How do I start counting when below a threshold and stop counting above the same threshold in a timeseries?

I modeled wind speed data for a whole year and now I want to count the amount of consecutive hours that the wind is below a certain threshold. I have to find out the maximum consecutive hours of wind below that threshold. The arrows show the possible longest periods below a threshold of 2. How can I find out which period is the longest and how long it is?

Thanks in advance!

Answer 1

[Umang Pandey]

Hi Hans,

I understand that you want to count the number of consecutive hours based on a certain threshold.

The following MATLAB Answer where the user wants to count the number of times the signal is above certain threshold, might be of help:

https://www.mathworks.com/matlabcentral/answers/1877942-is-there-a-way-to-put-some-threshold-and-count-the-number-of-times-whenever-the-signal-becomes-above?s_tid=answers_rc1-3_p3_Topic

Best,

Umang

Answer 2

[Fabio Freschi]

Below you can find a possible solution that does not require any toolbox

% signal length
N = 100;

% threshold
t = 0.5;

% your signal
x = rand(N,1);

% plot
figure, hold on
plot(1:N,x,'o-');
plot([1 N],[t t])

% find values below threshold
y = x < t;

% plot
plot(find(y),x(y),'*');

% find changes in vector y
f = find(diff([0; y; 0] == 0));

% start indices of consecutive values below threshold
s = f(1:2:end);

% count the number of consecutive values below threshold
c = f(2:2:end)-s;

% report on plot
text(s,x(s),num2str(c),'FontSize',14)