Header1200x385

facebook_page_plugin
× Welcome to the CPL Meteorology question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.

Linear Interpolation

  • Andrew
  • Topic Author

Andrew created the topic: Linear Interpolation

Im just trying to interpret this old chestnut.

Would it be fair to say that you would use HH for the time of issue and continue utilising this for the next 1.5 hours, then use the values for HH+3 for the next 3 hours etc? Would this be interpolation or am i missing something?

A crude drawing attached!

Thanks

Andrew
#1
Attachments:

Please Log in or Create an account to join the conversation.

  • Richard

Richard replied the topic: Re: Linear Interpolation

Hi Andrew,

Linear interpretation means to spread the difference between the two values evenly over the three hours between the spot times in the TAF. Having said that you of course simply round to the nearest whole value (oC or hPa)

For example, if you have 3 degrees of temperature difference between the values for HH and HH+3, you can assume the temperature will change by about 1 degree per hour.

If there's only 2 degrees of difference, you can assume that up until half way between HH and HH+3 the HH value will apply and after that the HH+3 value will be applicable.

The regs recently changed with respect to this. It used to be the value given applied to +/- 90 minutes either side of the time in exactly the way you described. Now though the system has gone to the more logical approach of simply spreading the expected change over the whole time period.

Here's a more concrete example from a fictitious TAF for 1820/1908:

TAF XXXXX ....... T 18 18 16 13 Q 1012 1011 1008 1004

You will NOT work out decimal values in the cockpit and you probably won't need to in the exam either. In practice you can eyeball the values quite easily. In my opinion it is easier and more realistic than the previous system.

This table gives you some examples of linear interpolation. Figures in bold are taken directly from the TAF.
___Time(code)___ Temperature QNH
2000 (HH)
18oC1012
2145
18oC1011 (QNH drops by 1hPa over 3 hours and 2145 is more than half way to the next spot value)
2300 (HH+3)
18oC1011
0030
17oC (Drops 2oC over 3 hours i.e. 1oC per 1.5 hours. So by 0030, the temperature will have dropped 1 degree)1010 (QNH dropping by 1hPa/hr between 2300 and 0200)
0200 (HH+6)
16oC1008
0300
15oC (temperature dropping about 1o/hr between 0200 and 0500)1007 (QNH dropping by 1.25 hPa/hr between 0200 and 0500. Answer rounded 1007)
0415
14oC (temperature dropping about 1o/hr. Forecast temperature will change to 13oC from 0430)1004 (QNH dropping by 1.25 hPa/hr between 0200 and 0500. QNH will be closer to 1004)
0500 (HH+9)
13oC1004
Now my head hurts :blink:

Cheers,

Rich
#2

Please Log in or Create an account to join the conversation.

  • Andrew
  • Topic Author

Andrew replied the topic: Re: Linear Interpolation

Thank Rich,

I was getting little confused on that one, and have my exam tomorrow so thought i better get some clarification.

I believe a question in the Exam Prep modules still uses the old method of 90min either side as you explained.

Thanks again for your help!!!

Andrew
#3

Please Log in or Create an account to join the conversation.

  • Andrew
  • Topic Author

Andrew replied the topic: Re: Linear Interpolation

Thank Rich,

I was getting little confused on that one, and have my exam tomorrow so thought i better get some clarification.

I believe a question in the Exam Prep modules still uses the old method of 90min either side as you explained.

Thanks again for your help on both questions!!!! :)

Andrew
#4

Please Log in or Create an account to join the conversation.

Time to create page: 0.143 seconds