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 2025-12-03.

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	11% 6% 2 6% 5% 4 5% 4 6% 8% 43%	126837
	Night	12% 6% 2 6% 4 4 5% 4 6% 8% 44%	123874
Transparency	Day	37% 6% 12% 19% 18% 7%	130612
	Night	36% 6% 11% 18% 20% 9%	128168
Seeing	Day	14% 9% 22% 25% 20% 10%	16462
	Night	13% 8% 22% 31% 21% 4	26474

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	15% 6% 2 5% 3 3 3 2 4 5% 53%	4056
	11th to 20th	Day	13% 6% 2 4 3 2 3 2 4 7% 54%	4060
	21st to end	Day	17% 7% 2 5% 4 3 3 3 5% 7% 44%	4668
February	1st to 10th	Day	15% 8% 2 6% 5% 4 5% 3 6% 6% 40%	2000
	11th to 20th	Day	19% 6% 2 6% 3 2 3 3 5% 7% 43%	1948
	21st to end	Day	15% 6% 3 5% 4 4 4 3 6% 7% 43%	1650
March	1st to 10th	Day	19% 8% 3 7% 4 4 4 3 5% 6% 38%	4746
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 5% 7% 53%	4980
	21st to end	Day	15% 6% 2 6% 4 4 4 3 4 6% 47%	5465
April	1st to 10th	Day	11% 5% 2 7% 5% 4 4 4 5% 6% 48%	2200
	11th to 20th	Day	12% 5% 1 5% 3 3 4 3 5% 7% 52%	2325
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 8% 53%	2570
May	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	5862
	11th to 20th	Day	14% 6% 3 6% 5% 4 5% 4 7% 9% 36%	5861
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 7% 9% 45%	6433
June	1st to 10th	Day	11% 5% 2 7% 5% 6% 8% 6% 7% 10% 32%	2600
	11th to 20th	Day	5% 5% 1 5% 5% 6% 7% 6% 8% 12% 40%	2600
	21st to end	Day	5% 5% 2 7% 6% 6% 6% 6% 8% 11% 36%	2600
July	1st to 10th	Day	9% 7% 2 9% 7% 7% 9% 7% 9% 9% 24%	5900
	11th to 20th	Day	3 4 2 6% 6% 6% 7% 6% 10% 14% 36%	5900
	21st to end	Day	6% 6% 3 8% 7% 7% 8% 7% 10% 13% 26%	6424
August	1st to 10th	Day	13% 6% 2 6% 5% 6% 7% 6% 8% 13% 29%	4234
	11th to 20th	Day	5% 4 2 6% 4 5% 5% 4 7% 10% 48%	3920
	21st to end	Day	7% 5% 2 6% 4 4 6% 5% 7% 11% 42%	4220
September	1st to 10th	Day	10% 6% 2 7% 5% 4 5% 6% 7% 9% 40%	2200
	11th to 20th	Day	12% 6% 2 6% 4 4 6% 5% 7% 8% 41%	2200
	21st to end	Day	13% 6% 2 8% 5% 5% 5% 3 6% 6% 43%	2200
October	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 6% 50%	2200
	11th to 20th	Day	9% 4 1 4 3 3 3 3 5% 6% 60%	2070
	21st to end	Day	10% 5% 2 5% 3 3 3 4 5% 6% 56%	2200
November	1st to 10th	Day	12% 6% 2 5% 4 3 3 3 3 6% 52%	1978
	11th to 20th	Day	12% 4 2 5% 3 3 3 3 5% 5% 55%	1890
	21st to end	Day	13% 6% 2 5% 4 4 4 4 5% 7% 47%	1890
December	1st to 10th	Day	13% 6% 3 5% 3 3 4 3 4 5% 51%	2948
	11th to 20th	Day	16% 6% 2 6% 3 4 4 3 4 5% 47%	3373
	21st to end	Day	20% 8% 2 6% 3 3 3 2 4 5% 43%	4466
January	1st to 10th	Night	14% 5% 2 4 3 3 3 3 5% 6% 53%	5889
	11th to 20th	Night	12% 6% 2 6% 4 5% 5% 4 5% 7% 45%	5900
	21st to end	Night	17% 8% 3 6% 3 3 3 3 4 7% 43%	6283
February	1st to 10th	Night	14% 7% 3 6% 3 4 4 3 6% 5% 45%	2400
	11th to 20th	Night	19% 6% 1 5% 4 3 4 3 4 5% 46%	2338
	21st to end	Night	14% 6% 2 6% 4 3 4 3 6% 6% 45%	1980
March	1st to 10th	Night	19% 8% 2 6% 3 4 4 3 5% 6% 40%	5107
	11th to 20th	Night	9% 5% 2 4 3 4 4 4 6% 8% 52%	4980
	21st to end	Night	13% 6% 2 5% 4 4 4 4 5% 6% 47%	5467
April	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 7% 49%	2200
	11th to 20th	Night	11% 4 2 5% 3 4 3 4 5% 8% 51%	2038
	21st to end	Night	6% 4 2 5% 4 4 5% 4 5% 8% 54%	1829
May	1st to 10th	Night	8% 6% 2 7% 5% 5% 4 5% 7% 11% 40%	4033
	11th to 20th	Night	15% 5% 2 6% 4 4 5% 4 7% 9% 38%	4032
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 8% 10% 46%	4421
June	1st to 10th	Night	9% 6% 2 8% 6% 5% 6% 5% 8% 11% 34%	1800
	11th to 20th	Night	4 5% 2 5% 5% 5% 6% 5% 8% 12% 44%	1800
	21st to end	Night	5% 5% 2 8% 6% 5% 7% 6% 8% 12% 37%	1800
July	1st to 10th	Night	11% 7% 4 9% 6% 7% 8% 6% 9% 11% 23%	4058
	11th to 20th	Night	3 4 2 6% 6% 7% 8% 7% 10% 14% 33%	4057
	21st to end	Night	7% 7% 4 9% 6% 7% 8% 6% 10% 12% 25%	4427
August	1st to 10th	Night	13% 7% 3 8% 5% 5% 7% 5% 8% 10% 29%	2915
	11th to 20th	Night	6% 4 2 6% 4 4 5% 5% 7% 12% 44%	3124
	21st to end	Night	6% 4 2 5% 4 4 4 4 7% 11% 47%	3642
September	1st to 10th	Night	9% 7% 2 7% 5% 5% 6% 4 7% 11% 37%	2200
	11th to 20th	Night	12% 6% 2 6% 4 6% 6% 4 6% 7% 43%	2200
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	2200
October	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 51%	2200
	11th to 20th	Night	11% 4 1 4 3 2 2 2 4 5% 61%	2330
	21st to end	Night	10% 4 1 5% 3 3 4 3 5% 5% 56%	2640
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 55%	2535
	11th to 20th	Night	13% 6% 2 4 3 3 2 2 4 6% 56%	2730
	21st to end	Night	13% 5% 3 5% 5% 4 4 3 5% 6% 48%	2730
December	1st to 10th	Night	12% 6% 2 4 3 3 3 3 4 5% 55%	4277
	11th to 20th	Night	15% 6% 2 5% 4 3 3 3 4 5% 50%	4822
	21st to end	Night	19% 8% 3 6% 4 4 4 3 4 5% 43%	6490

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	35% 3 4 17% 33% 9%	4057
	11th to 20th	Day	32% 3 4 16% 32% 13%	4060
	21st to end	Day	42% 3 7% 16% 21% 11%	4668
February	1st to 10th	Day	44% 5% 8% 16% 20% 9%	2000
	11th to 20th	Day	42% 4 6% 17% 24% 8%	1958
	21st to end	Day	40% 5% 7% 17% 21% 11%	1650
March	1st to 10th	Day	46% 5% 6% 13% 21% 8%	4746
	11th to 20th	Day	29% 4 9% 15% 21% 22%	4980
	21st to end	Day	40% 4 7% 18% 19% 12%	5465
April	1st to 10th	Day	36% 6% 9% 21% 18% 10%	2200
	11th to 20th	Day	32% 4 8% 21% 24% 12%	2325
	21st to end	Day	29% 5% 11% 24% 24% 6%	2570
May	1st to 10th	Day	39% 6% 15% 22% 16% 3	5897
	11th to 20th	Day	42% 6% 18% 21% 11% 1	5865
	21st to end	Day	33% 8% 16% 22% 21% 1	6447
June	1st to 10th	Day	42% 9% 16% 19% 12% 2	2600
	11th to 20th	Day	32% 9% 18% 23% 16% 2	2600
	21st to end	Day	38% 10% 18% 19% 13% 2	2600
July	1st to 10th	Day	49% 11% 20% 16% 3 1	5835
	11th to 20th	Day	33% 11% 29% 23% 3 1	5831
	21st to end	Day	45% 12% 21% 18% 3	6240
August	1st to 10th	Day	44% 9% 19% 24% 3	4221
	11th to 20th	Day	30% 7% 16% 33% 13% 1	3958
	21st to end	Day	34% 8% 17% 29% 12% 1	4220
September	1st to 10th	Day	38% 9% 16% 18% 15% 3	2200
	11th to 20th	Day	38% 8% 10% 21% 18% 5%	2200
	21st to end	Day	41% 6% 8% 20% 18% 6%	2200
October	1st to 10th	Day	33% 4 6% 14% 28% 15%	3590
	11th to 20th	Day	16% 2 5% 16% 27% 33%	3349
	21st to end	Day	21% 3 5% 17% 38% 16%	3538
November	1st to 10th	Day	34% 4 8% 13% 26% 15%	1978
	11th to 20th	Day	31% 4 6% 17% 26% 15%	1890
	21st to end	Day	36% 4 8% 17% 23% 10%	1890
December	1st to 10th	Day	37% 3 7% 16% 26% 10%	2948
	11th to 20th	Day	40% 3 6% 22% 22% 7%	3370
	21st to end	Day	44% 3 5% 20% 24% 3	4466
January	1st to 10th	Night	33% 4 6% 17% 32% 9%	5889
	11th to 20th	Night	38% 4 5% 18% 26% 10%	5900
	21st to end	Night	42% 3 5% 14% 26% 9%	6283
February	1st to 10th	Night	40% 4 7% 18% 21% 10%	2400
	11th to 20th	Night	41% 4 6% 15% 21% 11%	2339
	21st to end	Night	38% 5% 8% 16% 18% 15%	1980
March	1st to 10th	Night	45% 4 8% 15% 16% 12%	5107
	11th to 20th	Night	29% 5% 9% 13% 24% 20%	4980
	21st to end	Night	37% 4 7% 17% 21% 13%	5467
April	1st to 10th	Night	34% 5% 10% 19% 19% 13%	2200
	11th to 20th	Night	32% 4 8% 21% 24% 12%	2038
	21st to end	Night	28% 5% 11% 24% 23% 9%	1829
May	1st to 10th	Night	36% 8% 16% 22% 15% 3	4060
	11th to 20th	Night	40% 8% 18% 23% 10% 2	4033
	21st to end	Night	31% 6% 16% 26% 19% 2	4429
June	1st to 10th	Night	41% 10% 18% 17% 14% 1	1800
	11th to 20th	Night	30% 10% 19% 26% 13% 2	1800
	21st to end	Night	36% 12% 21% 16% 12% 4	1800
July	1st to 10th	Night	50% 12% 21% 13% 3 1	4015
	11th to 20th	Night	36% 11% 31% 18% 3	4006
	21st to end	Night	48% 14% 21% 15% 2	4297
August	1st to 10th	Night	46% 10% 19% 22% 3	2904
	11th to 20th	Night	31% 9% 19% 28% 12% 1	3152
	21st to end	Night	29% 7% 20% 31% 12% 1	3642
September	1st to 10th	Night	39% 10% 17% 17% 13% 4	2200
	11th to 20th	Night	39% 9% 11% 18% 19% 4	2200
	21st to end	Night	38% 6% 13% 17% 20% 6%	2200
October	1st to 10th	Night	35% 4 7% 14% 25% 16%	3590
	11th to 20th	Night	17% 2 4 12% 30% 34%	3790
	21st to end	Night	20% 3 4 19% 34% 20%	4252
November	1st to 10th	Night	32% 3 8% 14% 28% 15%	2535
	11th to 20th	Night	31% 3 6% 16% 28% 15%	2730
	21st to end	Night	37% 4 7% 18% 23% 11%	2730
December	1st to 10th	Night	33% 3 7% 19% 29% 9%	4277
	11th to 20th	Night	38% 3 10% 18% 23% 8%	4824
	21st to end	Night	45% 3 5% 20% 24% 3	6490

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	19% 23% 44% 12% 2	300
	11th to 20th	Day	21% 20% 33% 17% 7% 1	300
	21st to end	Day	25% 22% 37% 13% 3	330
February	1st to 10th	Day	19% 20% 39% 15% 6%	300
	11th to 20th	Day	32% 16% 32% 19% 1	300
	21st to end	Day	22% 19% 44% 12% 3	249
March	1st to 10th	Day	19% 21% 30% 23% 7% 1	410
	11th to 20th	Day	16% 16% 37% 21% 9% 1	500
	21st to end	Day	21% 14% 35% 22% 8% 1	550
April	1st to 10th	Day	18% 16% 30% 24% 11% 1	500
	11th to 20th	Day	13% 14% 29% 26% 14% 3	492
	21st to end	Day	12% 9% 25% 30% 18% 5%	500
May	1st to 10th	Day	9% 11% 28% 30% 15% 7%	503
	11th to 20th	Day	11% 6% 22% 31% 19% 11%	527
	21st to end	Day	12% 5% 18% 29% 24% 12%	573
June	1st to 10th	Day	12% 3 17% 29% 26% 13%	530
	11th to 20th	Day	7% 2 15% 27% 27% 22%	530
	21st to end	Day	6% 4 11% 31% 28% 20%	530
July	1st to 10th	Day	7% 3 6% 24% 34% 26%	525
	11th to 20th	Day	6% 2 5% 19% 38% 30%	524
	21st to end	Day	5% 2 7% 25% 35% 25%	576
August	1st to 10th	Day	15% 3 9% 22% 31% 20%	526
	11th to 20th	Day	7% 3 7% 21% 39% 23%	519
	21st to end	Day	14% 4 9% 26% 32% 17%	583
September	1st to 10th	Day	13% 2 12% 27% 31% 15%	530
	11th to 20th	Day	15% 2 12% 35% 26% 11%	530
	21st to end	Day	15% 4 12% 30% 28% 11%	530
October	1st to 10th	Day	16% 5% 24% 29% 18% 9%	530
	11th to 20th	Day	11% 7% 21% 32% 23% 7%	530
	21st to end	Day	12% 7% 28% 30% 20% 3	583
November	1st to 10th	Day	20% 5% 25% 30% 17% 3	456
	11th to 20th	Day	14% 14% 38% 22% 11% 2	330
	21st to end	Day	13% 18% 34% 23% 12% 1	330
December	1st to 10th	Day	21% 20% 36% 15% 7% 1	306
	11th to 20th	Day	23% 17% 36% 15% 8% 1	300
	21st to end	Day	29% 13% 28% 22% 7% 1	330
January	1st to 10th	Night	20% 15% 34% 26% 5% 1	800
	11th to 20th	Night	16% 14% 30% 28% 11%	800
	21st to end	Night	20% 13% 37% 25% 4	880
February	1st to 10th	Night	18% 11% 33% 31% 7%	800
	11th to 20th	Night	22% 14% 33% 28% 3	783
	21st to end	Night	18% 14% 36% 28% 4	660
March	1st to 10th	Night	15% 13% 34% 32% 7%	800
	11th to 20th	Night	14% 14% 33% 28% 11%	800
	21st to end	Night	13% 14% 33% 31% 9%	880
April	1st to 10th	Night	14% 11% 32% 31% 12%	800
	11th to 20th	Night	12% 9% 31% 32% 15% 1	656
	21st to end	Night	7% 6% 26% 41% 19% 1	600
May	1st to 10th	Night	8% 8% 24% 38% 21% 2	600
	11th to 20th	Night	9% 4 24% 33% 24% 6%	591
	21st to end	Night	10% 2 14% 39% 30% 5%	650
June	1st to 10th	Night	10% 3 14% 37% 33% 4	600
	11th to 20th	Night	5% 2 13% 32% 39% 10%	600
	21st to end	Night	7% 2 9% 33% 41% 8%	600
July	1st to 10th	Night	8% 2 5% 29% 39% 16%	585
	11th to 20th	Night	7% 1 3 22% 48% 18%	582
	21st to end	Night	7% 2 6% 30% 44% 12%	657
August	1st to 10th	Night	10% 1 6% 32% 39% 11%	597
	11th to 20th	Night	11% 2 4 25% 47% 10%	594
	21st to end	Night	11% 4 7% 28% 37% 12%	690
September	1st to 10th	Night	11% 3 8% 32% 38% 9%	800
	11th to 20th	Night	14% 3 10% 33% 36% 6%	800
	21st to end	Night	14% 2 13% 32% 36% 4	800
October	1st to 10th	Night	12% 4 20% 35% 25% 5%	800
	11th to 20th	Night	12% 6% 22% 31% 25% 3	800
	21st to end	Night	11% 6% 24% 33% 24% 2	880
November	1st to 10th	Night	15% 7% 20% 36% 21% 1	820
	11th to 20th	Night	15% 9% 27% 33% 15% 1	840
	21st to end	Night	15% 10% 30% 30% 15% 1	840
December	1st to 10th	Night	15% 11% 31% 29% 12% 1	809
	11th to 20th	Night	18% 9% 29% 32% 12% 1	800
	21st to end	Night	25% 10% 25% 31% 9%	880

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