<p>In the Zambretti's documentation on Winter the Z number falling is lower by 1 unit, and the Summer Rising improves the prospect to 1 unit, therefore we have to adjust:</p>

<p>Winter Z=Z+1
Summer Z=Z-1</p>

<p>To compensate the derivation.</p>

CC BY-SA 4.0
2020-05-27T09:24:38.747Z
Camilo's answer (and the article cited in the answer) states the opposite.
Jean-Marie Prival
CC BY-SA 4.0
2022-02-09T21:55:57.967Z
I checked the sources and verified that you have to substract 1 in winter. Although, I corrected my answer as this needs to be done only if the pressure is falling. Do you have a source to support your claim?
Camilo Rada
CC BY-SA 4.0
2020-05-26T14:57:31.723Z
2020-05-26T14:57:31.723Z
lcfelippe
<p>Such simple algorithms are empirical and for the same reason very local, so depending on your location they can have very bad performance. In the other hand, some people argue that properly calibrated they can be <a href="https://web.archive.org/web/20110610213848/http://www.meteormetrics.com/zambretti.htm" rel="nofollow noreferrer">over 90%</a> accurate.</p>
<p>Most of them are based in the Zambretti algorithm or some variant of it. This algorithm was originally implemented in a forecaster device produced by the firm Negretti and Zambra in the early XX century.</p>
<p><a href="https://i.sstatic.net/jfAuY.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/jfAuY.png" alt="enter image description here" /></a></p>
<p>It considers the absolute value of the pressure, the trend of the pressure, the season and the wind direction (although wind direction and season have a small impact in the output). The algorithm is well described and translated to formulas in <a href="https://web.archive.org/web/20110610213848/http://www.meteormetrics.com/zambretti.htm" rel="nofollow noreferrer">this article</a>, but in summary it works calculating a tabulated forecast number <span class="math-container">$Z$</span> as follows:</p>
<ol>
<li>From your measured pressure <span class="math-container">$P$</span>, temperature in Celsius <span class="math-container">$T$</span> and the altitude in meters <span class="math-container">$h$</span> compute the atmospheric pressure reduced to sea level <span class="math-container">$P_0$</span>. There are many formulas for this, a common one is:</li>
</ol>
<p><span class="math-container">$P_0=P\left(1-\frac{0.0065 h}{T + 0.0065 h+ 273.15}\right)^{-5.257}$</span></p>
<ol start="2">
<li>Compute the pressure trend and</li>
</ol>
<ul>
<li>If the pressure is <strong>falling</strong> compute the forecast number as <span class="math-container">$Z = 130-\frac{P_0}{81}$</span></li>
<li>If the pressure is <strong>steady</strong> compute the forecast number as <span class="math-container">$Z = 147 -\frac{5 P_0}{376}$</span></li>
<li>If the pressure is <strong>rising</strong> compute the forecast number as <span class="math-container">$Z = 179-\frac{2 P_0}{129}$</span></li>
</ul>
<ol start="3">
<li>Adjust <span class="math-container">$Z$</span> for wind direction:</li>
</ol>
<ul>
<li>For Northerly winds adjust <span class="math-container">$Z=Z+1$</span></li>
<li>For Southerly winds adjust <span class="math-container">$Z=Z-2$</span></li>
</ul>
<ol start="4">
<li>Adjust <span class="math-container">$Z$</span> for the season:</li>
</ol>
<ul>
<li>If Winter and pressure is falling, adjust: <span class="math-container">$Z=Z-1$</span></li>
<li>If Summer and pressure is rising, adjust: <span class="math-container">$Z=Z+1$</span></li>
</ul>
<ol start="5">
<li>Look up for the forecast from the following table:</li>
</ol>
<p><a href="https://i.sstatic.net/WCUeP.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WCUeP.jpg" alt="enter image description here" /></a></p>
<p>This was made for the UK, and many people have developed modified version to work in other areas. You will find many implementations online with downloadable code, as <a href="http://www.beteljuice.co.uk/zambretti/forecast.html" rel="nofollow noreferrer">this one in javascript</a>, or <a href="https://github.com/fandonov/weatherstation" rel="nofollow noreferrer">this one in C for Arduino UNO</a>, an explanation of this Arduino UNO implementation can be found <a href="https://cdn.hackaday.io/files/274581174763072/english_description.pdf" rel="nofollow noreferrer">here</a>.</p>

CC BY-SA 4.0
2019-03-02T13:49:09.670Z
Sir you are a life salver! Thank you so much.
Timur Mengazi
CC BY-SA 4.0
2022-07-20T23:59:39.923Z
P in mb or in Hg both seem to basically give Z values way off the chart, over 100, unless I'm missing something?
JeopardyTempest
CC BY-SA 4.0
2022-08-02T02:43:50.773Z
@camilo-rada for =130−0/81 if P0 = 100-hpa, Z = 117, so what the unit of pressure apply to this, I see the original page and it is the same.
DzungPV
CC BY-SA 4.0
2019-03-02T02:48:13.447Z
2022-02-09T21:53:47.200Z
2022-02-09T21:53:47.200Z
Camilo Rada
<p>I have a project for my school to make, and I'd like to have some materials of research/graphs of dependencies about how the weather is predicted (i.e. cloudy, sunny, rainy). I've been searching the internet for the past few days but couldn't find something rather helping.</p>

<p>I can use an Arduino UNO and a bunch of sensors (i.e. atmospheric pressure, temperature, humidity, etc.), but my main problem is how to accurately interpret the parameters measured.</p>

<weather-forecasting>
Weather forecast based on pressure, temperature and humidity only (for implementation in Arduino)
Timur Mengazi
# Weather forecast based on pressure, temperature and humidity only (for implementation in Arduino)

(Asked by: username_0 on 2019-03-01)

I have a project for my school to make, and I'd like to have some materials of research/graphs of dependencies about how the weather is predicted (i.e. cloudy, sunny, rainy). I've been searching the internet for the past few days but couldn't find something rather helping.

I can use an Arduino UNO and a bunch of sensors (i.e. atmospheric pressure, temperature, humidity, etc.), but my main problem is how to accurately interpret the parameters measured.

**Tags:** weather-forecasting


---

## 2 answers

### Answer by username_3 on 2019-03-02. Score: 9 | Accepted answer

Such simple algorithms are empirical and for the same reason very local, so depending on your location they can have very bad performance. In the other hand, some people argue that properly calibrated they can be [over 90%](https://web.archive.org/web/20110610213848/http://www.meteormetrics.com/zambretti.htm) accurate.

Most of them are based in the Zambretti algorithm or some variant of it. This algorithm was originally implemented in a forecaster device produced by the firm Negretti and Zambra in the early XX century.

[![enter image description here](https://i.sstatic.net/jfAuY.png)](https://i.sstatic.net/jfAuY.png)

It considers the absolute value of the pressure, the trend of the pressure, the season and the wind direction (although wind direction and season have a small impact in the output). The algorithm is well described and translated to formulas in [this article](https://web.archive.org/web/20110610213848/http://www.meteormetrics.com/zambretti.htm), but in summary it works calculating a tabulated forecast number $Z$ as follows:

1. From your measured pressure $P$, temperature in Celsius $T$ and the altitude in meters $h$ compute the atmospheric pressure reduced to sea level $P\_0$. There are many formulas for this, a common one is:

$P\_0=P\left(1-\frac{0.0065 h}{T + 0.0065 h+ 273.15}\right)^{-5.257}$

2. Compute the pressure trend and

* If the pressure is **falling** compute the forecast number as $Z = 130-\frac{P\_0}{81}$
* If the pressure is **steady** compute the forecast number as $Z = 147 -\frac{5 P\_0}{376}$
* If the pressure is **rising** compute the forecast number as $Z = 179-\frac{2 P\_0}{129}$

3. Adjust $Z$ for wind direction:

* For Northerly winds adjust $Z=Z+1$
* For Southerly winds adjust $Z=Z-2$

4. Adjust $Z$ for the season:

* If Winter and pressure is falling, adjust: $Z=Z-1$
* If Summer and pressure is rising, adjust: $Z=Z+1$

5. Look up for the forecast from the following table:

[![enter image description here](https://i.sstatic.net/WCUeP.jpg)](https://i.sstatic.net/WCUeP.jpg)

This was made for the UK, and many people have developed modified version to work in other areas. You will find many implementations online with downloadable code, as [this one in javascript](http://www.beteljuice.co.uk/zambretti/forecast.html), or [this one in C for Arduino UNO](https://github.com/fandonov/weatherstation), an explanation of this Arduino UNO implementation can be found [here](https://cdn.hackaday.io/files/274581174763072/english_description.pdf).

> **Comments:**
> **username_0**: Sir you are a life salver! Thank you so much.  
> **username_4**: P in mb or in Hg both seem to basically give Z values way off the chart, over 100, unless I'm missing something?  
> **username_5**: @username_3-username_3 for =130−0/81 if P0 = 100-hpa, Z = 117, so what the unit of pressure apply to this, I see the original page and it is the same.  

---

### Answer by username_1 on 2020-05-26. Score: -1

In the Zambretti's documentation on Winter the Z number falling is lower by 1 unit, and the Summer Rising improves the prospect to 1 unit, therefore we have to adjust:

Winter Z=Z+1
Summer Z=Z-1

To compensate the derivation.

> **Comments:**
> **username_2**: username_3's answer (and the article cited in the answer) states the opposite.  
> **username_3**: I checked the sources and verified that you have to substract 1 in winter. Although, I corrected my answer as this needs to be done only if the pressure is falling. Do you have a source to support your claim?  

---

