Gayle H. Riggsbee Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gayle H. Riggsbee Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	12% 6% 2 6% 5% 4 5% 4 6% 8% 42%	127440
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	124263
Transparency	Day	39% 6% 12% 19% 18% 6%	131073
	Night	38% 6% 11% 18% 21% 7%	128699
Seeing	Day	16% 9% 25% 25% 18% 7%	16962
	Night	16% 8% 25% 31% 18% 2	27077

By Month

Percentage Cloud Cover SE (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% 6% 2 4 3 3 3 2 3 5% 56%	4146
	11th to 20th	Day	12% 5% 2 4 3 3 3 3 3 4 59%	4114
	21st to end	Day	15% 8% 2 7% 4 4 3 3 4 5% 44%	4466
February	1st to 10th	Day	15% 7% 3 5% 5% 4 3 3 5% 7% 43%	1800
	11th to 20th	Day	21% 7% 2 5% 3 2 3 3 5% 5% 45%	1900
	21st to end	Day	19% 6% 2 7% 4 3 3 3 5% 5% 43%	1736
March	1st to 10th	Day	20% 7% 3 7% 4 4 3 3 4 5% 40%	4935
	11th to 20th	Day	8% 6% 2 6% 4 4 4 3 4 6% 54%	4980
	21st to end	Day	14% 7% 2 7% 4 4 3 3 4 5% 48%	5465
April	1st to 10th	Day	10% 4 2 5% 4 4 4 5% 6% 7% 50%	2214
	11th to 20th	Day	11% 6% 2 5% 3 4 4 4 5% 8% 48%	2374
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 5% 7% 49%	2490
May	1st to 10th	Day	9% 7% 2 8% 5% 6% 6% 5% 7% 10% 37%	5862
	11th to 20th	Day	18% 7% 3 7% 5% 5% 5% 5% 7% 9% 29%	5861
	21st to end	Day	12% 5% 2 6% 5% 4 5% 4 6% 8% 43%	6433
June	1st to 10th	Day	10% 6% 2 7% 6% 5% 6% 5% 7% 10% 36%	2600
	11th to 20th	Day	8% 5% 2 5% 6% 6% 6% 5% 7% 10% 39%	2600
	21st to end	Day	6% 5% 2 7% 6% 6% 7% 6% 8% 11% 37%	2600
July	1st to 10th	Day	10% 6% 3 9% 7% 7% 7% 6% 8% 10% 28%	5900
	11th to 20th	Day	6% 6% 2 7% 6% 7% 9% 6% 10% 13% 28%	5900
	21st to end	Day	8% 7% 3 8% 6% 7% 7% 6% 9% 11% 27%	6424
August	1st to 10th	Day	23% 8% 3 7% 5% 6% 6% 6% 7% 9% 20%	4234
	11th to 20th	Day	10% 6% 2 7% 6% 5% 6% 6% 8% 11% 32%	4174
	21st to end	Day	6% 3 1 5% 4 4 5% 5% 7% 12% 48%	4317
September	1st to 10th	Day	7% 5% 2 6% 6% 6% 5% 5% 7% 9% 42%	2265
	11th to 20th	Day	12% 6% 2 7% 5% 5% 6% 4 7% 8% 39%	2200
	21st to end	Day	19% 6% 2 6% 4 3 4 4 5% 7% 40%	2200
October	1st to 10th	Day	14% 5% 2 4 3 4 5% 3 4 6% 49%	2055
	11th to 20th	Day	12% 4 2 5% 3 3 3 3 3 5% 58%	2000
	21st to end	Day	13% 3 2 6% 3 3 4 2 3 5% 57%	2200
November	1st to 10th	Day	12% 5% 2 4 3 3 3 2 5% 5% 55%	2057
	11th to 20th	Day	14% 5% 2 4 2 3 3 3 5% 6% 55%	2000
	21st to end	Day	13% 5% 2 5% 4 3 4 3 5% 6% 51%	1890
December	1st to 10th	Day	12% 7% 2 6% 5% 4 3 3 4 5% 50%	3020
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 6% 45%	3463
	21st to end	Day	19% 6% 2 5% 3 3 3 3 4 6% 46%	4565
January	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 4 4 57%	6019
	11th to 20th	Night	11% 6% 2 5% 4 4 3 3 4 5% 53%	5979
	21st to end	Night	15% 9% 3 6% 4 3 3 2 4 5% 46%	6490
February	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 5% 5% 46%	2600
	11th to 20th	Night	19% 7% 3 6% 4 4 3 3 4 5% 43%	2383
	21st to end	Night	15% 6% 2 4 4 3 4 4 4 6% 48%	1892
March	1st to 10th	Night	19% 8% 3 6% 4 4 3 3 4 5% 41%	4918
	11th to 20th	Night	11% 4 1 4 3 3 4 4 5% 7% 54%	4980
	21st to end	Night	15% 4 2 5% 4 4 4 3 5% 6% 48%	5467
April	1st to 10th	Night	12% 6% 2 4 4 4 4 4 5% 7% 48%	2185
	11th to 20th	Night	10% 5% 2 5% 4 3 3 3 5% 7% 52%	1975
	21st to end	Night	11% 4 2 5% 5% 4 4 4 6% 7% 49%	1925
May	1st to 10th	Night	11% 6% 3 6% 4 4 4 4 6% 10% 42%	4033
	11th to 20th	Night	18% 6% 2 7% 4 4 5% 4 6% 9% 34%	4032
	21st to end	Night	13% 5% 2 6% 4 5% 5% 4 8% 9% 41%	4421
June	1st to 10th	Night	11% 7% 3 7% 6% 7% 6% 4 7% 9% 35%	1800
	11th to 20th	Night	9% 7% 3 7% 5% 6% 6% 5% 8% 10% 35%	1800
	21st to end	Night	7% 5% 2 7% 6% 6% 5% 6% 8% 12% 37%	1800
July	1st to 10th	Night	11% 8% 3 9% 6% 5% 6% 6% 9% 11% 25%	4058
	11th to 20th	Night	8% 6% 3 8% 7% 6% 7% 6% 10% 14% 25%	4057
	21st to end	Night	11% 9% 3 11% 7% 7% 7% 5% 8% 10% 22%	4427
August	1st to 10th	Night	24% 7% 3 7% 5% 5% 6% 4 7% 9% 23%	2915
	11th to 20th	Night	11% 7% 2 8% 5% 6% 6% 5% 9% 9% 33%	2875
	21st to end	Night	6% 5% 1 5% 4 4 5% 4 7% 11% 49%	3546
September	1st to 10th	Night	9% 5% 2 6% 5% 4 4 4 7% 10% 43%	2130
	11th to 20th	Night	13% 7% 2 7% 4 4 4 3 6% 7% 42%	2200
	21st to end	Night	17% 7% 2 5% 4 5% 5% 4 5% 6% 41%	2200
October	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 4 6% 51%	2345
	11th to 20th	Night	13% 4 2 5% 3 3 3 2 4 5% 55%	2400
	21st to end	Night	11% 5% 1 4 3 3 3 3 4 5% 57%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 2 4 5% 53%	2461
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 4 5% 54%	2615
	21st to end	Night	13% 5% 2 4 3 4 4 3 6% 7% 49%	2730
December	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 52%	4380
	11th to 20th	Night	16% 8% 2 6% 4 3 3 3 4 6% 45%	4952
	21st to end	Night	19% 6% 2 5% 3 3 4 3 5% 5% 45%	6633

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	33% 3 5% 19% 33% 7%	4147
	11th to 20th	Day	31% 3 4 14% 37% 11%	4114
	21st to end	Day	43% 3 4 18% 23% 10%	4466
February	1st to 10th	Day	41% 5% 8% 22% 18% 6%	1800
	11th to 20th	Day	43% 4 6% 18% 24% 5%	1910
	21st to end	Day	43% 5% 8% 16% 21% 8%	1736
March	1st to 10th	Day	46% 4 6% 13% 20% 11%	4935
	11th to 20th	Day	32% 4 7% 13% 26% 19%	4980
	21st to end	Day	40% 4 6% 19% 23% 7%	5465
April	1st to 10th	Day	32% 5% 10% 21% 26% 6%	2214
	11th to 20th	Day	34% 5% 8% 21% 22% 9%	2374
	21st to end	Day	33% 7% 11% 23% 22% 4	2490
May	1st to 10th	Day	41% 7% 18% 22% 11% 1	5897
	11th to 20th	Day	50% 6% 14% 17% 12% 1	5865
	21st to end	Day	38% 7% 13% 20% 19% 4	6447
June	1st to 10th	Day	39% 9% 16% 23% 11% 2	2600
	11th to 20th	Day	36% 10% 18% 21% 14% 2	2600
	21st to end	Day	35% 11% 16% 22% 14% 2	2600
July	1st to 10th	Day	48% 8% 16% 20% 7% 1	5835
	11th to 20th	Day	43% 10% 31% 14% 3	5831
	21st to end	Day	47% 10% 17% 19% 7%	6240
August	1st to 10th	Day	57% 8% 17% 13% 4	4221
	11th to 20th	Day	42% 10% 16% 19% 12% 1	4213
	21st to end	Day	28% 8% 15% 31% 15% 2	4317
September	1st to 10th	Day	36% 9% 14% 19% 19% 3	2265
	11th to 20th	Day	41% 8% 11% 17% 20% 3	2200
	21st to end	Day	43% 6% 10% 20% 15% 6%	2200
October	1st to 10th	Day	33% 5% 6% 19% 27% 10%	3341
	11th to 20th	Day	20% 3 5% 18% 27% 28%	3240
	21st to end	Day	27% 3 4 23% 26% 17%	3538
November	1st to 10th	Day	31% 4 8% 18% 30% 10%	2057
	11th to 20th	Day	30% 4 8% 20% 27% 9%	2000
	21st to end	Day	34% 4 9% 21% 23% 9%	1890
December	1st to 10th	Day	38% 3 5% 17% 26% 10%	3020
	11th to 20th	Day	41% 3 10% 19% 21% 7%	3460
	21st to end	Day	40% 3 8% 24% 22% 2	4565
January	1st to 10th	Night	31% 3 6% 16% 36% 7%	6019
	11th to 20th	Night	34% 4 6% 14% 34% 8%	5979
	21st to end	Night	42% 3 4 15% 28% 8%	6490
February	1st to 10th	Night	40% 4 8% 19% 21% 7%	2600
	11th to 20th	Night	45% 4 6% 16% 22% 7%	2384
	21st to end	Night	37% 5% 10% 17% 22% 9%	1892
March	1st to 10th	Night	45% 5% 6% 12% 26% 7%	4918
	11th to 20th	Night	30% 4 7% 13% 28% 19%	4980
	21st to end	Night	37% 4 8% 21% 21% 9%	5467
April	1st to 10th	Night	34% 6% 10% 20% 23% 8%	2185
	11th to 20th	Night	32% 5% 9% 22% 22% 10%	1975
	21st to end	Night	33% 7% 11% 25% 20% 5%	1925
May	1st to 10th	Night	38% 7% 16% 23% 15% 2	4060
	11th to 20th	Night	46% 7% 16% 19% 11%	4033
	21st to end	Night	36% 8% 14% 24% 15% 2	4429
June	1st to 10th	Night	43% 9% 14% 21% 11% 2	1800
	11th to 20th	Night	41% 10% 17% 19% 12% 1	1800
	21st to end	Night	36% 12% 18% 20% 12% 3	1800
July	1st to 10th	Night	49% 8% 21% 15% 5% 1	4015
	11th to 20th	Night	46% 13% 27% 10% 3 1	4006
	21st to end	Night	54% 10% 15% 16% 5%	4297
August	1st to 10th	Night	56% 7% 17% 15% 3 1	2904
	11th to 20th	Night	43% 9% 22% 18% 7% 1	2902
	21st to end	Night	29% 7% 13% 28% 19% 4	3546
September	1st to 10th	Night	35% 9% 15% 20% 15% 6%	2130
	11th to 20th	Night	41% 8% 13% 16% 20% 2	2200
	21st to end	Night	43% 7% 12% 18% 16% 5%	2200
October	1st to 10th	Night	34% 4 8% 21% 26% 6%	3839
	11th to 20th	Night	22% 3 4 17% 29% 25%	3899
	21st to end	Night	28% 3 5% 20% 26% 18%	4252
November	1st to 10th	Night	35% 4 7% 16% 26% 12%	2461
	11th to 20th	Night	33% 4 6% 17% 28% 12%	2615
	21st to end	Night	34% 4 7% 21% 26% 8%	2730
December	1st to 10th	Night	35% 4 7% 17% 28% 9%	4380
	11th to 20th	Night	42% 4 11% 16% 19% 9%	4954
	21st to end	Night	41% 3 9% 22% 22% 3	6633

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	20% 24% 42% 13% 2	330
	11th to 20th	Day	22% 22% 34% 17% 4	318
	21st to end	Day	21% 25% 35% 17% 2	330
February	1st to 10th	Day	18% 22% 39% 17% 5%	300
	11th to 20th	Day	38% 14% 39% 7% 2	300
	21st to end	Day	23% 20% 36% 18% 3	335
March	1st to 10th	Day	21% 18% 32% 22% 7%	500
	11th to 20th	Day	19% 18% 39% 18% 3 2	500
	21st to end	Day	21% 16% 35% 21% 7% 1	550
April	1st to 10th	Day	18% 17% 33% 22% 9% 1	500
	11th to 20th	Day	12% 16% 33% 23% 13% 3	492
	21st to end	Day	14% 10% 30% 28% 16% 4	500
May	1st to 10th	Day	10% 14% 36% 23% 14% 3	503
	11th to 20th	Day	17% 6% 24% 25% 20% 7%	527
	21st to end	Day	15% 6% 19% 24% 24% 11%	573
June	1st to 10th	Day	12% 3 20% 31% 26% 8%	530
	11th to 20th	Day	9% 4 17% 30% 27% 12%	530
	21st to end	Day	8% 2 15% 34% 32% 10%	530
July	1st to 10th	Day	9% 3 9% 22% 37% 20%	525
	11th to 20th	Day	9% 2 7% 27% 37% 18%	524
	21st to end	Day	9% 1 9% 31% 36% 13%	576
August	1st to 10th	Day	20% 2 14% 25% 23% 16%	526
	11th to 20th	Day	12% 2 11% 29% 28% 18%	519
	21st to end	Day	12% 2 13% 30% 27% 17%	583
September	1st to 10th	Day	9% 2 14% 34% 27% 14%	530
	11th to 20th	Day	14% 5% 14% 25% 32% 11%	530
	21st to end	Day	20% 5% 19% 28% 23% 5%	530
October	1st to 10th	Day	17% 5% 21% 29% 22% 8%	530
	11th to 20th	Day	13% 8% 25% 28% 20% 6%	530
	21st to end	Day	15% 9% 28% 25% 18% 5%	583
November	1st to 10th	Day	17% 8% 29% 29% 14% 4	535
	11th to 20th	Day	18% 14% 35% 23% 9% 2	440
	21st to end	Day	12% 15% 38% 23% 10% 2	330
December	1st to 10th	Day	25% 16% 37% 17% 4	330
	11th to 20th	Day	20% 13% 36% 22% 8% 2	330
	21st to end	Day	26% 12% 38% 17% 7% 1	363
January	1st to 10th	Night	19% 13% 35% 26% 6%	840
	11th to 20th	Night	18% 13% 35% 28% 6%	825
	21st to end	Night	19% 10% 39% 29% 3	880
February	1st to 10th	Night	19% 11% 36% 30% 5%	800
	11th to 20th	Night	23% 13% 39% 23% 2	783
	21st to end	Night	19% 14% 37% 27% 4	660
March	1st to 10th	Night	18% 11% 35% 31% 5%	800
	11th to 20th	Night	17% 13% 36% 26% 8% 1	800
	21st to end	Night	18% 11% 37% 28% 6%	880
April	1st to 10th	Night	16% 15% 33% 28% 8%	800
	11th to 20th	Night	13% 12% 32% 30% 12%	791
	21st to end	Night	12% 7% 28% 41% 12%	725
May	1st to 10th	Night	12% 8% 32% 36% 12%	600
	11th to 20th	Night	15% 5% 26% 37% 17% 1	591
	21st to end	Night	13% 3 16% 33% 32% 2	650
June	1st to 10th	Night	13% 3 18% 39% 25% 3	600
	11th to 20th	Night	12% 4 16% 33% 28% 7%	600
	21st to end	Night	10% 2 14% 37% 33% 5%	600
July	1st to 10th	Night	9% 1 11% 35% 36% 7%	585
	11th to 20th	Night	10% 1 8% 30% 44% 7%	582
	21st to end	Night	9% 1 11% 34% 40% 5%	657
August	1st to 10th	Night	16% 2 11% 26% 40% 6%	597
	11th to 20th	Night	14% 1 7% 33% 37% 7%	594
	21st to end	Night	12% 2 11% 30% 40% 5%	853
September	1st to 10th	Night	11% 2 12% 32% 37% 6%	800
	11th to 20th	Night	16% 2 15% 29% 35% 4	800
	21st to end	Night	21% 4 16% 31% 26% 3	800
October	1st to 10th	Night	15% 5% 19% 28% 28% 5%	800
	11th to 20th	Night	15% 6% 24% 32% 21% 2	800
	21st to end	Night	14% 7% 24% 31% 22% 2	880
November	1st to 10th	Night	17% 6% 24% 33% 16% 3	820
	11th to 20th	Night	15% 10% 30% 31% 12% 1	840
	21st to end	Night	15% 10% 30% 31% 13% 2	840
December	1st to 10th	Night	20% 10% 29% 33% 7% 1	840
	11th to 20th	Night	20% 8% 27% 36% 10%	840
	21st to end	Night	23% 13% 24% 30% 9%	924

Page generated at 2026-01-14 01:14:27. Version ac07

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