The Last Stand Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Last Stand Observatory clear sky chart from 2005-11-06 to 2024-11-08.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	101041
	Night	14% 6% 2 5% 4 3 4 3 5% 7% 47%	98158
Transparency	Day	40% 5% 11% 21% 15% 8%	104810
	Night	36% 5% 13% 21% 16% 9%	102516
Seeing	Day	13% 5% 19% 28% 23% 11%	14747
	Night	16% 5% 22% 35% 21% 2	24638

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	16% 11% 2 6% 4 4 3 3 4 5% 41%	2847
	11th to 20th	Day	17% 5% 3 4 3 2 2 2 3 6% 53%	2850
	21st to end	Day	23% 7% 2 5% 4 3 4 4 6% 8% 35%	3228
February	1st to 10th	Day	22% 8% 3 5% 4 4 3 3 4 5% 40%	1900
	11th to 20th	Day	20% 9% 2 6% 4 4 3 3 3 5% 42%	1849
	21st to end	Day	19% 7% 2 6% 4 3 4 3 4 6% 42%	1646
March	1st to 10th	Day	16% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	3436
	11th to 20th	Day	20% 10% 4 9% 6% 5% 4 4 4 4 29%	3480
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 6% 44%	3814
April	1st to 10th	Day	14% 7% 3 7% 5% 4 5% 4 5% 7% 40%	2090
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 3 5% 5% 38%	2098
	21st to end	Day	13% 8% 2 7% 6% 4 4 3 6% 5% 42%	2280
May	1st to 10th	Day	18% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	4077
	11th to 20th	Day	15% 7% 2 7% 5% 4 4 4 5% 7% 40%	4082
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 8% 10% 39%	4477
June	1st to 10th	Day	5% 4 2 6% 4 4 6% 5% 8% 12% 44%	2470
	11th to 20th	Day	6% 4 2 6% 6% 5% 7% 5% 7% 10% 43%	2470
	21st to end	Day	7% 6% 3 8% 7% 6% 8% 5% 8% 9% 33%	2470
July	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 5% 7% 11% 42%	4120
	11th to 20th	Day	7% 6% 3 8% 7% 6% 7% 5% 8% 11% 31%	4120
	21st to end	Day	12% 8% 3 7% 6% 6% 5% 5% 7% 11% 30%	4465
August	1st to 10th	Day	2 1 1 3 3 4 3 4 6% 9% 64%	4104
	11th to 20th	Day	3 2 1 5% 4 5% 7% 6% 9% 12% 46%	3927
	21st to end	Day	6% 4 2 7% 6% 6% 6% 5% 8% 11% 40%	3828
September	1st to 10th	Day	8% 7% 3 7% 6% 7% 6% 4 7% 10% 37%	2090
	11th to 20th	Day	11% 6% 2 6% 6% 5% 6% 5% 6% 9% 38%	2090
	21st to end	Day	7% 4 2 6% 5% 6% 5% 4 6% 8% 47%	2090
October	1st to 10th	Day	6% 4 2 4 4 5% 5% 4 6% 8% 53%	2076
	11th to 20th	Day	12% 6% 2 4 4 3 4 4 5% 6% 49%	1900
	21st to end	Day	10% 5% 2 6% 5% 4 3 3 5% 6% 52%	2090
November	1st to 10th	Day	13% 6% 2 6% 5% 5% 4 4 6% 7% 43%	1928
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 5% 6% 48%	1900
	21st to end	Day	17% 8% 3 7% 5% 4 4 3 6% 5% 39%	1743
December	1st to 10th	Day	17% 9% 2 6% 4 4 3 3 5% 5% 41%	1710
	11th to 20th	Day	16% 8% 3 6% 5% 4 3 3 5% 6% 40%	2161
	21st to end	Day	16% 7% 2 7% 3 3 4 2 4 5% 47%	3135
January	1st to 10th	Night	21% 8% 2 5% 3 3 3 2 4 6% 43%	4108
	11th to 20th	Night	17% 6% 2 4 2 2 2 2 3 6% 55%	4120
	21st to end	Night	24% 7% 2 5% 4 3 3 2 5% 7% 37%	4468
February	1st to 10th	Night	22% 8% 3 5% 3 4 3 2 5% 6% 40%	2280
	11th to 20th	Night	21% 7% 2 5% 3 2 3 2 4 6% 44%	2218
	21st to end	Night	21% 9% 2 5% 4 3 3 4 4 4 41%	1806
March	1st to 10th	Night	24% 11% 2 6% 3 4 4 4 5% 5% 31%	3419
	11th to 20th	Night	26% 9% 3 8% 5% 4 4 3 5% 5% 28%	3480
	21st to end	Night	15% 7% 3 6% 3 4 3 3 4 5% 48%	3817
April	1st to 10th	Night	15% 8% 3 7% 5% 3 5% 4 5% 6% 40%	2090
	11th to 20th	Night	18% 7% 3 5% 4 3 3 3 5% 7% 42%	2029
	21st to end	Night	16% 9% 2 5% 5% 4 4 3 5% 7% 40%	1899
May	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 5% 6% 30%	2823
	11th to 20th	Night	14% 8% 2 6% 3 4 4 3 5% 7% 44%	2823
	21st to end	Night	11% 8% 2 7% 5% 5% 5% 5% 8% 11% 34%	3090
June	1st to 10th	Night	6% 3 1 5% 3 3 4 3 6% 9% 57%	1710
	11th to 20th	Night	5% 4 1 4 4 4 5% 4 6% 8% 54%	1710
	21st to end	Night	6% 3 2 5% 4 5% 5% 5% 9% 11% 45%	1710
July	1st to 10th	Night	5% 4 1 4 3 4 4 4 6% 10% 53%	2850
	11th to 20th	Night	5% 5% 2 6% 5% 4 6% 4 7% 10% 46%	2850
	21st to end	Night	9% 7% 3 6% 5% 4 5% 4 6% 11% 41%	3098
August	1st to 10th	Night	1 1 1 1 2 2 3 5% 8% 76%	2836
	11th to 20th	Night	1 2 1 3 3 3 5% 4 7% 12% 59%	2892
	21st to end	Night	4 2 1 3 3 4 4 5% 7% 12% 55%	3827
September	1st to 10th	Night	6% 4 2 6% 4 4 5% 4 7% 9% 48%	2090
	11th to 20th	Night	9% 4 2 5% 4 5% 5% 4 6% 7% 49%	2090
	21st to end	Night	7% 4 2 5% 3 3 3 4 5% 8% 56%	2090
October	1st to 10th	Night	5% 4 2 3 3 3 3 3 5% 8% 61%	2090
	11th to 20th	Night	10% 5% 2 4 2 3 3 2 4 6% 60%	2274
	21st to end	Night	10% 5% 1 3 2 3 4 2 4 4 61%	2508
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 7% 52%	2311
	11th to 20th	Night	13% 6% 2 6% 4 3 3 4 5% 5% 50%	2280
	21st to end	Night	16% 7% 3 6% 4 4 4 3 4 5% 44%	2434
December	1st to 10th	Night	17% 8% 2 5% 4 4 4 3 4 6% 43%	2470
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 5% 5% 40%	3036
	21st to end	Night	16% 8% 2 6% 3 3 4 2 4 6% 46%	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	47% 3 5% 14% 20% 11%	2848
	11th to 20th	Day	35% 3 5% 22% 23% 12%	2850
	21st to end	Day	48% 3 3 14% 23% 9%	3228
February	1st to 10th	Day	49% 3 5% 13% 17% 13%	1900
	11th to 20th	Day	47% 3 7% 15% 17% 11%	1859
	21st to end	Day	45% 3 9% 16% 16% 11%	1646
March	1st to 10th	Day	49% 5% 10% 18% 13% 6%	3436
	11th to 20th	Day	57% 6% 6% 9% 15% 7%	3480
	21st to end	Day	41% 4 8% 19% 19% 9%	3814
April	1st to 10th	Day	44% 4 8% 17% 15% 11%	2090
	11th to 20th	Day	47% 3 7% 15% 17% 10%	2098
	21st to end	Day	42% 4 6% 22% 18% 8%	2280
May	1st to 10th	Day	49% 5% 12% 19% 11% 5%	4115
	11th to 20th	Day	43% 5% 8% 19% 19% 6%	4086
	21st to end	Day	35% 8% 12% 35% 10% 1	4489
June	1st to 10th	Day	31% 7% 15% 28% 18% 1	2470
	11th to 20th	Day	35% 6% 11% 27% 18% 4	2470
	21st to end	Day	45% 7% 13% 25% 9% 1	2470
July	1st to 10th	Day	37% 5% 28% 26% 5%	4056
	11th to 20th	Day	47% 6% 18% 21% 8%	4051
	21st to end	Day	48% 6% 19% 23% 3 1	4325
August	1st to 10th	Day	18% 4 13% 48% 16%	4090
	11th to 20th	Day	28% 6% 30% 29% 6% 1	3966
	21st to end	Day	38% 7% 18% 24% 11% 1	3827
September	1st to 10th	Day	42% 9% 14% 17% 14% 5%	2090
	11th to 20th	Day	44% 7% 11% 20% 15% 4	2090
	21st to end	Day	35% 6% 11% 19% 17% 12%	2090
October	1st to 10th	Day	20% 4 6% 14% 19% 37%	3453
	11th to 20th	Day	26% 3 5% 21% 27% 18%	3140
	21st to end	Day	32% 3 4 20% 28% 14%	3428
November	1st to 10th	Day	40% 5% 9% 18% 19% 10%	1928
	11th to 20th	Day	38% 4 9% 17% 19% 13%	1900
	21st to end	Day	46% 4 7% 15% 15% 12%	1743
December	1st to 10th	Day	47% 3 7% 18% 19% 6%	1710
	11th to 20th	Day	44% 4 6% 12% 22% 12%	2159
	21st to end	Day	42% 2 5% 19% 20% 13%	3135
January	1st to 10th	Night	45% 3 7% 13% 17% 17%	4108
	11th to 20th	Night	35% 3 4 18% 26% 15%	4120
	21st to end	Night	48% 3 5% 13% 22% 10%	4468
February	1st to 10th	Night	47% 3 8% 12% 16% 14%	2280
	11th to 20th	Night	44% 3 7% 16% 17% 14%	2218
	21st to end	Night	49% 3 6% 15% 17% 11%	1806
March	1st to 10th	Night	53% 5% 8% 14% 13% 5%	3419
	11th to 20th	Night	58% 5% 9% 13% 10% 6%	3480
	21st to end	Night	38% 4 11% 15% 20% 11%	3817
April	1st to 10th	Night	43% 4 12% 17% 14% 10%	2090
	11th to 20th	Night	43% 4 10% 16% 14% 14%	2029
	21st to end	Night	45% 4 8% 19% 15% 9%	1899
May	1st to 10th	Night	55% 6% 9% 17% 8% 5%	2850
	11th to 20th	Night	39% 6% 11% 20% 18% 6%	2824
	21st to end	Night	41% 8% 17% 28% 7%	3099
June	1st to 10th	Night	22% 6% 20% 32% 19% 1	1710
	11th to 20th	Night	27% 5% 15% 32% 17% 3	1710
	21st to end	Night	31% 8% 22% 28% 11% 1	1710
July	1st to 10th	Night	27% 6% 30% 34% 4	2805
	11th to 20th	Night	34% 7% 25% 25% 9%	2796
	21st to end	Night	38% 8% 25% 24% 4	2999
August	1st to 10th	Night	9% 4 20% 54% 14%	2826
	11th to 20th	Night	18% 6% 37% 34% 5%	2919
	21st to end	Night	22% 10% 27% 30% 10% 1	3827
September	1st to 10th	Night	31% 8% 22% 21% 13% 4	2090
	11th to 20th	Night	32% 8% 15% 26% 15% 4	2090
	21st to end	Night	27% 5% 15% 24% 17% 11%	2090
October	1st to 10th	Night	17% 2 8% 18% 18% 37%	3480
	11th to 20th	Night	17% 2 8% 24% 31% 18%	3773
	21st to end	Night	21% 2 5% 26% 31% 15%	4120
November	1st to 10th	Night	32% 5% 12% 20% 20% 11%	2311
	11th to 20th	Night	38% 3 10% 18% 18% 12%	2280
	21st to end	Night	42% 4 8% 18% 18% 11%	2434
December	1st to 10th	Night	42% 3 8% 22% 16% 9%	2470
	11th to 20th	Night	45% 4 7% 13% 17% 15%	3037
	21st to end	Night	41% 4 5% 15% 24% 10%	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	23% 5% 40% 23% 9% 1	270
	11th to 20th	Day	26% 10% 31% 24% 7% 1	270
	21st to end	Day	23% 14% 30% 21% 11%	297
February	1st to 10th	Day	27% 12% 30% 21% 9% 1	270
	11th to 20th	Day	24% 16% 31% 20% 7% 1	270
	21st to end	Day	25% 10% 43% 17% 5%	225
March	1st to 10th	Day	21% 13% 35% 24% 7%	270
	11th to 20th	Day	20% 12% 40% 19% 8%	270
	21st to end	Day	19% 15% 34% 24% 7% 1	297
April	1st to 10th	Day	19% 14% 36% 23% 7%	270
	11th to 20th	Day	22% 11% 33% 23% 9% 1	296
	21st to end	Day	17% 8% 35% 27% 11% 1	460
May	1st to 10th	Day	15% 9% 40% 25% 9% 1	648
	11th to 20th	Day	16% 3 31% 32% 15% 3	675
	21st to end	Day	11% 6% 28% 35% 18% 3	734
June	1st to 10th	Day	6% 3 15% 34% 32% 10%	680
	11th to 20th	Day	9% 1 11% 41% 30% 8%	680
	21st to end	Day	8% 3 9% 32% 31% 17%	680
July	1st to 10th	Day	8% 3 6% 30% 33% 21%	671
	11th to 20th	Day	4 1 4 30% 36% 24%	668
	21st to end	Day	6% 2 3 29% 38% 22%	739
August	1st to 10th	Day	4 1 3 28% 39% 26%	676
	11th to 20th	Day	4 1 2 28% 39% 26%	603
	21st to end	Day	9% 2 2 19% 31% 37%	330
September	1st to 10th	Day	9% 4 4 23% 32% 28%	300
	11th to 20th	Day	7% 2 5% 20% 35% 31%	300
	21st to end	Day	9% 2 4 27% 39% 19%	300
October	1st to 10th	Day	6% 1 10% 31% 39% 14%	300
	11th to 20th	Day	11% 2 11% 35% 29% 13%	300
	21st to end	Day	16% 7% 23% 28% 18% 8%	330
November	1st to 10th	Day	17% 3 17% 34% 22% 7%	291
	11th to 20th	Day	21% 4 20% 32% 19% 4	270
	21st to end	Day	23% 12% 29% 22% 11% 3	270
December	1st to 10th	Day	17% 6% 28% 31% 12% 5%	270
	11th to 20th	Day	22% 10% 31% 29% 8% 1	270
	21st to end	Day	22% 6% 37% 23% 10% 2	297
January	1st to 10th	Night	22% 7% 30% 31% 10%	950
	11th to 20th	Night	23% 9% 27% 33% 8%	950
	21st to end	Night	24% 8% 28% 35% 7%	1007
February	1st to 10th	Night	29% 10% 26% 31% 5%	760
	11th to 20th	Night	24% 9% 30% 31% 6%	744
	21st to end	Night	28% 10% 34% 25% 3	550
March	1st to 10th	Night	24% 8% 33% 31% 5%	570
	11th to 20th	Night	22% 7% 33% 31% 6%	570
	21st to end	Night	19% 6% 35% 33% 7%	627
April	1st to 10th	Night	18% 7% 38% 31% 6%	570
	11th to 20th	Night	23% 7% 32% 29% 9%	563
	21st to end	Night	20% 6% 35% 29% 8% 1	570
May	1st to 10th	Night	18% 6% 38% 31% 7%	570
	11th to 20th	Night	13% 2 29% 41% 15% 1	561
	21st to end	Night	11% 2 28% 43% 16% 1	617
June	1st to 10th	Night	7% 1 13% 49% 30% 1	570
	11th to 20th	Night	6% 1 16% 51% 24% 1	570
	21st to end	Night	7% 1 10% 39% 37% 5%	570
July	1st to 10th	Night	6% 2 5% 40% 41% 6%	555
	11th to 20th	Night	5% 1 3 32% 53% 6%	552
	21st to end	Night	4 1 4 38% 47% 6%	624
August	1st to 10th	Night	1 1 3 41% 47% 7%	567
	11th to 20th	Night	2 1 3 37% 50% 7%	564
	21st to end	Night	5% 1 3 27% 52% 11%	627
September	1st to 10th	Night	7% 1 3 31% 48% 10%	570
	11th to 20th	Night	9% 2 2 33% 47% 7%	570
	21st to end	Night	9% 1 8% 32% 44% 7%	570
October	1st to 10th	Night	6% 1 8% 39% 42% 4	570
	11th to 20th	Night	11% 2 20% 42% 23% 2	754
	21st to end	Night	12% 5% 22% 38% 21% 2	836
November	1st to 10th	Night	13% 3 19% 44% 19% 2	771
	11th to 20th	Night	17% 4 23% 38% 16% 1	760
	21st to end	Night	19% 6% 25% 35% 13% 1	914
December	1st to 10th	Night	21% 6% 25% 32% 15% 1	950
	11th to 20th	Night	22% 9% 28% 32% 9%	950
	21st to end	Night	22% 7% 28% 32% 10% 1	1045

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