Spencer
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spencer clear sky chart from 2005-11-06 to 2024-10-24.

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 7% 5% 4 4 4 6% 7% 34%	101013
	Night	16% 10% 3 7% 5% 4 4 3 5% 6% 35%	97638
Transparency	Day	49% 5% 10% 22% 13% 2	100825
	Night	49% 5% 9% 21% 14% 2	97496
Seeing	Day	22% 13% 25% 22% 13% 5%	14964
	Night	21% 11% 24% 28% 14% 2	25086

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	18% 21% 8% 10% 4 4 3 3 4 5% 20%	2836
	11th to 20th	Day	18% 19% 9% 12% 8% 4 3 2 3 3 20%	2840
	21st to end	Day	23% 22% 5% 10% 5% 3 3 3 4 5% 19%	3124
February	1st to 10th	Day	20% 15% 4 8% 6% 3 3 3 4 5% 28%	1710
	11th to 20th	Day	21% 15% 4 10% 4 4 3 3 4 5% 26%	1760
	21st to end	Day	23% 15% 4 7% 4 3 3 3 4 5% 29%	1570
March	1st to 10th	Day	20% 15% 4 8% 4 4 3 3 4 5% 28%	3418
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 5% 8% 39%	3480
	21st to end	Day	20% 10% 3 7% 5% 4 4 4 6% 7% 31%	3815
April	1st to 10th	Day	15% 11% 4 7% 5% 4 5% 4 5% 6% 34%	2109
	11th to 20th	Day	17% 9% 2 6% 5% 4 3 3 5% 8% 40%	2341
	21st to end	Day	20% 9% 3 6% 4 4 4 3 5% 7% 36%	2470
May	1st to 10th	Day	21% 11% 3 8% 5% 5% 5% 4 6% 7% 26%	4082
	11th to 20th	Day	19% 8% 3 8% 4 4 4 4 6% 8% 32%	4081
	21st to end	Day	10% 6% 2 6% 5% 6% 6% 4 7% 10% 37%	4476
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 7% 8% 39%	2470
	11th to 20th	Day	10% 5% 2 7% 6% 6% 6% 5% 7% 9% 37%	2470
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 10% 37%	2470
July	1st to 10th	Day	9% 5% 2 6% 5% 6% 6% 5% 6% 9% 41%	4120
	11th to 20th	Day	10% 6% 2 5% 5% 5% 5% 4 7% 9% 41%	4120
	21st to end	Day	18% 8% 3 8% 5% 5% 6% 4 6% 7% 30%	4466
August	1st to 10th	Day	10% 7% 3 8% 5% 6% 7% 5% 8% 10% 32%	4104
	11th to 20th	Day	11% 6% 3 7% 5% 6% 6% 5% 7% 9% 35%	4044
	21st to end	Day	6% 4 1 5% 4 5% 5% 4 6% 8% 50%	4381
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 6% 7% 46%	2153
	11th to 20th	Day	12% 4 2 7% 5% 5% 4 3 6% 8% 45%	2090
	21st to end	Day	20% 8% 3 6% 4 4 4 4 4 6% 39%	2090
October	1st to 10th	Day	15% 6% 3 5% 3 3 3 4 6% 8% 45%	1970
	11th to 20th	Day	16% 7% 2 6% 4 4 4 2 5% 6% 43%	1900
	21st to end	Day	21% 7% 3 6% 4 4 4 2 4 6% 40%	2010
November	1st to 10th	Day	13% 7% 2 5% 3 3 3 4 4 5% 50%	1751
	11th to 20th	Day	15% 12% 3 6% 5% 4 3 3 5% 6% 37%	1710
	21st to end	Day	17% 12% 3 7% 5% 5% 3 3 5% 6% 35%	1710
December	1st to 10th	Day	25% 14% 5% 9% 3 4 4 3 4 5% 24%	1595
	11th to 20th	Day	21% 15% 5% 10% 4 3 3 3 3 5% 28%	2153
	21st to end	Day	20% 19% 5% 8% 6% 5% 5% 4 5% 6% 17%	3124
January	1st to 10th	Night	17% 22% 6% 10% 6% 4 4 3 4 5% 20%	4109
	11th to 20th	Night	20% 16% 6% 12% 5% 5% 3 3 4 4 21%	4120
	21st to end	Night	26% 16% 5% 10% 6% 3 3 2 3 3 22%	4532
February	1st to 10th	Night	20% 13% 4 8% 5% 4 4 4 4 5% 28%	2470
	11th to 20th	Night	22% 12% 4 9% 5% 5% 4 3 4 5% 27%	2305
	21st to end	Night	22% 15% 3 8% 4 4 3 3 5% 4 27%	1884
March	1st to 10th	Night	21% 13% 3 6% 4 4 4 3 4 6% 31%	3433
	11th to 20th	Night	15% 10% 3 7% 4 4 4 3 5% 7% 37%	3480
	21st to end	Night	16% 8% 4 6% 5% 4 4 3 5% 6% 40%	3817
April	1st to 10th	Night	16% 11% 3 6% 6% 4 4 3 5% 7% 36%	2071
	11th to 20th	Night	14% 7% 3 6% 5% 4 4 4 5% 8% 40%	1803
	21st to end	Night	16% 7% 3 7% 5% 4 4 4 5% 6% 39%	1710
May	1st to 10th	Night	25% 11% 3 9% 5% 5% 4 3 5% 6% 24%	2813
	11th to 20th	Night	19% 10% 3 7% 5% 4 4 3 5% 7% 34%	2812
	21st to end	Night	13% 7% 3 6% 4 4 5% 4 7% 8% 39%	3079
June	1st to 10th	Night	10% 6% 2 6% 4 5% 5% 4 6% 10% 43%	1710
	11th to 20th	Night	8% 5% 2 7% 5% 5% 6% 5% 7% 9% 41%	1710
	21st to end	Night	11% 5% 2 5% 5% 5% 4 4 7% 9% 44%	1710
July	1st to 10th	Night	10% 5% 2 7% 5% 5% 5% 4 8% 9% 40%	2840
	11th to 20th	Night	10% 7% 3 6% 5% 5% 4 4 6% 9% 42%	2840
	21st to end	Night	12% 9% 3 8% 6% 4 4 5% 6% 7% 36%	3087
August	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 5% 7% 9% 33%	2825
	11th to 20th	Night	12% 6% 2 8% 4 4 5% 3 6% 8% 42%	2785
	21st to end	Night	7% 3 2 5% 4 4 5% 3 7% 10% 51%	3242
September	1st to 10th	Night	11% 6% 2 5% 4 4 5% 3 6% 7% 47%	2023
	11th to 20th	Night	11% 4 1 4 4 4 4 3 5% 8% 52%	2090
	21st to end	Night	16% 6% 2 6% 4 4 4 4 4 7% 42%	2090
October	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 5% 7% 48%	2209
	11th to 20th	Night	15% 6% 2 7% 3 4 4 3 4 6% 46%	2280
	21st to end	Night	22% 5% 2 6% 4 4 3 3 4 6% 41%	2413
November	1st to 10th	Night	13% 6% 2 6% 4 3 4 4 5% 5% 49%	2324
	11th to 20th	Night	19% 10% 3 6% 4 4 4 4 5% 7% 35%	2470
	21st to end	Night	17% 11% 3 7% 4 4 4 3 4 6% 37%	2470
December	1st to 10th	Night	23% 16% 5% 8% 4 4 4 4 4 4 25%	2508
	11th to 20th	Night	19% 17% 6% 9% 5% 3 4 3 4 5% 27%	3042
	21st to end	Night	19% 18% 5% 10% 6% 5% 4 3 4 5% 22%	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	67% 4 10% 13% 6%	2837
	11th to 20th	Day	72% 3 6% 14% 5%	2840
	21st to end	Day	69% 3 6% 16% 4 1	3124
February	1st to 10th	Day	60% 3 10% 18% 9% 1	1710
	11th to 20th	Day	62% 4 8% 16% 9% 1	1769
	21st to end	Day	58% 4 8% 18% 10% 1	1570
March	1st to 10th	Day	58% 4 8% 21% 9% 1	3418
	11th to 20th	Day	43% 5% 8% 28% 16% 1	3480
	21st to end	Day	52% 5% 7% 21% 13% 1	3815
April	1st to 10th	Day	49% 5% 10% 19% 14% 3	2109
	11th to 20th	Day	43% 4 10% 21% 18% 4	2341
	21st to end	Day	48% 5% 12% 21% 12% 3	2470
May	1st to 10th	Day	56% 6% 9% 19% 9% 1	4120
	11th to 20th	Day	49% 7% 10% 19% 13% 1	4085
	21st to end	Day	39% 8% 15% 21% 13% 4	4489
June	1st to 10th	Day	41% 6% 12% 23% 16% 1	2470
	11th to 20th	Day	41% 7% 14% 22% 15% 2	2470
	21st to end	Day	39% 8% 14% 23% 14% 2	2470
July	1st to 10th	Day	39% 6% 13% 26% 13% 3	4055
	11th to 20th	Day	39% 5% 10% 30% 14% 2	4051
	21st to end	Day	52% 5% 10% 17% 15% 2	4324
August	1st to 10th	Day	45% 6% 13% 26% 9%	4091
	11th to 20th	Day	43% 6% 9% 28% 14% 1	4083
	21st to end	Day	31% 5% 13% 29% 20% 2	4381
September	1st to 10th	Day	37% 6% 9% 21% 22% 5%	2153
	11th to 20th	Day	37% 5% 9% 26% 20% 2	2090
	21st to end	Day	46% 5% 8% 20% 19% 2	2090
October	1st to 10th	Day	38% 4 12% 22% 21% 3	1970
	11th to 20th	Day	43% 4 8% 26% 19% 1	1900
	21st to end	Day	46% 4 8% 23% 17% 2	2010
November	1st to 10th	Day	37% 4 8% 29% 19% 4	1751
	11th to 20th	Day	48% 3 7% 20% 18% 3	1710
	21st to end	Day	51% 4 8% 21% 15% 1	1710
December	1st to 10th	Day	64% 4 7% 20% 5% 1	1595
	11th to 20th	Day	60% 3 9% 18% 10%	2150
	21st to end	Day	66% 6% 10% 14% 4	3124
January	1st to 10th	Night	67% 4 8% 14% 6% 1	4109
	11th to 20th	Night	67% 4 8% 15% 5%	4120
	21st to end	Night	68% 3 7% 16% 5% 1	4532
February	1st to 10th	Night	58% 4 9% 19% 10% 1	2470
	11th to 20th	Night	62% 3 7% 17% 11% 1	2307
	21st to end	Night	60% 4 9% 16% 9% 3	1884
March	1st to 10th	Night	54% 4 7% 22% 13% 1	3433
	11th to 20th	Night	46% 5% 8% 25% 15% 1	3480
	21st to end	Night	45% 4 8% 25% 16% 2	3817
April	1st to 10th	Night	49% 4 9% 19% 16% 3	2071
	11th to 20th	Night	42% 5% 11% 20% 16% 6%	1803
	21st to end	Night	46% 4 10% 20% 18% 2	1710
May	1st to 10th	Night	60% 6% 8% 15% 10% 2	2840
	11th to 20th	Night	48% 8% 9% 23% 11% 2	2813
	21st to end	Night	40% 8% 13% 22% 12% 4	3087
June	1st to 10th	Night	36% 7% 13% 25% 17% 2	1710
	11th to 20th	Night	37% 10% 13% 20% 18% 2	1710
	21st to end	Night	35% 8% 15% 24% 16% 2	1710
July	1st to 10th	Night	39% 6% 12% 23% 16% 4	2795
	11th to 20th	Night	39% 6% 11% 29% 14% 2	2786
	21st to end	Night	46% 7% 9% 19% 15% 3	2988
August	1st to 10th	Night	47% 5% 14% 23% 9% 1	2814
	11th to 20th	Night	41% 4 9% 23% 22% 1	2812
	21st to end	Night	30% 4 12% 31% 21% 2	3242
September	1st to 10th	Night	35% 6% 13% 20% 19% 7%	2023
	11th to 20th	Night	32% 5% 11% 25% 24% 3	2090
	21st to end	Night	42% 4 9% 23% 21% 1	2090
October	1st to 10th	Night	35% 6% 11% 24% 20% 4	2209
	11th to 20th	Night	41% 3 9% 23% 21% 3	2280
	21st to end	Night	46% 4 6% 23% 17% 3	2413
November	1st to 10th	Night	37% 5% 10% 21% 22% 6%	2324
	11th to 20th	Night	48% 5% 7% 19% 18% 4	2470
	21st to end	Night	51% 3 9% 21% 15% 2	2470
December	1st to 10th	Night	63% 4 7% 19% 6% 1	2508
	11th to 20th	Night	61% 3 8% 18% 9% 1	3044
	21st to end	Night	65% 4 8% 16% 7%	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	31% 25% 27% 12% 3 1	270
	11th to 20th	Day	36% 27% 30% 7% 1	270
	21st to end	Day	32% 29% 27% 10% 2	297
February	1st to 10th	Day	34% 21% 33% 7% 3 1	270
	11th to 20th	Day	44% 25% 24% 6% 1	270
	21st to end	Day	40% 24% 28% 8%	225
March	1st to 10th	Day	30% 21% 28% 16% 5%	443
	11th to 20th	Day	28% 24% 28% 14% 5% 1	460
	21st to end	Day	32% 18% 32% 14% 5%	506
April	1st to 10th	Day	29% 19% 26% 18% 8%	460
	11th to 20th	Day	28% 16% 29% 17% 7% 4	452
	21st to end	Day	30% 15% 28% 17% 7% 2	460
May	1st to 10th	Day	28% 16% 27% 18% 8% 4	463
	11th to 20th	Day	24% 11% 30% 19% 13% 4	487
	21st to end	Day	14% 10% 23% 29% 16% 8%	529
June	1st to 10th	Day	15% 9% 26% 28% 17% 6%	490
	11th to 20th	Day	13% 8% 24% 31% 17% 6%	490
	21st to end	Day	11% 10% 31% 26% 16% 6%	490
July	1st to 10th	Day	14% 7% 20% 24% 18% 16%	485
	11th to 20th	Day	8% 5% 23% 36% 19% 9%	484
	21st to end	Day	18% 5% 18% 29% 20% 10%	532
August	1st to 10th	Day	13% 7% 19% 30% 21% 11%	486
	11th to 20th	Day	13% 6% 20% 30% 19% 11%	479
	21st to end	Day	11% 5% 15% 32% 26% 11%	539
September	1st to 10th	Day	15% 5% 18% 28% 25% 10%	490
	11th to 20th	Day	11% 3 13% 32% 28% 12%	490
	21st to end	Day	24% 10% 23% 24% 15% 5%	490
October	1st to 10th	Day	16% 7% 27% 26% 20% 4	490
	11th to 20th	Day	20% 13% 27% 24% 12% 3	490
	21st to end	Day	28% 13% 26% 22% 8% 2	507
November	1st to 10th	Day	21% 13% 30% 23% 10% 2	349
	11th to 20th	Day	30% 17% 26% 20% 7% 1	270
	21st to end	Day	24% 21% 31% 19% 3 1	270
December	1st to 10th	Day	36% 18% 25% 15% 5%	214
	11th to 20th	Day	28% 21% 33% 13% 5%	270
	21st to end	Day	35% 22% 26% 12% 4	297
January	1st to 10th	Night	29% 19% 29% 19% 4	760
	11th to 20th	Night	33% 18% 26% 21% 2	760
	21st to end	Night	28% 17% 30% 21% 3	836
February	1st to 10th	Night	28% 15% 29% 27% 2	760
	11th to 20th	Night	33% 20% 29% 18% 1	743
	21st to end	Night	33% 16% 32% 18% 2	628
March	1st to 10th	Night	26% 16% 29% 25% 4	760
	11th to 20th	Night	27% 16% 29% 20% 8% 1	760
	21st to end	Night	25% 16% 31% 23% 6%	836
April	1st to 10th	Night	24% 16% 32% 21% 6%	760
	11th to 20th	Night	19% 14% 31% 25% 11%	679
	21st to end	Night	18% 11% 34% 29% 8%	570
May	1st to 10th	Night	20% 9% 31% 27% 13%	570
	11th to 20th	Night	20% 8% 26% 31% 14% 1	561
	21st to end	Night	14% 4 23% 30% 25% 4	617
June	1st to 10th	Night	11% 3 22% 38% 24% 2	570
	11th to 20th	Night	12% 4 19% 36% 27% 3	570
	21st to end	Night	14% 4 19% 40% 21% 2	570
July	1st to 10th	Night	12% 4 15% 33% 30% 5%	555
	11th to 20th	Night	9% 3 15% 36% 33% 4	552
	21st to end	Night	12% 3 14% 36% 30% 5%	624
August	1st to 10th	Night	11% 4 10% 39% 33% 3	567
	11th to 20th	Night	11% 4 13% 38% 30% 5%	564
	21st to end	Night	12% 2 13% 35% 32% 6%	693
September	1st to 10th	Night	14% 3 14% 34% 30% 6%	760
	11th to 20th	Night	13% 2 15% 34% 33% 4	760
	21st to end	Night	17% 8% 21% 31% 21% 3	760
October	1st to 10th	Night	13% 5% 22% 32% 25% 3	760
	11th to 20th	Night	19% 9% 28% 28% 15% 2	760
	21st to end	Night	25% 10% 22% 30% 12% 1	805
November	1st to 10th	Night	16% 10% 26% 32% 16% 1	740
	11th to 20th	Night	25% 14% 26% 25% 9% 1	760
	21st to end	Night	25% 11% 24% 31% 8% 1	760
December	1st to 10th	Night	33% 15% 26% 22% 5%	760
	11th to 20th	Night	25% 17% 29% 24% 5%	760
	21st to end	Night	29% 15% 27% 27% 2	836

Page generated at 2024-10-21 02:19:59. Version ac07

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