Gillette
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gillette clear sky chart from 2005-11-06 to 2025-10-19.

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% 51%	135845
	Night	6% 7% 3 7% 4 4 4 4 6% 7% 48%	131340
Transparency	Day	31% 5% 12% 26% 20% 6%	130635
	Night	33% 6% 13% 26% 18% 5%	127569
Seeing	Day	10% 9% 29% 31% 17% 4	17238
	Night	10% 8% 35% 36% 11% 1	27177

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	3 8% 4 14% 8% 6% 5% 4 6% 7% 35%	3638
	11th to 20th	Day	2 8% 6% 10% 7% 7% 6% 4 6% 7% 37%	3640
	21st to end	Day	1 6% 3 8% 6% 3 3 3 5% 6% 56%	4250
February	1st to 10th	Day	6% 13% 7% 9% 6% 5% 5% 4 6% 6% 34%	1800
	11th to 20th	Day	4 11% 4 10% 6% 5% 5% 3 5% 7% 39%	1799
	21st to end	Day	4 9% 4 8% 6% 5% 5% 4 7% 6% 42%	1650
March	1st to 10th	Day	3 10% 4 6% 3 3 3 3 5% 7% 54%	4500
	11th to 20th	Day	5% 8% 2 5% 3 3 4 4 6% 9% 51%	4942
	21st to end	Day	12% 11% 3 8% 5% 4 4 4 6% 7% 36%	5806
April	1st to 10th	Day	9% 9% 3 6% 5% 5% 4 5% 6% 8% 42%	2400
	11th to 20th	Day	10% 12% 3 9% 5% 4 4 3 5% 6% 39%	2371
	21st to end	Day	15% 10% 2 5% 5% 4 4 4 6% 7% 38%	2450
May	1st to 10th	Day	13% 10% 2 5% 3 3 4 4 5% 7% 45%	6340
	11th to 20th	Day	12% 8% 2 6% 4 4 4 4 6% 7% 45%	6298
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 8% 46%	6925
June	1st to 10th	Day	7% 4 2 5% 3 4 5% 5% 7% 10% 49%	2800
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 7% 9% 49%	2800
	21st to end	Day	5% 3 1 3 2 3 4 4 6% 9% 61%	2800
July	1st to 10th	Day	4 3 1 4 3 4 5% 4 7% 9% 57%	6339
	11th to 20th	Day	5% 3 1 3 3 3 4 4 6% 9% 59%	6340
	21st to end	Day	4 3 1 4 3 3 4 3 6% 10% 58%	6903
August	1st to 10th	Day	6% 3 1 4 3 4 4 4 7% 10% 55%	6065
	11th to 20th	Day	4 2 1 4 3 4 4 4 7% 9% 59%	5676
	21st to end	Day	4 4 2 5% 3 4 4 3 5% 8% 59%	5984
September	1st to 10th	Day	6% 3 1 3 2 2 3 3 5% 7% 66%	2400
	11th to 20th	Day	7% 4 1 4 3 3 4 3 5% 7% 59%	2400
	21st to end	Day	9% 4 1 4 3 3 3 2 4 5% 62%	2160
October	1st to 10th	Day	7% 4 1 3 3 2 2 2 3 4 69%	3270
	11th to 20th	Day	9% 5% 2 4 3 3 3 3 4 7% 58%	3230
	21st to end	Day	9% 7% 2 5% 3 4 4 3 5% 8% 49%	3191
November	1st to 10th	Day	8% 6% 3 5% 5% 3 4 4 6% 9% 48%	1763
	11th to 20th	Day	5% 9% 3 5% 4 4 4 4 7% 7% 48%	1665
	21st to end	Day	5% 9% 2 6% 4 4 5% 4 6% 9% 48%	1600
December	1st to 10th	Day	2 9% 4 6% 4 4 4 3 4 5% 55%	2620
	11th to 20th	Day	3 4 2 6% 4 5% 5% 4 6% 8% 52%	3026
	21st to end	Day	3 8% 5% 8% 5% 5% 5% 4 7% 6% 43%	4004
January	1st to 10th	Night	3 10% 5% 11% 6% 5% 4 4 5% 7% 40%	6327
	11th to 20th	Night	3 7% 5% 13% 7% 5% 5% 3 6% 6% 39%	6340
	21st to end	Night	2 5% 3 8% 5% 4 4 3 6% 7% 53%	6772
February	1st to 10th	Night	4 11% 6% 12% 8% 6% 4 4 6% 7% 32%	2600
	11th to 20th	Night	3 13% 4 9% 5% 6% 5% 5% 5% 7% 37%	2487
	21st to end	Night	3 10% 3 8% 6% 5% 5% 4 6% 8% 44%	1980
March	1st to 10th	Night	4 9% 3 6% 4 3 3 3 6% 8% 51%	5374
	11th to 20th	Night	5% 7% 2 6% 4 4 4 4 5% 8% 51%	5054
	21st to end	Night	9% 9% 4 10% 6% 6% 5% 4 7% 8% 33%	5153
April	1st to 10th	Night	11% 10% 3 5% 4 4 4 4 6% 7% 43%	2000
	11th to 20th	Night	11% 11% 3 7% 5% 4 6% 4 6% 6% 38%	1977
	21st to end	Night	11% 10% 3 5% 4 4 5% 4 7% 7% 40%	1943
May	1st to 10th	Night	13% 8% 2 4 2 3 3 3 4 6% 51%	3640
	11th to 20th	Night	11% 7% 2 5% 4 4 4 4 6% 8% 42%	3617
	21st to end	Night	12% 7% 2 6% 4 4 4 4 5% 7% 44%	3972
June	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 9% 10% 46%	1600
	11th to 20th	Night	8% 4 2 6% 4 5% 5% 4 6% 8% 48%	1600
	21st to end	Night	5% 3 1 5% 2 2 4 3 5% 9% 61%	1600
July	1st to 10th	Night	4 4 1 4 4 3 5% 5% 7% 9% 55%	3640
	11th to 20th	Night	4 2 1 4 4 5% 5% 4 6% 10% 55%	3640
	21st to end	Night	5% 3 2 5% 4 4 4 4 7% 8% 54%	3970
August	1st to 10th	Night	6% 3 1 5% 4 4 5% 4 6% 8% 54%	3732
	11th to 20th	Night	4 3 1 5% 4 5% 5% 5% 7% 10% 52%	4173
	21st to end	Night	5% 4 2 6% 4 4 4 4 6% 8% 54%	4993
September	1st to 10th	Night	7% 3 1 3 2 2 3 3 5% 7% 64%	2000
	11th to 20th	Night	7% 4 1 5% 3 3 4 4 7% 7% 55%	2000
	21st to end	Night	9% 4 1 3 3 3 3 3 4 5% 61%	2235
October	1st to 10th	Night	8% 5% 1 3 2 2 2 2 3 4 68%	3920
	11th to 20th	Night	10% 6% 2 4 3 3 3 3 5% 6% 56%	3874
	21st to end	Night	8% 7% 2 6% 3 4 3 3 5% 7% 52%	4336
November	1st to 10th	Night	7% 7% 2 7% 4 4 4 5% 6% 7% 47%	2535
	11th to 20th	Night	5% 10% 3 7% 5% 5% 4 4 6% 7% 45%	2731
	21st to end	Night	5% 7% 3 7% 4 4 4 4 6% 8% 49%	2800
December	1st to 10th	Night	2 8% 3 7% 5% 4 4 4 5% 7% 50%	4570
	11th to 20th	Night	2 8% 3 7% 6% 5% 5% 5% 6% 8% 45%	5181
	21st to end	Night	4 10% 3 9% 5% 5% 4 4 5% 7% 44%	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	46% 6% 11% 26% 10%	3639
	11th to 20th	Day	45% 6% 14% 22% 11% 2	3640
	21st to end	Day	28% 5% 12% 33% 16% 5%	4250
February	1st to 10th	Day	48% 6% 14% 19% 11% 2	1800
	11th to 20th	Day	44% 7% 11% 23% 12% 3	1808
	21st to end	Day	40% 6% 14% 23% 14% 4	1650
March	1st to 10th	Day	30% 3 8% 26% 25% 8%	4500
	11th to 20th	Day	30% 5% 16% 31% 19% 1	4942
	21st to end	Day	46% 7% 15% 25% 7%	5806
April	1st to 10th	Day	39% 7% 15% 26% 12% 3	2400
	11th to 20th	Day	45% 5% 12% 22% 13% 2	2374
	21st to end	Day	43% 6% 15% 24% 12% 1	2450
May	1st to 10th	Day	38% 5% 12% 26% 16% 2	6340
	11th to 20th	Day	37% 6% 11% 26% 17% 3	6302
	21st to end	Day	33% 6% 13% 30% 16% 1	6926
June	1st to 10th	Day	27% 9% 19% 27% 14% 3	2800
	11th to 20th	Day	28% 8% 15% 24% 18% 6%	2800
	21st to end	Day	19% 7% 12% 28% 24% 10%	2800
July	1st to 10th	Day	22% 6% 8% 29% 26% 9%	6271
	11th to 20th	Day	21% 4 8% 26% 29% 12%	6265
	21st to end	Day	20% 5% 10% 22% 26% 17%	6705
August	1st to 10th	Day	27% 6% 12% 22% 20% 13%	4418
	11th to 20th	Day	23% 5% 9% 26% 28% 9%	4111
	21st to end	Day	14% 4 9% 25% 32% 17%	4311
September	1st to 10th	Day	19% 5% 7% 25% 31% 14%	2400
	11th to 20th	Day	25% 5% 7% 26% 26% 11%	2400
	21st to end	Day	25% 3 10% 30% 26% 6%	2160
October	1st to 10th	Day	22% 3 7% 18% 31% 18%	3270
	11th to 20th	Day	27% 4 11% 25% 28% 5%	3230
	21st to end	Day	33% 5% 13% 27% 18% 4	3191
November	1st to 10th	Day	32% 7% 14% 21% 21% 4	1763
	11th to 20th	Day	33% 6% 14% 27% 16% 4	1665
	21st to end	Day	32% 6% 15% 27% 17% 3	1600
December	1st to 10th	Day	31% 4 17% 29% 17% 1	2620
	11th to 20th	Day	28% 6% 18% 34% 13% 1	3024
	21st to end	Day	38% 5% 8% 31% 14% 4	4004
January	1st to 10th	Night	43% 5% 11% 31% 9% 1	6327
	11th to 20th	Night	44% 6% 14% 20% 14% 3	6340
	21st to end	Night	28% 6% 15% 29% 18% 4	6772
February	1st to 10th	Night	49% 7% 13% 18% 10% 3	2600
	11th to 20th	Night	44% 7% 14% 19% 13% 3	2488
	21st to end	Night	38% 6% 14% 22% 16% 5%	1980
March	1st to 10th	Night	30% 5% 10% 28% 21% 7%	5374
	11th to 20th	Night	30% 5% 17% 24% 22% 2	5054
	21st to end	Night	46% 8% 18% 19% 7%	5153
April	1st to 10th	Night	38% 8% 14% 25% 12% 3	2000
	11th to 20th	Night	43% 8% 11% 24% 13% 1	1977
	21st to end	Night	39% 7% 14% 27% 11% 1	1943
May	1st to 10th	Night	35% 5% 11% 31% 17% 1	3640
	11th to 20th	Night	38% 7% 14% 26% 13% 3	3618
	21st to end	Night	37% 8% 14% 28% 12%	3972
June	1st to 10th	Night	29% 10% 23% 21% 15% 2	1600
	11th to 20th	Night	32% 8% 18% 23% 15% 5%	1600
	21st to end	Night	21% 6% 14% 28% 25% 7%	1600
July	1st to 10th	Night	23% 5% 12% 30% 23% 7%	3600
	11th to 20th	Night	24% 6% 9% 26% 28% 7%	3592
	21st to end	Night	26% 5% 10% 21% 29% 8%	3856
August	1st to 10th	Night	27% 6% 14% 22% 20% 11%	2715
	11th to 20th	Night	28% 6% 13% 27% 20% 7%	3014
	21st to end	Night	21% 5% 10% 22% 28% 16%	3597
September	1st to 10th	Night	21% 3 8% 28% 28% 13%	2000
	11th to 20th	Night	27% 5% 11% 25% 23% 10%	2000
	21st to end	Night	26% 4 8% 26% 27% 8%	2235
October	1st to 10th	Night	23% 3 8% 22% 29% 16%	3920
	11th to 20th	Night	30% 4 8% 26% 27% 4	3874
	21st to end	Night	32% 4 11% 31% 14% 8%	4336
November	1st to 10th	Night	33% 6% 16% 23% 16% 6%	2535
	11th to 20th	Night	36% 5% 13% 26% 17% 4	2731
	21st to end	Night	31% 6% 16% 26% 18% 3	2800
December	1st to 10th	Night	32% 5% 15% 27% 19% 2	4570
	11th to 20th	Night	36% 6% 14% 33% 10% 1	5182
	21st to end	Night	40% 4 8% 33% 13% 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	7% 9% 31% 36% 14% 3	300
	11th to 20th	Day	9% 11% 43% 24% 10% 3	300
	21st to end	Day	8% 10% 46% 25% 8% 3	330
February	1st to 10th	Day	10% 13% 51% 22% 4	300
	11th to 20th	Day	8% 12% 41% 32% 5% 1	300
	21st to end	Day	6% 10% 40% 36% 8%	249
March	1st to 10th	Day	7% 13% 35% 33% 9% 3	300
	11th to 20th	Day	9% 12% 34% 30% 13% 2	468
	21st to end	Day	17% 15% 29% 27% 10% 4	550
April	1st to 10th	Day	17% 11% 33% 28% 10% 2	500
	11th to 20th	Day	14% 19% 31% 25% 10%	492
	21st to end	Day	20% 10% 25% 26% 16% 3	540
May	1st to 10th	Day	16% 12% 24% 26% 16% 5%	703
	11th to 20th	Day	19% 6% 26% 26% 18% 5%	725
	21st to end	Day	16% 10% 28% 27% 16% 4	789
June	1st to 10th	Day	6% 7% 26% 31% 24% 6%	730
	11th to 20th	Day	8% 11% 31% 32% 15% 3	730
	21st to end	Day	6% 5% 29% 36% 21% 4	730
July	1st to 10th	Day	5% 3 21% 37% 30% 5%	721
	11th to 20th	Day	3 3 22% 46% 21% 5%	718
	21st to end	Day	5% 2 24% 35% 29% 6%	794
August	1st to 10th	Day	5% 3 24% 38% 26% 5%	660
	11th to 20th	Day	7% 3 22% 34% 24% 10%	517
	21st to end	Day	3 5% 26% 34% 24% 7%	583
September	1st to 10th	Day	6% 4 24% 38% 21% 8%	530
	11th to 20th	Day	10% 5% 23% 31% 23% 8%	530
	21st to end	Day	10% 7% 23% 32% 19% 9%	335
October	1st to 10th	Day	15% 6% 22% 27% 16% 14%	330
	11th to 20th	Day	15% 9% 31% 28% 12% 6%	324
	21st to end	Day	11% 12% 33% 28% 14% 3	330
November	1st to 10th	Day	12% 10% 34% 31% 9% 5%	300
	11th to 20th	Day	9% 12% 43% 23% 11% 2	300
	21st to end	Day	11% 10% 40% 29% 9% 1	300
December	1st to 10th	Day	7% 14% 40% 31% 8% 1	300
	11th to 20th	Day	7% 14% 45% 26% 7%	300
	21st to end	Day	8% 16% 34% 30% 11% 1	330
January	1st to 10th	Night	9% 12% 37% 31% 11% 1	1000
	11th to 20th	Night	8% 14% 45% 24% 9% 1	1000
	21st to end	Night	9% 12% 41% 30% 7%	1015
February	1st to 10th	Night	8% 13% 45% 26% 7% 1	800
	11th to 20th	Night	9% 10% 41% 33% 7% 1	784
	21st to end	Night	7% 10% 38% 33% 10% 1	660
March	1st to 10th	Night	8% 8% 41% 35% 8% 1	800
	11th to 20th	Night	9% 7% 38% 38% 9% 1	800
	21st to end	Night	13% 10% 32% 35% 10% 1	738
April	1st to 10th	Night	17% 9% 33% 35% 8%	600
	11th to 20th	Night	16% 12% 31% 33% 6% 1	593
	21st to end	Night	17% 8% 30% 34% 11%	600
May	1st to 10th	Night	13% 9% 25% 37% 16% 1	600
	11th to 20th	Night	17% 6% 25% 35% 16% 1	591
	21st to end	Night	16% 7% 32% 32% 13% 1	650
June	1st to 10th	Night	9% 7% 31% 39% 13% 1	600
	11th to 20th	Night	10% 7% 40% 35% 8%	600
	21st to end	Night	6% 5% 33% 44% 11%	600
July	1st to 10th	Night	7% 3 31% 44% 15%	585
	11th to 20th	Night	5% 3 35% 48% 10%	582
	21st to end	Night	4 2 28% 52% 15%	657
August	1st to 10th	Night	5% 4 25% 50% 16% 1	597
	11th to 20th	Night	6% 4 23% 48% 18% 1	594
	21st to end	Night	5% 3 31% 44% 16% 1	660
September	1st to 10th	Night	8% 5% 22% 48% 16% 1	600
	11th to 20th	Night	9% 5% 27% 40% 19% 1	600
	21st to end	Night	11% 5% 29% 38% 14% 2	640
October	1st to 10th	Night	16% 6% 27% 36% 13% 1	800
	11th to 20th	Night	14% 10% 30% 34% 11% 1	792
	21st to end	Night	11% 8% 35% 35% 11% 1	964
November	1st to 10th	Night	9% 8% 38% 34% 10% 1	975
	11th to 20th	Night	9% 12% 40% 30% 8% 1	1000
	21st to end	Night	9% 9% 40% 35% 8%	1000
December	1st to 10th	Night	6% 11% 43% 33% 7%	1000
	11th to 20th	Night	7% 9% 44% 34% 6% 1	1000
	21st to end	Night	10% 11% 42% 29% 8%	1100

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