Maxwell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Maxwell Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	14% 7% 3 6% 4 4 5% 4 6% 8% 39%	138100
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 42%	134924
Transparency	Day	41% 7% 11% 21% 17% 4	141849
	Night	40% 6% 10% 21% 19% 5%	139248
Seeing	Day	16% 11% 23% 23% 18% 9%	16855
	Night	16% 9% 21% 29% 21% 4	26823

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	22% 9% 5% 5% 3 3 3 3 4 5% 39%	5258
	11th to 20th	Day	13% 5% 5% 7% 3 3 3 2 3 5% 52%	5280
	21st to end	Day	22% 10% 3 7% 4 4 4 3 4 5% 35%	6394
February	1st to 10th	Day	17% 9% 4 6% 4 3 3 4 5% 6% 39%	2100
	11th to 20th	Day	23% 8% 2 6% 5% 4 4 3 3 5% 37%	2048
	21st to end	Day	22% 7% 3 6% 5% 4 3 3 4 5% 39%	1730
March	1st to 10th	Day	20% 9% 3 8% 4 5% 5% 3 5% 6% 33%	4580
	11th to 20th	Day	13% 8% 3 6% 4 3 4 3 5% 6% 46%	4949
	21st to end	Day	14% 6% 3 7% 5% 4 4 3 5% 7% 42%	5740
April	1st to 10th	Day	13% 7% 2 5% 4 4 5% 4 6% 8% 43%	2520
	11th to 20th	Day	13% 6% 2 6% 4 3 4 4 5% 7% 47%	2494
	21st to end	Day	11% 5% 2 7% 4 5% 4 4 7% 8% 43%	2562
May	1st to 10th	Day	18% 7% 3 6% 4 5% 5% 4 7% 8% 33%	7616
	11th to 20th	Day	14% 7% 3 8% 5% 5% 5% 4 6% 8% 34%	7626
	21st to end	Day	19% 10% 4 8% 5% 5% 5% 4 5% 7% 29%	8391
June	1st to 10th	Day	10% 6% 2 7% 5% 6% 5% 5% 8% 10% 35%	2730
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 6% 9% 11% 42%	2730
	21st to end	Day	5% 6% 2 7% 5% 6% 5% 5% 8% 10% 41%	2730
July	1st to 10th	Day	7% 5% 2 7% 5% 6% 6% 5% 9% 12% 36%	6030
	11th to 20th	Day	7% 5% 3 7% 5% 7% 7% 6% 9% 13% 32%	6030
	21st to end	Day	10% 8% 3 9% 7% 6% 7% 6% 9% 12% 23%	6554
August	1st to 10th	Day	5% 3 1 4 4 5% 5% 5% 8% 11% 48%	4007
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	3849
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 7% 52%	4312
September	1st to 10th	Day	10% 6% 2 5% 5% 5% 5% 4 7% 9% 41%	2325
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 42%	2200
	21st to end	Day	12% 5% 2 5% 4 5% 4 4 6% 6% 47%	2200
October	1st to 10th	Day	11% 4 2 6% 4 3 4 2 4 5% 56%	2200
	11th to 20th	Day	10% 5% 1 5% 3 4 3 2 4 7% 56%	2200
	21st to end	Day	15% 6% 2 5% 3 4 3 3 4 5% 53%	2035
November	1st to 10th	Day	13% 5% 2 5% 4 3 3 3 4 5% 52%	1852
	11th to 20th	Day	12% 6% 2 4 4 3 5% 2 4 6% 51%	1890
	21st to end	Day	17% 6% 2 6% 4 3 4 3 5% 6% 44%	1890
December	1st to 10th	Day	19% 7% 3 5% 4 3 4 3 4 4 43%	3020
	11th to 20th	Day	21% 11% 3 7% 4 3 3 3 3 5% 36%	3463
	21st to end	Day	20% 9% 4 6% 5% 4 4 3 5% 6% 34%	4565
January	1st to 10th	Night	18% 9% 3 7% 4 3 4 2 4 5% 40%	7635
	11th to 20th	Night	16% 7% 3 7% 4 3 3 2 4 4 47%	7680
	21st to end	Night	21% 10% 2 6% 4 4 3 3 4 5% 39%	7768
February	1st to 10th	Night	17% 10% 3 5% 4 3 4 3 4 6% 42%	2520
	11th to 20th	Night	22% 8% 3 7% 4 3 3 2 4 4 39%	2458
	21st to end	Night	22% 7% 2 5% 4 5% 4 4 4 6% 37%	2076
March	1st to 10th	Night	23% 11% 3 7% 4 4 4 3 4 5% 31%	5493
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 47%	5217
	21st to end	Night	15% 7% 3 7% 5% 4 4 3 4 5% 42%	5460
April	1st to 10th	Night	11% 5% 2 5% 3 4 4 4 6% 9% 47%	2100
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 4 8% 50%	2075
	21st to end	Night	12% 5% 3 5% 4 4 4 4 5% 7% 48%	2058
May	1st to 10th	Night	15% 8% 3 8% 6% 5% 5% 4 6% 7% 33%	5237
	11th to 20th	Night	15% 7% 3 8% 5% 5% 5% 4 6% 7% 34%	5234
	21st to end	Night	16% 9% 3 9% 5% 4 4 4 6% 8% 31%	5761
June	1st to 10th	Night	8% 6% 2 6% 6% 5% 6% 4 7% 10% 41%	1890
	11th to 20th	Night	6% 3 2 4 4 4 4 4 8% 11% 51%	1890
	21st to end	Night	5% 5% 2 6% 4 5% 5% 4 7% 11% 46%	1890
July	1st to 10th	Night	6% 5% 2 7% 6% 5% 5% 5% 8% 10% 39%	4148
	11th to 20th	Night	7% 5% 2 6% 4 5% 5% 6% 10% 13% 37%	4147
	21st to end	Night	8% 7% 4 8% 7% 6% 6% 5% 8% 11% 30%	4517
August	1st to 10th	Night	3 3 1 4 3 4 4 4 8% 12% 55%	3137
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 6% 10% 51%	3200
	21st to end	Night	7% 4 1 5% 3 3 4 3 5% 7% 57%	3586
September	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 5% 6% 7% 47%	2055
	11th to 20th	Night	11% 6% 2 6% 3 4 5% 4 5% 10% 45%	2200
	21st to end	Night	10% 5% 2 6% 4 4 4 4 5% 6% 51%	2200
October	1st to 10th	Night	10% 4 2 6% 3 3 3 2 4 6% 59%	2200
	11th to 20th	Night	12% 5% 1 6% 4 3 3 3 4 5% 54%	2200
	21st to end	Night	14% 6% 2 4 3 3 3 3 4 5% 52%	2801
November	1st to 10th	Night	12% 5% 2 5% 4 4 4 3 4 5% 52%	2666
	11th to 20th	Night	13% 7% 2 4 3 3 3 3 4 6% 52%	2730
	21st to end	Night	16% 7% 2 5% 4 3 3 3 5% 6% 45%	2730
December	1st to 10th	Night	20% 8% 2 5% 3 3 3 3 4 5% 43%	4380
	11th to 20th	Night	22% 8% 3 5% 3 3 3 2 3 5% 41%	4952
	21st to end	Night	20% 9% 4 7% 4 4 4 3 5% 6% 34%	6633

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	48% 4 7% 22% 17% 2	5259
	11th to 20th	Day	37% 2 5% 20% 31% 5%	5280
	21st to end	Day	52% 3 6% 19% 15% 5%	6394
February	1st to 10th	Day	46% 4 7% 21% 19% 3	2100
	11th to 20th	Day	50% 4 5% 20% 15% 5%	2058
	21st to end	Day	48% 4 7% 15% 20% 6%	1730
March	1st to 10th	Day	52% 4 7% 12% 17% 7%	4580
	11th to 20th	Day	37% 5% 6% 20% 20% 13%	4949
	21st to end	Day	42% 5% 10% 25% 16% 3	5742
April	1st to 10th	Day	38% 7% 11% 23% 16% 5%	2520
	11th to 20th	Day	36% 5% 7% 22% 24% 6%	2494
	21st to end	Day	37% 6% 10% 28% 16% 2	2562
May	1st to 10th	Day	45% 7% 13% 21% 12% 2	7651
	11th to 20th	Day	45% 8% 14% 22% 11% 1	7630
	21st to end	Day	53% 8% 8% 17% 12% 2	8405
June	1st to 10th	Day	41% 9% 13% 21% 14% 2	2730
	11th to 20th	Day	31% 11% 19% 23% 15% 2	2730
	21st to end	Day	34% 11% 17% 22% 13% 3	2730
July	1st to 10th	Day	35% 12% 18% 24% 10% 1	5965
	11th to 20th	Day	36% 14% 25% 20% 4 1	5961
	21st to end	Day	48% 12% 19% 16% 4	6370
August	1st to 10th	Day	26% 9% 13% 36% 16% 1	3994
	11th to 20th	Day	32% 9% 16% 26% 13% 3	3885
	21st to end	Day	29% 7% 14% 26% 22% 2	4311
September	1st to 10th	Day	38% 8% 15% 18% 15% 5%	2325
	11th to 20th	Day	38% 8% 11% 21% 19% 3	2200
	21st to end	Day	36% 6% 12% 20% 20% 7%	2200
October	1st to 10th	Day	23% 4 5% 15% 39% 13%	3590
	11th to 20th	Day	20% 3 5% 19% 32% 20%	3568
	21st to end	Day	27% 4 7% 24% 29% 9%	3259
November	1st to 10th	Day	35% 4 8% 18% 28% 7%	1852
	11th to 20th	Day	35% 4 6% 22% 25% 8%	1890
	21st to end	Day	40% 4 7% 20% 24% 5%	1890
December	1st to 10th	Day	44% 3 5% 19% 24% 5%	3020
	11th to 20th	Day	52% 4 6% 20% 14% 4	3460
	21st to end	Day	49% 5% 9% 24% 12% 2	4565
January	1st to 10th	Night	47% 3 10% 21% 16% 2	7636
	11th to 20th	Night	42% 2 6% 18% 27% 5%	7680
	21st to end	Night	48% 4 5% 18% 18% 7%	7768
February	1st to 10th	Night	45% 4 8% 20% 20% 4	2520
	11th to 20th	Night	50% 4 6% 18% 15% 6%	2459
	21st to end	Night	48% 5% 6% 16% 18% 7%	2076
March	1st to 10th	Night	54% 4 6% 14% 17% 4	5493
	11th to 20th	Night	36% 4 7% 19% 21% 14%	5217
	21st to end	Night	44% 4 8% 22% 18% 4	5460
April	1st to 10th	Night	33% 6% 13% 24% 20% 5%	2100
	11th to 20th	Night	34% 4 6% 22% 23% 10%	2075
	21st to end	Night	35% 5% 12% 26% 19% 3	2058
May	1st to 10th	Night	46% 8% 12% 19% 13% 1	5264
	11th to 20th	Night	46% 8% 13% 20% 12% 1	5235
	21st to end	Night	49% 9% 11% 18% 12% 1	5771
June	1st to 10th	Night	36% 9% 14% 21% 18% 2	1890
	11th to 20th	Night	25% 9% 18% 29% 17% 2	1890
	21st to end	Night	29% 10% 18% 24% 15% 4	1890
July	1st to 10th	Night	36% 10% 20% 24% 9% 1	4105
	11th to 20th	Night	32% 13% 27% 21% 5% 1	4096
	21st to end	Night	44% 12% 23% 17% 4	4387
August	1st to 10th	Night	21% 8% 14% 38% 18% 1	3126
	11th to 20th	Night	27% 8% 19% 27% 16% 2	3230
	21st to end	Night	26% 7% 13% 24% 28% 3	3586
September	1st to 10th	Night	33% 10% 17% 18% 17% 5%	2055
	11th to 20th	Night	34% 8% 13% 19% 23% 3	2200
	21st to end	Night	33% 6% 10% 22% 22% 8%	2200
October	1st to 10th	Night	22% 4 6% 19% 32% 17%	3590
	11th to 20th	Night	21% 3 4 17% 34% 21%	3571
	21st to end	Night	24% 3 4 27% 29% 12%	4527
November	1st to 10th	Night	33% 4 9% 21% 24% 10%	2666
	11th to 20th	Night	33% 3 7% 20% 28% 10%	2730
	21st to end	Night	41% 4 5% 21% 23% 6%	2730
December	1st to 10th	Night	44% 3 5% 20% 25% 3	4380
	11th to 20th	Night	48% 3 5% 18% 23% 3	4954
	21st to end	Night	50% 5% 8% 24% 12% 1	6633

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	27% 25% 30% 15% 3	330
	11th to 20th	Day	25% 26% 32% 13% 5%	330
	21st to end	Day	22% 26% 36% 12% 3	363
February	1st to 10th	Day	23% 23% 34% 16% 4	330
	11th to 20th	Day	38% 20% 31% 10% 1 1	330
	21st to end	Day	31% 25% 31% 10% 3	273
March	1st to 10th	Day	25% 21% 35% 14% 4 1	330
	11th to 20th	Day	24% 22% 30% 17% 5% 1	477
	21st to end	Day	21% 17% 36% 19% 6% 1	594
April	1st to 10th	Day	20% 17% 28% 22% 12% 1	540
	11th to 20th	Day	16% 18% 30% 23% 12% 2	532
	21st to end	Day	15% 14% 32% 24% 13% 2	540
May	1st to 10th	Day	17% 12% 29% 25% 11% 5%	543
	11th to 20th	Day	11% 9% 28% 28% 15% 9%	567
	21st to end	Day	16% 8% 19% 27% 18% 12%	617
June	1st to 10th	Day	12% 6% 18% 29% 24% 12%	570
	11th to 20th	Day	5% 3 21% 27% 26% 18%	570
	21st to end	Day	9% 3 16% 23% 31% 18%	570
July	1st to 10th	Day	7% 3 9% 26% 34% 21%	565
	11th to 20th	Day	8% 1 10% 23% 31% 26%	564
	21st to end	Day	10% 2 10% 22% 33% 23%	616
August	1st to 10th	Day	11% 3 10% 24% 33% 19%	526
	11th to 20th	Day	9% 2 12% 28% 34% 16%	519
	21st to end	Day	15% 2 10% 23% 32% 19%	583
September	1st to 10th	Day	11% 3 13% 28% 31% 14%	530
	11th to 20th	Day	13% 3 14% 21% 33% 16%	530
	21st to end	Day	15% 5% 14% 28% 30% 9%	530
October	1st to 10th	Day	12% 5% 24% 28% 22% 9%	530
	11th to 20th	Day	11% 9% 26% 30% 21% 2	530
	21st to end	Day	19% 11% 25% 27% 15% 3	413
November	1st to 10th	Day	21% 10% 28% 26% 13% 2	330
	11th to 20th	Day	13% 18% 36% 25% 7%	330
	21st to end	Day	17% 18% 33% 19% 10% 2	330
December	1st to 10th	Day	29% 21% 27% 17% 5% 1	330
	11th to 20th	Day	26% 20% 30% 16% 6% 1	330
	21st to end	Day	29% 19% 32% 15% 5%	363
January	1st to 10th	Night	25% 17% 30% 23% 5%	840
	11th to 20th	Night	23% 19% 28% 23% 7%	840
	21st to end	Night	21% 16% 34% 26% 3	924
February	1st to 10th	Night	22% 14% 29% 31% 4	840
	11th to 20th	Night	27% 18% 28% 24% 3	823
	21st to end	Night	28% 16% 29% 25% 2	692
March	1st to 10th	Night	21% 15% 29% 30% 5%	840
	11th to 20th	Night	21% 12% 30% 30% 8% 1	840
	21st to end	Night	18% 14% 30% 28% 10%	873
April	1st to 10th	Night	14% 12% 29% 31% 13%	630
	11th to 20th	Night	16% 10% 29% 31% 13% 1	623
	21st to end	Night	13% 8% 24% 37% 17% 1	630
May	1st to 10th	Night	13% 8% 23% 34% 20% 2	630
	11th to 20th	Night	13% 5% 21% 33% 24% 5%	621
	21st to end	Night	10% 3 13% 37% 31% 6%	683
June	1st to 10th	Night	10% 4 12% 31% 37% 6%	630
	11th to 20th	Night	7% 1 10% 30% 39% 13%	630
	21st to end	Night	7% 2 9% 29% 42% 12%	630
July	1st to 10th	Night	9% 2 7% 23% 41% 18%	615
	11th to 20th	Night	9% 2 6% 22% 42% 19%	612
	21st to end	Night	8% 2 7% 23% 45% 14%	687
August	1st to 10th	Night	6% 3 5% 25% 48% 13%	597
	11th to 20th	Night	10% 2 10% 24% 44% 9%	594
	21st to end	Night	12% 2 7% 28% 40% 11%	660
September	1st to 10th	Night	9% 2 11% 27% 42% 9%	655
	11th to 20th	Night	13% 2 13% 28% 38% 7%	800
	21st to end	Night	10% 4 14% 30% 38% 3	800
October	1st to 10th	Night	12% 5% 17% 32% 29% 6%	800
	11th to 20th	Night	13% 8% 22% 35% 21% 1	800
	21st to end	Night	17% 9% 19% 34% 19% 2	880
November	1st to 10th	Night	13% 9% 21% 34% 19% 3	820
	11th to 20th	Night	15% 11% 28% 31% 14% 1	840
	21st to end	Night	21% 10% 26% 30% 12% 1	840
December	1st to 10th	Night	24% 13% 28% 26% 8%	840
	11th to 20th	Night	24% 11% 25% 31% 9%	840
	21st to end	Night	25% 15% 25% 28% 6%	924

Page generated at 2026-07-28 02:25:20. Version ac07

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