Hale Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hale Hill Observatory clear sky chart from 2005-11-06 to 2025-02-06.

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	14% 9% 3 7% 4 4 4 4 5% 7% 38%	108288
	Night	15% 10% 4 7% 4 4 4 3 5% 6% 38%	110093
Transparency	Day	44% 6% 11% 23% 15% 2	111937
	Night	46% 5% 10% 22% 15% 2	114583
Seeing	Day	18% 13% 28% 23% 14% 5%	15855
	Night	19% 11% 28% 27% 13% 2	25797

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	14% 15% 6% 11% 5% 4 5% 4 5% 5% 26%	3617
	11th to 20th	Day	12% 13% 7% 14% 9% 4 5% 4 4 5% 24%	3620
	21st to end	Day	21% 15% 5% 10% 4 3 3 2 4 5% 28%	4221
February	1st to 10th	Day	17% 13% 6% 8% 5% 4 4 3 4 6% 30%	1950
	11th to 20th	Day	15% 11% 4 10% 5% 5% 4 4 5% 6% 32%	1848
	21st to end	Day	17% 15% 4 9% 4 3 4 3 4 6% 32%	1570
March	1st to 10th	Day	22% 17% 3 7% 3 3 3 2 3 3 33%	3120
	11th to 20th	Day	15% 7% 3 5% 4 4 4 3 5% 7% 44%	3310
	21st to end	Day	15% 10% 5% 8% 6% 5% 5% 4 6% 6% 31%	4064
April	1st to 10th	Day	15% 11% 3 7% 4 4 5% 3 6% 7% 36%	2280
	11th to 20th	Day	18% 8% 3 6% 5% 5% 3 3 5% 6% 39%	2254
	21st to end	Day	19% 8% 2 7% 5% 5% 5% 3 4 6% 36%	2394
May	1st to 10th	Day	19% 9% 3 8% 4 4 6% 4 7% 8% 28%	4082
	11th to 20th	Day	20% 7% 2 6% 5% 4 4 3 5% 6% 37%	4081
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 7% 8% 39%	4632
June	1st to 10th	Day	11% 6% 2 5% 5% 5% 5% 3 6% 8% 43%	2660
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 8% 46%	2660
	21st to end	Day	11% 6% 2 6% 5% 4 6% 3 7% 9% 42%	2660
July	1st to 10th	Day	14% 6% 2 6% 5% 4 4 3 7% 10% 38%	4440
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 5% 7% 11% 45%	4440
	21st to end	Day	11% 6% 3 7% 5% 6% 6% 5% 8% 10% 33%	4730
August	1st to 10th	Day	9% 4 1 5% 4 3 5% 3 7% 10% 49%	4104
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 6% 8% 11% 38%	3743
	21st to end	Day	6% 4 2 5% 3 4 5% 3 6% 9% 54%	4180
September	1st to 10th	Day	12% 6% 2 5% 4 4 4 5% 5% 8% 46%	2278
	11th to 20th	Day	9% 4 2 6% 4 5% 5% 3 5% 7% 49%	2090
	21st to end	Day	12% 5% 2 5% 4 4 4 3 6% 6% 49%	2090
October	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 4 8% 51%	2090
	11th to 20th	Day	14% 6% 3 5% 4 4 4 4 5% 6% 45%	1939
	21st to end	Day	19% 9% 3 7% 4 4 3 4 4 6% 37%	1881
November	1st to 10th	Day	9% 6% 2 5% 4 4 4 3 5% 6% 51%	1762
	11th to 20th	Day	17% 10% 3 7% 4 3 3 4 4 6% 40%	1800
	21st to end	Day	19% 11% 4 7% 4 3 4 3 5% 5% 36%	1800
December	1st to 10th	Day	13% 12% 3 7% 4 3 3 2 3 5% 43%	2908
	11th to 20th	Day	16% 14% 6% 9% 5% 5% 5% 4 4 5% 27%	3008
	21st to end	Day	27% 18% 7% 9% 5% 4 4 3 4 5% 16%	3982
January	1st to 10th	Night	16% 15% 8% 12% 5% 5% 4 3 5% 5% 22%	6348
	11th to 20th	Night	13% 18% 7% 13% 6% 5% 5% 3 4 4 22%	6360
	21st to end	Night	21% 13% 5% 7% 5% 4 4 3 4 5% 29%	6748
February	1st to 10th	Night	19% 12% 5% 10% 5% 4 3 3 5% 5% 28%	2341
	11th to 20th	Night	15% 12% 4 10% 4 5% 4 3 4 6% 33%	2218
	21st to end	Night	20% 12% 4 9% 4 3 3 3 4 5% 32%	1884
March	1st to 10th	Night	22% 17% 3 5% 4 3 3 3 3 5% 32%	3733
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 4 6% 48%	3650
	21st to end	Night	18% 8% 3 7% 5% 4 4 3 4 6% 38%	3594
April	1st to 10th	Night	14% 10% 2 7% 4 4 4 3 5% 7% 41%	1900
	11th to 20th	Night	17% 9% 3 6% 4 3 3 3 4 7% 41%	1875
	21st to end	Night	16% 9% 3 7% 4 4 4 4 6% 6% 37%	1785
May	1st to 10th	Night	18% 10% 4 8% 5% 3 5% 4 5% 8% 29%	2813
	11th to 20th	Night	19% 7% 2 6% 3 3 3 3 5% 7% 42%	2812
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	2911
June	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 5% 8% 48%	1520
	11th to 20th	Night	9% 4 2 6% 4 3 4 4 5% 9% 49%	1520
	21st to end	Night	6% 5% 1 5% 4 4 5% 5% 7% 10% 47%	1520
July	1st to 10th	Night	10% 6% 2 5% 3 4 4 4 6% 8% 47%	2530
	11th to 20th	Night	6% 4 1 4 4 4 5% 4 6% 8% 55%	2530
	21st to end	Night	10% 7% 2 7% 6% 4 5% 4 7% 9% 38%	2828
August	1st to 10th	Night	8% 5% 2 6% 4 4 4 3 5% 8% 52%	2825
	11th to 20th	Night	7% 5% 2 6% 4 5% 4 4 6% 9% 49%	3070
	21st to end	Night	5% 3 1 5% 4 4 4 3 6% 9% 57%	3476
September	1st to 10th	Night	10% 6% 1 6% 4 4 3 4 5% 6% 50%	1900
	11th to 20th	Night	7% 4 2 5% 3 3 3 4 6% 7% 55%	2071
	21st to end	Night	13% 6% 2 5% 4 4 3 3 5% 7% 48%	2090
October	1st to 10th	Night	8% 4 2 5% 3 4 4 3 5% 6% 56%	2090
	11th to 20th	Night	16% 7% 2 4 3 4 3 3 4 6% 48%	2235
	21st to end	Night	18% 9% 2 7% 3 3 4 3 5% 7% 40%	2717
November	1st to 10th	Night	12% 8% 2 4 3 3 4 3 5% 6% 50%	2536
	11th to 20th	Night	14% 10% 3 7% 4 4 3 3 5% 7% 41%	2600
	21st to end	Night	20% 13% 4 7% 4 3 3 3 4 5% 35%	2600
December	1st to 10th	Night	16% 11% 4 8% 4 3 3 3 3 5% 40%	4268
	11th to 20th	Night	19% 13% 4 9% 5% 4 3 4 5% 5% 29%	5199
	21st to end	Night	29% 18% 6% 9% 4 4 4 3 3 4 18%	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	59% 4 7% 21% 8% 1	3618
	11th to 20th	Day	61% 5% 10% 12% 10% 2	3620
	21st to end	Day	61% 2 7% 19% 6% 3	4221
February	1st to 10th	Day	55% 5% 10% 19% 9% 2	1950
	11th to 20th	Day	52% 5% 10% 21% 10% 1	1858
	21st to end	Day	54% 4 8% 22% 10% 2	1570
March	1st to 10th	Day	56% 4 6% 18% 11% 3	3120
	11th to 20th	Day	40% 5% 11% 29% 14% 1	3310
	21st to end	Day	50% 8% 13% 21% 7% 1	4064
April	1st to 10th	Day	46% 7% 11% 21% 14% 1	2280
	11th to 20th	Day	45% 5% 10% 25% 12% 4	2254
	21st to end	Day	49% 6% 9% 23% 11% 3	2394
May	1st to 10th	Day	50% 8% 14% 21% 7%	4120
	11th to 20th	Day	47% 7% 10% 26% 10%	4085
	21st to end	Day	39% 8% 13% 18% 19% 3	4645
June	1st to 10th	Day	37% 7% 11% 25% 16% 3	2660
	11th to 20th	Day	33% 8% 12% 24% 20% 3	2660
	21st to end	Day	37% 8% 15% 19% 17% 4	2660
July	1st to 10th	Day	40% 6% 12% 24% 16% 2	4371
	11th to 20th	Day	30% 7% 14% 30% 19% 1	4365
	21st to end	Day	43% 8% 15% 19% 14% 2	4580
August	1st to 10th	Day	29% 7% 14% 32% 16% 1	4091
	11th to 20th	Day	36% 7% 12% 28% 16% 1	3779
	21st to end	Day	26% 6% 16% 31% 17% 4	4179
September	1st to 10th	Day	36% 7% 11% 24% 18% 4	2278
	11th to 20th	Day	33% 5% 12% 26% 22% 3	2090
	21st to end	Day	35% 5% 10% 24% 24% 2	2090
October	1st to 10th	Day	22% 4 7% 29% 36% 2	3480
	11th to 20th	Day	30% 4 9% 26% 30% 2	3207
	21st to end	Day	39% 3 9% 23% 25% 1	3081
November	1st to 10th	Day	33% 5% 12% 28% 17% 5%	1762
	11th to 20th	Day	46% 4 9% 21% 16% 4	1800
	21st to end	Day	50% 5% 8% 22% 14% 1	1800
December	1st to 10th	Day	45% 3 10% 28% 14%	2908
	11th to 20th	Day	60% 4 8% 21% 7% 1	3005
	21st to end	Day	71% 5% 9% 10% 5%	3982
January	1st to 10th	Night	64% 5% 9% 15% 7%	6348
	11th to 20th	Night	67% 4 8% 10% 8% 3	6360
	21st to end	Night	58% 3 9% 20% 9% 1	6748
February	1st to 10th	Night	58% 5% 11% 18% 7% 1	2341
	11th to 20th	Night	52% 4 10% 20% 12% 1	2219
	21st to end	Night	54% 6% 9% 19% 10% 2	1884
March	1st to 10th	Night	56% 4 7% 21% 11% 1	3733
	11th to 20th	Night	37% 6% 11% 23% 22% 1	3650
	21st to end	Night	46% 6% 11% 28% 8% 1	3594
April	1st to 10th	Night	44% 5% 10% 25% 14% 2	1900
	11th to 20th	Night	44% 4 10% 20% 18% 5%	1875
	21st to end	Night	45% 7% 9% 23% 13% 3	1785
May	1st to 10th	Night	51% 10% 13% 19% 7% 1	2840
	11th to 20th	Night	42% 6% 14% 27% 11% 1	2813
	21st to end	Night	35% 9% 13% 22% 17% 4	2919
June	1st to 10th	Night	34% 6% 11% 25% 21% 4	1520
	11th to 20th	Night	32% 6% 11% 24% 21% 6%	1520
	21st to end	Night	29% 8% 16% 21% 20% 5%	1520
July	1st to 10th	Night	35% 7% 10% 26% 20% 3	2490
	11th to 20th	Night	25% 7% 13% 31% 21% 2	2482
	21st to end	Night	40% 6% 15% 23% 14% 3	2738
August	1st to 10th	Night	30% 7% 13% 36% 12% 2	2814
	11th to 20th	Night	32% 6% 14% 28% 19% 1	3099
	21st to end	Night	24% 6% 14% 30% 22% 4	3476
September	1st to 10th	Night	33% 6% 11% 23% 20% 6%	1900
	11th to 20th	Night	27% 5% 10% 26% 27% 5%	2071
	21st to end	Night	34% 7% 10% 21% 23% 5%	2090
October	1st to 10th	Night	19% 5% 7% 21% 45% 3	3480
	11th to 20th	Night	26% 2 7% 26% 35% 3	3706
	21st to end	Night	35% 4 8% 23% 25% 5%	4467
November	1st to 10th	Night	36% 5% 9% 24% 22% 5%	2536
	11th to 20th	Night	44% 3 9% 22% 18% 3	2600
	21st to end	Night	52% 4 9% 22% 13% 1	2600
December	1st to 10th	Night	49% 3 11% 24% 13%	4268
	11th to 20th	Night	57% 5% 11% 17% 10%	5201
	21st to end	Night	72% 4 7% 12% 5%	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	27% 20% 34% 16% 3	300
	11th to 20th	Day	33% 21% 34% 11% 1	300
	21st to end	Day	29% 24% 34% 10% 2	330
February	1st to 10th	Day	28% 26% 32% 13% 1	285
	11th to 20th	Day	25% 30% 30% 12% 3	270
	21st to end	Day	32% 19% 39% 8% 1	225
March	1st to 10th	Day	24% 29% 34% 11% 1	270
	11th to 20th	Day	28% 21% 35% 11% 5% 1	355
	21st to end	Day	29% 20% 31% 16% 4	506
April	1st to 10th	Day	23% 19% 32% 19% 7% 1	460
	11th to 20th	Day	24% 19% 26% 22% 8% 1	452
	21st to end	Day	25% 19% 33% 14% 7% 1	460
May	1st to 10th	Day	21% 17% 31% 21% 7% 2	463
	11th to 20th	Day	22% 9% 32% 20% 13% 5%	487
	21st to end	Day	12% 8% 29% 27% 16% 8%	620
June	1st to 10th	Day	13% 7% 29% 30% 18% 4	680
	11th to 20th	Day	11% 7% 26% 34% 17% 5%	680
	21st to end	Day	12% 7% 30% 27% 17% 6%	680
July	1st to 10th	Day	13% 4 18% 27% 25% 13%	671
	11th to 20th	Day	7% 4 25% 32% 22% 9%	668
	21st to end	Day	12% 3 19% 34% 24% 8%	692
August	1st to 10th	Day	11% 4 18% 28% 29% 10%	486
	11th to 20th	Day	14% 4 23% 30% 22% 7%	479
	21st to end	Day	8% 3 15% 33% 29% 12%	539
September	1st to 10th	Day	11% 5% 19% 33% 25% 7%	490
	11th to 20th	Day	9% 3 18% 31% 29% 11%	490
	21st to end	Day	15% 10% 24% 23% 21% 7%	490
October	1st to 10th	Day	10% 9% 27% 33% 17% 4	490
	11th to 20th	Day	15% 17% 34% 21% 10% 2	377
	21st to end	Day	28% 17% 36% 17% 2	330
November	1st to 10th	Day	14% 24% 29% 21% 9% 3	300
	11th to 20th	Day	25% 22% 32% 16% 5% 1	300
	21st to end	Day	28% 16% 37% 16% 2 1	300
December	1st to 10th	Day	19% 28% 33% 15% 4 1	300
	11th to 20th	Day	27% 20% 37% 11% 4 1	300
	21st to end	Day	34% 19% 33% 11% 2 1	330
January	1st to 10th	Night	23% 18% 34% 21% 4	1000
	11th to 20th	Night	26% 21% 33% 18% 3	1000
	21st to end	Night	23% 19% 34% 22% 2	1060
February	1st to 10th	Night	25% 15% 38% 19% 2	781
	11th to 20th	Night	22% 20% 36% 20% 2	743
	21st to end	Night	27% 17% 36% 18% 2	628
March	1st to 10th	Night	26% 17% 32% 22% 2	760
	11th to 20th	Night	23% 15% 35% 20% 6% 1	760
	21st to end	Night	26% 10% 33% 25% 5%	703
April	1st to 10th	Night	20% 14% 32% 28% 6%	570
	11th to 20th	Night	20% 14% 29% 26% 10% 1	563
	21st to end	Night	20% 10% 38% 25% 7% 1	570
May	1st to 10th	Night	19% 9% 32% 29% 11%	570
	11th to 20th	Night	16% 8% 27% 30% 16% 3	561
	21st to end	Night	11% 4 25% 36% 21% 4	617
June	1st to 10th	Night	12% 4 23% 39% 22% 1	570
	11th to 20th	Night	11% 4 22% 43% 19% 1	570
	21st to end	Night	9% 3 23% 38% 26% 2	570
July	1st to 10th	Night	9% 2 15% 36% 31% 7%	555
	11th to 20th	Night	6% 4 16% 37% 33% 4	552
	21st to end	Night	8% 4 13% 42% 29% 4	624
August	1st to 10th	Night	8% 2 11% 41% 33% 5%	567
	11th to 20th	Night	7% 2 15% 37% 34% 4	564
	21st to end	Night	6% 3 15% 32% 39% 6%	627
September	1st to 10th	Night	14% 3 15% 38% 27% 4	570
	11th to 20th	Night	8% 4 16% 35% 33% 5%	741
	21st to end	Night	14% 8% 19% 31% 25% 3	760
October	1st to 10th	Night	11% 5% 25% 33% 23% 3	760
	11th to 20th	Night	19% 12% 28% 26% 14% 1	760
	21st to end	Night	22% 14% 33% 24% 7%	836
November	1st to 10th	Night	16% 11% 34% 29% 11% 1	780
	11th to 20th	Night	19% 18% 28% 27% 7% 1	800
	21st to end	Night	25% 18% 26% 25% 7%	800
December	1st to 10th	Night	25% 15% 37% 19% 4	805
	11th to 20th	Night	27% 13% 32% 22% 6% 1	1000
	21st to end	Night	32% 16% 31% 17% 3	1100

Page generated at 2025-02-03 03:05:56. Version ac07

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