<p>This is adding to Dawn's answer.</p>

<p>If you want to check your pressure readings against those of the synoptic charts, use the equation:</p>

<p>$P$ $=$ ${\rho}gh$</p>

<p>Where: </p>

<ul>
<li>$\rho$ is the density of air</li>
<li>$g$ is the the gravitational constant, $9.8$ $m/s^2$</li>
<li>$h$ is the height</li>
</ul>

<p>The average density of air at sea level is  is $1.225$ $kg/m^3$ and the elevation of Johannesburg is $1500$ $m$.</p>

<p>The equivalent column of air would have a pressure of</p>

<p>$P$ $=$ $1.225(9.8)(1500)$ $=$ $18$ $007$ $Pa$</p>

<p>Divide this by $100$ to get hectopascals gives $180$ $hPa$</p>

<p>If the synoptic chart gives a pressure of $1020$ $hPa$</p>

<p>Then, $1020 - 180$ $=$ $840$ $hPa$</p>

CC BY-SA 3.0
2016-01-28T09:47:20.677Z
2016-01-28T09:47:20.677Z
Fred
<p>I live in the northern suburbs of Johannesburg in South Africa.
I have noticed while working with synoptic weather charts in geography class that the isobars indicate that Johannesburg should have atmospheric pressure of around 1010-1020 hPa, just like most cities around the coast, even though Johannesburg is at 1500 m above sea level.
Weather service providers also indicate that Johannesburg's atmospheric pressure should be around that.</p>

<p>What confuses me is that my weather station at home (which I have been monitoring for several years now) never seems to show pressure ratings out of 840-860 hPa.
My outdoor watch gives me the same pressures.
I am convinced that my watch cannot be broken because during my holiday in coastal Cape Town, it gave pressure readings of the expected 1020~ hPa and I can "feel" the higher pressure that I am not used to.</p>

<p>Surely both of my apparatus cannot be wrong.</p>

<p>Is there an explanation for this?</p>

<meteorology><atmosphere>
Why are weather service atmospheric pressures systematically different from those I measure in Johannesburg?
RaymondSWalters
# Why are weather service atmospheric pressures systematically different from those I measure in Johannesburg?

(Asked by: username_0 on 2016-01-28)

I live in the northern suburbs of Johannesburg in South Africa.
I have noticed while working with synoptic weather charts in geography class that the isobars indicate that Johannesburg should have atmospheric pressure of around 1010-1020 hPa, just like most cities around the coast, even though Johannesburg is at 1500 m above sea level.
Weather service providers also indicate that Johannesburg's atmospheric pressure should be around that.

What confuses me is that my weather station at home (which I have been monitoring for several years now) never seems to show pressure ratings out of 840-860 hPa.
My outdoor watch gives me the same pressures.
I am convinced that my watch cannot be broken because during my holiday in coastal Cape Town, it gave pressure readings of the expected 1020~ hPa and I can "feel" the higher pressure that I am not used to.

Surely both of my apparatus cannot be wrong.

Is there an explanation for this?

**Tags:** meteorology, atmosphere


---

## 1 answers

### Answer by username_1 on 2016-01-28. Score: 12

This is adding to Dawn's answer.

If you want to check your pressure readings against those of the synoptic charts, use the equation:

$P$ $=$ ${\rho}gh$

Where:

* $\rho$ is the density of air
* $g$ is the the gravitational constant, $9.8$ $m/s^2$
* $h$ is the height

The average density of air at sea level is is $1.225$ $kg/m^3$ and the elevation of Johannesburg is $1500$ $m$.

The equivalent column of air would have a pressure of

$P$ $=$ $1.225(9.8)(1500)$ $=$ $18$ $007$ $Pa$

Divide this by $100$ to get hectopascals gives $180$ $hPa$

If the synoptic chart gives a pressure of $1020$ $hPa$

Then, $1020 - 180$ $=$ $840$ $hPa$


---

