Waterspouts and hailstorms occur infrequently , while snow and tornadoes are rare.是什么意思？

Waterspouts and hailstorms occur infrequently , while snow and tornadoes are rare.

水龙卷和冰雹偶有出现, 降雪和陆龙卷则属罕见.

and

conj. 和，与，而且，于是，然后，因此；

例句
I spend a lot on expensive jewelry and clothing...
我花很多钱购买昂贵的首饰和衣服。

hailstorms

n. 雹暴( hailstorm的名词复数 )；

例句
This area is hit by an average of about 5 hailstorms a year.
这个地区平均每年遭受大约五次冰雹袭击.

occur

vi. 发生，出现，闪现；

例句
Changes in temperature occur slowly and are constrained within relatively tight bounds.
温度变化缓慢，而且局限在相对较小的范围内。

infrequently

adv. 稀少地，珍贵地；

例句
When we are isolated and poor, we are not infrequently forgotten.
当我们一贫如洗, 离群索居的时候,往往是没人想到我们的.

while

conj. 在…期间，与…同时，虽然，

例句
A Dr Matthew Owens was reported missing while on an expedition to north-eastern Turkey.
据报道，一位叫马修·欧文斯的博士在前往土耳其东北部考察的途中失踪了。

snow

n. 雪，雪花，积雪，雪季；vt. 使纷纷落下，使变白，下雪，被雪覆盖，被雪阻挡；vi. 降雪；

例句
...closed roads and atrocious weather: snow in the mountains, torrential rain elsewhere.
道路封锁，天气恶劣：山区雪花纷飞，其他地方大雨倾盆

tornadoes

n. 龙卷风，旋风( tornado的名词复数 )；

例句
Waterspouts and hailstorms occur infrequently , while snow and tornadoes are rare.
水龙卷和冰雹偶有出现, 降雪和陆龙卷则属罕见.

are

vi. （用于第二人称单复数现在时，第一、三人称复数现在时）是；n. 公亩（等于100平方米）；

例句
We are scared to abandon ourselves to our feelings in case we seem weak or out of control.
我们不敢放纵自己的情感，以免显得太过软弱或缺乏自制。

rare

adj. 罕见的，特殊的，半熟的；半生的，稀薄的，经过稀化的，三分熟；

例句
It was one of his rare attempts at humour.
那是他极不常见的想要表现幽默的一次。