Hillier Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hillier Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	16% 10% 3 8% 5% 5% 4 4 5% 7% 32%	126691
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 31%	122337
Transparency	Day	51% 6% 12% 21% 10% 1	130182
	Night	53% 6% 12% 19% 10% 1	126933
Seeing	Day	23% 17% 30% 19% 8% 2	15599
	Night	23% 14% 30% 24% 8% 1	24667

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% 17% 6% 13% 6% 6% 4 3 4 4 23%	3597
	11th to 20th	Day	12% 17% 8% 14% 6% 5% 4 4 4 5% 21%	3570
	21st to end	Day	13% 17% 7% 13% 7% 6% 4 3 4 5% 20%	4213
February	1st to 10th	Day	17% 14% 5% 13% 6% 5% 5% 4 5% 5% 23%	1800
	11th to 20th	Day	13% 16% 5% 11% 7% 5% 5% 3 4 6% 27%	1756
	21st to end	Day	20% 14% 5% 10% 6% 5% 4 3 6% 4 23%	1650
March	1st to 10th	Day	19% 14% 4 10% 5% 4 4 3 4 5% 27%	4518
	11th to 20th	Day	11% 13% 4 8% 5% 5% 6% 4 6% 7% 32%	4980
	21st to end	Day	19% 11% 4 8% 4 4 3 3 3 5% 38%	5737
April	1st to 10th	Day	20% 13% 3 8% 4 4 4 2 4 6% 32%	2400
	11th to 20th	Day	21% 10% 3 6% 5% 4 4 3 4 5% 36%	2374
	21st to end	Day	23% 10% 3 8% 5% 3 4 3 5% 5% 32%	2510
May	1st to 10th	Day	24% 10% 3 8% 5% 5% 4 4 5% 6% 26%	6346
	11th to 20th	Day	16% 10% 3 9% 5% 4 5% 3 5% 7% 32%	6318
	21st to end	Day	19% 8% 3 7% 4 4 4 4 5% 8% 33%	6949
June	1st to 10th	Day	21% 8% 3 8% 5% 4 5% 4 5% 7% 30%	2800
	11th to 20th	Day	14% 6% 2 7% 4 5% 5% 4 6% 8% 38%	2800
	21st to end	Day	18% 8% 2 7% 5% 5% 6% 4 5% 7% 33%	2800
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 10% 35%	6360
	11th to 20th	Day	10% 5% 2 7% 5% 4 5% 4 7% 9% 42%	6360
	21st to end	Day	9% 6% 2 5% 4 5% 5% 4 7% 10% 44%	6926
August	1st to 10th	Day	16% 8% 3 7% 5% 5% 4 3 5% 8% 37%	4418
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 6% 9% 38%	4119
	21st to end	Day	15% 6% 2 6% 4 4 6% 4 6% 9% 39%	4312
September	1st to 10th	Day	13% 5% 2 7% 4 4 5% 5% 6% 7% 42%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 7% 47%	2376
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 6% 40%	2023
October	1st to 10th	Day	20% 8% 3 7% 5% 3 3 3 4 6% 37%	1900
	11th to 20th	Day	20% 9% 3 8% 5% 5% 4 3 5% 6% 31%	1900
	21st to end	Day	25% 11% 2 8% 5% 4 4 3 5% 5% 27%	1960
November	1st to 10th	Day	15% 9% 3 8% 6% 5% 6% 5% 5% 6% 33%	1761
	11th to 20th	Day	17% 13% 4 9% 5% 5% 4 3 5% 5% 30%	1612
	21st to end	Day	21% 15% 4 10% 6% 5% 4 3 6% 5% 21%	1600
December	1st to 10th	Day	21% 18% 5% 12% 6% 5% 4 3 5% 6% 16%	2600
	11th to 20th	Day	18% 17% 5% 12% 6% 5% 4 3 3 4 23%	2986
	21st to end	Day	22% 17% 5% 13% 5% 3 3 3 3 4 22%	3960
January	1st to 10th	Night	14% 18% 8% 13% 6% 5% 3 2 4 4 22%	6349
	11th to 20th	Night	11% 17% 7% 15% 8% 6% 5% 3 4 5% 19%	6315
	21st to end	Night	16% 16% 6% 13% 6% 6% 4 3 4 5% 20%	6738
February	1st to 10th	Night	15% 17% 6% 10% 6% 5% 5% 3 5% 6% 23%	2600
	11th to 20th	Night	14% 14% 6% 11% 6% 5% 4 3 5% 5% 28%	2529
	21st to end	Night	15% 15% 5% 9% 6% 5% 4 3 4 5% 28%	1980
March	1st to 10th	Night	21% 14% 4 9% 4 5% 4 3 3 4 30%	5318
	11th to 20th	Night	14% 12% 3 7% 4 5% 4 4 7% 8% 30%	4979
	21st to end	Night	18% 11% 3 9% 5% 4 4 3 5% 5% 34%	5194
April	1st to 10th	Night	21% 10% 4 8% 5% 4 4 4 4 6% 32%	2000
	11th to 20th	Night	20% 9% 3 7% 5% 4 3 3 4 5% 37%	1975
	21st to end	Night	21% 10% 3 7% 5% 4 4 3 5% 6% 33%	1905
May	1st to 10th	Night	25% 10% 3 8% 5% 4 5% 4 5% 6% 25%	3594
	11th to 20th	Night	15% 8% 3 7% 4 4 4 3 5% 7% 39%	3575
	21st to end	Night	18% 9% 3 7% 4 5% 4 3 6% 8% 34%	3926
June	1st to 10th	Night	17% 7% 3 6% 6% 5% 6% 3 6% 7% 34%	1600
	11th to 20th	Night	15% 6% 3 5% 4 4 4 4 7% 8% 40%	1600
	21st to end	Night	17% 6% 3 6% 5% 4 6% 4 6% 8% 35%	1600
July	1st to 10th	Night	12% 7% 3 8% 6% 4 5% 5% 6% 10% 35%	3600
	11th to 20th	Night	12% 5% 3 6% 4 4 5% 4 6% 8% 43%	3600
	21st to end	Night	9% 6% 2 6% 5% 4 4 4 6% 7% 48%	3926
August	1st to 10th	Night	10% 7% 3 7% 4 5% 5% 4 6% 8% 43%	2714
	11th to 20th	Night	12% 7% 3 7% 4 5% 4 3 6% 8% 42%	2896
	21st to end	Night	10% 5% 2 7% 5% 5% 6% 5% 6% 9% 42%	3586
September	1st to 10th	Night	11% 6% 2 7% 5% 4 3 4 4 7% 47%	2000
	11th to 20th	Night	13% 5% 2 4 3 3 4 4 5% 6% 52%	1981
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 6% 43%	2154
October	1st to 10th	Night	17% 8% 4 7% 4 4 4 3 5% 6% 37%	2280
	11th to 20th	Night	21% 8% 2 7% 5% 4 4 4 5% 7% 32%	2280
	21st to end	Night	24% 10% 3 8% 5% 4 4 3 5% 6% 29%	2637
November	1st to 10th	Night	18% 10% 3 6% 5% 3 4 4 4 6% 36%	2537
	11th to 20th	Night	17% 12% 4 8% 5% 4 4 3 5% 5% 33%	2785
	21st to end	Night	21% 14% 4 10% 5% 5% 4 3 4 7% 24%	2800
December	1st to 10th	Night	18% 18% 5% 13% 7% 5% 5% 3 4 5% 17%	4580
	11th to 20th	Night	22% 14% 4 10% 5% 4 4 3 4 4 26%	5208
	21st to end	Night	26% 17% 4 12% 5% 4 3 3 5% 4 19%	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	64% 6% 11% 15% 4 1	3598
	11th to 20th	Day	65% 5% 10% 11% 8% 1	3600
	21st to end	Day	66% 5% 9% 10% 9% 1	4213
February	1st to 10th	Day	63% 5% 9% 17% 5% 1	1800
	11th to 20th	Day	58% 5% 10% 17% 8% 1	1765
	21st to end	Day	64% 5% 8% 12% 9% 1	1650
March	1st to 10th	Day	60% 4 9% 12% 11% 3	4521
	11th to 20th	Day	51% 6% 17% 14% 11% 1	4980
	21st to end	Day	50% 4 7% 19% 18% 3	5737
April	1st to 10th	Day	54% 4 7% 18% 13% 3	2400
	11th to 20th	Day	51% 5% 7% 19% 17% 2	2374
	21st to end	Day	54% 5% 8% 16% 14% 3	2510
May	1st to 10th	Day	59% 6% 9% 17% 9% 2	6357
	11th to 20th	Day	51% 6% 13% 19% 10% 1	6322
	21st to end	Day	48% 7% 12% 22% 11% 1	6949
June	1st to 10th	Day	52% 7% 10% 19% 12% 1	2800
	11th to 20th	Day	42% 7% 11% 25% 12% 2	2800
	21st to end	Day	49% 8% 12% 19% 11% 1	2800
July	1st to 10th	Day	42% 6% 13% 27% 11%	6290
	11th to 20th	Day	37% 6% 14% 28% 14%	6285
	21st to end	Day	35% 6% 13% 37% 9%	6728
August	1st to 10th	Day	46% 5% 14% 24% 10% 1	4404
	11th to 20th	Day	42% 6% 15% 27% 9% 1	4158
	21st to end	Day	41% 6% 15% 27% 11%	4312
September	1st to 10th	Day	39% 8% 14% 27% 12%	2400
	11th to 20th	Day	35% 7% 12% 31% 14%	2376
	21st to end	Day	43% 7% 14% 25% 11%	2023
October	1st to 10th	Day	46% 5% 12% 30% 8%	3160
	11th to 20th	Day	46% 5% 16% 26% 8%	3140
	21st to end	Day	48% 5% 17% 23% 7%	3211
November	1st to 10th	Day	49% 6% 15% 21% 8% 1	1761
	11th to 20th	Day	56% 5% 11% 17% 10% 1	1612
	21st to end	Day	64% 8% 12% 11% 6%	1600
December	1st to 10th	Day	70% 6% 13% 9% 2	2600
	11th to 20th	Day	66% 5% 10% 14% 5%	2986
	21st to end	Day	67% 4 9% 15% 5%	3960
January	1st to 10th	Night	66% 5% 11% 13% 4	6349
	11th to 20th	Night	68% 6% 10% 10% 6%	6360
	21st to end	Night	68% 5% 8% 8% 10% 1	6738
February	1st to 10th	Night	62% 5% 11% 15% 6% 1	2600
	11th to 20th	Night	58% 5% 11% 19% 7% 1	2531
	21st to end	Night	59% 5% 9% 17% 10% 1	1980
March	1st to 10th	Night	60% 4 7% 16% 10% 3	5328
	11th to 20th	Night	49% 7% 18% 16% 8% 3	4980
	21st to end	Night	52% 4 8% 22% 13% 2	5194
April	1st to 10th	Night	53% 4 9% 19% 13% 2	2000
	11th to 20th	Night	51% 5% 7% 19% 17% 2	1975
	21st to end	Night	52% 6% 8% 16% 15% 3	1905
May	1st to 10th	Night	58% 7% 9% 15% 8% 3	3600
	11th to 20th	Night	44% 6% 15% 19% 14% 1	3576
	21st to end	Night	48% 6% 14% 18% 14% 1	3927
June	1st to 10th	Night	47% 9% 12% 19% 12% 1	1600
	11th to 20th	Night	40% 6% 13% 21% 17% 2	1600
	21st to end	Night	46% 10% 11% 20% 13% 1	1600
July	1st to 10th	Night	43% 6% 16% 19% 15% 1	3560
	11th to 20th	Night	37% 7% 13% 32% 11%	3552
	21st to end	Night	35% 6% 13% 31% 14% 1	3809
August	1st to 10th	Night	39% 7% 14% 27% 12% 1	2704
	11th to 20th	Night	40% 6% 16% 29% 9% 1	2923
	21st to end	Night	37% 7% 16% 28% 11% 1	3586
September	1st to 10th	Night	37% 8% 10% 30% 14% 1	2000
	11th to 20th	Night	32% 6% 15% 34% 12% 1	1981
	21st to end	Night	41% 6% 12% 29% 11% 1	2154
October	1st to 10th	Night	41% 6% 12% 29% 12%	3800
	11th to 20th	Night	43% 6% 15% 27% 9%	3779
	21st to end	Night	48% 6% 16% 22% 8%	4336
November	1st to 10th	Night	49% 5% 15% 21% 9% 1	2537
	11th to 20th	Night	52% 7% 13% 18% 10% 2	2785
	21st to end	Night	61% 7% 13% 15% 4	2800
December	1st to 10th	Night	70% 6% 12% 10% 2	4580
	11th to 20th	Night	62% 5% 9% 14% 10% 1	5208
	21st to end	Night	69% 4 7% 11% 9%	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	26% 23% 36% 13% 3	200
	11th to 20th	Day	33% 18% 31% 16% 2 1	200
	21st to end	Day	26% 22% 34% 15% 3	275
February	1st to 10th	Day	32% 26% 30% 11% 1	300
	11th to 20th	Day	30% 29% 32% 8% 1	303
	21st to end	Day	33% 20% 34% 11% 2	414
March	1st to 10th	Day	27% 21% 33% 16% 3	500
	11th to 20th	Day	29% 21% 32% 14% 3	500
	21st to end	Day	33% 22% 27% 13% 6%	550
April	1st to 10th	Day	32% 24% 27% 12% 5%	500
	11th to 20th	Day	33% 21% 23% 13% 8% 2	492
	21st to end	Day	30% 20% 30% 12% 6% 1	500
May	1st to 10th	Day	24% 19% 31% 18% 6% 2	503
	11th to 20th	Day	20% 16% 37% 19% 6% 2	527
	21st to end	Day	21% 15% 31% 22% 9% 2	573
June	1st to 10th	Day	22% 16% 33% 20% 7% 1	530
	11th to 20th	Day	15% 17% 34% 22% 11% 2	530
	21st to end	Day	20% 14% 33% 20% 9% 2	530
July	1st to 10th	Day	13% 8% 33% 29% 15% 2	525
	11th to 20th	Day	12% 11% 33% 27% 14% 3	524
	21st to end	Day	14% 8% 33% 31% 10% 3	576
August	1st to 10th	Day	15% 10% 27% 31% 13% 3	526
	11th to 20th	Day	14% 7% 30% 28% 15% 6%	519
	21st to end	Day	16% 11% 31% 24% 11% 7%	583
September	1st to 10th	Day	14% 13% 30% 24% 12% 6%	530
	11th to 20th	Day	11% 8% 24% 26% 18% 12%	522
	21st to end	Day	21% 11% 26% 25% 14% 3	490
October	1st to 10th	Day	24% 17% 26% 18% 11% 4	490
	11th to 20th	Day	27% 20% 28% 17% 6% 2	490
	21st to end	Day	31% 19% 26% 20% 4	470
November	1st to 10th	Day	26% 19% 30% 18% 6% 1	395
	11th to 20th	Day	25% 22% 33% 12% 6% 2	212
	21st to end	Day	32% 23% 33% 11% 3	200
December	1st to 10th	Day	34% 18% 34% 12% 4	200
	11th to 20th	Day	34% 27% 30% 8% 3	200
	21st to end	Day	34% 19% 28% 15% 3 1	220
January	1st to 10th	Night	28% 20% 32% 16% 4	800
	11th to 20th	Night	30% 19% 27% 20% 4 1	800
	21st to end	Night	21% 16% 36% 22% 5%	880
February	1st to 10th	Night	26% 21% 33% 18% 2	800
	11th to 20th	Night	23% 22% 37% 16% 3	783
	21st to end	Night	26% 20% 31% 19% 3	660
March	1st to 10th	Night	26% 16% 33% 21% 4	800
	11th to 20th	Night	26% 20% 30% 19% 4 1	800
	21st to end	Night	26% 15% 30% 22% 6% 1	880
April	1st to 10th	Night	28% 17% 32% 21% 3	800
	11th to 20th	Night	28% 18% 28% 18% 7% 1	791
	21st to end	Night	26% 14% 29% 23% 8% 1	705
May	1st to 10th	Night	26% 9% 27% 27% 9% 2	400
	11th to 20th	Night	16% 12% 35% 29% 7%	394
	21st to end	Night	14% 10% 37% 27% 12%	433
June	1st to 10th	Night	18% 9% 30% 35% 8% 1	400
	11th to 20th	Night	18% 10% 30% 31% 11% 1	400
	21st to end	Night	18% 7% 31% 31% 13%	400
July	1st to 10th	Night	9% 4 33% 40% 12% 1	390
	11th to 20th	Night	13% 4 28% 39% 15% 1	388
	21st to end	Night	10% 5% 31% 41% 12% 1	438
August	1st to 10th	Night	12% 4 27% 37% 20%	474
	11th to 20th	Night	13% 3 24% 34% 23% 2	602
	21st to end	Night	13% 7% 30% 33% 15% 2	880
September	1st to 10th	Night	15% 7% 32% 31% 16% 1	800
	11th to 20th	Night	16% 7% 20% 30% 24% 4	793
	21st to end	Night	18% 11% 26% 25% 18% 2	760
October	1st to 10th	Night	21% 13% 28% 21% 16% 2	760
	11th to 20th	Night	26% 13% 30% 24% 7% 1	760
	21st to end	Night	30% 14% 29% 23% 5%	836
November	1st to 10th	Night	26% 16% 29% 21% 7%	780
	11th to 20th	Night	26% 16% 32% 17% 8%	800
	21st to end	Night	31% 18% 28% 17% 6%	800
December	1st to 10th	Night	32% 18% 29% 18% 4	800
	11th to 20th	Night	29% 18% 31% 18% 3 1	800
	21st to end	Night	34% 16% 28% 19% 2	880

Page generated at 2025-09-16 01:49:11. Version ac07

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