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 2025-05-26.

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% 7% 2 6% 4 4 4 4 6% 8% 50%	117868
	Night	4 6% 2 6% 4 4 4 4 6% 7% 52%	120263
Transparency	Day	30% 7% 14% 26% 18% 5%	117611
	Night	29% 7% 15% 26% 19% 5%	120109
Seeing	Day	13% 19% 35% 24% 9% 1	16603
	Night	10% 17% 35% 29% 8% 1	26681

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	1 6% 3 9% 5% 6% 5% 4 7% 8% 46%	3638
	11th to 20th	Day	1 6% 4 11% 4 5% 5% 3 5% 7% 47%	3640
	21st to end	Day	1 5% 2 7% 3 4 3 3 5% 6% 62%	4032
February	1st to 10th	Day	4 9% 4 8% 5% 6% 5% 4 5% 7% 42%	1800
	11th to 20th	Day	3 9% 4 10% 5% 4 5% 4 5% 7% 44%	1904
	21st to end	Day	2 9% 4 8% 5% 5% 4 5% 6% 7% 46%	1650
March	1st to 10th	Day	3 9% 3 6% 4 3 3 3 5% 6% 54%	4500
	11th to 20th	Day	4 9% 2 5% 3 3 3 3 5% 7% 56%	4681
	21st to end	Day	11% 14% 3 7% 4 3 4 3 6% 7% 37%	5969
April	1st to 10th	Day	9% 13% 4 6% 4 3 4 4 6% 7% 42%	2400
	11th to 20th	Day	11% 12% 3 7% 5% 4 5% 4 6% 6% 37%	2371
	21st to end	Day	14% 11% 3 7% 5% 4 3 4 5% 7% 36%	2525
May	1st to 10th	Day	13% 11% 4 9% 5% 4 5% 4 6% 6% 33%	4362
	11th to 20th	Day	13% 8% 2 5% 4 4 4 3 6% 8% 42%	4528
	21st to end	Day	14% 8% 2 5% 5% 4 5% 4 5% 8% 40%	4897
June	1st to 10th	Day	8% 5% 2 5% 4 5% 4 5% 6% 10% 46%	2660
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 6% 10% 47%	2660
	21st to end	Day	5% 3 1 4 3 4 4 5% 8% 7% 56%	2660
July	1st to 10th	Day	4 4 1 6% 5% 6% 6% 5% 7% 10% 47%	4430
	11th to 20th	Day	2 2 1 5% 5% 5% 6% 5% 8% 10% 51%	4430
	21st to end	Day	3 2 1 3 4 4 6% 4 7% 8% 56%	4802
August	1st to 10th	Day	8% 4 2 5% 3 4 6% 5% 7% 11% 47%	4411
	11th to 20th	Day	6% 3 2 5% 4 4 5% 5% 7% 8% 50%	3835
	21st to end	Day	4 3 2 5% 3 3 4 3 6% 7% 60%	4180
September	1st to 10th	Day	7% 4 1 4 3 2 3 3 5% 7% 61%	2280
	11th to 20th	Day	7% 4 2 4 3 3 4 3 4 6% 61%	2209
	21st to end	Day	10% 5% 1 4 3 2 2 3 3 6% 61%	2090
October	1st to 10th	Day	6% 5% 1 2 2 2 2 2 3 5% 71%	3299
	11th to 20th	Day	6% 5% 2 5% 3 3 2 3 4 6% 61%	3119
	21st to end	Day	6% 8% 2 5% 4 4 4 4 5% 8% 50%	3091
November	1st to 10th	Day	5% 5% 2 5% 3 4 6% 4 6% 8% 52%	1763
	11th to 20th	Day	2 5% 3 6% 4 4 3 4 5% 8% 55%	1800
	21st to end	Day	3 7% 3 6% 3 4 4 4 6% 10% 50%	1602
December	1st to 10th	Day	2 5% 2 8% 4 3 4 3 4 6% 59%	2620
	11th to 20th	Day	1 4 2 6% 4 5% 5% 4 7% 10% 55%	3026
	21st to end	Day	2 8% 4 9% 4 4 5% 3 5% 7% 49%	4004
January	1st to 10th	Night	2 7% 4 11% 6% 5% 4 4 5% 6% 46%	6327
	11th to 20th	Night	1 4 3 8% 6% 5% 5% 4 7% 8% 51%	6340
	21st to end	Night	1 3 2 6% 4 3 4 3 4 7% 63%	6974
February	1st to 10th	Night	3 8% 4 8% 5% 5% 5% 5% 7% 7% 44%	2600
	11th to 20th	Night	2 7% 3 7% 6% 6% 6% 4 7% 7% 46%	2382
	21st to end	Night	2 8% 3 8% 4 4 4 4 6% 7% 49%	1980
March	1st to 10th	Night	3 9% 3 6% 4 4 4 4 5% 6% 54%	5374
	11th to 20th	Night	3 7% 2 6% 4 4 5% 4 6% 9% 52%	5298
	21st to end	Night	7% 12% 3 8% 5% 5% 5% 5% 6% 8% 37%	4984
April	1st to 10th	Night	10% 9% 3 6% 5% 3 4 4 7% 8% 42%	2000
	11th to 20th	Night	10% 10% 2 6% 4 4 4 3 4 7% 46%	1977
	21st to end	Night	11% 8% 3 7% 4 5% 4 4 4 7% 43%	1874
May	1st to 10th	Night	10% 9% 4 8% 5% 4 4 4 5% 6% 42%	2825
	11th to 20th	Night	7% 5% 3 5% 4 4 4 4 6% 9% 49%	2597
	21st to end	Night	13% 9% 2 5% 3 4 4 4 6% 8% 42%	2806
June	1st to 10th	Night	8% 5% 2 5% 5% 4 4 4 7% 11% 45%	1520
	11th to 20th	Night	9% 5% 2 5% 4 5% 6% 4 7% 8% 45%	1520
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 8% 57%	1520
July	1st to 10th	Night	6% 4 2 5% 5% 4 5% 4 5% 6% 54%	2540
	11th to 20th	Night	2 2 1 4 3 4 5% 4 8% 11% 56%	2540
	21st to end	Night	3 3 1 3 4 3 4 4 7% 9% 60%	2761
August	1st to 10th	Night	4 4 2 7% 6% 5% 6% 5% 7% 8% 46%	2528
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 5% 8% 9% 49%	2988
	21st to end	Night	3 3 1 5% 4 4 4 4 6% 7% 58%	3487
September	1st to 10th	Night	7% 4 1 4 3 2 3 3 4 6% 62%	1900
	11th to 20th	Night	6% 4 1 4 4 4 4 4 5% 7% 58%	1971
	21st to end	Night	9% 6% 1 5% 2 3 3 2 4 5% 60%	2090
October	1st to 10th	Night	6% 5% 1 3 2 2 2 2 3 4 71%	3662
	11th to 20th	Night	6% 5% 2 5% 3 3 3 3 5% 7% 59%	3778
	21st to end	Night	6% 7% 3 6% 3 4 3 3 5% 7% 54%	4465
November	1st to 10th	Night	5% 7% 2 6% 3 4 5% 5% 6% 8% 50%	2535
	11th to 20th	Night	4 6% 2 5% 4 3 4 3 6% 7% 57%	2600
	21st to end	Night	3 6% 3 6% 4 4 4 4 6% 8% 52%	2795
December	1st to 10th	Night	2 5% 2 5% 4 4 4 3 6% 7% 58%	4570
	11th to 20th	Night	1 4 3 7% 4 5% 6% 5% 6% 8% 51%	5181
	21st to end	Night	2 9% 4 10% 5% 5% 4 3 5% 7% 46%	6974

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% 7% 15% 27% 14% 4	3639
	11th to 20th	Day	32% 8% 16% 20% 15% 8%	3640
	21st to end	Day	21% 6% 16% 30% 20% 7%	4032
February	1st to 10th	Day	40% 8% 15% 21% 14% 3	1800
	11th to 20th	Day	37% 8% 15% 22% 13% 4	1914
	21st to end	Day	34% 8% 16% 23% 16% 4	1650
March	1st to 10th	Day	28% 5% 10% 28% 20% 8%	4500
	11th to 20th	Day	25% 6% 15% 30% 22% 1	4681
	21st to end	Day	43% 8% 16% 23% 10%	5969
April	1st to 10th	Day	40% 7% 16% 23% 11% 3	2400
	11th to 20th	Day	44% 6% 12% 20% 16% 2	2374
	21st to end	Day	44% 8% 15% 25% 9%	2525
May	1st to 10th	Day	44% 12% 15% 19% 8% 3	4362
	11th to 20th	Day	36% 8% 15% 27% 12% 2	4532
	21st to end	Day	37% 10% 17% 25% 10% 1	4897
June	1st to 10th	Day	31% 10% 20% 25% 11% 2	2660
	11th to 20th	Day	31% 8% 18% 25% 14% 5%	2660
	21st to end	Day	21% 7% 16% 28% 21% 6%	2660
July	1st to 10th	Day	28% 8% 11% 29% 21% 2	4361
	11th to 20th	Day	24% 7% 12% 26% 24% 7%	4355
	21st to end	Day	21% 6% 11% 22% 24% 15%	4651
August	1st to 10th	Day	29% 7% 18% 22% 18% 6%	4396
	11th to 20th	Day	27% 6% 15% 26% 20% 6%	3873
	21st to end	Day	21% 4 11% 20% 33% 11%	4179
September	1st to 10th	Day	22% 4 11% 26% 26% 11%	2280
	11th to 20th	Day	24% 5% 11% 26% 26% 8%	2209
	21st to end	Day	25% 5% 11% 33% 24% 4	2090
October	1st to 10th	Day	18% 4 10% 21% 33% 13%	3299
	11th to 20th	Day	25% 3 12% 33% 22% 5%	3119
	21st to end	Day	32% 5% 13% 29% 18% 3	3091
November	1st to 10th	Day	26% 9% 14% 26% 21% 5%	1763
	11th to 20th	Day	25% 7% 13% 29% 20% 6%	1800
	21st to end	Day	28% 6% 18% 29% 16% 4	1602
December	1st to 10th	Day	26% 5% 19% 30% 18% 2	2620
	11th to 20th	Day	23% 6% 21% 33% 16% 1	3024
	21st to end	Day	34% 5% 9% 27% 22% 3	4004
January	1st to 10th	Night	37% 6% 13% 25% 16% 4	6327
	11th to 20th	Night	29% 9% 16% 22% 15% 8%	6340
	21st to end	Night	20% 7% 16% 28% 24% 5%	6974
February	1st to 10th	Night	35% 9% 18% 23% 13% 3	2600
	11th to 20th	Night	34% 8% 16% 24% 15% 3	2382
	21st to end	Night	31% 8% 14% 25% 17% 5%	1980
March	1st to 10th	Night	28% 7% 10% 27% 18% 9%	5374
	11th to 20th	Night	27% 6% 17% 24% 24% 2	5298
	21st to end	Night	41% 10% 21% 20% 8% 1	4984
April	1st to 10th	Night	37% 8% 16% 24% 13% 3	2000
	11th to 20th	Night	38% 6% 14% 23% 16% 3	1977
	21st to end	Night	39% 8% 15% 25% 12% 1	1874
May	1st to 10th	Night	38% 10% 14% 23% 13% 2	2825
	11th to 20th	Night	28% 8% 18% 36% 9% 2	2598
	21st to end	Night	37% 9% 15% 29% 9% 1	2806
June	1st to 10th	Night	30% 9% 24% 24% 11% 1	1520
	11th to 20th	Night	33% 9% 19% 22% 13% 3	1520
	21st to end	Night	22% 7% 19% 28% 19% 6%	1520
July	1st to 10th	Night	27% 6% 11% 29% 23% 5%	2500
	11th to 20th	Night	20% 5% 16% 32% 21% 6%	2492
	21st to end	Night	20% 6% 14% 28% 23% 8%	2674
August	1st to 10th	Night	32% 6% 18% 21% 20% 4	2520
	11th to 20th	Night	29% 6% 15% 25% 21% 4	3015
	21st to end	Night	23% 5% 12% 24% 30% 6%	3487
September	1st to 10th	Night	24% 4 8% 27% 28% 9%	1900
	11th to 20th	Night	24% 5% 13% 25% 26% 7%	1971
	21st to end	Night	27% 4 8% 27% 26% 7%	2090
October	1st to 10th	Night	20% 3 8% 19% 38% 11%	3662
	11th to 20th	Night	26% 3 10% 35% 22% 4	3778
	21st to end	Night	30% 5% 13% 27% 18% 8%	4465
November	1st to 10th	Night	28% 7% 16% 22% 20% 6%	2535
	11th to 20th	Night	26% 5% 14% 29% 20% 6%	2600
	21st to end	Night	27% 6% 15% 29% 19% 4	2795
December	1st to 10th	Night	24% 6% 17% 31% 19% 3	4570
	11th to 20th	Night	28% 7% 17% 33% 13% 1	5182
	21st to end	Night	36% 6% 11% 28% 17% 3	6974

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	4 22% 37% 23% 11% 3	300
	11th to 20th	Day	12% 27% 35% 16% 8% 1	300
	21st to end	Day	12% 35% 38% 11% 3 1	330
February	1st to 10th	Day	12% 31% 46% 11% 1	300
	11th to 20th	Day	8% 31% 45% 14% 2	300
	21st to end	Day	17% 30% 40% 12% 1	249
March	1st to 10th	Day	10% 32% 37% 15% 5% 1	300
	11th to 20th	Day	16% 23% 41% 15% 5% 1	350
	21st to end	Day	22% 27% 31% 15% 5% 1	550
April	1st to 10th	Day	22% 28% 35% 12% 2	500
	11th to 20th	Day	23% 29% 36% 10% 2	492
	21st to end	Day	21% 20% 31% 21% 6%	625
May	1st to 10th	Day	18% 21% 32% 19% 8% 2	703
	11th to 20th	Day	22% 17% 27% 22% 10% 2	725
	21st to end	Day	19% 18% 32% 21% 9% 1	760
June	1st to 10th	Day	9% 14% 33% 29% 13% 2	680
	11th to 20th	Day	14% 18% 38% 23% 7%	680
	21st to end	Day	9% 12% 35% 30% 13% 1	680
July	1st to 10th	Day	6% 6% 33% 38% 14% 2	671
	11th to 20th	Day	4 7% 41% 35% 13% 1	668
	21st to end	Day	6% 7% 30% 39% 17% 2	739
August	1st to 10th	Day	7% 7% 31% 39% 15% 1	676
	11th to 20th	Day	5% 8% 35% 37% 12% 2	517
	21st to end	Day	5% 12% 33% 36% 12% 2	539
September	1st to 10th	Day	8% 12% 34% 32% 11% 3	490
	11th to 20th	Day	10% 12% 34% 29% 12% 4	419
	21st to end	Day	16% 17% 34% 21% 8% 4	300
October	1st to 10th	Day	18% 17% 25% 24% 12% 3	300
	11th to 20th	Day	17% 24% 35% 16% 6% 2	300
	21st to end	Day	12% 30% 32% 18% 6% 2	330
November	1st to 10th	Day	13% 26% 35% 19% 5% 2	300
	11th to 20th	Day	15% 27% 35% 18% 5%	300
	21st to end	Day	10% 25% 42% 16% 6% 1	300
December	1st to 10th	Day	13% 30% 35% 16% 6%	300
	11th to 20th	Day	13% 30% 44% 11% 3	300
	21st to end	Day	12% 29% 38% 14% 5%	330
January	1st to 10th	Night	7% 24% 36% 23% 9% 1	1000
	11th to 20th	Night	5% 30% 40% 18% 6% 1	1000
	21st to end	Night	7% 30% 38% 21% 3	1100
February	1st to 10th	Night	8% 26% 41% 20% 5%	800
	11th to 20th	Night	5% 23% 42% 22% 6% 1	784
	21st to end	Night	8% 20% 42% 25% 5% 1	660
March	1st to 10th	Night	9% 22% 36% 27% 6% 1	800
	11th to 20th	Night	8% 16% 37% 31% 6% 1	798
	21st to end	Night	15% 16% 33% 28% 7%	660
April	1st to 10th	Night	16% 15% 36% 30% 4	600
	11th to 20th	Night	18% 20% 32% 26% 5%	593
	21st to end	Night	17% 16% 30% 26% 11%	600
May	1st to 10th	Night	14% 13% 32% 30% 10% 1	600
	11th to 20th	Night	17% 10% 31% 28% 13% 1	591
	21st to end	Night	20% 12% 32% 27% 8%	634
June	1st to 10th	Night	10% 11% 32% 34% 13%	570
	11th to 20th	Night	12% 14% 36% 31% 7%	570
	21st to end	Night	6% 7% 36% 43% 8% 1	570
July	1st to 10th	Night	7% 5% 29% 39% 20% 1	555
	11th to 20th	Night	6% 4 34% 47% 9% 1	552
	21st to end	Night	4 2 30% 53% 12%	624
August	1st to 10th	Night	5% 5% 28% 48% 13% 1	567
	11th to 20th	Night	6% 4 27% 44% 17% 1	564
	21st to end	Night	5% 5% 26% 46% 17% 1	627
September	1st to 10th	Night	8% 6% 25% 43% 16% 1	570
	11th to 20th	Night	10% 9% 25% 39% 16% 2	570
	21st to end	Night	14% 13% 25% 34% 13% 1	570
October	1st to 10th	Night	15% 14% 28% 29% 13% 1	674
	11th to 20th	Night	13% 15% 32% 30% 9% 1	760
	21st to end	Night	12% 17% 39% 24% 8% 2	1043
November	1st to 10th	Night	9% 17% 41% 25% 6% 2	975
	11th to 20th	Night	10% 21% 39% 24% 5% 1	1000
	21st to end	Night	7% 21% 40% 27% 5%	1000
December	1st to 10th	Night	7% 27% 40% 20% 5% 1	1000
	11th to 20th	Night	7% 22% 39% 26% 6%	1000
	21st to end	Night	9% 26% 38% 20% 6% 1	1100

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