Stevenage has seen a 151% jump in vehicle break-ins over the last three months.
Police say the increase in items like mobile phones and bags being stolen since April, compared to the same period last year, could be down to people leaving their windows open in the hot weather.
In Stevenage there were 161 cases, compared to 66 previously - although the county's average rise is around ten percent.
Break-ins to cars and vans in St Albans is also up - by 39 percent (from 169 incidents to 239).
Vans are also being targeted for tools and equipment stored in them.
Inspector Andrew Palfreyman Hertfordshire Constabulary's Crime Reduction Unit said: "We tend to see more thefts from vehicles when the weather is hot, as people often leave windows open to keep their vehicles cool.
If you leave valuable items in your vehicle on view, this can encourage thieves to try and break in. Make sure that any items are not on view, such as coats, bags, phones and try to remove anything of value from the vehicle when left overnight".
He continued: "Vans are also being targeted for tools and equipment stored in them. Criminals can use a variety of methods to bypass vehicle security systems, so I urge the removal or securing of tools. We are running crime prevention events across the county helping people to secure their vehicles and security marking valuables".