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 2024-01-12.

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	15% 10% 4 7% 6% 3 5% 5% 5% 7% 33%	74389
	Night	16% 10% 4 7% 6% 3 4 4 5% 6% 34%	75252
Transparency	Day	49% 7% 12% 20% 11% 1	74097
	Night	50% 6% 11% 20% 12% 1	75065
Seeing	Day	18% 11% 28% 22% 14% 6%	14329
	Night	20% 10% 29% 29% 11% 1	24422

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	16% 16% 7% 11% 8% 3 5% 5% 5% 5% 20%	1517
	11th to 20th	Day	16% 17% 8% 12% 8% 4 5% 4 4 5% 17%	1448
	21st to end	Day	16% 19% 9% 12% 7% 4 5% 3 3 4 18%	1710
February	1st to 10th	Day	14% 14% 8% 11% 10% 4 6% 4 4 5% 21%	1620
	11th to 20th	Day	15% 13% 7% 11% 10% 4 5% 4 5% 4 23%	1710
	21st to end	Day	15% 15% 8% 11% 6% 2 4 4 4 6% 24%	1480
March	1st to 10th	Day	15% 11% 5% 8% 8% 4 4 5% 5% 7% 29%	1823
	11th to 20th	Day	13% 13% 6% 8% 6% 3 4 3 5% 6% 32%	1980
	21st to end	Day	20% 13% 4 8% 6% 4 4 3 4 6% 29%	2178
April	1st to 10th	Day	17% 12% 6% 6% 6% 3 4 5% 5% 6% 30%	2115
	11th to 20th	Day	18% 10% 3 6% 5% 4 5% 5% 5% 7% 34%	2266
	21st to end	Day	18% 9% 3 7% 6% 3 5% 4 5% 7% 34%	2340
May	1st to 10th	Day	17% 9% 4 6% 6% 4 5% 5% 5% 6% 33%	2340
	11th to 20th	Day	19% 7% 3 5% 5% 3 4 4 5% 7% 36%	2301
	21st to end	Day	13% 6% 2 6% 5% 3 4 5% 6% 10% 40%	2563
June	1st to 10th	Day	11% 5% 2 5% 6% 3 5% 5% 6% 9% 42%	2520
	11th to 20th	Day	12% 6% 2 7% 5% 3 5% 6% 5% 8% 41%	2520
	21st to end	Day	12% 5% 2 5% 6% 4 5% 7% 7% 10% 38%	2520
July	1st to 10th	Day	7% 5% 2 4 5% 4 5% 5% 7% 9% 47%	2520
	11th to 20th	Day	10% 4 2 5% 6% 3 5% 5% 6% 9% 43%	2520
	21st to end	Day	8% 5% 2 4 6% 4 6% 6% 7% 9% 42%	2627
August	1st to 10th	Day	10% 4 2 5% 6% 3 6% 6% 6% 9% 41%	2327
	11th to 20th	Day	12% 5% 3 5% 6% 3 6% 6% 6% 9% 38%	2303
	21st to end	Day	10% 6% 2 6% 5% 4 5% 5% 5% 10% 42%	2532
September	1st to 10th	Day	12% 6% 2 7% 6% 5% 4 5% 7% 9% 38%	2052
	11th to 20th	Day	11% 5% 2 4 5% 2 4 5% 6% 9% 46%	1980
	21st to end	Day	14% 7% 3 6% 5% 3 5% 6% 7% 8% 37%	1980
October	1st to 10th	Day	15% 7% 2 6% 6% 3 4 4 5% 6% 41%	1944
	11th to 20th	Day	19% 11% 3 7% 7% 3 5% 5% 4 7% 31%	1800
	21st to end	Day	22% 12% 3 5% 6% 3 4 4 4 7% 29%	1810
November	1st to 10th	Day	13% 10% 3 6% 6% 3 5% 4 6% 8% 36%	1672
	11th to 20th	Day	15% 13% 6% 7% 6% 3 5% 4 4 7% 29%	1710
	21st to end	Day	20% 14% 5% 8% 5% 3 5% 4 4 6% 27%	1645
December	1st to 10th	Day	14% 15% 10% 11% 8% 3 4 3 3 5% 22%	1520
	11th to 20th	Day	16% 16% 6% 10% 6% 3 5% 5% 5% 6% 22%	1713
	21st to end	Day	24% 22% 8% 11% 6% 3 3 3 4 4 11%	2783
January	1st to 10th	Night	18% 13% 6% 10% 7% 4 4 5% 4 6% 23%	2648
	11th to 20th	Night	18% 16% 8% 11% 6% 3 4 5% 5% 5% 19%	2541
	21st to end	Night	13% 15% 9% 12% 8% 3 5% 4 5% 5% 21%	2643
February	1st to 10th	Night	17% 16% 7% 9% 8% 4 4 4 4 5% 22%	2340
	11th to 20th	Night	15% 14% 6% 10% 8% 4 5% 4 4 5% 26%	2134
	21st to end	Night	18% 12% 5% 8% 7% 3 4 4 5% 6% 28%	1776
March	1st to 10th	Night	16% 9% 5% 7% 7% 4 4 5% 4 5% 33%	2136
	11th to 20th	Night	12% 11% 4 7% 5% 3 5% 5% 4 6% 39%	1980
	21st to end	Night	20% 9% 4 6% 5% 3 5% 4 4 7% 34%	2178
April	1st to 10th	Night	16% 10% 4 6% 5% 3 5% 4 5% 7% 36%	1859
	11th to 20th	Night	19% 8% 2 6% 4 2 5% 5% 5% 6% 37%	1643
	21st to end	Night	18% 8% 2 5% 6% 3 4 4 5% 7% 38%	1620
May	1st to 10th	Night	17% 9% 3 7% 6% 4 5% 4 6% 7% 34%	1620
	11th to 20th	Night	20% 6% 2 5% 5% 3 3 4 4 7% 41%	1592
	21st to end	Night	10% 5% 2 4 6% 3 6% 5% 4 10% 45%	1708
June	1st to 10th	Night	12% 5% 2 5% 5% 2 4 3 5% 9% 47%	1440
	11th to 20th	Night	10% 5% 2 6% 4 2 5% 4 7% 8% 46%	1367
	21st to end	Night	10% 5% 3 5% 5% 3 4 4 5% 9% 48%	1440
July	1st to 10th	Night	7% 4 2 4 5% 2 5% 4 5% 9% 54%	1440
	11th to 20th	Night	10% 4 2 5% 4 3 4 5% 6% 8% 51%	1440
	21st to end	Night	7% 3 2 4 5% 3 4 6% 6% 10% 51%	1688
August	1st to 10th	Night	8% 4 1 5% 5% 3 5% 5% 8% 10% 47%	1611
	11th to 20th	Night	9% 3 2 5% 6% 4 4 4 6% 9% 48%	1598
	21st to end	Night	8% 5% 3 4 4 3 4 4 5% 8% 52%	1822
September	1st to 10th	Night	11% 6% 2 5% 4 3 5% 4 6% 7% 47%	1890
	11th to 20th	Night	12% 4 1 3 5% 2 3 4 5% 6% 54%	1980
	21st to end	Night	14% 6% 3 5% 5% 2 4 6% 5% 7% 44%	1980
October	1st to 10th	Night	15% 7% 2 6% 6% 3 3 4 4 6% 44%	2016
	11th to 20th	Night	20% 10% 4 7% 5% 3 4 4 5% 6% 33%	2160
	21st to end	Night	22% 11% 3 6% 6% 3 4 4 5% 5% 31%	2542
November	1st to 10th	Night	14% 9% 3 6% 5% 2 4 4 5% 6% 42%	2406
	11th to 20th	Night	16% 12% 4 7% 6% 4 5% 6% 6% 6% 30%	2470
	21st to end	Night	21% 13% 5% 7% 5% 3 3 5% 4 6% 28%	2530
December	1st to 10th	Night	18% 15% 7% 11% 6% 3 5% 5% 4 5% 20%	2660
	11th to 20th	Night	15% 15% 7% 11% 7% 3 4 4 5% 6% 22%	3470
	21st to end	Night	26% 23% 8% 9% 6% 3 3 3 3 4 12%	4884

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% 13% 6% 1	1518
	11th to 20th	Day	69% 7% 11% 9% 4	1448
	21st to end	Day	72% 4 6% 11% 5% 1	1710
February	1st to 10th	Day	65% 6% 9% 13% 7%	1620
	11th to 20th	Day	64% 5% 9% 13% 8% 1	1720
	21st to end	Day	62% 5% 10% 15% 7% 2	1480
March	1st to 10th	Day	54% 6% 13% 17% 8% 1	1823
	11th to 20th	Day	52% 5% 11% 18% 12% 2	1980
	21st to end	Day	58% 5% 9% 18% 9% 1	2178
April	1st to 10th	Day	55% 5% 11% 18% 11%	2115
	11th to 20th	Day	49% 6% 11% 19% 13% 2	2266
	21st to end	Day	50% 6% 13% 16% 14% 2	2340
May	1st to 10th	Day	51% 6% 11% 21% 9% 2	2340
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2305
	21st to end	Day	40% 7% 14% 23% 14% 2	2563
June	1st to 10th	Day	37% 8% 12% 27% 14% 2	2520
	11th to 20th	Day	40% 7% 11% 23% 17% 2	2520
	21st to end	Day	38% 9% 13% 22% 16% 1	2520
July	1st to 10th	Day	32% 8% 11% 26% 18% 4	2451
	11th to 20th	Day	35% 8% 13% 24% 17% 2	2445
	21st to end	Day	35% 9% 13% 22% 18% 3	2481
August	1st to 10th	Day	37% 9% 14% 26% 13% 2	2314
	11th to 20th	Day	40% 10% 12% 24% 13% 2	2303
	21st to end	Day	38% 7% 11% 25% 17% 2	2532
September	1st to 10th	Day	42% 8% 13% 23% 12% 2	2052
	11th to 20th	Day	34% 7% 13% 32% 13% 1	1980
	21st to end	Day	42% 7% 13% 24% 13%	1980
October	1st to 10th	Day	44% 6% 11% 25% 14% 1	1944
	11th to 20th	Day	53% 6% 13% 18% 10% 1	1800
	21st to end	Day	55% 6% 13% 17% 9%	1810
November	1st to 10th	Day	46% 6% 13% 22% 12% 1	1672
	11th to 20th	Day	54% 5% 13% 19% 8% 1	1710
	21st to end	Day	59% 5% 12% 17% 7% 1	1645
December	1st to 10th	Day	65% 5% 10% 15% 4	1520
	11th to 20th	Day	63% 6% 13% 14% 4	1709
	21st to end	Day	79% 4 9% 6% 3	2783
January	1st to 10th	Night	63% 5% 10% 15% 6% 1	2648
	11th to 20th	Night	67% 6% 11% 11% 6%	2541
	21st to end	Night	65% 4 11% 13% 6% 1	2643
February	1st to 10th	Night	65% 6% 10% 12% 6% 1	2340
	11th to 20th	Night	60% 5% 10% 15% 9% 1	2135
	21st to end	Night	57% 5% 12% 16% 8% 2	1776
March	1st to 10th	Night	52% 5% 10% 22% 10% 1	2136
	11th to 20th	Night	46% 6% 11% 21% 15% 2	1980
	21st to end	Night	53% 5% 9% 21% 11% 2	2178
April	1st to 10th	Night	48% 5% 10% 21% 14% 2	1859
	11th to 20th	Night	47% 6% 10% 17% 18% 2	1643
	21st to end	Night	46% 6% 11% 19% 16% 3	1620
May	1st to 10th	Night	48% 6% 11% 21% 11% 2	1620
	11th to 20th	Night	44% 5% 10% 20% 18% 3	1593
	21st to end	Night	36% 8% 14% 24% 15% 3	1709
June	1st to 10th	Night	35% 6% 11% 29% 18% 3	1440
	11th to 20th	Night	34% 8% 14% 25% 18% 2	1367
	21st to end	Night	34% 6% 13% 27% 19% 1	1440
July	1st to 10th	Night	29% 7% 11% 29% 22% 4	1400
	11th to 20th	Night	31% 6% 13% 29% 19% 3	1392
	21st to end	Night	28% 8% 16% 24% 21% 3	1594
August	1st to 10th	Night	30% 9% 14% 31% 15% 1	1600
	11th to 20th	Night	32% 9% 12% 32% 13% 3	1598
	21st to end	Night	31% 6% 13% 28% 20% 1	1822
September	1st to 10th	Night	35% 7% 12% 27% 17% 2	1890
	11th to 20th	Night	31% 6% 10% 30% 20% 1	1980
	21st to end	Night	38% 7% 12% 28% 14% 1	1980
October	1st to 10th	Night	42% 5% 10% 24% 18% 1	2016
	11th to 20th	Night	52% 6% 11% 18% 12% 1	2160
	21st to end	Night	55% 5% 11% 20% 9% 1	2542
November	1st to 10th	Night	43% 5% 12% 25% 15% 1	2406
	11th to 20th	Night	53% 6% 13% 19% 8% 1	2470
	21st to end	Night	57% 5% 12% 19% 7%	2530
December	1st to 10th	Night	65% 6% 10% 13% 5%	2660
	11th to 20th	Night	61% 7% 12% 13% 6% 1	3473
	21st to end	Night	76% 5% 10% 6% 2	4884

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% 36% 15% 7%	270
	11th to 20th	Day	22% 13% 46% 15% 3 1	243
	21st to end	Day	29% 16% 33% 16% 5% 1	264
February	1st to 10th	Day	28% 20% 39% 12% 1	240
	11th to 20th	Day	28% 19% 39% 13% 1	240
	21st to end	Day	28% 30% 31% 9% 2	198
March	1st to 10th	Day	25% 23% 35% 14% 3	263
	11th to 20th	Day	21% 14% 34% 23% 7%	420
	21st to end	Day	29% 13% 31% 21% 6% 1	462
April	1st to 10th	Day	20% 15% 33% 21% 10% 1	420
	11th to 20th	Day	25% 18% 30% 16% 6% 6%	412
	21st to end	Day	26% 12% 30% 20% 9% 2	420
May	1st to 10th	Day	25% 12% 27% 22% 12% 3	423
	11th to 20th	Day	21% 9% 29% 21% 13% 6%	447
	21st to end	Day	16% 6% 23% 28% 19% 9%	517
June	1st to 10th	Day	12% 7% 29% 30% 17% 4	630
	11th to 20th	Day	14% 8% 29% 26% 18% 5%	630
	21st to end	Day	12% 8% 27% 30% 17% 6%	630
July	1st to 10th	Day	8% 5% 22% 28% 24% 12%	621
	11th to 20th	Day	10% 6% 27% 29% 19% 9%	618
	21st to end	Day	9% 4 26% 30% 23% 9%	568
August	1st to 10th	Day	9% 4 21% 26% 25% 17%	446
	11th to 20th	Day	13% 4 17% 27% 24% 14%	439
	21st to end	Day	12% 6% 21% 26% 23% 13%	495
September	1st to 10th	Day	14% 8% 21% 26% 21% 11%	450
	11th to 20th	Day	10% 4 22% 22% 28% 14%	450
	21st to end	Day	18% 9% 22% 22% 21% 8%	450
October	1st to 10th	Day	19% 8% 28% 22% 17% 7%	450
	11th to 20th	Day	21% 17% 28% 26% 8% 1	450
	21st to end	Day	30% 15% 35% 16% 3	325
November	1st to 10th	Day	17% 20% 35% 16% 10% 1	270
	11th to 20th	Day	21% 22% 33% 18% 5% 1	270
	21st to end	Day	29% 19% 35% 13% 4	239
December	1st to 10th	Day	27% 21% 29% 15% 8%	180
	11th to 20th	Day	25% 21% 41% 10% 2 1	182
	21st to end	Day	28% 25% 33% 14% 1	297
January	1st to 10th	Night	22% 16% 36% 21% 5% 1	950
	11th to 20th	Night	25% 14% 35% 21% 5% 1	908
	21st to end	Night	20% 11% 34% 28% 6%	861
February	1st to 10th	Night	26% 13% 37% 21% 3	720
	11th to 20th	Night	23% 15% 33% 25% 4	703
	21st to end	Night	25% 17% 36% 20% 3	592
March	1st to 10th	Night	22% 13% 34% 24% 6% 1	720
	11th to 20th	Night	18% 15% 35% 26% 5% 1	720
	21st to end	Night	23% 11% 30% 27% 7% 1	792
April	1st to 10th	Night	21% 14% 34% 26% 4 1	599
	11th to 20th	Night	23% 9% 34% 25% 8% 2	533
	21st to end	Night	22% 8% 33% 29% 8%	540
May	1st to 10th	Night	21% 6% 31% 34% 7% 1	540
	11th to 20th	Night	21% 5% 30% 31% 13% 1	531
	21st to end	Night	12% 4 23% 40% 20% 2	584
June	1st to 10th	Night	12% 4 24% 46% 14% 1	540
	11th to 20th	Night	12% 4 25% 43% 16%	467
	21st to end	Night	12% 4 20% 46% 16% 2	540
July	1st to 10th	Night	9% 2 16% 40% 29% 4	525
	11th to 20th	Night	11% 2 19% 42% 23% 3	522
	21st to end	Night	5% 2 21% 47% 22% 2	591
August	1st to 10th	Night	9% 2 15% 44% 28% 3	537
	11th to 20th	Night	10% 1 21% 35% 29% 4	534
	21st to end	Night	11% 3 17% 34% 31% 4	594
September	1st to 10th	Night	12% 3 21% 35% 25% 4	630
	11th to 20th	Night	12% 3 18% 35% 26% 5%	720
	21st to end	Night	16% 7% 20% 31% 21% 5%	720
October	1st to 10th	Night	18% 5% 28% 29% 17% 2	720
	11th to 20th	Night	24% 13% 28% 27% 7% 1	720
	21st to end	Night	28% 11% 33% 24% 4	792
November	1st to 10th	Night	20% 11% 30% 28% 11% 1	740
	11th to 20th	Night	23% 15% 34% 21% 7% 1	760
	21st to end	Night	27% 12% 33% 23% 5%	760
December	1st to 10th	Night	24% 18% 33% 20% 6%	760
	11th to 20th	Night	23% 15% 38% 21% 3 1	912
	21st to end	Night	31% 13% 36% 18% 2	1045

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