In addition to the current sensor value, each observation also includes the previous value. We can query for a set of current and previous sensor values using a single request, allowing us to identify locations where the weather has transitioned from one state to another.

For instance, to identify areas in Syracuse, NY where the roads have transitioned from dry to moist, our query looks like this:

https://api.helios.earth/v1/observations?city=syracuse&sensors[road_weather][data]=1&sensors[road_weather][prev]=0

We can also specify ranges for both the current and previous sensor values, allowing us to query for broad variations in weather transitions. With the following query, we can identify locations where the road weather transitioned from either dry, moist, wet, or ponding to to either partial or fully-covered snow, indicating that a cold front may be moving through the area:

https://api.helios.earth/v1/observations?sensors[road_weather][data][min]=6&sensors[road_weather][prev][max]=3

Lastly, we can aggregate our search results by the previous sensor values to quickly determine the current transitional state across our result set. In this case, we will query for all observations in Syracuse, NY that reported wet roads. We will also aggregate our results by the previous road weather value to determine how many of those locations reported new values versus unchanged values. (Note: since we are not querying for previous values, we can use the shorthand notation for our sensor query.)

https://api.helios.earth/v1/observations?city=syracuse&sensors[road_weather]=2&aggs=sensors.road_weather.prev