Mount Fritz Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Fritz Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	4 5% 2 5% 4 4 5% 4 6% 8% 52%	140152
	Night	3 5% 2 5% 3 3 3 3 5% 8% 59%	138864
Transparency	Day	26% 7% 11% 24% 23% 9%	135079
	Night	22% 5% 12% 27% 25% 9%	134956
Seeing	Day	13% 26% 32% 20% 7% 2	17440
	Night	10% 29% 29% 22% 8% 1	27620

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	1 4 4 7% 4 3 4 3 5% 6% 60%	4720
	11th to 20th	Day	2 1 6% 4 4 4 4 6% 8% 62%	4837
	21st to end	Day	4 2 6% 4 4 4 3 5% 7% 62%	5844
February	1st to 10th	Day	5% 3 7% 4 4 4 3 6% 8% 57%	1890
	11th to 20th	Day	4 2 6% 3 5% 5% 4 6% 9% 55%	1974
	21st to end	Day	1 6% 2 6% 3 3 3 3 6% 9% 59%	1730
March	1st to 10th	Day	2 9% 4 6% 3 3 4 3 5% 7% 54%	4600
	11th to 20th	Day	2 10% 4 8% 4 4 5% 4 7% 8% 46%	5067
	21st to end	Day	3 7% 2 6% 4 4 6% 4 6% 9% 49%	5787
April	1st to 10th	Day	3 7% 2 5% 4 4 4 4 6% 8% 52%	2400
	11th to 20th	Day	7% 8% 4 7% 4 4 4 4 7% 7% 43%	2371
	21st to end	Day	7% 9% 3 7% 5% 4 6% 6% 7% 9% 39%	2400
May	1st to 10th	Day	10% 9% 3 6% 4 4 5% 4 6% 8% 42%	5919
	11th to 20th	Day	7% 7% 2 6% 5% 6% 6% 5% 7% 8% 42%	6298
	21st to end	Day	6% 6% 2 5% 4 4 6% 5% 8% 9% 45%	6925
June	1st to 10th	Day	4 5% 2 4 4 5% 6% 6% 9% 9% 46%	2800
	11th to 20th	Day	5% 4 2 4 4 4 5% 5% 7% 10% 51%	2800
	21st to end	Day	3 3 1 5% 5% 5% 6% 5% 7% 8% 51%	2800
July	1st to 10th	Day	4 3 1 4 4 5% 5% 5% 8% 9% 51%	6339
	11th to 20th	Day	2 3 1 5% 5% 6% 7% 6% 8% 10% 46%	6340
	21st to end	Day	5% 4 2 6% 6% 6% 6% 5% 7% 8% 46%	6823
August	1st to 10th	Day	7% 4 2 5% 4 5% 6% 6% 8% 10% 43%	5779
	11th to 20th	Day	3 3 2 6% 5% 5% 5% 5% 6% 9% 52%	5463
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 6% 8% 44%	5984
September	1st to 10th	Day	4 5% 2 4 3 4 4 4 6% 8% 56%	2400
	11th to 20th	Day	5% 3 1 4 4 4 5% 4 7% 7% 55%	2396
	21st to end	Day	6% 5% 2 5% 3 3 3 3 4 5% 62%	2180
October	1st to 10th	Day	3 5% 1 3 2 2 2 3 3 4 71%	3270
	11th to 20th	Day	5% 3 2 5% 3 3 4 4 4 7% 60%	3250
	21st to end	Day	3 6% 2 4 3 3 3 3 4 6% 63%	3422
November	1st to 10th	Day	2 4 1 4 3 3 3 3 5% 7% 65%	1853
	11th to 20th	Day	1 5% 1 3 3 3 3 3 6% 9% 63%	1890
	21st to end	Day	1 5% 2 4 4 4 4 4 6% 8% 59%	1703
December	1st to 10th	Day	2 1 3 2 2 2 3 4 7% 72%	2700
	11th to 20th	Day	2 4 1 4 3 3 2 3 5% 8% 66%	3106
	21st to end	Day	1 6% 3 8% 4 4 4 3 6% 8% 54%	4092
January	1st to 10th	Night	6% 3 6% 3 3 3 3 5% 9% 58%	8201
	11th to 20th	Night	3 1 6% 4 3 3 3 5% 7% 65%	8212
	21st to end	Night	1 4 2 6% 5% 4 3 3 4 6% 63%	8370
February	1st to 10th	Night	5% 2 5% 3 4 4 3 5% 8% 60%	2730
	11th to 20th	Night	5% 3 6% 4 5% 5% 4 5% 6% 58%	2531
	21st to end	Night	1 7% 3 6% 3 3 3 3 4 6% 63%	2076
March	1st to 10th	Night	1 9% 4 6% 4 3 3 3 4 6% 56%	5494
	11th to 20th	Night	2 10% 3 5% 2 3 3 3 5% 8% 56%	5082
	21st to end	Night	4 7% 3 6% 3 3 3 3 5% 8% 53%	5173
April	1st to 10th	Night	3 7% 2 5% 3 4 4 4 6% 7% 56%	2000
	11th to 20th	Night	6% 9% 3 6% 3 2 3 3 5% 7% 52%	1977
	21st to end	Night	9% 10% 2 5% 3 4 3 4 5% 7% 48%	2000
May	1st to 10th	Night	10% 8% 2 6% 4 4 4 4 5% 6% 48%	4057
	11th to 20th	Night	7% 6% 2 5% 4 4 5% 4 6% 8% 50%	3617
	21st to end	Night	8% 7% 2 4 3 3 4 3 5% 7% 54%	3972
June	1st to 10th	Night	5% 4 1 3 2 3 4 3 7% 10% 58%	1600
	11th to 20th	Night	5% 2 1 3 3 3 4 4 6% 8% 62%	1600
	21st to end	Night	4 3 1 3 2 2 4 3 7% 8% 64%	1600
July	1st to 10th	Night	3 3 1 4 3 3 3 4 6% 8% 60%	3640
	11th to 20th	Night	3 3 1 3 3 4 5% 5% 8% 11% 54%	3640
	21st to end	Night	3 3 1 4 3 4 5% 5% 7% 8% 58%	4046
August	1st to 10th	Night	7% 4 2 4 3 3 4 4 7% 10% 52%	4033
	11th to 20th	Night	3 3 1 4 3 4 4 4 7% 9% 58%	4373
	21st to end	Night	6% 5% 1 4 3 4 4 4 6% 9% 53%	4993
September	1st to 10th	Night	5% 3 1 5% 3 3 3 3 5% 7% 62%	2000
	11th to 20th	Night	5% 3 2 3 3 2 3 2 5% 7% 64%	2003
	21st to end	Night	5% 4 1 3 2 4 3 3 5% 6% 64%	2215
October	1st to 10th	Night	3 4 1 3 2 2 2 2 3 5% 74%	3920
	11th to 20th	Night	5% 5% 1 4 3 3 3 3 5% 7% 61%	3898
	21st to end	Night	3 6% 2 4 2 2 3 3 4 6% 64%	4343
November	1st to 10th	Night	2 4 1 4 2 3 3 3 5% 7% 67%	2665
	11th to 20th	Night	1 4 1 3 3 3 3 3 6% 7% 66%	2730
	21st to end	Night	1 4 2 5% 3 3 3 3 6% 6% 62%	2914
December	1st to 10th	Night	1 2 1 3 2 2 2 2 4 6% 75%	4710
	11th to 20th	Night	1 4 2 4 3 2 3 2 4 8% 67%	5321
	21st to end	Night	2 6% 3 6% 3 3 4 3 5% 9% 55%	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	24% 5% 12% 29% 26% 5%	4723
	11th to 20th	Day	19% 4 14% 27% 25% 10%	4837
	21st to end	Day	21% 6% 10% 28% 25% 10%	5844
February	1st to 10th	Day	26% 5% 16% 27% 20% 6%	1890
	11th to 20th	Day	23% 6% 13% 30% 22% 6%	1983
	21st to end	Day	24% 4 11% 26% 26% 8%	1730
March	1st to 10th	Day	27% 5% 7% 24% 28% 7%	4600
	11th to 20th	Day	31% 8% 11% 26% 21% 3	5067
	21st to end	Day	25% 7% 15% 38% 14% 1	5787
April	1st to 10th	Day	28% 6% 13% 26% 19% 8%	2400
	11th to 20th	Day	36% 6% 12% 22% 19% 5%	2374
	21st to end	Day	38% 8% 10% 27% 16% 1	2400
May	1st to 10th	Day	37% 7% 9% 24% 18% 4	5919
	11th to 20th	Day	33% 8% 12% 20% 21% 6%	6302
	21st to end	Day	27% 10% 13% 23% 24% 4	6926
June	1st to 10th	Day	28% 9% 18% 22% 16% 6%	2800
	11th to 20th	Day	27% 7% 11% 19% 24% 13%	2800
	21st to end	Day	25% 9% 11% 20% 23% 12%	2800
July	1st to 10th	Day	22% 9% 9% 15% 24% 21%	6271
	11th to 20th	Day	26% 11% 12% 22% 17% 12%	6265
	21st to end	Day	30% 10% 10% 16% 18% 15%	6626
August	1st to 10th	Day	34% 11% 13% 18% 15% 9%	4208
	11th to 20th	Day	27% 9% 11% 17% 20% 16%	3956
	21st to end	Day	26% 8% 11% 18% 25% 12%	4311
September	1st to 10th	Day	25% 7% 7% 20% 27% 14%	2400
	11th to 20th	Day	25% 7% 8% 22% 26% 12%	2396
	21st to end	Day	25% 4 7% 20% 34% 9%	2180
October	1st to 10th	Day	18% 4 5% 17% 34% 22%	3270
	11th to 20th	Day	23% 6% 9% 31% 24% 7%	3250
	21st to end	Day	22% 4 7% 29% 31% 6%	3422
November	1st to 10th	Day	20% 5% 12% 28% 28% 8%	1853
	11th to 20th	Day	19% 4 12% 31% 26% 8%	1890
	21st to end	Day	23% 5% 11% 30% 22% 9%	1703
December	1st to 10th	Day	13% 4 11% 36% 29% 6%	2700
	11th to 20th	Day	17% 5% 11% 38% 27% 3	3104
	21st to end	Day	27% 5% 10% 28% 20% 9%	4092
January	1st to 10th	Night	21% 5% 10% 34% 23% 7%	8202
	11th to 20th	Night	17% 4 16% 26% 26% 12%	8212
	21st to end	Night	21% 4 11% 24% 28% 12%	8370
February	1st to 10th	Night	21% 5% 17% 29% 22% 6%	2730
	11th to 20th	Night	23% 4 15% 31% 23% 5%	2532
	21st to end	Night	23% 2 10% 27% 28% 10%	2076
March	1st to 10th	Night	27% 5% 7% 20% 30% 10%	5494
	11th to 20th	Night	25% 4 11% 21% 29% 9%	5082
	21st to end	Night	27% 5% 17% 33% 16% 1	5173
April	1st to 10th	Night	25% 6% 12% 29% 21% 7%	2000
	11th to 20th	Night	31% 4 14% 28% 19% 5%	1977
	21st to end	Night	33% 4 11% 30% 21% 1	2000
May	1st to 10th	Night	35% 5% 11% 28% 16% 5%	4054
	11th to 20th	Night	29% 5% 15% 28% 18% 4	3618
	21st to end	Night	26% 5% 14% 29% 23% 3	3972
June	1st to 10th	Night	21% 5% 23% 29% 17% 6%	1600
	11th to 20th	Night	18% 6% 14% 22% 22% 17%	1600
	21st to end	Night	17% 6% 11% 24% 27% 15%	1600
July	1st to 10th	Night	20% 6% 9% 20% 28% 17%	3600
	11th to 20th	Night	19% 7% 19% 27% 18% 10%	3592
	21st to end	Night	21% 7% 15% 23% 19% 15%	3931
August	1st to 10th	Night	25% 9% 18% 23% 19% 6%	2928
	11th to 20th	Night	22% 8% 18% 21% 20% 11%	3168
	21st to end	Night	18% 7% 14% 24% 29% 8%	3597
September	1st to 10th	Night	21% 4 11% 22% 27% 15%	2000
	11th to 20th	Night	20% 4 10% 27% 26% 13%	2003
	21st to end	Night	21% 4 7% 27% 32% 10%	2215
October	1st to 10th	Night	15% 3 7% 23% 33% 21%	3920
	11th to 20th	Night	21% 5% 11% 30% 26% 7%	3898
	21st to end	Night	20% 5% 8% 28% 31% 9%	4343
November	1st to 10th	Night	17% 5% 13% 28% 29% 9%	2665
	11th to 20th	Night	17% 4 12% 32% 27% 8%	2730
	21st to end	Night	21% 5% 11% 30% 26% 7%	2914
December	1st to 10th	Night	12% 3 13% 36% 31% 6%	4710
	11th to 20th	Night	15% 6% 10% 35% 30% 4	5322
	21st to end	Night	25% 5% 12% 25% 26% 8%	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	20% 35% 27% 12% 4 2	330
	11th to 20th	Day	19% 47% 22% 11% 2	330
	21st to end	Day	17% 47% 25% 8% 2	363
February	1st to 10th	Day	20% 51% 21% 6% 1 1	330
	11th to 20th	Day	15% 50% 26% 7% 2	330
	21st to end	Day	25% 44% 23% 7% 1	273
March	1st to 10th	Day	18% 38% 31% 12% 2	330
	11th to 20th	Day	19% 33% 33% 11% 4	518
	21st to end	Day	17% 37% 33% 10% 3	550
April	1st to 10th	Day	17% 39% 28% 13% 2	500
	11th to 20th	Day	24% 37% 27% 10% 2 1	492
	21st to end	Day	20% 31% 30% 13% 4 1	500
May	1st to 10th	Day	21% 27% 32% 15% 5% 1	638
	11th to 20th	Day	18% 17% 35% 20% 8% 2	725
	21st to end	Day	12% 19% 33% 26% 9% 2	789
June	1st to 10th	Day	7% 16% 33% 27% 14% 3	730
	11th to 20th	Day	8% 18% 39% 25% 8% 2	730
	21st to end	Day	6% 15% 38% 26% 12% 3	730
July	1st to 10th	Day	8% 6% 31% 34% 17% 3	721
	11th to 20th	Day	2 7% 39% 30% 16% 6%	718
	21st to end	Day	5% 8% 33% 34% 16% 5%	764
August	1st to 10th	Day	8% 11% 38% 32% 9% 2	527
	11th to 20th	Day	6% 9% 40% 31% 12% 3	517
	21st to end	Day	7% 15% 34% 34% 9% 2	583
September	1st to 10th	Day	7% 15% 38% 29% 10% 2	530
	11th to 20th	Day	7% 17% 41% 25% 10% 1	526
	21st to end	Day	9% 20% 32% 23% 12% 4	330
October	1st to 10th	Day	9% 19% 31% 29% 9% 3	330
	11th to 20th	Day	14% 34% 31% 14% 5% 1	330
	21st to end	Day	12% 37% 32% 15% 4 1	363
November	1st to 10th	Day	15% 37% 28% 13% 5% 2	330
	11th to 20th	Day	15% 42% 30% 11% 2 1	330
	21st to end	Day	15% 40% 34% 10% 1 1	330
December	1st to 10th	Day	24% 38% 25% 9% 3 1	330
	11th to 20th	Day	23% 47% 24% 5% 1	330
	21st to end	Day	22% 45% 22% 9% 1 1	363
January	1st to 10th	Night	11% 45% 26% 13% 3 1	1050
	11th to 20th	Night	16% 50% 21% 9% 3	1038
	21st to end	Night	10% 49% 27% 10% 3	924
February	1st to 10th	Night	9% 52% 26% 10% 3	840
	11th to 20th	Night	10% 44% 31% 14% 1	824
	21st to end	Night	14% 46% 29% 10% 1	692
March	1st to 10th	Night	10% 40% 34% 12% 4	840
	11th to 20th	Night	13% 33% 30% 20% 5%	828
	21st to end	Night	11% 33% 34% 18% 3 1	745
April	1st to 10th	Night	14% 30% 38% 17% 2	600
	11th to 20th	Night	13% 29% 36% 17% 6% 1	593
	21st to end	Night	15% 25% 30% 23% 6% 1	600
May	1st to 10th	Night	15% 16% 37% 24% 8% 2	600
	11th to 20th	Night	17% 9% 32% 27% 13% 1	591
	21st to end	Night	9% 12% 31% 31% 15% 1	650
June	1st to 10th	Night	5% 10% 30% 37% 16% 2	600
	11th to 20th	Night	6% 10% 38% 36% 10%	600
	21st to end	Night	5% 8% 29% 39% 19% 1	600
July	1st to 10th	Night	7% 4 19% 42% 25% 3	585
	11th to 20th	Night	2 3 24% 46% 23% 3	582
	21st to end	Night	4 3 17% 46% 27% 3	657
August	1st to 10th	Night	5% 4 24% 46% 20% 2	597
	11th to 20th	Night	4 4 24% 44% 21% 3	594
	21st to end	Night	4 5% 23% 42% 23% 4	660
September	1st to 10th	Night	7% 6% 25% 42% 17% 4	600
	11th to 20th	Night	6% 10% 31% 37% 14% 3	600
	21st to end	Night	6% 15% 28% 36% 12% 2	615
October	1st to 10th	Night	8% 19% 36% 26% 10% 1	800
	11th to 20th	Night	8% 33% 30% 22% 7% 1	800
	21st to end	Night	10% 32% 39% 14% 5% 1	935
November	1st to 10th	Night	9% 36% 35% 15% 4 1	1025
	11th to 20th	Night	11% 44% 31% 11% 2 1	1050
	21st to end	Night	9% 40% 34% 14% 3	1050
December	1st to 10th	Night	13% 47% 27% 12% 2	1050
	11th to 20th	Night	13% 48% 26% 10% 3	1050
	21st to end	Night	14% 47% 25% 10% 3	1155

Page generated at 2026-03-15 00:23:52. Version ac07

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