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 2025-03-27.

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% 41%	109395
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	109972
Transparency	Day	39% 7% 10% 22% 18% 4	113186
	Night	38% 6% 10% 21% 20% 5%	114319
Seeing	Day	16% 11% 23% 22% 19% 9%	15033
	Night	16% 9% 21% 29% 21% 4	25148

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	19% 9% 5% 4 3 3 3 3 5% 5% 42%	4056
	11th to 20th	Day	16% 6% 6% 8% 3 3 3 2 3 5% 45%	4060
	21st to end	Day	22% 10% 3 6% 3 3 3 3 4 6% 37%	4944
February	1st to 10th	Day	18% 10% 4 6% 4 3 3 4 5% 6% 38%	2000
	11th to 20th	Day	23% 9% 2 6% 5% 4 4 3 3 5% 36%	1948
	21st to end	Day	22% 7% 3 6% 5% 4 3 3 4 5% 38%	1650
March	1st to 10th	Day	25% 9% 3 7% 4 4 4 3 5% 6% 31%	3220
	11th to 20th	Day	15% 8% 3 5% 3 3 4 3 4 6% 45%	3491
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 4 6% 7% 38%	3992
April	1st to 10th	Day	13% 7% 2 5% 5% 4 5% 4 5% 7% 43%	2280
	11th to 20th	Day	14% 7% 2 6% 4 3 4 4 5% 7% 45%	2254
	21st to end	Day	11% 5% 2 6% 4 5% 4 4 7% 8% 45%	2318
May	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 7% 8% 40%	4082
	11th to 20th	Day	16% 7% 3 7% 5% 4 5% 4 6% 8% 36%	4081
	21st to end	Day	11% 6% 2 6% 4 5% 5% 4 6% 9% 42%	4476
June	1st to 10th	Day	10% 6% 2 7% 5% 6% 6% 5% 8% 10% 35%	2470
	11th to 20th	Day	5% 4 1 5% 5% 5% 6% 6% 8% 11% 45%	2470
	21st to end	Day	6% 6% 2 7% 5% 6% 5% 5% 7% 10% 42%	2470
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 7% 6% 9% 13% 30%	4120
	11th to 20th	Day	8% 6% 3 7% 5% 6% 7% 6% 9% 11% 34%	4120
	21st to end	Day	14% 10% 4 10% 8% 6% 6% 5% 7% 8% 21%	4466
August	1st to 10th	Day	5% 3 1 4 4 5% 5% 5% 8% 11% 49%	3884
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	3729
	21st to end	Day	7% 4 2 5% 4 4 5% 5% 6% 7% 52%	4180
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 4 7% 9% 40%	2209
	11th to 20th	Day	12% 5% 2 6% 4 6% 5% 4 7% 8% 42%	2090
	21st to end	Day	12% 5% 2 5% 4 4 4 4 5% 6% 48%	2090
October	1st to 10th	Day	11% 4 2 5% 4 3 4 2 4 5% 57%	2090
	11th to 20th	Day	10% 5% 1 5% 3 4 3 2 5% 7% 54%	2090
	21st to end	Day	14% 6% 2 5% 3 4 3 3 4 5% 53%	1934
November	1st to 10th	Day	13% 5% 2 5% 4 3 3 3 4 5% 53%	1762
	11th to 20th	Day	13% 5% 2 4 5% 4 5% 2 4 6% 50%	1800
	21st to end	Day	17% 6% 2 6% 4 3 4 3 5% 6% 44%	1800
December	1st to 10th	Day	19% 7% 3 5% 4 3 3 3 4 4 44%	2930
	11th to 20th	Day	21% 11% 3 8% 4 3 3 3 3 5% 35%	3373
	21st to end	Day	20% 9% 4 6% 5% 3 4 3 5% 6% 34%	4466
January	1st to 10th	Night	17% 10% 3 7% 4 3 4 2 4 4 41%	5889
	11th to 20th	Night	18% 8% 3 8% 4 3 3 2 3 4 44%	5900
	21st to end	Night	19% 10% 2 5% 3 4 4 3 4 6% 41%	6007
February	1st to 10th	Night	18% 10% 3 5% 4 3 4 3 4 6% 42%	2400
	11th to 20th	Night	22% 8% 3 7% 4 3 3 2 4 4 39%	2338
	21st to end	Night	22% 7% 2 5% 4 5% 4 4 4 6% 37%	1980
March	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 4 5% 31%	3853
	11th to 20th	Night	13% 6% 2 4 4 4 4 3 6% 7% 47%	3675
	21st to end	Night	16% 8% 3 8% 5% 4 5% 3 4 5% 40%	3806
April	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 6% 8% 47%	1900
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 4 7% 48%	1875
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 6% 50%	1862
May	1st to 10th	Night	12% 5% 3 6% 5% 4 4 4 6% 7% 42%	2813
	11th to 20th	Night	16% 7% 2 6% 5% 4 5% 4 5% 7% 39%	2812
	21st to end	Night	7% 4 2 6% 5% 4 4 5% 7% 10% 45%	3079
June	1st to 10th	Night	8% 6% 3 6% 6% 5% 6% 4 7% 9% 41%	1710
	11th to 20th	Night	5% 3 1 4 4 4 4 5% 8% 11% 53%	1710
	21st to end	Night	6% 5% 2 6% 4 5% 5% 4 7% 11% 46%	1710
July	1st to 10th	Night	7% 6% 3 7% 7% 5% 6% 6% 8% 10% 35%	2840
	11th to 20th	Night	9% 5% 2 5% 4 5% 4 5% 9% 12% 40%	2840
	21st to end	Night	11% 9% 4 10% 8% 7% 7% 5% 7% 8% 25%	3087
August	1st to 10th	Night	3 3 1 4 3 4 4 4 8% 12% 56%	3040
	11th to 20th	Night	7% 4 2 5% 4 3 4 4 6% 10% 51%	3100
	21st to end	Night	6% 4 1 5% 3 3 4 3 5% 7% 58%	3476
September	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 5% 6% 8% 47%	1952
	11th to 20th	Night	11% 7% 2 6% 3 4 5% 4 5% 10% 43%	2090
	21st to end	Night	10% 5% 2 6% 4 4 3 4 5% 6% 52%	2090
October	1st to 10th	Night	10% 4 2 5% 3 3 3 2 4 6% 60%	2090
	11th to 20th	Night	13% 5% 2 6% 4 3 3 3 4 5% 53%	2090
	21st to end	Night	14% 6% 2 4 3 3 3 3 4 5% 52%	2660
November	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 4 6% 51%	2536
	11th to 20th	Night	13% 7% 2 4 3 3 3 3 5% 6% 52%	2600
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 6% 46%	2600
December	1st to 10th	Night	20% 7% 2 5% 3 3 3 3 4 5% 44%	4250
	11th to 20th	Night	23% 8% 3 5% 3 3 3 2 3 5% 41%	4822
	21st to end	Night	20% 9% 4 7% 4 4 4 3 5% 6% 34%	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	45% 4 6% 21% 22% 2	4057
	11th to 20th	Day	44% 3 5% 22% 25% 2	4060
	21st to end	Day	48% 4 7% 20% 18% 3	4944
February	1st to 10th	Day	47% 4 7% 20% 18% 3	2000
	11th to 20th	Day	51% 4 5% 19% 16% 5%	1958
	21st to end	Day	48% 4 7% 15% 20% 6%	1650
March	1st to 10th	Day	54% 5% 7% 11% 15% 9%	3220
	11th to 20th	Day	39% 5% 6% 22% 20% 7%	3491
	21st to end	Day	44% 6% 12% 20% 15% 4	3992
April	1st to 10th	Day	39% 7% 10% 22% 17% 6%	2280
	11th to 20th	Day	39% 6% 7% 20% 23% 6%	2254
	21st to end	Day	36% 6% 10% 29% 17% 3	2318
May	1st to 10th	Day	39% 8% 13% 24% 14% 2	4120
	11th to 20th	Day	43% 8% 12% 25% 11%	4085
	21st to end	Day	36% 9% 11% 25% 18% 2	4489
June	1st to 10th	Day	41% 10% 13% 21% 14% 1	2470
	11th to 20th	Day	29% 10% 19% 25% 16% 2	2470
	21st to end	Day	35% 11% 16% 21% 14% 4	2470
July	1st to 10th	Day	38% 13% 21% 21% 6% 1	4055
	11th to 20th	Day	38% 13% 20% 23% 6% 1	4051
	21st to end	Day	57% 10% 13% 14% 5%	4324
August	1st to 10th	Day	25% 9% 13% 36% 16% 1	3871
	11th to 20th	Day	32% 9% 16% 26% 14% 3	3765
	21st to end	Day	29% 7% 14% 26% 22% 2	4179
September	1st to 10th	Day	38% 8% 15% 18% 15% 5%	2209
	11th to 20th	Day	39% 8% 11% 20% 20% 3	2090
	21st to end	Day	36% 6% 11% 20% 20% 7%	2090
October	1st to 10th	Day	22% 4 5% 15% 40% 14%	3480
	11th to 20th	Day	21% 3 6% 19% 31% 20%	3458
	21st to end	Day	26% 4 7% 23% 30% 10%	3158
November	1st to 10th	Day	35% 4 7% 18% 28% 8%	1762
	11th to 20th	Day	36% 4 6% 21% 26% 8%	1800
	21st to end	Day	41% 4 6% 19% 25% 5%	1800
December	1st to 10th	Day	43% 4 5% 19% 24% 5%	2930
	11th to 20th	Day	53% 4 6% 20% 14% 4	3370
	21st to end	Day	49% 4 9% 24% 12% 2	4466
January	1st to 10th	Night	47% 3 7% 20% 20% 3	5889
	11th to 20th	Night	45% 3 6% 19% 23% 5%	5900
	21st to end	Night	45% 4 5% 19% 21% 6%	6007
February	1st to 10th	Night	45% 4 8% 19% 20% 4	2400
	11th to 20th	Night	50% 4 6% 17% 16% 7%	2339
	21st to end	Night	48% 5% 6% 16% 18% 7%	1980
March	1st to 10th	Night	54% 5% 7% 13% 15% 6%	3853
	11th to 20th	Night	36% 5% 7% 21% 21% 10%	3675
	21st to end	Night	46% 5% 8% 20% 19% 2	3806
April	1st to 10th	Night	34% 6% 12% 23% 20% 5%	1900
	11th to 20th	Night	36% 5% 6% 20% 22% 11%	1875
	21st to end	Night	34% 5% 11% 27% 19% 4	1862
May	1st to 10th	Night	37% 9% 12% 23% 17% 2	2840
	11th to 20th	Night	42% 9% 13% 24% 12% 1	2813
	21st to end	Night	30% 10% 14% 26% 19% 2	3087
June	1st to 10th	Night	36% 10% 14% 21% 17% 2	1710
	11th to 20th	Night	22% 9% 18% 30% 18% 3	1710
	21st to end	Night	30% 9% 18% 22% 16% 5%	1710
July	1st to 10th	Night	39% 10% 22% 19% 8% 2	2795
	11th to 20th	Night	32% 11% 24% 24% 7% 2	2786
	21st to end	Night	53% 12% 13% 16% 6% 1	2988
August	1st to 10th	Night	20% 8% 14% 38% 18% 1	3029
	11th to 20th	Night	27% 8% 19% 27% 16% 2	3130
	21st to end	Night	26% 7% 13% 24% 28% 3	3476
September	1st to 10th	Night	32% 10% 18% 19% 17% 5%	1952
	11th to 20th	Night	35% 8% 13% 18% 23% 3	2090
	21st to end	Night	32% 6% 9% 22% 22% 8%	2090
October	1st to 10th	Night	21% 4 6% 19% 33% 18%	3480
	11th to 20th	Night	21% 3 4 17% 34% 21%	3461
	21st to end	Night	24% 3 4 27% 29% 12%	4386
November	1st to 10th	Night	33% 4 8% 21% 23% 10%	2536
	11th to 20th	Night	34% 3 7% 19% 28% 10%	2600
	21st to end	Night	40% 4 6% 21% 24% 6%	2600
December	1st to 10th	Night	43% 3 5% 20% 26% 3	4250
	11th to 20th	Night	48% 4 5% 18% 22% 4	4824
	21st to end	Night	50% 5% 8% 24% 12% 1	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	26% 26% 31% 14% 2	300
	11th to 20th	Day	25% 25% 32% 13% 5%	300
	21st to end	Day	21% 26% 37% 13% 4	330
February	1st to 10th	Day	24% 24% 32% 16% 4	300
	11th to 20th	Day	39% 20% 29% 10% 1 1	300
	21st to end	Day	31% 25% 33% 9% 2	249
March	1st to 10th	Day	27% 22% 34% 13% 4 1	300
	11th to 20th	Day	25% 21% 30% 17% 6% 1	440
	21st to end	Day	22% 18% 35% 18% 7% 1	530
April	1st to 10th	Day	21% 17% 27% 21% 13% 2	460
	11th to 20th	Day	17% 18% 29% 23% 11% 1	452
	21st to end	Day	16% 12% 33% 24% 13% 1	460
May	1st to 10th	Day	15% 11% 30% 26% 12% 6%	463
	11th to 20th	Day	11% 7% 27% 29% 17% 10%	487
	21st to end	Day	13% 6% 19% 29% 20% 14%	529
June	1st to 10th	Day	12% 6% 18% 28% 24% 12%	490
	11th to 20th	Day	5% 2 18% 26% 27% 21%	490
	21st to end	Day	10% 3 16% 24% 31% 17%	490
July	1st to 10th	Day	8% 3 9% 26% 33% 20%	485
	11th to 20th	Day	8% 2 10% 23% 31% 26%	484
	21st to end	Day	11% 2 10% 21% 33% 23%	532
August	1st to 10th	Day	10% 3 10% 25% 34% 19%	486
	11th to 20th	Day	9% 2 13% 29% 34% 13%	479
	21st to end	Day	15% 2 9% 21% 33% 20%	539
September	1st to 10th	Day	11% 3 12% 28% 32% 14%	490
	11th to 20th	Day	14% 3 14% 21% 31% 16%	490
	21st to end	Day	16% 5% 13% 27% 30% 9%	490
October	1st to 10th	Day	12% 4 23% 29% 22% 10%	490
	11th to 20th	Day	12% 10% 27% 29% 20% 2	490
	21st to end	Day	18% 10% 23% 28% 17% 3	378
November	1st to 10th	Day	21% 9% 29% 26% 13% 2	300
	11th to 20th	Day	14% 19% 35% 24% 7%	300
	21st to end	Day	17% 17% 33% 20% 11% 2	300
December	1st to 10th	Day	30% 20% 27% 17% 6% 1	300
	11th to 20th	Day	27% 21% 30% 16% 6% 1	300
	21st to end	Day	29% 19% 32% 15% 5%	330
January	1st to 10th	Night	25% 17% 31% 22% 5%	800
	11th to 20th	Night	23% 19% 28% 23% 7%	800
	21st to end	Night	20% 16% 34% 27% 3	880
February	1st to 10th	Night	22% 14% 29% 31% 4	800
	11th to 20th	Night	27% 18% 27% 25% 3	783
	21st to end	Night	28% 15% 29% 26% 2	660
March	1st to 10th	Night	22% 15% 29% 30% 5%	800
	11th to 20th	Night	21% 12% 30% 29% 8% 1	800
	21st to end	Night	18% 14% 30% 28% 10%	817
April	1st to 10th	Night	14% 13% 29% 31% 14%	570
	11th to 20th	Night	17% 11% 29% 29% 13% 1	563
	21st to end	Night	13% 8% 26% 36% 16% 1	570
May	1st to 10th	Night	11% 7% 24% 33% 22% 3	570
	11th to 20th	Night	12% 4 21% 32% 25% 6%	561
	21st to end	Night	8% 3 12% 37% 33% 7%	617
June	1st to 10th	Night	10% 3 13% 31% 37% 6%	570
	11th to 20th	Night	5% 1 9% 31% 40% 14%	570
	21st to end	Night	7% 1 9% 30% 41% 11%	570
July	1st to 10th	Night	9% 2 8% 25% 40% 16%	555
	11th to 20th	Night	9% 2 6% 23% 40% 19%	552
	21st to end	Night	9% 2 7% 24% 45% 14%	624
August	1st to 10th	Night	6% 1 5% 26% 49% 13%	567
	11th to 20th	Night	10% 2 11% 25% 44% 8%	564
	21st to end	Night	11% 2 7% 27% 42% 11%	627
September	1st to 10th	Night	9% 2 11% 26% 42% 9%	622
	11th to 20th	Night	14% 2 13% 28% 36% 7%	760
	21st to end	Night	10% 4 14% 30% 38% 3	760
October	1st to 10th	Night	12% 4 16% 32% 29% 7%	760
	11th to 20th	Night	14% 8% 23% 34% 21% 1	760
	21st to end	Night	17% 9% 19% 33% 20% 2	836
November	1st to 10th	Night	13% 9% 21% 33% 20% 3	780
	11th to 20th	Night	16% 11% 28% 30% 14% 1	800
	21st to end	Night	21% 10% 25% 31% 12% 1	800
December	1st to 10th	Night	24% 14% 28% 26% 9%	800
	11th to 20th	Night	24% 11% 26% 30% 9%	800
	21st to end	Night	25% 14% 26% 28% 6%	880

Page generated at 2025-03-24 08:04:27. Version ac07

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