Clay Center Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clay Center Observatory clear sky chart from 2005-11-06 to 2024-04-03.

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 5% 5% 7% 40%	78706
	Night	17% 7% 3 5% 5% 3 4 4 5% 6% 41%	81414
Transparency	Day	30% 36% 8% 15% 10% 2	78411
	Night	31% 35% 8% 15% 9% 1	81243
Seeing	Day	24% 17% 33% 18% 7% 2	14235
	Night	22% 15% 34% 23% 6% 1	23282

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	14% 7% 2 6% 5% 3 3 5% 4 6% 46%	2517
	11th to 20th	Day	17% 7% 3 7% 4 2 5% 5% 5% 5% 42%	2520
	21st to end	Day	21% 10% 3 6% 4 3 4 4 5% 6% 34%	2772
February	1st to 10th	Day	20% 7% 3 4 4 4 5% 5% 5% 7% 37%	1781
	11th to 20th	Day	13% 6% 3 6% 4 4 4 5% 5% 7% 42%	1848
	21st to end	Day	18% 9% 3 4 5% 3 4 5% 5% 7% 38%	1570
March	1st to 10th	Day	17% 8% 2 4 5% 3 5% 4 6% 8% 38%	1897
	11th to 20th	Day	16% 6% 3 5% 5% 3 4 5% 5% 7% 40%	2009
	21st to end	Day	19% 8% 2 5% 5% 3 3 4 5% 5% 40%	2408
April	1st to 10th	Day	18% 9% 3 6% 5% 3 5% 4 4 7% 37%	2184
	11th to 20th	Day	17% 7% 2 4 5% 3 4 5% 5% 8% 39%	2154
	21st to end	Day	21% 6% 2 5% 5% 3 4 4 6% 7% 36%	2340
May	1st to 10th	Day	24% 8% 3 6% 5% 3 4 4 5% 7% 31%	2340
	11th to 20th	Day	18% 7% 3 5% 5% 3 4 5% 4 6% 41%	2318
	21st to end	Day	15% 5% 2 5% 5% 3 4 4 5% 8% 44%	2725
June	1st to 10th	Day	20% 5% 3 5% 4 4 5% 5% 7% 9% 33%	2520
	11th to 20th	Day	17% 6% 3 5% 5% 3 5% 5% 6% 8% 38%	2520
	21st to end	Day	15% 6% 3 6% 6% 3 6% 5% 6% 9% 35%	2520
July	1st to 10th	Day	13% 5% 2 4 5% 4 6% 5% 5% 9% 40%	2520
	11th to 20th	Day	12% 6% 3 5% 5% 4 5% 6% 7% 10% 39%	2520
	21st to end	Day	11% 5% 3 5% 5% 3 6% 6% 6% 8% 40%	2744
August	1st to 10th	Day	10% 6% 2 6% 5% 4 6% 7% 7% 9% 38%	2341
	11th to 20th	Day	12% 6% 3 6% 5% 4 5% 6% 6% 8% 39%	2287
	21st to end	Day	12% 5% 2 5% 5% 3 4 5% 6% 9% 44%	2376
September	1st to 10th	Day	14% 5% 2 4 5% 3 5% 4 5% 8% 46%	2147
	11th to 20th	Day	15% 6% 2 4 3 2 3 3 4 6% 52%	1980
	21st to end	Day	18% 4 3 5% 4 2 4 3 5% 7% 46%	1980
October	1st to 10th	Day	25% 5% 2 4 4 3 3 3 4 6% 41%	1980
	11th to 20th	Day	15% 6% 2 5% 4 2 4 4 5% 6% 47%	1764
	21st to end	Day	20% 7% 3 4 4 3 4 4 5% 6% 39%	1782
November	1st to 10th	Day	17% 6% 3 3 4 3 3 4 3 5% 49%	1671
	11th to 20th	Day	18% 7% 3 4 4 2 4 3 5% 5% 46%	1710
	21st to end	Day	18% 7% 2 4 4 3 4 4 4 6% 44%	1710
December	1st to 10th	Day	18% 7% 3 5% 5% 3 5% 4 5% 6% 40%	1573
	11th to 20th	Day	21% 7% 3 4 3 2 4 3 3 6% 44%	1906
	21st to end	Day	21% 10% 3 6% 5% 3 5% 3 4 5% 35%	2772
January	1st to 10th	Night	17% 8% 3 4 3 2 3 3 4 5% 47%	4429
	11th to 20th	Night	15% 9% 3 8% 6% 3 3 3 4 5% 40%	4440
	21st to end	Night	21% 13% 4 7% 6% 3 4 4 4 5% 30%	4884
February	1st to 10th	Night	17% 9% 2 6% 5% 3 4 4 5% 7% 38%	2396
	11th to 20th	Night	15% 8% 3 6% 5% 3 5% 4 5% 6% 40%	2218
	21st to end	Night	18% 9% 3 4 4 2 4 4 4 6% 42%	1884
March	1st to 10th	Night	18% 7% 3 5% 5% 3 4 4 5% 6% 40%	2270
	11th to 20th	Night	15% 9% 3 6% 5% 3 4 5% 5% 6% 38%	2170
	21st to end	Night	18% 9% 3 5% 5% 3 3 4 4 5% 41%	2185
April	1st to 10th	Night	17% 8% 3 5% 6% 3 4 5% 5% 7% 37%	1821
	11th to 20th	Night	18% 7% 2 5% 4 3 5% 4 5% 6% 40%	1753
	21st to end	Night	22% 6% 3 5% 4 3 4 4 5% 6% 38%	1620
May	1st to 10th	Night	23% 9% 3 5% 5% 2 4 4 5% 6% 32%	1620
	11th to 20th	Night	18% 6% 3 5% 5% 3 6% 5% 5% 8% 37%	1590
	21st to end	Night	17% 8% 3 6% 5% 2 3 3 6% 9% 38%	1550
June	1st to 10th	Night	18% 8% 3 6% 4 3 6% 5% 5% 7% 37%	1440
	11th to 20th	Night	18% 7% 3 6% 4 3 5% 5% 5% 8% 36%	1440
	21st to end	Night	16% 4 3 4 4 2 5% 6% 5% 10% 41%	1440
July	1st to 10th	Night	12% 5% 3 6% 6% 3 4 6% 7% 9% 40%	1440
	11th to 20th	Night	11% 6% 3 6% 6% 4 5% 5% 6% 11% 36%	1440
	21st to end	Night	12% 5% 3 5% 5% 4 6% 5% 5% 9% 41%	1574
August	1st to 10th	Night	9% 5% 2 5% 6% 3 4 5% 7% 10% 44%	1580
	11th to 20th	Night	10% 6% 3 6% 6% 4 7% 6% 7% 8% 38%	1609
	21st to end	Night	13% 5% 2 4 5% 3 3 4 5% 8% 47%	1980
September	1st to 10th	Night	12% 5% 3 5% 5% 4 5% 4 6% 6% 45%	1812
	11th to 20th	Night	14% 5% 2 3 3 2 3 3 4 7% 53%	1980
	21st to end	Night	17% 5% 2 5% 5% 2 4 5% 6% 5% 45%	1980
October	1st to 10th	Night	21% 6% 2 5% 4 3 4 3 4 5% 42%	1980
	11th to 20th	Night	15% 5% 2 5% 4 3 5% 4 5% 7% 46%	2193
	21st to end	Night	21% 7% 3 5% 4 3 3 4 4 6% 39%	2574
November	1st to 10th	Night	14% 5% 2 5% 5% 3 3 4 5% 5% 48%	2407
	11th to 20th	Night	17% 7% 3 5% 4 2 4 4 4 5% 46%	2470
	21st to end	Night	17% 6% 2 4 4 2 4 4 5% 6% 45%	2470
December	1st to 10th	Night	19% 7% 4 5% 5% 2 4 4 5% 6% 39%	2603
	11th to 20th	Night	19% 7% 3 4 4 2 5% 4 5% 6% 41%	3288
	21st to end	Night	20% 10% 4 6% 4 3 3 4 4 6% 36%	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	32% 26% 7% 26% 9%	2518
	11th to 20th	Day	36% 26% 7% 20% 10% 1	2520
	21st to end	Day	43% 25% 8% 18% 6% 1	2772
February	1st to 10th	Day	32% 36% 7% 13% 11% 2	1781
	11th to 20th	Day	27% 36% 10% 16% 9% 2	1857
	21st to end	Day	34% 36% 7% 11% 9% 2	1570
March	1st to 10th	Day	30% 36% 9% 14% 9% 2	1900
	11th to 20th	Day	30% 37% 8% 14% 8% 2	2009
	21st to end	Day	30% 35% 5% 12% 13% 4	2408
April	1st to 10th	Day	30% 37% 7% 12% 12% 2	2184
	11th to 20th	Day	30% 37% 8% 13% 10% 2	2154
	21st to end	Day	29% 37% 7% 13% 10% 3	2340
May	1st to 10th	Day	35% 37% 5% 12% 10% 1	2340
	11th to 20th	Day	29% 38% 7% 12% 12% 3	2322
	21st to end	Day	23% 39% 9% 17% 10% 2	2725
June	1st to 10th	Day	33% 39% 9% 14% 6%	2520
	11th to 20th	Day	30% 39% 8% 12% 9% 1	2520
	21st to end	Day	31% 39% 10% 13% 6% 1	2520
July	1st to 10th	Day	30% 40% 8% 14% 7% 1	2450
	11th to 20th	Day	26% 41% 9% 16% 8% 1	2445
	21st to end	Day	26% 41% 8% 15% 8% 2	2591
August	1st to 10th	Day	27% 41% 9% 14% 8%	2327
	11th to 20th	Day	28% 39% 7% 16% 9% 1	2287
	21st to end	Day	26% 39% 6% 14% 12% 2	2376
September	1st to 10th	Day	25% 38% 7% 14% 14% 1	2147
	11th to 20th	Day	22% 37% 7% 19% 14% 2	1980
	21st to end	Day	28% 37% 5% 13% 15% 2	1980
October	1st to 10th	Day	29% 37% 7% 15% 12% 1	1980
	11th to 20th	Day	27% 37% 8% 19% 9% 1	1764
	21st to end	Day	28% 38% 10% 13% 10% 2	1782
November	1st to 10th	Day	24% 40% 7% 17% 11% 1	1671
	11th to 20th	Day	25% 39% 7% 16% 12% 1	1710
	21st to end	Day	27% 42% 9% 15% 7%	1710
December	1st to 10th	Day	29% 41% 8% 11% 9% 1	1573
	11th to 20th	Day	32% 32% 9% 11% 12% 4	1906
	21st to end	Day	41% 26% 8% 17% 7%	2772
January	1st to 10th	Night	31% 24% 10% 25% 8% 2	4429
	11th to 20th	Night	37% 25% 8% 19% 9% 2	4440
	21st to end	Night	48% 25% 7% 13% 7% 1	4884
February	1st to 10th	Night	29% 36% 9% 15% 9% 1	2396
	11th to 20th	Night	30% 36% 7% 15% 10% 2	2220
	21st to end	Night	31% 35% 6% 18% 9% 2	1884
March	1st to 10th	Night	31% 35% 6% 16% 10% 2	2280
	11th to 20th	Night	33% 36% 9% 12% 9% 1	2171
	21st to end	Night	32% 34% 5% 12% 13% 4	2185
April	1st to 10th	Night	29% 37% 8% 14% 11% 2	1821
	11th to 20th	Night	32% 36% 5% 13% 12% 2	1753
	21st to end	Night	31% 38% 6% 13% 10% 3	1620
May	1st to 10th	Night	35% 37% 6% 10% 9% 3	1620
	11th to 20th	Night	30% 39% 7% 11% 9% 4	1591
	21st to end	Night	27% 39% 10% 12% 11% 1	1551
June	1st to 10th	Night	33% 38% 10% 13% 6%	1440
	11th to 20th	Night	33% 38% 8% 13% 9%	1440
	21st to end	Night	29% 40% 11% 13% 7%	1440
July	1st to 10th	Night	29% 41% 9% 15% 7%	1400
	11th to 20th	Night	28% 40% 8% 18% 6% 1	1392
	21st to end	Night	27% 42% 6% 17% 7% 1	1486
August	1st to 10th	Night	27% 39% 9% 17% 7%	1570
	11th to 20th	Night	28% 39% 9% 14% 9%	1609
	21st to end	Night	26% 38% 8% 17% 9% 1	1980
September	1st to 10th	Night	27% 39% 7% 14% 13% 1	1812
	11th to 20th	Night	22% 38% 7% 15% 15% 2	1980
	21st to end	Night	26% 37% 6% 15% 13% 2	1980
October	1st to 10th	Night	29% 37% 7% 15% 11% 2	1980
	11th to 20th	Night	29% 38% 8% 18% 7%	2193
	21st to end	Night	30% 38% 7% 13% 10% 1	2574
November	1st to 10th	Night	22% 41% 9% 14% 12% 2	2407
	11th to 20th	Night	26% 40% 7% 15% 11% 1	2470
	21st to end	Night	26% 41% 8% 16% 8% 1	2470
December	1st to 10th	Night	29% 41% 8% 14% 7% 1	2603
	11th to 20th	Night	29% 33% 12% 11% 14% 1	3288
	21st to end	Night	41% 25% 7% 20% 7%	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	31% 23% 26% 16% 4	180
	11th to 20th	Day	30% 24% 34% 11% 1 1	180
	21st to end	Day	25% 26% 34% 12% 3 1	198
February	1st to 10th	Day	31% 21% 35% 12% 2	383
	11th to 20th	Day	28% 28% 34% 9% 1	456
	21st to end	Day	30% 21% 33% 13% 3	382
March	1st to 10th	Day	31% 21% 33% 12% 3	460
	11th to 20th	Day	30% 24% 31% 11% 4 1	460
	21st to end	Day	29% 23% 32% 8% 7% 1	506
April	1st to 10th	Day	33% 23% 24% 13% 6% 1	428
	11th to 20th	Day	26% 27% 30% 14% 4	412
	21st to end	Day	30% 19% 33% 15% 2 1	420
May	1st to 10th	Day	35% 24% 26% 11% 3 1	423
	11th to 20th	Day	23% 20% 38% 13% 5%	447
	21st to end	Day	18% 16% 36% 22% 6% 1	485
June	1st to 10th	Day	23% 12% 39% 19% 6% 1	450
	11th to 20th	Day	17% 15% 42% 19% 5% 2	450
	21st to end	Day	16% 12% 40% 22% 8% 1	450
July	1st to 10th	Day	18% 9% 33% 27% 11% 2	445
	11th to 20th	Day	15% 8% 32% 32% 10% 3	444
	21st to end	Day	11% 9% 39% 32% 8% 1	488
August	1st to 10th	Day	11% 11% 34% 33% 10% 2	446
	11th to 20th	Day	16% 10% 32% 27% 14% 2	439
	21st to end	Day	15% 9% 33% 27% 12% 4	495
September	1st to 10th	Day	20% 11% 28% 25% 12% 4	450
	11th to 20th	Day	21% 14% 29% 18% 12% 7%	450
	21st to end	Day	23% 12% 31% 22% 8% 4	450
October	1st to 10th	Day	29% 12% 26% 20% 8% 5%	450
	11th to 20th	Day	19% 16% 35% 19% 9% 2	396
	21st to end	Day	27% 16% 31% 17% 8% 1	396
November	1st to 10th	Day	21% 17% 34% 16% 10% 2	365
	11th to 20th	Day	27% 20% 33% 15% 5% 1	370
	21st to end	Day	28% 19% 31% 15% 6% 1	370
December	1st to 10th	Day	27% 27% 30% 12% 4	233
	11th to 20th	Day	33% 23% 34% 7% 2	180
	21st to end	Day	26% 24% 33% 12% 4 2	198
January	1st to 10th	Night	22% 19% 39% 17% 3	760
	11th to 20th	Night	28% 19% 36% 16% 2	760
	21st to end	Night	22% 19% 36% 19% 3	836
February	1st to 10th	Night	25% 18% 37% 19% 1	760
	11th to 20th	Night	24% 20% 40% 15% 1	743
	21st to end	Night	26% 21% 34% 17% 3	628
March	1st to 10th	Night	26% 18% 36% 17% 4	760
	11th to 20th	Night	22% 22% 37% 16% 3	760
	21st to end	Night	25% 19% 33% 18% 4	836
April	1st to 10th	Night	25% 20% 32% 20% 4	729
	11th to 20th	Night	23% 19% 34% 19% 4	689
	21st to end	Night	25% 17% 36% 20% 2	540
May	1st to 10th	Night	29% 17% 31% 19% 4	540
	11th to 20th	Night	21% 18% 39% 19% 3	529
	21st to end	Night	20% 8% 39% 28% 5% 1	389
June	1st to 10th	Night	20% 8% 37% 28% 7% 1	360
	11th to 20th	Night	19% 8% 40% 28% 6%	360
	21st to end	Night	16% 6% 41% 32% 5%	360
July	1st to 10th	Night	14% 4 37% 37% 8%	350
	11th to 20th	Night	14% 4 31% 41% 9% 1	348
	21st to end	Night	13% 4 35% 39% 9%	394
August	1st to 10th	Night	12% 5% 37% 37% 10%	506
	11th to 20th	Night	13% 4 32% 40% 10% 1	545
	21st to end	Night	15% 5% 24% 39% 14% 2	792
September	1st to 10th	Night	15% 5% 34% 34% 11%	720
	11th to 20th	Night	17% 8% 30% 26% 17% 2	720
	21st to end	Night	21% 8% 28% 30% 11% 2	720
October	1st to 10th	Night	25% 10% 27% 24% 12% 2	720
	11th to 20th	Night	18% 13% 35% 23% 11% 1	720
	21st to end	Night	27% 14% 33% 20% 6% 1	792
November	1st to 10th	Night	18% 14% 34% 24% 8% 1	740
	11th to 20th	Night	22% 20% 32% 18% 7%	760
	21st to end	Night	23% 20% 34% 18% 5% 1	760
December	1st to 10th	Night	24% 18% 35% 17% 5%	760
	11th to 20th	Night	25% 18% 33% 21% 2 1	760
	21st to end	Night	26% 18% 35% 19% 2	836

Page generated at 2024-03-31 06:08:55. Version ac07

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