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 2024-10-24.

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% 6% 2 6% 4 4 5% 4 6% 8% 40%	100782
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	97955
Transparency	Day	40% 7% 11% 21% 17% 4	100591
	Night	39% 6% 10% 21% 19% 5%	97815
Seeing	Day	16% 11% 22% 23% 19% 9%	14558
	Night	16% 9% 21% 29% 21% 4	24530

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% 12% 3 5% 4 4 4 3 6% 6% 35%	2836
	11th to 20th	Day	21% 7% 4 8% 4 3 3 2 3 4 40%	2840
	21st to end	Day	25% 10% 2 5% 3 3 3 2 4 5% 39%	3448
February	1st to 10th	Day	19% 10% 4 6% 3 3 3 4 4 6% 38%	1900
	11th to 20th	Day	22% 8% 3 6% 5% 4 4 3 3 5% 37%	1848
	21st to end	Day	24% 7% 3 6% 5% 4 3 3 4 5% 37%	1570
March	1st to 10th	Day	25% 9% 3 7% 4 4 4 3 5% 6% 31%	3120
	11th to 20th	Day	15% 8% 3 5% 3 3 4 3 4 6% 45%	3384
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 6% 7% 37%	3926
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	15% 6% 2 5% 3 4 3 3 4 5% 53%	1862
November	1st to 10th	Day	13% 5% 2 4 4 3 3 3 4 5% 55%	1672
	11th to 20th	Day	13% 5% 2 4 5% 4 5% 2 4 5% 51%	1710
	21st to end	Day	17% 6% 2 6% 4 3 4 3 5% 6% 43%	1710
December	1st to 10th	Day	22% 8% 3 6% 5% 4 4 4 5% 5% 35%	1710
	11th to 20th	Day	18% 8% 3 7% 5% 4 4 3 4 5% 41%	2153
	21st to end	Day	19% 9% 4 7% 5% 4 5% 3 5% 7% 32%	3124
January	1st to 10th	Night	20% 11% 2 6% 4 3 4 3 4 5% 38%	4109
	11th to 20th	Night	20% 8% 3 7% 4 3 3 2 4 5% 41%	4120
	21st to end	Night	23% 13% 3 5% 3 4 3 2 3 5% 37%	4203
February	1st to 10th	Night	18% 10% 3 5% 4 3 4 3 4 6% 42%	2280
	11th to 20th	Night	22% 8% 2 6% 4 3 3 3 5% 4 40%	2218
	21st to end	Night	23% 7% 2 5% 4 5% 4 4 4 5% 36%	1884
March	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 4 5% 32%	3733
	11th to 20th	Night	13% 6% 2 4 4 4 4 3 6% 7% 47%	3562
	21st to end	Night	16% 8% 3 8% 5% 5% 5% 3 4 5% 39%	3733
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% 7% 2 4 3 3 3 3 4 5% 52%	2557
November	1st to 10th	Night	12% 4 2 4 4 4 4 3 4 5% 53%	2406
	11th to 20th	Night	12% 7% 2 4 3 3 3 3 4 6% 52%	2470
	21st to end	Night	17% 7% 2 5% 4 3 3 3 5% 6% 45%	2470
December	1st to 10th	Night	20% 7% 2 6% 4 5% 4 4 5% 7% 36%	2470
	11th to 20th	Night	18% 6% 2 5% 3 4 4 3 4 5% 46%	3042
	21st to end	Night	19% 9% 4 8% 5% 5% 5% 4 6% 7% 30%	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	49% 4 6% 18% 21% 2	2837
	11th to 20th	Day	49% 3 6% 18% 22% 2	2840
	21st to end	Day	49% 4 7% 20% 18% 3	3448
February	1st to 10th	Day	48% 4 6% 20% 19% 3	1900
	11th to 20th	Day	50% 4 5% 19% 16% 5%	1858
	21st to end	Day	50% 5% 7% 14% 19% 6%	1570
March	1st to 10th	Day	54% 5% 7% 11% 14% 9%	3120
	11th to 20th	Day	39% 5% 6% 22% 20% 7%	3384
	21st to end	Day	44% 6% 12% 20% 15% 4	3926
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	31% 5% 7% 21% 27% 9%	2090
	11th to 20th	Day	30% 4 9% 22% 25% 11%	2090
	21st to end	Day	35% 4 5% 18% 29% 8%	1862
November	1st to 10th	Day	33% 4 7% 18% 30% 8%	1672
	11th to 20th	Day	36% 4 6% 22% 25% 8%	1710
	21st to end	Day	41% 4 6% 19% 25% 5%	1710
December	1st to 10th	Day	51% 4 7% 16% 19% 4	1710
	11th to 20th	Day	47% 4 6% 20% 19% 5%	2150
	21st to end	Day	51% 5% 8% 19% 14% 2	3124
January	1st to 10th	Night	48% 4 6% 18% 22% 2	4109
	11th to 20th	Night	47% 3 7% 20% 17% 6%	4120
	21st to end	Night	51% 4 5% 18% 19% 3	4203
February	1st to 10th	Night	45% 4 8% 19% 20% 5%	2280
	11th to 20th	Night	49% 4 6% 18% 16% 7%	2219
	21st to end	Night	50% 5% 6% 15% 17% 7%	1884
March	1st to 10th	Night	54% 5% 7% 13% 15% 6%	3733
	11th to 20th	Night	36% 6% 8% 22% 21% 9%	3562
	21st to end	Night	47% 5% 8% 20% 18% 2	3733
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	28% 4 7% 24% 27% 10%	2090
	11th to 20th	Night	34% 4 7% 20% 24% 12%	2090
	21st to end	Night	35% 4 5% 19% 28% 10%	2557
November	1st to 10th	Night	32% 4 8% 21% 24% 10%	2406
	11th to 20th	Night	34% 3 7% 19% 28% 10%	2470
	21st to end	Night	41% 4 6% 20% 24% 6%	2470
December	1st to 10th	Night	48% 5% 7% 19% 18% 3	2470
	11th to 20th	Night	41% 4 6% 18% 27% 4	3044
	21st to end	Night	51% 6% 8% 20% 14% 2	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	24% 27% 32% 14% 3	270
	11th to 20th	Day	26% 26% 30% 13% 5%	270
	21st to end	Day	21% 27% 36% 12% 4	297
February	1st to 10th	Day	26% 23% 33% 15% 3	270
	11th to 20th	Day	39% 20% 30% 10% 1 1	270
	21st to end	Day	35% 26% 31% 7% 2	225
March	1st to 10th	Day	27% 20% 36% 13% 4 1	270
	11th to 20th	Day	24% 21% 29% 18% 6% 1	403
	21st to end	Day	23% 18% 34% 18% 7% 1	506
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	19% 11% 23% 29% 15% 3	354
November	1st to 10th	Day	20% 10% 29% 26% 13% 3	270
	11th to 20th	Day	14% 20% 36% 24% 7%	270
	21st to end	Day	17% 17% 33% 20% 11% 2	270
December	1st to 10th	Day	31% 20% 25% 17% 6% 1	270
	11th to 20th	Day	25% 21% 31% 16% 6% 1	270
	21st to end	Day	29% 19% 33% 15% 4	297
January	1st to 10th	Night	25% 17% 30% 22% 5%	760
	11th to 20th	Night	23% 19% 28% 23% 7%	760
	21st to end	Night	21% 17% 33% 26% 3	836
February	1st to 10th	Night	23% 13% 29% 31% 4	760
	11th to 20th	Night	27% 18% 28% 25% 3	743
	21st to end	Night	29% 16% 29% 24% 2	628
March	1st to 10th	Night	21% 15% 29% 30% 5%	760
	11th to 20th	Night	21% 12% 30% 28% 8% 1	760
	21st to end	Night	18% 14% 30% 27% 10%	790
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	18% 9% 19% 32% 20% 2	805
November	1st to 10th	Night	14% 8% 21% 33% 20% 3	740
	11th to 20th	Night	16% 11% 29% 30% 14% 1	760
	21st to end	Night	21% 10% 25% 31% 13% 1	760
December	1st to 10th	Night	24% 13% 28% 26% 9%	760
	11th to 20th	Night	23% 11% 26% 30% 9%	760
	21st to end	Night	26% 14% 26% 28% 6%	836

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