Grubbs Road Observatory
Clear Sky Chart
History

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

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	13% 7% 2 6% 4 4 4 4 6% 7% 43%	101018
	Night	13% 6% 2 5% 4 4 4 3 5% 7% 46%	98188
Transparency	Day	39% 4 10% 25% 18% 4	104691
	Night	37% 3 9% 26% 20% 5%	102656
Seeing	Day	15% 7% 22% 26% 21% 10%	15455
	Night	16% 6% 25% 32% 18% 2	25234

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	15% 11% 3 8% 4 3 3 3 5% 5% 40%	2527
	11th to 20th	Day	18% 9% 2 6% 4 4 3 3 4 4 44%	2698
	21st to end	Day	24% 11% 4 7% 4 3 3 3 4 5% 33%	3135
February	1st to 10th	Day	19% 10% 3 5% 4 3 4 3 5% 5% 38%	1866
	11th to 20th	Day	18% 10% 2 6% 4 4 3 3 4 5% 39%	1848
	21st to end	Day	20% 11% 2 6% 5% 4 4 3 4 6% 35%	1570
March	1st to 10th	Day	20% 12% 3 7% 3 3 3 3 4 5% 35%	3120
	11th to 20th	Day	16% 6% 2 6% 3 3 3 3 4 6% 49%	3347
	21st to end	Day	13% 7% 3 6% 4 5% 5% 3 5% 6% 42%	4133
April	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 5% 8% 45%	2280
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 2 5% 6% 41%	2254
	21st to end	Day	17% 8% 2 5% 5% 4 4 3 5% 8% 40%	2280
May	1st to 10th	Day	15% 6% 2 6% 5% 4 5% 5% 7% 10% 33%	3920
	11th to 20th	Day	16% 7% 2 6% 5% 5% 5% 4 6% 8% 36%	4081
	21st to end	Day	11% 6% 3 6% 5% 5% 5% 4 7% 8% 40%	4697
June	1st to 10th	Day	9% 5% 1 5% 4 5% 5% 5% 7% 10% 44%	2660
	11th to 20th	Day	5% 3 2 5% 4 5% 5% 5% 8% 10% 48%	2660
	21st to end	Day	5% 5% 2 5% 5% 5% 6% 5% 8% 10% 46%	2660
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 8% 11% 48%	4440
	21st to end	Day	9% 9% 4 9% 7% 6% 5% 4 6% 8% 33%	4630
August	1st to 10th	Day	4 3 1 4 3 3 4 4 6% 10% 57%	3791
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 6% 9% 45%	3729
	21st to end	Day	5% 3 1 4 3 4 4 3 6% 9% 57%	4180
September	1st to 10th	Day	10% 5% 2 6% 3 4 4 5% 6% 7% 47%	2280
	11th to 20th	Day	11% 5% 2 6% 5% 4 4 3 5% 8% 47%	2178
	21st to end	Day	13% 5% 2 4 4 3 4 3 4 7% 51%	2090
October	1st to 10th	Day	9% 4 1 5% 3 3 3 3 5% 5% 59%	2040
	11th to 20th	Day	13% 6% 2 5% 3 3 3 2 4 5% 54%	1900
	21st to end	Day	19% 7% 1 4 3 3 4 2 4 5% 48%	2024
November	1st to 10th	Day	12% 5% 2 6% 4 3 4 4 3 6% 53%	1762
	11th to 20th	Day	16% 7% 2 5% 3 3 3 2 4 5% 50%	1710
	21st to end	Day	18% 8% 3 5% 4 5% 4 3 4 5% 42%	1710
December	1st to 10th	Day	18% 8% 2 5% 5% 4 5% 3 5% 4 41%	1677
	11th to 20th	Day	17% 9% 2 6% 4 4 4 4 5% 6% 39%	1918
	21st to end	Day	19% 11% 3 8% 5% 4 5% 3 5% 6% 32%	2783
January	1st to 10th	Night	18% 8% 2 5% 3 4 4 3 5% 7% 41%	4428
	11th to 20th	Night	16% 8% 3 7% 4 5% 4 3 4 5% 41%	4269
	21st to end	Night	25% 9% 3 5% 3 3 4 2 4 5% 38%	4532
February	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 5% 6% 36%	2313
	11th to 20th	Night	18% 8% 3 7% 4 3 3 3 4 5% 42%	2218
	21st to end	Night	18% 8% 3 8% 4 4 5% 3 4 5% 38%	1884
March	1st to 10th	Night	22% 10% 3 6% 4 4 3 2 4 5% 38%	3733
	11th to 20th	Night	14% 7% 2 5% 4 4 3 3 4 6% 49%	3640
	21st to end	Night	13% 7% 2 7% 5% 5% 4 3 5% 6% 42%	3498
April	1st to 10th	Night	12% 5% 2 4 3 4 4 4 5% 7% 49%	1900
	11th to 20th	Night	15% 6% 3 5% 3 3 4 3 5% 7% 47%	1875
	21st to end	Night	16% 7% 2 6% 4 4 4 3 6% 7% 41%	1900
May	1st to 10th	Night	19% 7% 2 6% 4 2 3 3 5% 8% 41%	2972
	11th to 20th	Night	13% 6% 2 7% 4 4 4 4 5% 6% 45%	2812
	21st to end	Night	9% 5% 2 6% 4 5% 6% 4 7% 9% 42%	2849
June	1st to 10th	Night	9% 6% 2 6% 3 4 4 4 7% 7% 47%	1520
	11th to 20th	Night	4 3 1 4 3 3 4 5% 7% 8% 58%	1520
	21st to end	Night	3 3 1 3 4 5% 5% 4 8% 12% 54%	1520
July	1st to 10th	Night	8% 5% 2 5% 5% 4 5% 4 7% 10% 46%	2530
	11th to 20th	Night	5% 3 2 4 3 4 4 4 6% 10% 55%	2530
	21st to end	Night	7% 8% 3 9% 6% 5% 5% 5% 7% 8% 38%	2929
August	1st to 10th	Night	3 2 1 4 3 4 4 3 6% 8% 62%	3134
	11th to 20th	Night	5% 3 2 5% 4 4 5% 4 6% 10% 53%	3100
	21st to end	Night	4 4 1 3 3 3 3 3 5% 8% 63%	3476
September	1st to 10th	Night	9% 5% 2 5% 3 3 2 3 4 7% 57%	1900
	11th to 20th	Night	9% 5% 2 4 3 4 4 3 6% 7% 54%	1981
	21st to end	Night	10% 3 2 4 3 3 3 3 4 7% 56%	2090
October	1st to 10th	Night	8% 3 1 4 2 3 3 2 4 6% 62%	2137
	11th to 20th	Night	12% 5% 1 5% 4 3 3 3 4 5% 56%	2280
	21st to end	Night	18% 7% 2 4 3 3 3 3 5% 5% 48%	2574
November	1st to 10th	Night	13% 5% 2 5% 4 3 3 3 4 6% 54%	2536
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 52%	2477
	21st to end	Night	17% 7% 2 4 4 3 3 2 4 6% 48%	2470
December	1st to 10th	Night	19% 8% 2 5% 4 4 4 3 5% 4 41%	2498
	11th to 20th	Night	17% 7% 3 5% 5% 4 4 3 4 5% 44%	3279
	21st to end	Night	17% 10% 3 9% 5% 4 4 4 4 6% 33%	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	48% 2 6% 27% 14% 2	2528
	11th to 20th	Day	45% 3 7% 28% 15% 2	2698
	21st to end	Day	55% 2 9% 21% 12% 1	3135
February	1st to 10th	Day	48% 3 9% 25% 13% 2	1866
	11th to 20th	Day	48% 4 9% 21% 13% 5%	1858
	21st to end	Day	51% 3 8% 19% 13% 5%	1570
March	1st to 10th	Day	53% 2 6% 19% 13% 7%	3120
	11th to 20th	Day	39% 3 8% 26% 18% 5%	3347
	21st to end	Day	43% 3 12% 23% 19% 1	4133
April	1st to 10th	Day	38% 3 11% 27% 18% 3	2280
	11th to 20th	Day	44% 3 9% 23% 18% 4	2254
	21st to end	Day	44% 3 10% 27% 15% 1	2280
May	1st to 10th	Day	45% 5% 15% 21% 13% 1	3955
	11th to 20th	Day	45% 5% 12% 25% 13% 1	4085
	21st to end	Day	41% 5% 11% 23% 19% 1	4710
June	1st to 10th	Day	34% 5% 14% 26% 21% 1	2660
	11th to 20th	Day	28% 5% 12% 31% 21% 3	2660
	21st to end	Day	31% 6% 16% 24% 17% 5%	2660
July	1st to 10th	Day	36% 7% 14% 26% 14% 3	4371
	11th to 20th	Day	28% 6% 12% 38% 13% 3	4365
	21st to end	Day	49% 5% 12% 24% 8% 1	4484
August	1st to 10th	Day	22% 5% 15% 31% 25% 1	3778
	11th to 20th	Day	35% 5% 13% 27% 19% 2	3765
	21st to end	Day	24% 5% 11% 36% 21% 3	4179
September	1st to 10th	Day	34% 5% 10% 26% 20% 4	2280
	11th to 20th	Day	36% 5% 7% 24% 25% 3	2178
	21st to end	Day	34% 4 7% 25% 25% 5%	2090
October	1st to 10th	Day	18% 2 6% 17% 42% 15%	3391
	11th to 20th	Day	24% 2 6% 22% 29% 17%	3140
	21st to end	Day	30% 2 5% 27% 29% 6%	3314
November	1st to 10th	Day	33% 4 8% 26% 24% 5%	1762
	11th to 20th	Day	38% 2 6% 23% 25% 5%	1710
	21st to end	Day	45% 3 5% 26% 18% 4	1710
December	1st to 10th	Day	46% 3 10% 24% 14% 2	1677
	11th to 20th	Day	46% 3 8% 24% 17% 2	1915
	21st to end	Day	55% 2 11% 19% 11% 1	2783
January	1st to 10th	Night	44% 2 7% 32% 14% 1	4428
	11th to 20th	Night	47% 1 7% 26% 15% 2	4269
	21st to end	Night	49% 2 8% 25% 14% 1	4532
February	1st to 10th	Night	50% 3 8% 22% 15% 1	2313
	11th to 20th	Night	45% 3 7% 22% 18% 4	2219
	21st to end	Night	48% 3 9% 19% 17% 5%	1884
March	1st to 10th	Night	51% 3 7% 15% 17% 8%	3733
	11th to 20th	Night	39% 2 9% 24% 17% 9%	3640
	21st to end	Night	43% 2 9% 28% 16% 1	3498
April	1st to 10th	Night	35% 3 10% 27% 20% 4	1900
	11th to 20th	Night	38% 3 9% 22% 21% 7%	1875
	21st to end	Night	41% 4 8% 26% 18% 3	1900
May	1st to 10th	Night	43% 4 15% 25% 12% 2	3002
	11th to 20th	Night	40% 5% 14% 23% 17% 1	2813
	21st to end	Night	37% 4 12% 29% 16% 2	2857
June	1st to 10th	Night	34% 5% 13% 24% 23% 2	1520
	11th to 20th	Night	23% 4 13% 31% 23% 6%	1520
	21st to end	Night	22% 5% 18% 28% 20% 7%	1520
July	1st to 10th	Night	32% 6% 15% 29% 16% 3	2490
	11th to 20th	Night	24% 5% 14% 36% 17% 4	2482
	21st to end	Night	43% 6% 14% 24% 13%	2836
August	1st to 10th	Night	19% 5% 15% 36% 24% 1	3123
	11th to 20th	Night	27% 4 15% 31% 18% 4	3130
	21st to end	Night	20% 5% 10% 37% 25% 4	3476
September	1st to 10th	Night	29% 4 8% 30% 23% 6%	1900
	11th to 20th	Night	30% 6% 8% 27% 25% 4	1981
	21st to end	Night	30% 4 10% 25% 25% 6%	2090
October	1st to 10th	Night	16% 3 5% 16% 42% 18%	3566
	11th to 20th	Night	21% 2 4 18% 40% 15%	3779
	21st to end	Night	28% 2 6% 25% 29% 10%	4234
November	1st to 10th	Night	33% 3 6% 26% 26% 6%	2536
	11th to 20th	Night	34% 3 6% 24% 24% 9%	2477
	21st to end	Night	40% 2 7% 25% 23% 3	2470
December	1st to 10th	Night	47% 3 9% 23% 16% 2	2498
	11th to 20th	Night	43% 3 8% 23% 21% 3	3281
	21st to end	Night	52% 3 8% 21% 13% 3	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% 13% 37% 19% 4 1	270
	11th to 20th	Day	25% 17% 37% 17% 4	270
	21st to end	Day	21% 15% 42% 17% 5%	297
February	1st to 10th	Day	25% 14% 36% 19% 4 1	270
	11th to 20th	Day	28% 18% 39% 13% 2	270
	21st to end	Day	34% 14% 34% 16% 3	225
March	1st to 10th	Day	23% 12% 40% 18% 7%	270
	11th to 20th	Day	22% 17% 40% 19% 3	270
	21st to end	Day	23% 14% 36% 19% 7% 1	487
April	1st to 10th	Day	19% 11% 32% 27% 10%	460
	11th to 20th	Day	17% 17% 27% 25% 11% 2	452
	21st to end	Day	19% 12% 31% 24% 10% 4	460
May	1st to 10th	Day	17% 12% 26% 30% 11% 4	463
	11th to 20th	Day	17% 6% 21% 28% 18% 9%	487
	21st to end	Day	10% 4 22% 28% 23% 12%	659
June	1st to 10th	Day	10% 5% 16% 26% 27% 16%	680
	11th to 20th	Day	6% 1 14% 36% 32% 12%	680
	21st to end	Day	6% 3 18% 35% 24% 13%	680
July	1st to 10th	Day	7% 2 9% 27% 31% 24%	671
	11th to 20th	Day	7% 2 10% 27% 35% 19%	668
	21st to end	Day	8% 3 12% 26% 33% 18%	739
August	1st to 10th	Day	6% 2 8% 29% 36% 18%	492
	11th to 20th	Day	11% 3 10% 30% 30% 15%	479
	21st to end	Day	10% 2 7% 22% 37% 22%	539
September	1st to 10th	Day	9% 3 11% 26% 34% 18%	490
	11th to 20th	Day	11% 1 7% 26% 34% 20%	490
	21st to end	Day	17% 3 18% 24% 26% 12%	490
October	1st to 10th	Day	9% 2 18% 33% 31% 7%	440
	11th to 20th	Day	14% 8% 32% 25% 14% 7%	300
	21st to end	Day	25% 9% 29% 23% 12% 2	330
November	1st to 10th	Day	16% 9% 30% 27% 15% 3	300
	11th to 20th	Day	11% 11% 35% 28% 14% 1	270
	21st to end	Day	23% 10% 27% 27% 10% 2	270
December	1st to 10th	Day	21% 14% 31% 21% 11% 2	270
	11th to 20th	Day	24% 11% 33% 21% 11%	270
	21st to end	Day	24% 18% 34% 20% 4	297
January	1st to 10th	Night	23% 10% 35% 27% 5%	950
	11th to 20th	Night	24% 11% 38% 23% 4	950
	21st to end	Night	22% 9% 39% 28% 2	1045
February	1st to 10th	Night	25% 8% 36% 28% 4	793
	11th to 20th	Night	21% 13% 40% 23% 2	743
	21st to end	Night	22% 15% 39% 22% 2	628
March	1st to 10th	Night	22% 9% 34% 31% 5%	760
	11th to 20th	Night	21% 11% 34% 29% 6%	665
	21st to end	Night	19% 11% 31% 29% 9%	627
April	1st to 10th	Night	14% 9% 38% 31% 7%	570
	11th to 20th	Night	17% 9% 29% 32% 12% 1	563
	21st to end	Night	18% 4 34% 32% 11% 1	570
May	1st to 10th	Night	18% 7% 26% 34% 14% 1	570
	11th to 20th	Night	14% 4 20% 39% 20% 2	561
	21st to end	Night	8% 4 16% 40% 29% 3	617
June	1st to 10th	Night	12% 3 11% 39% 31% 4	570
	11th to 20th	Night	5% 1 10% 47% 34% 3	570
	21st to end	Night	4 12% 44% 35% 5%	570
July	1st to 10th	Night	6% 2 8% 28% 45% 10%	555
	11th to 20th	Night	8% 1 8% 29% 45% 9%	552
	21st to end	Night	6% 2 8% 33% 42% 9%	624
August	1st to 10th	Night	5% 2 5% 33% 47% 8%	567
	11th to 20th	Night	8% 1 8% 37% 41% 5%	564
	21st to end	Night	7% 2 4 30% 47% 8%	627
September	1st to 10th	Night	11% 1 9% 33% 40% 6%	570
	11th to 20th	Night	11% 1 9% 33% 39% 7%	651
	21st to end	Night	11% 3 16% 36% 30% 6%	760
October	1st to 10th	Night	9% 3 17% 37% 30% 4	760
	11th to 20th	Night	14% 4 27% 34% 18% 2	760
	21st to end	Night	20% 7% 27% 34% 11% 1	836
November	1st to 10th	Night	15% 5% 27% 38% 14% 1	780
	11th to 20th	Night	16% 7% 32% 34% 11%	763
	21st to end	Night	20% 8% 30% 31% 10% 1	760
December	1st to 10th	Night	23% 8% 29% 31% 8% 1	788
	11th to 20th	Night	22% 6% 31% 32% 8%	950
	21st to end	Night	26% 11% 31% 28% 4	1045

Page generated at 2024-11-08 05:44:47. Version ac07

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