Hale Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hale Mountain Observatory clear sky chart from 2005-11-06 to 2024-11-25.

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	11% 7% 3 6% 4 4 4 4 5% 7% 45%	101416
	Night	11% 7% 3 6% 4 3 4 3 5% 6% 50%	98144
Transparency	Day	37% 6% 11% 24% 19% 4	105067
	Night	34% 5% 11% 24% 21% 6%	102633
Seeing	Day	16% 14% 30% 25% 12% 2	15495
	Night	15% 15% 31% 26% 11% 1	25326

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	11% 13% 4 6% 3 2 3 3 4 5% 45%	2847
	11th to 20th	Day	9% 7% 2 8% 4 3 3 3 4 5% 53%	2850
	21st to end	Day	20% 10% 3 5% 3 2 3 2 3 5% 44%	3135
February	1st to 10th	Day	16% 10% 3 7% 6% 4 3 3 4 3 41%	1710
	11th to 20th	Day	14% 11% 3 6% 3 4 3 3 4 6% 43%	1695
	21st to end	Day	16% 11% 3 7% 4 4 3 3 4 5% 40%	1703
March	1st to 10th	Day	14% 8% 2 6% 5% 3 4 3 4 5% 45%	3436
	11th to 20th	Day	12% 7% 2 6% 4 4 4 4 4 5% 47%	3480
	21st to end	Day	16% 10% 3 8% 5% 5% 4 3 4 6% 35%	3814
April	1st to 10th	Day	12% 7% 3 7% 5% 4 4 4 5% 6% 43%	2177
	11th to 20th	Day	16% 8% 2 7% 5% 4 4 3 5% 6% 40%	2254
	21st to end	Day	17% 11% 3 7% 5% 4 3 3 4 5% 39%	2286
May	1st to 10th	Day	18% 9% 3 8% 5% 5% 5% 3 5% 5% 36%	4082
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 4 5% 7% 40%	4082
	21st to end	Day	14% 7% 3 8% 6% 5% 5% 4 6% 7% 35%	4477
June	1st to 10th	Day	9% 6% 3 6% 5% 5% 5% 4 7% 9% 42%	2470
	11th to 20th	Day	7% 4 2 6% 5% 5% 5% 5% 6% 9% 46%	2470
	21st to end	Day	4 4 2 4 5% 5% 5% 5% 6% 9% 52%	2470
July	1st to 10th	Day	9% 5% 1 5% 4 5% 5% 4 7% 11% 43%	4120
	11th to 20th	Day	4 4 2 6% 4 4 5% 4 6% 10% 52%	4120
	21st to end	Day	7% 5% 2 6% 5% 6% 6% 5% 8% 11% 39%	4465
August	1st to 10th	Day	4 3 1 4 4 4 5% 4 7% 9% 55%	4104
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 5% 9% 46%	3993
	21st to end	Day	4 4 2 5% 4 5% 5% 4 6% 8% 54%	4180
September	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 5% 7% 53%	2280
	11th to 20th	Day	10% 4 2 6% 4 4 5% 4 8% 8% 46%	2094
	21st to end	Day	7% 4 2 5% 4 5% 4 4 5% 7% 54%	2076
October	1st to 10th	Day	8% 5% 2 5% 3 2 3 3 5% 6% 56%	1900
	11th to 20th	Day	12% 4 2 5% 3 2 2 2 3 5% 59%	1900
	21st to end	Day	17% 7% 3 4 3 2 3 2 4 4 51%	2090
November	1st to 10th	Day	10% 7% 2 4 3 3 2 3 4 6% 55%	1958
	11th to 20th	Day	10% 7% 2 6% 3 3 3 2 5% 6% 53%	1946
	21st to end	Day	14% 9% 2 6% 2 3 3 3 5% 5% 47%	1746
December	1st to 10th	Day	14% 7% 3 6% 5% 4 3 3 4 6% 46%	1710
	11th to 20th	Day	12% 11% 3 7% 5% 2 4 2 4 5% 42%	2161
	21st to end	Day	14% 15% 8% 7% 4 3 4 3 4 5% 32%	3135
January	1st to 10th	Night	10% 9% 5% 9% 5% 5% 4 3 4 4 42%	4108
	11th to 20th	Night	10% 6% 3 6% 3 3 4 3 4 5% 53%	4120
	21st to end	Night	18% 11% 4 6% 3 2 2 2 3 5% 43%	4532
February	1st to 10th	Night	14% 10% 3 7% 4 3 3 3 5% 5% 42%	2470
	11th to 20th	Night	13% 8% 3 6% 4 3 3 3 4 5% 47%	2388
	21st to end	Night	13% 9% 3 6% 4 4 4 3 4 5% 46%	1749
March	1st to 10th	Night	16% 9% 3 6% 4 3 4 3 5% 4 43%	3419
	11th to 20th	Night	13% 8% 2 5% 3 3 4 3 5% 5% 49%	3480
	21st to end	Night	12% 8% 3 8% 5% 4 4 3 5% 7% 42%	3817
April	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 6% 48%	2001
	11th to 20th	Night	12% 8% 3 5% 4 4 3 3 5% 6% 49%	1875
	21st to end	Night	15% 8% 3 6% 4 4 5% 3 4 6% 43%	1891
May	1st to 10th	Night	13% 9% 3 6% 3 4 4 3 5% 7% 44%	2823
	11th to 20th	Night	11% 8% 2 7% 4 4 4 3 6% 8% 42%	2823
	21st to end	Night	14% 7% 2 6% 5% 4 4 4 6% 8% 40%	3090
June	1st to 10th	Night	5% 4 1 5% 3 3 4 4 6% 9% 56%	1710
	11th to 20th	Night	4 3 1 4 4 4 4 4 7% 9% 57%	1710
	21st to end	Night	3 2 1 3 3 3 4 4 5% 9% 63%	1710
July	1st to 10th	Night	9% 4 2 6% 3 4 4 4 6% 9% 49%	2850
	11th to 20th	Night	4 2 1 4 3 3 3 3 5% 9% 64%	2850
	21st to end	Night	5% 5% 2 6% 4 4 4 4 7% 10% 49%	3098
August	1st to 10th	Night	3 2 1 4 4 3 5% 4 6% 10% 58%	2836
	11th to 20th	Night	6% 4 2 4 4 3 4 4 6% 8% 53%	2829
	21st to end	Night	3 3 1 4 3 3 3 3 6% 8% 64%	3476
September	1st to 10th	Night	8% 4 2 5% 3 3 3 3 5% 6% 58%	1900
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 6% 8% 51%	2081
	21st to end	Night	7% 5% 2 5% 4 3 4 2 5% 6% 57%	2090
October	1st to 10th	Night	6% 3 1 4 3 3 4 3 5% 7% 62%	2273
	11th to 20th	Night	12% 4 1 4 3 3 3 3 4 4 58%	2280
	21st to end	Night	17% 7% 2 4 2 2 3 2 4 5% 52%	2508
November	1st to 10th	Night	11% 6% 2 4 2 2 3 3 3 4 58%	2340
	11th to 20th	Night	10% 6% 2 4 4 3 3 3 4 6% 56%	2450
	21st to end	Night	14% 8% 2 5% 3 3 3 2 4 5% 52%	2529
December	1st to 10th	Night	12% 9% 3 6% 4 3 4 3 4 6% 47%	2470
	11th to 20th	Night	12% 9% 3 7% 5% 4 4 3 4 4 44%	3036
	21st to end	Night	14% 13% 8% 8% 4 3 4 2 3 4 36%	4532

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	42% 4 6% 24% 22% 3	2848
	11th to 20th	Day	34% 5% 13% 25% 19% 4	2850
	21st to end	Day	44% 3 7% 20% 20% 6%	3135
February	1st to 10th	Day	48% 4 9% 19% 16% 3	1710
	11th to 20th	Day	44% 3 8% 23% 19% 3	1704
	21st to end	Day	47% 4 7% 17% 17% 8%	1703
March	1st to 10th	Day	41% 5% 7% 18% 18% 11%	3436
	11th to 20th	Day	38% 7% 10% 22% 20% 4	3480
	21st to end	Day	48% 6% 10% 22% 13% 1	3814
April	1st to 10th	Day	41% 6% 10% 21% 18% 5%	2177
	11th to 20th	Day	45% 5% 10% 20% 16% 4	2254
	21st to end	Day	50% 5% 7% 21% 15% 2	2286
May	1st to 10th	Day	50% 7% 8% 20% 15% 1	4120
	11th to 20th	Day	41% 9% 10% 25% 14% 1	4086
	21st to end	Day	46% 8% 13% 18% 14% 1	4489
June	1st to 10th	Day	37% 9% 14% 26% 13% 2	2470
	11th to 20th	Day	34% 9% 13% 26% 16% 1	2470
	21st to end	Day	27% 8% 15% 29% 17% 4	2470
July	1st to 10th	Day	31% 8% 21% 25% 13% 2	4056
	11th to 20th	Day	26% 6% 12% 36% 18% 2	4051
	21st to end	Day	36% 10% 17% 28% 9% 1	4325
August	1st to 10th	Day	24% 7% 15% 27% 23% 4	4090
	11th to 20th	Day	34% 6% 16% 28% 15% 1	4032
	21st to end	Day	26% 8% 15% 35% 13% 2	4179
September	1st to 10th	Day	32% 5% 10% 25% 19% 9%	2280
	11th to 20th	Day	32% 7% 11% 23% 22% 5%	2094
	21st to end	Day	29% 6% 10% 21% 28% 6%	2076
October	1st to 10th	Day	18% 3 6% 13% 40% 19%	3160
	11th to 20th	Day	19% 2 4 18% 40% 16%	3140
	21st to end	Day	30% 3 10% 29% 23% 5%	3428
November	1st to 10th	Day	31% 5% 8% 25% 22% 8%	1958
	11th to 20th	Day	33% 3 7% 25% 22% 10%	1946
	21st to end	Day	39% 3 8% 21% 25% 4	1746
December	1st to 10th	Day	39% 5% 9% 24% 20% 3	1710
	11th to 20th	Day	44% 3 6% 20% 23% 4	2159
	21st to end	Day	52% 4 8% 19% 14% 3	3135
January	1st to 10th	Night	44% 4 8% 19% 22% 3	4108
	11th to 20th	Night	32% 5% 15% 22% 24% 3	4120
	21st to end	Night	44% 3 10% 18% 20% 5%	4532
February	1st to 10th	Night	44% 4 9% 23% 15% 5%	2470
	11th to 20th	Night	39% 4 8% 24% 20% 5%	2389
	21st to end	Night	41% 3 7% 17% 21% 9%	1749
March	1st to 10th	Night	44% 5% 7% 19% 15% 10%	3419
	11th to 20th	Night	38% 6% 8% 25% 18% 5%	3480
	21st to end	Night	40% 6% 12% 25% 14% 2	3817
April	1st to 10th	Night	38% 5% 9% 21% 20% 8%	2001
	11th to 20th	Night	37% 5% 8% 22% 19% 8%	1875
	21st to end	Night	43% 4 9% 23% 19% 2	1891
May	1st to 10th	Night	41% 8% 10% 21% 17% 4	2850
	11th to 20th	Night	38% 9% 14% 23% 16% 1	2824
	21st to end	Night	40% 8% 15% 21% 13% 2	3099
June	1st to 10th	Night	23% 9% 15% 32% 19% 3	1710
	11th to 20th	Night	23% 6% 14% 33% 21% 3	1710
	21st to end	Night	19% 6% 15% 30% 24% 7%	1710
July	1st to 10th	Night	30% 8% 18% 23% 16% 4	2805
	11th to 20th	Night	18% 5% 11% 40% 23% 2	2796
	21st to end	Night	29% 8% 18% 31% 13% 1	2999
August	1st to 10th	Night	21% 7% 16% 32% 23% 2	2826
	11th to 20th	Night	28% 6% 19% 29% 15% 2	2856
	21st to end	Night	19% 6% 13% 39% 22% 3	3476
September	1st to 10th	Night	26% 5% 10% 28% 21% 10%	1900
	11th to 20th	Night	30% 6% 11% 22% 24% 6%	2081
	21st to end	Night	28% 5% 8% 22% 30% 7%	2090
October	1st to 10th	Night	14% 3 7% 13% 36% 27%	3793
	11th to 20th	Night	20% 3 4 17% 37% 20%	3779
	21st to end	Night	29% 3 7% 29% 24% 9%	4120
November	1st to 10th	Night	31% 4 8% 23% 25% 10%	2340
	11th to 20th	Night	30% 3 7% 23% 26% 10%	2450
	21st to end	Night	37% 2 7% 21% 26% 6%	2529
December	1st to 10th	Night	39% 4 9% 20% 22% 5%	2470
	11th to 20th	Night	43% 3 7% 20% 21% 6%	3037
	21st to end	Night	51% 3 10% 17% 15% 4	4532

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% 26% 34% 11% 3	270
	11th to 20th	Day	26% 31% 34% 7% 2	270
	21st to end	Day	23% 33% 30% 11% 3	297
February	1st to 10th	Day	30% 26% 35% 8%	270
	11th to 20th	Day	33% 29% 29% 8% 1	270
	21st to end	Day	40% 27% 27% 6%	225
March	1st to 10th	Day	24% 28% 33% 15% 1	270
	11th to 20th	Day	34% 23% 33% 9% 1	270
	21st to end	Day	34% 29% 29% 6% 2	297
April	1st to 10th	Day	26% 26% 32% 12% 4 1	357
	11th to 20th	Day	26% 22% 37% 12% 3	452
	21st to end	Day	27% 21% 35% 13% 4	466
May	1st to 10th	Day	18% 21% 41% 16% 4	653
	11th to 20th	Day	16% 16% 36% 23% 8% 1	675
	21st to end	Day	13% 12% 39% 21% 11% 3	734
June	1st to 10th	Day	10% 7% 28% 35% 18% 2	680
	11th to 20th	Day	7% 6% 30% 39% 16% 2	680
	21st to end	Day	4 6% 34% 36% 18% 3	680
July	1st to 10th	Day	10% 3 22% 36% 22% 7%	671
	11th to 20th	Day	4 3 24% 41% 24% 4	668
	21st to end	Day	8% 1 17% 40% 28% 7%	739
August	1st to 10th	Day	7% 3 20% 43% 23% 4	676
	11th to 20th	Day	11% 4 18% 41% 23% 3	643
	21st to end	Day	7% 2 17% 36% 31% 8%	539
September	1st to 10th	Day	11% 2 17% 40% 25% 6%	490
	11th to 20th	Day	6% 2 30% 38% 20% 4	304
	21st to end	Day	12% 7% 30% 34% 16% 2	300
October	1st to 10th	Day	9% 13% 32% 30% 12% 3	300
	11th to 20th	Day	17% 15% 41% 22% 3 1	300
	21st to end	Day	29% 18% 33% 16% 2 1	330
November	1st to 10th	Day	17% 18% 43% 17% 5% 1	300
	11th to 20th	Day	20% 23% 33% 19% 4	300
	21st to end	Day	26% 27% 29% 16% 2	282
December	1st to 10th	Day	22% 26% 36% 11% 3 2	270
	11th to 20th	Day	26% 25% 37% 9% 2 1	270
	21st to end	Day	23% 31% 36% 8% 2	297
January	1st to 10th	Night	19% 22% 37% 18% 4	950
	11th to 20th	Night	22% 24% 34% 17% 3	950
	21st to end	Night	19% 24% 33% 21% 3	1045
February	1st to 10th	Night	23% 22% 34% 17% 3	950
	11th to 20th	Night	21% 27% 36% 14% 2	914
	21st to end	Night	27% 24% 30% 17% 2	493
March	1st to 10th	Night	20% 20% 35% 23% 2	570
	11th to 20th	Night	24% 19% 32% 22% 3	570
	21st to end	Night	19% 25% 38% 15% 4	627
April	1st to 10th	Night	18% 24% 35% 20% 4	570
	11th to 20th	Night	17% 21% 37% 19% 5%	563
	21st to end	Night	19% 17% 40% 17% 7%	570
May	1st to 10th	Night	14% 13% 41% 25% 7%	570
	11th to 20th	Night	15% 12% 32% 30% 11% 1	561
	21st to end	Night	11% 11% 34% 27% 15% 2	617
June	1st to 10th	Night	6% 6% 24% 39% 23% 1	570
	11th to 20th	Night	5% 2 36% 40% 16% 2	570
	21st to end	Night	3 2 29% 43% 21% 3	570
July	1st to 10th	Night	5% 1 19% 41% 30% 3	555
	11th to 20th	Night	5% 1 20% 43% 28% 5%	552
	21st to end	Night	5% 1 16% 41% 34% 4	624
August	1st to 10th	Night	4 1 20% 42% 30% 3	567
	11th to 20th	Night	8% 2 20% 43% 24% 3	564
	21st to end	Night	6% 2 19% 39% 29% 4	627
September	1st to 10th	Night	10% 2 17% 44% 24% 2	570
	11th to 20th	Night	13% 3 19% 40% 21% 5%	570
	21st to end	Night	9% 6% 26% 35% 21% 2	570
October	1st to 10th	Night	7% 7% 33% 30% 21% 2	753
	11th to 20th	Night	15% 14% 36% 26% 8%	760
	21st to end	Night	22% 14% 33% 22% 8% 1	836
November	1st to 10th	Night	14% 16% 37% 24% 8%	780
	11th to 20th	Night	17% 17% 37% 23% 6%	850
	21st to end	Night	22% 20% 32% 20% 6%	973
December	1st to 10th	Night	21% 19% 36% 20% 4	950
	11th to 20th	Night	21% 21% 32% 21% 4	950
	21st to end	Night	20% 25% 33% 18% 4	1045

Page generated at 2024-11-22 07:02:23. Version ac07

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