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 2026-02-01.

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	16% 7% 2 6% 4 4 4 4 6% 7% 41%	127474
	Night	16% 7% 2 6% 4 3 4 3 5% 6% 43%	124543
Transparency	Day	26% 39% 8% 15% 11% 2	131001
	Night	26% 38% 7% 15% 11% 3	129090
Seeing	Day	20% 12% 30% 24% 11% 4	16587
	Night	19% 9% 31% 29% 10% 1	25880

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	13% 7% 2 4 3 3 3 3 5% 5% 52%	3677
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 4 5% 46%	3650
	21st to end	Day	19% 10% 4 7% 4 3 3 3 4 5% 40%	4489
February	1st to 10th	Day	15% 9% 3 7% 5% 4 4 3 5% 6% 41%	1830
	11th to 20th	Day	17% 8% 3 6% 4 3 3 3 5% 5% 42%	1948
	21st to end	Day	17% 8% 3 6% 4 4 3 4 4 5% 41%	1650
March	1st to 10th	Day	22% 6% 1 4 3 3 4 3 4 5% 46%	4476
	11th to 20th	Day	12% 10% 3 6% 4 4 5% 3 5% 6% 43%	4786
	21st to end	Day	17% 7% 2 5% 4 3 4 3 5% 6% 43%	5920
April	1st to 10th	Day	19% 8% 2 5% 3 4 5% 3 5% 7% 39%	2400
	11th to 20th	Day	15% 6% 2 5% 4 5% 4 4 6% 7% 41%	2374
	21st to end	Day	17% 6% 2 5% 4 4 5% 4 7% 7% 40%	2400
May	1st to 10th	Day	23% 10% 3 7% 4 4 4 4 6% 7% 28%	5885
	11th to 20th	Day	25% 9% 2 5% 3 3 4 3 4 6% 35%	6196
	21st to end	Day	21% 7% 2 5% 4 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	14% 6% 2 5% 4 5% 6% 4 7% 9% 38%	2800
	11th to 20th	Day	14% 6% 2 6% 5% 4 5% 4 6% 9% 39%	2800
	21st to end	Day	10% 5% 2 5% 5% 6% 6% 5% 7% 10% 41%	2800
July	1st to 10th	Day	10% 5% 2 7% 5% 5% 6% 5% 7% 10% 37%	6360
	11th to 20th	Day	12% 8% 3 7% 5% 6% 6% 6% 7% 10% 29%	6360
	21st to end	Day	11% 6% 3 6% 4 5% 6% 4 7% 9% 39%	6926
August	1st to 10th	Day	17% 6% 2 7% 5% 4 5% 5% 7% 9% 32%	4193
	11th to 20th	Day	14% 5% 3 6% 4 5% 5% 4 6% 8% 41%	3850
	21st to end	Day	9% 4 2 5% 3 4 5% 5% 7% 10% 45%	4312
September	1st to 10th	Day	14% 6% 3 5% 3 3 3 3 5% 8% 46%	2400
	11th to 20th	Day	11% 6% 2 6% 3 3 4 4 5% 8% 49%	2311
	21st to end	Day	18% 7% 3 6% 4 4 4 4 5% 6% 39%	2200
October	1st to 10th	Day	19% 5% 1 5% 3 3 4 3 5% 6% 45%	2095
	11th to 20th	Day	16% 6% 2 4 3 2 3 2 4 6% 51%	2000
	21st to end	Day	18% 6% 3 5% 3 3 3 2 4 6% 47%	2059
November	1st to 10th	Day	13% 6% 2 4 3 3 4 3 5% 6% 51%	1851
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 5% 5% 48%	1890
	21st to end	Day	16% 5% 2 5% 4 3 5% 4 4 7% 45%	1843
December	1st to 10th	Day	12% 6% 2 6% 3 3 4 4 5% 6% 48%	2680
	11th to 20th	Day	22% 8% 2 4 2 2 3 2 4 5% 46%	3066
	21st to end	Day	15% 6% 3 6% 4 4 4 4 5% 7% 42%	4048
January	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 53%	6489
	11th to 20th	Night	15% 8% 2 6% 3 3 3 3 4 5% 49%	6455
	21st to end	Night	18% 9% 2 6% 4 3 4 3 5% 6% 41%	6703
February	1st to 10th	Night	17% 10% 2 6% 3 3 3 3 5% 6% 41%	2568
	11th to 20th	Night	19% 8% 2 5% 4 3 3 3 5% 5% 42%	2338
	21st to end	Night	18% 7% 2 6% 4 3 3 3 4 6% 44%	1980
March	1st to 10th	Night	18% 9% 3 5% 3 2 3 2 4 5% 46%	5364
	11th to 20th	Night	13% 12% 3 5% 3 3 3 3 4 5% 45%	5168
	21st to end	Night	17% 8% 3 7% 4 4 4 3 5% 5% 41%	5011
April	1st to 10th	Night	18% 9% 3 6% 5% 3 4 3 5% 5% 40%	2000
	11th to 20th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 42%	1975
	21st to end	Night	17% 6% 3 5% 3 4 4 4 5% 7% 43%	2000
May	1st to 10th	Night	22% 10% 3 7% 4 5% 4 4 5% 8% 27%	4054
	11th to 20th	Night	24% 9% 3 6% 4 3 4 3 5% 7% 33%	3727
	21st to end	Night	22% 8% 2 6% 4 4 3 3 5% 8% 35%	3926
June	1st to 10th	Night	16% 7% 3 6% 4 4 4 4 5% 8% 40%	1600
	11th to 20th	Night	15% 6% 3 5% 5% 4 5% 4 5% 9% 40%	1600
	21st to end	Night	9% 5% 2 7% 5% 6% 6% 5% 8% 10% 40%	1600
July	1st to 10th	Night	9% 8% 3 8% 5% 5% 6% 5% 6% 10% 35%	3600
	11th to 20th	Night	13% 9% 3 8% 5% 4 5% 4 6% 10% 33%	3600
	21st to end	Night	12% 6% 3 7% 4 4 4 4 7% 8% 41%	3926
August	1st to 10th	Night	22% 8% 2 6% 4 4 4 3 6% 8% 32%	2921
	11th to 20th	Night	12% 6% 2 7% 4 4 4 4 6% 8% 44%	3197
	21st to end	Night	8% 5% 2 5% 4 4 4 3 5% 7% 53%	3586
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 4 4 7% 45%	2000
	11th to 20th	Night	13% 5% 2 5% 3 4 3 3 5% 6% 51%	2087
	21st to end	Night	20% 8% 2 6% 3 3 4 3 6% 6% 40%	2200
October	1st to 10th	Night	19% 6% 2 5% 4 4 3 2 5% 5% 44%	2301
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	18% 7% 2 5% 3 3 3 2 4 5% 47%	2780
November	1st to 10th	Night	12% 5% 2 5% 4 3 3 2 5% 6% 52%	2667
	11th to 20th	Night	17% 6% 2 6% 4 3 3 3 4 5% 48%	2730
	21st to end	Night	14% 6% 2 5% 4 4 4 4 4 7% 47%	2772
December	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 4 6% 49%	4720
	11th to 20th	Night	23% 6% 2 5% 3 3 3 2 4 6% 43%	5348
	21st to end	Night	17% 8% 3 5% 3 4 4 3 5% 6% 42%	7150

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	23% 26% 5% 23% 20% 2	3678
	11th to 20th	Day	29% 27% 6% 19% 14% 5%	3680
	21st to end	Day	37% 29% 7% 17% 9% 1	4489
February	1st to 10th	Day	19% 63% 6% 8% 4	1830
	11th to 20th	Day	19% 61% 2 9% 8% 1	1957
	21st to end	Day	18% 62% 5% 9% 5% 2	1650
March	1st to 10th	Day	29% 30% 7% 14% 12% 7%	4479
	11th to 20th	Day	29% 30% 7% 20% 11% 3	4786
	21st to end	Day	29% 31% 8% 9% 18% 5%	5920
April	1st to 10th	Day	19% 62% 4 7% 6% 2	2400
	11th to 20th	Day	15% 63% 6% 9% 7%	2374
	21st to end	Day	15% 62% 5% 8% 8% 1	2400
May	1st to 10th	Day	42% 32% 11% 9% 5%	5896
	11th to 20th	Day	38% 29% 7% 18% 7%	6200
	21st to end	Day	35% 31% 7% 16% 11% 1	6949
June	1st to 10th	Day	18% 62% 4 8% 8%	2800
	11th to 20th	Day	15% 63% 4 11% 6%	2800
	21st to end	Day	13% 62% 6% 9% 9%	2800
July	1st to 10th	Day	33% 33% 8% 15% 11%	6290
	11th to 20th	Day	37% 32% 15% 11% 5%	6285
	21st to end	Day	28% 28% 9% 17% 17% 1	6728
August	1st to 10th	Day	32% 39% 11% 13% 4	4179
	11th to 20th	Day	24% 39% 9% 22% 5%	3886
	21st to end	Day	15% 38% 10% 23% 13% 1	4312
September	1st to 10th	Day	16% 58% 7% 10% 8% 1	2400
	11th to 20th	Day	15% 58% 5% 11% 10% 1	2311
	21st to end	Day	24% 58% 5% 8% 5%	2200
October	1st to 10th	Day	19% 37% 9% 14% 16% 4	3407
	11th to 20th	Day	11% 36% 7% 16% 25% 5%	3240
	21st to end	Day	13% 36% 11% 19% 19% 2	3297
November	1st to 10th	Day	17% 53% 7% 11% 10% 2	1851
	11th to 20th	Day	16% 54% 3 11% 14% 2	1890
	21st to end	Day	16% 54% 6% 16% 6% 1	1843
December	1st to 10th	Day	24% 36% 10% 19% 10% 2	2680
	11th to 20th	Day	31% 30% 5% 23% 9% 3	3066
	21st to end	Day	31% 29% 7% 18% 14% 2	4048
January	1st to 10th	Night	24% 26% 7% 18% 19% 5%	6489
	11th to 20th	Night	28% 26% 6% 19% 14% 6%	6500
	21st to end	Night	34% 28% 7% 14% 14% 3	6703
February	1st to 10th	Night	18% 62% 6% 9% 3 1	2568
	11th to 20th	Night	18% 60% 3 8% 10% 1	2340
	21st to end	Night	18% 61% 4 7% 6% 3	1980
March	1st to 10th	Night	33% 29% 5% 16% 9% 8%	5374
	11th to 20th	Night	29% 30% 9% 19% 9% 3	5169
	21st to end	Night	34% 30% 4 9% 20% 3	5011
April	1st to 10th	Night	20% 62% 3 7% 7% 1	2000
	11th to 20th	Night	15% 63% 4 9% 8% 1	1975
	21st to end	Night	17% 62% 3 8% 8% 1	2000
May	1st to 10th	Night	41% 31% 10% 12% 4 2	4060
	11th to 20th	Night	40% 30% 8% 13% 9%	3728
	21st to end	Night	35% 31% 9% 15% 9% 1	3927
June	1st to 10th	Night	17% 63% 2 7% 10%	1600
	11th to 20th	Night	16% 63% 4 11% 6%	1600
	21st to end	Night	15% 62% 6% 9% 8% 1	1600
July	1st to 10th	Night	35% 30% 9% 16% 9% 1	3560
	11th to 20th	Night	37% 30% 13% 15% 4	3552
	21st to end	Night	30% 28% 11% 16% 14% 1	3809
August	1st to 10th	Night	37% 38% 12% 9% 5%	2911
	11th to 20th	Night	25% 38% 9% 22% 6%	3227
	21st to end	Night	16% 37% 6% 25% 16% 1	3586
September	1st to 10th	Night	16% 58% 6% 11% 7% 1	2000
	11th to 20th	Night	16% 57% 4 10% 11% 2	2087
	21st to end	Night	24% 58% 4 9% 5%	2200
October	1st to 10th	Night	23% 37% 7% 16% 15% 2	3769
	11th to 20th	Night	11% 36% 6% 19% 24% 4	3899
	21st to end	Night	13% 36% 8% 22% 18% 3	4479
November	1st to 10th	Night	18% 54% 6% 11% 9% 2	2667
	11th to 20th	Night	17% 54% 4 9% 12% 3	2730
	21st to end	Night	16% 55% 6% 13% 9% 1	2772
December	1st to 10th	Night	25% 35% 7% 22% 9% 2	4720
	11th to 20th	Night	31% 31% 6% 18% 11% 3	5348
	21st to end	Night	33% 28% 8% 15% 13% 3	7150

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	25% 17% 40% 13% 4 1	220
	11th to 20th	Day	31% 27% 27% 12% 3	220
	21st to end	Day	24% 23% 35% 15% 2 1	344
February	1st to 10th	Day	24% 21% 37% 15% 2 1	330
	11th to 20th	Day	28% 19% 38% 14% 1	496
	21st to end	Day	27% 20% 33% 17% 3	414
March	1st to 10th	Day	25% 17% 35% 17% 5%	500
	11th to 20th	Day	19% 20% 39% 16% 5% 1	500
	21st to end	Day	26% 21% 32% 17% 4 1	550
April	1st to 10th	Day	25% 19% 34% 15% 6% 1	500
	11th to 20th	Day	22% 16% 34% 18% 8% 2	492
	21st to end	Day	24% 14% 36% 19% 6% 2	500
May	1st to 10th	Day	28% 12% 32% 18% 7% 2	503
	11th to 20th	Day	23% 10% 36% 22% 7% 2	527
	21st to end	Day	22% 7% 25% 26% 15% 5%	573
June	1st to 10th	Day	18% 7% 28% 28% 15% 4	530
	11th to 20th	Day	15% 7% 33% 29% 12% 3	530
	21st to end	Day	13% 5% 30% 32% 17% 3	530
July	1st to 10th	Day	12% 4 24% 33% 19% 8%	525
	11th to 20th	Day	9% 4 21% 36% 22% 7%	524
	21st to end	Day	12% 5% 27% 34% 17% 5%	576
August	1st to 10th	Day	12% 4 24% 35% 18% 6%	526
	11th to 20th	Day	13% 5% 19% 33% 22% 8%	519
	21st to end	Day	11% 4 26% 31% 18% 9%	583
September	1st to 10th	Day	17% 7% 28% 24% 16% 8%	530
	11th to 20th	Day	13% 4 26% 33% 15% 10%	530
	21st to end	Day	24% 7% 26% 26% 12% 4	530
October	1st to 10th	Day	23% 8% 23% 25% 15% 7%	530
	11th to 20th	Day	18% 12% 30% 24% 13% 3	530
	21st to end	Day	20% 13% 33% 21% 11% 3	505
November	1st to 10th	Day	15% 14% 33% 24% 12% 2	425
	11th to 20th	Day	20% 17% 32% 22% 9%	430
	21st to end	Day	21% 14% 35% 22% 7% 1	383
December	1st to 10th	Day	24% 22% 34% 12% 6% 1	220
	11th to 20th	Day	30% 20% 31% 13% 6%	220
	21st to end	Day	26% 17% 31% 21% 5% 1	242
January	1st to 10th	Night	20% 14% 41% 21% 4	840
	11th to 20th	Night	25% 15% 33% 23% 4	840
	21st to end	Night	20% 15% 41% 21% 2	924
February	1st to 10th	Night	22% 12% 39% 24% 3	801
	11th to 20th	Night	25% 16% 41% 17% 1	783
	21st to end	Night	23% 17% 36% 20% 3	660
March	1st to 10th	Night	21% 13% 39% 22% 4	800
	11th to 20th	Night	22% 12% 39% 22% 5% 1	800
	21st to end	Night	22% 10% 40% 23% 5%	880
April	1st to 10th	Night	23% 12% 37% 23% 5%	800
	11th to 20th	Night	21% 10% 34% 29% 6%	791
	21st to end	Night	20% 9% 34% 31% 6% 1	800
May	1st to 10th	Night	24% 9% 30% 28% 8% 1	600
	11th to 20th	Night	24% 7% 34% 30% 6%	468
	21st to end	Night	21% 3 19% 39% 17% 2	433
June	1st to 10th	Night	19% 4 23% 37% 16% 1	400
	11th to 20th	Night	16% 4 22% 40% 19%	400
	21st to end	Night	10% 2 21% 51% 16%	400
July	1st to 10th	Night	13% 5% 15% 46% 19% 3	390
	11th to 20th	Night	12% 4 14% 43% 26% 2	388
	21st to end	Night	11% 3 19% 45% 21%	438
August	1st to 10th	Night	13% 2 14% 47% 22% 2	590
	11th to 20th	Night	13% 3 15% 41% 25% 3	790
	21st to end	Night	13% 3 16% 40% 23% 4	880
September	1st to 10th	Night	16% 4 22% 36% 20% 3	800
	11th to 20th	Night	16% 3 17% 36% 23% 5%	800
	21st to end	Night	23% 4 21% 32% 18% 2	800
October	1st to 10th	Night	22% 5% 25% 27% 18% 4	800
	11th to 20th	Night	18% 10% 27% 33% 12% 1	800
	21st to end	Night	21% 8% 36% 24% 10% 1	880
November	1st to 10th	Night	15% 9% 35% 29% 11% 1	820
	11th to 20th	Night	20% 13% 32% 25% 9% 1	840
	21st to end	Night	16% 13% 36% 26% 8% 1	840
December	1st to 10th	Night	24% 14% 35% 19% 7% 1	840
	11th to 20th	Night	23% 11% 37% 25% 4	840
	21st to end	Night	22% 11% 38% 26% 3	924

Page generated at 2026-01-29 01:21:44. Version ac07

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