Graystar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Graystar Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	16% 6% 2 5% 5% 3 4 5% 5% 8% 42%	74790
	Night	17% 6% 3 5% 5% 3 4 4 5% 7% 42%	74969
Transparency	Day	40% 6% 13% 23% 16% 2	74495
	Night	42% 6% 12% 22% 16% 2	74798
Seeing	Day	19% 12% 31% 25% 11% 2	13933
	Night	20% 11% 30% 28% 10% 1	23165

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	15% 5% 2 5% 6% 3 5% 4 4 6% 44%	1517
	11th to 20th	Day	15% 8% 3 5% 5% 3 5% 4 6% 6% 39%	1472
	21st to end	Day	15% 7% 3 5% 4 4 4 4 5% 8% 41%	1692
February	1st to 10th	Day	18% 8% 3 6% 4 3 4 4 4 7% 38%	1668
	11th to 20th	Day	16% 6% 2 6% 6% 2 4 4 4 7% 44%	1748
	21st to end	Day	20% 7% 3 5% 5% 2 5% 5% 5% 6% 37%	1480
March	1st to 10th	Day	14% 6% 3 6% 4 2 4 5% 4 7% 45%	1797
	11th to 20th	Day	15% 6% 2 5% 5% 3 4 6% 7% 8% 39%	1862
	21st to end	Day	19% 8% 3 6% 4 2 4 3 4 6% 40%	2376
April	1st to 10th	Day	18% 7% 3 6% 4 3 4 4 5% 7% 40%	2160
	11th to 20th	Day	18% 5% 2 6% 4 3 4 4 5% 8% 42%	2134
	21st to end	Day	19% 6% 2 5% 4 3 4 4 5% 7% 40%	2207
May	1st to 10th	Day	23% 7% 3 5% 5% 3 3 4 5% 8% 36%	2340
	11th to 20th	Day	20% 6% 3 4 4 3 4 5% 6% 8% 38%	2409
	21st to end	Day	16% 5% 2 4 4 3 4 5% 6% 9% 42%	2725
June	1st to 10th	Day	19% 5% 2 4 5% 2 5% 6% 6% 8% 38%	2520
	11th to 20th	Day	15% 6% 3 5% 5% 3 5% 5% 6% 8% 39%	2520
	21st to end	Day	11% 5% 2 5% 5% 3 4 5% 5% 9% 44%	2520
July	1st to 10th	Day	10% 5% 2 5% 6% 4 6% 5% 7% 8% 42%	2520
	11th to 20th	Day	9% 6% 2 4 5% 3 5% 6% 7% 9% 43%	2520
	21st to end	Day	12% 5% 3 6% 5% 4 5% 6% 7% 9% 40%	2744
August	1st to 10th	Day	11% 5% 2 4 5% 3 5% 6% 6% 11% 42%	2366
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 5% 7% 9% 41%	2126
	21st to end	Day	12% 6% 2 5% 5% 3 4 5% 6% 8% 43%	2376
September	1st to 10th	Day	14% 5% 2 5% 5% 2 5% 4 5% 8% 45%	2160
	11th to 20th	Day	12% 5% 2 4 4 2 3 4 4 8% 50%	2057
	21st to end	Day	19% 6% 3 5% 4 3 4 4 6% 6% 42%	1980
October	1st to 10th	Day	20% 5% 2 5% 4 3 4 3 4 7% 41%	1922
	11th to 20th	Day	17% 4 2 3 5% 3 4 4 5% 7% 46%	1800
	21st to end	Day	19% 7% 2 5% 4 3 4 4 5% 7% 41%	1811
November	1st to 10th	Day	15% 5% 2 4 5% 2 3 4 4 5% 52%	1671
	11th to 20th	Day	17% 6% 2 4 3 2 3 4 4 6% 49%	1710
	21st to end	Day	14% 6% 2 5% 3 3 4 4 5% 8% 46%	1682
December	1st to 10th	Day	19% 6% 3 5% 6% 3 4 5% 5% 8% 38%	1520
	11th to 20th	Day	21% 6% 3 3 4 2 3 4 4 6% 45%	1906
	21st to end	Day	18% 9% 3 6% 6% 4 5% 5% 6% 7% 33%	2772
January	1st to 10th	Night	16% 6% 3 5% 4 2 4 3 4 6% 46%	2649
	11th to 20th	Night	20% 7% 2 5% 4 2 5% 5% 5% 7% 39%	2577
	21st to end	Night	15% 7% 3 5% 4 2 4 4 4 6% 46%	2660
February	1st to 10th	Night	18% 9% 3 4 5% 2 4 4 4 6% 41%	2290
	11th to 20th	Night	19% 7% 3 5% 5% 2 4 4 4 6% 41%	2098
	21st to end	Night	20% 7% 2 4 5% 3 4 4 3 6% 43%	1776
March	1st to 10th	Night	17% 6% 2 4 4 2 4 4 4 7% 46%	2150
	11th to 20th	Night	17% 6% 3 5% 5% 3 4 5% 5% 7% 41%	2093
	21st to end	Night	18% 7% 3 5% 5% 3 3 4 4 6% 41%	1980
April	1st to 10th	Night	19% 7% 4 5% 6% 3 5% 4 4 6% 39%	1800
	11th to 20th	Night	19% 7% 2 4 4 2 5% 5% 5% 6% 40%	1775
	21st to end	Night	19% 7% 3 6% 4 3 4 3 4 6% 41%	1751
May	1st to 10th	Night	21% 9% 3 6% 4 3 4 5% 5% 6% 33%	1620
	11th to 20th	Night	20% 8% 3 6% 5% 3 5% 3 5% 6% 36%	1500
	21st to end	Night	18% 8% 2 6% 5% 3 4 4 6% 8% 38%	1550
June	1st to 10th	Night	18% 6% 3 4 5% 3 5% 5% 5% 7% 39%	1440
	11th to 20th	Night	14% 7% 3 5% 4 2 5% 6% 6% 10% 38%	1440
	21st to end	Night	11% 4 4 5% 5% 4 6% 5% 6% 10% 41%	1440
July	1st to 10th	Night	13% 5% 3 5% 7% 3 5% 5% 6% 10% 38%	1440
	11th to 20th	Night	11% 6% 4 5% 5% 4 5% 6% 6% 9% 40%	1440
	21st to end	Night	10% 6% 2 5% 6% 3 5% 5% 6% 11% 42%	1574
August	1st to 10th	Night	8% 5% 2 5% 6% 3 5% 5% 7% 10% 43%	1551
	11th to 20th	Night	12% 6% 3 5% 6% 4 5% 5% 7% 8% 38%	1774
	21st to end	Night	13% 5% 2 4 5% 3 4 5% 6% 9% 43%	1980
September	1st to 10th	Night	15% 5% 2 6% 4 4 5% 6% 5% 7% 43%	1800
	11th to 20th	Night	13% 5% 3 4 5% 3 3 3 4 6% 52%	1901
	21st to end	Night	18% 5% 2 6% 5% 3 5% 3 4 7% 43%	1980
October	1st to 10th	Night	20% 7% 3 5% 5% 2 4 3 4 5% 41%	2035
	11th to 20th	Night	16% 4 2 5% 4 3 3 4 5% 7% 48%	2160
	21st to end	Night	22% 6% 2 5% 4 2 4 3 4 6% 42%	2543
November	1st to 10th	Night	13% 5% 2 5% 5% 3 4 3 4 6% 51%	2407
	11th to 20th	Night	20% 5% 2 4 4 2 3 3 4 6% 48%	2470
	21st to end	Night	15% 4 2 5% 4 3 4 4 5% 7% 48%	2493
December	1st to 10th	Night	21% 6% 2 4 5% 3 5% 3 5% 7% 40%	2660
	11th to 20th	Night	20% 7% 2 4 4 3 4 4 5% 6% 44%	3288
	21st to end	Night	20% 8% 3 5% 4 2 4 4 5% 5% 38%	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	42% 5% 12% 27% 11% 3	1518
	11th to 20th	Day	42% 6% 15% 23% 12% 2	1472
	21st to end	Day	42% 3 14% 25% 13% 3	1692
February	1st to 10th	Day	45% 6% 16% 21% 12% 1	1668
	11th to 20th	Day	43% 5% 10% 24% 16% 2	1757
	21st to end	Day	46% 5% 11% 20% 14% 3	1480
March	1st to 10th	Day	38% 5% 15% 24% 15% 3	1800
	11th to 20th	Day	39% 5% 14% 25% 15% 2	1862
	21st to end	Day	45% 4 9% 19% 18% 4	2376
April	1st to 10th	Day	40% 5% 11% 21% 18% 4	2160
	11th to 20th	Day	39% 5% 11% 24% 17% 3	2134
	21st to end	Day	42% 5% 10% 21% 18% 4	2207
May	1st to 10th	Day	47% 6% 10% 20% 16% 2	2340
	11th to 20th	Day	43% 6% 12% 21% 16% 2	2413
	21st to end	Day	40% 8% 17% 22% 12% 2	2725
June	1st to 10th	Day	41% 7% 14% 23% 14% 1	2520
	11th to 20th	Day	40% 9% 13% 23% 13% 2	2520
	21st to end	Day	35% 9% 15% 26% 16% 1	2520
July	1st to 10th	Day	37% 9% 14% 23% 17% 1	2450
	11th to 20th	Day	32% 9% 15% 27% 14% 2	2445
	21st to end	Day	36% 9% 12% 25% 16% 2	2591
August	1st to 10th	Day	31% 9% 17% 28% 14% 1	2352
	11th to 20th	Day	34% 9% 16% 26% 15% 1	2126
	21st to end	Day	35% 8% 13% 22% 20% 2	2376
September	1st to 10th	Day	36% 7% 15% 21% 18% 2	2160
	11th to 20th	Day	31% 5% 14% 23% 24% 2	2057
	21st to end	Day	42% 6% 11% 19% 21% 2	1980
October	1st to 10th	Day	42% 6% 11% 22% 17% 2	1922
	11th to 20th	Day	36% 6% 12% 25% 20% 1	1800
	21st to end	Day	42% 6% 12% 21% 17% 3	1811
November	1st to 10th	Day	36% 4 14% 24% 20% 2	1671
	11th to 20th	Day	39% 4 13% 22% 20% 3	1710
	21st to end	Day	38% 5% 16% 29% 10% 2	1682
December	1st to 10th	Day	46% 6% 15% 19% 14% 1	1520
	11th to 20th	Day	42% 4 16% 22% 15% 2	1906
	21st to end	Day	49% 6% 12% 23% 9%	2772
January	1st to 10th	Night	41% 4 11% 26% 15% 2	2649
	11th to 20th	Night	45% 6% 15% 23% 10% 1	2577
	21st to end	Night	41% 4 11% 24% 18% 2	2660
February	1st to 10th	Night	45% 6% 14% 22% 11% 2	2290
	11th to 20th	Night	45% 5% 10% 20% 15% 4	2100
	21st to end	Night	45% 4 9% 23% 17% 3	1776
March	1st to 10th	Night	41% 4 13% 24% 15% 4	2160
	11th to 20th	Night	42% 5% 10% 22% 16% 5%	2094
	21st to end	Night	43% 5% 8% 19% 18% 6%	1980
April	1st to 10th	Night	45% 5% 10% 19% 17% 4	1800
	11th to 20th	Night	43% 6% 9% 19% 22% 1	1775
	21st to end	Night	45% 5% 9% 19% 17% 5%	1751
May	1st to 10th	Night	50% 7% 9% 18% 14% 2	1620
	11th to 20th	Night	50% 5% 10% 16% 17% 2	1501
	21st to end	Night	44% 9% 14% 19% 12% 2	1551
June	1st to 10th	Night	44% 5% 10% 21% 18% 1	1440
	11th to 20th	Night	39% 9% 14% 25% 12% 1	1440
	21st to end	Night	38% 9% 13% 25% 14% 2	1440
July	1st to 10th	Night	41% 9% 14% 21% 13% 2	1400
	11th to 20th	Night	38% 9% 11% 28% 13% 1	1392
	21st to end	Night	37% 8% 15% 24% 14% 3	1486
August	1st to 10th	Night	34% 9% 19% 23% 14% 1	1541
	11th to 20th	Night	39% 9% 14% 25% 11% 1	1774
	21st to end	Night	36% 9% 14% 22% 17% 2	1980
September	1st to 10th	Night	42% 6% 10% 23% 17% 2	1800
	11th to 20th	Night	34% 5% 11% 23% 23% 3	1901
	21st to end	Night	43% 5% 8% 21% 20% 3	1980
October	1st to 10th	Night	46% 6% 10% 21% 15% 3	2035
	11th to 20th	Night	37% 5% 13% 28% 16% 1	2160
	21st to end	Night	47% 4 11% 21% 16% 1	2543
November	1st to 10th	Night	38% 4 13% 22% 19% 4	2407
	11th to 20th	Night	41% 4 11% 22% 18% 4	2470
	21st to end	Night	37% 5% 14% 28% 14% 2	2493
December	1st to 10th	Night	46% 6% 14% 20% 13% 1	2660
	11th to 20th	Night	43% 4 14% 19% 17% 3	3288
	21st to end	Night	48% 5% 13% 22% 11% 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	24% 22% 32% 17% 4	180
	11th to 20th	Day	20% 27% 33% 18% 3	168
	21st to end	Day	21% 18% 40% 15% 4 1	224
February	1st to 10th	Day	27% 20% 35% 16% 1	288
	11th to 20th	Day	24% 20% 39% 16% 2	416
	21st to end	Day	28% 20% 33% 16% 3	346
March	1st to 10th	Day	20% 23% 35% 19% 3	420
	11th to 20th	Day	18% 19% 41% 18% 3 1	420
	21st to end	Day	26% 18% 33% 18% 5% 1	462
April	1st to 10th	Day	21% 21% 34% 17% 6% 1	420
	11th to 20th	Day	23% 13% 33% 22% 8%	412
	21st to end	Day	26% 17% 35% 17% 5%	420
May	1st to 10th	Day	29% 15% 32% 19% 4 1	423
	11th to 20th	Day	20% 12% 36% 25% 6% 1	447
	21st to end	Day	19% 6% 28% 28% 15% 4	485
June	1st to 10th	Day	20% 7% 30% 28% 13% 2	450
	11th to 20th	Day	15% 9% 32% 29% 12% 2	450
	21st to end	Day	13% 6% 33% 30% 14% 3	450
July	1st to 10th	Day	14% 4 22% 36% 19% 4	445
	11th to 20th	Day	9% 4 22% 41% 19% 5%	444
	21st to end	Day	13% 7% 27% 34% 17% 3	488
August	1st to 10th	Day	12% 5% 24% 35% 18% 5%	446
	11th to 20th	Day	13% 5% 22% 38% 18% 4	439
	21st to end	Day	13% 5% 26% 35% 14% 6%	495
September	1st to 10th	Day	20% 5% 26% 26% 19% 5%	450
	11th to 20th	Day	14% 6% 27% 27% 18% 8%	450
	21st to end	Day	20% 8% 28% 25% 14% 4	450
October	1st to 10th	Day	26% 7% 25% 22% 16% 4	450
	11th to 20th	Day	16% 14% 30% 25% 14% 1	450
	21st to end	Day	23% 12% 36% 19% 8% 2	410
November	1st to 10th	Day	15% 13% 29% 29% 11% 2	365
	11th to 20th	Day	21% 22% 28% 22% 6% 1	370
	21st to end	Day	18% 16% 33% 25% 7% 1	342
December	1st to 10th	Day	19% 21% 36% 17% 6% 1	180
	11th to 20th	Day	31% 19% 31% 16% 4	180
	21st to end	Day	25% 19% 34% 16% 6% 1	198
January	1st to 10th	Night	20% 19% 38% 20% 3	760
	11th to 20th	Night	26% 17% 30% 24% 3	737
	21st to end	Night	19% 17% 39% 22% 2	792
February	1st to 10th	Night	21% 18% 36% 22% 3	720
	11th to 20th	Night	24% 20% 37% 17% 2	703
	21st to end	Night	24% 19% 35% 18% 3	592
March	1st to 10th	Night	22% 16% 37% 19% 5%	720
	11th to 20th	Night	22% 13% 38% 22% 4 1	720
	21st to end	Night	21% 14% 36% 23% 6%	792
April	1st to 10th	Night	22% 16% 34% 22% 6%	720
	11th to 20th	Night	23% 10% 33% 29% 6%	711
	21st to end	Night	23% 11% 32% 28% 6% 1	671
May	1st to 10th	Night	24% 11% 30% 25% 9% 1	540
	11th to 20th	Night	23% 10% 33% 28% 6%	439
	21st to end	Night	21% 4 19% 35% 18% 3	389
June	1st to 10th	Night	20% 5% 25% 34% 16%	360
	11th to 20th	Night	15% 5% 22% 41% 17% 1	360
	21st to end	Night	11% 4 23% 44% 16% 1	360
July	1st to 10th	Night	15% 3 19% 45% 17% 1	350
	11th to 20th	Night	13% 4 18% 42% 21% 3	348
	21st to end	Night	12% 4 20% 42% 22% 1	394
August	1st to 10th	Night	10% 1 23% 43% 21% 1	477
	11th to 20th	Night	15% 3 17% 38% 25% 2	710
	21st to end	Night	15% 3 15% 39% 24% 3	792
September	1st to 10th	Night	16% 4 23% 37% 16% 4	720
	11th to 20th	Night	16% 4 21% 33% 20% 5%	720
	21st to end	Night	21% 3 23% 31% 19% 2	720
October	1st to 10th	Night	24% 8% 24% 25% 16% 3	720
	11th to 20th	Night	17% 12% 28% 30% 13% 1	720
	21st to end	Night	25% 10% 33% 24% 8%	792
November	1st to 10th	Night	15% 12% 31% 29% 11% 1	740
	11th to 20th	Night	22% 14% 32% 24% 8% 1	760
	21st to end	Night	17% 17% 32% 26% 7% 1	760
December	1st to 10th	Night	25% 16% 34% 19% 5% 1	760
	11th to 20th	Night	23% 17% 32% 23% 4	760
	21st to end	Night	24% 14% 38% 21% 3	836

Page generated at 2024-01-11 22:33:50. Version ac07

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