Northern Cross Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northern Cross Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 10% 4 8% 5% 4 4 4 5% 7% 34%	125643
	Night	15% 11% 4 9% 5% 4 4 3 5% 6% 34%	124125
Transparency	Day	49% 5% 11% 21% 12% 1	129212
	Night	50% 5% 11% 21% 12% 1	128626
Seeing	Day	18% 12% 29% 22% 13% 5%	16807
	Night	19% 10% 29% 29% 11% 1	26744

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	11% 19% 7% 19% 5% 4 3 3 3 4 22%	3617
	11th to 20th	Day	14% 18% 6% 19% 9% 9% 5% 3 4 3 10%	3620
	21st to end	Day	22% 19% 7% 14% 5% 4 5% 2 3 4 15%	4303
February	1st to 10th	Day	14% 15% 6% 12% 8% 5% 6% 3 4 5% 23%	1800
	11th to 20th	Day	14% 14% 7% 13% 8% 5% 4 3 5% 4 23%	1906
	21st to end	Day	14% 16% 6% 12% 5% 4 4 3 4 6% 24%	1650
March	1st to 10th	Day	14% 12% 5% 10% 6% 4 4 3 5% 6% 31%	4559
	11th to 20th	Day	12% 13% 5% 8% 5% 4 4 3 5% 7% 34%	4980
	21st to end	Day	19% 16% 4 9% 5% 4 4 3 4 6% 28%	5465
April	1st to 10th	Day	17% 13% 4 7% 6% 5% 5% 3 5% 6% 30%	2351
	11th to 20th	Day	17% 10% 3 7% 4 5% 4 4 5% 6% 35%	2522
	21st to end	Day	17% 9% 3 8% 5% 4 4 3 5% 7% 35%	2600
May	1st to 10th	Day	20% 9% 3 6% 4 4 4 4 5% 6% 35%	5862
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 8% 35%	5861
	21st to end	Day	16% 7% 2 6% 4 3 4 3 6% 7% 42%	6521
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 4 6% 9% 41%	2800
	11th to 20th	Day	11% 6% 2 8% 4 5% 5% 4 6% 8% 40%	2800
	21st to end	Day	12% 5% 2 6% 6% 5% 5% 5% 7% 10% 38%	2800
July	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	6360
	11th to 20th	Day	10% 4 1 6% 4 4 5% 4 6% 8% 48%	6360
	21st to end	Day	9% 5% 2 5% 4 5% 5% 4 6% 9% 46%	6616
August	1st to 10th	Day	14% 4 2 6% 5% 4 5% 4 6% 8% 42%	4234
	11th to 20th	Day	13% 5% 2 6% 4 5% 5% 5% 6% 9% 41%	4174
	21st to end	Day	9% 5% 2 7% 5% 5% 6% 5% 6% 9% 41%	4588
September	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 7% 8% 40%	2279
	11th to 20th	Day	10% 5% 2 5% 4 3 5% 4 6% 8% 49%	2200
	21st to end	Day	14% 7% 3 7% 5% 4 5% 4 7% 8% 36%	2200
October	1st to 10th	Day	15% 6% 2 6% 5% 4 4 3 5% 6% 44%	2160
	11th to 20th	Day	18% 11% 2 8% 5% 4 4 4 4 7% 33%	1980
	21st to end	Day	21% 12% 3 6% 5% 4 4 4 5% 7% 29%	1910
November	1st to 10th	Day	13% 11% 2 8% 5% 4 4 4 6% 7% 36%	1762
	11th to 20th	Day	15% 14% 5% 8% 5% 4 4 3 5% 7% 28%	1800
	21st to end	Day	19% 15% 5% 9% 5% 4 4 3 4 5% 26%	1731
December	1st to 10th	Day	14% 15% 9% 12% 7% 4 5% 3 5% 5% 21%	2600
	11th to 20th	Day	17% 22% 7% 10% 6% 4 4 3 4 4 18%	2690
	21st to end	Day	24% 22% 7% 12% 5% 4 3 2 3 4 14%	3982
January	1st to 10th	Night	14% 14% 7% 14% 6% 4 4 3 4 5% 24%	6348
	11th to 20th	Night	12% 15% 9% 16% 9% 7% 6% 3 5% 4 14%	6360
	21st to end	Night	17% 17% 9% 12% 5% 4 5% 3 4 4 20%	6657
February	1st to 10th	Night	16% 17% 6% 10% 7% 5% 5% 3 4 4 23%	2600
	11th to 20th	Night	14% 15% 5% 12% 7% 4 4 3 5% 5% 26%	2378
	21st to end	Night	17% 12% 4 10% 6% 4 4 3 5% 6% 27%	1980
March	1st to 10th	Night	15% 12% 3 9% 4 4 4 3 5% 6% 35%	5293
	11th to 20th	Night	10% 10% 3 7% 4 4 4 4 5% 8% 41%	4980
	21st to end	Night	17% 11% 3 7% 5% 3 4 3 4 6% 36%	5467
April	1st to 10th	Night	17% 12% 3 7% 4 3 4 4 5% 6% 35%	2065
	11th to 20th	Night	19% 9% 2 7% 4 3 5% 4 5% 6% 37%	1827
	21st to end	Night	17% 9% 2 6% 5% 4 4 3 5% 7% 38%	1800
May	1st to 10th	Night	20% 11% 3 8% 4 4 4 3 5% 6% 33%	4033
	11th to 20th	Night	19% 8% 3 6% 4 4 3 3 4 6% 41%	4032
	21st to end	Night	14% 6% 2 6% 5% 4 5% 3 4 7% 44%	4333
June	1st to 10th	Night	12% 5% 2 6% 5% 4 3 3 5% 9% 47%	1600
	11th to 20th	Night	11% 6% 2 7% 4 3 4 4 7% 8% 45%	1521
	21st to end	Night	10% 6% 2 6% 4 3 4 3 5% 9% 49%	1600
July	1st to 10th	Night	9% 4 2 5% 4 4 5% 4 7% 10% 47%	3618
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 6% 8% 50%	3617
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 8% 49%	4240
August	1st to 10th	Night	12% 4 2 5% 3 4 4 4 6% 8% 49%	2915
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 8% 49%	2875
	21st to end	Night	9% 6% 2 6% 5% 4 4 3 5% 7% 49%	3307
September	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 5% 7% 49%	2100
	11th to 20th	Night	10% 4 1 4 4 3 4 3 4 7% 56%	2200
	21st to end	Night	14% 7% 3 6% 5% 3 4 4 5% 6% 43%	2200
October	1st to 10th	Night	14% 6% 2 6% 5% 3 3 3 4 6% 47%	2240
	11th to 20th	Night	19% 11% 3 8% 4 4 4 3 5% 6% 35%	2377
	21st to end	Night	21% 11% 2 7% 6% 4 4 3 5% 6% 32%	2684
November	1st to 10th	Night	14% 9% 3 7% 4 3 4 4 5% 6% 41%	2536
	11th to 20th	Night	16% 13% 3 8% 5% 5% 5% 4 6% 5% 30%	2600
	21st to end	Night	20% 15% 4 8% 5% 4 3 4 4 6% 27%	2664
December	1st to 10th	Night	13% 15% 7% 16% 5% 5% 5% 3 4 5% 21%	4580
	11th to 20th	Night	15% 22% 7% 12% 7% 4 3 3 4 5% 18%	5502
	21st to end	Night	24% 23% 8% 10% 5% 4 3 2 3 4 14%	6996

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	66% 5% 10% 14% 5% 1	3618
	11th to 20th	Day	78% 6% 8% 5% 3	3620
	21st to end	Day	75% 4 9% 8% 4	4303
February	1st to 10th	Day	63% 6% 9% 14% 8%	1800
	11th to 20th	Day	65% 4 8% 14% 8% 1	1916
	21st to end	Day	61% 5% 10% 16% 7% 2	1650
March	1st to 10th	Day	54% 4 12% 18% 9% 2	4559
	11th to 20th	Day	50% 4 16% 21% 8% 1	4980
	21st to end	Day	58% 5% 9% 16% 11% 2	5465
April	1st to 10th	Day	54% 5% 11% 18% 11% 1	2351
	11th to 20th	Day	49% 5% 11% 20% 12% 2	2522
	21st to end	Day	48% 6% 13% 18% 14% 1	2600
May	1st to 10th	Day	50% 5% 10% 19% 13% 3	5897
	11th to 20th	Day	46% 7% 12% 21% 13% 1	5865
	21st to end	Day	41% 5% 9% 22% 20% 3	6535
June	1st to 10th	Day	37% 8% 12% 27% 14% 2	2800
	11th to 20th	Day	41% 7% 11% 23% 16% 2	2800
	21st to end	Day	39% 9% 12% 23% 16% 1	2800
July	1st to 10th	Day	39% 5% 13% 26% 15% 1	6291
	11th to 20th	Day	33% 5% 13% 30% 18% 1	6285
	21st to end	Day	35% 5% 14% 23% 21% 1	6428
August	1st to 10th	Day	38% 7% 12% 25% 16% 1	4221
	11th to 20th	Day	39% 7% 9% 32% 11% 1	4213
	21st to end	Day	39% 5% 14% 28% 12% 1	4587
September	1st to 10th	Day	40% 7% 12% 26% 12% 2	2279
	11th to 20th	Day	32% 6% 13% 32% 15% 1	2200
	21st to end	Day	44% 7% 12% 23% 14%	2200
October	1st to 10th	Day	33% 4 9% 33% 20% 1	3524
	11th to 20th	Day	42% 4 10% 26% 18%	3220
	21st to end	Day	47% 4 13% 24% 11% 1	3122
November	1st to 10th	Day	47% 5% 13% 23% 12% 1	1762
	11th to 20th	Day	55% 5% 12% 19% 8% 1	1800
	21st to end	Day	60% 5% 11% 16% 6% 1	1731
December	1st to 10th	Day	66% 4 15% 12% 3	2600
	11th to 20th	Day	70% 4 9% 12% 5%	2686
	21st to end	Day	76% 3 7% 8% 5%	3982
January	1st to 10th	Night	62% 4 9% 18% 6% 2	6348
	11th to 20th	Night	71% 5% 9% 9% 5%	6360
	21st to end	Night	67% 4 10% 12% 6%	6657
February	1st to 10th	Night	64% 6% 10% 13% 7% 1	2600
	11th to 20th	Night	60% 5% 10% 17% 9% 1	2379
	21st to end	Night	56% 5% 12% 16% 8% 1	1980
March	1st to 10th	Night	50% 5% 13% 22% 10% 1	5293
	11th to 20th	Night	40% 4 15% 24% 15% 1	4980
	21st to end	Night	50% 4 12% 19% 14% 2	5467
April	1st to 10th	Night	49% 5% 9% 21% 13% 2	2065
	11th to 20th	Night	46% 6% 10% 18% 18% 2	1827
	21st to end	Night	46% 6% 11% 19% 17% 2	1800
May	1st to 10th	Night	52% 5% 10% 16% 12% 5%	4060
	11th to 20th	Night	46% 5% 9% 26% 11% 3	4033
	21st to end	Night	39% 6% 10% 23% 17% 4	4341
June	1st to 10th	Night	35% 5% 10% 28% 19% 2	1600
	11th to 20th	Night	35% 8% 13% 25% 17% 2	1521
	21st to end	Night	34% 5% 14% 26% 20% 1	1600
July	1st to 10th	Night	33% 5% 13% 28% 17% 4	3580
	11th to 20th	Night	32% 4 13% 29% 20% 1	3572
	21st to end	Night	35% 5% 14% 22% 24% 2	4115
August	1st to 10th	Night	34% 6% 14% 28% 18% 1	2904
	11th to 20th	Night	34% 6% 9% 38% 11% 2	2902
	21st to end	Night	35% 5% 12% 33% 15% 1	3307
September	1st to 10th	Night	34% 6% 12% 30% 17% 2	2100
	11th to 20th	Night	29% 5% 11% 31% 22% 2	2200
	21st to end	Night	40% 6% 12% 27% 15% 1	2200
October	1st to 10th	Night	27% 3 9% 33% 27% 1	3656
	11th to 20th	Night	44% 4 10% 22% 19%	3876
	21st to end	Night	42% 4 11% 30% 13% 1	4422
November	1st to 10th	Night	43% 5% 11% 25% 14% 1	2536
	11th to 20th	Night	54% 6% 12% 18% 9% 1	2600
	21st to end	Night	59% 5% 11% 18% 6%	2664
December	1st to 10th	Night	65% 5% 13% 13% 3	4580
	11th to 20th	Night	68% 5% 9% 12% 5% 1	5505
	21st to end	Night	74% 4 8% 7% 6%	6996

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% 18% 37% 15% 7%	300
	11th to 20th	Day	24% 16% 45% 12% 2 1	300
	21st to end	Day	30% 18% 30% 16% 5% 1	330
February	1st to 10th	Day	27% 22% 36% 12% 2 1	300
	11th to 20th	Day	27% 20% 38% 14% 1	300
	21st to end	Day	29% 27% 33% 8% 2	249
March	1st to 10th	Day	25% 24% 35% 14% 2	327
	11th to 20th	Day	23% 15% 34% 23% 6%	500
	21st to end	Day	29% 14% 31% 20% 5% 1	550
April	1st to 10th	Day	22% 16% 34% 20% 8% 1	500
	11th to 20th	Day	26% 18% 31% 15% 5% 5%	492
	21st to end	Day	25% 13% 30% 20% 9% 2	500
May	1st to 10th	Day	25% 13% 28% 21% 10% 2	503
	11th to 20th	Day	20% 9% 29% 22% 13% 6%	527
	21st to end	Day	17% 8% 24% 26% 17% 8%	609
June	1st to 10th	Day	12% 9% 30% 30% 16% 4	730
	11th to 20th	Day	14% 8% 30% 27% 16% 4	730
	21st to end	Day	12% 9% 28% 31% 15% 5%	730
July	1st to 10th	Day	9% 6% 23% 28% 23% 11%	721
	11th to 20th	Day	10% 7% 28% 29% 18% 9%	718
	21st to end	Day	9% 3 25% 31% 24% 8%	664
August	1st to 10th	Day	9% 3 21% 26% 27% 15%	526
	11th to 20th	Day	13% 4 18% 28% 24% 12%	519
	21st to end	Day	10% 6% 21% 27% 23% 13%	583
September	1st to 10th	Day	13% 8% 22% 24% 21% 12%	530
	11th to 20th	Day	8% 4 20% 22% 31% 16%	530
	21st to end	Day	18% 8% 23% 23% 21% 7%	530
October	1st to 10th	Day	17% 8% 27% 23% 18% 7%	530
	11th to 20th	Day	20% 15% 30% 26% 8% 2	522
	21st to end	Day	29% 16% 35% 15% 5%	359
November	1st to 10th	Day	17% 22% 35% 16% 9% 1	300
	11th to 20th	Day	23% 22% 32% 17% 5% 1	300
	21st to end	Day	30% 19% 34% 13% 4	265
December	1st to 10th	Day	26% 21% 31% 15% 8%	200
	11th to 20th	Day	27% 23% 38% 10% 2	203
	21st to end	Day	28% 24% 33% 14% 1	330
January	1st to 10th	Night	22% 16% 36% 20% 5% 1	1000
	11th to 20th	Night	24% 16% 35% 21% 4 1	1000
	21st to end	Night	22% 12% 35% 26% 5%	956
February	1st to 10th	Night	25% 13% 37% 22% 3	800
	11th to 20th	Night	23% 15% 34% 24% 3	783
	21st to end	Night	23% 18% 37% 19% 2	660
March	1st to 10th	Night	22% 14% 35% 23% 5% 1	800
	11th to 20th	Night	18% 17% 35% 25% 5% 1	800
	21st to end	Night	23% 12% 31% 26% 7% 1	880
April	1st to 10th	Night	22% 15% 34% 24% 4 1	665
	11th to 20th	Night	22% 11% 34% 24% 7% 2	593
	21st to end	Night	21% 9% 34% 29% 8%	600
May	1st to 10th	Night	21% 7% 32% 32% 7% 1	600
	11th to 20th	Night	21% 6% 30% 30% 12% 1	591
	21st to end	Night	12% 5% 24% 39% 18% 2	650
June	1st to 10th	Night	13% 5% 25% 44% 14% 1	600
	11th to 20th	Night	13% 4 25% 42% 16%	521
	21st to end	Night	12% 4 22% 45% 16% 2	600
July	1st to 10th	Night	9% 2 17% 41% 27% 4	585
	11th to 20th	Night	11% 2 20% 41% 23% 3	582
	21st to end	Night	6% 2 20% 47% 23% 2	657
August	1st to 10th	Night	9% 2 15% 44% 27% 3	597
	11th to 20th	Night	11% 1 21% 35% 27% 4	594
	21st to end	Night	11% 3 17% 36% 30% 4	660
September	1st to 10th	Night	12% 4 21% 33% 26% 4	700
	11th to 20th	Night	11% 3 17% 34% 30% 5%	800
	21st to end	Night	17% 7% 20% 32% 20% 5%	800
October	1st to 10th	Night	17% 5% 27% 31% 18% 2	800
	11th to 20th	Night	23% 12% 28% 27% 8% 1	793
	21st to end	Night	26% 11% 33% 24% 5%	836
November	1st to 10th	Night	20% 11% 30% 28% 10% 1	780
	11th to 20th	Night	22% 16% 33% 22% 7% 1	800
	21st to end	Night	28% 12% 34% 22% 5%	800
December	1st to 10th	Night	23% 18% 32% 20% 6%	800
	11th to 20th	Night	24% 15% 38% 20% 2 1	961
	21st to end	Night	31% 13% 36% 18% 2	1100

Page generated at 2025-10-16 01:50:28. Version ac07

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