Matilda's observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Matilda's observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	2 4 2 5% 4 4 4 4 6% 8% 57%	140480
	Night	2 5% 2 5% 3 3 4 4 5% 7% 59%	139850
Transparency	Day	15% 38% 7% 16% 18% 6%	135477
	Night	16% 37% 7% 16% 18% 6%	135860
Seeing	Day	10% 22% 30% 24% 12% 2	17353
	Night	6% 18% 34% 28% 13% 1	27498

By Month

Percentage Cloud Cover SW (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	4 2 8% 5% 4 4 3 5% 6% 60%	4757
	11th to 20th	Day	3 2 6% 3 2 2 2 4 5% 70%	5340
	21st to end	Day	1 5% 3 7% 3 4 3 3 5% 6% 61%	5844
February	1st to 10th	Day	5% 2 7% 3 3 3 3 5% 6% 62%	1917
	11th to 20th	Day	1 6% 2 6% 3 3 3 3 6% 7% 60%	2049
	21st to end	Day	1 8% 3 5% 3 3 4 3 4 7% 59%	1730
March	1st to 10th	Day	1 6% 3 6% 3 3 3 3 5% 6% 61%	4665
	11th to 20th	Day	1 8% 3 8% 5% 5% 5% 4 6% 8% 46%	5110
	21st to end	Day	1 6% 3 6% 4 4 4 4 6% 7% 56%	5889
April	1st to 10th	Day	2 6% 3 6% 3 4 4 4 6% 8% 54%	2520
	11th to 20th	Day	2 7% 3 5% 3 4 4 4 6% 7% 54%	2491
	21st to end	Day	4 7% 2 5% 3 4 4 4 6% 8% 53%	2520
May	1st to 10th	Day	3 6% 2 6% 4 4 5% 4 6% 9% 52%	5560
	11th to 20th	Day	3 5% 2 5% 4 4 5% 4 6% 7% 57%	6138
	21st to end	Day	2 3 1 4 3 4 4 4 6% 8% 62%	6925
June	1st to 10th	Day	3 3 2 3 3 3 5% 4 8% 10% 56%	2800
	11th to 20th	Day	3 2 1 2 3 3 4 4 6% 7% 65%	2800
	21st to end	Day	3 3 2 5% 5% 4 5% 5% 8% 10% 51%	2800
July	1st to 10th	Day	2 2 1 4 3 4 5% 4 7% 7% 62%	6339
	11th to 20th	Day	1 2 1 4 4 5% 7% 7% 10% 13% 45%	6259
	21st to end	Day	3 3 2 5% 5% 6% 8% 7% 10% 10% 40%	6401
August	1st to 10th	Day	4 3 1 5% 5% 6% 7% 6% 9% 10% 43%	5651
	11th to 20th	Day	2 2 1 4 3 4 6% 6% 9% 10% 53%	5366
	21st to end	Day	4 4 2 5% 5% 6% 7% 6% 10% 12% 40%	5984
September	1st to 10th	Day	5% 4 1 5% 5% 5% 5% 4 7% 8% 51%	2400
	11th to 20th	Day	5% 3 2 4 4 3 5% 4 6% 6% 59%	2390
	21st to end	Day	3 3 1 4 3 3 3 3 5% 7% 64%	2200
October	1st to 10th	Day	3 3 1 3 2 2 2 2 3 6% 73%	3275
	11th to 20th	Day	3 3 2 3 3 5% 4 4 6% 6% 62%	3250
	21st to end	Day	2 3 1 3 2 2 2 2 4 5% 73%	3548
November	1st to 10th	Day	1 3 1 4 2 2 2 3 4 8% 70%	1879
	11th to 20th	Day	1 4 2 3 2 2 2 3 4 7% 70%	1890
	21st to end	Day	2 5% 2 5% 3 3 3 3 4 6% 64%	1890
December	1st to 10th	Day	1 3 1 4 2 1 2 2 3 7% 74%	2705
	11th to 20th	Day	3 1 3 3 3 3 3 6% 10% 64%	3106
	21st to end	Day	4 2 5% 4 3 3 4 5% 8% 62%	4092
January	1st to 10th	Night	5% 3 8% 4 4 3 3 4 7% 58%	8201
	11th to 20th	Night	3 2 6% 3 3 3 2 4 7% 68%	7668
	21st to end	Night	1 6% 3 5% 3 3 3 3 4 6% 63%	8370
February	1st to 10th	Night	1 6% 3 7% 3 3 2 2 3 7% 63%	2700
	11th to 20th	Night	2 8% 3 6% 3 3 3 3 5% 7% 58%	2458
	21st to end	Night	1 9% 3 6% 3 3 2 3 4 6% 60%	2076
March	1st to 10th	Night	1 7% 4 6% 3 3 3 3 4 5% 61%	5416
	11th to 20th	Night	2 9% 4 7% 4 3 3 3 4 6% 55%	5110
	21st to end	Night	2 10% 3 6% 3 3 4 3 4 6% 56%	5313
April	1st to 10th	Night	3 7% 2 5% 3 3 3 4 5% 7% 59%	2100
	11th to 20th	Night	2 8% 2 5% 3 3 4 4 6% 8% 56%	2077
	21st to end	Night	5% 8% 2 5% 3 3 4 3 5% 7% 55%	2100
May	1st to 10th	Night	4 7% 2 5% 4 4 5% 3 5% 8% 55%	4640
	11th to 20th	Night	3 4 1 4 3 4 4 4 6% 8% 58%	3904
	21st to end	Night	4 4 2 5% 3 4 4 4 4 6% 60%	3972
June	1st to 10th	Night	2 2 1 3 3 4 5% 5% 7% 9% 59%	1600
	11th to 20th	Night	1 2 1 3 3 2 3 4 5% 7% 68%	1600
	21st to end	Night	3 5% 1 4 4 4 5% 4 9% 9% 52%	1600
July	1st to 10th	Night	2 3 1 5% 5% 6% 6% 5% 7% 8% 52%	3640
	11th to 20th	Night	2 2 1 4 5% 7% 7% 6% 10% 12% 44%	3703
	21st to end	Night	3 4 2 6% 5% 6% 7% 7% 10% 12% 38%	4472
August	1st to 10th	Night	5% 3 2 6% 5% 5% 6% 6% 8% 9% 44%	4160
	11th to 20th	Night	2 2 1 4 4 4 7% 6% 10% 15% 45%	4483
	21st to end	Night	6% 4 2 6% 6% 6% 7% 6% 10% 12% 34%	4993
September	1st to 10th	Night	4 4 2 5% 4 4 5% 6% 7% 9% 52%	2000
	11th to 20th	Night	3 2 1 3 3 3 4 3 5% 8% 64%	2009
	21st to end	Night	4 3 1 3 2 3 3 3 4 6% 67%	2200
October	1st to 10th	Night	3 3 1 3 2 3 3 3 4 6% 70%	3911
	11th to 20th	Night	3 3 1 3 3 3 4 3 6% 7% 64%	3898
	21st to end	Night	3 3 1 3 2 2 2 2 3 4 75%	4252
November	1st to 10th	Night	1 4 1 2 2 2 2 2 4 6% 74%	2615
	11th to 20th	Night	1 4 1 2 2 2 2 2 4 7% 72%	2730
	21st to end	Night	2 6% 2 4 3 3 3 4 5% 7% 62%	2730
December	1st to 10th	Night	1 4 2 5% 2 2 3 2 3 4 72%	4700
	11th to 20th	Night	1 4 1 3 2 2 3 3 4 7% 70%	5321
	21st to end	Night	1 6% 3 6% 4 4 4 3 5% 6% 59%	7128

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	20% 26% 6% 25% 18% 4	4760
	11th to 20th	Day	14% 25% 8% 20% 26% 7%	5340
	21st to end	Day	20% 25% 5% 14% 27% 9%	5844
February	1st to 10th	Day	8% 59% 6% 12% 10% 6%	1917
	11th to 20th	Day	11% 59% 4 12% 10% 4	2059
	21st to end	Day	11% 60% 2 7% 14% 6%	1730
March	1st to 10th	Day	14% 31% 4 7% 30% 13%	4665
	11th to 20th	Day	22% 36% 5% 13% 20% 4	5110
	21st to end	Day	16% 32% 6% 33% 12% 2	5889
April	1st to 10th	Day	12% 61% 3 13% 10% 2	2520
	11th to 20th	Day	12% 61% 3 9% 11% 4	2494
	21st to end	Day	12% 61% 4 9% 12% 2	2520
May	1st to 10th	Day	20% 30% 8% 18% 16% 7%	5560
	11th to 20th	Day	16% 32% 5% 12% 26% 8%	6142
	21st to end	Day	12% 30% 8% 15% 27% 7%	6926
June	1st to 10th	Day	8% 64% 6% 10% 8% 4	2800
	11th to 20th	Day	7% 63% 3 8% 10% 10%	2800
	21st to end	Day	9% 63% 5% 8% 11% 5%	2800
July	1st to 10th	Day	13% 27% 6% 12% 18% 24%	6271
	11th to 20th	Day	18% 27% 19% 26% 6% 3	6184
	21st to end	Day	23% 28% 14% 20% 12% 3	6217
August	1st to 10th	Day	25% 39% 19% 12% 4 2	4113
	11th to 20th	Day	14% 39% 14% 14% 14% 7%	3882
	21st to end	Day	23% 39% 15% 13% 9% 1	4311
September	1st to 10th	Day	11% 61% 6% 10% 8% 3	2400
	11th to 20th	Day	10% 58% 4 11% 13% 5%	2390
	21st to end	Day	9% 58% 4 10% 15% 4	2200
October	1st to 10th	Day	6% 36% 2 9% 31% 17%	3275
	11th to 20th	Day	18% 35% 5% 21% 16% 4	3250
	21st to end	Day	10% 35% 2 13% 33% 6%	3548
November	1st to 10th	Day	8% 53% 3 13% 18% 5%	1879
	11th to 20th	Day	9% 56% 3 13% 16% 5%	1890
	21st to end	Day	11% 55% 6% 12% 11% 5%	1890
December	1st to 10th	Day	6% 34% 4 25% 26% 4	2705
	11th to 20th	Day	8% 33% 5% 24% 27% 3	3104
	21st to end	Day	14% 27% 6% 23% 23% 7%	4092
January	1st to 10th	Night	22% 23% 5% 29% 17% 4	8202
	11th to 20th	Night	14% 27% 7% 22% 20% 10%	7668
	21st to end	Night	20% 24% 5% 15% 27% 9%	8370
February	1st to 10th	Night	9% 60% 5% 9% 12% 5%	2700
	11th to 20th	Night	12% 58% 5% 12% 10% 3	2458
	21st to end	Night	11% 59% 2 9% 14% 5%	2076
March	1st to 10th	Night	18% 30% 2 7% 31% 11%	5416
	11th to 20th	Night	20% 32% 5% 15% 23% 5%	5110
	21st to end	Night	21% 30% 8% 29% 9% 2	5313
April	1st to 10th	Night	11% 59% 6% 14% 9% 2	2100
	11th to 20th	Night	11% 61% 3 10% 11% 3	2077
	21st to end	Night	13% 60% 5% 12% 8% 2	2100
May	1st to 10th	Night	20% 29% 9% 20% 17% 5%	4637
	11th to 20th	Night	14% 32% 7% 18% 19% 11%	3905
	21st to end	Night	15% 30% 11% 12% 24% 8%	3972
June	1st to 10th	Night	8% 64% 7% 9% 8% 4	1600
	11th to 20th	Night	6% 63% 5% 8% 10% 10%	1600
	21st to end	Night	10% 62% 5% 9% 11% 4	1600
July	1st to 10th	Night	19% 27% 9% 13% 14% 18%	3600
	11th to 20th	Night	22% 27% 23% 21% 4 3	3655
	21st to end	Night	26% 29% 16% 15% 13% 2	4344
August	1st to 10th	Night	31% 39% 14% 9% 5% 2	3022
	11th to 20th	Night	17% 41% 16% 11% 10% 5%	3243
	21st to end	Night	25% 39% 15% 13% 7%	3597
September	1st to 10th	Night	11% 59% 6% 12% 9% 3	2000
	11th to 20th	Night	8% 58% 5% 10% 15% 4	2009
	21st to end	Night	9% 58% 3 11% 16% 3	2200
October	1st to 10th	Night	10% 36% 2 12% 29% 11%	3911
	11th to 20th	Night	17% 35% 6% 19% 18% 5%	3898
	21st to end	Night	10% 35% 2 17% 29% 6%	4252
November	1st to 10th	Night	7% 53% 3 13% 19% 4	2615
	11th to 20th	Night	8% 54% 2 15% 17% 5%	2730
	21st to end	Night	12% 55% 5% 13% 12% 4	2730
December	1st to 10th	Night	10% 34% 3 27% 22% 4	4700
	11th to 20th	Night	9% 32% 5% 22% 28% 5%	5322
	21st to end	Night	20% 27% 8% 17% 24% 5%	7128

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	11% 27% 35% 19% 7% 1	330
	11th to 20th	Day	10% 27% 40% 19% 4 1	330
	21st to end	Day	5% 22% 44% 19% 10%	363
February	1st to 10th	Day	10% 37% 32% 17% 4 1	330
	11th to 20th	Day	14% 42% 29% 11% 5%	330
	21st to end	Day	19% 33% 32% 14% 2	273
March	1st to 10th	Day	16% 35% 26% 18% 4 1	356
	11th to 20th	Day	16% 32% 32% 15% 4 1	540
	21st to end	Day	22% 38% 29% 9% 2 1	594
April	1st to 10th	Day	16% 37% 28% 16% 3 1	540
	11th to 20th	Day	21% 42% 27% 8% 2	532
	21st to end	Day	19% 39% 29% 10% 2	540
May	1st to 10th	Day	14% 35% 34% 14% 3	543
	11th to 20th	Day	9% 24% 35% 23% 8% 1	745
	21st to end	Day	9% 25% 31% 25% 10% 1	789
June	1st to 10th	Day	3 15% 33% 29% 17% 2	730
	11th to 20th	Day	4 19% 35% 25% 14% 3	730
	21st to end	Day	5% 15% 29% 30% 17% 4	730
July	1st to 10th	Day	4 6% 26% 36% 23% 4	721
	11th to 20th	Day	3 5% 18% 36% 32% 6%	689
	21st to end	Day	5% 3 20% 41% 26% 4	575
August	1st to 10th	Day	6% 6% 31% 33% 21% 3	527
	11th to 20th	Day	4 7% 27% 39% 19% 3	517
	21st to end	Day	6% 7% 28% 35% 18% 5%	583
September	1st to 10th	Day	7% 8% 27% 34% 19% 4	530
	11th to 20th	Day	6% 15% 37% 28% 11% 3	520
	21st to end	Day	5% 15% 29% 31% 15% 5%	330
October	1st to 10th	Day	8% 25% 22% 25% 15% 5%	330
	11th to 20th	Day	11% 25% 30% 21% 8% 5%	330
	21st to end	Day	14% 22% 27% 23% 11% 3	363
November	1st to 10th	Day	10% 30% 32% 18% 8% 2	330
	11th to 20th	Day	7% 25% 32% 22% 12% 1	330
	21st to end	Day	11% 25% 31% 23% 8% 1	330
December	1st to 10th	Day	12% 26% 36% 17% 8% 1	330
	11th to 20th	Day	6% 29% 39% 21% 4 1	330
	21st to end	Day	17% 30% 34% 15% 3	363
January	1st to 10th	Night	5% 22% 35% 27% 9% 1	1050
	11th to 20th	Night	5% 21% 36% 26% 10% 1	845
	21st to end	Night	4 19% 39% 28% 9% 1	924
February	1st to 10th	Night	5% 24% 43% 22% 5%	840
	11th to 20th	Night	7% 30% 39% 18% 5%	824
	21st to end	Night	10% 29% 44% 16% 2	692
March	1st to 10th	Night	7% 29% 39% 20% 5%	840
	11th to 20th	Night	7% 21% 40% 25% 6% 1	840
	21st to end	Night	7% 29% 41% 18% 4	795
April	1st to 10th	Night	9% 31% 42% 15% 3	630
	11th to 20th	Night	9% 27% 40% 20% 4	623
	21st to end	Night	10% 25% 40% 20% 5%	630
May	1st to 10th	Night	11% 17% 37% 27% 7%	630
	11th to 20th	Night	8% 16% 37% 27% 11% 1	609
	21st to end	Night	7% 18% 36% 27% 10% 1	650
June	1st to 10th	Night	3 9% 35% 33% 20% 1	600
	11th to 20th	Night	2 12% 40% 34% 12% 1	600
	21st to end	Night	4 6% 31% 35% 23% 2	600
July	1st to 10th	Night	5% 3 21% 40% 29% 1	585
	11th to 20th	Night	3 2 14% 40% 37% 4	582
	21st to end	Night	5% 1 10% 35% 41% 8%	657
August	1st to 10th	Night	5% 3 14% 37% 37% 5%	597
	11th to 20th	Night	6% 3 19% 44% 26% 3	594
	21st to end	Night	5% 2 19% 41% 29% 4	660
September	1st to 10th	Night	4 5% 21% 42% 23% 4	600
	11th to 20th	Night	4 8% 30% 33% 23% 3	600
	21st to end	Night	4 12% 26% 38% 19% 1	600
October	1st to 10th	Night	6% 16% 31% 32% 13% 2	791
	11th to 20th	Night	5% 16% 36% 27% 14% 2	800
	21st to end	Night	7% 19% 31% 26% 14% 3	880
November	1st to 10th	Night	6% 19% 33% 28% 13% 1	975
	11th to 20th	Night	4 21% 39% 24% 10% 1	1050
	21st to end	Night	7% 22% 30% 28% 11% 1	1050
December	1st to 10th	Night	8% 23% 37% 24% 8% 1	1050
	11th to 20th	Night	5% 26% 37% 24% 8%	1050
	21st to end	Night	8% 23% 40% 24% 5% 1	1155

Page generated at 2026-05-13 23:58:53. Version ac07

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