Halliday Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Halliday Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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% 6% 2 5% 5% 3 5% 5% 6% 8% 42%	74768
	Night	16% 6% 2 5% 4 3 4 4 5% 7% 43%	74980
Transparency	Day	26% 39% 9% 15% 10% 1	74473
	Night	27% 38% 8% 15% 11% 2	74809
Seeing	Day	19% 11% 30% 25% 12% 4	13944
	Night	19% 8% 31% 30% 11% 1	23248

By Month

Percentage Cloud Cover NE (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	14% 7% 3 5% 5% 3 4 5% 6% 5% 44%	1517
	11th to 20th	Day	14% 8% 3 5% 4 3 5% 5% 6% 6% 41%	1464
	21st to end	Day	17% 7% 2 4 4 3 4 4 5% 7% 42%	1750
February	1st to 10th	Day	16% 8% 4 6% 5% 2 4 4 4 6% 40%	1647
	11th to 20th	Day	17% 7% 3 5% 4 3 4 4 5% 5% 43%	1748
	21st to end	Day	18% 7% 4 6% 5% 3 4 4 5% 5% 40%	1480
March	1st to 10th	Day	14% 6% 2 5% 5% 2 4 4 5% 6% 46%	1797
	11th to 20th	Day	15% 7% 2 5% 5% 3 5% 5% 5% 7% 41%	1898
	21st to end	Day	20% 7% 3 5% 5% 3 4 4 3 6% 40%	2354
April	1st to 10th	Day	18% 7% 3 4 4 4 5% 4 5% 7% 41%	2160
	11th to 20th	Day	15% 5% 2 5% 5% 3 5% 5% 6% 8% 42%	2134
	21st to end	Day	19% 5% 2 4 5% 3 5% 5% 7% 7% 39%	2160
May	1st to 10th	Day	20% 8% 2 5% 5% 3 4 5% 6% 8% 35%	2339
	11th to 20th	Day	21% 5% 2 5% 5% 3 5% 5% 5% 7% 37%	2426
	21st to end	Day	16% 6% 2 5% 4 3 4 5% 6% 9% 40%	2725
June	1st to 10th	Day	14% 6% 2 4 5% 4 6% 6% 7% 10% 37%	2520
	11th to 20th	Day	14% 5% 3 5% 6% 3 6% 5% 6% 9% 38%	2520
	21st to end	Day	10% 4 2 4 5% 4 6% 7% 7% 10% 41%	2520
July	1st to 10th	Day	11% 4 2 5% 5% 4 6% 7% 8% 9% 38%	2520
	11th to 20th	Day	8% 5% 2 4 5% 4 6% 7% 8% 10% 42%	2520
	21st to end	Day	11% 5% 3 5% 6% 4 6% 6% 7% 10% 37%	2744
August	1st to 10th	Day	8% 5% 3 6% 6% 3 5% 7% 6% 10% 41%	2315
	11th to 20th	Day	12% 5% 2 5% 4 4 6% 6% 7% 8% 42%	2126
	21st to end	Day	12% 5% 2 5% 4 3 5% 5% 6% 10% 41%	2376
September	1st to 10th	Day	15% 6% 3 4 4 2 4 4 5% 8% 45%	2160
	11th to 20th	Day	12% 5% 2 6% 3 2 4 5% 5% 8% 48%	2081
	21st to end	Day	17% 6% 3 5% 4 2 4 5% 5% 6% 41%	1980
October	1st to 10th	Day	21% 5% 2 4 4 2 4 4 5% 6% 43%	1886
	11th to 20th	Day	16% 5% 2 4 4 2 3 3 4 6% 50%	1800
	21st to end	Day	19% 6% 3 5% 3 2 3 3 4 6% 45%	1854
November	1st to 10th	Day	14% 5% 2 4 4 2 4 4 5% 6% 51%	1671
	11th to 20th	Day	17% 5% 2 4 4 3 4 4 5% 5% 48%	1710
	21st to end	Day	16% 4 2 5% 4 2 6% 4 4 8% 45%	1668
December	1st to 10th	Day	17% 7% 3 6% 4 2 4 5% 5% 6% 41%	1520
	11th to 20th	Day	19% 7% 2 4 3 2 4 4 5% 6% 45%	1906
	21st to end	Day	17% 6% 3 5% 5% 3 5% 6% 5% 7% 38%	2772
January	1st to 10th	Night	17% 6% 3 4 4 2 3 4 4 6% 47%	2649
	11th to 20th	Night	20% 6% 3 5% 4 3 4 4 4 7% 41%	2574
	21st to end	Night	15% 7% 2 5% 4 2 5% 3 5% 7% 45%	2601
February	1st to 10th	Night	17% 9% 3 5% 4 2 3 4 5% 6% 40%	2310
	11th to 20th	Night	18% 9% 3 5% 4 2 4 3 5% 6% 41%	2098
	21st to end	Night	19% 7% 2 5% 4 2 3 4 4 6% 44%	1776
March	1st to 10th	Night	16% 6% 2 5% 4 2 4 3 4 7% 48%	2150
	11th to 20th	Night	18% 6% 3 6% 4 3 4 4 4 5% 42%	2056
	21st to end	Night	19% 7% 3 6% 4 2 3 3 4 5% 42%	2001
April	1st to 10th	Night	17% 8% 3 5% 5% 2 4 4 5% 6% 42%	1800
	11th to 20th	Night	16% 7% 3 6% 5% 2 5% 4 5% 6% 43%	1775
	21st to end	Night	19% 6% 3 5% 4 2 4 5% 5% 7% 42%	1800
May	1st to 10th	Night	20% 9% 3 6% 4 4 4 4 5% 7% 33%	1620
	11th to 20th	Night	19% 8% 3 5% 4 3 4 5% 5% 8% 36%	1483
	21st to end	Night	18% 7% 2 5% 5% 3 4 3 6% 8% 38%	1550
June	1st to 10th	Night	16% 7% 4 5% 4 3 4 5% 5% 8% 40%	1440
	11th to 20th	Night	14% 5% 3 4 5% 3 5% 6% 5% 8% 39%	1440
	21st to end	Night	9% 5% 2 6% 5% 3 6% 6% 7% 11% 41%	1440
July	1st to 10th	Night	10% 6% 3 7% 4 3 6% 6% 6% 9% 39%	1440
	11th to 20th	Night	7% 7% 2 5% 5% 4 4 5% 6% 10% 44%	1440
	21st to end	Night	9% 5% 3 5% 6% 4 5% 6% 8% 9% 41%	1574
August	1st to 10th	Night	9% 7% 2 6% 5% 3 5% 5% 7% 9% 42%	1603
	11th to 20th	Night	10% 6% 3 6% 6% 3 4 6% 6% 9% 42%	1774
	21st to end	Night	12% 6% 2 5% 5% 3 4 5% 6% 8% 44%	1980
September	1st to 10th	Night	15% 6% 2 5% 5% 3 4 5% 5% 7% 44%	1800
	11th to 20th	Night	13% 5% 2 4 3 3 4 4 5% 6% 52%	1877
	21st to end	Night	19% 7% 2 5% 4 2 3 4 5% 6% 42%	1980
October	1st to 10th	Night	21% 6% 2 5% 4 3 4 3 5% 5% 43%	2070
	11th to 20th	Night	16% 5% 2 4 4 3 3 4 5% 6% 50%	2160
	21st to end	Night	18% 7% 2 5% 5% 2 3 3 4 5% 46%	2502
November	1st to 10th	Night	13% 5% 2 4 4 2 3 3 5% 6% 52%	2407
	11th to 20th	Night	18% 6% 2 5% 4 3 3 3 4 5% 47%	2470
	21st to end	Night	14% 5% 2 4 4 3 4 5% 4 8% 48%	2508
December	1st to 10th	Night	20% 6% 2 5% 5% 3 3 4 4 6% 41%	2660
	11th to 20th	Night	18% 5% 2 5% 5% 2 4 4 5% 7% 44%	3288
	21st to end	Night	17% 9% 4 5% 4 3 4 4 5% 7% 39%	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	25% 41% 9% 17% 8% 1	1518
	11th to 20th	Day	27% 43% 9% 12% 7% 1	1464
	21st to end	Day	27% 37% 9% 17% 9% 2	1750
February	1st to 10th	Day	28% 36% 7% 17% 11% 1	1647
	11th to 20th	Day	27% 38% 8% 17% 9% 1	1757
	21st to end	Day	30% 38% 6% 15% 8% 3	1480
March	1st to 10th	Day	25% 37% 9% 19% 9% 2	1800
	11th to 20th	Day	26% 38% 8% 16% 9% 3	1898
	21st to end	Day	30% 38% 6% 14% 12% 2	2354
April	1st to 10th	Day	27% 38% 8% 12% 11% 4	2160
	11th to 20th	Day	26% 37% 9% 14% 10% 3	2134
	21st to end	Day	28% 38% 7% 11% 12% 3	2160
May	1st to 10th	Day	29% 40% 8% 14% 9% 1	2339
	11th to 20th	Day	29% 39% 7% 12% 11% 1	2430
	21st to end	Day	26% 40% 10% 14% 9% 2	2725
June	1st to 10th	Day	25% 41% 11% 14% 8% 1	2520
	11th to 20th	Day	26% 42% 10% 15% 6% 2	2520
	21st to end	Day	24% 41% 11% 15% 8% 1	2520
July	1st to 10th	Day	24% 42% 10% 13% 9% 1	2450
	11th to 20th	Day	22% 42% 10% 18% 8% 1	2445
	21st to end	Day	24% 43% 10% 15% 8% 1	2591
August	1st to 10th	Day	23% 40% 12% 17% 8%	2301
	11th to 20th	Day	25% 40% 9% 14% 11% 1	2126
	21st to end	Day	27% 40% 9% 12% 10% 1	2376
September	1st to 10th	Day	23% 38% 9% 14% 14% 3	2160
	11th to 20th	Day	21% 37% 9% 15% 16% 2	2081
	21st to end	Day	27% 39% 6% 12% 14% 2	1980
October	1st to 10th	Day	26% 38% 7% 16% 14% 1	1886
	11th to 20th	Day	26% 37% 8% 15% 13% 1	1800
	21st to end	Day	27% 38% 7% 15% 12% 1	1854
November	1st to 10th	Day	21% 41% 8% 18% 12% 1	1671
	11th to 20th	Day	26% 40% 8% 14% 11% 1	1710
	21st to end	Day	27% 41% 8% 16% 8% 1	1668
December	1st to 10th	Day	26% 40% 11% 15% 8%	1520
	11th to 20th	Day	29% 33% 8% 18% 11% 1	1906
	21st to end	Day	33% 28% 10% 18% 10%	2772
January	1st to 10th	Night	25% 40% 8% 17% 9% 1	2649
	11th to 20th	Night	29% 41% 9% 15% 6% 1	2574
	21st to end	Night	25% 38% 7% 19% 10% 1	2601
February	1st to 10th	Night	29% 36% 9% 14% 11% 1	2310
	11th to 20th	Night	29% 38% 8% 14% 9% 3	2100
	21st to end	Night	27% 36% 6% 17% 13% 2	1776
March	1st to 10th	Night	24% 36% 9% 18% 11% 2	2160
	11th to 20th	Night	32% 37% 6% 12% 10% 3	2057
	21st to end	Night	29% 37% 6% 14% 11% 4	2001
April	1st to 10th	Night	29% 37% 7% 12% 12% 3	1800
	11th to 20th	Night	31% 36% 6% 11% 14% 2	1775
	21st to end	Night	27% 38% 6% 13% 13% 4	1800
May	1st to 10th	Night	32% 38% 6% 12% 10% 2	1620
	11th to 20th	Night	29% 38% 7% 11% 12% 3	1484
	21st to end	Night	28% 40% 9% 12% 9% 1	1551
June	1st to 10th	Night	29% 39% 8% 13% 10% 1	1440
	11th to 20th	Night	26% 40% 9% 15% 8% 2	1440
	21st to end	Night	24% 41% 12% 14% 8% 1	1440
July	1st to 10th	Night	29% 41% 8% 14% 7% 1	1400
	11th to 20th	Night	23% 41% 8% 17% 9% 1	1392
	21st to end	Night	23% 40% 11% 15% 10% 2	1486
August	1st to 10th	Night	25% 39% 14% 13% 8% 1	1593
	11th to 20th	Night	26% 38% 10% 14% 11% 1	1774
	21st to end	Night	28% 40% 10% 12% 9% 1	1980
September	1st to 10th	Night	27% 38% 6% 13% 13% 2	1800
	11th to 20th	Night	21% 37% 8% 16% 15% 2	1877
	21st to end	Night	28% 37% 5% 13% 15% 2	1980
October	1st to 10th	Night	27% 38% 6% 14% 13% 1	2070
	11th to 20th	Night	27% 36% 9% 17% 9% 1	2160
	21st to end	Night	28% 37% 7% 15% 11% 1	2502
November	1st to 10th	Night	19% 40% 8% 16% 14% 2	2407
	11th to 20th	Night	26% 40% 6% 14% 12% 1	2470
	21st to end	Night	24% 40% 9% 15% 11% 1	2508
December	1st to 10th	Night	26% 40% 9% 15% 9% 1	2660
	11th to 20th	Night	27% 34% 10% 13% 14% 2	3288
	21st to end	Night	34% 28% 11% 18% 8%	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	22% 16% 42% 15% 4 1	180
	11th to 20th	Day	28% 28% 25% 15% 4	166
	21st to end	Day	23% 19% 37% 17% 3 2	250
February	1st to 10th	Day	25% 22% 36% 15% 2 1	267
	11th to 20th	Day	25% 17% 42% 15% 1	416
	21st to end	Day	29% 18% 33% 18% 1	346
March	1st to 10th	Day	22% 17% 37% 19% 4	420
	11th to 20th	Day	20% 18% 40% 16% 5% 1	420
	21st to end	Day	27% 20% 29% 18% 5% 1	462
April	1st to 10th	Day	22% 20% 34% 16% 7% 1	420
	11th to 20th	Day	19% 15% 34% 21% 9% 2	412
	21st to end	Day	26% 14% 35% 17% 5% 2	420
May	1st to 10th	Day	27% 12% 32% 19% 8% 2	423
	11th to 20th	Day	22% 9% 36% 23% 8% 2	447
	21st to end	Day	22% 5% 23% 28% 17% 6%	485
June	1st to 10th	Day	17% 6% 27% 30% 15% 4	450
	11th to 20th	Day	15% 7% 34% 28% 13% 4	450
	21st to end	Day	13% 4 29% 34% 17% 4	450
July	1st to 10th	Day	13% 2 22% 33% 21% 8%	445
	11th to 20th	Day	9% 3 20% 36% 24% 8%	444
	21st to end	Day	13% 5% 26% 34% 18% 5%	488
August	1st to 10th	Day	11% 3 23% 37% 20% 7%	446
	11th to 20th	Day	13% 4 18% 34% 23% 8%	439
	21st to end	Day	12% 3 23% 32% 19% 10%	495
September	1st to 10th	Day	18% 6% 26% 24% 17% 10%	450
	11th to 20th	Day	13% 4 25% 30% 16% 11%	450
	21st to end	Day	23% 7% 26% 26% 14% 5%	450
October	1st to 10th	Day	26% 8% 20% 25% 16% 6%	450
	11th to 20th	Day	17% 11% 29% 27% 14% 3	450
	21st to end	Day	22% 11% 34% 21% 10% 3	432
November	1st to 10th	Day	15% 11% 32% 25% 13% 2	365
	11th to 20th	Day	19% 16% 33% 22% 10%	370
	21st to end	Day	19% 10% 36% 24% 8% 2	328
December	1st to 10th	Day	24% 19% 36% 11% 8% 1	180
	11th to 20th	Day	29% 19% 31% 15% 6%	180
	21st to end	Day	26% 13% 32% 23% 5% 1	198
January	1st to 10th	Night	20% 12% 42% 21% 4	760
	11th to 20th	Night	26% 11% 35% 24% 4	736
	21st to end	Night	19% 14% 42% 22% 3	792
February	1st to 10th	Night	23% 11% 39% 25% 3	720
	11th to 20th	Night	24% 16% 42% 17% 1	703
	21st to end	Night	23% 17% 37% 20% 3	592
March	1st to 10th	Night	21% 12% 39% 23% 4	720
	11th to 20th	Night	22% 10% 40% 22% 4 1	720
	21st to end	Night	22% 9% 39% 24% 6%	792
April	1st to 10th	Night	22% 12% 36% 24% 6%	720
	11th to 20th	Night	20% 8% 34% 32% 6%	711
	21st to end	Night	21% 8% 33% 31% 5% 1	720
May	1st to 10th	Night	25% 8% 29% 29% 9% 1	540
	11th to 20th	Night	23% 7% 33% 30% 6%	422
	21st to end	Night	21% 2 18% 39% 18% 2	389
June	1st to 10th	Night	19% 4 21% 37% 18% 1	360
	11th to 20th	Night	15% 4 21% 41% 19%	360
	21st to end	Night	9% 2 20% 53% 16%	360
July	1st to 10th	Night	14% 4 15% 45% 21% 3	350
	11th to 20th	Night	12% 3 14% 43% 26% 2	348
	21st to end	Night	10% 2 20% 45% 22% 1	394
August	1st to 10th	Night	12% 2 14% 48% 22% 3	529
	11th to 20th	Night	13% 3 14% 41% 25% 3	710
	21st to end	Night	14% 3 14% 41% 24% 4	792
September	1st to 10th	Night	16% 3 21% 36% 21% 4	720
	11th to 20th	Night	16% 3 18% 35% 23% 6%	720
	21st to end	Night	22% 4 21% 32% 19% 2	720
October	1st to 10th	Night	23% 5% 24% 26% 17% 4	720
	11th to 20th	Night	17% 8% 27% 35% 12% 1	720
	21st to end	Night	23% 7% 36% 24% 10% 1	792
November	1st to 10th	Night	15% 8% 34% 29% 12% 1	740
	11th to 20th	Night	20% 12% 32% 26% 10% 1	760
	21st to end	Night	16% 11% 38% 26% 8% 1	760
December	1st to 10th	Night	25% 13% 35% 19% 8% 1	760
	11th to 20th	Night	21% 10% 39% 26% 4	760
	21st to end	Night	21% 10% 40% 26% 3	836

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