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 2024-11-05.

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	17% 10% 3 8% 5% 4 4 4 5% 7% 33%	101558
	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 32%	97473
Transparency	Day	50% 6% 12% 21% 10% 1	105067
	Night	52% 6% 12% 20% 9% 1	102053
Seeing	Day	23% 16% 30% 20% 8% 2	14511
	Night	23% 14% 30% 24% 8% 1	23613

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% 10% 6% 6% 4 3 4 4 25%	2517
	11th to 20th	Day	13% 15% 6% 13% 6% 6% 5% 4 4 5% 22%	2520
	21st to end	Day	17% 19% 6% 13% 7% 6% 4 4 4 4 17%	2941
February	1st to 10th	Day	17% 14% 4 13% 6% 5% 5% 4 5% 5% 23%	1710
	11th to 20th	Day	13% 16% 5% 10% 6% 5% 5% 3 4 6% 28%	1665
	21st to end	Day	20% 14% 4 9% 6% 5% 5% 3 6% 4 23%	1570
March	1st to 10th	Day	20% 14% 4 9% 6% 4 4 3 4 5% 28%	3144
	11th to 20th	Day	12% 16% 5% 9% 5% 5% 5% 3 5% 5% 30%	3480
	21st to end	Day	14% 9% 4 7% 5% 4 3 3 4 6% 42%	4003
April	1st to 10th	Day	20% 13% 3 8% 4 4 4 2 4 7% 32%	2280
	11th to 20th	Day	21% 9% 3 6% 4 4 4 3 4 5% 36%	2254
	21st to end	Day	23% 9% 3 8% 5% 3 4 3 5% 5% 31%	2385
May	1st to 10th	Day	20% 8% 3 8% 5% 4 4 4 5% 6% 32%	4426
	11th to 20th	Day	17% 10% 3 8% 5% 4 4 3 5% 7% 33%	4398
	21st to end	Day	14% 6% 2 6% 5% 4 5% 4 6% 9% 39%	4837
June	1st to 10th	Day	22% 8% 3 8% 5% 4 5% 4 5% 7% 30%	2660
	11th to 20th	Day	15% 6% 2 7% 4 5% 5% 4 6% 8% 38%	2660
	21st to end	Day	18% 8% 2 7% 5% 5% 6% 4 5% 8% 32%	2660
July	1st to 10th	Day	11% 6% 3 7% 5% 5% 5% 5% 7% 10% 37%	4440
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 7% 9% 40%	4440
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 9% 41%	4814
August	1st to 10th	Day	16% 8% 3 7% 5% 5% 4 3 5% 7% 36%	4282
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 6% 9% 37%	3991
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 9% 39%	4180
September	1st to 10th	Day	13% 5% 2 7% 4 5% 5% 5% 5% 7% 42%	2280
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 6% 7% 46%	2280
	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 7% 6% 5% 5% 5% 5% 6% 33%	1708
	11th to 20th	Day	17% 14% 4 9% 5% 5% 4 3 5% 5% 29%	1532
	21st to end	Day	21% 15% 4 10% 6% 5% 4 3 6% 5% 22%	1520
December	1st to 10th	Day	23% 17% 5% 11% 6% 4 4 3 5% 6% 17%	1520
	11th to 20th	Day	21% 15% 5% 10% 5% 4 4 3 4 5% 24%	1906
	21st to end	Day	26% 17% 5% 12% 5% 3 3 2 3 3 20%	2772
January	1st to 10th	Night	17% 15% 7% 11% 6% 5% 4 3 5% 5% 21%	4429
	11th to 20th	Night	13% 16% 5% 14% 8% 6% 5% 3 4 5% 20%	4440
	21st to end	Night	19% 18% 6% 12% 7% 6% 4 3 3 4 20%	4710
February	1st to 10th	Night	15% 16% 6% 10% 6% 5% 5% 3 5% 6% 22%	2470
	11th to 20th	Night	14% 14% 6% 10% 6% 5% 4 3 5% 5% 29%	2400
	21st to end	Night	16% 15% 5% 8% 6% 5% 4 3 4 5% 29%	1884
March	1st to 10th	Night	21% 12% 4 9% 4 4 4 3 4 4 32%	3692
	11th to 20th	Night	19% 17% 4 8% 5% 4 3 3 5% 6% 26%	3479
	21st to end	Night	15% 10% 3 8% 4 4 3 3 5% 5% 39%	3628
April	1st to 10th	Night	21% 10% 4 7% 5% 4 4 4 4 5% 33%	1900
	11th to 20th	Night	20% 9% 3 7% 5% 4 3 3 4 5% 37%	1875
	21st to end	Night	21% 10% 3 7% 4 4 4 3 5% 6% 33%	1811
May	1st to 10th	Night	22% 10% 3 7% 5% 4 4 4 5% 6% 30%	2514
	11th to 20th	Night	20% 9% 3 6% 4 4 4 3 5% 6% 37%	2495
	21st to end	Night	14% 7% 3 6% 4 4 4 3 6% 9% 42%	2738
June	1st to 10th	Night	17% 7% 3 6% 6% 5% 6% 3 6% 7% 34%	1520
	11th to 20th	Night	14% 7% 3 5% 4 4 4 4 7% 8% 40%	1520
	21st to end	Night	18% 6% 3 6% 5% 4 5% 4 6% 8% 34%	1520
July	1st to 10th	Night	10% 7% 3 8% 6% 5% 5% 5% 6% 9% 36%	2520
	11th to 20th	Night	13% 5% 2 6% 4 4 5% 4 6% 8% 42%	2520
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 5% 6% 6% 44%	2739
August	1st to 10th	Night	11% 7% 3 7% 4 5% 4 4 6% 7% 42%	2630
	11th to 20th	Night	12% 7% 3 7% 4 5% 4 3 6% 8% 41%	2805
	21st to end	Night	10% 5% 2 7% 5% 5% 6% 5% 6% 8% 42%	3476
September	1st to 10th	Night	12% 6% 2 7% 5% 4 3 4 4 7% 46%	1900
	11th to 20th	Night	13% 5% 2 4 3 3 4 4 5% 6% 51%	1900
	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	19% 10% 3 6% 5% 3 4 4 4 6% 36%	2470
	11th to 20th	Night	17% 12% 3 8% 5% 4 4 4 5% 5% 32%	2645
	21st to end	Night	21% 13% 4 10% 5% 5% 4 3 4 7% 24%	2660
December	1st to 10th	Night	20% 20% 5% 11% 6% 4 4 3 5% 4 19%	2660
	11th to 20th	Night	21% 15% 4 9% 6% 4 4 3 4 4 27%	3288
	21st to end	Night	30% 16% 4 11% 5% 4 3 3 4 3 16%	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	61% 8% 14% 11% 5% 1	2518
	11th to 20th	Day	64% 7% 11% 11% 7% 1	2520
	21st to end	Day	71% 6% 10% 9% 4 1	2941
February	1st to 10th	Day	63% 5% 9% 18% 4 1	1710
	11th to 20th	Day	58% 5% 10% 17% 9% 1	1674
	21st to end	Day	64% 6% 9% 12% 9% 1	1570
March	1st to 10th	Day	59% 5% 10% 17% 9% 1	3147
	11th to 20th	Day	54% 6% 14% 14% 10% 2	3480
	21st to end	Day	43% 5% 9% 23% 16% 4	4003
April	1st to 10th	Day	54% 4 7% 18% 13% 3	2280
	11th to 20th	Day	50% 5% 7% 19% 16% 2	2254
	21st to end	Day	55% 5% 8% 15% 14% 3	2385
May	1st to 10th	Day	51% 7% 10% 20% 10% 2	4440
	11th to 20th	Day	48% 7% 13% 21% 8% 1	4402
	21st to end	Day	40% 8% 12% 24% 14% 1	4837
June	1st to 10th	Day	52% 7% 10% 18% 12% 1	2660
	11th to 20th	Day	43% 7% 11% 24% 12% 2	2660
	21st to end	Day	49% 8% 13% 18% 11% 1	2660
July	1st to 10th	Day	40% 7% 13% 27% 12% 1	4370
	11th to 20th	Day	38% 7% 16% 31% 9%	4365
	21st to end	Day	38% 8% 13% 31% 11%	4661
August	1st to 10th	Day	47% 5% 14% 23% 10% 1	4268
	11th to 20th	Day	42% 6% 15% 27% 10%	4030
	21st to end	Day	41% 6% 15% 27% 11%	4180
September	1st to 10th	Day	39% 8% 14% 26% 13%	2280
	11th to 20th	Day	36% 7% 12% 30% 15%	2280
	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% 7% 15% 21% 8% 1	1708
	11th to 20th	Day	57% 5% 11% 16% 10% 1	1532
	21st to end	Day	63% 8% 12% 11% 6%	1520
December	1st to 10th	Day	68% 7% 11% 10% 3 1	1520
	11th to 20th	Day	64% 6% 14% 13% 3	1906
	21st to end	Day	72% 5% 7% 13% 3	2772
January	1st to 10th	Night	64% 7% 14% 11% 3	4429
	11th to 20th	Night	66% 8% 12% 10% 4	4440
	21st to end	Night	69% 6% 9% 10% 5% 1	4710
February	1st to 10th	Night	62% 6% 12% 14% 6% 1	2470
	11th to 20th	Night	57% 5% 11% 19% 7%	2402
	21st to end	Night	58% 6% 9% 16% 10% 1	1884
March	1st to 10th	Night	56% 5% 9% 21% 9% 1	3702
	11th to 20th	Night	57% 7% 14% 13% 8% 1	3480
	21st to end	Night	46% 4 8% 22% 17% 3	3628
April	1st to 10th	Night	53% 4 9% 19% 13% 2	1900
	11th to 20th	Night	50% 5% 7% 18% 17% 2	1875
	21st to end	Night	52% 6% 9% 16% 15% 3	1811
May	1st to 10th	Night	52% 9% 10% 18% 9% 3	2520
	11th to 20th	Night	47% 7% 12% 21% 10% 1	2496
	21st to end	Night	39% 7% 14% 21% 18% 2	2739
June	1st to 10th	Night	47% 9% 13% 19% 11% 1	1520
	11th to 20th	Night	41% 7% 13% 21% 17% 2	1520
	21st to end	Night	46% 10% 12% 19% 13% 1	1520
July	1st to 10th	Night	43% 7% 13% 21% 15% 1	2480
	11th to 20th	Night	38% 7% 13% 34% 8%	2472
	21st to end	Night	39% 8% 15% 23% 15% 1	2651
August	1st to 10th	Night	40% 7% 14% 26% 12% 1	2620
	11th to 20th	Night	40% 6% 16% 30% 8%	2832
	21st to end	Night	37% 7% 16% 28% 11% 1	3476
September	1st to 10th	Night	37% 8% 11% 30% 14% 1	1900
	11th to 20th	Night	33% 6% 15% 33% 12% 1	1900
	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% 10% 1	2470
	11th to 20th	Night	52% 7% 13% 17% 9% 2	2645
	21st to end	Night	61% 7% 13% 15% 5%	2660
December	1st to 10th	Night	68% 7% 10% 12% 3	2660
	11th to 20th	Night	61% 5% 11% 15% 7% 1	3288
	21st to end	Night	72% 6% 8% 12% 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	27% 22% 33% 14% 3	180
	11th to 20th	Day	35% 18% 28% 16% 2 1	180
	21st to end	Day	26% 19% 35% 17% 3	247
February	1st to 10th	Day	31% 27% 30% 12% 1	270
	11th to 20th	Day	29% 29% 35% 7% 1	272
	21st to end	Day	34% 20% 33% 12% 2	382
March	1st to 10th	Day	26% 21% 33% 17% 3	460
	11th to 20th	Day	28% 21% 33% 14% 3	460
	21st to end	Day	31% 23% 27% 12% 6%	506
April	1st to 10th	Day	32% 22% 28% 13% 6%	460
	11th to 20th	Day	32% 21% 23% 14% 8% 2	452
	21st to end	Day	30% 20% 29% 13% 6% 2	460
May	1st to 10th	Day	23% 20% 30% 19% 7% 2	463
	11th to 20th	Day	20% 16% 36% 20% 7% 2	487
	21st to end	Day	20% 14% 33% 22% 9% 2	529
June	1st to 10th	Day	24% 14% 32% 21% 8% 1	490
	11th to 20th	Day	15% 17% 32% 22% 11% 2	490
	21st to end	Day	21% 14% 33% 20% 10% 3	490
July	1st to 10th	Day	14% 8% 31% 30% 15% 2	485
	11th to 20th	Day	13% 11% 32% 26% 14% 3	484
	21st to end	Day	14% 8% 32% 33% 11% 3	532
August	1st to 10th	Day	17% 10% 27% 30% 13% 3	486
	11th to 20th	Day	14% 6% 30% 28% 15% 6%	479
	21st to end	Day	16% 11% 32% 24% 11% 7%	539
September	1st to 10th	Day	14% 13% 31% 24% 12% 6%	490
	11th to 20th	Day	12% 9% 25% 24% 18% 12%	490
	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	25% 20% 29% 19% 6% 1	378
	11th to 20th	Day	28% 21% 32% 12% 5% 3	192
	21st to end	Day	29% 23% 34% 11% 3	180
December	1st to 10th	Day	33% 19% 34% 11% 3	180
	11th to 20th	Day	33% 27% 30% 8% 2	180
	21st to end	Day	36% 20% 28% 13% 2 1	198
January	1st to 10th	Night	29% 20% 31% 17% 4	760
	11th to 20th	Night	30% 19% 28% 19% 4 1	760
	21st to end	Night	21% 15% 36% 22% 5%	836
February	1st to 10th	Night	26% 21% 33% 18% 2	760
	11th to 20th	Night	22% 21% 38% 16% 3	743
	21st to end	Night	26% 20% 31% 19% 4	628
March	1st to 10th	Night	26% 16% 33% 21% 4	760
	11th to 20th	Night	26% 21% 29% 19% 5% 1	760
	21st to end	Night	25% 15% 31% 23% 6% 1	836
April	1st to 10th	Night	27% 17% 32% 21% 3	760
	11th to 20th	Night	27% 19% 28% 18% 7% 1	751
	21st to end	Night	26% 14% 29% 23% 8% 1	671
May	1st to 10th	Night	26% 8% 27% 27% 9% 2	380
	11th to 20th	Night	17% 11% 35% 29% 7%	374
	21st to end	Night	13% 10% 38% 27% 12%	411
June	1st to 10th	Night	17% 8% 30% 36% 8% 1	380
	11th to 20th	Night	17% 9% 29% 32% 12% 1	380
	21st to end	Night	19% 7% 30% 31% 14%	380
July	1st to 10th	Night	9% 5% 33% 40% 12% 1	370
	11th to 20th	Night	13% 4 28% 38% 15% 1	368
	21st to end	Night	10% 4 31% 41% 13% 1	416
August	1st to 10th	Night	12% 4 28% 36% 19%	450
	11th to 20th	Night	13% 4 24% 34% 23% 2	571
	21st to end	Night	13% 7% 30% 33% 15% 2	836
September	1st to 10th	Night	15% 7% 32% 31% 15% 1	760
	11th to 20th	Night	17% 7% 20% 29% 23% 4	760
	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%	760
	11th to 20th	Night	26% 17% 31% 17% 8%	760
	21st to end	Night	31% 17% 28% 18% 6%	760
December	1st to 10th	Night	32% 18% 28% 18% 3	760
	11th to 20th	Night	29% 18% 32% 18% 3 1	760
	21st to end	Night	34% 17% 28% 19% 2	836

Page generated at 2024-11-03 02:11:28. Version ac07

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