Grants
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grants clear sky chart from 2005-11-06 to 2024-11-03.

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	2 4 2 5% 4 4 5% 4 6% 7% 57%	100588
	Night	2 4 2 5% 4 4 4 4 6% 7% 59%	98608
Transparency	Day	24% 3 7% 20% 27% 18%	100351
	Night	23% 3 8% 21% 28% 17%	98434
Seeing	Day	7% 11% 23% 28% 22% 10%	14558
	Night	4 7% 22% 40% 24% 3	25024

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% 2 9% 3 3 4 3 6% 5% 57%	2858
	11th to 20th	Day	1 2 1 4 3 3 4 4 6% 7% 65%	2860
	21st to end	Day	3 7% 3 8% 4 5% 5% 4 5% 8% 49%	3146
February	1st to 10th	Day	1 6% 3 5% 3 4 3 3 5% 6% 61%	1710
	11th to 20th	Day	2 6% 2 5% 4 3 5% 3 5% 9% 56%	1759
	21st to end	Day	1 5% 2 5% 4 4 5% 4 7% 6% 58%	1627
March	1st to 10th	Day	1 3 2 5% 3 4 4 4 6% 8% 61%	3444
	11th to 20th	Day	2 8% 3 9% 6% 5% 6% 5% 6% 7% 43%	3490
	21st to end	Day	1 4 2 6% 5% 4 5% 4 6% 8% 55%	3825
April	1st to 10th	Day	1 4 1 3 4 3 4 4 6% 6% 64%	2259
	11th to 20th	Day	2 4 2 4 4 4 4 4 5% 6% 62%	2251
	21st to end	Day	3 3 1 4 3 3 4 4 5% 6% 64%	2280
May	1st to 10th	Day	1 2 1 3 2 3 3 3 4 5% 71%	3847
	11th to 20th	Day	2 4 2 5% 5% 4 5% 5% 7% 7% 54%	4071
	21st to end	Day	1 2 1 3 2 3 3 2 3 5% 74%	4477
June	1st to 10th	Day	2 2 1 3 3 3 5% 4 5% 8% 63%	2521
	11th to 20th	Day	1 2 1 3 3 3 3 3 4 6% 70%	2589
	21st to end	Day	3 4 2 5% 4 5% 5% 5% 7% 7% 53%	2470
July	1st to 10th	Day	2 3 2 5% 4 5% 5% 4 6% 7% 58%	4110
	11th to 20th	Day	2 3 1 5% 5% 6% 8% 7% 10% 11% 41%	4110
	21st to end	Day	3 4 2 6% 6% 6% 7% 7% 10% 11% 39%	4454
August	1st to 10th	Day	3 4 2 7% 7% 6% 7% 7% 10% 11% 37%	4092
	11th to 20th	Day	3 3 2 5% 4 5% 6% 5% 8% 9% 49%	3794
	21st to end	Day	4 4 2 7% 7% 7% 6% 6% 8% 10% 39%	4180
September	1st to 10th	Day	5% 4 1 7% 5% 6% 5% 4 6% 7% 49%	2280
	11th to 20th	Day	4 5% 2 5% 4 4 5% 4 6% 6% 56%	2248
	21st to end	Day	3 3 1 4 4 3 3 3 5% 7% 65%	2014
October	1st to 10th	Day	4 4 2 4 4 3 4 4 4 5% 61%	1900
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 4 73%	1900
	21st to end	Day	2 2 1 3 2 2 3 2 4 6% 74%	2090
November	1st to 10th	Day	2 2 1 4 2 2 2 2 5% 6% 73%	1879
	11th to 20th	Day	1 2 1 3 3 3 3 3 4 7% 71%	1868
	21st to end	Day	2 4 2 5% 5% 4 3 3 4 5% 63%	1621
December	1st to 10th	Day	3 5% 1 5% 3 3 4 3 5% 6% 63%	1520
	11th to 20th	Day	2 6% 2 5% 4 5% 5% 3 7% 9% 53%	1926
	21st to end	Day	1 4 2 5% 4 5% 4 4 6% 7% 59%	3118
January	1st to 10th	Night	1 5% 2 7% 4 4 4 4 6% 8% 56%	4097
	11th to 20th	Night	1 2 1 3 3 2 3 3 5% 6% 70%	4110
	21st to end	Night	3 8% 3 7% 4 4 5% 4 6% 8% 49%	4521
February	1st to 10th	Night	1 4 2 5% 3 3 3 3 5% 8% 64%	2470
	11th to 20th	Night	2 5% 2 5% 4 4 4 3 5% 7% 59%	2306
	21st to end	Night	1 4 1 4 3 3 4 3 5% 8% 66%	1822
March	1st to 10th	Night	1 2 1 4 3 2 3 2 4 6% 71%	3430
	11th to 20th	Night	4 8% 3 9% 5% 4 4 3 6% 5% 48%	3490
	21st to end	Night	1 3 2 4 3 3 3 4 7% 8% 64%	3828
April	1st to 10th	Night	1 3 1 3 3 3 3 4 5% 6% 67%	1919
	11th to 20th	Night	3 4 1 3 3 3 3 3 5% 6% 65%	1877
	21st to end	Night	1 3 1 3 3 3 3 3 5% 5% 69%	1900
May	1st to 10th	Night	2 3 1 3 2 2 2 2 4 5% 74%	3119
	11th to 20th	Night	3 4 1 5% 4 4 4 4 5% 6% 59%	2834
	21st to end	Night	1 1 1 2 2 2 2 2 3 4 81%	3110
June	1st to 10th	Night	2 3 2 3 3 3 3 3 7% 8% 62%	1639
	11th to 20th	Night	3 2 1 3 2 2 3 3 4 5% 70%	1569
	21st to end	Night	3 4 2 6% 5% 4 5% 6% 7% 8% 51%	1710
July	1st to 10th	Night	3 3 1 5% 4 4 4 4 6% 6% 59%	2860
	11th to 20th	Night	2 4 2 6% 5% 6% 8% 7% 10% 12% 39%	2860
	21st to end	Night	3 4 2 7% 6% 7% 7% 7% 9% 12% 35%	3109
August	1st to 10th	Night	2 3 2 9% 7% 8% 8% 7% 10% 13% 31%	2847
	11th to 20th	Night	3 4 2 8% 7% 6% 7% 6% 9% 10% 38%	3042
	21st to end	Night	3 3 2 7% 5% 7% 8% 7% 11% 10% 36%	3487
September	1st to 10th	Night	5% 5% 2 7% 4 4 5% 4 6% 8% 50%	1900
	11th to 20th	Night	3 3 2 5% 4 4 4 3 5% 7% 59%	1928
	21st to end	Night	2 2 1 4 3 3 3 3 4 6% 67%	2166
October	1st to 10th	Night	4 4 1 4 4 3 4 3 4 6% 63%	2280
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 76%	2280
	21st to end	Night	2 3 1 3 3 3 2 2 4 4 75%	2508
November	1st to 10th	Night	2 2 1 3 2 3 3 2 4 6% 74%	2244
	11th to 20th	Night	1 1 1 3 2 2 3 2 5% 6% 74%	2288
	21st to end	Night	3 4 1 4 4 4 3 3 4 6% 63%	2555
December	1st to 10th	Night	2 3 1 4 2 3 3 2 5% 6% 68%	2660
	11th to 20th	Night	3 5% 2 5% 4 4 4 3 5% 8% 57%	3271
	21st to end	Night	2 5% 2 5% 4 4 4 3 5% 8% 59%	4572

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	27% 3 6% 31% 27% 7%	2859
	11th to 20th	Day	18% 1 11% 38% 25% 6%	2860
	21st to end	Day	31% 2 7% 25% 28% 7%	3146
February	1st to 10th	Day	22% 4 7% 24% 29% 15%	1710
	11th to 20th	Day	25% 2 4 26% 29% 14%	1768
	21st to end	Day	21% 4 3 21% 31% 20%	1627
March	1st to 10th	Day	19% 4 2 21% 37% 17%	3444
	11th to 20th	Day	37% 4 3 21% 23% 10%	3490
	21st to end	Day	22% 5% 4 20% 39% 10%	3825
April	1st to 10th	Day	18% 3 4 19% 36% 20%	2259
	11th to 20th	Day	21% 3 2 16% 35% 23%	2254
	21st to end	Day	19% 3 4 19% 36% 19%	2280
May	1st to 10th	Day	14% 3 2 14% 39% 28%	3847
	11th to 20th	Day	25% 5% 5% 15% 31% 19%	4074
	21st to end	Day	12% 3 3 8% 25% 49%	4477
June	1st to 10th	Day	18% 3 4 16% 29% 30%	2521
	11th to 20th	Day	16% 3 2 9% 24% 47%	2589
	21st to end	Day	28% 4 5% 15% 22% 27%	2470
July	1st to 10th	Day	24% 5% 7% 11% 18% 36%	4046
	11th to 20th	Day	30% 4 14% 31% 16% 6%	4041
	21st to end	Day	33% 5% 17% 30% 12% 2	4314
August	1st to 10th	Day	37% 4 20% 19% 12% 8%	4078
	11th to 20th	Day	28% 3 12% 26% 19% 12%	3831
	21st to end	Day	37% 5% 14% 21% 18% 5%	4179
September	1st to 10th	Day	32% 4 8% 17% 23% 16%	2280
	11th to 20th	Day	28% 4 7% 15% 25% 21%	2248
	21st to end	Day	21% 2 4 17% 31% 24%	2014
October	1st to 10th	Day	25% 3 5% 19% 24% 24%	1900
	11th to 20th	Day	17% 2 2 13% 34% 33%	1900
	21st to end	Day	13% 2 5% 20% 40% 20%	2090
November	1st to 10th	Day	12% 2 3 24% 37% 21%	1879
	11th to 20th	Day	15% 2 4 22% 40% 17%	1868
	21st to end	Day	24% 1 3 18% 36% 17%	1621
December	1st to 10th	Day	21% 3 6% 30% 27% 13%	1520
	11th to 20th	Day	27% 3 8% 26% 28% 9%	1924
	21st to end	Day	23% 3 7% 22% 27% 18%	3118
January	1st to 10th	Night	25% 3 8% 30% 29% 7%	4097
	11th to 20th	Night	15% 1 10% 36% 31% 6%	4110
	21st to end	Night	32% 2 7% 26% 26% 6%	4521
February	1st to 10th	Night	19% 2 8% 25% 32% 14%	2470
	11th to 20th	Night	22% 3 5% 26% 29% 16%	2307
	21st to end	Night	17% 2 4 21% 38% 17%	1822
March	1st to 10th	Night	15% 3 2 23% 41% 16%	3430
	11th to 20th	Night	36% 3 4 22% 26% 9%	3490
	21st to end	Night	17% 2 4 29% 39% 8%	3828
April	1st to 10th	Night	17% 2 3 23% 35% 20%	1919
	11th to 20th	Night	17% 3 3 17% 34% 26%	1877
	21st to end	Night	15% 3 5% 21% 37% 19%	1900
May	1st to 10th	Night	12% 4 3 11% 44% 27%	3119
	11th to 20th	Night	23% 4 7% 17% 29% 20%	2835
	21st to end	Night	9% 3 4 8% 26% 51%	3110
June	1st to 10th	Night	19% 3 4 18% 26% 30%	1639
	11th to 20th	Night	16% 2 2 10% 22% 47%	1569
	21st to end	Night	29% 4 9% 14% 16% 28%	1710
July	1st to 10th	Night	26% 5% 8% 12% 15% 34%	2815
	11th to 20th	Night	32% 5% 17% 28% 12% 6%	2806
	21st to end	Night	38% 6% 24% 21% 9% 2	3011
August	1st to 10th	Night	41% 5% 21% 15% 10% 7%	2838
	11th to 20th	Night	38% 4 17% 16% 12% 12%	3071
	21st to end	Night	38% 6% 20% 19% 12% 6%	3487
September	1st to 10th	Night	32% 4 11% 19% 19% 16%	1900
	11th to 20th	Night	24% 4 9% 16% 24% 22%	1928
	21st to end	Night	19% 2 6% 15% 31% 26%	2166
October	1st to 10th	Night	22% 4 6% 19% 26% 23%	2280
	11th to 20th	Night	14% 2 2 14% 35% 32%	2280
	21st to end	Night	15% 2 4 19% 41% 19%	2508
November	1st to 10th	Night	13% 2 2 26% 40% 18%	2244
	11th to 20th	Night	12% 2 5% 25% 38% 19%	2288
	21st to end	Night	21% 2 4 21% 37% 16%	2555
December	1st to 10th	Night	18% 2 8% 30% 30% 12%	2660
	11th to 20th	Night	25% 2 9% 25% 29% 9%	3272
	21st to end	Night	21% 4 6% 23% 30% 15%	4572

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% 11% 29% 30% 18% 3	270
	11th to 20th	Day	8% 12% 28% 31% 16% 5%	270
	21st to end	Day	5% 10% 27% 31% 18% 8%	297
February	1st to 10th	Day	6% 16% 27% 32% 18% 2	270
	11th to 20th	Day	10% 11% 28% 37% 13% 1	270
	21st to end	Day	12% 22% 29% 23% 12% 1	282
March	1st to 10th	Day	12% 21% 30% 23% 13% 2	460
	11th to 20th	Day	13% 19% 30% 25% 12% 2	460
	21st to end	Day	13% 26% 32% 18% 8% 2	506
April	1st to 10th	Day	11% 19% 34% 21% 11% 4	460
	11th to 20th	Day	13% 21% 34% 18% 11% 3	452
	21st to end	Day	11% 18% 33% 25% 11% 2	460
May	1st to 10th	Day	6% 17% 38% 25% 11% 3	463
	11th to 20th	Day	5% 14% 32% 26% 15% 8%	488
	21st to end	Day	7% 14% 30% 29% 16% 4	529
June	1st to 10th	Day	5% 6% 21% 33% 28% 7%	541
	11th to 20th	Day	4 7% 20% 34% 26% 9%	609
	21st to end	Day	7% 4 22% 35% 22% 9%	490
July	1st to 10th	Day	5% 2 13% 31% 33% 16%	486
	11th to 20th	Day	5% 6% 24% 36% 29%	484
	21st to end	Day	6% 2 6% 23% 36% 28%	531
August	1st to 10th	Day	5% 1 8% 27% 36% 22%	487
	11th to 20th	Day	5% 3 9% 29% 31% 24%	477
	21st to end	Day	6% 2 10% 30% 31% 21%	539
September	1st to 10th	Day	7% 3 13% 29% 33% 16%	490
	11th to 20th	Day	5% 4 19% 32% 28% 11%	490
	21st to end	Day	5% 8% 21% 29% 24% 14%	414
October	1st to 10th	Day	4 9% 20% 26% 26% 16%	300
	11th to 20th	Day	7% 8% 21% 28% 24% 12%	300
	21st to end	Day	8% 9% 22% 25% 26% 10%	330
November	1st to 10th	Day	5% 12% 25% 35% 20% 3	276
	11th to 20th	Day	6% 7% 17% 37% 27% 7%	270
	21st to end	Day	9% 13% 27% 26% 19% 6%	270
December	1st to 10th	Day	7% 12% 21% 34% 23% 3	270
	11th to 20th	Day	6% 17% 26% 33% 14% 5%	270
	21st to end	Day	9% 14% 30% 31% 13% 3	297
January	1st to 10th	Night	3 8% 25% 42% 20% 3	760
	11th to 20th	Night	4 9% 27% 40% 17% 2	760
	21st to end	Night	6% 9% 24% 42% 18% 2	836
February	1st to 10th	Night	4 9% 29% 41% 15% 2	760
	11th to 20th	Night	5% 10% 32% 43% 10% 1	744
	21st to end	Night	3 14% 35% 41% 7%	628
March	1st to 10th	Night	4 12% 30% 40% 13% 1	760
	11th to 20th	Night	6% 10% 25% 41% 17% 1	760
	21st to end	Night	5% 12% 32% 38% 13%	836
April	1st to 10th	Night	4 16% 30% 41% 9% 1	589
	11th to 20th	Night	5% 18% 31% 35% 11%	563
	21st to end	Night	3 13% 34% 39% 10% 1	570
May	1st to 10th	Night	4 9% 31% 45% 11%	570
	11th to 20th	Night	4 7% 27% 40% 21% 2	561
	21st to end	Night	3 5% 30% 44% 18% 1	617
June	1st to 10th	Night	3 3 18% 52% 22% 1	570
	11th to 20th	Night	4 3 22% 49% 22% 1	570
	21st to end	Night	3 3 15% 42% 34% 3	570
July	1st to 10th	Night	6% 1 8% 37% 42% 5%	555
	11th to 20th	Night	4 6% 24% 57% 9%	552
	21st to end	Night	5% 1 5% 21% 55% 12%	624
August	1st to 10th	Night	5% 5% 24% 54% 12%	567
	11th to 20th	Night	6% 1 5% 30% 51% 7%	564
	21st to end	Night	4 6% 29% 54% 7%	627
September	1st to 10th	Night	6% 1 7% 36% 45% 4	570
	11th to 20th	Night	5% 1 11% 42% 38% 4	598
	21st to end	Night	3 2 13% 41% 36% 5%	760
October	1st to 10th	Night	4 3 19% 43% 27% 3	760
	11th to 20th	Night	3 4 18% 50% 23% 3	760
	21st to end	Night	4 6% 21% 40% 24% 5%	836
November	1st to 10th	Night	2 4 18% 48% 25% 3	748
	11th to 20th	Night	3 6% 20% 43% 25% 4	768
	21st to end	Night	6% 12% 22% 35% 22% 3	950
December	1st to 10th	Night	6% 9% 22% 45% 16% 2	950
	11th to 20th	Night	6% 9% 29% 39% 16% 1	950
	21st to end	Night	5% 9% 29% 43% 12% 1	861

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