为什么我的Arduino不能超过10？(Why can't my Arduino compare above 10?)

 我正在进行Arduino项目，在那里我不断测量气压，并使用Adafruit_BMP085库计算第一次测量的相对高度。 
 
 我显示当前值和峰值。 它基本上可以工作，但只要当前高度超过10米就停止更新峰值。 
 
 草图看起来像这样： 
 
#include <Adafruit_BMP085.h>

String currentAltitude;
String peakAltitude;
int32_t groundpressure;

Adafruit_BMP085 bmp;

void setup() {
    groundpressure = bmp.readPressure();
}

void loop() {
    currentAltitude = bmp.readAltitude(groundpressure);
    if (currentAltitude > peakAltitude) { 
        peakAltitude = currentAltitude;
    }
    Serial.println("Current: " + currentAltitude + "m");
    Serial.println("Peak: " + peakAltitude + "m");
    delay(10);
}
 
 我得到的（当我提高传感器/或增加气压时）是这样的： 
 
Current: 0m
Peak: 0m

Current: 4m
Peak: 4m

Current: 11m
Peak: 4m (still)
 
 为什么它停止比较。 是因为我正在比较的变量类型？ 

I'm on an Arduino project where I constantly measure the the air pressure and calculate a relative height from the first measurement using the Adafruit_BMP085 library.
 
I display the current value and a peak value. It basically works, but as soon as the current height exceeds 10m it stops updating the peak value.
 
Sketch looks something like this:
 
#include <Adafruit_BMP085.h>

String currentAltitude;
String peakAltitude;
int32_t groundpressure;

Adafruit_BMP085 bmp;

void setup() {
    groundpressure = bmp.readPressure();
}

void loop() {
    currentAltitude = bmp.readAltitude(groundpressure);
    if (currentAltitude > peakAltitude) { 
        peakAltitude = currentAltitude;
    }
    Serial.println("Current: " + currentAltitude + "m");
    Serial.println("Peak: " + peakAltitude + "m");
    delay(10);
}
 
What I get (when I raise the sensor/ or increase air pressure) is something like this:
 
Current: 0m
Peak: 0m

Current: 4m
Peak: 4m

Current: 11m
Peak: 4m (still)
 
Why does it stop comparing. Is it due to the type of variable I'm comparing?