(KTXL) — Tropical storms and hurricanes rarely reach California, but they have happened before and Hurricane Hilary could possibly reach the state by the weekend.

The last time a tropical storm made landfall in the state was near San Pedro on Sept. 25, 1939, according to the National Weather Service.

A hurricane reached San Diego in 1858, causing $2 million in damages to shipping, shore structures, power and communication lines, and crops. 

Tropical storms and hurricanes form in the Atlantic and Pacific oceans, and their number usually increases in August, peaks in mid-September, and continues until the month of November, according to the National Weather Service.

Differences between a hurricane and a tropical storm

Hurricanes and tropical storms are both tropical cyclones that are rotating systems of clouds that form over tropical or subtropical waters. Their classification is determined by wind speed. 

According to the NOAA, Hurricanes gain energy from warm ocean waters and these storms are classified as hurricanes when wind speeds reach 74 miles per hour or greater. 

A tropical storm is a tropical cyclone with winds ranging from 39 to 73 mph. 

A weather system with speeds of 38 mph or less is considered a tropical depression.

What are the different hurricane categories?

The hurricane categories are rated 1 to 5 on the Saffir-Simpson Hurricane Wind Scale, which is based on the storm’s maximum sustained wind speed and the potential damage it can cause. 

However, it doesn’t take into account other potential hazards such as storm surge, rainfall flooding and tornadoes, according to the National Hurricane Center.

Here are the different hurricane categories and what they mean, according to the Saffir-Simpson Hurricane Wind Scale: 

Category 1

These hurricanes are considered “very dangerous with winds that will produce some damage” and have sustained winds of 74 to 95 mph. 

Damage can occur including the removal of roof shingles, vinyl siding and gutters on homes. Large tree branches can snap and power lines and poles could be damaged, potentially causing outages.

Category 2

Hurricanes in this category have “extremely dangerous winds” that can cause “extensive damage” and have sustained winds of 96 to 110 mph. 

Homes can sustain major roof and siding damage, along with shallowly rooted trees snapping or causing them to be uprooted.

Category 3

“Devasting damage will occur” with these hurricanes, as they have sustained winds of 111 to 129 mph. 

Homes could have major damage or removal of roof decking. Many trees are likely to be snapped or uprooted and electricity and water could be unavailable for several days to weeks after the storm passes, depending on the damages.

Category 4

These hurricanes can cause “catastrophic damage” with sustained winds of 130 to 156 mph.

Homes can sustain severe damage with the loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted along with power poles being downed.

Power outages could last weeks to months, depending on the extent of the damages.

Category 5

Like category 4, this hurricane causes “catastrophic damage, but have sustained winds of 157 mph or higher. 

Homes are most likely to be destroyed and have total roof failure and wall collapse. Residential areas could be isolated by fallen trees and power poles and power outages can last weeks to months.

How do these storms form? 

For a tropical storm or hurricane to develop, the following conditions must be in place, according to the NOAA: 

•Warm ocean waters that are at least 80 degrees
•An atmosphere becomes unstable with differences in temperature where temperatures decrease with height
•Moist air near the mid-level of the atmosphere
•Must be at least 200 miles (with rare exceptions) north or south of the equator for it to spin
•Little change in wind speed or direction with height