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 2024-11-30.

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% 2 6% 4 4 4 4 5% 7% 40%	101496
	Night	18% 7% 3 6% 4 4 4 3 5% 6% 41%	98113
Transparency	Day	40% 6% 13% 23% 16% 2	105041
	Night	42% 5% 12% 23% 15% 2	102657
Seeing	Day	19% 13% 31% 24% 10% 2	15139
	Night	20% 11% 30% 28% 10% 1	24263

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	16% 7% 2 5% 4 3 3 3 4 5% 47%	2517
	11th to 20th	Day	16% 9% 2 5% 4 6% 5% 4 5% 6% 39%	2520
	21st to end	Day	22% 11% 4 8% 4 4 4 2 4 5% 32%	2964
February	1st to 10th	Day	17% 9% 2 6% 4 4 3 4 4 7% 39%	1760
	11th to 20th	Day	15% 6% 2 7% 5% 3 3 3 5% 6% 45%	1848
	21st to end	Day	20% 8% 3 6% 5% 3 4 4 5% 6% 37%	1570
March	1st to 10th	Day	27% 8% 2 6% 3 3 3 3 4 6% 36%	3116
	11th to 20th	Day	9% 5% 2 6% 4 4 4 5% 7% 9% 46%	3292
	21st to end	Day	20% 7% 3 6% 3 3 3 2 5% 5% 43%	4166
April	1st to 10th	Day	19% 7% 2 6% 4 3 4 3 5% 7% 38%	2280
	11th to 20th	Day	18% 6% 2 6% 4 4 4 3 5% 7% 41%	2254
	21st to end	Day	19% 7% 2 5% 4 4 4 4 6% 7% 40%	2330
May	1st to 10th	Day	27% 10% 3 5% 3 3 3 3 5% 8% 31%	4106
	11th to 20th	Day	25% 7% 2 6% 5% 4 3 3 5% 7% 32%	4287
	21st to end	Day	15% 5% 2 5% 4 4 4 4 6% 9% 44%	4837
June	1st to 10th	Day	18% 5% 2 5% 4 4 4 5% 6% 8% 38%	2660
	11th to 20th	Day	14% 7% 3 5% 5% 4 5% 4 6% 8% 40%	2660
	21st to end	Day	11% 6% 2 6% 5% 4 4 4 6% 9% 44%	2660
July	1st to 10th	Day	10% 7% 3 7% 5% 6% 6% 4 7% 8% 37%	4440
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 5% 7% 9% 40%	4440
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 8% 34%	4814
August	1st to 10th	Day	18% 6% 3 7% 4 4 4 4 6% 10% 34%	4158
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 7% 9% 41%	3730
	21st to end	Day	8% 5% 2 5% 4 4 6% 4 6% 9% 48%	4180
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 5% 7% 46%	2280
	11th to 20th	Day	11% 5% 2 5% 3 3 3 4 5% 8% 50%	2171
	21st to end	Day	20% 7% 3 6% 3 3 3 3 5% 6% 40%	2090
October	1st to 10th	Day	19% 5% 2 5% 4 4 4 3 4 7% 42%	2028
	11th to 20th	Day	16% 4 2 4 4 3 4 3 5% 7% 48%	1900
	21st to end	Day	18% 7% 2 5% 3 4 3 3 5% 7% 43%	1911
November	1st to 10th	Day	14% 5% 2 5% 4 3 3 3 4 5% 53%	1761
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 4 6% 49%	1800
	21st to end	Day	15% 7% 2 6% 3 4 3 3 4 8% 45%	1768
December	1st to 10th	Day	18% 6% 3 6% 5% 4 4 4 5% 8% 38%	1520
	11th to 20th	Day	21% 7% 2 4 3 2 2 3 4 6% 45%	1906
	21st to end	Day	18% 9% 3 7% 5% 5% 4 4 6% 7% 32%	2772
January	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 6% 47%	4429
	11th to 20th	Night	17% 9% 3 6% 4 3 3 3 4 5% 44%	4440
	21st to end	Night	24% 11% 3 6% 4 3 4 3 4 5% 34%	4688
February	1st to 10th	Night	17% 9% 3 5% 4 3 4 3 5% 6% 42%	2418
	11th to 20th	Night	18% 8% 3 6% 5% 3 4 3 4 5% 41%	2218
	21st to end	Night	20% 7% 2 5% 4 4 3 3 3 6% 42%	1884
March	1st to 10th	Night	23% 11% 3 6% 3 3 3 3 3 5% 37%	3724
	11th to 20th	Night	13% 5% 3 6% 5% 4 4 3 5% 6% 45%	3663
	21st to end	Night	21% 9% 3 5% 3 3 3 3 5% 4 41%	3466
April	1st to 10th	Night	19% 7% 3 6% 5% 4 5% 3 5% 6% 38%	1900
	11th to 20th	Night	19% 8% 3 5% 4 4 5% 4 6% 6% 39%	1875
	21st to end	Night	18% 8% 2 6% 4 3 4 3 4 6% 42%	1848
May	1st to 10th	Night	23% 11% 5% 8% 3 4 3 3 5% 7% 28%	2834
	11th to 20th	Night	24% 9% 3 7% 5% 4 4 3 4 5% 31%	2636
	21st to end	Night	17% 9% 2 6% 3 3 4 3 5% 8% 41%	2738
June	1st to 10th	Night	18% 7% 2 4 5% 4 4 3 5% 7% 40%	1520
	11th to 20th	Night	14% 7% 3 6% 3 4 4 5% 6% 10% 39%	1520
	21st to end	Night	11% 5% 3 6% 4 5% 6% 4 7% 9% 39%	1520
July	1st to 10th	Night	12% 10% 4 8% 6% 5% 5% 4 6% 8% 33%	2520
	11th to 20th	Night	14% 7% 3 7% 4 5% 5% 4 7% 9% 36%	2520
	21st to end	Night	18% 9% 3 6% 5% 4 4 3 5% 8% 35%	2739
August	1st to 10th	Night	23% 6% 3 6% 4 5% 4 4 6% 7% 31%	2737
	11th to 20th	Night	14% 6% 3 7% 5% 5% 4 4 6% 7% 40%	3097
	21st to end	Night	9% 5% 2 5% 4 4 4 3 5% 8% 52%	3476
September	1st to 10th	Night	14% 5% 1 6% 4 5% 5% 5% 5% 7% 44%	1900
	11th to 20th	Night	12% 5% 2 5% 4 3 3 2 4 6% 52%	2007
	21st to end	Night	17% 6% 2 7% 4 4 4 3 4 6% 41%	2090
October	1st to 10th	Night	19% 8% 3 6% 4 3 4 3 4 5% 42%	2149
	11th to 20th	Night	15% 4 2 5% 3 3 3 3 5% 6% 50%	2280
	21st to end	Night	21% 6% 2 5% 3 3 3 3 4 6% 44%	2685
November	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 4 5% 52%	2537
	11th to 20th	Night	19% 6% 2 4 3 3 2 3 4 6% 48%	2600
	21st to end	Night	15% 4 2 5% 4 5% 3 4 5% 6% 47%	2623
December	1st to 10th	Night	21% 7% 2 5% 4 4 4 3 5% 7% 39%	2660
	11th to 20th	Night	20% 7% 2 5% 3 3 4 3 5% 6% 43%	3288
	21st to end	Night	20% 9% 3 6% 4 4 4 3 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	40% 3 11% 30% 13% 2	2518
	11th to 20th	Day	44% 5% 15% 23% 12% 1	2520
	21st to end	Day	53% 5% 10% 21% 9% 2	2964
February	1st to 10th	Day	44% 6% 16% 21% 12% 1	1760
	11th to 20th	Day	42% 5% 10% 24% 17% 2	1857
	21st to end	Day	46% 5% 12% 20% 13% 3	1570
March	1st to 10th	Day	47% 5% 12% 24% 10% 2	3119
	11th to 20th	Day	29% 7% 14% 33% 15% 2	3292
	21st to end	Day	43% 4 9% 17% 22% 5%	4166
April	1st to 10th	Day	41% 5% 11% 21% 18% 4	2280
	11th to 20th	Day	39% 6% 12% 24% 17% 3	2254
	21st to end	Day	41% 5% 10% 21% 18% 4	2330
May	1st to 10th	Day	54% 7% 13% 14% 11% 1	4120
	11th to 20th	Day	50% 6% 15% 18% 9% 1	4291
	21st to end	Day	37% 7% 13% 24% 17% 2	4837
June	1st to 10th	Day	40% 7% 13% 23% 15% 1	2660
	11th to 20th	Day	39% 8% 13% 25% 13% 2	2660
	21st to end	Day	34% 8% 15% 25% 16% 1	2660
July	1st to 10th	Day	44% 10% 11% 18% 17% 1	4370
	11th to 20th	Day	37% 9% 18% 22% 13% 1	4365
	21st to end	Day	43% 9% 13% 19% 15% 1	4661
August	1st to 10th	Day	42% 9% 17% 24% 9%	4144
	11th to 20th	Day	34% 8% 15% 32% 10% 1	3766
	21st to end	Day	28% 7% 14% 29% 20% 1	4180
September	1st to 10th	Day	36% 7% 15% 21% 20% 2	2280
	11th to 20th	Day	31% 5% 14% 24% 24% 2	2171
	21st to end	Day	44% 6% 11% 18% 19% 2	2090
October	1st to 10th	Day	33% 6% 13% 22% 22% 4	3366
	11th to 20th	Day	24% 4 12% 24% 32% 4	3140
	21st to end	Day	28% 5% 16% 26% 22% 3	3123
November	1st to 10th	Day	35% 4 14% 24% 21% 2	1761
	11th to 20th	Day	38% 4 12% 22% 21% 3	1800
	21st to end	Day	39% 4 16% 29% 10% 2	1768
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	40% 4 11% 31% 11% 3	4429
	11th to 20th	Night	45% 4 13% 24% 13% 2	4440
	21st to end	Night	53% 4 9% 21% 12% 1	4688
February	1st to 10th	Night	44% 6% 14% 23% 12% 2	2418
	11th to 20th	Night	45% 5% 10% 20% 17% 4	2220
	21st to end	Night	44% 4 10% 22% 17% 4	1884
March	1st to 10th	Night	51% 3 10% 22% 11% 2	3734
	11th to 20th	Night	38% 5% 13% 26% 15% 3	3664
	21st to end	Night	46% 4 8% 15% 21% 6%	3466
April	1st to 10th	Night	46% 5% 10% 19% 17% 4	1900
	11th to 20th	Night	44% 6% 9% 19% 21% 1	1875
	21st to end	Night	44% 6% 9% 19% 18% 5%	1848
May	1st to 10th	Night	55% 8% 11% 16% 10% 1	2840
	11th to 20th	Night	55% 6% 13% 15% 10% 1	2637
	21st to end	Night	41% 8% 14% 20% 15% 2	2739
June	1st to 10th	Night	43% 5% 10% 21% 19% 1	1520
	11th to 20th	Night	38% 9% 14% 26% 12% 1	1520
	21st to end	Night	39% 9% 13% 25% 13% 2	1520
July	1st to 10th	Night	46% 9% 13% 19% 12% 1	2480
	11th to 20th	Night	40% 9% 15% 26% 10% 1	2472
	21st to end	Night	47% 8% 14% 16% 14% 2	2651
August	1st to 10th	Night	50% 8% 18% 15% 8% 1	2727
	11th to 20th	Night	39% 9% 14% 27% 10% 1	3127
	21st to end	Night	28% 8% 12% 33% 18% 1	3476
September	1st to 10th	Night	41% 6% 10% 23% 18% 1	1900
	11th to 20th	Night	34% 5% 11% 24% 23% 3	2007
	21st to end	Night	45% 5% 8% 20% 19% 2	2090
October	1st to 10th	Night	38% 6% 11% 23% 20% 2	3591
	11th to 20th	Night	24% 4 10% 26% 31% 4	3779
	21st to end	Night	31% 4 12% 28% 22% 2	4423
November	1st to 10th	Night	36% 4 13% 22% 20% 4	2537
	11th to 20th	Night	41% 4 10% 22% 19% 4	2600
	21st to end	Night	38% 5% 13% 28% 14% 2	2623
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% 28% 33% 17% 3	180
	21st to end	Day	22% 17% 40% 15% 5% 2	252
February	1st to 10th	Day	25% 20% 37% 16% 1	320
	11th to 20th	Day	23% 21% 39% 15% 2	456
	21st to end	Day	27% 20% 33% 16% 4	382
March	1st to 10th	Day	22% 21% 33% 20% 3	460
	11th to 20th	Day	17% 22% 39% 17% 3 1	460
	21st to end	Day	26% 19% 33% 17% 5% 1	506
April	1st to 10th	Day	22% 21% 34% 16% 6% 1	460
	11th to 20th	Day	24% 14% 34% 20% 8%	452
	21st to end	Day	25% 16% 35% 18% 5% 1	460
May	1st to 10th	Day	29% 16% 31% 18% 5% 1	463
	11th to 20th	Day	21% 11% 36% 26% 6% 1	487
	21st to end	Day	18% 7% 30% 28% 14% 3	529
June	1st to 10th	Day	19% 7% 32% 28% 12% 2	490
	11th to 20th	Day	14% 9% 33% 29% 12% 2	490
	21st to end	Day	13% 7% 35% 28% 13% 3	490
July	1st to 10th	Day	13% 5% 24% 35% 19% 4	485
	11th to 20th	Day	10% 4 25% 39% 18% 4	484
	21st to end	Day	13% 7% 29% 33% 16% 3	532
August	1st to 10th	Day	14% 6% 25% 33% 18% 5%	486
	11th to 20th	Day	13% 6% 23% 37% 18% 4	479
	21st to end	Day	12% 6% 27% 35% 14% 6%	539
September	1st to 10th	Day	19% 6% 27% 26% 19% 4	490
	11th to 20th	Day	14% 6% 26% 29% 17% 8%	490
	21st to end	Day	21% 10% 28% 24% 13% 4	490
October	1st to 10th	Day	24% 8% 26% 22% 15% 4	490
	11th to 20th	Day	15% 14% 31% 24% 13% 2	490
	21st to end	Day	22% 12% 35% 21% 9% 2	444
November	1st to 10th	Day	15% 15% 31% 27% 11% 2	395
	11th to 20th	Day	21% 21% 30% 22% 6% 1	400
	21st to end	Day	19% 17% 33% 24% 6% 1	370
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	25% 18% 31% 23% 3	760
	21st to end	Night	20% 17% 39% 22% 2	836
February	1st to 10th	Night	21% 18% 37% 22% 3	760
	11th to 20th	Night	24% 20% 38% 17% 2	743
	21st to end	Night	25% 18% 34% 19% 4	628
March	1st to 10th	Night	23% 16% 36% 20% 5%	760
	11th to 20th	Night	21% 14% 38% 21% 4 1	760
	21st to end	Night	22% 15% 35% 22% 5%	836
April	1st to 10th	Night	22% 16% 35% 22% 5%	760
	11th to 20th	Night	23% 10% 33% 28% 6%	751
	21st to end	Night	22% 11% 32% 28% 6% 1	708
May	1st to 10th	Night	24% 10% 31% 25% 9% 1	570
	11th to 20th	Night	23% 10% 32% 29% 6%	463
	21st to end	Night	21% 4 20% 35% 18% 3	411
June	1st to 10th	Night	20% 5% 25% 35% 15%	380
	11th to 20th	Night	14% 5% 23% 41% 16% 1	380
	21st to end	Night	11% 6% 23% 43% 16% 1	380
July	1st to 10th	Night	14% 3 19% 45% 17% 1	370
	11th to 20th	Night	13% 4 18% 42% 20% 2	368
	21st to end	Night	13% 5% 21% 41% 21%	416
August	1st to 10th	Night	12% 2 23% 42% 20% 1	505
	11th to 20th	Night	15% 3 18% 37% 24% 2	750
	21st to end	Night	15% 3 16% 39% 24% 3	836
September	1st to 10th	Night	16% 5% 24% 36% 16% 4	760
	11th to 20th	Night	16% 4 21% 33% 22% 5%	760
	21st to end	Night	21% 4 24% 31% 18% 2	760
October	1st to 10th	Night	24% 8% 24% 25% 16% 3	760
	11th to 20th	Night	16% 12% 29% 29% 13% 1	760
	21st to end	Night	24% 10% 33% 24% 9%	836
November	1st to 10th	Night	14% 12% 32% 29% 11% 1	780
	11th to 20th	Night	21% 14% 33% 24% 7% 1	800
	21st to end	Night	18% 17% 32% 26% 7% 1	800
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 2024-11-27 17:30:50. Version ac07

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