90W 45N Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 90W 45N Park clear sky chart from 2005-11-06 to 2025-03-12.

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	13% 9% 3 8% 5% 4 5% 4 5% 7% 36%	108976
	Night	13% 10% 4 9% 5% 4 4 4 5% 6% 36%	110241
Transparency	Day	46% 5% 12% 24% 12% 1	112481
	Night	47% 5% 11% 23% 13% 1	114876
Seeing	Day	17% 10% 29% 25% 14% 5%	16403
	Night	17% 9% 32% 30% 11% 1	26479

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	7% 12% 6% 16% 7% 5% 4 3 4 7% 28%	3617
	11th to 20th	Day	8% 13% 5% 12% 6% 6% 7% 7% 7% 6% 22%	3620
	21st to end	Day	9% 15% 7% 15% 4 4 4 3 4 7% 28%	4092
February	1st to 10th	Day	9% 12% 5% 10% 5% 6% 6% 5% 5% 7% 30%	1800
	11th to 20th	Day	10% 12% 4 9% 6% 5% 4 4 6% 8% 33%	1807
	21st to end	Day	11% 11% 4 10% 7% 6% 6% 4 5% 7% 28%	1650
March	1st to 10th	Day	13% 10% 3 5% 3 4 3 3 4 6% 46%	3220
	11th to 20th	Day	8% 9% 4 10% 6% 5% 4 4 6% 8% 36%	3417
	21st to end	Day	20% 13% 4 11% 6% 4 3 3 4 6% 27%	4086
April	1st to 10th	Day	15% 14% 4 8% 5% 4 4 3 5% 7% 31%	2280
	11th to 20th	Day	20% 11% 3 7% 5% 4 4 3 5% 6% 31%	2254
	21st to end	Day	19% 11% 3 7% 5% 5% 4 4 5% 6% 30%	2364
May	1st to 10th	Day	17% 6% 3 6% 5% 4 5% 3 5% 7% 39%	4399
	11th to 20th	Day	16% 6% 3 6% 3 4 4 3 6% 7% 43%	4399
	21st to end	Day	17% 8% 2 7% 5% 4 5% 4 5% 6% 36%	4824
June	1st to 10th	Day	13% 5% 3 6% 5% 4 4 3 6% 8% 42%	2660
	11th to 20th	Day	15% 7% 3 6% 5% 5% 4 4 6% 8% 37%	2660
	21st to end	Day	12% 6% 3 5% 5% 5% 5% 4 7% 10% 39%	2660
July	1st to 10th	Day	14% 4 2 6% 4 5% 6% 6% 7% 9% 38%	4440
	11th to 20th	Day	6% 3 1 5% 4 4 5% 5% 8% 10% 49%	4440
	21st to end	Day	8% 6% 2 7% 4 5% 4 4 6% 8% 46%	4813
August	1st to 10th	Day	13% 6% 2 6% 4 5% 5% 4 6% 7% 42%	4408
	11th to 20th	Day	14% 5% 2 5% 3 4 4 4 6% 8% 45%	3948
	21st to end	Day	13% 7% 3 7% 5% 4 5% 4 6% 8% 39%	4180
September	1st to 10th	Day	12% 7% 3 6% 4 3 4 4 6% 7% 45%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 44%	2248
	21st to end	Day	14% 7% 3 7% 4 3 5% 3 7% 7% 39%	2077
October	1st to 10th	Day	15% 7% 2 6% 4 3 5% 4 7% 7% 40%	1900
	11th to 20th	Day	19% 11% 3 8% 5% 3 4 3 5% 7% 32%	1900
	21st to end	Day	18% 11% 4 8% 5% 3 3 3 5% 7% 33%	1905
November	1st to 10th	Day	15% 8% 3 7% 5% 4 4 4 6% 7% 38%	1762
	11th to 20th	Day	14% 13% 4 11% 6% 5% 5% 3 6% 6% 27%	1666
	21st to end	Day	14% 13% 4 8% 5% 4 5% 3 6% 6% 32%	1600
December	1st to 10th	Day	8% 11% 5% 17% 8% 7% 6% 4 5% 4 24%	2610
	11th to 20th	Day	12% 19% 7% 10% 6% 4 4 3 4 5% 27%	3008
	21st to end	Day	21% 21% 5% 13% 5% 4 4 4 4 4 16%	3982
January	1st to 10th	Night	8% 12% 5% 12% 10% 4 3 3 4 5% 34%	6348
	11th to 20th	Night	7% 13% 5% 13% 7% 6% 7% 6% 7% 5% 24%	6360
	21st to end	Night	9% 15% 8% 14% 5% 5% 4 3 4 5% 29%	6881
February	1st to 10th	Night	8% 12% 5% 10% 6% 6% 4 3 6% 7% 33%	2600
	11th to 20th	Night	9% 10% 3 9% 5% 5% 5% 4 6% 6% 38%	2479
	21st to end	Night	13% 12% 3 10% 5% 4 4 3 4 7% 33%	1980
March	1st to 10th	Night	10% 7% 3 6% 4 4 5% 3 5% 8% 45%	3853
	11th to 20th	Night	9% 8% 3 10% 7% 5% 4 4 5% 6% 38%	3600
	21st to end	Night	21% 11% 4 12% 6% 5% 4 2 3 4 28%	3544
April	1st to 10th	Night	19% 11% 3 7% 5% 4 4 3 4 7% 34%	1900
	11th to 20th	Night	22% 8% 2 4 4 4 3 4 5% 5% 38%	1875
	21st to end	Night	19% 8% 2 7% 4 4 3 3 5% 7% 37%	1813
May	1st to 10th	Night	16% 9% 3 8% 4 4 4 4 5% 7% 36%	2506
	11th to 20th	Night	14% 6% 2 5% 3 4 4 3 6% 7% 47%	2506
	21st to end	Night	15% 7% 2 7% 5% 4 4 4 6% 7% 40%	2743
June	1st to 10th	Night	13% 5% 2 6% 4 3 4 4 6% 7% 46%	1520
	11th to 20th	Night	11% 5% 2 7% 5% 5% 6% 5% 6% 8% 39%	1520
	21st to end	Night	11% 5% 2 5% 5% 4 4 4 6% 9% 46%	1520
July	1st to 10th	Night	14% 4 2 6% 4 4 4 4 6% 8% 44%	2530
	11th to 20th	Night	5% 4 1 4 3 3 5% 3 6% 10% 56%	2530
	21st to end	Night	7% 5% 2 6% 4 4 4 3 6% 7% 52%	2750
August	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 6% 7% 45%	2517
	11th to 20th	Night	12% 5% 2 5% 3 4 3 3 5% 8% 49%	2886
	21st to end	Night	13% 6% 2 6% 4 4 4 3 5% 6% 44%	3476
September	1st to 10th	Night	10% 4 2 6% 3 4 3 4 5% 6% 53%	1900
	11th to 20th	Night	11% 6% 2 6% 3 4 3 3 6% 6% 50%	1912
	21st to end	Night	12% 7% 2 5% 4 3 4 3 5% 8% 46%	2099
October	1st to 10th	Night	14% 5% 2 6% 3 4 4 4 5% 6% 47%	2280
	11th to 20th	Night	15% 10% 3 7% 4 4 4 4 5% 7% 37%	2280
	21st to end	Night	18% 11% 3 8% 5% 4 4 4 5% 6% 32%	2692
November	1st to 10th	Night	15% 10% 3 7% 4 3 4 3 5% 7% 41%	2536
	11th to 20th	Night	14% 13% 4 10% 5% 4 5% 3 4 6% 33%	2730
	21st to end	Night	13% 12% 4 9% 6% 5% 4 4 5% 7% 32%	2800
December	1st to 10th	Night	10% 10% 4 10% 6% 5% 5% 4 7% 9% 30%	4580
	11th to 20th	Night	12% 17% 7% 14% 6% 4 4 3 4 5% 25%	5199
	21st to end	Night	23% 16% 5% 11% 5% 4 3 3 4 5% 21%	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	57% 3 9% 20% 11%	3618
	11th to 20th	Day	58% 5% 8% 16% 12% 1	3620
	21st to end	Day	58% 3 11% 19% 9%	4092
February	1st to 10th	Day	52% 5% 11% 21% 11% 1	1800
	11th to 20th	Day	48% 6% 11% 24% 11% 1	1816
	21st to end	Day	55% 4 11% 19% 9% 2	1650
March	1st to 10th	Day	40% 3 13% 22% 19% 3	3220
	11th to 20th	Day	43% 7% 12% 24% 13% 1	3417
	21st to end	Day	59% 4 8% 20% 7% 1	4086
April	1st to 10th	Day	53% 4 11% 19% 12%	2280
	11th to 20th	Day	54% 5% 8% 20% 11% 2	2254
	21st to end	Day	54% 5% 11% 18% 12% 1	2364
May	1st to 10th	Day	43% 6% 10% 27% 13%	4440
	11th to 20th	Day	40% 5% 12% 31% 12% 1	4403
	21st to end	Day	47% 6% 10% 22% 15% 1	4837
June	1st to 10th	Day	38% 6% 14% 29% 12% 1	2660
	11th to 20th	Day	44% 6% 12% 24% 13% 1	2660
	21st to end	Day	39% 5% 18% 26% 11% 1	2660
July	1st to 10th	Day	40% 7% 16% 27% 9% 1	4371
	11th to 20th	Day	27% 7% 14% 35% 15% 2	4365
	21st to end	Day	36% 5% 13% 28% 17% 1	4662
August	1st to 10th	Day	40% 5% 15% 26% 13% 1	4393
	11th to 20th	Day	36% 5% 13% 34% 12%	3987
	21st to end	Day	43% 6% 12% 28% 11% 1	4179
September	1st to 10th	Day	38% 5% 15% 28% 14% 1	2280
	11th to 20th	Day	36% 5% 19% 31% 8% 1	2248
	21st to end	Day	43% 5% 13% 29% 10%	2077
October	1st to 10th	Day	29% 3 10% 36% 21% 1	3160
	11th to 20th	Day	45% 4 11% 24% 15% 1	3140
	21st to end	Day	46% 4 15% 27% 8%	3117
November	1st to 10th	Day	45% 5% 12% 24% 13% 2	1762
	11th to 20th	Day	56% 4 11% 20% 8% 1	1666
	21st to end	Day	52% 4 14% 23% 5% 1	1600
December	1st to 10th	Day	62% 4 11% 18% 5%	2610
	11th to 20th	Day	60% 4 8% 15% 12% 1	3005
	21st to end	Day	72% 3 6% 12% 7%	3982
January	1st to 10th	Night	54% 3 9% 21% 13% 1	6348
	11th to 20th	Night	59% 5% 9% 14% 14% 1	6360
	21st to end	Night	58% 4 10% 18% 10%	6881
February	1st to 10th	Night	50% 4 10% 23% 11% 2	2600
	11th to 20th	Night	45% 5% 11% 24% 12% 2	2480
	21st to end	Night	51% 4 13% 19% 10% 3	1980
March	1st to 10th	Night	38% 4 14% 27% 14% 3	3853
	11th to 20th	Night	44% 6% 10% 25% 14% 1	3600
	21st to end	Night	61% 3 7% 19% 9% 1	3544
April	1st to 10th	Night	51% 4 8% 19% 17% 1	1900
	11th to 20th	Night	46% 5% 9% 22% 14% 3	1875
	21st to end	Night	46% 5% 11% 22% 14% 2	1813
May	1st to 10th	Night	45% 6% 15% 17% 16% 1	2530
	11th to 20th	Night	35% 6% 11% 32% 15% 2	2507
	21st to end	Night	42% 7% 15% 19% 16% 1	2751
June	1st to 10th	Night	36% 6% 14% 30% 14% 1	1520
	11th to 20th	Night	41% 6% 13% 24% 14% 2	1520
	21st to end	Night	34% 6% 14% 31% 12% 3	1520
July	1st to 10th	Night	37% 6% 18% 23% 13% 3	2490
	11th to 20th	Night	24% 6% 15% 37% 17% 2	2482
	21st to end	Night	30% 6% 13% 29% 20% 2	2662
August	1st to 10th	Night	37% 6% 14% 26% 15% 2	2508
	11th to 20th	Night	33% 6% 15% 31% 15% 1	2913
	21st to end	Night	39% 5% 13% 25% 16% 1	3476
September	1st to 10th	Night	31% 5% 12% 33% 18% 1	1900
	11th to 20th	Night	34% 4 14% 35% 13%	1912
	21st to end	Night	36% 6% 12% 31% 14% 1	2099
October	1st to 10th	Night	28% 3 8% 36% 23% 3	3800
	11th to 20th	Night	36% 4 10% 31% 18%	3779
	21st to end	Night	51% 4 9% 23% 13%	4430
November	1st to 10th	Night	45% 3 12% 24% 15% 1	2536
	11th to 20th	Night	52% 4 13% 18% 11% 1	2730
	21st to end	Night	52% 4 14% 23% 6% 1	2800
December	1st to 10th	Night	49% 5% 20% 21% 5% 1	4580
	11th to 20th	Night	63% 3 8% 16% 10%	5201
	21st to end	Night	66% 3 6% 15% 9%	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	14% 13% 31% 29% 10% 2	300
	11th to 20th	Day	22% 11% 36% 23% 7% 1	300
	21st to end	Day	18% 11% 34% 28% 7% 2	330
February	1st to 10th	Day	15% 14% 44% 17% 7% 3	300
	11th to 20th	Day	17% 18% 37% 18% 9%	300
	21st to end	Day	16% 16% 41% 24% 3	249
March	1st to 10th	Day	17% 15% 40% 21% 7%	300
	11th to 20th	Day	18% 12% 40% 18% 9% 3	273
	21st to end	Day	23% 13% 31% 22% 9% 2	453
April	1st to 10th	Day	23% 16% 29% 25% 6% 1	460
	11th to 20th	Day	30% 11% 26% 22% 9% 2	452
	21st to end	Day	24% 13% 30% 21% 10% 3	463
May	1st to 10th	Day	20% 14% 30% 25% 9% 2	653
	11th to 20th	Day	19% 12% 28% 23% 14% 4	675
	21st to end	Day	19% 10% 25% 27% 13% 6%	734
June	1st to 10th	Day	11% 9% 28% 31% 15% 6%	680
	11th to 20th	Day	16% 10% 30% 27% 13% 4	680
	21st to end	Day	13% 9% 28% 29% 14% 7%	680
July	1st to 10th	Day	9% 4 20% 30% 27% 11%	671
	11th to 20th	Day	9% 6% 26% 30% 19% 10%	668
	21st to end	Day	6% 4 27% 30% 24% 9%	739
August	1st to 10th	Day	10% 4 26% 28% 23% 9%	676
	11th to 20th	Day	14% 5% 18% 27% 26% 9%	611
	21st to end	Day	14% 8% 22% 28% 22% 6%	539
September	1st to 10th	Day	17% 6% 25% 24% 22% 7%	490
	11th to 20th	Day	12% 7% 22% 34% 19% 5%	490
	21st to end	Day	18% 8% 26% 28% 15% 5%	477
October	1st to 10th	Day	17% 14% 25% 25% 14% 6%	300
	11th to 20th	Day	21% 19% 30% 23% 6% 1	300
	21st to end	Day	23% 15% 40% 16% 5% 1	330
November	1st to 10th	Day	21% 12% 32% 26% 7% 2	300
	11th to 20th	Day	18% 19% 35% 23% 4 1	300
	21st to end	Day	20% 13% 41% 20% 7%	300
December	1st to 10th	Day	15% 11% 45% 21% 8%	300
	11th to 20th	Day	19% 16% 41% 17% 6% 1	300
	21st to end	Day	29% 16% 32% 18% 4 2	330
January	1st to 10th	Night	15% 11% 37% 28% 8% 1	1000
	11th to 20th	Night	17% 12% 38% 26% 6% 2	1000
	21st to end	Night	17% 9% 37% 30% 6% 1	1100
February	1st to 10th	Night	14% 13% 37% 29% 7% 1	1000
	11th to 20th	Night	14% 12% 41% 26% 7%	925
	21st to end	Night	19% 11% 38% 26% 6%	660
March	1st to 10th	Night	17% 11% 33% 31% 7% 1	800
	11th to 20th	Night	15% 14% 34% 28% 8% 1	639
	21st to end	Night	21% 9% 31% 28% 9% 2	627
April	1st to 10th	Night	23% 10% 34% 25% 7% 1	570
	11th to 20th	Night	26% 8% 32% 27% 7%	563
	21st to end	Night	22% 9% 31% 29% 8% 1	570
May	1st to 10th	Night	21% 10% 32% 29% 6% 1	570
	11th to 20th	Night	20% 5% 31% 34% 9% 1	561
	21st to end	Night	16% 6% 26% 37% 15% 1	617
June	1st to 10th	Night	16% 7% 26% 36% 14% 1	570
	11th to 20th	Night	13% 6% 30% 40% 11% 1	570
	21st to end	Night	13% 5% 22% 42% 17% 1	570
July	1st to 10th	Night	8% 3 21% 39% 27% 3	555
	11th to 20th	Night	9% 5% 23% 36% 25% 3	552
	21st to end	Night	6% 2 19% 46% 24% 3	624
August	1st to 10th	Night	10% 2 20% 43% 24% 2	567
	11th to 20th	Night	14% 2 20% 32% 29% 3	564
	21st to end	Night	15% 4 20% 36% 22% 3	627
September	1st to 10th	Night	12% 3 24% 37% 19% 5%	570
	11th to 20th	Night	12% 4 24% 37% 20% 3	582
	21st to end	Night	14% 8% 29% 31% 16% 2	760
October	1st to 10th	Night	15% 8% 29% 29% 17% 2	760
	11th to 20th	Night	21% 11% 32% 28% 8%	760
	21st to end	Night	22% 10% 32% 27% 9% 1	836
November	1st to 10th	Night	21% 9% 35% 27% 8% 1	780
	11th to 20th	Night	20% 11% 39% 22% 7%	930
	21st to end	Night	20% 13% 36% 26% 5% 1	1000
December	1st to 10th	Night	18% 11% 39% 24% 7% 1	1000
	11th to 20th	Night	21% 12% 34% 26% 7% 1	1000
	21st to end	Night	26% 12% 32% 25% 5%	1100

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