Aim High Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Aim High Observatory clear sky chart from 2005-11-06 to 2024-11-10.

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	11% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	101030
	Night	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	98069
Transparency	Day	37% 7% 12% 19% 18% 7%	104849
	Night	37% 6% 11% 18% 19% 9%	102389
Seeing	Day	14% 9% 22% 25% 20% 10%	15051
	Night	13% 8% 22% 31% 22% 4	25062

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	16% 7% 2 6% 4 3 4 3 5% 7% 44%	2836
	11th to 20th	Day	12% 6% 2 4 3 2 3 2 4 7% 55%	2840
	21st to end	Day	19% 8% 3 5% 4 3 3 3 4 6% 43%	3256
February	1st to 10th	Day	15% 8% 2 6% 5% 5% 4 3 5% 6% 40%	1900
	11th to 20th	Day	18% 6% 2 6% 4 2 3 3 5% 7% 43%	1848
	21st to end	Day	16% 7% 3 5% 4 4 4 3 6% 7% 42%	1570
March	1st to 10th	Day	20% 9% 3 7% 4 4 3 3 5% 5% 36%	3302
	11th to 20th	Day	9% 4 2 5% 5% 4 4 4 6% 8% 49%	3480
	21st to end	Day	17% 5% 2 5% 5% 4 4 4 4 7% 43%	3815
April	1st to 10th	Day	11% 5% 2 7% 5% 4 4 4 5% 5% 48%	2090
	11th to 20th	Day	12% 5% 1 5% 3 3 4 3 6% 7% 50%	2207
	21st to end	Day	6% 4 1 4 4 4 4 4 5% 8% 54%	2441
May	1st to 10th	Day	8% 5% 2 6% 5% 5% 4 4 6% 8% 47%	4082
	11th to 20th	Day	14% 7% 3 7% 5% 4 5% 4 6% 8% 38%	4081
	21st to end	Day	6% 3 2 6% 4 5% 6% 5% 7% 9% 48%	4476
June	1st to 10th	Day	11% 5% 2 7% 5% 5% 8% 6% 7% 10% 33%	2470
	11th to 20th	Day	5% 5% 1 5% 5% 6% 7% 5% 8% 12% 41%	2470
	21st to end	Day	6% 5% 2 8% 6% 6% 6% 6% 8% 11% 36%	2470
July	1st to 10th	Day	11% 7% 3 10% 8% 7% 9% 7% 9% 9% 21%	4120
	11th to 20th	Day	4 5% 2 8% 7% 7% 8% 7% 9% 13% 29%	4120
	21st to end	Day	7% 7% 3 9% 8% 8% 8% 7% 10% 12% 21%	4466
August	1st to 10th	Day	12% 5% 2 6% 5% 7% 7% 6% 8% 13% 30%	4104
	11th to 20th	Day	6% 4 2 6% 4 5% 5% 4 7% 10% 48%	3798
	21st to end	Day	7% 5% 2 6% 4 4 6% 5% 8% 11% 42%	4091
September	1st to 10th	Day	11% 6% 2 7% 5% 4 5% 6% 7% 9% 39%	2090
	11th to 20th	Day	13% 6% 2 6% 4 5% 6% 5% 7% 8% 39%	2090
	21st to end	Day	13% 5% 2 8% 5% 5% 5% 4 6% 6% 43%	2090
October	1st to 10th	Day	12% 5% 2 4 4 3 4 3 5% 6% 52%	2090
	11th to 20th	Day	10% 4 1 4 3 3 3 3 5% 6% 59%	1967
	21st to end	Day	9% 4 2 5% 3 3 3 4 4 6% 57%	2090
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 3 6% 52%	1873
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 6% 53%	1710
	21st to end	Day	14% 6% 2 5% 4 4 4 4 5% 7% 46%	1710
December	1st to 10th	Day	16% 6% 3 5% 4 4 5% 3 5% 6% 43%	1710
	11th to 20th	Day	18% 6% 2 6% 3 4 4 3 4 5% 46%	2153
	21st to end	Day	18% 8% 3 7% 4 3 3 2 4 5% 43%	3124
January	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 6% 7% 42%	4109
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 5% 5% 8% 48%	4120
	21st to end	Night	19% 9% 3 6% 4 4 4 3 3 5% 41%	4395
February	1st to 10th	Night	14% 7% 3 6% 3 4 4 3 6% 5% 44%	2280
	11th to 20th	Night	18% 6% 1 5% 4 3 4 3 4 5% 47%	2218
	21st to end	Night	15% 6% 2 6% 4 3 4 3 6% 6% 44%	1884
March	1st to 10th	Night	22% 10% 3 6% 3 3 3 3 5% 5% 38%	3551
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 6% 8% 49%	3480
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 5% 6% 42%	3817
April	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 7% 49%	2090
	11th to 20th	Night	11% 4 1 5% 3 4 4 4 6% 8% 49%	1935
	21st to end	Night	6% 4 2 4 4 4 4 4 5% 7% 56%	1738
May	1st to 10th	Night	5% 4 2 7% 4 5% 4 5% 7% 11% 45%	2813
	11th to 20th	Night	13% 5% 2 7% 4 5% 5% 4 5% 8% 43%	2812
	21st to end	Night	6% 5% 2 5% 3 3 4 4 7% 9% 51%	3079
June	1st to 10th	Night	9% 6% 2 8% 6% 5% 5% 5% 9% 11% 33%	1710
	11th to 20th	Night	3 5% 2 5% 5% 5% 5% 5% 9% 12% 45%	1710
	21st to end	Night	5% 5% 2 8% 6% 6% 6% 6% 8% 11% 36%	1710
July	1st to 10th	Night	12% 7% 3 8% 6% 7% 8% 6% 9% 11% 23%	2840
	11th to 20th	Night	4 4 2 7% 6% 7% 8% 7% 11% 13% 30%	2840
	21st to end	Night	8% 8% 3 9% 7% 7% 8% 7% 11% 11% 22%	3087
August	1st to 10th	Night	12% 6% 3 8% 6% 5% 7% 5% 8% 11% 29%	2825
	11th to 20th	Night	6% 4 2 6% 4 4 5% 5% 7% 12% 44%	3026
	21st to end	Night	6% 4 2 5% 4 5% 4 4 7% 12% 48%	3530
September	1st to 10th	Night	9% 7% 2 7% 5% 5% 6% 4 7% 10% 36%	2090
	11th to 20th	Night	13% 6% 2 6% 4 6% 6% 4 6% 7% 41%	2090
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 6% 7% 43%	2090
October	1st to 10th	Night	11% 5% 2 4 3 4 4 3 5% 7% 52%	2090
	11th to 20th	Night	11% 5% 1 4 3 2 2 2 4 5% 60%	2213
	21st to end	Night	10% 4 1 5% 3 3 4 3 5% 6% 56%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 5% 5% 54%	2405
	11th to 20th	Night	13% 6% 2 5% 3 3 2 3 4 6% 54%	2470
	21st to end	Night	14% 5% 3 5% 5% 4 4 2 5% 6% 48%	2470
December	1st to 10th	Night	13% 6% 2 5% 3 4 4 4 5% 6% 47%	2470
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 5% 49%	3042
	21st to end	Night	18% 9% 3 7% 5% 4 4 3 4 6% 37%	4532

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	40% 4 5% 18% 24% 9%	2837
	11th to 20th	Day	31% 4 5% 17% 31% 13%	2840
	21st to end	Day	44% 3 5% 13% 23% 12%	3256
February	1st to 10th	Day	45% 5% 7% 15% 19% 9%	1900
	11th to 20th	Day	41% 4 6% 16% 24% 8%	1858
	21st to end	Day	41% 5% 7% 16% 20% 10%	1570
March	1st to 10th	Day	49% 5% 8% 15% 15% 8%	3302
	11th to 20th	Day	32% 5% 11% 16% 24% 12%	3480
	21st to end	Day	41% 5% 9% 19% 17% 9%	3815
April	1st to 10th	Day	36% 6% 9% 20% 18% 10%	2090
	11th to 20th	Day	33% 4 8% 20% 23% 12%	2207
	21st to end	Day	28% 5% 11% 24% 26% 6%	2441
May	1st to 10th	Day	33% 7% 13% 24% 19% 4	4120
	11th to 20th	Day	42% 7% 11% 23% 15% 2	4085
	21st to end	Day	28% 8% 17% 24% 21% 2	4489
June	1st to 10th	Day	42% 9% 16% 19% 12% 2	2470
	11th to 20th	Day	31% 9% 17% 24% 16% 2	2470
	21st to end	Day	38% 10% 18% 18% 13% 3	2470
July	1st to 10th	Day	52% 12% 20% 12% 3 1	4055
	11th to 20th	Day	40% 13% 21% 21% 3 1	4051
	21st to end	Day	50% 15% 18% 13% 4	4324
August	1st to 10th	Day	43% 9% 20% 24% 4	4091
	11th to 20th	Day	30% 7% 15% 33% 14% 1	3836
	21st to end	Day	33% 8% 17% 29% 12% 1	4091
September	1st to 10th	Day	39% 10% 16% 17% 15% 3	2090
	11th to 20th	Day	40% 8% 10% 19% 18% 5%	2090
	21st to end	Day	41% 6% 8% 20% 19% 6%	2090
October	1st to 10th	Day	33% 4 6% 14% 28% 15%	3480
	11th to 20th	Day	16% 2 5% 17% 26% 34%	3246
	21st to end	Day	20% 3 5% 17% 38% 16%	3428
November	1st to 10th	Day	35% 4 8% 13% 25% 15%	1873
	11th to 20th	Day	33% 4 6% 18% 25% 15%	1710
	21st to end	Day	37% 5% 8% 17% 23% 10%	1710
December	1st to 10th	Day	42% 5% 11% 15% 19% 9%	1710
	11th to 20th	Day	42% 4 5% 15% 26% 8%	2150
	21st to end	Day	46% 4 6% 16% 24% 4	3124
January	1st to 10th	Night	40% 5% 7% 15% 22% 11%	4109
	11th to 20th	Night	33% 4 7% 21% 25% 10%	4120
	21st to end	Night	46% 4 5% 10% 25% 9%	4395
February	1st to 10th	Night	41% 4 8% 17% 20% 11%	2280
	11th to 20th	Night	40% 4 6% 15% 22% 12%	2219
	21st to end	Night	39% 5% 9% 16% 17% 14%	1884
March	1st to 10th	Night	49% 5% 9% 14% 14% 8%	3551
	11th to 20th	Night	32% 5% 11% 13% 26% 12%	3480
	21st to end	Night	41% 6% 8% 16% 18% 11%	3817
April	1st to 10th	Night	34% 6% 10% 19% 20% 13%	2090
	11th to 20th	Night	33% 4 8% 21% 23% 12%	1935
	21st to end	Night	27% 5% 11% 24% 24% 9%	1738
May	1st to 10th	Night	29% 10% 15% 24% 18% 5%	2840
	11th to 20th	Night	37% 9% 14% 24% 14% 2	2813
	21st to end	Night	26% 7% 14% 27% 21% 3	3087
June	1st to 10th	Night	41% 10% 18% 17% 13%	1710
	11th to 20th	Night	29% 10% 19% 27% 14% 2	1710
	21st to end	Night	37% 12% 21% 14% 12% 4	1710
July	1st to 10th	Night	50% 14% 21% 11% 3 1	2795
	11th to 20th	Night	38% 12% 28% 18% 4	2786
	21st to end	Night	49% 17% 17% 12% 3 1	2988
August	1st to 10th	Night	45% 10% 20% 22% 3	2814
	11th to 20th	Night	30% 9% 18% 29% 12% 1	3054
	21st to end	Night	28% 7% 20% 31% 12% 1	3530
September	1st to 10th	Night	40% 10% 17% 15% 13% 4	2090
	11th to 20th	Night	41% 9% 11% 17% 18% 4	2090
	21st to end	Night	38% 6% 13% 16% 21% 6%	2090
October	1st to 10th	Night	34% 4 7% 13% 26% 16%	3480
	11th to 20th	Night	17% 2 4 12% 30% 34%	3673
	21st to end	Night	20% 3 4 19% 34% 20%	4120
November	1st to 10th	Night	33% 3 8% 14% 26% 15%	2405
	11th to 20th	Night	33% 3 6% 15% 27% 16%	2470
	21st to end	Night	38% 4 7% 18% 23% 10%	2470
December	1st to 10th	Night	37% 6% 10% 18% 23% 7%	2470
	11th to 20th	Night	39% 3 6% 14% 29% 9%	3044
	21st to end	Night	49% 4 6% 17% 20% 4	4532

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	18% 23% 43% 13% 2	270
	11th to 20th	Day	21% 19% 34% 17% 7% 1	270
	21st to end	Day	25% 24% 37% 11% 3	297
February	1st to 10th	Day	21% 22% 37% 14% 6%	270
	11th to 20th	Day	30% 16% 33% 20% 1	270
	21st to end	Day	24% 20% 43% 12% 3	225
March	1st to 10th	Day	17% 20% 30% 25% 7% 1	374
	11th to 20th	Day	16% 15% 38% 21% 9% 1	460
	21st to end	Day	21% 14% 34% 22% 8% 1	506
April	1st to 10th	Day	18% 16% 30% 23% 11% 2	460
	11th to 20th	Day	14% 15% 30% 24% 14% 3	452
	21st to end	Day	12% 10% 26% 30% 18% 5%	460
May	1st to 10th	Day	10% 11% 28% 29% 15% 7%	463
	11th to 20th	Day	10% 5% 20% 31% 21% 12%	487
	21st to end	Day	11% 4 17% 30% 25% 13%	529
June	1st to 10th	Day	12% 2 15% 29% 27% 14%	490
	11th to 20th	Day	8% 3 16% 25% 26% 23%	490
	21st to end	Day	7% 5% 11% 33% 27% 17%	490
July	1st to 10th	Day	8% 3 6% 24% 34% 25%	485
	11th to 20th	Day	7% 2 5% 20% 36% 31%	484
	21st to end	Day	5% 3 8% 26% 34% 24%	532
August	1st to 10th	Day	12% 3 9% 22% 33% 21%	486
	11th to 20th	Day	7% 3 8% 21% 39% 22%	479
	21st to end	Day	13% 3 7% 25% 34% 17%	539
September	1st to 10th	Day	13% 2 12% 25% 32% 15%	490
	11th to 20th	Day	16% 3 12% 33% 26% 11%	490
	21st to end	Day	14% 4 13% 29% 28% 12%	490
October	1st to 10th	Day	15% 4 24% 29% 18% 10%	490
	11th to 20th	Day	10% 7% 22% 33% 21% 7%	490
	21st to end	Day	11% 7% 27% 30% 22% 4	539
November	1st to 10th	Day	20% 6% 25% 29% 17% 3	417
	11th to 20th	Day	14% 16% 39% 21% 9% 1	270
	21st to end	Day	13% 15% 35% 23% 13% 1	270
December	1st to 10th	Day	21% 20% 34% 16% 7% 1	270
	11th to 20th	Day	24% 17% 37% 15% 8%	270
	21st to end	Day	28% 14% 29% 21% 7% 1	297
January	1st to 10th	Night	20% 15% 33% 26% 6% 1	760
	11th to 20th	Night	16% 14% 30% 28% 11%	760
	21st to end	Night	20% 14% 36% 25% 5%	836
February	1st to 10th	Night	18% 11% 32% 31% 7%	760
	11th to 20th	Night	22% 13% 33% 28% 4	743
	21st to end	Night	18% 14% 36% 28% 3	628
March	1st to 10th	Night	14% 13% 33% 33% 7%	760
	11th to 20th	Night	15% 13% 32% 28% 12%	760
	21st to end	Night	13% 14% 32% 31% 10%	836
April	1st to 10th	Night	14% 10% 32% 31% 13%	760
	11th to 20th	Night	13% 9% 30% 32% 15% 1	623
	21st to end	Night	7% 7% 27% 40% 18% 1	570
May	1st to 10th	Night	7% 9% 24% 37% 21% 2	570
	11th to 20th	Night	9% 4 24% 33% 25% 6%	561
	21st to end	Night	9% 2 13% 40% 31% 6%	617
June	1st to 10th	Night	10% 3 12% 37% 34% 4	570
	11th to 20th	Night	3 2 13% 33% 39% 11%	570
	21st to end	Night	7% 2 9% 34% 39% 8%	570
July	1st to 10th	Night	8% 3 6% 29% 39% 15%	555
	11th to 20th	Night	8% 1 3 22% 48% 17%	552
	21st to end	Night	7% 1 6% 30% 44% 12%	624
August	1st to 10th	Night	8% 1 6% 32% 40% 12%	567
	11th to 20th	Night	11% 2 4 26% 47% 10%	564
	21st to end	Night	10% 4 7% 29% 39% 12%	655
September	1st to 10th	Night	11% 3 8% 29% 40% 9%	760
	11th to 20th	Night	14% 3 10% 31% 36% 6%	760
	21st to end	Night	13% 2 13% 31% 36% 4	760
October	1st to 10th	Night	12% 4 19% 35% 25% 5%	760
	11th to 20th	Night	12% 6% 22% 32% 24% 3	760
	21st to end	Night	11% 6% 24% 33% 25% 2	836
November	1st to 10th	Night	15% 7% 20% 36% 21% 1	779
	11th to 20th	Night	15% 10% 27% 33% 14% 1	760
	21st to end	Night	16% 9% 29% 29% 15% 1	760
December	1st to 10th	Night	14% 11% 31% 29% 13% 1	760
	11th to 20th	Night	18% 9% 29% 31% 12% 1	760
	21st to end	Night	25% 10% 25% 31% 9%	836

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