Hazeldale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hazeldale Observatory clear sky chart from 2005-11-06 to 2024-01-17.

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	11% 6% 3 6% 6% 4 6% 6% 6% 8% 39%	75146
	Night	11% 5% 2 5% 5% 3 4 5% 6% 8% 46%	74654
Transparency	Day	41% 8% 11% 17% 16% 6%	74884
	Night	35% 8% 13% 19% 18% 8%	74436
Seeing	Day	14% 9% 20% 25% 22% 10%	13520
	Night	13% 7% 21% 30% 23% 6%	23052

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	17% 7% 3 4 4 2 4 4 5% 6% 43%	1896
	11th to 20th	Day	15% 6% 3 5% 5% 3 4 3 4 6% 46%	1860
	21st to end	Day	20% 5% 3 5% 4 3 3 4 3 5% 44%	1980
February	1st to 10th	Day	19% 7% 2 5% 5% 3 4 4 4 5% 40%	1800
	11th to 20th	Day	18% 7% 3 5% 4 2 4 4 6% 7% 42%	1748
	21st to end	Day	17% 8% 4 6% 6% 3 5% 4 5% 6% 36%	1480
March	1st to 10th	Day	15% 5% 3 6% 4 4 5% 5% 5% 7% 41%	1831
	11th to 20th	Day	13% 8% 4 7% 6% 3 4 4 6% 7% 38%	1980
	21st to end	Day	12% 5% 3 5% 5% 4 5% 5% 6% 7% 44%	2310
April	1st to 10th	Day	12% 6% 3 6% 5% 4 5% 6% 6% 8% 39%	2160
	11th to 20th	Day	16% 6% 3 6% 4 3 4 5% 6% 7% 40%	2134
	21st to end	Day	10% 5% 2 5% 6% 3 4 5% 7% 7% 47%	2160
May	1st to 10th	Day	12% 4 2 5% 5% 4 5% 6% 5% 8% 45%	2160
	11th to 20th	Day	10% 6% 2 6% 6% 4 6% 6% 6% 7% 40%	2124
	21st to end	Day	7% 5% 3 6% 6% 5% 8% 7% 8% 11% 34%	2350
June	1st to 10th	Day	9% 6% 3 7% 8% 5% 7% 8% 7% 10% 30%	2340
	11th to 20th	Day	5% 4 3 7% 7% 6% 7% 8% 8% 13% 32%	2391
	21st to end	Day	8% 6% 3 8% 10% 6% 8% 8% 10% 9% 25%	2367
July	1st to 10th	Day	9% 7% 4 9% 10% 6% 10% 7% 8% 9% 22%	2340
	11th to 20th	Day	8% 6% 4 8% 8% 5% 9% 9% 7% 10% 24%	2225
	21st to end	Day	8% 6% 3 8% 8% 5% 9% 7% 7% 10% 28%	2350
August	1st to 10th	Day	6% 5% 4 7% 9% 5% 8% 8% 9% 11% 30%	2148
	11th to 20th	Day	11% 7% 4 7% 9% 6% 8% 7% 7% 10% 24%	2125
	21st to end	Day	10% 6% 3 6% 8% 6% 8% 8% 7% 9% 29%	2376
September	1st to 10th	Day	9% 5% 3 7% 8% 5% 7% 7% 6% 8% 34%	2160
	11th to 20th	Day	10% 7% 3 7% 6% 4 5% 6% 6% 8% 38%	2079
	21st to end	Day	9% 4 2 6% 6% 3 5% 6% 5% 6% 48%	1980
October	1st to 10th	Day	6% 4 2 5% 5% 4 5% 5% 6% 6% 53%	1958
	11th to 20th	Day	7% 4 3 5% 6% 2 4 5% 5% 7% 54%	1800
	21st to end	Day	10% 4 2 5% 4 3 3 3 5% 7% 55%	1980
November	1st to 10th	Day	9% 5% 2 4 4 2 4 5% 6% 6% 54%	1858
	11th to 20th	Day	11% 4 2 4 4 2 3 4 5% 6% 55%	1739
	21st to end	Day	12% 7% 2 6% 5% 3 4 4 5% 7% 44%	1710
December	1st to 10th	Day	15% 7% 3 4 5% 3 4 4 4 7% 43%	1710
	11th to 20th	Day	15% 9% 3 7% 5% 3 4 4 5% 6% 39%	2153
	21st to end	Day	17% 7% 2 5% 5% 3 5% 4 5% 6% 39%	3384
January	1st to 10th	Night	17% 6% 3 3 4 2 3 3 4 6% 47%	2269
	11th to 20th	Night	16% 7% 2 4 3 2 4 3 4 6% 48%	2244
	21st to end	Night	19% 7% 3 5% 4 2 3 4 4 5% 44%	2376
February	1st to 10th	Night	19% 6% 2 5% 4 3 4 4 4 6% 43%	2160
	11th to 20th	Night	19% 7% 3 5% 4 2 3 4 5% 6% 43%	2098
	21st to end	Night	18% 8% 3 7% 5% 2 4 4 5% 7% 36%	1776
March	1st to 10th	Night	15% 6% 3 5% 4 3 4 4 4 7% 45%	2142
	11th to 20th	Night	15% 6% 3 5% 5% 3 5% 4 6% 8% 40%	1980
	21st to end	Night	12% 5% 2 4 4 2 4 4 4 6% 52%	2044
April	1st to 10th	Night	15% 6% 3 5% 4 2 4 4 6% 6% 46%	1800
	11th to 20th	Night	13% 6% 2 5% 6% 2 4 5% 5% 7% 45%	1775
	21st to end	Night	7% 4 2 4 4 2 4 4 6% 7% 57%	1800
May	1st to 10th	Night	9% 3 2 3 3 3 4 4 5% 9% 54%	1800
	11th to 20th	Night	6% 4 2 6% 5% 3 5% 5% 6% 8% 49%	1769
	21st to end	Night	6% 3 2 4 5% 3 5% 6% 8% 13% 46%	1916
June	1st to 10th	Night	8% 5% 3 5% 5% 4 6% 6% 7% 12% 39%	1620
	11th to 20th	Night	3 3 2 4 5% 4 5% 7% 8% 13% 48%	1560
	21st to end	Night	7% 4 2 5% 6% 4 6% 6% 7% 10% 42%	1582
July	1st to 10th	Night	5% 5% 3 5% 4 3 7% 7% 9% 13% 40%	1620
	11th to 20th	Night	4 4 2 6% 5% 4 7% 8% 10% 13% 38%	1728
	21st to end	Night	5% 4 2 6% 6% 3 5% 7% 9% 13% 40%	1968
August	1st to 10th	Night	2 3 1 4 4 4 6% 7% 7% 12% 48%	1790
	11th to 20th	Night	5% 4 2 5% 7% 4 7% 6% 11% 13% 36%	1776
	21st to end	Night	7% 4 2 6% 6% 4 6% 7% 8% 11% 39%	1980
September	1st to 10th	Night	7% 5% 2 6% 5% 3 5% 7% 8% 11% 40%	1800
	11th to 20th	Night	8% 4 2 6% 5% 3 5% 6% 6% 9% 46%	1862
	21st to end	Night	7% 6% 2 4 6% 3 5% 5% 5% 7% 51%	1980
October	1st to 10th	Night	6% 4 1 4 6% 2 3 3 5% 8% 58%	1998
	11th to 20th	Night	7% 3 2 3 4 2 4 3 5% 7% 60%	2160
	21st to end	Night	11% 5% 2 4 4 2 3 4 4 5% 56%	2376
November	1st to 10th	Night	10% 5% 2 4 4 2 3 3 4 6% 58%	2220
	11th to 20th	Night	11% 4 2 4 3 3 3 4 4 6% 56%	2436
	21st to end	Night	12% 6% 2 5% 4 3 4 4 4 6% 49%	2470
December	1st to 10th	Night	15% 6% 3 5% 4 2 4 3 5% 7% 46%	2470
	11th to 20th	Night	15% 8% 3 4 5% 3 4 4 5% 6% 43%	3042
	21st to end	Night	18% 9% 3 6% 5% 3 4 4 4 5% 40%	4267

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	42% 5% 6% 16% 22% 10%	1897
	11th to 20th	Day	40% 4 6% 15% 24% 10%	1860
	21st to end	Day	44% 4 4 16% 21% 11%	1980
February	1st to 10th	Day	46% 5% 8% 12% 19% 10%	1800
	11th to 20th	Day	41% 4 8% 18% 18% 11%	1758
	21st to end	Day	48% 5% 8% 14% 16% 9%	1480
March	1st to 10th	Day	41% 5% 9% 18% 19% 7%	1831
	11th to 20th	Day	45% 6% 8% 13% 19% 8%	1980
	21st to end	Day	37% 6% 12% 18% 20% 7%	2310
April	1st to 10th	Day	40% 7% 11% 16% 18% 8%	2160
	11th to 20th	Day	42% 6% 8% 17% 19% 8%	2134
	21st to end	Day	34% 6% 10% 21% 22% 7%	2160
May	1st to 10th	Day	35% 6% 10% 21% 21% 7%	2160
	11th to 20th	Day	40% 8% 12% 21% 18% 1	2128
	21st to end	Day	39% 10% 14% 20% 14% 3	2350
June	1st to 10th	Day	46% 11% 13% 21% 8% 1	2340
	11th to 20th	Day	39% 10% 17% 21% 10% 2	2391
	21st to end	Day	48% 13% 15% 16% 7% 1	2367
July	1st to 10th	Day	53% 14% 16% 13% 3 1	2276
	11th to 20th	Day	49% 15% 19% 14% 3 1	2158
	21st to end	Day	47% 12% 19% 17% 5% 1	2219
August	1st to 10th	Day	44% 14% 20% 15% 8%	2136
	11th to 20th	Day	52% 12% 15% 14% 5% 1	2125
	21st to end	Day	48% 11% 17% 14% 9% 2	2376
September	1st to 10th	Day	44% 10% 15% 16% 11% 4	2160
	11th to 20th	Day	42% 7% 15% 15% 14% 7%	2079
	21st to end	Day	36% 7% 11% 19% 18% 9%	1980
October	1st to 10th	Day	30% 6% 11% 18% 21% 14%	1958
	11th to 20th	Day	30% 5% 9% 16% 27% 14%	1800
	21st to end	Day	30% 4 6% 16% 26% 17%	1980
November	1st to 10th	Day	29% 5% 9% 17% 27% 12%	1858
	11th to 20th	Day	31% 4 6% 17% 28% 13%	1739
	21st to end	Day	39% 4 7% 17% 20% 12%	1710
December	1st to 10th	Day	41% 5% 9% 17% 20% 7%	1710
	11th to 20th	Day	45% 4 8% 12% 23% 8%	2150
	21st to end	Day	45% 6% 8% 16% 21% 4	3384
January	1st to 10th	Night	38% 4 8% 16% 24% 10%	2269
	11th to 20th	Night	40% 3 7% 17% 23% 10%	2244
	21st to end	Night	44% 4 6% 12% 25% 10%	2376
February	1st to 10th	Night	42% 4 9% 17% 19% 10%	2160
	11th to 20th	Night	43% 4 7% 14% 20% 12%	2099
	21st to end	Night	47% 6% 10% 13% 13% 12%	1776
March	1st to 10th	Night	39% 5% 9% 15% 22% 10%	2142
	11th to 20th	Night	41% 6% 12% 15% 19% 7%	1980
	21st to end	Night	33% 6% 11% 20% 23% 7%	2044
April	1st to 10th	Night	39% 6% 10% 17% 18% 10%	1800
	11th to 20th	Night	38% 6% 9% 21% 17% 9%	1775
	21st to end	Night	26% 6% 12% 23% 26% 9%	1800
May	1st to 10th	Night	28% 6% 11% 27% 23% 6%	1800
	11th to 20th	Night	31% 8% 18% 22% 19% 2	1770
	21st to end	Night	27% 11% 18% 26% 15% 3	1917
June	1st to 10th	Night	34% 12% 19% 24% 10% 1	1620
	11th to 20th	Night	24% 13% 23% 29% 10% 2	1560
	21st to end	Night	34% 12% 23% 22% 9% 1	1582
July	1st to 10th	Night	31% 18% 26% 20% 5%	1575
	11th to 20th	Night	28% 19% 30% 18% 3 1	1672
	21st to end	Night	31% 14% 27% 23% 4	1858
August	1st to 10th	Night	24% 15% 32% 21% 8%	1778
	11th to 20th	Night	34% 17% 26% 17% 5% 1	1776
	21st to end	Night	35% 15% 22% 17% 7% 4	1980
September	1st to 10th	Night	35% 11% 19% 17% 12% 6%	1800
	11th to 20th	Night	32% 11% 18% 16% 14% 8%	1862
	21st to end	Night	32% 6% 14% 22% 16% 10%	1980
October	1st to 10th	Night	26% 6% 12% 19% 22% 16%	1998
	11th to 20th	Night	24% 5% 9% 21% 26% 14%	2160
	21st to end	Night	31% 3 6% 16% 27% 18%	2376
November	1st to 10th	Night	30% 3 8% 19% 25% 16%	2220
	11th to 20th	Night	30% 4 7% 17% 28% 15%	2436
	21st to end	Night	36% 5% 7% 20% 20% 13%	2470
December	1st to 10th	Night	38% 6% 9% 20% 20% 7%	2470
	11th to 20th	Night	43% 4 9% 12% 19% 14%	3044
	21st to end	Night	46% 5% 8% 17% 18% 6%	4267

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	26% 15% 37% 15% 6% 1	270
	11th to 20th	Day	25% 15% 36% 18% 5%	258
	21st to end	Day	27% 21% 38% 12% 3	264
February	1st to 10th	Day	31% 18% 33% 14% 5%	240
	11th to 20th	Day	30% 21% 28% 17% 3	240
	21st to end	Day	19% 30% 37% 12% 2	198
March	1st to 10th	Day	15% 22% 43% 18% 2	240
	11th to 20th	Day	26% 20% 36% 13% 5%	240
	21st to end	Day	19% 18% 31% 22% 10% 1	396
April	1st to 10th	Day	15% 17% 33% 24% 10% 1	420
	11th to 20th	Day	22% 15% 28% 22% 11% 2	412
	21st to end	Day	11% 11% 30% 30% 15% 2	420
May	1st to 10th	Day	13% 9% 28% 25% 21% 4	423
	11th to 20th	Day	12% 6% 21% 32% 22% 9%	447
	21st to end	Day	8% 2 15% 35% 26% 13%	485
June	1st to 10th	Day	12% 4 12% 32% 24% 16%	450
	11th to 20th	Day	6% 2 13% 33% 30% 15%	501
	21st to end	Day	10% 3 14% 29% 25% 18%	630
July	1st to 10th	Day	9% 2 7% 27% 36% 19%	621
	11th to 20th	Day	8% 2 4 21% 39% 26%	505
	21st to end	Day	8% 1 6% 20% 40% 24%	488
August	1st to 10th	Day	5% 2 6% 20% 35% 32%	446
	11th to 20th	Day	10% 3 4 16% 39% 27%	439
	21st to end	Day	14% 3 7% 20% 32% 23%	495
September	1st to 10th	Day	8% 2 8% 26% 38% 19%	450
	11th to 20th	Day	11% 4 9% 28% 34% 14%	450
	21st to end	Day	11% 4 12% 29% 34% 9%	450
October	1st to 10th	Day	6% 6% 19% 38% 25% 6%	428
	11th to 20th	Day	6% 8% 26% 33% 23% 4	270
	21st to end	Day	19% 10% 24% 33% 9% 4	297
November	1st to 10th	Day	17% 6% 26% 36% 16% 1	270
	11th to 20th	Day	17% 13% 32% 26% 11% 1	270
	21st to end	Day	17% 16% 39% 20% 7% 2	270
December	1st to 10th	Day	19% 14% 33% 25% 7% 1	270
	11th to 20th	Day	23% 17% 36% 19% 6%	270
	21st to end	Day	29% 14% 31% 18% 8% 1	297
January	1st to 10th	Night	22% 12% 31% 28% 6%	760
	11th to 20th	Night	20% 11% 30% 31% 7%	748
	21st to end	Night	24% 14% 29% 28% 5%	792
February	1st to 10th	Night	21% 11% 35% 27% 6%	720
	11th to 20th	Night	23% 14% 29% 27% 6%	703
	21st to end	Night	22% 16% 35% 24% 2	592
March	1st to 10th	Night	17% 14% 33% 29% 7%	702
	11th to 20th	Night	18% 8% 30% 33% 11%	540
	21st to end	Night	14% 10% 32% 34% 10% 1	594
April	1st to 10th	Night	18% 10% 28% 31% 13%	540
	11th to 20th	Night	14% 9% 33% 28% 16% 1	533
	21st to end	Night	9% 6% 28% 40% 17% 1	540
May	1st to 10th	Night	11% 4 22% 43% 20% 1	540
	11th to 20th	Night	7% 3 18% 42% 25% 5%	531
	21st to end	Night	6% 2 14% 36% 36% 7%	584
June	1st to 10th	Night	10% 2 9% 32% 38% 9%	540
	11th to 20th	Night	3 1 7% 31% 44% 13%	540
	21st to end	Night	9% 1 7% 27% 41% 16%	540
July	1st to 10th	Night	6% 2 4 18% 49% 21%	525
	11th to 20th	Night	4 1 2 14% 50% 29%	522
	21st to end	Night	5% 1 3 19% 50% 23%	591
August	1st to 10th	Night	2 1 1 17% 49% 29%	537
	11th to 20th	Night	6% 2 14% 49% 29%	534
	21st to end	Night	8% 2 6% 17% 44% 23%	594
September	1st to 10th	Night	8% 3 4 22% 45% 18%	540
	11th to 20th	Night	10% 3 7% 27% 44% 9%	602
	21st to end	Night	9% 3 9% 34% 37% 7%	720
October	1st to 10th	Night	7% 2 17% 38% 31% 5%	720
	11th to 20th	Night	9% 5% 21% 36% 26% 3	720
	21st to end	Night	14% 6% 21% 35% 22% 2	792
November	1st to 10th	Night	13% 6% 17% 41% 20% 2	740
	11th to 20th	Night	15% 7% 28% 33% 15% 2	760
	21st to end	Night	15% 8% 28% 33% 15% 1	760
December	1st to 10th	Night	17% 11% 28% 29% 14% 1	760
	11th to 20th	Night	21% 8% 30% 32% 9%	760
	21st to end	Night	26% 11% 30% 26% 7%	836

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