6% 46%	2186
	21st to end	Night	21% 7% 3 6% 5% 2 4 4 4 6% 39%	2574
November	1st to 10th	Night	14% 5% 2 5% 5% 3 4 4 4 6% 48%	2407
	11th to 20th	Night	17% 7% 3 5% 4 3 3 3 4 5% 45%	2470
	21st to end	Night	16% 6% 2 3 4 2 3 5% 5% 7% 45%	2470
December	1st to 10th	Night	19% 7% 3 6% 5% 3 4 4 4 5% 41%	2611
	11th to 20th	Night	19% 6% 2 4 4 3 4 4 5% 7% 42%	3288
	21st to end	Night	20% 11% 3 6% 4 2 3 4 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	43% 4 13% 21% 15% 4	1900
	11th to 20th	Day	41% 5% 11% 24% 15% 4	1990
	21st to end	Day	47% 4 8% 17% 18% 6%	2389
April	1st to 10th	Day	45% 4 11% 19% 17% 4	2160
	11th to 20th	Day	41% 5% 11% 21% 19% 3	2134
	21st to end	Day	49% 4 9% 18% 16% 4	2330
May	1st to 10th	Day	52% 5% 9% 18% 13% 3	2340
	11th to 20th	Day	44% 5% 10% 19% 18% 3	2330
	21st to end	Day	40% 7% 14% 23% 13% 2	2725
June	1st to 10th	Day	46% 7% 13% 22% 11% 1	2520
	11th to 20th	Day	45% 6% 12% 21% 14% 2	2520
	21st to end	Day	43% 8% 13% 22% 13% 1	2520
July	1st to 10th	Day	41% 9% 10% 23% 15% 3	2450
	11th to 20th	Day	43% 8% 13% 23% 11% 2	2445
	21st to end	Day	40% 9% 13% 20% 15% 4	2591
August	1st to 10th	Day	42% 8% 15% 21% 13% 1	2336
	11th to 20th	Day	42% 8% 13% 21% 14% 1	2242
	21st to end	Day	38% 8% 12% 21% 19% 2	2376
September	1st to 10th	Day	40% 7% 10% 21% 19% 2	2160
	11th to 20th	Day	34% 6% 9% 22% 25% 3	1984
	21st to end	Day	41% 4 9% 20% 24% 2	1980
October	1st to 10th	Day	46% 6% 8% 21% 17% 4	1980
	11th to 20th	Day	37% 5% 10% 27% 18% 2	1770
	21st to end	Day	44% 4 13% 19% 15% 4	1782
November	1st to 10th	Day	40% 5% 12% 21% 19% 3	1671
	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	43% 5% 10% 23% 16% 4	2280
	11th to 20th	Night	45% 5% 10% 18% 18% 4	2189
	21st to end	Night	46% 4 8% 17% 19% 6%	2118
April	1st to 10th	Night	47% 5% 10% 18% 16% 3	1800
	11th to 20th	Night	44% 6% 9% 19% 19% 3	1775
	21st to end	Night	47% 5% 10% 19% 14% 5%	1630
May	1st to 10th	Night	54% 5% 10% 15% 13% 3	1620
	11th to 20th	Night	46% 6% 12% 19% 13% 4	1582
	21st to end	Night	46% 9% 13% 19% 12% 1	1551
June	1st to 10th	Night	46% 8% 12% 20% 13% 1	1440
	11th to 20th	Night	45% 6% 12% 23% 13% 1	1440
	21st to end	Night	39% 8% 15% 24% 13% 1	1440
July	1st to 10th	Night	40% 10% 13% 22% 14% 1	1400
	11th to 20th	Night	42% 9% 14% 26% 9% 1	1392
	21st to end	Night	40% 7% 14% 25% 12% 2	1486
August	1st to 10th	Night	38% 9% 15% 24% 12% 2	1558
	11th to 20th	Night	40% 11% 14% 23% 12% 1	1654
	21st to end	Night	34% 8% 11% 26% 20% 1	1980
September	1st to 10th	Night	43% 6% 10% 22% 17% 2	1800
	11th to 20th	Night	35% 5% 9% 22% 26% 2	1975
	21st to end	Night	43% 5% 8% 19% 21% 3	1980
October	1st to 10th	Night	45% 6% 10% 21% 14% 4	1980
	11th to 20th	Night	38% 6% 12% 28% 16% 1	2186
	21st to end	Night	47% 5% 11% 20% 15% 2	2574
November	1st to 10th	Night	37% 6% 12% 21% 19% 4	2407
	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	29% 20% 33% 11% 5% 1	490
April	1st to 10th	Day	30% 22% 28% 14% 6% 1	420
	11th to 20th	Day	25% 22% 36% 14% 3 1	412
	21st to end	Day	31% 20% 29% 15% 2 2	420
May	1st to 10th	Day	32% 21% 29% 13% 3 1	423
	11th to 20th	Day	20% 16% 43% 16% 5%	447
	21st to end	Day	19% 14% 33% 25% 8% 2	485
June	1st to 10th	Day	21% 12% 36% 20% 10% 2	450
	11th to 20th	Day	17% 15% 38% 21% 7% 1	450
	21st to end	Day	16% 14% 35% 26% 7% 3	450
July	1st to 10th	Day	16% 8% 31% 28% 15% 2	445
	11th to 20th	Day	15% 9% 28% 30% 15% 2	444
	21st to end	Day	13% 8% 40% 28% 10% 1	488
August	1st to 10th	Day	11% 9% 34% 32% 13% 2	446
	11th to 20th	Day	14% 7% 33% 29% 13% 3	439
	21st to end	Day	11% 10% 34% 25% 14% 6%	495
September	1st to 10th	Day	20% 8% 27% 25% 15% 4	450
	11th to 20th	Day	16% 12% 30% 24% 11% 8%	450
	21st to end	Day	23% 11% 34% 20% 9% 3	450
October	1st to 10th	Day	28% 13% 25% 21% 9% 4	450
	11th to 20th	Day	18% 17% 32% 21% 10% 2	410
	21st to end	Day	23% 17% 32% 19% 7% 2	396
November	1st to 10th	Day	20% 16% 33% 19% 8% 3	365
	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	25% 19% 35% 17% 3	821
April	1st to 10th	Night	24% 21% 34% 18% 3	720
	11th to 20th	Night	23% 17% 37% 19% 4	711
	21st to end	Night	25% 17% 37% 17% 3	550
May	1st to 10th	Night	28% 13% 33% 21% 4	540
	11th to 20th	Night	22% 16% 38% 20% 3	520
	21st to end	Night	21% 8% 34% 29% 7%	389
June	1st to 10th	Night	20% 7% 32% 33% 9%	360
	11th to 20th	Night	21% 8% 33% 33% 6%	360
	21st to end	Night	18% 3 34% 38% 7%	360
July	1st to 10th	Night	14% 5% 32% 36% 13%	350
	11th to 20th	Night	15% 5% 30% 37% 13% 1	348
	21st to end	Night	14% 4 32% 40% 10%	394
August	1st to 10th	Night	13% 4 34% 42% 8%	494
	11th to 20th	Night	13% 5% 29% 40% 13%	590
	21st to end	Night	15% 6% 25% 38% 15% 2	792
September	1st to 10th	Night	17% 5% 32% 34% 11% 1	720
	11th to 20th	Night	18% 7% 28% 30% 15% 2	720
	21st to end	Night	21% 7% 29% 30% 12% 2	720
October	1st to 10th	Night	26% 11% 27% 23% 12% 1	720
	11th to 20th	Night	19% 11% 34% 24% 11% 1	720
	21st to end	Night	26% 15% 34% 19% 6%	792
November	1st to 10th	Night	17% 14% 35% 23% 10% 1	740
	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 2024-03-24 23:37:01. Version ac07

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