Grimes Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grimes Observing Site clear sky chart from 2005-11-06 to 2026-09-29.

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	6% 6% 2 6% 4 4 4 4 6% 8% 49%	158289
	Night	5% 6% 2 6% 4 4 4 4 6% 8% 51%	150685
Transparency	Day	30% 6% 14% 26% 18% 5%	153066
	Night	29% 6% 15% 27% 18% 5%	146919
Seeing	Day	13% 19% 34% 23% 8% 1	18561
	Night	10% 17% 35% 29% 8% 1	28422

By Month

Percentage Cloud Cover SW (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	2 6% 3 8% 5% 5% 5% 4 7% 9% 46%	4720
	11th to 20th	Day	1 5% 4 9% 4 5% 5% 4 7% 9% 49%	4740
	21st to end	Day	1 4 2 7% 5% 4 4 3 6% 7% 56%	5230
February	1st to 10th	Day	4 9% 4 8% 6% 6% 5% 4 5% 7% 42%	1890
	11th to 20th	Day	3 9% 4 10% 5% 4 5% 4 5% 7% 45%	2002
	21st to end	Day	2 8% 4 8% 5% 5% 4 5% 5% 7% 47%	1730
March	1st to 10th	Day	3 9% 3 6% 4 3 3 3 5% 6% 54%	4600
	11th to 20th	Day	4 9% 2 5% 3 3 3 3 5% 7% 56%	4784
	21st to end	Day	11% 14% 3 7% 4 3 4 3 6% 7% 38%	6101
April	1st to 10th	Day	9% 13% 4 6% 4 4 4 4 6% 7% 41%	2520
	11th to 20th	Day	11% 11% 3 7% 4 4 5% 4 6% 7% 38%	2491
	21st to end	Day	14% 12% 3 7% 5% 4 3 4 5% 7% 36%	2651
May	1st to 10th	Day	10% 9% 3 7% 4 4 4 4 5% 7% 43%	7841
	11th to 20th	Day	14% 8% 2 5% 4 4 3 3 6% 8% 43%	8191
	21st to end	Day	15% 8% 2 5% 4 4 5% 4 5% 9% 39%	9026
June	1st to 10th	Day	8% 5% 2 6% 4 4 4 5% 6% 10% 46%	2940
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 10% 48%	2940
	21st to end	Day	6% 3 1 4 3 4 4 5% 8% 7% 55%	2940
July	1st to 10th	Day	3 3 1 5% 4 5% 7% 6% 8% 11% 46%	8249
	11th to 20th	Day	5% 3 1 4 4 4 5% 4 6% 8% 56%	8250
	21st to end	Day	6% 3 2 4 4 5% 6% 5% 7% 9% 49%	8957
August	1st to 10th	Day	6% 3 1 4 3 4 5% 4 6% 10% 54%	8142
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 10% 49%	7220
	21st to end	Day	6% 5% 2 5% 3 4 5% 5% 8% 11% 47%	7751
September	1st to 10th	Day	6% 4 1 4 3 2 3 3 5% 7% 62%	2520
	11th to 20th	Day	8% 4 2 4 3 3 4 4 5% 6% 58%	2441
	21st to end	Day	10% 5% 1 4 3 2 3 3 4 6% 59%	2288
October	1st to 10th	Day	6% 5% 1 2 2 2 2 2 3 4 70%	3403
	11th to 20th	Day	7% 6% 2 5% 3 3 2 2 4 6% 60%	3218
	21st to end	Day	5% 8% 2 5% 4 4 4 4 5% 8% 51%	3190
November	1st to 10th	Day	4 5% 2 5% 3 4 6% 5% 6% 8% 52%	1853
	11th to 20th	Day	2 5% 2 6% 4 4 3 4 5% 8% 55%	1890
	21st to end	Day	3 6% 3 7% 3 4 4 4 7% 9% 50%	1682
December	1st to 10th	Day	2 5% 2 8% 4 3 4 3 4 6% 59%	2700
	11th to 20th	Day	1 4 2 6% 4 4 4 4 7% 10% 55%	3106
	21st to end	Day	2 8% 4 8% 4 4 5% 3 5% 7% 49%	4092
January	1st to 10th	Night	2 8% 4 11% 5% 4 4 3 5% 7% 47%	8201
	11th to 20th	Night	1 3 2 6% 6% 5% 5% 4 7% 9% 53%	8250
	21st to end	Night	1 4 2 7% 4 3 4 3 4 6% 61%	9026
February	1st to 10th	Night	3 8% 4 8% 5% 5% 5% 5% 7% 8% 44%	2730
	11th to 20th	Night	2 7% 3 7% 6% 6% 5% 4 7% 7% 47%	2504
	21st to end	Night	2 8% 3 8% 4 3 4 4 6% 7% 50%	2076
March	1st to 10th	Night	3 9% 3 6% 4 4 4 4 5% 6% 53%	5494
	11th to 20th	Night	3 7% 2 6% 4 4 5% 4 6% 9% 51%	5415
	21st to end	Night	7% 12% 3 7% 5% 5% 5% 5% 6% 8% 37%	5094
April	1st to 10th	Night	10% 8% 3 6% 4 3 4 4 7% 8% 42%	2100
	11th to 20th	Night	10% 10% 2 5% 4 4 5% 3 4 7% 46%	2077
	21st to end	Night	11% 8% 3 7% 4 5% 4 4 4 7% 43%	1968
May	1st to 10th	Night	7% 7% 3 6% 4 4 4 4 5% 6% 50%	5091
	11th to 20th	Night	9% 7% 2 6% 4 5% 4 3 5% 8% 47%	4701
	21st to end	Night	14% 10% 2 5% 3 4 4 3 6% 7% 41%	5181
June	1st to 10th	Night	7% 5% 2 5% 4 4 4 4 6% 11% 46%	1680
	11th to 20th	Night	8% 5% 2 4 4 5% 6% 4 8% 8% 46%	1680
	21st to end	Night	6% 3 1 4 3 5% 5% 4 6% 8% 55%	1680
July	1st to 10th	Night	5% 4 2 6% 5% 4 5% 5% 6% 9% 51%	4740
	11th to 20th	Night	3 3 1 4 3 3 4 3 6% 9% 60%	4740
	21st to end	Night	6% 4 2 6% 5% 4 5% 4 7% 9% 49%	5152
August	1st to 10th	Night	4 4 1 5% 4 4 5% 4 6% 7% 57%	4680
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 7% 9% 50%	5620
	21st to end	Night	5% 4 2 6% 5% 4 5% 4 7% 9% 49%	6465
September	1st to 10th	Night	7% 3 1 4 3 2 4 3 4 6% 62%	2100
	11th to 20th	Night	7% 5% 2 4 4 4 4 4 5% 7% 54%	2179
	21st to end	Night	9% 6% 1 5% 2 3 3 2 4 5% 59%	2288
October	1st to 10th	Night	6% 5% 1 3 2 2 2 2 3 4 71%	3778
	11th to 20th	Night	6% 5% 2 5% 3 3 3 3 5% 7% 58%	3898
	21st to end	Night	6% 7% 3 6% 3 4 3 3 5% 7% 54%	4608
November	1st to 10th	Night	5% 7% 2 6% 3 4 5% 5% 6% 8% 51%	2665
	11th to 20th	Night	4 6% 2 5% 4 3 4 3 6% 7% 56%	2730
	21st to end	Night	3 6% 3 6% 4 4 4 4 6% 8% 52%	2935
December	1st to 10th	Night	2 5% 2 5% 4 4 5% 3 6% 8% 57%	4710
	11th to 20th	Night	1 4 3 7% 4 5% 6% 5% 6% 8% 51%	5321
	21st to end	Night	2 8% 4 10% 5% 5% 4 3 5% 7% 47%	7128

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	32% 5% 17% 27% 15% 4	4723
	11th to 20th	Day	28% 7% 17% 23% 16% 8%	4740
	21st to end	Day	25% 6% 15% 29% 19% 6%	5230
February	1st to 10th	Day	40% 8% 14% 21% 14% 3	1890
	11th to 20th	Day	37% 7% 15% 23% 14% 4	2012
	21st to end	Day	33% 7% 16% 24% 16% 4	1730
March	1st to 10th	Day	28% 5% 10% 28% 21% 8%	4600
	11th to 20th	Day	25% 6% 16% 29% 22% 1	4784
	21st to end	Day	42% 7% 16% 23% 10%	6101
April	1st to 10th	Day	40% 7% 16% 23% 10% 3	2520
	11th to 20th	Day	43% 6% 12% 20% 17% 2	2494
	21st to end	Day	45% 7% 14% 24% 9% 1	2651
May	1st to 10th	Day	37% 8% 13% 28% 13% 2	7841
	11th to 20th	Day	37% 6% 11% 25% 17% 4	8195
	21st to end	Day	39% 7% 14% 25% 14% 1	9027
June	1st to 10th	Day	31% 9% 19% 27% 12% 2	2940
	11th to 20th	Day	29% 8% 18% 25% 15% 5%	2940
	21st to end	Day	23% 7% 16% 27% 21% 6%	2940
July	1st to 10th	Day	28% 7% 13% 29% 20% 3	8181
	11th to 20th	Day	24% 4 11% 26% 20% 13%	8175
	21st to end	Day	29% 6% 13% 22% 18% 12%	8759
August	1st to 10th	Day	24% 5% 14% 20% 24% 13%	6446
	11th to 20th	Day	33% 6% 17% 23% 16% 5%	5688
	21st to end	Day	21% 5% 22% 20% 24% 8%	6078
September	1st to 10th	Day	22% 4 11% 27% 26% 11%	2520
	11th to 20th	Day	26% 5% 11% 25% 25% 8%	2441
	21st to end	Day	26% 4 10% 32% 24% 4	2288
October	1st to 10th	Day	19% 4 9% 21% 33% 13%	3403
	11th to 20th	Day	26% 3 12% 33% 22% 5%	3218
	21st to end	Day	31% 5% 13% 29% 18% 3	3190
November	1st to 10th	Day	26% 8% 15% 26% 20% 4	1853
	11th to 20th	Day	25% 6% 13% 30% 20% 6%	1890
	21st to end	Day	28% 6% 18% 29% 16% 4	1682
December	1st to 10th	Day	26% 5% 19% 30% 18% 1	2700
	11th to 20th	Day	23% 6% 21% 33% 16% 1	3104
	21st to end	Day	33% 5% 9% 28% 21% 3	4092
January	1st to 10th	Night	36% 5% 16% 27% 13% 3	8202
	11th to 20th	Night	25% 8% 20% 23% 16% 8%	8250
	21st to end	Night	23% 5% 16% 27% 23% 6%	9026
February	1st to 10th	Night	35% 8% 18% 24% 13% 3	2730
	11th to 20th	Night	33% 8% 16% 24% 16% 4	2504
	21st to end	Night	30% 8% 14% 26% 17% 5%	2076
March	1st to 10th	Night	29% 7% 10% 27% 18% 9%	5494
	11th to 20th	Night	27% 6% 17% 24% 23% 2	5415
	21st to end	Night	40% 10% 20% 20% 9% 1	5094
April	1st to 10th	Night	37% 7% 16% 24% 12% 2	2100
	11th to 20th	Night	37% 6% 14% 23% 17% 3	2077
	21st to end	Night	39% 7% 15% 24% 14% 1	1968
May	1st to 10th	Night	32% 7% 12% 31% 17% 1	5092
	11th to 20th	Night	34% 5% 15% 30% 14% 2	4702
	21st to end	Night	40% 6% 13% 27% 14%	5182
June	1st to 10th	Night	30% 9% 24% 25% 11% 1	1680
	11th to 20th	Night	32% 9% 19% 22% 14% 3	1680
	21st to end	Night	24% 6% 19% 27% 18% 6%	1680
July	1st to 10th	Night	29% 5% 12% 30% 21% 3	4700
	11th to 20th	Night	22% 3 13% 29% 21% 12%	4692
	21st to end	Night	31% 5% 16% 25% 17% 5%	5038
August	1st to 10th	Night	24% 4 14% 22% 27% 8%	3700
	11th to 20th	Night	32% 6% 21% 22% 16% 3	4427
	21st to end	Night	24% 5% 21% 25% 21% 4	5069
September	1st to 10th	Night	23% 4 9% 28% 28% 8%	2100
	11th to 20th	Night	28% 5% 13% 24% 25% 6%	2179
	21st to end	Night	28% 4 8% 28% 25% 7%	2288
October	1st to 10th	Night	20% 3 8% 20% 37% 11%	3778
	11th to 20th	Night	26% 3 10% 34% 22% 4	3898
	21st to end	Night	29% 5% 13% 27% 18% 8%	4608
November	1st to 10th	Night	28% 7% 16% 22% 21% 6%	2665
	11th to 20th	Night	26% 5% 15% 29% 20% 5%	2730
	21st to end	Night	27% 6% 15% 29% 19% 4	2935
December	1st to 10th	Night	24% 6% 17% 31% 19% 3	4710
	11th to 20th	Night	28% 7% 17% 33% 13% 2	5322
	21st to end	Night	35% 6% 11% 28% 17% 3	7128

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	8% 22% 35% 22% 10% 3	330
	11th to 20th	Day	16% 28% 33% 15% 7% 1	330
	21st to end	Day	13% 35% 38% 10% 3 1	363
February	1st to 10th	Day	12% 31% 45% 12% 1	330
	11th to 20th	Day	9% 30% 44% 14% 2	330
	21st to end	Day	17% 31% 38% 12% 1	273
March	1st to 10th	Day	12% 31% 35% 16% 5% 1	330
	11th to 20th	Day	18% 24% 39% 14% 5% 1	383
	21st to end	Day	22% 29% 30% 14% 4 1	594
April	1st to 10th	Day	23% 29% 36% 11% 2	540
	11th to 20th	Day	24% 30% 35% 10% 2	532
	21st to end	Day	23% 20% 30% 21% 5%	671
May	1st to 10th	Day	18% 21% 33% 18% 8% 2	753
	11th to 20th	Day	21% 17% 29% 22% 10% 2	775
	21st to end	Day	19% 18% 32% 20% 9% 1	844
June	1st to 10th	Day	10% 15% 33% 28% 12% 2	780
	11th to 20th	Day	14% 19% 37% 23% 7%	780
	21st to end	Day	12% 13% 34% 28% 12% 1	780
July	1st to 10th	Day	6% 6% 34% 38% 14% 2	771
	11th to 20th	Day	5% 6% 41% 35% 12% 1	768
	21st to end	Day	6% 8% 32% 36% 15% 2	849
August	1st to 10th	Day	7% 8% 32% 38% 15% 1	776
	11th to 20th	Day	6% 8% 35% 37% 12% 2	601
	21st to end	Day	6% 12% 34% 35% 11% 2	627
September	1st to 10th	Day	8% 14% 34% 32% 10% 3	570
	11th to 20th	Day	11% 15% 34% 27% 11% 3	491
	21st to end	Day	15% 16% 36% 21% 9% 4	354
October	1st to 10th	Day	19% 18% 25% 23% 11% 3	330
	11th to 20th	Day	21% 23% 33% 15% 6% 2	330
	21st to end	Day	12% 31% 32% 17% 6% 2	363
November	1st to 10th	Day	13% 26% 36% 18% 5% 2	330
	11th to 20th	Day	15% 26% 36% 17% 6%	330
	21st to end	Day	10% 23% 43% 16% 6% 1	330
December	1st to 10th	Day	16% 31% 33% 15% 5%	330
	11th to 20th	Day	16% 30% 42% 10% 3	330
	21st to end	Day	15% 30% 37% 13% 5%	363
January	1st to 10th	Night	8% 24% 36% 22% 9% 1	1050
	11th to 20th	Night	7% 30% 39% 18% 6% 1	1050
	21st to end	Night	7% 31% 38% 20% 3	1155
February	1st to 10th	Night	8% 26% 41% 20% 4	840
	11th to 20th	Night	6% 22% 42% 23% 6% 1	824
	21st to end	Night	9% 20% 42% 24% 5% 1	692
March	1st to 10th	Night	10% 22% 35% 26% 6% 1	840
	11th to 20th	Night	9% 17% 37% 30% 6% 1	838
	21st to end	Night	15% 16% 34% 28% 7%	693
April	1st to 10th	Night	16% 14% 37% 29% 3	630
	11th to 20th	Night	18% 20% 32% 26% 4	623
	21st to end	Night	18% 15% 29% 27% 11%	630
May	1st to 10th	Night	14% 12% 32% 31% 10% 1	630
	11th to 20th	Night	16% 11% 31% 28% 13% 1	621
	21st to end	Night	20% 11% 32% 28% 8%	683
June	1st to 10th	Night	10% 12% 32% 34% 12%	630
	11th to 20th	Night	12% 13% 38% 30% 7%	630
	21st to end	Night	7% 8% 36% 41% 7% 1	630
July	1st to 10th	Night	7% 5% 29% 40% 19% 1	615
	11th to 20th	Night	6% 3 33% 48% 9%	612
	21st to end	Night	4 2 32% 50% 12%	690
August	1st to 10th	Night	6% 5% 29% 48% 12% 1	627
	11th to 20th	Night	6% 4 27% 45% 17% 1	624
	21st to end	Night	6% 5% 27% 46% 16% 1	693
September	1st to 10th	Night	8% 6% 26% 43% 16% 1	630
	11th to 20th	Night	11% 10% 25% 38% 15% 2	630
	21st to end	Night	14% 13% 24% 35% 14%	624
October	1st to 10th	Night	15% 13% 28% 30% 13% 1	710
	11th to 20th	Night	14% 15% 32% 29% 9% 1	800
	21st to end	Night	12% 16% 39% 23% 8% 1	1098
November	1st to 10th	Night	9% 18% 40% 25% 6% 2	1025
	11th to 20th	Night	10% 21% 39% 25% 5% 1	1050
	21st to end	Night	7% 20% 40% 27% 5%	1050
December	1st to 10th	Night	8% 27% 41% 19% 5% 1	1050
	11th to 20th	Night	8% 22% 39% 25% 6%	1050
	21st to end	Night	10% 25% 39% 20% 6% 1	1155

Page generated at 2026-09-26 02:18:40. Version ac07

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