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 2024-01-13.

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% 6% 2 5% 5% 3 5% 5% 5% 8% 43%	74790
	Night	14% 6% 2 5% 4 3 4 4 5% 7% 46%	74966
Transparency	Day	40% 4 11% 25% 17% 3	74498
	Night	38% 4 10% 25% 19% 4	74779
Seeing	Day	15% 7% 22% 25% 21% 10%	14280
	Night	16% 6% 25% 32% 18% 3	24187

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	20% 7% 3 7% 5% 2 4 3 5% 5% 40%	1517
	11th to 20th	Day	21% 9% 3 6% 5% 3 4 4 4 5% 37%	1559
	21st to end	Day	14% 8% 4 7% 5% 3 3 3 3 5% 44%	1782
February	1st to 10th	Day	19% 9% 4 5% 5% 3 4 4 5% 6% 38%	1768
	11th to 20th	Day	19% 10% 3 5% 5% 3 4 4 4 6% 37%	1748
	21st to end	Day	21% 10% 3 6% 5% 3 4 4 4 5% 35%	1480
March	1st to 10th	Day	19% 8% 2 5% 4 2 3 3 4 6% 43%	1800
	11th to 20th	Day	21% 8% 3 7% 5% 3 4 5% 4 5% 35%	1903
	21st to end	Day	20% 8% 3 6% 5% 4 5% 4 5% 6% 34%	2358
April	1st to 10th	Day	13% 6% 2 5% 5% 3 4 5% 6% 8% 46%	2160
	11th to 20th	Day	15% 8% 4 6% 6% 2 5% 3 5% 6% 41%	2134
	21st to end	Day	17% 7% 3 5% 5% 3 4 4 5% 8% 39%	2160
May	1st to 10th	Day	16% 6% 2 5% 6% 3 5% 5% 5% 9% 38%	2245
	11th to 20th	Day	14% 6% 2 6% 6% 3 6% 4 5% 7% 40%	2301
	21st to end	Day	8% 6% 3 6% 7% 4 6% 6% 7% 9% 37%	2654
June	1st to 10th	Day	9% 4 2 5% 4 3 5% 6% 7% 10% 45%	2520
	11th to 20th	Day	5% 3 2 4 5% 3 6% 7% 8% 10% 48%	2520
	21st to end	Day	5% 4 2 4 5% 4 6% 6% 7% 10% 47%	2520
July	1st to 10th	Day	7% 4 2 6% 6% 5% 6% 7% 8% 11% 38%	2520
	11th to 20th	Day	6% 4 2 5% 6% 3 5% 5% 8% 10% 44%	2520
	21st to end	Day	6% 5% 2 5% 6% 4 6% 6% 6% 11% 44%	2641
August	1st to 10th	Day	6% 5% 2 5% 5% 4 5% 6% 8% 12% 42%	2154
	11th to 20th	Day	9% 6% 2 6% 5% 3 6% 5% 6% 9% 43%	2125
	21st to end	Day	7% 4 2 5% 5% 4 6% 5% 6% 11% 45%	2376
September	1st to 10th	Day	10% 5% 2 5% 4 3 5% 6% 6% 8% 47%	2160
	11th to 20th	Day	10% 5% 2 5% 5% 3 5% 4 5% 8% 48%	2064
	21st to end	Day	12% 4 2 3 4 2 4 3 4 7% 54%	1980
October	1st to 10th	Day	9% 4 1 4 4 2 3 4 5% 5% 58%	1933
	11th to 20th	Day	14% 6% 2 5% 4 2 3 3 4 5% 53%	1800
	21st to end	Day	20% 7% 1 3 4 2 4 3 4 5% 47%	1918
November	1st to 10th	Day	11% 4 2 5% 4 2 4 4 4 6% 55%	1672
	11th to 20th	Day	16% 6% 3 5% 4 2 3 3 4 5% 50%	1710
	21st to end	Day	18% 7% 3 5% 4 3 4 4 4 5% 42%	1710
December	1st to 10th	Day	18% 8% 2 4 5% 3 5% 4 5% 5% 41%	1677
	11th to 20th	Day	17% 8% 3 5% 5% 3 4 4 5% 6% 40%	1918
	21st to end	Day	19% 11% 3 6% 6% 3 5% 4 4 5% 32%	2783
January	1st to 10th	Night	18% 9% 3 5% 4 3 3 3 4 6% 42%	2648
	11th to 20th	Night	20% 8% 4 7% 4 3 4 3 4 5% 37%	2460
	21st to end	Night	17% 7% 3 5% 4 2 5% 4 5% 5% 44%	2574
February	1st to 10th	Night	20% 9% 3 6% 5% 3 4 3 4 6% 36%	2191
	11th to 20th	Night	19% 7% 3 7% 5% 3 3 4 4 5% 42%	2098
	21st to end	Night	19% 8% 3 7% 5% 2 5% 4 4 6% 37%	1776
March	1st to 10th	Night	18% 7% 3 4 5% 3 3 4 4 5% 44%	2160
	11th to 20th	Night	19% 8% 3 5% 4 3 4 3 4 5% 41%	2070
	21st to end	Night	17% 8% 4 7% 5% 4 5% 4 5% 5% 36%	1998
April	1st to 10th	Night	12% 5% 2 4 3 3 4 5% 5% 8% 50%	1800
	11th to 20th	Night	14% 6% 3 4 4 2 4 3 5% 7% 47%	1775
	21st to end	Night	16% 6% 2 5% 5% 3 5% 4 6% 8% 42%	1800
May	1st to 10th	Night	16% 6% 2 5% 5% 2 4 4 5% 7% 44%	1712
	11th to 20th	Night	13% 6% 2 6% 6% 3 4 6% 4 7% 43%	1592
	21st to end	Night	7% 4 2 5% 5% 4 6% 6% 7% 10% 43%	1618
June	1st to 10th	Night	9% 6% 2 5% 4 3 4 5% 7% 7% 48%	1440
	11th to 20th	Night	4 2 1 4 4 3 4 5% 6% 9% 58%	1440
	21st to end	Night	3 3 1 3 3 4 5% 5% 7% 13% 54%	1440
July	1st to 10th	Night	5% 4 2 4 5% 3 6% 5% 7% 10% 48%	1440
	11th to 20th	Night	6% 3 2 3 4 3 5% 5% 7% 11% 52%	1440
	21st to end	Night	5% 5% 1 5% 4 3 5% 5% 7% 10% 52%	1673
August	1st to 10th	Night	4 3 2 5% 5% 4 5% 5% 7% 10% 51%	1780
	11th to 20th	Night	6% 3 2 4 4 3 4 5% 6% 9% 52%	1776
	21st to end	Night	6% 5% 2 4 4 3 4 5% 6% 9% 52%	1980
September	1st to 10th	Night	9% 5% 2 4 3 2 3 3 4 7% 57%	1800
	11th to 20th	Night	8% 4 2 4 3 3 4 4 6% 7% 54%	1876
	21st to end	Night	9% 3 2 4 4 2 3 4 4 7% 58%	1980
October	1st to 10th	Night	8% 4 1 3 3 2 4 3 4 6% 61%	2024
	11th to 20th	Night	13% 5% 1 4 4 2 3 3 4 5% 54%	2160
	21st to end	Night	18% 7% 2 4 4 2 3 3 5% 5% 47%	2438
November	1st to 10th	Night	12% 4 2 4 4 2 3 3 4 6% 56%	2406
	11th to 20th	Night	14% 5% 2 4 4 2 4 3 4 6% 52%	2470
	21st to end	Night	17% 6% 3 4 4 3 3 3 4 6% 48%	2470
December	1st to 10th	Night	20% 8% 2 5% 5% 3 4 4 5% 4 41%	2498
	11th to 20th	Night	17% 7% 3 4 5% 3 4 4 4 5% 44%	3279
	21st to end	Night	17% 10% 3 8% 6% 3 4 5% 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	47% 3 7% 26% 14% 3	1518
	11th to 20th	Day	51% 3 8% 25% 12% 1	1559
	21st to end	Day	44% 2 12% 25% 14% 2	1782
February	1st to 10th	Day	48% 3 9% 25% 13% 2	1768
	11th to 20th	Day	49% 4 10% 22% 12% 4	1758
	21st to end	Day	52% 3 8% 19% 12% 5%	1480
March	1st to 10th	Day	45% 3 9% 23% 17% 4	1800
	11th to 20th	Day	53% 4 9% 19% 12% 3	1903
	21st to end	Day	50% 4 12% 21% 12% 1	2358
April	1st to 10th	Day	37% 3 11% 27% 19% 2	2160
	11th to 20th	Day	44% 3 8% 23% 18% 3	2134
	21st to end	Day	45% 3 9% 26% 15% 1	2160
May	1st to 10th	Day	44% 5% 14% 23% 13% 1	2245
	11th to 20th	Day	43% 5% 12% 22% 16% 2	2305
	21st to end	Day	41% 7% 16% 23% 11% 2	2654
June	1st to 10th	Day	33% 5% 13% 26% 21% 1	2520
	11th to 20th	Day	28% 6% 12% 31% 20% 3	2520
	21st to end	Day	31% 6% 15% 25% 17% 6%	2520
July	1st to 10th	Day	37% 8% 15% 22% 15% 4	2451
	11th to 20th	Day	33% 7% 15% 26% 15% 5%	2445
	21st to end	Day	34% 5% 16% 32% 12% 1	2494
August	1st to 10th	Day	32% 7% 17% 28% 15% 1	2141
	11th to 20th	Day	36% 6% 13% 27% 16% 1	2125
	21st to end	Day	32% 6% 14% 27% 16% 5%	2376
September	1st to 10th	Day	34% 6% 10% 27% 19% 3	2160
	11th to 20th	Day	35% 5% 7% 25% 25% 3	2064
	21st to end	Day	32% 4 7% 26% 26% 6%	1980
October	1st to 10th	Day	29% 3 10% 24% 27% 6%	1933
	11th to 20th	Day	35% 3 6% 24% 24% 9%	1800
	21st to end	Day	40% 3 7% 25% 20% 5%	1918
November	1st to 10th	Day	31% 4 8% 27% 25% 6%	1672
	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	45% 2 9% 25% 17% 2	2648
	11th to 20th	Night	51% 2 8% 23% 14% 1	2460
	21st to end	Night	43% 3 9% 27% 16% 2	2574
February	1st to 10th	Night	50% 3 9% 22% 15% 1	2191
	11th to 20th	Night	46% 3 8% 23% 18% 3	2099
	21st to end	Night	49% 3 9% 19% 16% 5%	1776
March	1st to 10th	Night	43% 4 7% 17% 21% 8%	2160
	11th to 20th	Night	47% 3 9% 22% 15% 3	2070
	21st to end	Night	49% 3 11% 22% 13% 2	1998
April	1st to 10th	Night	34% 3 11% 28% 20% 5%	1800
	11th to 20th	Night	38% 3 9% 23% 21% 6%	1775
	21st to end	Night	41% 4 8% 26% 18% 3	1800
May	1st to 10th	Night	40% 4 10% 25% 18% 3	1712
	11th to 20th	Night	40% 5% 14% 23% 16% 2	1593
	21st to end	Night	33% 6% 14% 28% 15% 3	1619
June	1st to 10th	Night	33% 5% 13% 23% 24% 1	1440
	11th to 20th	Night	23% 4 14% 32% 21% 6%	1440
	21st to end	Night	21% 5% 18% 28% 19% 7%	1440
July	1st to 10th	Night	29% 8% 16% 27% 16% 4	1400
	11th to 20th	Night	26% 5% 19% 27% 16% 7%	1392
	21st to end	Night	27% 5% 15% 35% 17% 1	1580
August	1st to 10th	Night	28% 7% 17% 30% 18% 1	1769
	11th to 20th	Night	29% 5% 14% 29% 21% 3	1776
	21st to end	Night	27% 6% 14% 27% 19% 6%	1980
September	1st to 10th	Night	30% 4 8% 32% 21% 6%	1800
	11th to 20th	Night	29% 6% 8% 28% 25% 5%	1876
	21st to end	Night	28% 3 9% 26% 27% 7%	1980
October	1st to 10th	Night	25% 4 8% 26% 29% 8%	2024
	11th to 20th	Night	34% 2 6% 22% 28% 7%	2160
	21st to end	Night	40% 3 6% 22% 23% 7%	2438
November	1st to 10th	Night	31% 3 7% 27% 27% 6%	2406
	11th to 20th	Night	34% 3 6% 24% 24% 9%	2470
	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	26% 16% 35% 19% 4	248
	21st to end	Day	17% 16% 45% 16% 5%	264
February	1st to 10th	Day	25% 14% 39% 18% 4 1	240
	11th to 20th	Day	30% 20% 38% 11% 2	240
	21st to end	Day	34% 13% 34% 17% 3	198
March	1st to 10th	Day	23% 12% 40% 18% 6%	240
	11th to 20th	Day	24% 17% 38% 18% 3	240
	21st to end	Day	24% 14% 34% 20% 8% 1	444
April	1st to 10th	Day	17% 10% 33% 28% 11%	420
	11th to 20th	Day	18% 14% 27% 25% 12% 2	412
	21st to end	Day	20% 11% 30% 25% 10% 3	420
May	1st to 10th	Day	18% 12% 26% 31% 10% 4	423
	11th to 20th	Day	18% 6% 21% 27% 18% 9%	447
	21st to end	Day	10% 3 22% 28% 24% 13%	608
June	1st to 10th	Day	10% 4 15% 25% 28% 17%	630
	11th to 20th	Day	5% 1 13% 36% 32% 13%	630
	21st to end	Day	6% 3 18% 35% 25% 14%	630
July	1st to 10th	Day	7% 2 8% 24% 33% 26%	621
	11th to 20th	Day	7% 2 10% 26% 35% 20%	618
	21st to end	Day	6% 2 11% 26% 35% 20%	684
August	1st to 10th	Day	7% 2 6% 30% 37% 17%	452
	11th to 20th	Day	13% 2 8% 28% 32% 16%	439
	21st to end	Day	11% 2 6% 22% 37% 23%	495
September	1st to 10th	Day	8% 3 9% 26% 34% 20%	450
	11th to 20th	Day	10% 1 7% 25% 36% 21%	450
	21st to end	Day	15% 3 17% 24% 28% 13%	450
October	1st to 10th	Day	10% 2 19% 33% 30% 6%	403
	11th to 20th	Day	15% 9% 33% 26% 14% 4	270
	21st to end	Day	27% 8% 29% 23% 11% 2	297
November	1st to 10th	Day	14% 8% 31% 27% 16% 4	270
	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% 37% 23% 4	915
	21st to end	Night	21% 9% 39% 28% 2	990
February	1st to 10th	Night	25% 8% 36% 28% 3	751
	11th to 20th	Night	22% 13% 41% 22% 2	703
	21st to end	Night	23% 14% 37% 23% 3	592
March	1st to 10th	Night	22% 9% 34% 30% 5%	720
	11th to 20th	Night	21% 11% 33% 29% 6%	630
	21st to end	Night	19% 11% 29% 30% 10%	594
April	1st to 10th	Night	13% 9% 38% 33% 8%	540
	11th to 20th	Night	17% 9% 27% 33% 12% 1	533
	21st to end	Night	18% 4 33% 32% 12% 1	540
May	1st to 10th	Night	18% 6% 27% 34% 14% 1	540
	11th to 20th	Night	14% 4 21% 39% 20% 2	531
	21st to end	Night	8% 3 15% 40% 30% 4	584
June	1st to 10th	Night	12% 3 11% 39% 32% 4	540
	11th to 20th	Night	5% 1 10% 46% 35% 3	540
	21st to end	Night	4 12% 44% 35% 5%	540
July	1st to 10th	Night	6% 2 7% 29% 46% 11%	525
	11th to 20th	Night	7% 1 8% 29% 45% 10%	522
	21st to end	Night	5% 2 8% 32% 44% 9%	591
August	1st to 10th	Night	5% 1 4 33% 48% 8%	537
	11th to 20th	Night	8% 1 7% 36% 43% 6%	534
	21st to end	Night	8% 3 4 30% 47% 8%	594
September	1st to 10th	Night	11% 1 8% 33% 41% 6%	540
	11th to 20th	Night	10% 1 9% 32% 40% 7%	616
	21st to end	Night	10% 2 15% 36% 31% 6%	720
October	1st to 10th	Night	9% 3 17% 37% 29% 4	720
	11th to 20th	Night	15% 4 28% 35% 17% 2	720
	21st to end	Night	21% 7% 26% 34% 11% 1	792
November	1st to 10th	Night	14% 4 26% 40% 14% 1	740
	11th to 20th	Night	16% 7% 32% 34% 11%	760
	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 2024-01-10 18:35:23. Version ac07

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